Great visualization of the unintuitive nature of special relativity. It's kind of mind blowing to realize that if we could accelerate our spaceship at a mere 1G continuously, we could visit the center of the Milky Way in under 20 years (spaceship time), totally do-able in a human lifespan. Of course 27900 years would have passed on Earth, so you wouldn't be able to tell anyone about your vacation.
I suppose you would also have to worry about surviving the trip - every atom in the interstellar medium would damage your ship, likely penetrating the entire way through, and electromagnetic repulsion would only work with charged particles.
Every proton would have the energy of a baseball. It's bananas. Granted, space is really empty, but it's not that empty, somewhere around a hundred atoms per cubic meter. Your ship might look like a very, very long shooting star. Probably dialing the speed down a touch would be worth it for whatever your shielding material is, but who knows? We're talking miracle engines here. I think in the "Valkyrie" ship-on-a-string concept, they had some sort of magnetic thingummy that generated more power as more stuff smacked into it, then as they decelerated they let out this sort of gas mist to go ahead of the ship and smack into things.
If you’re doing 2e8 m/s, your ship is long and thin with a 1m3 nose cone, and space has 100 baseballs per m3 then you’re being hit by 2e10 baseballs a second. How do I get an idea for what 20 billion baseballs (2TJ) feels like?
Apparently bullets are about an order of magnitude more energetic than baseballs (800J vs 80J) so I guess I could try to build my intuition based on being shot ~2 billion times a second instead. A kiloton of TNT is 4GJ, so it’s also like a 500 kiloton bomb going off every second.
Dropping 5e5kg of rock into the worlds biggest dumpster truck at 10m/s yields 25MJ of chaos so it’s also like a parking lot of 80,000 of those being filled with a continuous stream of rubble. That’s probably the best analogy given that we’re talking about machinery — your spaceship needs to have the build resilience of tens of thousands of dumpster trucks but condensed into the cross sectional area of a dinner table.
It's clearly not a solvable problem with current ideas about technology. No amount of ice, rock, magnetism, or Magic Unobtanium is going to make this practical.
I'm not even sure about warp drives. Bending spacetime one way means it has to squish back the other way. The energy released might not affect the ship - possibly - but I'd be surprised if it didn't affect the spacetime it had just passed through.
Some kind of new physics might make all of this possible, but - by definition - we have no idea what that might be.
Worse, it has deleterious effects on the spacetime inside the bubble too. The leading edge of the bubble has exactly the same curvature as a white hole, and the back edge is exactly like a black hole. The result is a beam of Hawking radiation between them. Expect temperatures on the order of 10³⁵K if you manage to stabilize the bubble, but note that this amount of radiation is much more than enough on its own to destabilize it. <https://en.wikipedia.org/wiki/Alcubierre_drive#Survivability...>
So not only do you need “exotic” matter with negative mass to bend spacetime into a warp bubble, the bubble itself creates so much radiation that spacetime is flattened back out again.
It's been literal decades since I saw it, but the premise of K-Pax always seemed neat: aliens move their consciousness around through some sort of superliminal signal[1], but it looks an awful lot like madness to us humans.
Until he starts solving cosmological problems . . I sort of felt like the truth of the matter should have been more ambiguous than was presented. More Shining, less Friday the 13th.
I remember the premise being far superior to the film, but maybe the novel is worth a look.
The theoretical Alcubierre drive basically collects all of the material you would intersect with in a gravitational well ahead of the vehicle as you travel, which is great!
...until you stop, releasing all of that mass as a gargantuan amount of energy at your landing site.
Am I wrong to think it seems probable that such things aren't realistically possible given the fact that the universe seems to be so lifeless?
If the practical limits of rocket technology don't allow life to much beyond their own solar system then given the vastness of space that would be a good reason why we don't see any evidence of intergalactic civilisations or large feats of engineering. All other explanations for why the universe seems to lifeless seem to rely on elaborate hypotheticals like us being an early civilisation, us being extremely lucky/improbable in other ways, or that alien life is anti-social. But it always seemed to me that the best explanation is probably just that such things are not possible.
I mean there's a chance there's some new physics out there, but you'd think if there was star wars level tech out there (warp drives, etc) then something out there would have built one already and would rather quickly spread outwards...
There is so much we don't know. But I would happily engage in speculation:
- Without needing to make it to the closest star, we have big problems here. If we solve those problems before we leave our solar system, we may be changed beyond recognition. We may not be biological any longer, for example, or at least not forcibly so, and traveling as solid matter may seem silly to our future descendants.
- We don't understand well enough the nature of reality. For all we know, our machines and organisms made of atoms and molecules may be, by far, more inefficient and wasteful than an equivalent process at some other layer or scale. Like somebody who discovers themselves living inside a match box in a forgotten attic, we may decide to move to the more spacious main floor of the castle.
- A variation of the above: maybe space-time itself is something we use inefficiently. It could be that a way to stop being troubled by the slow speed of light is by lowering our own "life" speed, increasing our volume to span entire solar systems, and decrease our density so much that your ancestors would confuse us with sparse interstellar matter. Or, at the opposite end, it could be that we find a way to move our entire future civilization to a cubic centimeter of space and a few microseconds that feel like eons.
I don't think you need such extreme relativistic speeds to obtain the desired result though. The above assertion that every proton would have the energy of a baseball puts us at a rather incredible speed. A quick Google gives the energy of a baseball pitched at 90 mph to be around 117.4 J. For a proton to have the equivalent relativistic kinetic energy would put it at 99.999999999999999999999918% the speed of light!!
Now let's take a much more reasonable speed like 10% the speed of light. Assuming the 100 protons per cubic meter figure above, each square metre of ship now only needs to dissipate 2.27 mW of energy. 10% the speed of light is enough time to reach Alpha Centauri within a single lifetime (42 years). And fast enough to visit every part of the galaxy in less than a million years. We could even imagine generational ships travelling at 1% the speed of light (now the energy dissipation demands are 2.25 μW per square metre of ship surface). That's still under 10 million years to colonise the entire galaxy.
If intelligent life is abundant in the galaxy then I don't think the speed of spaceships at least offers a fundamentally insurmountable technical challenge for that life to spread everywhere.
Sending humans across thousands of light years seems almost impossible , but sending von Neumann probes throughout the galaxy should be possible with some reasonable improvement to our technology.
Yes. That's my assumption: a sufficiently advanced race won't send members of its own species into the inhospitable furthest reaches of space, but rather probes that can report data back to the home-world. That was always my issue with the Kardashev Scale: isn't the technological level of a species better dictated by how little energy they use to accomplish some goal?
Von Neumann probes don’t just send information back, they reproduce themselves. Anything that can manufacture a copy of itself from materials scavenged in unexplored territory can probably build anything else you want as well. A Kardashev II civilization would build a Dyson swarm around it’s own star (or one near by if they are cautious) and use a fraction of its power to send self–replicating Von Neumann probes to a few hundred or thousand nearby stars and galaxies. Those probes would build not just copies of themselves but new Dyson swarms to launch them with. Once the Dyson swarm is built there is plenty of energy available to do all kinds of things, like moving planets around, terraforming them, and seeding them with life.
> How do I get an idea for what 20 billion baseballs (2TJ) feels like?
In the 90s, the show Special Relativity’s Funniest Home Videos would often have guys getting hit in the crotch 20B times with a baseball. Honestly, it never gets old.
> A kiloton of TNT is 4GJ, so it’s also like a 500 kiloton bomb going off every second.
Funnily, my first reaction to this comparison is that it makes it seem more plausible to me that this is possible. After all, a Star Destroyer can take gigaton level hits!
But the problem with that is Star Destroyers are fictional.
I've been spending too much time on spacebattles.com.
The canon energy levels given for Star Wars weaponry are so inconsistent with the presentation that one either ignores canon or accepts that the Star Wars galaxy has fundamentally different physics.
Extreme geek caution: I've been running a Star Wars pen and paper RPG since . . oh dear. . 2015, in an ancient simulationist[1] game system, and the only way I've been able to make anything consistent is dialing everything down to WW2 levels. 7.62x5X, .50 BMG, 46 cm/45 Type 94 capital ship weapons. All except for exceptional plot items, like lightsabers and doomsday weapons, which behave . . well, they're magic. Sensors are likewise pretty primitive, enhanced with canon exotics like Kronau radiation, so engagement ranges are (relatively) piddling, hostiles zip by each other all the time. Electronics and technology in general must be barely understood by literally anyone, with the powerful assemblies - hyperdrive cores, droid motivators, repulsorlift "sand" - being exotics, possibly xenotechnology from the deep past mined by xenoarchaeologists[2], but hooked together by varying degrees of "competent 1950s electrician".
In short, it's a fantasy setting with guns that players get excited about. And a community of worldbuilders that is, let's not dice words here, insane. That second point is huge if you're not 24 or otherwise gifted with a combination of hubris and spare time.
[1] All the kids today with their streamlined narrative-focused games! Seriously, though, I get it. The physics simulationist in me that brings me to HERO System says more about me as a person than my players.
[2] Archaeology a much more valuable degree in the Star Wars universe.
Your description reminds me of Star Fleet Battles. As a young teen I made the mistake of treating SFB like Star Trek, while also making the mistake of not realising that Star Trek itself was just doing space combat as ${year of filming}-era naval battles with latex and lightbulbs.
SFB is 1900-1945 naval warfare, but themed with every plot point from TOS and TAS and probably some novels too, so has the Kzinti, Tholian webs, Klingon stasis field generators, and two distinct weapons where the TV series uses just photon torpedoes.
Could you repeatedly send disposable ‘dozer’ drone ships towards your destination to help clear the way, and potentially devise a means of keeping the clearing free of stray atoms?
No, interstellar atoms (or other particles) aren't static, they aren't just hanging in one place. Everything is moving. Some particles are moving very fast.
I’m British with, funnily enough, a Duke’s County cricket ball on my desk.
My nephew is Californian and I gave him one for his sixth birthday. Mine felt a bit lighter so we did an experiment over Zoom together to measure the density. He’s certainly a bit young for that — “it’s the same size but yours is heavier” — but there’s no harm in influencing them from an early age.
Of relevance to your comment: I made a point of doing it in grams and millimetres.
Actually, the American SOP is the original because the British changed the definition of the gallon in 1824, which you will notice is long after the Revolutionary War. No one in America cared, so we kept on using our customary units.
I like that old Arthur C Clarke novel, The Songs of Distant Earth[0], where they travel with an ablative shield in front made of ice, and they can make new shield segments by finding planets with water.
It is not a bad book but I'd say for sure over-hyped. As much as I enjoyed reading it, I would not recommend it as "must have" if someone was into "The Martian". It is not a "must have" more like it is OK for general public and for hardcore sci-fi fans I would say that I can see how it could be disappointing. But yeah no one can easily appease hardcore fans anyway :)
Any recommendations on a mind melter?!?!? Something that is logically consistent is my only criterion (not much of a mind melter if it's incosistent within its own world) ... and the writing not being at a 10th grade level (distracting).
yes, the writing was annoying in parts (especially the Earth flashbacks).
What I liked about it was 1) if you don't know anything about it (I didn't even read the back cover) the surprises evolve nicely; and most importantly, 2) it was a story of humans cooperating with another species rather than either subjugating or resisting subjugation
Yeah I totally agree, the surprises were great.
I knew literally nothing except for the title and the author and that it could have something to do with space, as he wrote the Martian.
I hadn't read any Andy Weir before, but someone recommended it to me so just started reading.
I’d like to see a map of the known universe visualizing atomic density on a log scale.
I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. And on the other hand, I’ve seen claims that “space is really big” as you mentioned; but that claim has always seemed dubious.
One nice illustration of "space is really big" is a fact that if you take the cube with the side equal to the distance from the Sun to the nearest star and fill it with water, then the mass of this cube will be roughly equal to the mass of the whole visible Universe.
Another good one for me is that a cubic light year or butter would immediately collapse into a black hole with a Schwarzchild radius larger than the observable universe.
It's impossible for your fact and the fact you're replying to both be true. Water is denser than butter, and the nearest star to the sun is about 4.3ly away; if your fact were true, the universe would be a black hole.
A cubic lightyear is about 8.468e+50 liters, and butter weighs 911 g/L, giving the mass of a cubic lightyear of butter to be 7.714348e+50, whose Schwarzchild radius is about 121,103,293 lightyears, about 100x smaller than the radius of the known universe.
> if your fact were true, the universe would be a black hole
... maybe it is? Hear my pet theory out.
Extrapolating backwards from the expansion of our universe, the Big Bang model posits a hyperdense state that exceeds black hole levels originating from a singularity, yet it's thought that somehow it did not collapse back, handwaving it as "physics as we know it did not apply".
But maybe physics as we know it does apply. Notably physics as we know it does not imply a specific direction for the arrow of time.
So our universe might very well be a black hole, but we have time backwards compared to the usual way we think of black holes: what we think of as the origin of time and space is what we think of as the irremediable end of time and space in a black hole.
> a black hole, but we have time backwards compared to the usual way we think of black holes
Observations of our universe are straightforwardly understood -- and predicted -- by laying matter fields on an expanding Robertson-Walker metric. The same observations are not at all easy to understand by laying matter fields on a time-reversed Oppenheimer-Snyder-like black hole metric.
The first thing you run into is that at the largest scales (i.e., where the solid angles subtended by galaxy clusters are small for observers like us) visible matter is arranged roughly isotropically and roughly homogeneously: we detect typical spiral galaxies (and more importantly various atomic line transitions associated with them, like the <https://en.wikipedia.org/wiki/Lyman-alpha_forest>) at all sorts of redshifts.
Your homework would be to generate lightlike geodesics that can reproduce these observations at any time in a black-hole-like metric. If you can do that at for a single spacelike slice of your black hole, you then would want to work on evolving that slice using e.g. the <https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...>.
Just scratching the surface of how you would go about doing that would be an interesting research project for a layperson. Among other things, you would end up learning a lot more about what's in your second paragraph, and likely develop an idea about how much work is involved in writing down even a simple "pet theory" of physical cosomology that accords with observational data. Or at least you'd have a better idea of what observational data there is that needs to be accounted for. You'd also confront all sorts of open questions about the interiors of black holes where there is significant matter; that would be timely given the recent preprint by Roy Kerr at <https://arxiv.org/abs/2312.00841>.
I like it! Not sure it works though. We observe the expansion of the universe accelerating, with gravity too weak to counter it. Reversing that would mean it's collapsing faster than the gravitational attraction of it's contents can account for. So either way, gravity isn't enough to explain what we observe.
IANAP, but possibly because you're measuring different attributes of the same thing - i.e., mass - but we as a species don't really understand the fundamentals of what mass actually is.
Before ANGRY KEYBOARD SOUNDS commence, I'm not saying we don't know what mass is, I'm saying fundamentals. I.e., why is mass. What causes it to come into being? Bulk entanglement, i.e., a function of probability or "mass as destiny"? Tiny signals? Lots of rubber sheeting? Etc.
The way it was explained to me is that at the big bang space itself was expanding at faster than the speed of light. So over blackhole density could evolve into less than blackhole density.
> if your fact were true, the universe would be a black hole.
The mass of ordinary matter in the universe is 2×10^53 kilograms, which would have a Schwarzchild radius of 31.39 billion light years. The explanation from popular science communicators on this topic have never satisfied me.
Your maths is correct for one cubic light year of butter. Proxima Centauri is 4.247 light years away, and that gives such a cube of water[0] a mass of 6.468×10^52 kilograms[1], which would have a Schwarzchild radius of 10.15 billion light years.
[0] At STP, which isn't realistic at all
[1] Close enough; I think it was Brian Cox who once joked that in cosmology it is standard practice to approximate π as 1.
This sounded completely false to me initially. When I think of taking mass and turning it into black holes, it involves squashing the mass into a strictly smaller volume.
Thus I would've expected the radius of the black hole to be necessarily smaller than the dimensions of the butter cube.
However, I now realize my mistake - in examples like squashing mount Everest or earth or a neutron star into a black hole, we're starting with masses that are stable/in equilibrium. This would not be the case for a cubic light year of butter!
Further, it looks like the radius is directly proportional to the mass. Given that mass grows cubic with respect to dimension, it's expected the radius of the black hole would eventually outgrow the cube of butter if made sufficiently large...
100 atoms per cubic meter is on the lower end of typical densities in the interstellar medium. It can get many orders of magnitude denser than that.
Outside galaxies you have better chances of surviving high speeds. The intergalactic medium is only 1-10 particles per cubic meter in the web of gas we call the warm–hot intergalactic medium, and possibly less outside of that.
> I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light.
It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor.
The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast.
Of all the stuff on your ship that's not fuel, you'd probably need quite a bit of water for survival needs.
So you would make your spaceship relatively long and thin (to maximize internal volume for a given frontal area), and you would store your fuel (and water) in front of you to serve as exactly that shield.
Bussard ramjets will have problems at higher relativistic speeds. The fundamental issue[1] is that from the perspective of a near light speed system, everything else is near-frozen - which includes things like neutron capture and electromagnetism.
It's sort of funny, but dozens or hundreds of orders of magnitude below, the same sort of dynamics are at work in air-breathing ramjets. The impact velocity of the medium is starting to tell, and the exhaust velocity isn't particularly more energetic than what's threatening to ionize the air around your leading edges.
[1] Well, aside from the fact you're exceeding the average velocity of your exhaust mass
Your fuel is the outermost layer of your shields (modulo whatever is necessary to keep the fuel in place. But you might use magnetic fields perhaps).
That's basically free shielding: you have to carry the fuel around anyway, so you might as well put it to good use. If you run a nuclear fusion reactor, you won't really lose much of the mass of your fuel, unless you want to. Eg you could use the helium you produce as the reaction mass for your ion drive. (I haven't done the numbers to see how the required mass per second for your ion drive compares to the helium mass per second a nuclear fusion reactor would spit out.)
Because it's a free shield, you don't really get to complain about your shield being gradually consumed.
Of course, you can have some extra shielding further inside. You would keep your water forward of your people, but behind your fuel. So your water would not bear the brunt.
Hydrogen doesn't really get all that radioactive: you can use chemical means to remove any helium or so you might accidentally produce; and hydrogen's isotopes are both pretty short lived and relatively easy to separate. (At least much easier than eg enriching uranium.)
Your water and food is also only a very small fraction of the overall mass of your rocket: as always, the vast majority is made up of fuel.
Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel.
And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water. So if any tritium forms (which is an extremely common by-product of any fusion reactions which might happen, and the most common decay product of heavier hydrogen isotopes), it will render your water quite poisonous for human consumption, virtually irrevocably.
> And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water.
On the contrary, it's quite easy to do if there is any significant fraction of tritium present. The proportional mass difference of ³H vs. ¹H is x3, which alters the chemistry enough to make separation easy. You can use fractional distillation or electrolysis even for ²H — even mere hobbyists involved in the DIY fusion reactor scene sometimes extract deuterium from water this way, tritium would be easier.
With a half-life of 12 years and the time interstellar voyages take, you can just let your tritium sit around for a while. (In addition to what the other comment said about separation actually being relatively easy.)
Depending on your fusion reactor, you might actually highly value any tritium produced, instead of seeing it as a nuisance.
> Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel.
With a nuclear engine you make a difference between fuel and propellant. When you fuse your hydrogen into helium, you still have the helium afterwards.
I haven't run the numbers to see how your fuel consumption would compare to your propellant consumption for a reasonable fusion powered rocket. Though I suspect that you also need oodles of propellant, given how the rocket equation works.
For the first part of your journey you could rely on passive shielding via your propellant. For the latter part, you could use more costly active shielding like a big magnetic field or ablative shields in front of your main rocket etc.
Basically, you would still want to use your propellant as free shielding as much as possible.
The vacuum in solar space is around 10^7 atoms per cubic metre; the vacuum of interstellar space is around 10^6 atoms per cubic metre; and the vacuum of intergalactic space is around 1 atom per cubic metre.
Even if you managed to avoid matter, at some point, photons will start to become a problem. As you continue to accelerate, eventually, the cosmic microwave background itself will become deadly X-rays.
X-ray sources would turn to gamma rays. Not that it’s any better that X-rays. Other comments suggested lead plates. It would quickly get irradiated and would probably need to get shed as soon as you got to a destination.
No accounting for particles yet, which you'll also keep hitting, making your ship's materials radioactive and causing lots of secondary particle showers, bremsstrahlung and the likes.
First the particles will act like radiation, then they'll start causing matter-antimatter pair creation with your hull, then you'll get some exotic heavy quarks popping into existence, then you'll get some Higgs particles forming and at some point questions like "what is the mass of my ship" stop making sense.
No--x-rays will not make anything radioactive. Gamma rays can only do so when they're in the 2GeV range or higher (indirectly, through pair production.)
It's particles that make things radioactive, mostly neutrons as they aren't repelled by the nucleus and thus have a much easier time getting in. And while it would be hard to construct a shield out of it helium is effectively immune to becoming radioactive under neutron bombardment. And while lead isn't immune the reaction sequence produces nothing that won't be contained by the lead and it self-regenerates, enough bombardment returns it to where it started.
> somewhere around a hundred atoms per cubic meter.
Wikipedia says that intergalactic space contains less than one hydrogen atom per cubic meter; and that most of the baryonic matter in intergalactic space consists of hydrogen and helium atoms. If I've understood it correctly...
Well, I don't think the discussion was restricted to interstellar space; for example there's been quite a bit of chat about how long it would take, at 1G (on the astronaut's watch) to reach the edge of the (known) universe.
> Every proton would have the energy of a baseball
I wonder if you could capture that energy and use it to generate thrust.
Most of the energy is coming from your thrust so it'd be a lossy process however if you're able to capture all of the energy then there won't be anything left to damage the ship.
Isn't this kinda like mounting a fan on your car's roof to charge the battery? You have accelerated your spaceship to 0.995C (or whatever) and now you are encountering space dust at a phenomenal rate. Some of that dust is moving away from you, some of it toward you, some of it is at rest. On average it's all just sitting there unaware that your vessel is about to smack into it. The energy is in the difference between your speed and the particle's. If you try to harness it, you slow down.
(I'm asking genuinely here. My analogy might be wrong because it's too classical!)
The old Bussard ramjet concept was to capture these high velocity protons (with some kind of magnetic field), cause them to fuse, and use the fusion energy for propulsion.
There are a few engineering difficulties with the idea but it makes for some good SF stories...
Fantastic book. I read it for the first time about a year ago but I still think about it once every week or two. Thought about it as soon as the "spaceship" started to accelerate towards light speed.
Relativity effects will start to bite. The proton's going to stop being interested in your magnetic field[1], and you're approaching the velocity of the exhaust mass of the fusion reaction.
[1] The momentum vector just completely flattens almost any other physical characteristic. I'm not sure there's even enough time for nuclear fusion to take place.
There's two different kinds of energy here - the kinetic energy of a moving thing hitting you in opposition is a problem. No way to capture that as far as I know - it's like running into a wall and asking how it can help you go faster.
But then there's e=mc^2, so if the stuff you're running into is the fuel source for your fusion engine (could be a fission engine, but unlikely you'll run into heavy atoms like Uranium or Plutonium) then you have an unlimited source of energy...
So maybe sort of? Running into things slows you down, but then you capture that mass and release the energy out of it to go faster... because of the nature of e=mc^2 you'll usually get more energy out of something if you convert its mass than what you lose by running into that mass.
I’m imagining a ship with a hole in it and a piece of fuel at the backside where the particles hit. It would slow you down first since you’re tethered to the particles, then the explosion would push you forward.
The issue is that the energy for that explosion comes from the slowing down of your ship, so it doesn’t work.
The energy that comes from a mass-energy conversion greatly exceeds kinetic energy losses.
For the same reason that the power of an atomic bomb does not depend on how fast you smash the sections together - it depends on how much mass is converted to energy in the resulting reaction.
Imagine you drive into a wall of tnt, break through it, and as you exit it explodes and gives you an extra boost. Yes, you’ve lost some speed at the beginning but you’ve gained much more
They're saying a fastball pitch-- 340 joules or so of energy.
The thing is, you need to be going implausibly fast to have a proton have a fastball level of energy. Even at .99999999 C, a hydrogen atom still has less than a microjoule of energy. You need a lot of 9's to get up to 340 joules.
1/sqrt(1-(0.99999999)^2) * (1 atomic mass unit * (speed of light)^2
1.05529895 × 10-6 joules
Where'd you get 100atoms/m^3? I remember in astrophysics we learned it was closer to 1 proton/m^3, but maybe that was the universe average (which includes the vast and desolate intergalactic medium)
"Although the density of atoms in the interstellar medium is usually far below that in the best laboratory vacuums, the mean free path between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles."
Because the entire Milky Way is mostly at rest relative to itself.
If you are charging full speed ahead into its center, you are going against just about everything, including energetic particles no doubt coming the from the crowded center of the galaxy.
More like bridges and skyscrapers, perhaps with an ablating sheild at the lead end, maybe an electromagnetic field to divert particles away.
If the thrust is high enough, a tenth of a G to 1.5 maybe, the ship has to "stand up" on the thrusting engine in the same manner as a building must stand up over it's footprint, supporting itself against the force of (artifical) gravity.
If it's higher thrust (as the human meatsacks are suspended in a fluid they've also swallowed ??) then the ship has to look even more like a heavy load brutilist building.
It's more stable (and structurally leaner, I think) to instead have the engines at the front in tractor configuration, as in the interstellar ships in Avatar movies.
Unless you could somehow make an Alcubierre warp drive.
Of course…even if that was possible, it’s conjectured that the colonists already at your destination won’t appreciate you boiling the atmosphere when you hit them with blue shifted radiation.
> it’s conjectured that the colonists already at your destination won’t appreciate you boiling the atmosphere when you hit them with blue shifted radiation.
There's a general principle known as Jon's Law (not sure where the name comes from) that any powerful space drive is by definition a weapon of mass destruction.
You see this in The Expanse when incredibly powerful fusion torchship rockets (a major part of the Expanse 'verse) are attached to asteroids and these are used to kinetically bombard inner planets. The results are far worse than a nuclear attack, from kinetic energy alone.
Anything capable of traveling close to the speed of light would be "death star" level planet killer. We're talking smashing through the crust and boiling off the atmosphere or if it were massive maybe even fragmenting the planet. Obviously anything even wilder like an Alcubierre Drive would be likewise. Anything capable of going to the stars within a human lifetime could annihilate worlds.
Even present-day chemical rockets could be pretty destructive. Get something massive that won't burn up (like a rod of tungsten) up to interplanetary velocities and you can approach the yield of a small tactical nuke from just kinetic energy. This has been studied at least on paper by militaries. I think the phrase "rods from God" was used by DARPA at one point for the rod of tungsten idea.
This is glossed over in the vast majority of space sci-fi. Nobody even asks in Star Trek what happens if you point the Enterprise at a planet and say "warp 9, engage!" I'm guessing it would go poorly for the Enterprise but even worse for the planet.
Most hyperdrives just need to be Neptune’s distance from a star to work - two light hours; the Q-II needs to be five - Pluto’s! This means it can get you from any given human world in Known Space to any other in no more than eleven hours, but also no less than ten hours for any world outside the system.
There’s no intermediate setting. With most hyperdrives, a pilot can leave the helm unattended most of the time. If one does so in a Q-II for more than two minutes, they’re almost certain to crash into a star. It doesn't have an on-off switch, either, it has a grip that has to be kept or the drive turns off.
> There's a general principle known as Jon's Law (not sure where the name comes from) that any powerful space drive is by definition a weapon of mass destruction.
Also known as the Kzinti Lesson, from Larry Niven's Known Space series. I've not read where in that series this term is first introduced, but they're somewhere in all that.
> Obviously anything even wilder like an Alcubierre Drive would be likewise
Not yet known; the original Alcubierre Drive is a toy model that demonstrates the point, but has so many problems with it that, as is, it definitely won't work.
Something else along similar lines that does work? The only thing it won't act like when it hits something, is like being hit by normal matter that's actually moving at the speed of light, because if it did it would also be an infinite free energy source.
I'm thinking of the Lensmen series--the stardrive ejects non-interacting particles and thus doesn't tear things up. And since it's inertialess you can't use it to accelerate an impactor. However, you can go grab something that's already moving how you want, slap an inertialess drive on it and reposition it so that when the drive is turned off it's heading for your target. When it's a planet you fling around that can be pretty dangerous. And when it's a FTL planet (everything is Newtonian, this doesn't cause issues other than for the crew--can't allow one atom of native matter into your ship, can't use one atom from your ship in the drive on the planet) the results are spectacular.
(Note that they also have normal reaction drives in the Lensmen universe--the stardrive will get you to your objective but you still need to match velocities with it. And that does tear things up and we see it's use as a weapon, although not against a peer-class opponent.)
> Even present-day chemical rockets could be pretty destructive. Get something massive that won't burn up (like a rod of tungsten) up to interplanetary velocities and you can approach the yield of a small tactical nuke from just kinetic energy. This has been studied at least on paper by militaries. I think the phrase "rods from God" was used by DARPA at one point for the rod of tungsten idea.
Also known as "Project Thor", it was devised by Jerry Pournelle before he became a science fiction author. More on various iterations of the concept can be found in Wikipedia:
I think that law is basically Newton's second law of motion. F=ma essentially tells you that anything that decelerates a lot, such as a very fast spaceship crashing into the rock of your planet, pushes with an awful lot of force before it stops.
Edit: to be slightly more pedantic, the right form that still remains true with relativity is F = dp/dt, i.e. the force the spaceship would exert is equal to its change in momentum.
in niven's "known space" universe that was known as the "kzinti lesson"; the kzinti
were a warlike race that thought humanity would be easy pickings because their telepathic spies said they had a civilisation completely at peace. turns out humanity figured out really fast that their mining lasers, fusion drives, etc could be used as weapons when the hostiles showed up.
> Even present-day chemical rockets could be pretty destructive. Get something massive that won't burn up (like a rod of tungsten) up to interplanetary velocities and you can approach the yield of a small tactical nuke from just kinetic energy.
Of course, that energy didn't come for free: if the rod came from earth, your rockets have to provide the energy.
> This has been studied at least on paper by militaries.
I thought these weapons were real and deployed. Specifically, I understand that hypersonic missiles don't really need an explosive warhead; a hypersonic tungsten rod would make a bigger explosion than any conventional warhead.
Hypersonic missiles are different. I think the idea with those is whether they have a warhead or not they come in so fast you can't possibly shoot them down. The US, Russia, and China are all either confirmed or rumored to have hypersonic delivery systems like this.
The "rods from God" concept is the idea of creating an artificial meteorite as a weapon that comes in from space. These may or may not already exist, but if they do they'd be secret and would probably violate some treaties.
Regarding the speed: I read somewhere that hypersonics create a "wall" of plasma in front of them. The plasma neither transmits nor reflects radio, so the missile becomes invisible to radar, at least from the front. I have no idea whether that's true.
> Unless you could somehow make an Alcubierre warp drive.
Even if you can make it, even though it's theoretically possible that the warp bubble could move through space faster than the speed of light, it's a separate and completely open question as to how you might actually get it to move that fast to begin with.
They implied they avoided that because warping into a gravity well would cause some vague catastrophe.
Of course, the real reason was far more sinister: it’s way more dramatic to slowly creep up on the planet while listening to the captain’s log monologue to start the episode.
If nothing else, if you miss just a little dropping out of warp inside a planet, must be a Very Bad Thing in-universe. The ramifications would be seen even out-of-universe. ;-)
Sure, if your probability distribution looks at every point in the universe equally. But we're talking about introducing error in a situation where you're _trying_ to drop yourself right next to a planet, so the areas nearest to your target have a much greater probability.
The volume of space up to only geostationary orbit is about 177 times bigger than the volume of the earth.
Given that Star Trek's impulse drives are already traveling at up to around 0.9c, parking somewhere between the earth and the moon (which is about 1 light-second out), the ratio of space to volume of earth becomes 219,648.
That ratio growth with the cube of the distance to the planet.
If we’re talking about the known universe, the odds of your near-light navigation accidentally clipping a not-mapped-yet planet are certainly not zero.
Our gaze into the heavens is much better at spotting stars than their dark orbiting bodies, and we have fingers left over from one hand counting the number of observation platforms observing deep space from above our shimmering atmosphere.
Indeed. We can't even agree on whether or not there's an extra Neptune-sized gas giant out beyond the orbit of Pluto, and that's right here in our OWN solar system!
So? Even ten extra Jupiters or thousand extra suns would take up only a tiny amount of space compared to the size of the solar system out to Pluto:
The distance from the sun to Pluto is about 5.9 billion km. The radius of the sun is about 696,340 km. The ratio of radii is about 8,473. Cube that to get the ratio of volumes, and you get 608,263,848,559 for the ratio of volume in a sphere out to Pluto vs volume of the sun.
(Doing the numbers, I'm actually surprised: I had expected the ratio of radii to be bigger than 8,473. But I'm not surprised that the sun barely takes up any space.)
Arguably, the entire heliosphere is part of the Sun's atmosphere, and it reaches well beyond Pluto. If I were in a relativistic spaceship, I think I'd want to apply the brakes well before slamming into the heliosphere.
What do you call the heliosphere of a star that isn't the Sun? The stellosphere?
"Stellosphere" is wrong, because "stella" is Latin, and "sphere" is from Greek. It should be "asterosphere", but that's a word I've never seen nor heard.
> If we’re talking about the known universe, the odds of your near-light navigation accidentally clipping a not-mapped-yet planet are certainly not zero.
If we're still talking about Star Trek, then on a solar scale those ships can stop on a dime. They're not going to hit an unmapped planet while putting around.
It doesn't matter whether we can see those dark orbiting bodies: we observe minuscule gravitational impact on the stars, and that places a very sharp upper limit on the amount of mass that's outside of the star in a solar system.
The sun contains roughly 99.8% of the mass of the solar system, and is by far the largest object in it. But you wouldn't hit the sun randomly either. Space is just so damn large.
Sure, there are plenty of practical problems that would make the trip impossible. The fuel mass alone that would have to be shot out the back to make that trip would be something on the order of 800 million times the mass of the payload (you). So a 100kg person sitting in a 100kg spaceship would require something like 160 billion kg of fuel, assuming zero energy loss in burning the fuel. Relativistic rocket calculators are fun!
Fun fact: if you got 250 MPG in a space car loaded up with as much gasoline as there is water in the ocean, you could could drive to the edge of the observable universe.
Momentum but no mass, if I recall my physics correctly (low certainty).
Wait, hang on, we have access to an appreciable-chunk of the world's knowledge at our fingertips...
https://en.wikipedia.org/wiki/Photon - "Photons are massless[...]In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p[...]Current commonly accepted physical theories imply or assume the photon to be strictly massless"
Typically momentum is thought as mass times velocity, and since photons do have momentum, there was a desire to give them some kind of "relativistic mass".
In more recent times, it has been seen as easier to use just one concept of mass, and to redefine momentum entirely. So, photons have a mass of 0, and we don't need to specify "rest mass". But they do have momentum.
no, the earth is actually big. 47 trillion times heavier than that mass. At 90% c that 160 billion kg would still be a small fraction of the planets (moving 10,000x slower, at 30 km/s) kinetic energy. 500,000x less.
Of course that .06 m/s velocity change would be near instant, so bad things would happen. Probably a humanity-ending but not life-ending disaster. Global tsunamis and incredibly large tectonic changes, for sure. Imagine the entire water column of the marianas trench jumping up in the air and slamming into the ground below.
If the energy was transferred at a single point, it'll be the worst extinction event ever (4000x worse than chicxulub) but I'd bet single-celled and maybe even some multicellular life would survive. Anything bigger than a mouse is fucked, though.
Yes because ions are... ions. You don't see photons knocking the planet out of orbit, do you? They can exert force but they don't exert that much force.
But if you're trying to avoid self-propulsion and want to launch from Earth, say, even a 200kg craft, anywhere "close to the speed of light", then that will most probably require a significant enough amount of force to knock the planet out of orbit.
If you can apply small force over a long time, that will get you up to speed, too.
Someone did the math in the thread, and suggested that a constant 1g of acceleration would get you to the centre of the galaxy in 20 years (as measured by the clocks traveling on your spaceships). 1g of acceleration for 200kg is about 1962 Newton.
(This back of the envelope calculation assumes you have eg someone fire a laser at your ship to give you the energy you need. If you need to bring your own fuel, the rocket equation increases the total mass needed. But the same principle still applies: something like an ion drive has very little force, even if the top speed it can reach can be enormous.)
I think everyone is collectively ignoring that I'm specifically talking about accelerating the craft from earth, not using an ion drive or similar form of propulsion on-board. As that's what is implied by the comment that I responded to: "If you could shoot it out at close to speed of light"
Just need to build a parabolic reflector on one side of it and point it towards the opposite direction where we want to go. When the reflector shoots to far from the same, tilt the reflectors, drop down closer to the Sun by gravity, tilt them back, do it again.
Yes. And that's not the only way to make this work.
Btw, the sun is also an extremely inefficient engine. With a bit of extra engineering we could probably scoop up hydrogen from the sun, and 'burn' it much more efficiently.
Fun thought, we’re kind of all on one right now! Regardless of how you choose to define “human,” we’ve been around for much less time than a single revolution around the Milky Way. We get no say in the route, and our star is feeding us rather than fueling our travel, but still a wild thought.
Or stopping. It takes the same amount of time to slow down as it did to speed up. Presumably, you'd have to rotate 180° so you are now thrusting in the opposite direction. So at the speeds you've reached to get there in 20 years (ship time), you'd just race on by.
> takes the same amount of time to slow down as it did to speed up
Assuming you're going somewhere, you can use atmospheres and gravity to slow down. Decelerating should take less time than accelerating in practical contexts.
That is doable for "normal" speeds, not one where you accelerate 1G for twenty years, reach relativistic speeds that make you travel for hundreds of thousands of light-years in merely 20 years ship time
> doable for "normal" speeds, not one where you accelerate 1G for twenty years
It's still a lot of energy you can bleed off, particularly if you're aiming for a system with gas giants. I'm not suggesting one only rely on passive deceleration. But especially given it's fuel saved at the very end of the journey, fuel you no longer need to accelerate and decelerate for the entire duration of the trip, the savings could be sizeable.
the top of this thread is filled with a discussion of the almost unimaginably catastrophic consequences of a ship moving at 0.9C hitting atoms in interstellar space.
trying to decelerate by "braking" anywhere close to a gravitionally significant mass sounds like a guarantee to total destruction from the impact of "stuff" (even individual photons).
You decelerate from 0.9C to 50 km/s conventionally, more if you can aerobrake or line up multiple slingshots, and that last 0.00001% with gravity assist.
It saves you more than that in fuel, because the fuel you'd have used on that last bit of deceleration needed to be accelerated and decelerated the entire way from 0 to 0.9C back to close to zero.
> even 2Gs of force wouldn't be tolerable for humans for more than an hour
You can tolerate 2G for hours, particularly if everyone's oriented eyeballs in. That said, both aerobraking and gravity assists are intermittent accleerations.
so you've been traveling at some large speed for quite some time, and you are now proposing to come to a near stop to come into orbit around some far away planet? and what magical tech have you forgotten to tell us about that allows that sudden deceleration to not liquefy the bags of meat inside the ship?
I remember a StackOverflow question about a one-ton tungsten(?) ball heading towards Earth from the edge of the solar system, I forget how many nines after the decimal point in it's velocity. The question was what what would happen to Earth--and the very surprising answer was basically nothing. The ball would be vaporized very quickly by the interplanetary medium and disperse long before reaching Earth. Some of the energy would hit but it wouldn't be enough to notice.
My ship has a quantum shield that changes the location of incoming atoms; putting them just a bit to the safe side of the ship. Doesn't work with molecules though but that's coming up on v2.
I wonder how fast the interstellar medium of gas etc. drifts and flows... could sacrificial sweeper-ships drill out a less-dense-holes for other traffic on a reasonable timescale?
I read a paper on the topic a few years back. My recollection is that once you go faster than about 0.3c it becomes impossible to shed heat faster than you gain it from collisions with Helium. I'll try to dig it up.
The conservation of momentum means that whatever system you devise, the spaceship would have to eventually withstand the forces related to the total dP/dt required to get the obstacles out of the way...
You could change the distribution of the forces at best but I'm not sure whether that could be enough...
There exists nothing, even theoretically that you can put on the spaceship to get you that 1G continuous acceleration.
I don't remember exactly what is the maximum achievable delta V but if I remember well it is not a high portion of the speed of light, more like 60% of speed of light. And that assuming you have crazy things like carry a small black hole with you and use it to convert mass into energy at 40% efficiency.
It's worth saying that 1G is heavy acceleration to maintain, nothing "mere" about it. When you run the numbers the amount of energy involved basically adds up to needing a rocket made entirely of antimatter, and a similar mass of 'normal' matter to react with.
Keep in mind that when talking about long-distance journeys in space under any sort of constant acceleration, the numbers are generally in thousandths of 1G.
It would be cheaper in every sense to turn something like a giant asteroid into a rotating habitat ship than achieve a constant 1g to even the closest systems. Realistically interstellar travel is not a thing that (biologically modern) humans will ever be suited for, robotic probes don't need thousands of kg of food and water to stay alive, don't need artificial gravity, air, or entertainment. The fact that the trip is inevitably 1-way won't bother probes and robots either.
You are right. One addendum: many humans would be bothered by one-way trips, but humanity is large, and in absolute numbers there are still plenty of volunteers for one-way trips to the planets and stars.
When talking about how efficient a given reaction mass engine, such as a rocket, can be the unit is specific impulse in seconds. That unit represents how long a given engine carrying a given propellant can maintain 1g acceleration.
For context, the most powerful chemical rockets peak around 450s-530s. A nuclear rocket of the sort we can build today would be more than twice that value, and super-efficient ion thrusters can have IspS in the tens of thousands of seconds.
But we're talking about an engine with a specific impulse measured in decades, and as far as anyone knows that means having catastrophic amounts of antimatter. I don't think plucky explorers on a one-way trip are going to have access to a small moon's worth of antimatter, and if they did, imagine how many more interesting things they could do with it than fly to nowhere?
Like power an entire planet for centuries, or use it as the energy budget for a megastructure project like a Dyson swarm.
We're talking about a truly incomprehensible amount of energy, not just to carry the rocket and its own fuel, but the tens of thousands of kg of water and food for even a modest compliment of people.
And how many in that group are actually fit to spend many years living that lifestyle without losing their mind and getting into deadly fights with one another?
And who's covering the great expense of building these generational colony ships and training their only inhabitants, only to have them zip away never to be heard from again? (With no benefit other than believing there's a slight chance we've succeeded in making our species multi-plantary)?
As humanity becomes more numerous and richer, the fraction of humanity you need that have such strange ideas and desire keeps shrinking.
(Btw, I share your intuition that the number of people willing to sign up for a one-way trip to the stars, or even just Mars, is fairly low. However, if you already look at people who are dedicated enough to become astronauts, I suspect the additional filter of asking for one-way-trip volunteers for a mission to the stars isn't all that severe. My purely speculative guess is that at least 10-20% of current astronauts would be willing to sign up.)
Yes. There's currently way more people who want to be astronauts, than people who can be astronauts. So you get some extreme selection effects; and those tend to bring out weird people.
I was really thinking of astronauts who have actually been to space; I wonder whether the experience disposes people to kookiness. I suspect that trainee/aspiring astronauts might suppress their kookiness.
Rotation wont really work unless the structure is gigantic (many miles in diameter) otherwise you will suffer from dizziness. Rotating spaceships like in many movies are probably making everyone on board sick.
This is true, it's why I suggest using a large asteroid for just that reason. Either way though, I promise you that altering a human's vestibular system to accommodate spin is a MUCH easier problem to solve than generating gigatons of antimatter fuel.
Its a true mind f*. A round trip would take the best part out of a normal human lifetime but it would appear that you have travelled over 55 thousand years into the future when you return the earth...assuming it's still there.
Time travel seems so easy and we could do it with the tools we have now for the most part. Maybe we’ll never go into the past but relatively contemporary humans are going to be all over the future in all kinds of places.
After the first time it happens people in the future can start to expect a human from the past to keep visiting every so often.
If that did start happening, it's not like the people from the past would just pop up suddenly. We would be aware of their journey. It's not like a time machine where someone just disappears from the past and appears in the future.
Sure, and we would probably be able to tell when they were from based on the approaching crafts. It’s going to be amazing to be able to step 50,000 years into the future to see how humanity has turned out. I’d give up anything to have a chance at that.
If you stop to think, a trip to the future is a trip to immortality because you can go to a time where the technology to live forever will be available.
Unfortunately humanity is constantly on the verge of self destruction so I would expect to come back to a radioactive smoldering place with some survivors roaming around still trying to finish one another with sticks and stones.
Assuming a straight line and assuming 1G is your max acceleration: you accelerate for 10 years, reach your maximum velocity then flip and do the same thing in the other direction. You'll reach your destination with 0 velocity.
Do you mean that someone in the space ship could, without extending their own lifespan beyond what is already expected, use the space ship to "travel" to Earth's distant future?
Kip Thorne said it's theoretically possible. One end of the wormhole is on earth, the other is on the spaceship. Then you fly the spaceship and hop into the wormhole and arrive into the future on Earth.
You don't even need the wormhole. You just accelerate away to relativistic speeds then turn around and come back. It doesn't matter which direction you're going, just that you're going _fast_.
Iirc to get to Proxima Centauri at a constant 1g would require a fairly impressive mass of antimatter, to continue to the edge of the observable universe would be so many more orders of magnitude more energy than that it's barely worth discussing. Even though we can't reach c, as we start to approach it the energy requirements rise catastrophically, along with the risk of colliding with some random mote of Hydrogen and remembering that your relative velocity is .99999c, in the moment before you and your ship are atomized.
What if the front one is only a small fraction of the thrust at the rear? Would that be enough to "clear the path". Surely it would cost some efficiency but it may beat he alternative
In other words the space-equivalent of a supercavitating torpedo like the Shkval [1], more or less an underwater rocket which creates its own gas bubble around the projectile by deflecting part of the exhaust stream to nozzles on the front of the device.
This is already an unrealistic thought experiment, since continuous 1g acceleration for 20 years is so ridiculous that it requires a magic engine (i.e. consuming most of the ship mass in a matter-antimatter reaction) so if we're at it, we can also assume magic shielding which will also shield the crew from all that antimatter and its annihilation.
It's not a coincidence that the original post relativistic spaceship doesn't even bother considering larger accelerations than 0.1g, since achieving even that is a wild assumption.
You can’t “get to” light speed, that’s one of the big punchlines in relativity.
If you pick an acceleration equal to the acceleration we experience on Earth (aka 1g, aka ~9.8m/s^2), you hit relativistic speeds (speeds at which you need to take into account the effects of relativity to do anything) surprisingly fast. On the order of hundreds of days. So, it is not really a matter of safe acceleration on a long space trip. Instead you have to worry about the actual speed you are traveling at—even though space is very empty, there are still atoms floating around out there, and you’ll be moving at very high speeds relative to them, leading to interesting collisions.
It's kind of funny that the actual big punchline is that light speed matters at all. There would be no meme of "reaching lightspeed" without relativity, despite that meme originating from relativity specifically mentioning lightspeed as something you can't reach.
> Instead you have to worry about the actual speed you are traveling at
That's the other big punchline in relativity: There's no one "actual speed" because that implies that there is a single "important" frame of reference wherein those atoms are floating around waiting to be hit by a spaceship.
> A comoving observer is the only observer who will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.
What if you define the "important" reference frame as the speed of light's (in the same direction you are going). Since c is universally constant, it seems like a reasonable privileged reference.
Doesn't really work this way. A lot of the wonkiness in SR is tied to the fact that the speed of light is the same, measured in _any_ reference frame.
So, say you're on earth and you measure the speed of light... you find that it's c (~3x10^8 m/s).
Now you get on a spaceship and accelerate to 0.5c with respect to earth, and you measure the speed of light relative to your spaceship... still c!
In this way, you can't really define a reference frame with a speed "the same as the speed of light". And if you try, you'll run into nasty infinities in all your equations that will cause them to blow up and stop being useful.
So depending on how you measure, you’re always stationary or moving near light speed, or somewhere in between, depending on your measurement reference (the thing you’re moving relative to)?
How is there a speed limit at all, if that’s the case? You can accelerate to 0.5c and then toss an apple out the window and say you’re moving at the speed of an apple tossed out of the window, relative to the apple. You have all of c available as headroom again? You can accelerate up to 0.5c again, relative to the apple you tossed out the window?
I am imagining you will say that it will seem like this is what is happening to folks in the spaceship, but what’s really happening is that time is slowing for the spaceship and it’s passengers, and that they still can’t reach c. Fine. But c relative to what? There is no absolute c because there are no truly fixed points, so c relative to what?
There is no underlying reference frame. All motion is relative. Everyone, no matter how fast they are already going, will measure the speed of light as c. Accelerate to .99c and shine a flashlight in front of you. That light is moving ahead of you at the speed of c. Because to you, you are not moving.
That's true for the laws of physics, yes, but our universe does have a 'natural' frame of reference.
> A comoving observer is the only observer who will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.
> You can accelerate to 0.5c and then toss an apple out the window and say you’re moving at the speed of an apple tossed out of the window, relative to the apple. You have all of c available as headroom again? You can accelerate up to 0.5c again, relative to the apple you tossed out the window?
Yes you can. You can even do it with 0.6c for both those speeds.
it's either "relative to any observer." or "relative to any inertial reference frame". no matter where you go (on the ship, on a planet you pass by, on another ship) you will never see the apple travel as fast as the photons coming out of your flashlight. Depending on where the observer is, they will see the apple accelerate to 0.5c (if they are aboard the ship) or they will see it gain mass (or rather, see you throw it more slowly as if it had gained mass), contract in the direction it's thrown, and slow down (due to time dilation...relative to the moving frame).
The case I don't know how to answer is two apples thrown at each other, each with a speed greater than 0.5c.
Suppose there is a starting point A from which your ship is moving away from. At the same time, a photon is shot out from A. You can take the distance traversed between A and the ship as D1, and the distance traversed by the photon as D2. Then your "percentage of C" is D1/D2.
How can you get the distance D2? I'm not sure. I guess we have to pretend it's also a ship that is traveling at C that can emit information to us (also at C :p )
Not a physicist, but my impression is that you're always going 0 percent of the speed of light (in all directions) from your own frame of reference. All you notice is that our solar system is moving away, faster.
I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front.
> I guess you'd notice a change in light frequency based on the light in front/behind. Redder behind, bluer in front.
The light in question is not uniform like white noise; the spectral power distribution has relatively light and dark lines in them as a result of the physics of the bright sources and intervening gas and dust. Those features also get redshifted.
If one is moving relative to the sun, one would pay attention to the sun's Fraunhofer lines <https://physics.weber.edu/palen/clearinghouse/labs/Solarspec...>, which would be Doppler shifted to different wavelengths. These lines also appear in reflected light from bodies in the solar system; if you were flying towards Pluto you would see a corresponding blueshift of the reflected Fraunhofer lines (plus some additional structure related to the chemistry of Pluto; it has some luminescence, as does our moon, as do the leaves of plants, and luminescence tends to impinge on the narrower Fraunhofer lines).
Indeed, measuring the Doppler shifts of multiple known-chemistry light sources is a useful technique in navigation of spacecraft within our solar system; it can in principle do better than precision measurement of angles to multiple light sources.
The spectral distortions of the CMB are certainly interesting, but it's hard to imagine their utility for spacecraft navigation within the Milky Way, rather than helping to physical cosmologists understand why there even is a Milky Way.
In the solar system we have kind-of the opposite problem: in order to get reliable anisotropy data of the Milky Way, probes like WMAP need excellent almanac data for the ephemeris of Jupiter (it's a bright reflector of sunlight and its cloud-tops at ~70 kPa are ~22 GHz microwave-bright; I gather other outer planets are used too, but the details are beyond me) to check its 22-GHz-band detection of the CMB Doppler shift in the directions it looks.
In short, a reference frame moving at c is pretty much a logical absurdity, because by definition it would mean having massive objects move at c as well.
The tl;dr is that often one wants to know about an event on the past or future light cone of an initial event, like connecting the detection of a soft X-ray with an inverse Compton scattering at some astrophysical source like a black hole's corona or high-energy galaxy cluster electrons (the Sunyaev-Zel'dovich effect), where one wants to trace out the evolution of the scattered photon's momentum across cosmological distances. Coordinate systems along the photon's path from the thermal emission source (the black hole accretion disc or the surface of last scattering) can facilitate this. These coordinate systems are reference frames moving at c.
As long as one is working with covariant descriptions of matter, taking a notion like a photon's "affine time" or similar for a classical electromagnetic wave into (signed) momentum imparted to some object on a timelike path (such an object will have rest mass) is straightforward. The justifications mainly originate with Newman & Penrose in the early 1960s.
Here's a nice (and very fresh -- it's an incomplete draft) technical note that details one use of the Newman-Penrose formalism in the flat spacetime of Special Relativity. Note the extract from Chandrasekhar's 1983 textbook in section 7.3 (the author sources from a 1998 reprinted edition). <https://astromontgeron.fr/SR-Penrose.pdf> (PDF). The observations in §7.7 and §7.9 would be enough for me, if I hadn't already internalized the ideas further below.
I'll raise a couple of important and noncontroversial points from Jacques Fric's note: a photon -- whose frequency is proportional to its momentum -- oscillates some number of times along its null path lending a useful affine parametrization of the geodesic (photon motion is for all practical purposes always geodesic) that takes into account the spacetime curvature along the geodesic. Deploying the NP formalism in such a situation gives a nice analogy between motion through curved spacetime and motion through a refractive medium. And finally, covariant results in the NP coordinate system are readily interconvertible with covariant results in Minkowski coordinates.
Coordinate systems built along null geodesics -- typically lightlike Fermi-normal coordinates (FNC) -- also find applications in generalizations of the Jacobi equation, geodesic deviation, conjugate points, and so forth.
Now a step back to your comment. Null-basis FNC approaches let one calculate the momentum of a massless force carrier at any point along its evolution from p_{emited} to p_{observed}, and find cosmological applications (redshift of an astrophysical megamaser, the Lyman-alpha forest, etc) and microscopic ones (for the latter see ref [1] at <https://physics.stackexchange.com/questions/62488/local-iner...>).
In reading that stack exchange answer you'll want to know that 'Penrose showed that any [Einsteinian] spacetime [...] has a limit which is a plane wave, which can be thought of as a "first order approximation" to the spacetime along a null geodesic.' (from <https://link.springer.com/referenceworkentry/10.1007/1-4020-...>).
Pointing to references rooted in string theory and supersymmetry are not endorsements of those families of microscopic theories; they're just the most accessible examples of the practical use of lightlike FNCs for small locally relativistic systems.
So, not a logical absurdity, not useless, and not even especially uncommon.
Relativity (the fact that the laws of motion are the same in all inertial frames of reference) is itself a crucial part of Newtonian mechanics, except that Einstein's theories of relativity were such a big deal that we now tend to reserve the word "relativity" for his stuff, and call the old thing "Galilean invariance" or similar.
dumb question - why does going faster make it more likely you encounter fast protons? couldn’t protons in any reference frame be going quite fast relative to you?
Nah, most of the random atoms floating around in space are going to be travelling very roughly as fast as the things (stars, planets, etc..) around them -- because anything travelling much faster is likely to eventually bump into something and lose some of its momentum.
no a proton is a hydrogen nucleus (or at least can be seen as one) and has a charge. travelling near lightspeed you would have to worry most about uncharged atoms/molecules (because of their mass) and neutrons, neither of which can be deflected.
~850 years of ship time at max acceleration and the universe is 10^30 years old.
At this point, only white dwarfs, neutron stars, and various stellar remnants remain. Galaxies have dissipated, and stars like our Sun are a distant memory. (If any still exist to remember them. Which is, I'll add, not out of the question. Life can cling to structures built around black holes, white dwarfs, etc., and extract energy from those bodies for a very long time.) We're a long, long way from "heat death" -- from the Universe becoming an undifferentiated entropic sea of particles -- but it is already a very different place.
You could imagine a technically advanced society using it as a sort of penal death sentence. "You were not safe around our people, so we are sending you to the end of time."
In Skyrim they didn't send Alduin into the end of time, just into the future (also there is going to be an Elder Scrolls VI so V can't be the end of time, but will likely be the end of the fifth era). It was one of the reasons that the Felldir the Old was not onboard with the plan to use the Elder Scroll on Alduin. They didn't know how far into the future he would be sent. Most people on Nirn at the time of Skyrim didn't believe that dragons were real, much less Alduin who signals the end times. The Dragonborn ultimately ends up defeating Alduin, but because Alduin is immortal (in a way that the other dragons are not) he will return at a time deemed by his father Akatosh and actually destroy all of existence in that universe.
Going back to your original point it's a little different than sending him to the end of the universe, they sent him forward in time, but they actually had no idea how far or where there were sending him.
the unrealistic part of the whole narrative is: in that distant future he returns, another "hero" might step up and just kill him again, so he can't just "destroy all existence", or even Tamriel after some hundreds of years has modern/advanced military tech and will just obliterate a spawning dragon completely.
If something can be "killed" by a dude with an axe, it ain't a world-ending superpower for me. I get that Alduin could harass the known world by eating people or burn down villages if left unchecked, but "end of the world" for me is kinda different scale.
I think that is largely a difference between the lore and the gameplay. Alduin in lore literally eats all the matter in existence like a black hole. In the game he kinda goes against his purpose and wants to rule and feed on the souls in Sovngarde.
In case you're interested in these kinds of relativistic effects, there is a toolkit called OpenRelativity http://gamelab.mit.edu/research/openrelativity/ that you can use, for your simulated near-luminal speed graphics.
I think it's perhaps better to abandon ideas about going at high relativistic speeds to bridge large distances.
There's a more practical idea in nature: become a seed. It's how living things traverse vast distances and overcome intractable resource constraints during transit.
Compact oneself into an inert crystalized form, aim rocket, launch, wait. All you've gotta do is prevent chemical/structural breakdown for a few millennia or eons. Upon arrival to destination, have a machine turn-on and reconstitute the body and voila, you're there, all refreshed and ready to explore and virtually no time has passed.
Of course to do that one would have had to master biology, or maybe skip biology entirely and opt to copy oneself into a machine format, but that is likely easier than travelling at a sizeable fraction of c?
I had no idea you could achieve relevant amounts of time dilation so "easily" with relatively small (1m/s) of constant thrust - such that interstellar travel becomes somewhat viable provided you can continuously accelerate for the entire trip. Googling it, it would take approximately three years of observed time for a traveller to get to alpha centauri at 1g.
Could any somewhat-plausible technologies (fusion etc) get us to that point? I don't know how to do relatavistic maths, but it seems that directly converting mass to energy and ignoring relatavistic effects requires 50 000kg to accelerate 100 000 kg (with a constant relationship, as the C^2 cancels out) to the speed of light.
One gee over the course of months is insane. Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission nuclear gas rockets might max out at 12% but those numbers are dodgy as we don't have sharp numbers on gas core reactors.
As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I imagine a sort of neo-Polynesian culture, with travellers never quite knowing how the place will look if they ever come back that way.
> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations.
I guess this all depends on how you define simultaneity. Say I got on a relativistic rocket traveling near c and colonized Planet X in 10 years ship time and say 20,000 years Earth time. I put up my TV antenna as soon as I land, and I'd still be receiving Earth transmissions from approximately 10 years after I left, no? So even though civilization is long gone from Earth's point of view, from my point of view, I can keep up with the latest news as if I were still living there. So from my point of view I did not outlive anything.
If I were to go back, yes, everything would be gone.
That's not how it works. If the planet is N light years away from earth, the transmissions you receive would be those sent 20000-n years ago. along the way you'd get most of the messages sent during the 20k years as very red-shifted light, and extremely rapidly from your pov.
I think you may have that wrong (If I'm understanding you correctly).
If you traveled at the speed of light then you'd be at the planet in 0 time your time, and you would arrive with the first of the broadcasts over those 20k years. So once on the planet you'd get to watch all 20k years of broadcasts.
If you traveled at 1/2 the speed of light (not focusing on your dilation for the moment), then you'd still beat 50% of the transmissions and have 10k earth years of broadcasts to watch.
I think the question is what % of the speed of light is a gamma factor of 20k/10 = 2000. That's something like 99.9999999% of the speed of light.
Meaning you would get there before 99.99999% of the broadcasts had arrived and you'd be able to watch just about all of them in real time over the next (just less than) 20k years.
Assuming those two planets are roughly in the same reference frame, the only way 20k years could pass on earth w.r.t. your arrival is if the destination planet is 20k light years away.
If the destination planet is moving relativistically away from earth at an appreciable percentage of the speed of light then I couldn't say what the math would be. Maybe still the same, maybe not.
While traveling near c, I believe you'd be receiving Earth transmissions from approximately the same time you left, Earth clocks would appear to slow down to a crawl. But if you land on another planet and change reference frame to be more "stationary" again relative to Earth, I think a massive amount of Earth time would suddenly appear to have elapsed very quickly during the process of slowing/landing because of the huge shift in simultaneity.
Pretty sure only those 10 ship years worth of transmissions would suddenly appear. You're still 20K light years away so the rest of those years' transmissions are still in flight.
Hmm I suppose you're right. I guess you'd see very few transmissions during the journey due to time dilation, and then those 10 years would arrive relatively quickly when slowing/stopping?
I was thinking of this being the first half of the twin paradox, but perhaps for the apparent "time gap" in the spacetime diagram to appear, it's necessary for the traveling twin to turn and head back towards Earth quickly to shift simultaneity plane back in the other direction.
In the book series The Expanse, that was pretty much the only unrealistic technology that the humans had, which allowed them to colonize the solar system. Sure, there was some might-as-well-be-magic alien stuff later in the book, but by page 1, humans have colonized the solar system, purely with a hand-wavy engine that can generate sustained thrust without needing tons of reaction mass and without turning your exhaust port into an unimaginable hell cannon.
No, the exhaust port definitely was an unimaginable hell cannon. The characters spoke several times about slagging a ship or a surface base in their drive plume.
I'm going to embarrass myself horribly here, but I've only seen the series, so I'm not a hundred percent clear on how much acceleration they're pulling in cruise[1]. If it's something like .3 g then they're fairly close[2] to the limits of something like a fusion pulse drive (tiny nodules of deuterium or whatever, ejected out the back of the ship, then lasered into fusing, then the reaction pushes on the whatever - magnetic field, pusher planet, etc). That would make the solar system something more like what the Atlantic Ocean was in the days of sail.
They'd need refuels every stop though. Like with early coal fired steamers back in the day.
I got a funny feeling that without much better genetic muckety muck, the hard limit will be the human organism itself. Someone will get to the stars = if we don't screw everything up - but the someone won't be human, or maybe nothing like human.
[1] I know in emergencies they pull mad g, but it doesn't seem like something they keep up for long. Would still burn through their deuterium in no time though.
The ships cruise anywhere from 0.3 to 1G, the lower end mainly for comfort of people who grew up in space. But, the fictional drive is so efficient it could sustain 10s of G for extended periods (weeks, months).
Didn’t the guy who invented the drive essentially squish himself flat (too many G’s to lift his hand and reach the ‘off’ button, anyway) with his first prototype being efficient enough that it was still going during the story?
There's a scene about the discovery: He disables voice control (or switches it to Chinese) so at high-G he cannot hit the stop button, nor command it to stop.
Eh, there is a ton of unrealistic technology in book 1, the stealth ship for instance is impossible. By the authors own words they don't think of the books as being Hard Sci-Fi, harder then Star Trek sure.
What makes you think the stealth ship is impossible? They explain the concept pretty well in the first book: extensive cooling systems to soak up waste heat, large liquid tanks to store that heat internally for a while, and radar-absorbing coatings. We literally have all of those pieces today, it’d just cost unfathomable amounts of money to build with our current technology.
As a bonus, they even talk about the radar-absorbing coating not being perfect, and being able to detect the stealth ship as an object a few Kelvin above the background radiation when they pumped their radar into it.
I don't know when that was written, but it definitely feels like The Expanse authors read that exact page and built their stealth system to address those points - the stealth ships in the books aren't permanently stealthy (they have to radiate their heat from the internal heatsinks after some amount of time, seemingly on the order of months), and are immediately detected when they light up their engines (and hence coast until they reach their destination).
The main characters also manage to find a stealth ship after coming across coordinates to it - it was parked and completely offline (and unmanned) in the orbit of a small asteroid, which would further obscure any signature it might give off.
Unfortunately for cool space battles, no this still doesn't work for the reasons outlined.
First off you have to remember that when a ship turns it's engine on it's going to be visible effectively to the entire solar system unless there is a planet or something in the way. We already have the capability that we would notice a ship moving in the asteroid belt from Earth and in the Expanse it's going to be even higher. So everyone already will have known when you turned your engines on and can run the calculations to see where you are while you are coasting, so even within the confines of the fiction that shouldn't work.
But let's ignore that and say that for some reason no one was looking when you turned your engines on. Assuming your ship is somehow running with all systems off it's going to be roughly 300 C hotter then the vacuum around it, even with a heatsink like you mention (and a heatsink that can store that heat for months is also unrealistic) you're going to be sticking out like a sore thumb to any infa-red. You're just too hot.
That's basically the plot of the Forever War. When you drop out of hyper drive (or whatever they called it) you never know if you are going to be battling a ship from your future or your past. The very society that you fight for is completely alien. It's incredibly alienating.
What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.
I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed?
I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy to sustain in either case is probably measured in Dyson spheres, or some fraction of all energy in the universe pretty quickly too.
That's the drive of the eponymous generation ship in Kim Stanley Robinson's Aurora. It sends a mirror ahead of itself, then the Sol-Mercury lasers bounce off the mirror, back on the ship to slow it down.
I could see the light spacecraft with their matter-antimatter engines laying down "lightways", like railroads in the old west. Once a lightway is established, then it's much less of a big deal to get from system to system. Apart from the tens of thousands of years in subjective time, of course.
Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.
Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.
Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.
> As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations.
Spin, a novel by Robert Charles Wilson, touches on this from a planetary perspective. Quite the interesting read, though I won't say more here for fear of spoilers.
What do you think about the gravity well "drive" in three body problem trilogy? I thought it was fairly clever, especially given the stories describing it beforehand (maelstrom iirc?)
> This is the primary reason for selecting the annihilation of a proton (p+) and antiproton (p–-); the products include neutral and charged pions (πo, π+, π-), and the charged pions can be trapped and directed by magnetic fields to produce thrust. However, the pions produced in the annihilation reaction do possess (rest) mass (about 22% of the initial protonantiproton annihilation pair rest mass for charged pions, 14% for the neutral pions), so not all of the protonantiproton mass is converted into energy. This results in an energy density of the proton-antiproton reaction of
"only" 64% of the ideal limit, or 5.8x1016 J/kg.
I love that someone wrote and published a serious paper where the end design involved first stage thrust of 550 million ft-lbs and the total ship weighs something like 17 million metric tons and if I'm reading page 26 correctly, the payload on the ship has to be 7,500 kilometers from the ignition source to survive.
While there may be a 100% anilation with matter-antimatter reactions the resulting thrust is not perfectly efficient due to energy consumed as the rest mass of charged and uncharged pions, energy consumed as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and energy consumed as neutrinos and gamma rays.
I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.
Even the GPS satellites have to take into special relativity - they have to correct for a tiny amount of time per day due to time dilation causing the moving clocks on the satellites to tick slightly slower than the stationary ones on Earth.
Curiously, general-relativistic effects needed to be accounted for as well.
Quoting Wikipedia, GPS “must account for the gravitational redshift in its timing system, and physicists have analyzed timing data from the GPS to confirm other tests. When the first satellite was launched, some engineers resisted the prediction that a noticeable gravitational time dilation would occur, so the first satellite was launched without the clock adjustment that was later built into subsequent satellites. It showed the predicted shift of 38 microseconds per day. This rate of discrepancy is sufficient to substantially impair function of GPS within hours if not accounted for.”
It isn't possible without a magic reactionless drive; it might even require a zero-energy drive since the kinetic energy is huge and needs to come from somewhere.
The rocket equation kills you. A reasonable fusion rocket would require an enormous (like an entire planet worth) to continuously accelerate to relativistic speeds. Antimatter drives are the only option and still require large amounts of reaction mass since have to absorb the energy to throw it out the back, or use a laser.
You need a hydrogen 'scoop' and fusion. If you can accelerate to 0.01c, you can collect enough hydrogen for 1000 MW fusion power from 32 km2 (6 km diameter) scoop area. Faster you fly, more you get. You can accelerate until interstellar particles start to do real damage.
Well the obvious solution would be to figure out a mechanism to capture the particles that bombard your ship and figure out a way to turn those particles into thrust so you don’t need to take a bunch of fuel with you, which I get it, is another hard problem to solve, but maybe not impossible. We have done some incredible things before.
Theoretically it works, yes, though not nearly as well as initially thought. The engineering and the required materials, however, are still questionable and way beyond our current understanding of physics and material science.
It's a similar story for solar sails, unfortunately, though progress is being made in that area. Personally, I think that's the only realistic way of interstellar travel given our current knowledge of physics and engineering.
Even if you can figure out the problem of thrust, you still have to deal with particles slowly swiss-cheesing your ship.
And even beyond that, you either witness the ships leaving and never hear back or you embark on one and comeback to whatever Earth is in thousands of years from now.
After an observed 12 years of constant 1g acceleration, you will be 113,000 lightyears from your point of departure, which is enough distance to cross from earth to the opposite side of our galaxy.
Of course, you are now traveling at 99.999999996% of the speed of light. If you actually wanted to stop at the other side of the galaxy to take a look around, you would need to turn around at the halfway point to decelerate, which roughly doubles the travel time.
Acceleration and Deceleration are symmetrical. It takes the same amount of delta-v to accelerate between up to 0.999c as it takes to decelerate all the way back down to 0. Though as you burn fuel your ship gets lighter and the actual energy usage to maintain that constant delta-v of 1G will go down.
As for the energy used to accelerate from 0C to 0.009C compared to 0.99C to 0.999C, I'm not sure. I know the time taken (from an external reference frame) changes, but part of me suspects the total energy stays the same and the difference in time taken is caused entirely by time dilation. However, I suspect I might be messing up reference frames, I don't actually know the equations.
For practical values, nuclear pulse propulsion would only get you up to a few percent of the speed of light.
The best nuclear thermal, gas core, would give you 7000 seconds[0]. A "dusty plasma" rocket (suspend the nuclear fuel in dust form in a magnetic field) might get 100,000 seconds, or just over a day of accelerating at 1g.
[0] for the benefit of non-space nerds: the unit of measure is "lb-force seconds per lb-mass of fuel", which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket.
> which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket.
I think you mean “a very large fraction of the total mass”… generally the best efficiency comes if your fuel mass fraction is high, as it means there is little overhead of things in your spacecraft of things that are not fuel (the mass of the engine, etc)
No, I mean small, because I'm talking about the quality of the approximation not the way to maximise delta-v.
If you have 1 gram of fuel and a 1 ton payload and that fuel has 1e6 seconds of Isp, you can accelerate the ship for 1 second at 9.8m/s/s.
If you have 1 ton of fuel and a 1 gram payload and the same fuel and burn at 1 gram/second, the first second is mostly spent accelerating the fuel, which means you're no longer able to just approximate the Isp as "seconds at 1g" in a nice linear fashion — it starts off at 1 gee in this example, but ends up at 10^6 gee in the last moment, a million seconds later.
Nuclear thermal or pulse, fission or fusion, aren't enough for interstellar relativistic speeds. They have way too low specific impulse. They don't even have enough for constant acceleration inside the solar system unless the acceleration is really low or the fuel ratio is huge.
> unless the acceleration is really low or the fuel ratio is huge
You’d need kilotons of nuclear fuel and a 10^3 fuel ratio. But that’s plausible.
The economically-plausible answer is antimatter, where the ratio stays in the single digits and starts permitting deceleration. But I wouldn’t call that technologically plausible at this time.
"A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. "
"...The third mission uses a three-stage sail for a roundtrip manned exploration of e Eridani at 10.8 light years distance."
Holy cow! I just read the energy requirements of that third mission. Total electricity generation capacity of the entire world is about 12TW today [1], whereas that 3rd mission would require a "formidable" - as the author admits - 45,000TW at least. A mere three orders of magnitude more :)
I have a confused question here. I asked chatGPT what the approx terawatt output of the sun is. It claims it is 384.6 terawatts.
I find that hard to reconcile with the earth having 12TW production. Given that your number is probably OK, that would suggest the sun output calc is way off.
I wonder if any non-AI contributors have better ballpark estimates for the sun's energy output..
Hmm, I found a non-chatGPT source, which claims the same..
https://www.rmg.co.uk/file/2277/download?token=KyEQPN9O
If this relation is really true, I am somewhat shocked - us burning 1/30th of the suns energy output, can that really be true..?? If so, scary.
And also 'illuminating' concerning how unsustainable what we are doing is..?
That's a very nice figure for context. So the energy required would be about 26% of that. That means it's more power than we could ever hope to capture from solar alone, as only about 29% of the surface is land and some of that isn't particularly great for solar (looking at you rain forests and Antarctica:)
Unless we use space-based systems instead, which would make that a kind of power requirement trivial, ignoring the costs of course ;)
Taking the US as an example: depending on who you ask, we could generate enough power for the US with somewhere between 10000 square miles (Elon math) and 21250 square miles (pretty common number given by multiple other sources) of PV.
That is about a fifth of the state of Nevada. A lot of PV, to be sure, but far from beyond hope.
And that assumes nothing but PV, which is unrealistic. We capture a bunch of the sun's energy as wind and rain.
which has to be one of the most epic sci-fi novels ever since it is about a starship that gets its brakes damaged in a crash so their answer to every problem they face is to go faster!
I just finished reading Tau Zero after hearing about it here and also recommend it!
I expected relativity to play a role, but the time dilation the crew experienced was on a _far_ grander scale than I imagined. The book also left me thinking of favorite time travel/relativity classics like Interstellar and, going back, Star Trek IV: The Voyage Home (my favorite Star Trek movie).
I had fun with setting the "throttle_value" variable to something insane, like 30 (1 is the max on the slider), to see how close to c you need to travel at for the web app itself to disintegrate.
Apparently, it takes just over two lines of "9"s on a 1920px display for this to happen. The speed wraps back around to zero, and the world time becomes NaN as an added bonus for (nearly) breaking the laws of physics.
For the comments saying we "just" need to maintain a 1G acceleration, should point out that as the ship approaches the speed of light, its mass increases [0].
And as the mass increases, so does the thrust required to maintain that acceleration. So that engine better be able to tap into some magical energy source, otherwise it can't maintain the acceleration at the higher speeds. :)
The last time I studied special relativity was a while ago, but I think your mass only increases in other frames of reference.
If, for example, we consider the speed of your ship from the point of view of someone left on earth, then your ship should appear heavier from that person's perspective as you accelerate. But from your point of view, you still weigh the same, and so does your ship. In fact, you would see earth going through the dramatic "weight gain".
Is the ship gaining mass (and shortening, and having its time slowed) in its own frame of reference? No, right? So wouldn't it be that the 1G acceleration from the viewpoint of the ship remains from the internal perspective, even though near the speed of light, more and more of that energy goes into higher mass and slower time?
I'm reading up again to refresh my relativity knowledge. Though:
> even though near the speed of light, more and more of that energy goes into higher mass and slower time?
This to me does not sound very different from saying the (relativistic) mass to be accelerated is larger, and is what's stopping a 1G (or any constant) acceleration to be maintained to bring the ship to the speed of light.
The concept may have been an overly simplified pedagogical tool though that I need to upgrade from. (I'll keep the comment chain intact since it's educational to me).
My (previously unexamined) assumption was that the slower time exactly offsets the lower acceleration in terms of speed gained, such that in the reference frame of the accelerating ship, experiments would continue to show 1G acceleration.
The "increasing mass" stuff is more to make it easier for calculations and learning (with imo terrible consequence to physical intuition) than an actual physical concept.
It doesn't run in my browser and I want to thank the author for giving a proper error message! So often, pages just break for various reasons and you're just sitting there "is it gonna load?" or "is this what I'm supposed to be seeing?"
This is the first time I see "xyz is not supported", which both helps me understand what's wrong and lets me switch to a popular browser to see the content. And the site works and looks great there (on mobile)! Thanks OP :)
Yikes. At around 90%c, it's starting to look like a sunny day. Any physicists care to weigh in on the accuracy of this? I can search for the energy density of starlight in interstellar space... but how fast does one need to go to hit that "sunny day" point?
I'd think the small amount of atoms per cubic meter in interstellar space would make your day very sunny long before you got up to 90%C and the photons out there started to matter.
According to the simulation moving relativistically in addition to shifting the frequency also increases the intensity significantly. Something I had never heard about until now.
I'd love to find a reference for it as I found that surprising too.
I wonder how long one would enjoy the view of that sunny day when all the visible light, infrared and high radio frequencies coming our way are turned into high energy x-ray and gamma frequencies due to the doppler effect.
The effect is in the way you see light rays appear to arrive because of your motion. It doesn't really change that those things are really an absolute direction or that you're going in one direction.
[Show HN] Admittedly much less visually engaging :), but some time back I was interested in the twin paradox, so I made a website where you fly a toyish spaceship between three stars and see how the various clocks and the space itself change. It's flat spacetime, so relatively easy to simulate and everything just runs in JS: https://twinparadox.org
The original motivation was, can one make a game with relativistic spaceships, conceptually speaking? The issue is that if a player calls another player IRL it'll be like instantaneous communication... In flat spacetime, though contradictory to its postulates, one can choose a preferred frame of reference, say the planets, and allow instantaneous communication within that plane, not leading to any inconsistencies in a game, I think. Curved spacetime doesn't admit a notion of simultaneity, so that's completely out, no black holes in a game :( Never really got to the point of modelling multiple ships though, the life caught up. So it's just a single ship on the website for the moment.
This is a relativistic spaceflight simulator which shows what it would look like if you were on a spaceship traveling close to the speed of light. It is loosely based on my original code from 2012 implementing relativistic effects in my game The Polynomial.
What happens when you accelerate to a speed at which a photon with a wavelength as large as the observable universe (in the world frame) gets blueshifted to a wavelength shorter than the Plank length in your frame of reference?
So, this does seem to take the angle I'm flying at into account. If I'm accelerating to 99.9% the speed of light, and I yaw 180deg the view, I start "decelerating".
What happens if I keep maximum acceleration and pitch up 90deg? The software seems to keep flying on the original vector. Is this a bug or is it intended to be a simulation of how light would behave under those circumstances?
Not the way I see it. If I accelerate modestly (0.2m/s2) to just above 30% of c where the movement is noticable enough and then put the direction of travel just beyond one of the corners (e.g. just beyond the top right) by turning the view then the direction of travel will move slowly towards the center of the screen on its own - giving a slight sense of turning in the beginning - as one would expect of current velocity vector and the velocity accummulated from acceleration vector (always towards the center) add up. At velocities close to c the already huge velocity vector dominates and no chance of perpendicular (or any angled) acceleration make any effect on the velocity vector direction: the diagonal of the two can never go beyond c. The velocity vector locks in. (probably why those traveling near the speed of light don't age much, their particles cannot interact that freely with each other when their movement is locked in to a common travel vector?).
And here I'd observe that the acceleration value is measured in the local system where time slows down relative to the observer's system, while the speed is measured in the observer's system (naturaly, my speed relative to me is always zero).
Sure, from the observer's system (home planet "stationary" the speed is measured from) it can't go faster than c... but that's where I don't think it's accurate. From the flyer's perspective, if you're close to c (as viewed from home) and suddenly accelerate at a 90deg angle, that shouldn't bleed off your speed forward, or be limited by your speed forward. Only from the distant viewer's perspective would those two velocities combined max out at c. From your perspective they wouldn't.
The near c component of the velocity remains near c with applying 90 deg acceleration, hence that the velocity vector will remain towards this near c regardless of 90 deg acceleration characteristics. For a significant change one need to apply acceleration that has very significant deceleration component to this near c velocity vector (significant amount of >90deg acceleration applied, best to be close to 180deg).
yeah, and that's the way it would appear to a motionless observer. Buy not how it would appear to you. Remember, once you have achieved .99c velocity, no longer accelerating, you are standing still. From your perspective, any new acceleration at a 90deg angle should begin accumulating light from that direction.
Normally the light of the big bang is too dim (frequency too low) to be seen, but if you travel really fast the blue shift effect will make it visible, and since this light is uniformly distributed in the sky, it will appear as while light everywhere.
Recently I saw this video explaining an other weird phenomenon of relativity. Simultaneous events aren't really simultaneous in all frames of references https://youtu.be/YAmHAKdyV1o?si=JXkv2AvCIrYN9ZdZ
> Now you are staring into the big bang in all its glory; make sure to wear safety squints.
Wait what? That's not true right? All the light we see is concentrated in one point but it's not really the big bang that we see, it's an illusion, is that correct?
Can anyone explain what is happening as you get a few nines in your velocity?
Yes, that's the microwave background being blueshifted, but why is it growing across the screen? Shouldn't the angle that's blueshifted be less and less as you go faster? At lower speeds we do see the starbow effect, passing stars going down the spectrum as the angle increases.
And what becomes of the stars as you're going fast enough? Yes, most of the energy is shifted out of view but don't stars emit some at such low energies?
These are good questions, but the answer is essentially "not in scope for this visualization". And, also, really hard to visualize with reasonably correct physics at higher "nines".
The tool does not deal with gravitation. We know that our local patch of the Milky Way (out to a few hundred lightyears) is full of kHz-Hz waves (LIGO, Virgo et al.), and nHz waves (pulsar timing arrays). Known sources at these frequencies are extragalactic, so there will be lots of these waves outside the Milky Way too. At significant boosts, these gravitational waves will impose visual distortions. The "twinkling" that pulsar timing arrays detect over the course of many months would be detectable over the course of much shorter periods of ship time.
Gravitational microlensing is likely to create uncomfortably hot spots (low-energy caustics from our usual point of view) for an ultraboosted spaceship.
We also strongly suspect there are lots and lots of black holes massive enough to impose a significant gravitational redshift on background light from the early universe, releasing far IR radiation from near the photon sphere (note, this isn't Hawking radiation, just delayed and redshifted background) that an ultraboosted observer should be able to spot.
(Work in this area is usually about considering what an ultraboosted massive particle does as it passes a massive observer. For example, in the 1970s the Aichelburg-Sexl ultraboost tried to capture what a neutral cosmic ray would do -- gravitationally -- to a spherical gas cloud along its path. In essence there is an exchange of momentum: the ultraboosted particle loses some, and small parts of the gas cloud gains some. Increasing the mass and structural complexity of the ultraboosted observer inevitably runs into inelasticities. That admits a hand-wavy expectation that even below the threshold at which subatomic physics of the ship becomes relevant, the ultraboosted spaceship is liable to become so very hot that its thermal spectrum masks practically all the light from everything ahead of it.)
As you raised in your final question, there is good reason to believe there is plenty of very deep IR electromagnetic radiation out there from all sorts of sources. Most models of cosmic inflation will produce gravitational and electromagnetic radiation with all manner of wavelengths, many of which are stretched to cosmological lengths. We don't really have a good view of any of that because presently we can't yet make a sufficiently cold ultra-long-wavelength (sub-30 MHz; cf. FARSIDE, LCRT, GO-LoW) detector and don't even have good ideas about how to look at astrophysical sources below kHz. There's probably lots of bodies (e.g. magnetic interactions with exoplanet atmospheres and gas clouds) that will become optically bright as the CMBR is blueshifted out of the range of human vision.
The visualization also does not offer up a physical observer who would feel variations in the galactic magnetic field (cf. the Sokolov-Ternov effect with enough "nines"), the cosmic neutrino background, and two-photon physics from ultraboosted CMBR and starlight interacting with the observer's photon shock front.
The visualization is pretty, it captures some gross aspects of being relativistically boosted, but as many scattered threads in the discussion discuss, it's probably not a useful hint about what one of our descendants might experience if somehow they could reach "a few nines".
This is something I've wanted to try in VR one day. Does anyone know if there is an app/game available for one of the popular headsets? Is there a technical limit to how realistic the effect could be?
You won't be able to perceive anything "3D" if you're aiming for accuracy.
Even though we can perceive the "depth" of the stars through motion in this simulation, every star is still going to look equally far away, because their positions won't change between the left and right eyes. The same way we can't perceive the sun as further away than the moon -- depth perception merely tells us they're both "max far away".
You could always introduce depth perception by faking your eyes to be light-years apart, but of course that's not how it would really be in the spaceship...
Yeah I figured that would be the case. I still think it would be really cool effect to see anyway if you could go fast enough and experience the motion parallax of the stars. No offense to this project as it's really impressive.
Not exactly what youre looking for, but this type of game (using relavistic physics) has been attempted. Heres one built in unity that could be easily ported to a Quest line device.
Playing No Man's Sky in VR gives this vibe. Sitting is fine. The first star field motion might induce a bit of vertigo, especially if your machine struggles and you get frame drops and stutters as it is loading.
The rate that time is progressing is also going to change this effect a LOT.
Years are passing in seconds in this websites simulation, if you were experiencing it all in "real time" everything would appear pretty much stationary for quite a while.
Even at like 95% the speed of light, you're talking about years of time required to move between stars - nothing will appear to change much day to day until you actually get close to one of them.
I kept getting closer and closer to the speed of light, but before I crossed even 2 million light years (I think), I was staring at the Big Bang. Then I remembered that the diameter of the observable universe is about 90 billion light years (give or take), and I realized that I’m not going to be able to reach one end anywhere. There’s nothing that makes everything seem so small and insignificant like astronomy.
Maybe if you could start by persuading a small black hole to go where you want and following in its wake (or lack of wake actually). The black hole swallows everything in your path and grows more powerful. Perhaps if you had an array of three or more black holes you could steer the group by differentially feeding them somehow. If the initial black hole were an orbiting pair your spaceship could orbit a lagrange point and be swept along.
Black holes aren't immune to the effects of gravity from other objects just because they're black holes. It'd be no different than trying to use a trio of stars as your gravity engine (well, more space efficient). I.e. you still have to accelerate the stars, you still have to deal with slower things the stars "eat" slowing them down, if the stars pull your ship it's slowing them down. Effectively, you've just increased the mass of your spaceship by however much mass the black holes have and then still had to find something to accelerate the whole lot instead of your spaceship.
Humans are humble. We only ride our star in about 200 km/s so we can take the scenery route. It takes mere 100 million earth years to get to the other side of our galaxy.
It's not very clear where our black hole in the center of the milky way is taking us though.
The viz shows visible light, but you'll be basking in gamma and more energetic photons as they've all shifted up. It does note that infra red shifts into visible light. I wa swondering whether the perspective change is accurate as the length contraction should've brought everything much closer.
I don't care so much about the physics side, it's just so bloody pretty. Is there anything like this that will play infinitely, with similar sliders to tweak the visuals? It'll look so good on an OLED.
Don't forget to accelerate in the opposite direction (move it down) once to see the starlight shift out of visual range! It's fun until about 95% of c then there is just blackness.
Of course it’s not microwaves anymore, but it’s still the CMB, that is, decoupling/recombination, not the big bang. Unless you count the CMB as the big bang.
You’re driving and see a huge patch of the fog suddenly turn red, I would argue you’re seeing break lights not fog. But that gets into definitions not just physics.
Similarly, the CMB is energy from the Big Bang as last scattered ~380,000 years later. IMO, that’s the Big Bang, but reasonable people can obviously disagree.
Everything is energy from the big bang, including you and me. But the photons of the CMB don’t represent anything that already existed immediately after the big bang. Protons and neutrons and hydrogen and helium were created in between, and inflation (probably) happened in between. The universe at the time of last scattering was a whole different world than a split-second after the big bang.
The Big Bang generally refers not simply to the instant the universe was created but an early high energy state. The term had long been in use before it was discovered that there needed to be an initial singularity. https://en.wikipedia.org/wiki/History_of_the_Big_Bang_theory
Or as an astrophysicist said ~”You can refer to the Big Bang as the first instant or a fairly arbitrary period after that initial event. The universe cooling down enough for matter is just as reasonable an end as the universe cooling down for atoms.”
The acceleration/deceleration always points forward. If you've turned in other directions, then you can end up with some lateral velocity. You can cancel it out by decelerating until your speed hits a minimum, turning roughly 90 degrees, and decelerating along that axis as well. (If you still have residual speed, just repeat with more roughly-perpendicular axes.)
Isn't that correct though? The light gets redshifted away from the visible spectrum quite fast and after not very long there's nothing to see. Unlike when going forward where there's plenty of extreme red light that can be blueshifted into the visible spectrum (and eventually past it).
It's the aberration effects that were the most unintuitive for me. Specifically the difference when you are looking straight ahead in your direction of travel in how the stars appear to move when you are moving at constant speed and when you are accelerating.
At constant speed when a distant star first appears it moves directly away from the center point. When accelerating distant stars first move toward the center point then move away.
If you constantly increase the acceleration you can hold a star that is not at the center point stationary for a while, which is totally not something I would have expected.
I'd like to see something like this except instead of a seemingly random assortment of stars the stars are on a regular grid, connected by glowing filaments along the grid lines. That would make it easier to see what the heck is going on.
