Modern physics(and engineering) is kind of absurd. And I mean that in a good way. LIGO? I didn't believe at all that it would ever work. Even when they got detections I kinda thought they were chasing their own tails. Now the evidence is pretty much rock solid that the data is real(multiple facilities, correlations with light observations for neutron star mergers, etc).
Then I heard about LISA, which essentially is building this thing in fucking space(different geometry, but same basic concept), with probes somehow orbiting in unison and firing lasers at eachother over ridiculous distances(2.5 million kilometers!). I thought they were raving mad. But pathfinder, the POC, seemed to work, and now they're building the thing. Planned for 2037, but still.
When I heard about this project in a spacetime video a few years ago IIRC, again I thought okay, this is too far. It's never gonna work, there'll be too much noise yadda yadda. And it now it looks they might have done it.
At this point, if physicists say something is possible, I listen no matter how impossible it sounds.
This discovery didn't actually involve LIGO, nor any other feats of impressive physical engineering. It was made by observing neutron stars and finding patterns in their unexpected perturbations.
Neutron star rotation is so consistent that they are used to calibrate atomic clocks[0]. However some of them were glitching and not rotating as expected, but the glitches were consistent between each other. It turns out they aren't actually glitching, but spacetime is being distorted by massive gravitational waves.
Indeed, and part of the magic is knowing jupiters position with a very high precision. Said precision was delivered by the Juno probe in 2016 or so. With that precision we now know the earth position with much more accuracy. The trick is the jupiter and sun orbit around a point outside the surface of the sun. Said point (barycenter) is the point the earth rotates around.
The increased positional accuracy of the earth allows teasing out the distortion from to various pulsars to use the galaxy as a ligo like gravity wave detector.
My physics professor, David Blair, sketched out the design for LIGO and other detectors back ~1980 and the kinds of technology that would first need to be created in order to get there.
The big money and the big builds tend to be in the US (for now, at least, but empires always shift centres over the long time periods) but the ideas come from all over the world.
I watch pretty much all their videos as they come out, and indeed this one was... pretty out there(pun intended). Somehow I have no doubt it'll be built one day.
For now my mind is sufficiently blown by JWST taking spectra of exoplanet atmospheres.
For more materials, measurement, and spacecraft engineering challenges in verifying gravity theory, check out Gravity Probe B [1]. 40+ year effort (spoiler: it confirmed the geodetic and frame-dragging effects of General Relativity).
> how comes they can't get this fusion thing to work?
Fusion energy involves conjuring physics which don’t exist in that form in the known universe. (Stellar fusion happens at lower rates than what we’re targeting. The only reason it works is because stars are so huge.)
I think a useful fact to contextualize how much more intense we're hoping to make fusion, is that the volumetric power density (watts per cubic meter) of sol's core (276 watts/m^3) is on the scale of a well run compost pile (50~200)[0]. Since we don't want to make an object as big as the sun's core, we need to make something far, far, far more intense than the sun's core, which is understandably quite hard.
Unfortunately this is the correct answer, almost word for word how I was going to phrase it. Do long as other forms of energy are inexpensive, we will continue to abuse them no matter the damage.
Fussion is easy, hyndrogen nukes were detonated decades ago. Ask them about LENR (Low Energy or Lattice Enabled Nuclear Reactor). Cold Fusion topic is reputation-damaging for scientists for decades. I banned multiple times here for "participation in discussion". Now, LENR is used to boost fusors performance by two order of magnitude (for medical purpose).
What do these waves look like as they pass through us? Acoustic-like compression and expansion of particles as molecules temporarily reorient toward a “down” that is ever-so-slightly off from the Center of mass of the Earth?
Also, I assume that these waves are very gentle sinusoids? Could the opposite — a high-amplitude gravitational square wave — be possible? What would it do to the things it passes through?
The gravitational way has a direction (say, z) in which its propagating. Within the plane perpendicular to that direction (x-y), a circular ring of particles will at at one moment experience squeezing in one direction (x) and stretching the perpendicular direction (y). As the wave passes through and you move from the peak of the wave to the trough, the directions reverse, so the first direction (x) stretches and the other direction (y) squeezes. By "stretching" and "squeezing" I mean instantaneous additional (positive and negative) acceleration on top of the (much, much larger) acceleration from the background gravitational field provided by the Earth.
Just as a child can swing their legs at the resonant frequency of a swing to pump up their sinusoidal amplitude, a very weak gravitational wave can pump up a ring oscillator if it's oscillating at the ring's resonant frequency.
Exactly square gravitational waves are of course not possible, just as for electromagnetic waves. (They would have infinite energy at the corners.) But in principle you could get a close approximation. However, spacetime is incredibly stiff, and I think all the known real-world sources produce pretty smooth waves. I presume most violent events are mergers of existing black holes, and essentially always result from a smooth in-spiral rather, say, a sharp collision event. This is what the "chirp" signal looks like to the LIGO detector:
The effects of a square wave would be roughly as you would expect: instead of smoothly pumping up an oscillator, it would give it a sharp kick, just as with electromagnetism.
> Just as a child can swing their legs at the resonant frequency of a swing to pump up their sinusoidal amplitude, a very weak gravitational wave can pump up a ring oscillator if it's oscillating at the ring's resonant frequency.
So is it possible that a passing gravitational wave could initiate some natural process that otherwise might have not happened?
I was thinking more along the lines of a chemical or nuclear reaction (or some yet-to-be-conceptualized space-time reaction) at near an initiation point, but not quite there.
I assume the gravity wave could push the reaction to initiate by warping a subatomic element (like an electron orbital) into an otherwise impossible configuration on a scale of picometers for a split second.
The nuclear and electromagnetic forces in the vicinity are stupendously stronger than gravity at small scales. One could imagine that some electromagnetic or nuclear process might be heavily suppressed by some symmetry such that it could only be unlocked by gravitational forces (which would break that symmetry), but nothing like that comes to my mind. Basically, anything that gravity could do could be done by electromagnetic and nuclear forces vastly faster.
These gravity waves aren't strong enough to pull us off Earth. Everything on Earth is already subjected to gravity, and it doesn't cause nuclear reactions. Gravity waves won't cause impossible configurations to become possible.
Well it depends. How big a wave are we talking about? The ones so far have been very distant. But when to 100 solar mass black holes merge and form a 90 solar mass black hole they release 10 solar mass of energy (E=mc^2 strikes again). Being close to such a merge would be lethal.
However there's nothing that we know of that could happen close by, so the risk is near zero. Apparently the black hole at the center of the milky way is going to merge with another super massive blackhole in Andromeda in 4.5 billion years or so.
I think you are overestimating the amount of power in these gravitational waves by several dozen orders of magnitude.
To even detect these, we need to observe multiple pulsars over long periods of time in order to find minute effects only visible at galactic scales due to the nanohertz frequency of these waves. In other words, spacetime is being stretched and compressed at subatomic scales on a sinusoidal wave with a period of a month or so.
It’s a bit like asking if cosmic rays from the Pinwheel Galaxy are affecting cancer rates.
Makes sense. I am a bit of a drive by tourist on this; such that the only reason I asked this was the analogy of a kid pumping their feet stuck on me and messed up my assumed scale on effects.
I don't see it as impossible... Cosmic events frequently give rise to events that might not otherwise happen.
The real question is, is the change meaningful? If you have a tsunami but it doesn't change anything meaningful, is it an interesting observation outside of the event itself?
Acoustic waves propagate through what are essentially elastic deformations of the material they travel through. Can gravitational waves be thought of as propagating by elastically deforming spacetime?
If this analogy holds, then can it be taken further? Acoustic waves dissipate their energy insofar as they trigger plastic deformation in a material. Could gravitational waves plastically deform... spacetime itself? Or would they just be deforming the material? Or is gravitational energy not dissipated into other forms of energy at all?
> Or is gravitational energy not dissipated into other forms of energy at all?
Extremely weakly. The mechanism is similar to acoustic waves, but the coupling constant is so small, that the amount of dissipated power would be insignificantly small.
In theory, you can use gravitational waves to extract energy. For example, you can wait for the "compression" part of the wave, and push a cart uphill during it. Then let it slide downhill when the compression peak passes. You'll be able to extract some useful energy, because the distance that you pushed the car uphill will be shorter than the "normal" distance.
You can make more elaborate systems on this principle. E.g. a tuned resonator: two orbiting masses with a period selected to match the frequency of the gravitational waves.
"r" is the orbital radius, "G" is the Newton's constant, "M1" and "M2" are the masses of the orbiting objects.
It's actually kinda amazing that once you substitute in all the values and do the math, all the scary large powers just somehow cancel out to leave a small macroscopic number (I remember getting around 60 watts).
Edit: note, that both objects radiate. So it's 60 watts for both the Earth and the Sun, around 120W in total.
Agreed; it's very serendipitous that you can basically light up a room with the gravitational energy that the sun/earth pair radiate out into space.
Light up 1 room with an incandescent bulb, or your entire flat with LED bulbs nowadays. I would be very interested in seeing some napkin math, based on power efficiency progress and "rate of technological innovation", that attempted to project when we could feasibly run the equivalent of our present-day human civilization purely off of gravitational waves/radiation.
Can I ask how you got there? G^4/c^5 makes sense, but the rest loses me.
(Eventually after all the typing below I think that I traced the 32/5 and (M1*M2)^2 * (M1 + M2) to (eqn 16) in Peters & Matthews 1963 maybe? (e=0, a->r) https://doi.org/10.1103/PhysRev.131.435 (stick sci-hub.se in front of that if you need to). The authors take an approach comparable to the textbooks below.)
Super-quick textbook review. Practically all of them start with the quadrupole moment and try to justify an energy which is quadratic in derivatives of that while still within a linear theory. Carroll and Mathtias Blau take slightly different-from-each-other paths through the transverse-traceless TT-gauge to P = dE/dt = -2/5 \frac{G^4 M^5}{r^5} (c=1) for a circular equal-mass soft binary. Wald uses the radiation gauge and so eqn 4.4.58 looks fairly different, and comes with the amusing Waldian line "A lengthy calculation (where many terms which integrate to zero are dropped) yields the final result,". Sigh. Blau's development looks a lot like MTW, but the latter gives us .... Exercise 36.6 ("Apply the full formalism ... to a binary star system with circular orbits. Calculate ... the total power radiated; the total angular momentum radiated ..."). Gee, thanks thick textbook. FWIW, 90 seconds of that (mostly trying to make sure both sides have the same dimension rather than extracting a power in watts because bad/lazy reasons and anyway I always think you had about the right order of magnitude) doesn't take me to anything like the form of your calculation.
Rather than flip through other textbooks, let me rely on my maybe-shaky memory and say that most of them, at least the modern editions, follow the TT gauge approach and come up with an equation in a form similar to Carroll.
What stands out here is that Earth-sun has a large mass ratio and noncircular orbit, and it's not really a binary system anyway, and so will defy these textbook schemes. Secondly all of these take P in the far field, because linearization. (Compare that with your edit).
I also got way off into the weeds wanting to work with chirp mass (rather than q=m_1/m_2) which is what GW obs data analyses use because extracting the individual masses is hard. I also know a bit about EMRI BHBs (extreme mass-ratio inspiral) and in those dissipation is dealt with differently from textbooks even for soft binaries (e.g. soft->hard roughly PN & perturbative methods, EOB, GSF, numrel) absolutely none of which is of immediate practical use here.
So that's some of what motivates my question about the origin of your calculation.
ETA: I think most of what happened here is that my brain reads equations as words, and I simply forgot that I could actually rearrange teh ltteres! TGIF :-)
I used a paper from 1968 ( https://doi.org/10.1098/rspa.1968.0004 ), and treating the centripetal acceleration due to orbital motion as uniform acceleration. It's good enough for the Earth-Sun system, but it'll fail in the case of something like close orbiting pulsars.
The earth going around the sun produces all the tides in all the oceans of our planet, excluding the lunar tides. The combined power of the tides is tremendous.
Could there exist a material that resistively dissipates gravitational flux more strongly than normally, in the same sense that ferromagnetic materials resistively dissipate magnetic flux more strongly than normally? Something something "gravitational permeability"?
(I guess such a material would interact with gravitational waves as if it had more mass than it does, without that affecting its inertial mass?)
Acoustic waves dissipate their energy into non-acoustic (molecular) degrees of freedom. In the absence of matter, I think there are no other degrees of freedom for gravitational waves to dissipate into, unless they are strong enough to form a black hole (which they can do under extreme conditions, I think).
(This is waaaay outside my expertise though, so take my answer with a grain of salt. Everything I've said in this thread is basically based off the rudimentary understanding from taking a GR course in grad school. I've never done research on this topic.)
In principle yes, gravitational waves dissipate energy into ordinary matter they pass through. However the coupling is extremely weak (that gravitational waves are so hard to detect is testament to this).
In fact this weak coupling is what makes GWs so interesting for observational astronomy: They propagate from the source to our detectors virtually unchanged. (This is in contrast to EM radiation, which is very easy to scatter.) For example the farthest we can see back with EM radiation is about 200k years after the Big Bang, when the plasma of the early universe recombined into neutral hydrogen. By contrast gravitational waves can see back to the Big Bang itself, so it is a truly unique source of information as compared with light.
I would rate the weak coupling as a far second or third point of interest behind linear signal fall-off vs inverse square for most other kinds of signal such as electromagnetic waves.
Gamma ray burst is twice as far away? It's four times dimmer. A thousand times as far? A million times more dim.
Gravitational wave signal from <event> is twice as far away? Makes it twice as hard to detect. A thousand times as far away? Only a thousand times as hard to detect.
To be clear, gravitational waves follow the same inverse square law as EM radiation, in terms of power. This is after all just a statement of conservation of energy.
The linear drop-off you're referring to is when we look at it in terms of field strength (in this case the spacetime strain). Since power is proportional to field squared, this implies a linear drop-off in the field. It just so happens that for GWs it's easier to detect the field, whereas for (most) EM radiation it's easier to detect power.
There are field-detection methods for EM radiation as well, which are useful for weak signals. Homodyne and heterodyne detection are good examples.
Hard or impossible to visualize, but as the temporal dimension is so much larger, most of the deformity is in the time dimension.
But yes, spacetime itself is jiggly like Jell-O.
As we can't visualize 4d spacetime, most analogies will be wrong. But as photons don't experience time themselves, thinking about the geodesic path is probably less error prone than thinking of it as squishing physical objects.
Objects being pulled towards slower flows is the intuition that matches the math best for me.
Photons travel at the speed of light, and at that speed, any "subjective" time is zero. In Einstein's theory of special relativity, the faster you go, the slower your proper time appears to an external observer. At the speed of light, this effect reaches infinity.
> the faster you go, the slower your proper time appears to an external observer
From my perspective, it takes about 8 minutes for a photon from the sun to hit my eye. From the perspective of the photon, a little time has passed, no? Doesn't the atmosphere and passing through my glasses slow it down a wee bit? Can the photon "know" that its position has changed between emission and absorption? From the photons point of view, I must be very, very close to the sun, right?
It takes time for a photon to move in your reference frame, but time within the photon's own reference frame is not advancing at all during that. Within the photon's reference frame, the photon exists instantaneously, simultaneously, at its emitter and absorber. Its whole existence "brings together" the spacetime it was emitted from and the spacetime it is absorbed into, at a single 4D pinch-point. It's like the whole universe is squished flat into two hyperplanes of "everything behind the photon at time of emission" and "everything ahead of the photon at time of absorption", and those two hyperplanes have no distance between them.
> Doesn't the atmosphere and passing through my glasses slow it down a wee bit?
When a photon is travelling through anything other than vacuum, it's not "slowed down." It's repeatedly being absorbed and re-emitted. (Or rather, it's being absorbed, and new photons that happen to be mostly equivalent are being emitted.) The refractive index of a material is effectively a measurement of the likelihood of absorption, times the average per-particle time-delay between absorption and re-emission.
No because a single photon doesn't *experience" the whole universe but only the points where it's emitted and absorbed and you could say all the points in between along the geodesic between the emission and absorption events
So if it is never absorbed, flying on thru the vast emptiness of space, does it experience the sum total of the existence of the universe ? (along that geodesic, of course)
Ah yeah, along that geodesic, which is far from the total of the universe. But I think I now understand that meant "total duration of the existence of the universe".
I think an answer to that question depends of what you mean by total duration of the universe.
If the photon never gets absorbed by anything, then it goes in forever. If it exists then the universe still exists forever since at least one photon exists forever.
No. From the photons perspective, there is no concept of time. Phase speed, group speed, shadows going faster than the speed of light, etc.. will all complicate using the concepts used to teach diffraction
Massless particles being required to travel at the speed of light is perhaps a lens to think about it.
There is a lot of confusion in this thread. One is the topic of photons vs proper time. tl;dr use affine time for things that move at c, and proper time for things that move at less than c, and remember that nobody's coordinate time is in any sense the time in relativity.
Some quotes from this thread:
> photons [have] no concept of time
and earlier
> Nothing that travels at light speed experiences time. For a photon, emission and absorption is a single event.
and other commenters in the same thread
> Photons ... "subjective" time is zero. In Einstein's theory of special relativity, the faster you go the slower your proper time appears to an external observer
> time within the photon's own reference frame is not advancing at all
and even Don Lincoln in a linked video in this thread: "we have to be careful since the equations of relativity don't apply for travelling at the speed of light, but hopefully you see that this limit trick allows us to get arbitrarily close. So I think we can see that a photon experiences no time ..." Thus everyone above is in good company with these slogans. However, Don Lincoln almost certainly knows he needs to correct s/the/these/ (in the context of the lim v->c analysis in the video), and that his conclusion needs to be understood as "no proper time" in that context. But we also all know that it's a youtube pop sci outreach video, not a university lecture or crucial vital factual no-fake-news hackernews thread.
So, let me make the counter-propostion: photons evolve on their worldlines, so must experience some time.
Additionally, elastic Rayleigh scattering supports the idea that there may be one or more point-coincidences along the worldline of a photon. There can also be non-scattering point-coincidences where the photon's momentum energy is some fraction less than 1/1 of the energy-density (the stress-energy) at some point in spacetime along its worldline even if the photon does not interact non-gravitationally with the rest of the stress-energy there (e.g. at that point there could be one or more of a neutrino, free neutron, dark matter particle, or another photon). We should be able to describe such a point-concidence in coordinates adapted to our photon's worldline, just as we could adapt them to e.g. the free neutron's worldline.
Relativity gives us (for all practical purposes, fapp) total coordinate freedom. Point-coincidence physics are invariant under changes of coordinates.
So we can label any curve any way we like, without changing the physics of anything touching that curve.
Proper time \tau solves the timelike geodesic equation, which makes \tau handy for labelling points along a timelike geodesic, but \Delta\tau = 0 on null geodesics, so is not suitable for them.
There is a unique labelling of points along a null geodesic that does solve the geodesic equation, and that is the affine parameter. See https://physics.stackexchange.com/questions/17509/what-is-th... to save me a bunch of typing. Note that as the third answer says one can use the affine parameter to calculate and explain the gravitational or cosmological redshift as a consequence of the null geodesics picked out by the Einstein Field Equations.
That (and the equivalence principle) is also a satisfying way of understanding the relation E = hf (see the first equation at <https://en.wikipedia.org/wiki/Photon_energy#Formulas>) in a lab-scale patch of flat spacetime.
Otherwise, how do you explain any \Delta f if photons "have no concept of time"? You and others in this thread appear to have been arguing that in Special Relativity the standard inertial frame for massive particles is inappropriate for showing the time-evolution of massless particles. That's true. But the point of relativity is that we can deploy (fapp) any system of coordinates and if we are doing covariant physics (i.e. using tensors; one might start with chapter 11 of J.D. Jackson's textbook which is freely available online (and 2nd ed is on the Internet Archive) and is very widely used in teaching) then it almost doesn't matter what system of coordinates we use.
Almost: we can choose practically useless coordinates, like labelling a curve in a non-monotonic way, or labelling points non-uniquely. In fact, any f(\tau) does both of those on a null geodesic: every point gets labelled with a 0. That's not the photon's fault, that's the fault of trying to use an inappropriate system of coordinates. To be fair, such coordinates seem like obvious choices by a person familiar with their use in inertial frames for massive objects, but who then may be misled into thinking the inappropriateness of the coordinates for objects on null geodesics determines the physics of those objects.
Unfortunately, this mistake is very common, and has led to poor slogans which have been repeated many times by several people in this discussion.
If one wants to sloganize, "proper time is inappropriate for photons because they are massless" (cf. §1.2 on the inverse square law and photon mass in Jackson) "but just as nobody's proper time is preferred in relativity, neither is any proper time; and for photons affine time is a useful substitute".
> Massless particles being required to travel at the speed of light is perhaps a lens to think about it.
Indeed, and I just did that for you, although Jackson and I would flip that around to say that c is the speed of massless particles and experimentally (and for theoretical reasons) photons are massless.
FWIW (I'm probably the only one ever likely to read this), in Ch. 7.0 Lightman, Press, Price, and Teukolsky's Problem Book in Relativity and Gravitation puts the previous comment in reverse (which I love and will steal). The entire brief section quoted below is glorious in its economy of English, too.
Forgive the lazy \latex anyone who actually sees this, including future me.
"If u is the tangent vector to a curve, a tensor Q is said to be parallel propagated along the curve if \nabla_u Q = 0. If the tangent vector is itself parallel propagated, \nabla_u u = 0 (tangent vector "covariantly constant") the curve is a geodesic, the generalization of a straight line in flat space. If x^\alpha(\lambda) is the geodesic (with u^\alpha = dx^\alpha / d\lambda) then the components of the geodesic equation are
"Here \lambda must be an affine parameter along the curve; for non-null curves this means \lambda must be proportional to the proper length.
"If a curve is timelike, u is its tangent vector, and a := \nabla_u u = Du/d\tau, then a vector V is said to be Fermi-Walker transported along u if \nabla_u V = (u \otimes a - a \otimes u) \cdot V."
See also problem 7.11 and its solution.
Also of interest is Matthias Blau et al. 2006, "Fermi coordinates and Penrose limits". doi:10.1088/0264-9381/23/11/020 hep-th/0603109 which adapts Fermi coordinates to null geodesics. (abs. "(Fermi coordinates are direct measures of geodesic distance in space-time)... We describe in some detail the construction of Fermi coordinates" §4, "We now come to the general construction of Fermi coordinates associated to a null geodesic \gamma in a space-time with Lorentzian metric g_munu. Along \gamma we introduce a parallel transported pseudo-orthonormal frame ... Fermi coordinate are uniquely determined by a choice of pseudo-orthonormal frame along the null geodesic \gamma" "For many (in particular more advanced) purposes it is useful to rephrase the above construction of Fermi coordinates in terms of the Synge world function").
This isn't quite true. At a macro level, light slows down. At a micro level, photons travel at the speed of light, get absorbed and re-emitted, and change directions. These effects average out to looking like photons traveling slower.
At the micro level there are infinitely many paths the photon follows and most of them involve it traveling considerably faster than the speed of light. While they cancel out, if you omit them then the calculations don’t give correct results, so they’re happening inasmuch as anything can be said to happen scientifically. If you want to object that that contradicts GR well there is a Nobel prize or ten waiting for whoever squares that circle.
I did, and enjoyed it as a good pop-sci outreach video, and do not fault it given that's what it is. The lim v->c analysis is standard and well-presented, but his conclusion about photons' time is liable to confuse people (including other physicists who aren't relativists). I refer you to my comment elsewhere in this thread <https://news.ycombinator.com/item?id=36537015> which has a paragraph about this video (your link having partially motivated my comment), and which explains that it is common for relativists to use affine time for massless particles.
"However, spacetime is incredibly stiff, and I think all the known real-world sources produce pretty smooth waves."
In English, why does stiffness correlate to smooth waves? What does stiff spacetime mean? I'd have thought a square wave would be "stiff" as it's quite the opposite of smooth.
In order for a square wave to travel through a material, it would need to allow for infinitely high frequencies (see the animation on wikipedia[0] for a visual demonstation). The lower the maximum frequency possible, the more every wave will resemble a sine wave because it's the most basic shape: any non-smooth wave has higher-frequency components to give it its shape. Filter out those higher frequencies, and you're left with the basic smooth sine wave again.
A stiff material tends to dampen high frequencies, simply because it cannot deform fast enough to follow the wave's shape. In a way, the medium acts as a low-pass filter; compare, fow example, how fast you can clap your hands in air vs in water: the stiffness of water slows down your movements so you cannot reach high clapping frequencies.
> A stiff material tends to dampen high frequencies
All materials roll off their frequency response. But this is backwards - stiffer materials have higher resonant frequencies given equal density. It’s basically a word to describe a high spring constant.
I think what the GP was stating here were two independent reasons why we would find it hard to know what would happen if a gravitational square-ish wave interacted with spacetime: 1. spacetime is "stiff", and 2. there are no gravitational square-ish wave generators around to observe.
The two things are kind of related, though. One "natural" way to create a square wave in nature, is to "interrupt" a material transmitting a sinusoid wave, at the peak of its transmittance. And one way to do that, is to break through the modulus of elasticity of the material transmitting the wave, such that it switches from the elastic-deformation domain (transmitting the wave) to the plastic-deformation domain (ripping apart.)
Imagine a speaker cone tearing at the peak of a high-amplitude drum beat. The cone pushes "out" — and then doesn't push back "in", because instead the air behind it rips forward through it. The air created by the speaker cone wants to rush back "in", but now there's no longer a speaker cone acting as a waveguide for the inward flow, so the natural turbulence cancels out much of the "falling" energy of the wave, making it look much more like a square-wave drop.
I believe that the GP is saying that, because spacetime has such a high effective "modulus of elasticity", we haven't yet observed any practical way to perturb so as to create the conditions that would generate a gravitational square wave.
Imagine using a flick of your wrist to create a traveling transverse square wave in a string vs. in a long metal dowel. It's actually not so hard to create a smooth sinusoidal wave in a metal dowel. (It will be long wavelength, but it will be easily visible if you put your eye near one end and look down it.) But it would be impossible for you to make a square wave.
Thank you for that! As an EE and radio enthusiast, I feel like I have a fairly good grasp of how gravitational waves behave from that description. The way in which the mechanics of these different energies all sort of share characteristics is rather beautiful.
Could you set up massive pendulums to convert this gravitational wave energy into mechanical energy? Would such a device diminish the strength of the wave downstream?
Yes! In principle at least, and only miniscule/undetectable amounts of energy. The first gravitational wave detectors build were these "Weber bars" [1] - big block of aluminum that were supposed to resonant mechanically (like a bell) when a gravitation wave of the right frequency passed through them.
Looks like these things haven't detected a real gravitational wave, but if a strong enough one at the right frequency came through, they might start ringing like (very quiet) bells!
I recommend looking into LIGO and other similar experiments. They use laser interferometry to accurately measure the distance between two points, to an extreme degree. From LIGO's website:
Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is "Differential Arm" motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways.
As described above, as the lengths of the arms change, so too does the distance traveled by each laser beam. A beam in a shorter arm will return to the beam splitter before a beam in a longer arm--as the wave passes, each arm oscillates between being the shorter arm and the longer arm. When they arrive back at the beamsplitter (where they re-merge), the light waves no longer meet up nicely; they are out of phase. Instead, they shift in and out of alignment for as long as the wave is passing.
>*Gravitational waves cause space itself to stretch*
Please ELI5 specifically and empiracally what "Space Itself" actually is.
Would it be possible to build a 'galactic clock & Compass' - a "clock" to the regular pulses of a pulsar and the galactic direction the pulsar is in relation to the terrestrial compass (magnetic) on earth...?
What is the pulsar with the most reliable timings?
Space itself means the metric of spacetime: how you measure distances. Locally, if there was no gravity, you could think of space as a plain reticule where distances would be calculated through eucledian geometry: that means cartesian distances, parallel lines would be parallel and the angles of a triangle would sum 180, and the shortest path between two points is the straight line.
General Relativity successfully establishes that the presence of mass distorts this, so it defines a mathematical object (the Einstein tensor) that reacts to the distribution of mass and energy and precisely describes the changes to the metric. For example it can model how the mass of the sun distorts the space so that light from distant stars appear to follow a curved path because very close to the sun a curved path is now the shortest path.
The Einstein tensor defines how distances --and time-- are measured and it's the best mathematical model that we have about what "space itself" is. Future theoretical advances could take us forward and demonstrate that space itself emerges from other more fundamental elements, but this needs bridging quantum mechanics and gravity. We don't really know what space is made of, but scientists have precisely modelled how it reacts to mass (and energy) with utmost precision.
NB: At cosmic scales the exercise becomes more difficult, as there is an expansion of the metric of spacetime that is not due to the presence of mass, in fact it is caused by the _absence_ of mass as it seems to be due the energy of empty space: the phenomenon called Dark Energy.
If this is ELI5, we hang around very different kinds of five year olds :-)
Joking aside, thank you for the deeper explanation. The idea that spacetime isn’t fundamental is a very non-intuitive concept given how we’ve evolved to interact with the world. Any suggested reading on this topic for laypeople?
> Any suggested reading on this topic for laypeople?
Carlo Rovelli is a bona fide theoretical physicist, very involved in the development of Quantum Loop Gravity (one of the attempted approaches to bridge GR and QM). Turns out he is also a good pop-sci writer, so I would begin there. His book "The Order of Time" deals with the nature of time, which is not about the nature of space, but then again reading it you see that the mental gimnastics are similar.
I also found useful contributions from regular contributors at /r/cosmology (thank you /u/jazzwhizz) but it's less straightforward and alas, Reddit has its own issues.
I find /r/cosmotology a hairy subject hard to swallow...
I think they push string theory too much, and try too hard to braid it into the fabric of our societies, with their little shops and what not... It gets everywhere, and tomorrow is their favorite day "Friday"!
let's think about space like a giant, invisible playground. Normally, it's flat like your bedroom floor, where you can measure how far your toys are from each other with a ruler straight across. That's like when there's no gravity in space.
But guess what happens when something really heavy, like a big bowling ball (that's like a star or planet) comes into your playground? It makes everything around it bend and curve. So, the distance between your toys is no longer a straight line. It's like when you throw a ball, it doesn't go straight, it goes in a curve.
This bending is what a really smart guy named Einstein explained in a thing called General Relativity. He came up with a way to measure how space bends around heavy stuff.
And you know what else? There's this really weird stuff called Dark Energy that's everywhere but we can't see it. It makes space grow bigger and bigger, not because of heavy stuff, but because there's a lot of empty room. Scientists are still trying to understand this, but it's like blowing up a balloon: even though there's no heavy stuff inside, the balloon still gets bigger!
Out of curiosity, from someone who has never worked with non-euclidean geometry, what does it mean for a path to be curved in non-Euclidean space? My outsider understanding of curvature is that the inside of a curve is shorter than the outside of the curve, whereas a line has the same length on either side (assuming we give these curves and lines some thickness). But, if the shortest path can be curved, what do we mean by curved?
I've found it's easier to think about this stuff in two dimensions. The surface of a sphere (or the Earth!) has non-Euclidean geometry.
Imagine two people standing some distance apart from each other at the equator. They both begin walking in straight-line paths due south. At first, their paths are parallel. But as they move toward the south pole, they begin to drift closer to each other, as though their paths were curving towards each other. When they reach the south pole, they bump into each other. But they were both walking straight forward following the shortest path to the south pole the whole time. The curvature of the surface causes their initially-parallel paths to converge.[1]
On a plane (which has Euclidean geometry), initially-parallel paths never converge.
[1] Don't take this too literally; the real planet Earth is three-dimensional, and its gravity keeps us on the surface. But mathematically, it's possible to describe a curved two-dimensional space without referring to any higher dimensions. When I talk about "the surface of a sphere", that's what I mean -- the surface is the entire 2D space.
Maybe it's worth adding that in this way of thinking (intrinsic geometry of the surface), great-circle paths have exactly the property the GP brought up about straight lines: neighboring paths aren't shorter on one side and longer on the other. (If you think of them as 3-d paths then there's a shorter path below vs. longer above, but that's not part of the intrinsic geometry.)
If two people are in parallel, they will make two parallel circles. If two people aimed at a singe point, they are not in parallel.
Space-time is 4d array: array of framebuffers. You can stretch your mathematical model all day long, but you knowledge must be mapped to reality somehow. In model we have space-time, while in real world we have "physical vaccum" ("something nothing" or "phaccuum", for short). I prefer to name that thing "ether", because I like that word.
> If two people are in parallel, they will make two parallel circles. If two people aimed at a singe point, they are not in parallel.
In spherical geometry, the equivalent of a straight line is a great circle. There are no parallel great circles. That's why I used the phrase "initially parallel" -- at the starting point, both people's paths are at a 90-degree angle to the great circle connecting their locations.
I didn't want to get into "locally flat" vs. "globally curved" in something that started as an ELI5 thread.
> In spherical geometry, the equivalent of a straight line is a great circle.
Yes, of course. If we substitute parallel lines with straight lines in spherical geometry and mix 2D and 3D spaces, then our mental model will be nonsensical but cute.
We found no evidence of fourth dimension in the real world, so we cannot map this cute mathemagical model to reality.
Picture curves on the surface of the earth. They seem flat locally, but if you go a mile north, a mile east, a mile south, and a mile west, you don't end up _exactly_ where you start. (In the northern hemisphere you end up a little east of where you start; in the southern, a little west.)
Same thing in general relativity: the metric tensor measures the failure of closed loops on each axis to not close perfectly, the way they would in Euclidean space.
Basically even as a small creature on earth you can 'figure out' about the curvature by carefully measuring small-ish loops. The same is true for spacetime, but the loops' deformities are even smaller.
It's not too complicated. Get a round ball of some sort. When you draw on the surface, that's a "non-Euclidean space".
Take a straight line down from the "north pole" of your ball to its equator. Draw another straight line around a quarter of the equator. Draw a third line back to the pole. You've just drawn a triangle with 3 straight lines and the angles add to 270 degrees.
A non straight line is just not the shortest distance between two points on that surface.
Distance from center as described by the path - so if the distance to center is held, but the trajectory is changed, the line will be expressed as a curve around a tehtered point to the center.
If the 'tether' is a gravitational link (meaning that the teather, is a constant pull against the trajectory, regardless of the trajectory, the object will continue to curve around center.
I cannot be of help here but I'd say that your concept of curvature is too informal, but more formal maths can deal with it, take a look at the wikipedia page for "Geodesic", the maths are way above my head but the diagrams are cool :D
Super helps! thanks - while I knew a bunch of pieces and tidbits, you cemented it for me - thank you.
-
TensorFlow
>*General Relativity successfully establishes that the presence of mass distorts this, so it defines a mathematical object (the Einstein tensor) that reacts to the distribution of mass and energy and precisely describes the changes to the metric.*
This leads me to think that TensorFlow was attempting to map the 'weight' among topics of intersecting interests, sciences, etc... and seeing who the "tensor warping" was most strong with and adding higher eval weights to things that "gravitated" to one another based on the informational difference in distance?
(I dont know the nomenclature, but is that were using 'tensors' in AI/ML/whatever 'weights' come from?
A tensor is a mathematical object that has a lot of uses beyond general relativity, of course. It's a little tricky to visualize (I think there are good YouTube videos) but tensors can be thought of as multidimensional arrays, and also the whole tensor algebra can be done in terms of multidimensional array operations.
So reasoning about a neural network weights and operations in terms of tensors makes sense and I guess that's what the name Tensorflow comes from.
Yes, but your explaination helped me to better understand the other types of tensors in a way I can 'grok' it - as opposed to trying to be at the level of some google quant or whatever with ML understandings of weights.
It helped my put my own internal visualizations to the understanding.
and I had a weird peripheral memory on this from a thought I had whilst driving in 1999 where I was thinking of tensors in this way, but I didnt know what I was just daydreaming about... but apparently, it was einsteins tensors coupled with information theory - and while to me it was a pedestrians take on the premise - it turns out that that day dream was correct!
And it all ties back to when I was ten years old and meditating on the Mind of God -- It all tied into one another - and you gave me some cord to pull these experiences together with understanding which I havent had in 40 years... so that was nice. Thanks.
Anyone else recall daydreams from their past where their later understanding was confurmed, even though you were just "daydreaming at the time"?
Take a sheet of elastic material, like a balloon, but square. Suspend the corners in clamps on posts so that it’s “perfectly” flat. Draw a line that is the shortest distance between two points on the material: it will be a “straight” line like what you learnt in geometry class. Now place a marble in the center of the sheet and let it stretch the sheet. Now draw the shortest possible line connecting these two points: it will curve (and unless you chose by coincidence) it will likely not overlap with the first line: the “shortest distance” between the points has changed.
There’s another aspect to this: the expansion coefficient. One such model is that the coefficient depends only on time: as time passes, distances increase. To model this, draw the same line on the flat sheet of material, then expand it uniformly in all directions. The distance is still a straight line, but the line is longer after the expansion.
It's not so much that you see a slightly different "down," but that space itself is changing such that the distance between e.g. your head and feet is (very) slightly altered.
The two descriptions are equivalent. By the equivalence principle, the wave looks locally like neighboring bodies see different directions of down (and also slightly different strengths of the force of gravity in that direction).
Dumb, but earnest question: if space itself changes, how can the distance change? What is the distance a measure of, if not space itself? What's the yardstick, speed of light?
Gravitational waves really do change the time it takes for light to travel between two points. We use light travel-times to measure distances, thus we say that the distance between the points has changed.
If it feels counterintuitive for spacetime to be changing, that's good. It is outside our human experience and perception. The strongest gravitational waves ever observed by scientists passed through everyone who was alive in 2015. None of those people noticed before the instruments registered a detection.
>None of those people noticed before the instruments registered a detection.
IIRC, the reason no one noticed is that even the strongest gravitational waves are only going to "stretch" space by something less than the diameter of a hydrogen atom.
Correct me if I am wrong, the gravitational waves we measure have such small effect, that if we blew proton to the size of the Earth, it would get squished by distance smaller than a human hair.
Edit: from wikipedia LIGO page: "(interferometers) are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton"
I dont know why I remember the human hair analogy, perhaps I am confusing it with something else?
If the yardstick is light-speed, is there any meaningful distinction between saying that space itself changed and that there are local perturbations to the speed of light?
The thing I struggle with is that we normally think of things occupying space. If space itself gets distorted, then the size of those things should change, too. Or is that mental model a useful but ultimately incorrect way to think about the world?
Yes, and light is actually how the LIGO and similar detectors can measure such small changes in distance. Light emitted from a laser is split into two beams traveling different paths, and then merged back together.
When the light merges back together, if the two paths traveled took exactly the same distance (or an even multiple of the wavelength at least), then the beams add together constructively and you put back together the light from the laser.
But if one path becomes longer or shorter the other, the light is out of phase with itself (peaks of the waves no longer line up with each other) and you can detect the interference between them.
LIGO can detect a change in distance of less than one ten-thousandth the charge diameter of a proton.
The gravitational waves we detect are transverse waves. Longitudinal gravity waves have been proposed, discarded, and proposed again, I’m not sure what their current theoretical status is.
I was wondering if these waves could affect our brains, even subtly. Maybe cause a reaction in one part of one cell that tips the balance between a neuron firing or not.
Thanks for the link. I had issues with the WaPo article, but that tongue-in-cheek joke with dead-pan delivery wasn't one of them. Fortunately another commenter called it out as a joke as well.
Imagine if we could take advantage of the time-space potential/differences between gravitational areas to "skip" large parts of space. We would have to have a very precise gravity map, but could also get huge gravity potential boost!
Could we? Is there math that supports this? Wouldn't this require taking some, and then not losing it when the wave passes? I don't see much happening with the earth, as it's experiencing them, so I assume the "extraction" process would be significant and unique.
I was taught that all items fall at the same speed, due to the gravitational constant.
With these waves, does that mean that items will fall at ever so slightly differing speeds, as depending on the size of the wave there is a pull in the other direction, and the constant is slightly off?
And OT, but related: Since every item has a gravitational pull related to its mass, would a bigger item fall ever so slightly faster than a small one, as it is pulling itself to Earth in addition to Earth pulling it down.
No, gravitational waves do not affect the equivalence principle and do not change the gravitational constant. They are waves of change in the geometry of spacetime; freely falling objects show these spacetime geometry changes by changes in their relative motion.
Well in both your questions it depends on how you define speed. It is commonly understood to be the first derivative of position, or the quotient of distance divided by a time-span. But how do you define and measure distance and time-spans? Which reference frame do you use?
Also, the things falling in a vacuum at the same speed thing is a common misconception. The two arguments usually are that the mass of one body can be canceled out in both equations or that splitting a body into two won't make its halves fall slower.
To answer the question: Given all bodies have the same size, a constant distance to and mass of the reference body (usually earth) and everything starting at rest with no relative speed heavy and light bodies do experience the same acceleration, but a heavy body will collide sooner than a light body. Did it "fall faster"?
This is all firmly outside my technical discipline, but aren't there some theories that faster-than-light travel might be achieved by bending spacetime around a spacecraft, as opposed to trying to propel the spacecraft through space?
I really want to emphasize that this is entirely speculation, but is it possible that these gravitational waves could be the "ripples" produced in the wake of such faster-than-light travel, the same way a boat travelling through a body of water leaves ripples in the water behind it?
In the video, they mention that the models we have for FTL (by bending spacetime) wouldn't generate ripples in this way. We could however detect ripples from a really massive ship accelerating really really quickly.
> Whereas the original discovery spotted waves originating from the collision and merger of two star-sized black holes, the most likely source of the latest finding is the combined signal from many pairs of much larger black holes — millions or even billions of times the mass of the Sun — slowly orbiting each other in the hearts of distant galaxies. These waves are thousands of times stronger and longer than those found in 2015, with wavelengths of up to tens of light years. By contrast, the ripples detected since 2015 using a technique called interferometry are just tens or hundreds of kilometres long.
Seems like they probably know where these are coming from. I imagine, like your boat analogy, that we can observe massive natural oceans swells and wouldn't notice the wake of a boat as it moves across the ocean.
from a layman's perspective, this sounds crazy cool that they were able to "see" this in the data. seems like one of those things that would be easy to miss from being scoped in and only discoverable after zooming back out. waaaaay out.
Do they require a long list of impossible things to work? Also yes!
There are entirely valid solutions in general relativity which allow for an object in a pocket of spacetime with other spacetime warped around it in such a way that, more or less, space is moving but not the object.
However achieving the arrangement of spacetime to make this happens requires many things which are impossible, aren't known to exist, or require something like all the energy in the Universe to achieve.
Also there are no valid known solutions that transition from normal space to this special spacetime arrangement, so it could only exist if it always existed.
So it comes down to: we're pretty sure such things are not actually possible but we know where to look and what problems to solve if it were. Occasionally we see a paper which removes some of the impossible things from the list.
It's one of those "unlikely but maybe someday" kinds of things.
Which as shocking as it sounds still small compared to large gravitational waves.
The merger of black holes radiate something like 10% of that mass as gravitational waves. Start talking 2.5+ million solar masses black holes and ~500,000 solar mass worth of energy in gravitational waves seems plausible though obviously rare.
"I understand how the engines work now. It came to me in a dream. The engines don't move the ship at all. The ship stays where it is and the engines move the universe around it." ―Cubert Farnsworth
Theoretically it's impossible to travel through space at or more than speed of light. But space itself can move faster than light speed, and a warp drive would help something similar that you mentioned. That is possible theoretically. But the amount of energy or mass it needs is very high and no current technology (or in foreseeable future) can achieve it. So FTL remains a dream.
My hunch based on nothing is that we will achieve FTL no earlier than 2250.
It's fun to imagine the species in x-hundred years. My college physics professor once told us that in 500 years, physics professors will still teach Maxwell's equations in the format he was showing us. And honestly, I think he's right.
Somethings we will do the same way for hundreds of years, like the wheelbarrow will still exist in 500 years as it has for likely the previous 5,000.
Otoh, I doubt we will be going faster than light this millenium.
We still teach the Newton laws that are comparably old. We also teach that they are approximate, but work flawlessly for household-scale speeds and masses.
the only plausible possibility for a major change in Maxwell's laws is if we discover a magnetic monopole. It's fairly unlikely, but there isn't a solid reason one shouldn't exist as far as I know.
'space' isnt a substance that can move at any speed.
What's meant by the claim that 'space moves faster than light' is that extremely distant objects are moving away from each other, relative to each other, 'faster than light' -- which is permitted, so long as that distance can never be bridged by light.
The claim amounts to, in other words, that the universe is so large that we can compare objects at distances greater than those light could travel between them, and if we do that, they travel faster than light.
This is an "illusion in measurement" more than anything else. Nothing is travelling faster than light.
I think this simply refers to the metric expansion of the universe [1]. While nothing actually travels FTL, the distance between some objects really expands faster than FTL and is not an illusion.
The illusion is the impression that something moves faster than light.
'distances' arent literally 'expanding' -- this metaphor of expansion describes a shift in the matter distribution of the universe over time which seems like an expansion of an underlying substance 'space'.
This is the illusion. The metric is just that matter distribution. And the fact of its changing we call 'expansion'.
This metaphorical, substantival language, creates a lay impression that some physical object moves faster than light.
Yeah the only energy source that can produce enough energy to power an Alcubierre drive (warp drive) that humanity has ever even conceived is a matter-antimatter reactor. But we don’t even fully understand matter, much less antimatter, and are pretty far from that. 2250 at the earliest is not an unreasonable estimate.
Antimatter wouldn't be even close to powerful enough, and both matter and antimatter have got the wrong sign for every (or almost every, depending on which headline I trust) warp drive variation.
As antimatter is as powerful as one can get, it not being powerful enough is a good reason to think it's not going to work.
To simplify, the same way that a surfer on the sea can use the movement of the sea itself (waves) to surf!
An Alcubierre drive (they're theoretical) would basically constantly compress the time curve of spacetime in front of the craft, allowing the craft to "ride" this compression as it moves forward, which means that the local speed of light of the craft is faster than the speed of light of an external observer. Note that the main issue we have is to find something that can compress space, and then to have it have enough energy for it not to be trivial (because 110% of the speed of light, while technically FTL speed, is still very slow for interstellar travel). And of course, while the existence of something that does this spacetime curve compression fits the math we have, we've yet to find a material or technique that actually does so.
Imagine you are on a rubber ruler. You can move at most 1 mark per second on the ruler. This is true regardless of how much the ruler is stretched or compressed.
So to move from mark 1 to mark 100 will always take the same time at top speed, regardless of any stretching/compression.
Don’t take my word at all, but I think in your analogy you can’t imagine it as you’re only allowed to go 1 ruler tick a second, but imagine you can only move 1mm per second. If you compress the rubber ruler you traverse more ruler per second than before while still going the Same speed
The idea is similar to how we detect gravitational waves in interferometers, when space gets compressed, the distance the photon has to travel shrinks slightly.
To take your ruler example, if you compress it by 1mm, you can traverse its entire length traveling 1mm less than before, thus, from the reference frame of someone who can't see that you've compressed the ruler, you've traveled slightly faster.
This video might help understand where mass comes from and how to potentially modulate it, because mass can be thought of as bound energy creating voids in the gluon field:
The Higgs mechanism affects electrons, not quarks, and is only responsible for about 1% of matter's mass. Most mass comes from the binding energy between quarks, which creates flux tubes between quark-antiquark pairs. If we add more energy to pull quarks apart, eventually the total energy added exceeds the mass-energy equivalence of another quark-antiquark pair, so a new pair gets created from the vacuum. I believe this is related to the Casimir effect, but IANAP (physicist).
Keep in mind that the mass-energy equivalence also applies to time. So like in the movie Interstellar, when they go down to the water planet, gravity is so high that time passes slower for them than the guy in the orbiting ship. In other words, the ship sees the landing craft move slower and slower as it approaches the surface. This difference in the speed of time near a gravity well is what slows the inner edge of a satellite slightly more than the outer, curving it along the path of the orbit, which from the satellite's perspective feels like a straight line at that velocity, because it's weightless and feels no other acceleration other than tidal force. So if someone could move large amounts of energy into a confined space with some kind of flux capacitor (is this a pun? I don't even know anymore), they could slow time there and create a virtual mass through mass-energy equivalence by E=mc^2. If they did it in front of the satellite, it would begin to increase in velocity towards that mass. So this is sort of a warp drive mechanism, although I don't know how you'd confine it, and the energies involved would be planet-scale to achieve 1 g of acceleration like near the Earth.
Also if someone made a closed loop where electron-positron pairs were sent one way, then their energy was used to create quark-antiquark pairs sent the other way, there might be a 1% imbalance in mass due to the Higgs mechanism, which would add momentum to the loop opposite the direction of the heavier stream. Although due to conservation of momentum, I suspect that this wouldn't actually happen, because any momentum above light pressure should get lost to heat/entropy/etc. But it would be a fun experiment to try. The same experiment would also work just sending light energy photons one way and matter-antimatter pairs back the other way, but I've never seen a proof as to why this would or wouldn't beat light pressure. This would be a reactionless rocket, not a warp drive.
If there's a gravity field like a gluon field, just with slightly different rules, then I don't see why it couldn't be modulated. In fact, I think that the dark matter strands connecting galaxies are densities where perhaps something like slowed neutrinos or axions collect and slow time. They could even be places where gravity "flows" along eddies left over from the Big Bang, although this seems strange to us because gravity normally only flows into gravity wells. There's also currently no explanation for the Hubble constant in the expansion of the universe, so perhaps something is creating space over time. So I don't see why space couldn't be created behind a craft to push it forward. We just don't know how.
There are so many unexplored interactions like this, that I don't think any physicist can confidently say that warp drives, reactionless rockets and folding space are impossible. Which means that I give it 50/50 odds that some kind of sci-fi space engine will be invented within the next few decades, probably starting with a reactionless drive like the EmDrive, which (if it works) uses resonance to time the interaction of microwaves with the rebound of atoms in an asymmetric field, similar to the Biefeld-Brown effect explored by Thomas Townsend Brown in the 1920s, which was later found to just be an electrohydrodynamic (EHD) effect:
Unfortunately only physicists are privy to the mental associations which allow thought experiments like this. Textbooks leave us mainly theory and equations, not insights or abstractions. Physics formulas are like trying to understand the behavior of an app from its assembly language. So in a very real way, academic gatekeeping prevented almost everyone from contributing. For every divergent thinker like Einstein, there are 100 convergent thinkers who judge skeptically and crush ideas into oblivion.
I'm just an armchair warrior full of derivative ideas who has never invented anything, who would love to run experiments like these. But just like you the reader, I'll spend the rest of my life making CRUD apps to make rent because billionaires have all the money, instead of getting to be like Dr. Gillian Taylor in Star Trek IV, suddenly able to explore every possibility under the freedom of UBI. That was a joke, but not really.
>The Higgs mechanism affects electrons, not quarks
It effects both. The LHC produces Higgs particles through the annihilation of top-antitop pairs, which works because the top quark couples strongly to the Higgs field.
This is a terrible analogy, I apologize in advance because I don't fully understand it yet, but:
We think of empty space as empty because its symmetry isn't broken, so it looks transparent to our matter as we move through it. Similarly to how electrons pair up to form cooper pairs, which act like bosons and pass through the atoms of a superconductor without interacting:
But the Higgs field permeates space, so if we could pick and choose where to interact with it, we could "grab" it and propel ourselves. This would be analogous to a magnet levitating on a superconducting ring, just like this video but imagine that the track is the superconductor and the puck is the magnet:
If we built a puck with two electromagnets, we could power up one of them above the critical field of the superconductor and form a resistive section which the other magnet would be drawn to or repulsed from:
By alternating the strength of the two magnets, the track would stop behaving as empty space, and the puck could accelerate along it like a maglev train. A similar technique should work with the Higgs field.
I don't know how much the Higgs field "weighs", so I don't know how much of a reactive force we would get for the force applied. My guess is that it either wouldn't be higher than light pressure, or that the probability of an interaction would stay beneath what's required to beat light pressure. As in, this may be tied to how often a photon splits into an electron-positron pair, with the remaining energy bleeding off as heat or entropy. But it would be a fun experiment to run.
I just want to add that physics terminology and notation is too big to fit in the human mind, like trying to memorize a 100 digit phone number. The same problem exists in functional programming, where stuff like monads and y combinators just won't stick in a mind trained on imperative programming. So there's a very real limit to what we can understand. No matter how long we study this stuff, we can never connect all of the dots, or see faint relationships between distant concepts. IMHO this problem is getting worse with time, despite the advances in stuff like the Standard Model.
But AI doesn't have that problem. Within 10-20 years, it will infer how to modulate stuff like the weak force and gravity with electromagnetism in a practical way. If the aliens can do it we can do it! Or we can at least build machines to tackle the problem faster than we ever could. And if that's true, then why bother keeping research secret? It's gonna all come out eventually.
Edit: added the Higgs mechanism derivation from superconductivity. Also wanted to add that axions (if they exist) only interact with gravity and electromagnetism, making them a potential bridge between the two:
I'm kind of cringing today reading what I wrote. The ideas I'm talking about are more like leads than answers, probably following a logical fallacy down the rabbit hole at each decision point. If I could ask an AI, then I could avoid the pitfalls and come up with something plausible. I also wish we could cross out and edit any sentences which are wrong. It might be better to invent a warp drive on Stack Overflow.
I understand the gist of this research: pulsars emit radio waves at regular frequencies, so by monitoring radio waves received from pulsars in the sphere surrounding us, we can measure correlated anomalies in their frequencies and infer that they were caused by large gravitational waves that effectively changed the shape of the transmission medium. That makes sense.
But this is not measuring the gravitational wave itself. It's measuring the change in trajectory of the radio signals that are "riding" the wave. In the ocean analogy, it would be as if we were surrounded by a circle of floating turrets that each emitted floating darts at regular intervals in all directions. Then we would measure the time it took the darts to reach us, and from that we could infer the size of the waves the darts encountered along the way. But we never actually see the waves, only the darts.
So my question is: how can we tell the difference between one really big wave, and many really small waves that would sum to the same effect? In other words, we know there is some waveform(s) that changed the velocity vector of the radio signal. But if there are multiple arrangements of waves that would produce the same change in signal, how do we pick the right arrangement?
The timing of a single pulsar would not be reliable enough to detect gravitational waves. Instead, each collaboration monitors an array of dozens. As a result, they have found a signature called the Hellings–Downs curve, which predicts how, in the presence of gravitational waves coming from all possible directions, the correlation between pairs of pulsars varies as a function of their separation in the sky.
Not sure it answers your question but my impression is they simulate results of all possible effects and then see which one(s) the data correlates with. So if there are multiple causes that could produce identical effects then I doubt they could distinguish between them.
That makes intuitive sense. So I guess the more pulsars we measure, the more accurately we can disambiguate between different possible waveforms.
It reminds me of EEG (brain wave) measurement: a hairnet with 256 electrodes will have higher resolution than one with 128 electrodes (ignoring all the issues with interference of the skull).
These ‘monster’ gravitational waves have supposedly been ‘spotted’ by calculating disparities in pulsar timings. The article didn’t have as much information as I was hoping - were any of these waves detected/confirmed by LIGO?
As others pointed out, LIGO is too small compared to the wavelength of the GW, but even if LIGO was extremely long, the technical noises (seismic, control system noises, gravity gradient noise, etc.) at such low frequencies are extremely high for ground-based detectors
Probably a stupid question, but could you theoretically surf on gravitational waves? Obviously not on earth, but maybe near a binary black hole or something. If I imagine waves in a rubber-sheet mental model of gravity warping spacetime, it seems like surfing should be possible.
Sadly, this is not possible, at least AFAIK. The basic problem is that a gravitational wave won't push against you, like a water wave would; the wave will pass through you.
I'm curious what the amplitude of these waves are: what's the (order of magnitude) change in the distance to a 1000 light year distant pulsar, as the gravitational wave passes through?
Edit:
Being told by @Dr_CMingarelli on twitter that it's 10 meters pr lightyear.
> Each observatory has two light storage arms that are 4 kilometers in length. [...] A passing gravitational wave will slightly stretch one arm as it shortens the other. [...] Even with such long arms, the strongest gravitational waves will only change the distance between the ends of the arms by at most roughly 10⁻¹⁸ m.
With the nominal hair value being 75 µm, `apt install qalc` tells me that's
That's for the gravitational waves we can detect from LIGO. These new ones, as I understand it, are being detected by changes in pulsar frequency over 15 years of measurements - so they have a pretty different character than the ones wikipedia's talking about... I don't know how that translates to anything that makes any sense tho
Thinking about how gravitational waves pass through us all the time stretching us out and squishing us and changing how time passes has for some reason been one of the most comforting realizations I've ever had. I still don't quite have the right words for why, but to me it kind of answers questions like if we are in a simulation (my hunch is no) and weird head trip questions that can be depersonalizing like that from my perspective last Tuesday I started hallucinating all of this including everything I think is my history. Not that I think that, just something about the foam of turbulent gravity washing over all of us invisibly. Yeah, comforting.
Spacetime is incredibly resistant to deformation, hence the tiny displacements and the need for long-baseline laser interferometry to detect these waves.
How are we sure that pulsars are 100% consistent? How do we know the timing discrepancies are due to gravitational waves and not just tiny wobbles in the pulsar itself?
I'd suggest looking into Alcubierre Warp Drive, cool story on why the guy came up with it and shows how to wrap spacetime around a spaceship to make it go faster than light.
The ship wouldn't go faster than light because it wouldn't move at all, the spacetime around it would.
The channel below has quite a few other videos on the subject. I love it.
Roughly speaking, it's because the waves are emitted from the region outside the black holes, being created by the in-spiraling of two black holds before/during their merger. Once the black holes merge and settle down, they stop emitting gravitational waves.
But that's just a cartoon. Strictly speaking, the picture of a wave traveling with respect to a fixed background spacetime is only an accurate approximation when the wave is very weak. In the immediate neighborhood of a black hole merger, the approximation breaks down, and you just have to look how the whole spacetime itself is evolving (usually through simulation).
No, they are not caused by the galactic filaments. We don't know exactly yet what all is causing gravitational wave background (GWB), but a theory is that it could be caused by supermassive black holes, or primordial black holes from the early universe, etc.
When I heard about this project in a spacetime video a few years ago IIRC, again I thought okay, this is too far. It's never gonna work, there'll be too much noise yadda yadda. And it now it looks they might have done it.
At this point, if physicists say something is possible, I listen no matter how impossible it sounds.