For everyone that's disappointed, remember that thrust without reaction mass is still possible with enough electrical power and lasers, and doesn't appear to violate any known laws of physics.
It's also possible without any energy transmission using lasers etc. as long as you are in some non-flat spacetime (i.e. there is some mass in your vincinity). It's called "swimming in spacetime" [1]
[edit] You can find a less scientific / more illustrative introduction to the concept here [2].
> as long as you are in some non-flat spacetime (i.e. there is some mass in your vincinity)
I'm not an expert, but isn't that definitionally everywhere? Or is it that it works better when your vicinity is more curved? The paper is a bit beyond my ability to get anything from, and the summary on Wikipedia isn't much better, and also doesn't seem to match what you're implying it is.
> “Swimming in spacetime” is a geometrical motive principle that exploits the curved spacetime metric of the gravitational field to permit an extended body undergoing specific deformations in shape, to change position. In weak gravitational fields, like that of Earth, the change in position per deformation cycle would be far too small to detect, but the concept remains of interest as the only unambiguous example of reactionless motion in mainstream physics.
Photon drive is not "thrust without reaction mass", because photons do have mass (they have zero rest mass, but are never at rest). Mass of a photon is, unsurprisingly, E/c^2, where
E=h\omega. If photons had zero mass, they would not have been influenced by gravity, and bending of light by the solar mass has been measured in the beginning of 20th century.
You're using outdated terminology. Modern physicists use "mass" to refer to what was called "rest mass" shortly after relativity was developed in order to distinguish it from the new concepts they at the time called "relativistic mass" and "inertial mass". The term "relativistic mass" was motivated in part by the desire to still think of all gravitation as being sourced by some sort of mass, but the modern terminology is to say that gravity is sourced by the stress-energy tensor, of which (rest) mass is just one component.
Modern physicists unequivocally say "a photon has zero mass, but has non-zero momentum and energy".
I have a physics degree and still say that photons have zero rest mass, and then parenthetically mention that they never rest, precisely because of the issues of communication that tend to arise when we talk about things like thrust.
I'm always tailoring it to the audience because sometimes physics metaphors need to be unpacked when communicating. I have run into people who think that the "hole" that can move around in a lattice is an actual particle, not a quasiparticle, like it was a swimmy little positron just sitting in there, simply because the metaphoric shorthand had overtaken reality.
If I were talking physics grad to physics grad, I wouldn't do that. Anyone else? Yeah, brushing the complexities under the rug with shorthand leads to, well ... the idea that you can get thrust without kicking something in the other direction.
It is not false, because I made clear what mass I am talking about. Dogmatic nit-picking may satisfy your wish to feel superior to the great unwashed masses, but it is not particularly helpful.
I am aware of Okun's campaign to eliminate the term "realtivistic mass". I am not sure I am 100% on board with this, because the concept of "relativistic mass" is intuitive and practically useful (used by engineers working on particle accelerators, for ex.).
For instance, if a quantity of photons is put into reflective mirror box, when you put it into the scale, the measured weight will increase by E/c^2 (both before and after the photons are absorbed).
That terminology is outdated. Today, mass generally means invariant mass, corresponding to rest mass in case of 'massive' particles. The invariant mass of photons is 0, hence it's 'massless'.
The gamma factors hidden by the relativistic mass definition of course still occur, but they get folded into the 4-vector quantities instead, which happens naturally if you use proper time for your derivatives.
This way, we avoid having to introduce things like transverse and longitudinal mass, which was the explanation within the variable-mass picture as to why the spatial part of 4-acceleration generally isn't parallel to 3-acceleration.
The photon paths bend due to the curvature of space-time caused by gravity. This does not mean that they have non-zero mass. iirc, it has been proved that photons have non-zero _momentum_ but I am not sure about mass.
As I understand it, the relevant thing for gravity is energy. Mass is a particular form of energy, which photons do not have. It's energy that causes space to bend, but in most cases energy-in-the-form-of-mass completely dominates.
Scott Manly did an overview of potential ways of propulsion in space including some very wacky setups. The most promising ones seem to be using nuclear or fusion reactions to accelerate tiny amounts of mass to very high speeds. The engineering issues are substantial of course. But definitely goes a bit beyond the limitations of chemical processes (i.e. burning fuels). https://www.youtube.com/watch?v=QEZv_OXA_NI
I'd say beamed energy propulsion is more promising for high speeds. That's because the main problem becomes keeping the vehicle from destroying itself with waste heat, and beamed propulsion keeps the waste heat back at the beam souce. You can even actively cool an object with laser light (via anti-Stokes scattering and the like.)
In the end it's all about a very simple equation: E(kin) = ½mv² so it's all about exhaust velocity. The closer you can get that to its theoretical maximum (e.g. c), the less reaction mass is required.
But the question still remains: is there a practical non-Newtonian way? We already know of rather impractical ones, such has deforming spacetime.
I've wondered if it would be possible to build an efficient rocket engine that accelerates its reaction mass so hard that it gains an appreciable amount of relativistic mass? The fundamental problem with interstellar travel is the amount of reaction mass you need to make the trip in a reasonable amount of time. The rocket equation is a harsh mistress. If you were accelerating each gram of propellant so hard that it gained a metric ton of relativistic mass you could bypass it, assuming you had some sort of lightweight yet near limitless power source.
I mean, sure, ultimately mass==energy. It broadens your options but doesn't change the problem that you have to take it with you, it gets used up, and the more you take the more you have to use.
The difference is quantitative, but multiple magnitudes big: The ratio of impulse to mass is given by the exhaust velocity, and velocities for massive propellants barely reach 0.01% of the speed of light. Thus the thrust can be more than 10 000 times higher for the same propellant mass.
On the flip side, the exhaust velocity also determines the ratio of (kinetic) energy to impulse, so energy beam propulsion needs over 10 000 times more stored energy.
Energy beam propulsion is only viable if you have compact high-capacity energy storage available (antimatter?).
The thing is we know how to harvest solar energy while in space. Harvesting matter has been proposed with ram scoops but contrary to solar cells, these are just theoretical propositions.
Being able to use energy directly instead of propulsion mass would make a lot of travels more sustainable and with potentially lighter vehicles.
Neat idea. How much energy do you lose (assuming a vacuum)?
You could imagine some kind of dish that catches the energy, but that's the extent of my knowledge -- I don't know how you'd convert it back to propulsion.
Not entirely the right question. Usually in spaceflight you are overwhelmingly concerned with efficiency, since you have to carry your fuel with you. But with laser-boosted light sails, you leave your engines at home. At reasonable distances, (100+ AU) most of the beam is wasted, but the spacecraft doesn't care about that.
Your main losses will be just from inverse-square law. Even if your ship is reflecting a laser off its rear end, you can only focus the beam so much; at some point the spot size will be bigger than the ship, and you'll be in the inverse-square regime (same surface area, 1/r^2 energy within it).
That doesn't seem fundamental, though. You could have a series of relays that catch the beam, convert it to energy, and re-beam it out.
But I'm sure the efficiency would be awful, and if you have a chain of N of these things, now you're dropping off exponentially with N. And N is linear in distance. Hm... this isn't sounding like such a great workaround anymore.
Not to mention that the incoming beam would be shoving your relay forward. Does the outgoing beam push it backwards? I don't know how that works. (Even if it does, you'd be shoved forward proportionally to the energy loss.)
Bleagh. You'd be better off sending out a series of energy pellets well in advance that a traveling ship would scoop up along the way. That must be what Pac-Man was all about...!
> That doesn't seem fundamental, though. You could have a series of relays that catch the beam, convert it to energy, and re-beam it out.
That's a lot of hardware to send out, though. With a dedicated relay you could afford very large collecting surfaces, to compensate a bit for the conversion/retransmission efficiency loss, but there's a bigger problem: you can't just put a chain of relays on a line inside a planetary system. You have to put them in orbit of the Sun (even if by proxy of an orbit around a planet/moon). This means your initial line of relays will quickly drift out of alignment, making the path through them much longer than beaming straight at a ship that's transferring between planets or out of the system. You'd have to put rings of relays at various heights above the sun to guarantee a reasonably short path, and that would take a lot of relays. And work only for a single plane - if you want relayed power at arbitrary plane, you'd have to build shells of relays - so the amount of satellites you need to deploy just squared.
You don't even need a dish to catch the energy; just use a mirror that reflects the laser. You will lose energy to particles in the laser beam, and possibly also due to redshift.
Wouldn't you push your satellite in the opposite direction? Not saying that it isn't a good idea. The satellite might be in orbit, so it could use gravity to negate that.
Note that in contemporary usage, the term 'mass' without qualifier is normally understood as a reference to invariant mass (aka rest mass). Photons have energy, but no mass. They can be used for propulsion due to having nonzero momentum.
While the 'working mass' in this case is massless, nevertheless the spacecraft still loses its own actual mass as it accelerates, which is something you might not expect from something labeled as "thrust without reaction mass".
True. After sending out a photon of energy E, a spaceraft of mass m will have its (invariant!) mass reduced by a factor of sqrt(1 - 2E/mc²).
In terms of energies, this is all rather trivial conceptionally: Rest energy (aka 'mass') gets converted into the kinetic energies of the spacecraft and the photon.
I mean I would just load couple of metric tons of antimatter in the ship and then just use up picked up interstellar space mass (1 atom per cubic meter is not much, but you will be moving trough shitload of them) along the way to annihilate. the faster you move the faster you will go up to a couple of limits. BUT deceleration may be a bit of a problem.
In 2003 a scientist published a paper about cyclic changes in the shape of a quasi-rigid body on a curved manifold. He shown that it can lead to net translation and/or rotation of the body in the manifold, so in simple words "swimming in space". I guess this is only possible on very large structures.
https://en.wikipedia.org/wiki/Photon_rocket
https://en.wikipedia.org/wiki/Nuclear_photonic_rocket
https://en.wikipedia.org/wiki/Laser_propulsion