It’s entirely theoretical, someone’s pet project, I noticed it through a NewScientist article with the dumbest headline and bad choice of illustration, but still!
I suspect this guy is not entirely up to speed on his relativistic calculations and that it’s never even going to go on a test bench (being somewhat more difficult to construct than the EM-drive that NewScientist inexplicably chose to illustrate the article with), but the guy admits he’s out on a limb. And we all would love some breakthrough space tech, wouldn’t we.
So anyone know enough to shoot this guy down authoritatively? Or know where to find some expert opinions on this paper?
Personally I think he’s just missing something obvious and that proper calculations would show perfect symmetry, but I’m not a particle physcicist.
Not sure the problem is with the ‘slight relativistic speeds’ thingy, but how do you change the mass of particles by just changing velocity? I mean, I suppose if you COULD do that you could make a closed loop sci-fi reactionless engine, but that seems kind of a show stopper right there, especially since changing mass would release energy…but then how do you get that energy back later? Maybe one of the physics oriented dopers will wander in, but seems like a version of perpetual motion and violates conservation of energy, at least just based on my quick skim.
How do you move ions back and forth in the direction of travel?
I thought the mass of something increased as you approach the speed of light, and actually attaining light-speed meant it would have infinite mass. So you accelerate the particles to near-light speed, they increase in mass, and then somehow that pushes the ship forward. But I don’t understand how they push the ship - are they bouncing off the accelerator unit?
And where does the energy come from to accelerate the particles?
Mass as distinguished from rest mass is a function of the total energy. You can absolutely (relatively?;)) change the mass by making something go faster. Whether or not the rest of it works, IDK.
Missed the edit window. Replying to the above, it may be that the question of where the energy comes from to accelerate the particles is something he has left to others, in much the same way that someone designing a steam turbine engine doesn’t necessarily need to decide for the engineer whether the thermal energy to transition water to steam is being generated through nuclear power, fossil fuel, or waste heat from a fusion reactor.
ETA: But, again, I don’t know if the physics of it actually work, just addressing this one possible objection as perhaps beyond the scope of the problem he is setting out to solve.
That’s fair enough. How does the energy drive the ship? That is, how do you transfer the energy of motion of the ions into forward motion? It’s apparently not like a rocket, where the propellant pushes the rocket forward. How does it work?
Somehow or other, you pump energy in. Fair enough - how do you get the energy to do work?
This kind of sounds like the juggler on the bridge or the truck full of birds. How could you change the velocity of ions in a way that doesn’t counteract whatever effect you get from the ions changing their mass?
The bellhop keeps it. But only if he doesn’t know what day he is going to be executed.
Don’t particle accelerators add mass to the particles they affect? Does that push the accelerator in any particular direction? I believe they are circular - I wouldn’t think that counts as the particles going back and forth.
It sounded to me as if they were saying that they were getting their acceleration by changing the mass, not just saying that mass changes as a particle accelerates to relativistic speeds. I mean, if they were just tossing the ions out the back, it wouldn’t be a reactionless drive…it would just be a regular old ion propulsion drive that, admittedly, was accelerating the particles a bit faster than most of the current drives and thus getting more thrust, albeit using more energy to do so…or something.
ETA: Admittedly, I only skimmed it on my phone, so maybe I was missing something. That was the objection that passed through what pretends to be my mind at the time, but I might be totally misuderstanding the limited info saying how this is supposed to operate.
Yup, a lot of people think that. But it’s only true if, for some bizarre reason, you define the word “mass” to mean “energy”. In which case this idea still doesn’t work.
If you want a more detailed explanation: In Newtonian mechanics, the momentum of a particle is given by p = mv. In special relativity, however, it turns out that that’s only an approximation for low speeds, and for higher speeds, you need to instead use the formula p = mgammav, where gamma is a unitless value that’s approximately 1 at low speeds, but approaches infinity as v approaches c. Well, early on in the history of relativity, some folks decided that they really wanted to treat the Newtonian formula as correct, and so they re-defined “mass” (sometimes called “relativistic mass”) as mgamma, so the formula could still be p = mv. By this definition, mass increases, but it’s really not a very useful definition: It’s usually not a useful quantity to deal with, and anyway it’s just equal to the energy, so if you do need to talk about “relativistic mass”, you might as well just say “energy”. And even if you absolutely insist on forcing the Newtonian momentum formula to be “correct”, it works a lot better to attach the gamma factor to v, instead of m, creating a quantity called “proper velocity”, or u (which has a number of other applications): u = gammav, and therefore p = m*u.
As Andy L states, this would appear to be in clear violation of conservation of momentum. If you want your ship to go forward something has to go backwards. I’m not good enough with relativity to follow the math, of the original paper, but I notice it does discuss radiation being released. So it may be high energy photons shooting out the back are what is generating the thrust. I also noticed this little tidbit.
So this thing is running off of a 165MW reaction all of which needs to be recaptured except for the 10 watts that is providing momentum. I think they would be better off just shooting a laser out the back.
For those with the time, this is an interesting take on the concept of mass (by someone seeking to make the case that there is no such thing as relativistic mass, only mass, and that the only way to properly define mass in a way that almost makes sense is in terms that include all that relativistic stuff). The lecture is about 50 minutes, and in case you’re worried he’s some quack, the lecture is before The Royal Institution:
Yeah, this one is different than the EM drive, which is what I figure you are talking about. There have been a series of tests on the EM drive, which, I think, the latest ones pretty much found nothing. Though it’s been a while since I heard/looked at anything about it. I think the last video I saw on it was from Scott Manley who has a YouTube video channel that’s mostly about the Kerbal Space program.
Assuming it requires energy input to accelerate the particles, it’s not a claim of perpetual motion.
I am not nearly a good enough physicist to evaluate this idea on its merits, but it doesn’t seem totally impossible to me? A device that takes in energy and results in thrust might be possible?
Sure, conservation of momentum is required, but if you can change the masses of the things you’re pushing with, you could maybe still have conservation of relative momentum, just not Newtonian momentum.
It’s equivalent to a claim either of perpetual motion or that the universe has a preferred reference frame.
If the force of the drive for a given power input varies with velocity, then relativity is wrong and there must be some preferred reference frame, since there must be some velocity at which the drive is most efficient.
But if that’s not true–if the force of the drive is constant with velocity (for a given power), then you can turn the drive into a perpetual motion machine. Because kinetic energy goes up with the square of velocity, whereas the energy input is linear with time, if you wait long enough the KE will eventually exceed the input. You can then use some external device to capture the energy excess.
The problem with the conservation of momentum in this situation is that if you want to device to have momentum in some particular direction, there needs to be something with momentum in the opposite direction.
Energy is still conserved. But I read a few years (unfortunately I cannot recall where) that reactionless motion was possible near the event horizon of a black hole. I think that the dependence on the black hole had to be relative (no pun) but the effect would be near unobservable in a small gravitational filed. But at near light speed?