4 ms·

I believe that the dp=dE/c requirement for thrust isn't valid with static fields: simply consider an electromagnet a large distance D away from a permanent ma

I believe that the dp=dE/c requirement for thrust isn't valid with static fields: simply consider an electromagnet a large distance D away from a permanent magnet. For a very small energy cost (P<<F * c), you can generate a magnetic field to propel yourself forward. The opposing magnet won't move for D/c seconds, so you're transporting momentum at a much lower cost.* , which I believe is what lukev refers as "other logical possibilities" -- the car is another example of this. Of course, in free space magnetic field decays as 1/D^3, so eventually you will be better off spending F * c power to generate a force F. I do not disagree with "The device must emit something" though, since obviously momentum has to be carried by some particle, although I'm not a physicist to attest the specifics precisely. * - I believe in this case the energy cost is so low because you're "storing" energy in the field, energy you're spending acts more like a trigger. As a thought experiment consider a stream of photons carrying a large amount of power P through a cross section. Now suppose your "device" is a mirror aligned parallel to the stream. Now for an arbitrarily small energy cost, you can align the mirror to the stream and cause the photons to produce a force P/c, "triggering" the system to relocate energy.

gus_massa 12y ago
> For a very small energy cost (P<<F c), you can generate a magnetic field to propel yourself forward. The opposing magnet won't move for D/c seconds, so you're transporting momentum at a much lower cost.* The mechanical moment, i.e. the moment of the object you can see (for example magnets, electrons, ...), is not conserved. The total moment is conserved, i.e. the moment of the objects and the moment of the fields, in this case the electromagnetic field. The electromagnetic field is quantized, and the particles are the photons. So in this case the photons travel from the electromagnet to the static magnet, and they carry away some moment and energy, so the moment is conserved locally. It's important to consider the moment of the fields, because some of the photons don't go to the other magnet, but go to other directions and are never absorbed, the thecnical name is "radiation". If you don't consider it, then the system will lost some moment and some energy. > As a thought experiment consider a stream of photons carrying a large amount of power P through a cross section. Now suppose your "device" is a mirror aligned parallel to the stream. This is a solar sail. The solar sail is a real device. But I think that this experiment is not trying to build a complicated version of a solar sail. To test a solar sail, you should put the device in a strong light. I suppose that the test chamber is in the dark.

gus_massa 12y ago

> For a very small energy cost (P<<F c), you can generate a magnetic field to propel yourself forward. The opposing magnet won't move for D/c seconds, so you're transporting momentum at a much lower cost.* The mechanical moment, i.e. the moment of the object you can see (for example magnets, electrons, ...), is not conserved. The total moment is conserved, i.e. the moment of the objects and the moment of the fields, in this case the electromagnetic field. The electromagnetic field is quantized, and the particles are the photons. So in this case the photons travel from the electromagnet to the static magnet, and they carry away some moment and energy, so the moment is conserved locally. It's important to consider the moment of the fields, because some of the photons don't go to the other magnet, but go to other directions and are never absorbed, the thecnical name is "radiation". If you don't consider it, then the system will lost some moment and some energy. > As a thought experiment consider a stream of photons carrying a large amount of power P through a cross section. Now suppose your "device" is a mirror aligned parallel to the stream. This is a solar sail. The solar sail is a real device. But I think that this experiment is not trying to build a complicated version of a solar sail. To test a solar sail, you should put the device in a strong light. I suppose that the test chamber is in the dark.