Comment by Pyxl101 | Hacker News Reader

Comment by Pyxl101 | Hacker News Reader

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Why is it impossible? Is it the amount of thrust that's generated that's impossible, or the mere concept of generating thrust from electromagnetic waves?

The reason that I'm asking is because light has momentum. If I shine a beam of light out the back of my spacecraft, then I will accelerate in the opposite direction. Not a lot, just a little.

Was EmDrive impossible because it was claimed to generate way more thrust than could be explained from the energy levels involved? (Edit: It looks like an additional reason it's impossible is because the EmDrive was claimed not to emit radiation, and rather keep it bouncing around in the cavity. Thanks for the replies and explanations!)

Imagine that I attach a 1 megawatt laser in the 630 nm spectrum to my spacecraft. From some rough calculations based on (1), it looks like the laser will generate about 0.003 newtons of thrust (equal to the weight of 0.2 pounds on Earth), which is very little thrust, and impractical for accelerating a spacecraft over regular time scales.

(1) Momentum of a laser beam: http://umdberg.pbworks.com/w/page/50455623/Momentum%20of%20a...

Any reactionless drive better than a photon drive is probably a perpetual motion machine.

https://arxiv.org/abs/1506.00494

Again, this is incorrect interpretation of the cited paper.

The EmDrive claimed to violate conservation of momentum. To extend your analogy, instead of shining the light out the back of your spacecraft, you shined it inside the spacecraft at the back wall. It bounced around and came out as net positive thrust. Hence the extreme skepticism.

Ok, if you shine it on the back wall one might expect the radiation pressure balances out and there is zero net force.

What happens if you used a waveguide to turn it around 180 degrees to the front wall? Photons have no mass, so turning it in a waveguide would result in zero force on the waveguide right?

Photons have momentum and energy.

Right, but if you bend their trajectory, does the device doing the bending experience any net force?

Edit: just learned light cannot be bent by electric or magnetic fields, I must have been thinking of radiation particles.

You can absorb light and emit new photons, but the only way to bend their trajectory is bending spacetime with gravity. Needless to say, the EMDrive doesn’t contain a black hole.

It will exert a force. I don’t know how waveguides work, but momentum is conserved (and photons have it) so it must exert a force.

What you're talking about is a photon drive, which still obeys conservation of momentum. The EMDrive was (claimed to be) reactionless, and a genuine reactionless drive would shatter physics at the foundations.

In addition to the other replies, a working EMDrive would imply that the results of physics calculations would depend on your choice of coordinate system, because of https://en.wikipedia.org/wiki/Noether%27s_theorem, which would be absurd.

As I understand it yes. Momentum is always meant to be conserved. Light does have momentum, but since energy has mass the loss of energy to procuce the light allows it to work out. It's also a very small amount of thrust even relative to the EM drive.

It's the amount of supposed thrust in addition to the violation of conservation of momentum, many orders of magnitude above pure electromagnetic propulsion. Obviously, physicists know that photons carry momentum.

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