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It's a little more complicated than conservation of momentum, which is just a subset of conservation of energy useful for intuition about the non quantum world.

It's a little more complicated than conservation of momentum, which is just a subset of conservation of energy useful for intuition about the non quantum world. If you're converting energy into massive particles or photons for propulsion you can still have a "massless" drive while maintaining conservation of momentum because what you're really conserving is the total energy of the system. Research has previously shown that it is possible to create particles but it requires ungodly amounts of energy and a specially designed apparatus that separates the resulting particle and antiparticle using quantum effects before they can annihilate each other. There is even some research into using a vibrating "quantum mirror" that forces virtual photons to become real using only energy from quantum fluctuations but that's even more complicated and difficult to engineer. Either the EM drive has a unique combination of features that allows efficient creation and separation of particles using electricity, some other unknown quantum effects are present that can convert energy to momentum without an opposite reaction on another particle/field, or there is still a hidden error. Given the scientific knowledge we have today, it's extremely likely to be the latter but that doesn't preclude the possibility that we have discovered something novel.

gus_massa 10y ago
If you use massless particles like a photon, the theoretical maximum trust/energy ratio is 0.0033 mN/KW. The problem is that they say they got x4000 the theoretical maximum. If you use a particle with mass, the trust/energy ratio is much lower. You have to split some of the energy to create the particles and convert a smaller part to momentum (you must multiply by c or c^2 to get the right units). So creating particles with mass is worse, not better. If you push a bunch of particles that you have stored, you can get much better result, but it's no more a fuelless drive.
- deleted 10y ago
  [deleted]
tprice7 10y ago
I had never realized that conservation of momentum is just a special case of conservation of energy, thank you for sharing this interesting observation! (I see now that, since the change in energy when changing the frame of reference is determined by the dot product of the momentum with the velocity of the new frame of reference, conservation of momentum follows from conservation of energy in _all_ frames of reference)
cynicalkane 10y ago
Really annoying to see false expertise pop up again and again. > conservation of momentum, which is just a subset of conservation of energy This is false. Momentum and energy are two different things, even with mass-energy equivalence. In particular, momentum is a vector while energy is a scalar, and their mass-energy equivalent units (impulse vs energy) are also different. Also: pair production doesn't necessarily require some amazing quantum device to keep the pair separate. > There is even some research into using a vibrating "quantum mirror" that forces virtual photons to become real using only energy from quantum fluctuations but that's even more complicated and difficult to engineer. This is true if you consider conservation of energy to be an engineering problem. Under no scientific theory can you pluck energy from the vacuum.
- tprice7 10y ago
  Did you see my response to the parent, where I give a sketch of how conservation of momentum follows directly from conservation of energy in all frames of reference? What is your objection to that argument?
  - cynicalkane 10y ago
    Remove either conservation law and work out the equations for an elastic collision, and you'll see a new degree of freedom pop up. In classical mechanics they are logically independent, and since the space is affine, frames of reference don't help you. Your sketch takes as a premise, something that is actually a consequence of conservation laws: the way quantities change under frame-of-reference changes. You can get both by looking at the least action principle under frame of reference changes and using Noether's theorem, but that's not what your sketch is about.
    - tprice7 10y ago
      Is there some objective sense in which conservation of momentum is more fundamental than conservation of energy in a different reference frame? Since either can be derived from the other, how do you determine which is "actually a consequence" of which? (I ask out of genuine curiosity)
pharke 10y ago
Excuse my almost complete lack of knowledge of physics but when you say it is possible to create matter from energy but you must separate the particle/antiparticle pair is this not similar to what happened after the big bang and did that not result in an asymmetry of particle/antiparticles, at least as far as we can tell? Could it be that there is some other way to either separate the particle pairs or create particles sans antiparticles that does not require a complicated apparatus? Perhaps the EmDrive somehow approximates the conditions of the early universe in some miniscule manner. Honestly, I'm clutching at straws but I can't believe that the only way to create matter from energy is with some complicated device since the existence of the universe seems to say otherwise. As an example, compare our attempts at creating fusion with the 'solutions' that nature has come up with.
- TeMPOraL 10y ago
  The open question abut the universe here is where did all that missing antimatter go? Because we can't find it so far. EmDrive could hint at an answer, but on the other hand it also could not. We need to figure out what's going on with the drive (and whether it really isn't an error) to be sure. > compare our attempts at creating fusion with the 'solutions' that nature has come up with Well, if you just want to have a nature-like fusion, we've figured that out in the 40s and made it work in the 50s ;). https://en.wikipedia.org/wiki/Thermonuclear_weapon#History https://en.wikipedia.org/wiki/Thermonuclear_weapon#History Our attempts at fusion are now focused on getting useful energy out of it without incinerating everything around the reactor.

gus_massa 10y ago

If you use massless particles like a photon, the theoretical maximum trust/energy ratio is 0.0033 mN/KW. The problem is that they say they got x4000 the theoretical maximum. If you use a particle with mass, the trust/energy ratio is much lower. You have to split some of the energy to create the particles and convert a smaller part to momentum (you must multiply by c or c^2 to get the right units). So creating particles with mass is worse, not better. If you push a bunch of particles that you have stored, you can get much better result, but it's no more a fuelless drive.

deleted 10y ago
[deleted]

tprice7 10y ago

I had never realized that conservation of momentum is just a special case of conservation of energy, thank you for sharing this interesting observation! (I see now that, since the change in energy when changing the frame of reference is determined by the dot product of the momentum with the velocity of the new frame of reference, conservation of momentum follows from conservation of energy in _all_ frames of reference)

cynicalkane 10y ago

Really annoying to see false expertise pop up again and again. > conservation of momentum, which is just a subset of conservation of energy This is false. Momentum and energy are two different things, even with mass-energy equivalence. In particular, momentum is a vector while energy is a scalar, and their mass-energy equivalent units (impulse vs energy) are also different. Also: pair production doesn't necessarily require some amazing quantum device to keep the pair separate. > There is even some research into using a vibrating "quantum mirror" that forces virtual photons to become real using only energy from quantum fluctuations but that's even more complicated and difficult to engineer. This is true if you consider conservation of energy to be an engineering problem. Under no scientific theory can you pluck energy from the vacuum.

tprice7 10y ago
Did you see my response to the parent, where I give a sketch of how conservation of momentum follows directly from conservation of energy in all frames of reference? What is your objection to that argument?
- cynicalkane 10y ago
  Remove either conservation law and work out the equations for an elastic collision, and you'll see a new degree of freedom pop up. In classical mechanics they are logically independent, and since the space is affine, frames of reference don't help you. Your sketch takes as a premise, something that is actually a consequence of conservation laws: the way quantities change under frame-of-reference changes. You can get both by looking at the least action principle under frame of reference changes and using Noether's theorem, but that's not what your sketch is about.
  - tprice7 10y ago
    Is there some objective sense in which conservation of momentum is more fundamental than conservation of energy in a different reference frame? Since either can be derived from the other, how do you determine which is "actually a consequence" of which? (I ask out of genuine curiosity)

pharke 10y ago

Excuse my almost complete lack of knowledge of physics but when you say it is possible to create matter from energy but you must separate the particle/antiparticle pair is this not similar to what happened after the big bang and did that not result in an asymmetry of particle/antiparticles, at least as far as we can tell? Could it be that there is some other way to either separate the particle pairs or create particles sans antiparticles that does not require a complicated apparatus? Perhaps the EmDrive somehow approximates the conditions of the early universe in some miniscule manner. Honestly, I'm clutching at straws but I can't believe that the only way to create matter from energy is with some complicated device since the existence of the universe seems to say otherwise. As an example, compare our attempts at creating fusion with the 'solutions' that nature has come up with.

TeMPOraL 10y ago
The open question abut the universe here is where did all that missing antimatter go? Because we can't find it so far. EmDrive could hint at an answer, but on the other hand it also could not. We need to figure out what's going on with the drive (and whether it really isn't an error) to be sure. > compare our attempts at creating fusion with the 'solutions' that nature has come up with Well, if you just want to have a nature-like fusion, we've figured that out in the 40s and made it work in the 50s ;). https://en.wikipedia.org/wiki/Thermonuclear_weapon#History https://en.wikipedia.org/wiki/Thermonuclear_weapon#History Our attempts at fusion are now focused on getting useful energy out of it without incinerating everything around the reactor.