7 ms·

There seems to be a meme around the EmDrive where "it works" and "conservation of momentum is upheld" are mutually exclusive. Couldn't it be the case that there

by gfodor 10y ago

There seems to be a meme around the EmDrive where "it works" and "conservation of momentum is upheld" are mutually exclusive. Couldn't it be the case that there is some local phenomenon happening that we do not understand that still manages to maintain conservation of momentum? Isn't assuming that momentum conservation is violated a conjecture on the underlying physics of the device, which we don't know?

colechristensen 10y ago
It's only exciting if it violates the conservation of momentum. If it doesn't, it's not useful as a space thuster or a flying car, and whatever is causing the miniscule force is just some leaky edge case that's difficult to measure. It's boring if it doesn't violate momentum conservation.
- nkoren 10y ago
  I dunno. Accidentally discovering a way to, say, shove dark matter around with microwaves and do useful work with it would not necessarily violate momentum conservation, and wouldn't really be that boring. (Not that I think this is what the EM drive is doing, or that I'd bet on it with anything other than very long odds, but I'm glad someone is looking into it).
- int_19h 10y ago
  Is it really true said "leaky edge case" is applicable throughout our entire local space? e.g. the solar system? or even the vicinity of Earth?
akiselev 10y ago
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.
  - 10y ago
PaulHoule 10y ago
Not if it works by momentum being transferred through the power cable.
hinkley 10y ago
In high school my physics teacher taught us about induction and eddy currents. My group got to build a variation on a Gauss gun using a circular projectile that sat over a magnetic coil which an alternating current passed through. With the right frequency and voltage you could launch the ring across the room. My armchair crackpot theory is that's not what's going on here, but some other similar principle is at play. Some other physical law is behaving in a way similar to eddy currents, and the force comes from the field working against itself at a small delta-t
lj3 10y ago
> Couldn't it be the case that there is some local phenomenon happening that we do not understand that still manages to maintain conservation of momentum? Ever hear of the Luminiferous Aether? From what I can tell, it could explain this phenomenon. Most people consider it an outdated crackpot theory largely because it's at odds with the entirety of quantum theory and the theory of relativity. But, this is the sort of thing that makes you go back to square one and re-consider everything you thought you knew.
- dandelany 10y ago
  L.A. isn't only problematic because it's inconsistent with theory, but also because it's inconsistent with experimental evidence. Michelson & Morley debunked it pretty thoroughly without having to invoke quantum or relativity IMHO.
colordrops 10y ago
Coming from complete ignorance here, but wouldn't conservation of momentum be upheld if there is some medium that the emdrive is reacting against? For instance, a propeller driven motorboat doesn't need propellant to move, as it displaces water. Could there be some new force at work displacing (regular or dark) matter in the vicinity?
api 10y ago
What if it's moving dark matter or some other kind of unknown substrate?

by gfodor 10y ago

colechristensen 10y ago

It's only exciting if it violates the conservation of momentum. If it doesn't, it's not useful as a space thuster or a flying car, and whatever is causing the miniscule force is just some leaky edge case that's difficult to measure. It's boring if it doesn't violate momentum conservation.

nkoren 10y ago
I dunno. Accidentally discovering a way to, say, shove dark matter around with microwaves and do useful work with it would not necessarily violate momentum conservation, and wouldn't really be that boring. (Not that I think this is what the EM drive is doing, or that I'd bet on it with anything other than very long odds, but I'm glad someone is looking into it).
int_19h 10y ago
Is it really true said "leaky edge case" is applicable throughout our entire local space? e.g. the solar system? or even the vicinity of Earth?

akiselev 10y ago

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.
- 10y ago

PaulHoule 10y ago

Not if it works by momentum being transferred through the power cable.

hinkley 10y ago

In high school my physics teacher taught us about induction and eddy currents. My group got to build a variation on a Gauss gun using a circular projectile that sat over a magnetic coil which an alternating current passed through. With the right frequency and voltage you could launch the ring across the room. My armchair crackpot theory is that's not what's going on here, but some other similar principle is at play. Some other physical law is behaving in a way similar to eddy currents, and the force comes from the field working against itself at a small delta-t

lj3 10y ago

> Couldn't it be the case that there is some local phenomenon happening that we do not understand that still manages to maintain conservation of momentum? Ever hear of the Luminiferous Aether? From what I can tell, it could explain this phenomenon. Most people consider it an outdated crackpot theory largely because it's at odds with the entirety of quantum theory and the theory of relativity. But, this is the sort of thing that makes you go back to square one and re-consider everything you thought you knew.

dandelany 10y ago
L.A. isn't only problematic because it's inconsistent with theory, but also because it's inconsistent with experimental evidence. Michelson & Morley debunked it pretty thoroughly without having to invoke quantum or relativity IMHO.

colordrops 10y ago

Coming from complete ignorance here, but wouldn't conservation of momentum be upheld if there is some medium that the emdrive is reacting against? For instance, a propeller driven motorboat doesn't need propellant to move, as it displaces water. Could there be some new force at work displacing (regular or dark) matter in the vicinity?

api 10y ago

What if it's moving dark matter or some other kind of unknown substrate?