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I have been reading about the quantum vacuum thruster on Wikipedia and I think I understand the idea of virtual particles being created and destroyed but what I don't understand is how this is different from just shining a light and moving due to momentum from the light in the opposite direction.

Just to make sure I am understanding the concept correctly, here is a summary of what I think is going on:

  • Particles and antiparticles are continuously being created and destroyed in the quantum vacuum.

  • The thruster uses energy to exert a force on these particles.

  • An equal and opposite force pushes the thruster forward.

  • The particles annihilate each other.

If any of this is wrong please correct me.

It seems to me that in the end, only energy in the form of electromagnetic radiation exits the thruster. If momentum is conserved then the momentum imparted on the thruster must be due to the momentum of the electromagnetic radiation. How then, is this different from just creating this electromagnetic radiation in a more traditional way and using its momentum for thrust?

Thanks for the explanation

John Rennie
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fred
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If you shoot light out the backside of your craft then indeed you would move towards the front. But then you'd use up some energy in order to go because the light that leaves would carry energy as well as momentum. The energy of your craft would decrease and thus the mass of your craft would go down. You'd arrive at your destination with a less massive craft than you departed with. But some people might want to move without having to lose mass.

A quantum vacuum plasma thruster (or Q-thruster) is a name given for a variety of devices that claimed to be reactionless drives. A reactionless drive claims to emit no matter or radiation. It could arrive just as massive as it departed, without having to pick up fuel on the way there.

For example, if you have a cavity inside your craft and if you emited light from the top of the inside of the cavity towards the bottom of the inside of the cavity, then at first the top would go up but then when your light hit the bottom the bottom would start to go down. Since the light is always inside the cavity you could claim that no light or energy or momentum gets outside, so it's reactionless.

The drive you imagine where light comes out the bottom, is not a reactionless drive because you propose to emit radiation. And most people are not excited by proposed reactionless drives. So does it work? In some sense you could sit on one end of a box in space with a bunch of super balls and start throwing them to the opposite wall and the outside of the box will start moving. The center of mass doesn't move but since there used to be a pile of superballs on one side the outside can move one way as the center of mass of the superballs moves the other way. So sure, you could expect some very small motion of the container if the energy inside moves itself about. But nothing exciting. What do people see? Here's an example of a report.

An article published by Shawyer in Acta Astronautica summarises the existing tests on the EmDrive. Of seven tests, four produced a measured force in the intended direction and three produced thrust in the opposite direction. Furthermore, in one of the tests, thrust could be produced in either direction by varying the spring constants in the measuring apparatus

https://en.m.wikipedia.org/wiki/RF_resonant_cavity_thruster

Almost anyone that looks at such results thinks the results are noise. And the claims on the face of it look like obvious violations of conservation of momentum. So almost no one pays any attention at all (except in the sense of concern about science outreach and how public perception of science might be affected by phantastical claims).

And keep in mind that terms like "quantum vacuum plasma" are not standard scientific terms.

Timaeus
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  • If the mass does not decrease then how can momentum be conserved? Is conservation of momentum violated or am I misunderstanding the design? – fred Jul 01 '16 at 19:53
  • @fred The Wikipedia page lists the EM drive as an example of a Q-thruster and the very first thing it says about the EM drive is that it would violate conservation of momentum. If you violate conservation of momentum and then use non standard language as a justification then most people will stop listening right then and there. And that's exactly what most physicists did. – Timaeus Jul 01 '16 at 20:08
  • Also, if nothing is emitted, where does the spent energy go? Or is it supposed to be a perpetual motion machine. – fred Jul 01 '16 at 20:22
  • Wikipedia says "the Q-thruster conserves momentum by creating a wake or anisotropic state in the quantum vacuum". This means nothing to me but I guess this explains the momentum problem. – fred Jul 01 '16 at 20:25
  • @fred At first you had energy in your batteries, and vacuum fields inside your cavity. Later, it depends on your device. Later you could have depleted batteries and have energetic nonvacuum fields inside your cavity (if the walls are reflective) but who knows, these devices are not based on standard accepted mainstream peer reviewed science. But if you aren't concerned about violating conservation of momentum, then don't worry about conservation of energy either. – Timaeus Jul 01 '16 at 20:27
  • It really sounds like pseudo science at this point if conservation of energy and momentum are being disregarded. Thanks for making that apparent to me. – fred Jul 01 '16 at 20:30
  • @fred Science is about reproducible results subjected to peer review. Many things posted about these devices are not peer reviewed. And many of the alleged effects are different when attempted to be replicated. That's why I cited the evidence of the force basically being small and in random directions and the direction detected depending on the details of the device used to measure the force. – Timaeus Jul 01 '16 at 20:45
  • @fred It is entirely pseudoscience, and "the Q-thruster conserves momentum by creating a wake or anisotropic state in the quantum vacuum" means nothing to anyone. It's just words strung together for the purpose of defrauding taxpayers. –  Jul 01 '16 at 20:55