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Comment by electrograv | Hacker News Reader

Thank you!

Another question on a tangentially related topic: Could you perhaps shed more light on why there so much pessimistic skepticism towards any claims of working reaction-less propulsion drives (violating conservation of momentum), for example?

I understand the experience-based reasons for skepticism (since there have been many claims that don’t replicate), but why do there also seem to be so many physicists claiming with 100% confidence that no such thing could ever work, according to current physics theory?

I’m referring for example to skepticism about things like “EMDrive” from a theoretical perspective (e.g. the posts from physicists here: https://www.reddit.com/r/EmDrive/) as opposed to focusing on experimental failures to replicate.

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I'm not an expert in these things. But my recollection is that the solutions of general relativity that behave like warp drives all violate various energy conditions and hence are only possible in the presence of what physicists politely refer to as "exotic matter". The Alcubierre drive, for example, requires matter with a negative energy density. We're reasonably certain that the class of quantum field theories we use to describe matter (QED, QCD, the Standard Model itself,..) are not compatible with these requirements.

why do there also seem to be so many physicists claiming with 100% confidence that no such thing could ever work

Because it would violate conservation of momentum. Now wait a minute, you might think, haven't we just learned that such conservation laws don't hold in GR?

It's more complicated than that.

An analogy would be doing mechanics in a rotating reference frame. To make this work out, you have to account for things like centrifugal forces, with energy contributions that come 'out of nowhere' - as in, if you go to the non-rotating, inertial frame, they vanish: There's no real physical source providing that energy.

Something similar happens in general relativity, where you can get conservation laws via Noether's second theorem even if your spacetime is not static (or rather stationary, to be pedantic). You will generally get a contribution from the gravitational field, which cannot be localized, but also - in contrast to the previous example - cannot be made to vanish because global inertial frames do not exist in general.

One should perhaps also mention that reactionless motion supposedly is possible in classical physics: It's been claimed that a body can change its position without ejecting any propulsive mass through a series of deformations in an inhomogeneous gravitational field. There's a cute name for this, 'swimming in spacetime'.

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