The Em Drive - Does it violate conservation of momentum?

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S Petar Belic
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Joined: Mon Jun 15, 2015 12:31 pm

The Em Drive - Does it violate conservation of momentum?

Post by S Petar Belic »

For those who don't know what this is, the Em Drive is a seemingly new type of propulsion intended for spacecraft invented by Robert Shawyer - with no reaction mass. So far it seems to have produced very low thrust (microNewtons) while seemingly violating conservation of momentum which has produced a lot of controversy.

The reasons I don't immediately dismiss it as crank science is because in 2012 Northwestern Polytechnical University (China) claimed to verify the results and more recently NASA (last year) claimed to have reproduced this. Having said this, no peer-reviews seem to exist yet. However, a couple of days ago a german group has started sharing results which would seemingly show thrust.

Because it's meant to be 'truly reactionless' I have a little bit of understanding as to how it violates conservation of momentum, however I'd like to hear a 'real' physicist clarify this for me, i.e. in terms a 1st year physics student might understand.
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joe
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Re: The Em Drive - Does it violate conservation of momentum?

Post by joe »

It's always difficult to assess something that has not been formally published. Newspapers and popular magazines like New Scientist (relevant here) do not insist on the same levels of detail and rigour that are expected in scientific journals. As a result, there's a lot of skepticism about this story. There's an entry that looks reasonable on Wikipedia
https://en.wikipedia.org/wiki/EmDrive

Is it possible to produce thrust without reaction mass? Certainly yes: photons have momentum so nett emission of microwaves in one direction would produce thrust in the opposite direction. However, the values quoted "720 mN was reported at 2,500 W of input power" are too high for this explanation. (2.5 kW/c = 8 µN.)

Other explanations? 2.5 kW of electric power in a small device could possibly generate a lot of spurious effects, including asymmetric heating (& air currents), unbalanced magnetic forces. Suspension of Newton's second law would require at very least a formal scientific paper.
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