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posted by martyb on Friday June 17 2016, @12:39AM   Printer-friendly
from the invisible-light dept.

International Business Times writes:

A new peer-reviewed paper (open, DOI: 10.1063/1.4953807) on the EmDrive from Finland states that the controversial electromagnetic space propulsion technology does work due to microwaves fed into the device converting into photons that leak out of the closed cavity, producing an exhaust.

So how could something come out that you can't detect? Well, the photons bounce back and forth inside the metal cavity, and some of them end up going together in the same direction with the same speed, but they are 180 degrees out of phase. Invariably, when travelling together in this out-of-phase configuration, they cancel each other's electromagnetic field out completely.

That's the same as water waves travelling together so that the crest of one wave is exactly at the trough of the other and cancelling each other out. The water does not go away, it's still there, in the same way the pairs of photons are still there and carrying momentum even though you can't see them as light.

If you don't have electromagnetic properties on the waves as they have cancelled each other out, then they don't reflect from the cavity walls anymore. Instead they leak out of the cavity. So we have an exhaust – the photons are leaking out pair-wise.


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  • (Score: 3, Funny) by Anonymous Coward on Friday June 17 2016, @12:49AM

    by Anonymous Coward on Friday June 17 2016, @12:49AM (#361342)

    Space drive relies on conservation of momentum. Your space thingamajiggi shoots things backward, it moves forward, that way conservation of momentum is maintained.

    So, this "emDrive" thing, somehow push the craft forward without shooting stuff back, so people are saying that's bullshit. Now this Fin dude is saying it does shoot things back, hence its forward propulsion.

    Ok, carry on.

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  • (Score: 0) by Anonymous Coward on Friday June 17 2016, @01:13AM

    by Anonymous Coward on Friday June 17 2016, @01:13AM (#361348)

    EmDrive is legit.

    • (Score: 0) by Anonymous Coward on Friday June 17 2016, @01:23AM

      by Anonymous Coward on Friday June 17 2016, @01:23AM (#361353)

      That probably works better for liberal arts majors.

  • (Score: 3, Interesting) by Immerman on Friday June 17 2016, @01:50AM

    by Immerman (3985) on Friday June 17 2016, @01:50AM (#361359)

    Except his explanation doesn't hold water - if it were generating thrust on par with a photon rocket nobody would care about it. Photon rockets are well understood, and too weak to be useful for much. What makes the EmDrive interesting is that it seemingly has a thrust-to-power ratio orders of magnitude greater than is possible with a photon rocket.

    • (Score: 0) by Anonymous Coward on Friday June 17 2016, @02:15AM

      by Anonymous Coward on Friday June 17 2016, @02:15AM (#361367)

      It's a resonant photon rocket.

      • (Score: 3, Interesting) by Immerman on Friday June 17 2016, @04:15AM

        by Immerman (3985) on Friday June 17 2016, @04:15AM (#361403)

        That wouldn't change anything. Every erg of energy generates X photons with Y momentum. You can do all the fancy bouncing and resonating you want, but so long as conservation of momentum holds, when the photons finally exit the device they will depart with Y momentum, and the device will gain Y momentum in the opposite direction.

        • (Score: 0) by Anonymous Coward on Friday June 17 2016, @07:59AM

          by Anonymous Coward on Friday June 17 2016, @07:59AM (#361452)

          Immerman, you are blowing everyone's "liberal arts" paradigm with your physics and logic and sciency stuff. Please stop.

          • (Score: 2) by Immerman on Friday June 17 2016, @03:03PM

            by Immerman (3985) on Friday June 17 2016, @03:03PM (#361585)

            Hey, if you can't take the math, get out of the science discussion :-D

    • (Score: 1) by Prune on Saturday June 18 2016, @02:16AM

      by Prune (4334) on Saturday June 18 2016, @02:16AM (#361963)

      Wow! Brilliant! You sure showed all those physicist peer reviewers of this paper how stupid they are to not have found such a basic flaw and rejected the paper! Ah, it's good to know that Soylent's own Immerman is here to vet the work of these ignorant scientists — I can now sleep at night!

      • (Score: 2) by Immerman on Saturday June 18 2016, @05:36PM

        by Immerman (3985) on Saturday June 18 2016, @05:36PM (#362131)

        Hey, when has intelligence ever been a defense against your own stupidity?

        Fact: Shawyers initial claims were that he generated 0.016N with 850W, or about 20uN/W
        Fact: Nasa's tests found about 1/1000th the thrust, about 20nN/W
        Fact: the theoretical upper limit for a perfectly collimated photon drive is about 2nN/W

        So, even if this thing is in fact emitting photons in some new and interesting, virtually undetectable manner, which just happen to be perfectly collimated... it's still 10 times too weak to explain the even the weakest thrust we've measured. Maybe it actually does produce an exhaust in this manner, and the idea is certainly interesting, worthy of discussion, and may have serious implications both in other fields and in the practical considerations for EmDrive (if the EmDrive is proven to work).

        It's just not a candidate to explain the thrust that's been measured.