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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: 0) by Anonymous Coward on Friday June 17 2016, @05:35AM

    by Anonymous Coward on Friday June 17 2016, @05:35AM (#361421)

    > 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.

    This analogy says exactly the opposite of what is intended. Water waves transport energy (and probably momentum, I am not sure); flat water, which is equal to two waves superposed in opposite phase, does not. It unintentionally elucidates the paper because the author is explicitly saying that photons are *not* like water waves, in that they can carry momentum while being superposed in this way - from the paper: "Thus, our explanation complies with conservation of momentum but departs from the current consent about photons by regarding photons as indivisible and indestructible basic building blocks of nature."

  • (Score: 0) by Anonymous Coward on Friday June 17 2016, @07:50AM

    by Anonymous Coward on Friday June 17 2016, @07:50AM (#361449)
    Photons are not "indestructible". Far from it. Photons are created and destroyed all the time in interactions with matter. An excited electron in an atom creates a photon when it falls from a higher energy level to a lower one. There is also the photoelectric effect, where a photon hits an electron, destroying it in the process, but passing on its momentum. A high-energy photon can interact with a heavy atomic nucleus and materialise into an electron-positron pair. Similarly, a positron and an electron can collide and produce two high-energy photons.
    • (Score: 2) by HiThere on Friday June 17 2016, @07:04PM

      by HiThere (866) Subscriber Badge on Friday June 17 2016, @07:04PM (#361737) Journal

      Not clear. IIUC, some models say that photons are absorbed rather than destroyed by energy absorption, and thus when they are emitted, they are actually being re-emitted from a previously absorption.

      So creation and destruction may not happen. This probably depends on exactly what photons are. If they are just packets of energy, then it works fine, but photons also have spin, so I'm not sure that it does (or doesn't) work.

      *IF* photons are just packets of energy, or even essentially packets of energy, then they can neither be created nor destroyed, only absorbed or emitted. But polarization makes this messier, so perhaps you can disassemble a photon into energy (absorbed into one process) and spin (absorbed into another). If so, I've never heard of this being done, but I'm not a specialist in this area...or even in physics.

      Still, my first guess is that photons are neither created nor destroyed.

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