What happens with a photon when it moves ouit of a galaxy into the 'infinity void', where there's nothing but 'darkness'? Does the photon decay in the end when there are no interactions?

  • $\begingroup$ light quantum = photon? $\endgroup$
    – Mike G
    Jul 28 '19 at 4:00
  • $\begingroup$ Edited question. $\endgroup$ Jul 29 '19 at 3:57
  • 1
    $\begingroup$ The photon is more accurately defined as a wave if it doesn't hit anything. The wave expands theoretically forever, though is loses energy both by the inverse square law (frequency) and by the expansion of space (stretches the wavelength). There may be some kind of ultimate plank limit where the photon/wave is no longer detectible, but that would take a long time to get that far. So, ultimately the "photon" becomes undetectable if it travels far enough and if the expansion of space is universal. Does that answer your question? $\endgroup$
    – userLTK
    Jul 29 '19 at 4:57
  • $\begingroup$ Edited title a little $\endgroup$ Jul 29 '19 at 8:20
  • $\begingroup$ @userLTK Thank you. Is there some method to detect and observe this kind of photons? $\endgroup$ Jul 29 '19 at 12:03

In summary "no, why would it?"

If the universe continues to expand, then the photon will experience ongoing cosmological redshift, so it's wavelength (as measured by an observer moving with the general "flow" of galaxies in that part of the cosmos, will get longer, its energy lower, and it will, accordingly become more difficult to detect.

We can detect today photons emitted about 380 000 years after the big bang. At the time they were infrared light, but redshift has stretched them to microwave wavelengths today.

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    $\begingroup$ Although the peak wavelength of the CMB (when it was emitted) was in the near infrared, it had a significant component in the visible spectrum, as you can see in the spectrum diagram here. That answer also shows what it would have looked like (orange). $\endgroup$
    – PM 2Ring
    Jul 30 '19 at 8:33

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