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The largest frequency range is gamma rays, but does the EM spectrum 'stop' somewhere? Like is there a limit to how large a frequency can get? Or how small frequency can get? Is it one of those things that theoretically nothing is stopping it, but nothing in the universe can produce ways of such a frequency or beyond a certain limit?

Does light from galaxies redshift to the point where the wavelength is just insanely long? Long enough that we can't see them?

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  • $\begingroup$ duplicate: physics.stackexchange.com/q/43063 $\endgroup$
    – Paul
    Commented Aug 18, 2015 at 14:46
  • $\begingroup$ Maybe the longest EM wavelength is equal to the universe's causality horizon? How could the universe produce a wave that can't fit inside the causality horizon? $\endgroup$
    – Cham
    Commented Sep 19, 2019 at 1:59

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I'll start with the second of your questions. Yes, light from very distant galaxies gets redshifted to such long wavelengths that there practically isn't any light to see. The lower limit on frequency is zero. Obviously. Technically one could say there is no signal at $0\,Hz$, but that still put a lower boundary on the frequency. Objects on the edge of our cosmological horizon have their light redshifted almost infinitely by the time it reaches us.

As for upper bounds on frequency, there's two ways to think about this. Strictly speaking, there is no limit on how high a frequency can be. I could say the limit is that wavelength can't go below zero, but that would feel cheap and like cheating. A photon can have an arbitrarily high amount of energy. However, something else to consider is the limits on knowledge of frequency. If the wavelength goes below the Planck Length, we really don't have any measurement equipment that could theoretically ever measure it. Not only that, but the energy of a photon with this wavelength would be at the Planck Scale. At this scale, the Compton wavelength and the Schwarzschild radius of a Black Hole are about equal. That means any photon with this high a frequency has enough energy to spontaneously create a particle that immediately becomes a black hole and swallows the photon. This indicates two things: 1) We really need Quantum Gravity to adequately describe this energy scale and 2) we can be pretty sure that whatever is flying around with this energy is probably not photons.

So a lower limit on frequency is zero and an upper limit is that with a wavelength of one Planck Length. (At least, that's the upper limit until we find a nice theory of Quantum Gravity).

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  • $\begingroup$ Ah okay thanks, follow up: Why does this necessitate quantum gravity? i'm not saying we don't need it, by why does the photon of planck wavelength need quantum gravity? Is that a bit too long to answer here? $\endgroup$
    – Shaurya Bhave
    Commented Aug 18, 2015 at 21:38
  • $\begingroup$ Our current descriptions say a photon with a wavelength of the Planck length can spontaneously generate a particle with size and mass sufficient to be a black hole and, thus distort spacetime and consume the photon. This means that 1) gravity becomes significant at those quantum scales and 2) without some theory describing gravity adequately at quantum scales, we would expect that these tiny black holes would be flying around the universe (which is scary and thankfully not something we observe). So we need a theory that can adequately describe gravity on quantum scales... Quantum Gravity $\endgroup$
    – Jim
    Commented Aug 18, 2015 at 21:55
  • $\begingroup$ The high frequency photon creating a black hole that destroys the photon feels very weird to me. The new black hole should then produce Hawking radiation that rip-off the black hole, so the photon is back there again, and so on... Or the photon creates a wormhole and pass through it. Pretty sick! $\endgroup$
    – Cham
    Commented Sep 19, 2019 at 2:16
  • $\begingroup$ @Cham An interesting thought. The size of the black hole would mean that it would, as you say, evaporate very quickly. But the Hawking radiation wouldn't just be one photon at high energy, it would be many particles at low energy. So, if I might offer speculation, this would look like the photon spontaneously decays into many lower energy photons and particles. If true, this might explain why we don't see many photons at that energy, but it has worrying implications within special relativity.... which probably means I'm wrong $\endgroup$
    – Jim
    Commented Mar 4, 2021 at 13:23

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