9 upvotes, 2 direct replies (showing 2)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
is there either an upper or lower limit to the frequency of an electromagnetic wave?
Comment by miucat17 at 20/07/2022 at 14:56 UTC
20 upvotes, 2 direct replies
Regarding the lower limit, if the frequency approaches zero, this means that you get less and less oscillations of the electromagnetic field in a given time as well as a given space - so much that, in the end, you have no more oscillations at all. The result is simply a flat field that is constant in time, that is, a DC electrical current. So the lower limit of the frequency is zero.
The upper limit is a bit more tricky. Increasing the energy to infinity means that the energy density of the wave, assuming constant intensity, also goes to infinity. So we have ever-rising amounts of energy in an arbitrarily small space. If you look at this quantum-mechanically, this is the same thing as saying that the photons making up the wave have arbitrarily high energy. At some point, this will get to the point where quantum effects become relevant on the macroscopic level (because a single photon has a macroscopic amount of energy). It may be that at this point, the concept of waves stops to make sense, because the quantized electromagnetic field will behave differently (i.e. non-linearly) in this regime. This is speculation, however, AFAIK we do not know for sure what happens to quantum electrodynamics at very high energies.
Comment by Baloroth at 20/07/2022 at 15:38 UTC
5 upvotes, 0 direct replies
Not in theory, but there are practical limitations. Since the wavelength of a wave is inversely proportional to the frequency (i.e. low frequency equals large wavelength), you get a lower bound from the requirement that your wave fit inside the observable universe (due to causality: anything larger than the universe would be causally disconnected, so you'd need to have two parts of the wave that were produced by different unrelated events, so it wouldn't be a single wave).
At higher frequencies, the limit comes from the fact that energy is proportional to frequency: at very high frequencies, the photon has enough energy to interact with other photons (such as the cosmic microwave background) and produce electron-positron pairs, or possibly even a black hole. There could also be other quantum effects at very high energies, physics in that area is still poorly understood.