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The electromagnetic spectrum is commonly referred to as consisting of; radio-waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays - of increasing frequency from left to right.

But is it possible to get radiation of higher wavelength than radio waves, or lower wavelength than gamma rays - does it even exist? Or could they be produced?

Most interestingly, from the Planck–Einstein relation, $E = hf$, how high of an energy could you get for a very very high frequency radiation?

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  • $\begingroup$ I think you should ask: What is the highest frequency ever measure? And what would it take to go higher? $\endgroup$
    – Bill Alsept
    Commented Sep 19, 2021 at 17:28

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Higher energy gamma and longer wavelength radio?

Keep in mind that the different 'kinds' are merely human labeling conventions for a spectrum that is continuous in the mathematical sense. There is no feature of "radio" that distinguishes it objectively from microwaves. We just pick a boundary on the basis of some technological limitations that apply when we decide the difference and stick labels on.

The reason there aren't labels beyond radio and gamma is that there is no real need to label those bands.

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  • $\begingroup$ dmckee As you know, the wavelength of radio waves refers to the current oscillation in the antenna rod and has nothing to do with the wavelength of the photons emitted from the accelerated electrons. It is a modulated radiation. So the real limit for photons is in the IR range. $\endgroup$
    – HolgerFiedler
    Commented May 8, 2016 at 5:15
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    $\begingroup$ Nonsense. High $n$ Rydberg atom transitions are a fruitful astrophysics source and they are in the microwave range. There is no lower limit on the energy of a photon. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented May 8, 2016 at 5:18
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    $\begingroup$ I know basically nothing about physics, but I had a thought: If an object was traveling near the speed of light and emitted a high energy gamma wave into the direction it's heading towards, could an observer heading in the opposite direction (at any speed) receive such a short wavelength that there appears to be no period between waves at all, owing to the doppler effect? Or I guess, a period possibly shorter than a plank length, making the period meaningless. And if so, would that be the upper bound? $\endgroup$
    – Dragoon
    Commented Jan 6 at 5:46
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In addition to the answer by dmckee and to answer the question how high in energy you could get a photon it might be worth thinking about 'Gamma Ray Astronomy' where the highest energy photons are detected. The record highest photon energy observed is apparently currently 80 TeV, which corresponsds to a wavelength of $1.5 \times10^{-20}m$ wavelength (if I calculated it correctly). This is very short considering the 'size' of the hydrogen nucleus is ~$1.8\times10^{-15}$.....

...but, of course, as dmckee points out there is a continuous spectrum and no high or low energy limit to the energy of a photon.

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    $\begingroup$ There is a high-energy limit to the long-range (cosmologically speaking) propagation of high-energy photons. Above about 400 TeV the cross-section for light-light interactions with the CMB suddenly begins to become significant. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented May 7, 2016 at 23:07
  • $\begingroup$ @dmckee - thanks for comment. Again I learn from physics SE, many thanks $\endgroup$
    – tom
    Commented May 7, 2016 at 23:11
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There is a theoretical limit for how small the waves can be. it is when the waves are as long as the Planck length. But as far as I know there isn't a limit of how long the waves can be.

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  • $\begingroup$ It's been a long time since I've done any calculus, but suppose you have an object traveling near the speed of light which emits a photon in the exact opposite direction of it's heading, owing to Doppler, couldn't a formula be something like wavelength (to a stationary observer) equals the limit of x as velocity approaches the speed of light? $\endgroup$
    – Dragoon
    Commented Jan 6 at 6:04
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After 6 years you can pretty much answer your question on Wikipedia: Ultra-high-energy gamma rays are relevant in astrophysics and very large radio waves can be used to communicate with submarines.

Ultra-high-energy gamma ray

Extremely low frequency

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Wouldn't a wave smaller then the smallest be "in the same theoretical universe of discourse" mathematically negative? Looks like a good explanation for a time traveling Sci-Fi xD

Also, wave may be able to reach a higher speed then it's correlation to length. For example if traveling on a referent that is also traveling. A wave inside another one would be 2 time faster if we can alter it's spatial referent, for which quantum intrication could be the key. Don't forget that all those equations have or are directly correlated to a constant. Those are human made, at least human labelled. Thinking about it in separate questions was a good lead, what would happen over gamma ray speed/frequency, to an electromagnetic signal, imagining we have a transmitter that can achieve that?

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Beyond radio waves are mega-giga-super-long waves as the wavelength approaches infinity; the longer they are, the more they dissolve into nothingness because once half the wavelength $\frac{λ}{2}$ gets more or less bigger than the radius of the entire universe, they cannot really interact with anything. In other words, they practically cease to exist and their energy approaches 0.

Beyond gamma rays are mega-giga-ultra-death rays that are more and more ionizing. Wavelength could be made arbitrarily short as long as it is larger than 0, but good luck gathering amounts of energy to generate such rays. Main difference is that "usual" gamma rays just knock off electron clouds from atoms, break bonds and cause radiolysis of matter, while mega-giga-ultra-death rays instantly turn any state of matter into hot plasma.

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