What is the highest possible frequency, shortest wavelength, for an electromagnetic wave in free space, and what limits it? Is the answer different for EM waves in other materials or circumstances? How could waves of this frequency be generated and transmitted, again if that is possible?


5 Answers 5


String theory assumes that lorentz covariance is a perfect symmetry of our world. If that is true, it means a single photon is allowed to have an arbitrary energy, even greater than Planck length.

You need at least two photons that are not parallel to have a rest frame where something like a Planckian black hole might be generated that will absorb them. But single-photon states cannot be bounded in energy like this in a pure vacuum.

If the vacuum is not pure, presumably the ultra-planckian photon will react with background photons creating black holes in the rest frame and being absorbed by it.


It's theorized that the Planck length is the smallest meaningful unit of distance. A wave with that wavelength would have a frequency of $\approx 6.2\cdot 10^{34}\,\text{Hz}$. A gamma ray typically has a frequency of $>10^{19}\,\text{Hz}$. Since the energy of a photon is directly proportional to its frequency, this theoretical upper bound would require vastly more energetic processes than those we presently conceive of. The individual photons involved would each be carrying $41\,\text{joules}$, or $2.56\cdot 10^{20}\,\text{eV}$, of energy.

That's a lot of volts!

  • $\begingroup$ I think you dropped a factor of c somewhere in your calculation. $\endgroup$
    – Dave Tweed
    Commented Oct 30, 2012 at 16:38
  • 2
    $\begingroup$ well, but there you got a nice paradox: since string theory assumes that lorentz covariance is a perfect symmetry of the world, a single photon could have any energy, even greater than Planck energy. You need at least two photons to have a rest frame where a plancking black hole will form $\endgroup$
    – lurscher
    Commented Oct 30, 2012 at 17:43
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    $\begingroup$ The Planck length isn't actually the smallest meaningful unit of distance - that's a widely held misconception. But it is theorized that there is some size below which any object collapses to a black hole, and that size is probably on the order of the Planck length. $\endgroup$
    – David Z
    Commented Oct 30, 2012 at 17:46
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    $\begingroup$ @lurscher: It's not a paradox, because the answer doesn't assume string theory is correct. $\endgroup$
    – user4552
    Commented Sep 5, 2013 at 1:50
  • $\begingroup$ How do you get 6.2e34 hz? It's c/l, not 1/l. $\endgroup$
    – thang
    Commented Apr 8, 2015 at 9:00

The highest measured frequencies of EM waves are Gamma-rays and are typically produced from the decay of atomic nuclei. The most powerful sources of gamma-rays (and usually the sources with the shortest wavelength) are caused by astronomical events. Recently there was a very strong gamma-ray burst from Cygnus-A, . It is estimated that the gamma ray burst was the result of the black hole gobbling up something with three times the mass of the Earth.

There is no theoretical upper limit for the frequency of gamma-rays. To make one bigger than what we've seen so far will require starting with a super-massive black hole and something much larger than the Earth. Not quite reproducible in the laboratory.

  • $\begingroup$ I suspect that there might be an upper limit where the wavelength of the photon approaches the Planck length (1.6162e-35 m). If my math is correct, this single photon would have an energy on the order of 12.3 GJ. $\endgroup$
    – Dave Tweed
    Commented Oct 30, 2012 at 16:25
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    $\begingroup$ You probably either meant Cygnus A, the very distant radio galaxy, or Sagittarius A*, the black hole at the center of our galaxy. $\endgroup$
    – user10851
    Commented Feb 11, 2013 at 23:05
  • $\begingroup$ @ChrisWhite It's probably about this. Also I believe that even if there is no perfect definition of gamma-ray bursts then this event was definitely not one of them. $\endgroup$
    – Kuba
    Commented Sep 5, 2013 at 0:16
  • $\begingroup$ our spectrum is only limited by the sensors we can build. Anything beyond that is not visible to us. $\endgroup$
    – Ruskes
    Commented Oct 10, 2017 at 20:52

The theoretical maximum possible frequency threshold for small wavelength; it get's grainy in quantum scale, for electromagnetic radiation on Earth is the Planck frequency, which is approximately 1.85487×10^43 hertz. Which is the maximum conceivable inverse introduction of a EM wave from a source to it's successor or receiver in the maximum ideal threshold of time (Planck time) across a Planck unit of distance, this borders on 18 Trillion Quettahertz; Quetta being newly founded unit prefix...


At what frequency does the wavelength = the height of the wave amplitude ?

At 7.7646 x 1020 Hz. The associated wavelength and amplitude is 3.861 x 10−13 m. The electron Compton wavelength is 2.426 × 10−12 m, which is 2π times this amplitude.

The wave height = the physical space the waveheight occupy in space, similar to the wavelength. My assumption is that the wave height of the basic electromagnetic wave is fixed.

Correct. Take a look at some pictures of the electromagnetic spectrum, note that the amplitude is the same regardless of frequency, and note that the dimensionality of action h can be expressed as momentum x distance. Also note that the reduced Planck's constant ħ is h divided by 2π.

It's important to note that an electromagnetic wave is an electromagnetic wave. Some people will tell you it's an electric wave and a magnetic wave which generate one another, and therefore no medium is required. That's not true. Check out the Wikipedia electromagnetic wave article and note this:

"Also, E and B far-fields in free space, which as wave solutions depend primarily on these two Maxwell equations, are in-phase with each other. This is guaranteed since the generic wave solution is first order in both space and time, and the curl operator on one side of these equations results in first-order spatial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field".

What people call the electric wave is actually the spatial derivative of the electromagnetic wave, while the magnetic wave is actually the time derivative. They are merely two aspects of the same wave, not two different waves. And as Maxwell said, "light consists of transverse undulations in the same medium that is the cause of electric and magnetic phenomena". When an ocean wave travels through the sea, the sea waves. When a seismic wave travels through the ground, the ground waves. When a gravitational wave travels through space, space waves. See LIGO:

"Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein's mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that 'waves' of distorted space would radiate from the source (like the movement of waves away from a stone thrown into a pond)".

The same is true for an electromagnetic wave. When an electromagnetic wave travels through space, space waves.

from: John Duffield

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    $\begingroup$ Then space is the luminiferous aether? $\endgroup$
    – Darrel Lee
    Commented Feb 20, 2023 at 2:53

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