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  1. This asked: What is the minimum wavelength of electromagnetic radiation?

  2. And also this: What is the maximum possible frequency and wavelength?

The second question is contradictory; maximum frequency implies minimum wavelength.

I am asking the very opposite:

What is the minimum frequency and maximum wavelength of electromagnetic radiation?

The lowest measured/defined seems to be $3 \,\text{Hz}$ (ELF-waves), which means a wavelength $1/3$ of a light-second, $\sim10^8\,\text{m}$.

But this can't be the physical limit for the wavelength. Does such a physical limit for the wavelength exist (similar to how the speed of light is the speed limit)?

Vincent Thacker
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Jokela
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3 Answers3

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There is no theoretical physical limit on the wavelength, though there are some practical limits on the generation of very long wavelengths and their detection.

To generate a long wavelength requires an aerial of roughly one wavelength in size. The accelerated expansion of the universe due to dark energy means the size of the observable universe is tending to a constant, and that will presumably make it hard to generate any wavelengths longer than this size.

As for detection, we tend to measure the change in the electric field associated with an EM wave not its absolute value. As frequencies get lower we will need either increased intensity waves or ever more sensitive equipment. Both of these have practical limits, though I hesitate to speculate what they are.

John Rennie
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4

Adding to what others have said, here's a little proof that there is no maximum wavelength:

Let us assume that there is a maximum photon wavelength.

Observer B is moving away from Source A. Source A emits a photon of maximum wavelength towards Observer B. Due to the Doppler effect, the photon is red-shifted from Observer B's perspective, meaning it is observed to have longer wavelength.

This observed wavelength is now greater than the maximum wavelength. Contradiction.

Therefore there is no maximum photon wavelength, q.e.d.

Conversely, a photon would be blue-shifted if the observer is moving towards the source, by the same logic proving there is no minimum wavelength either.

But realistically there are limits on the frequencies which can be produced or observed. Extremely high energy photons can spontaneously produce a particle/antiparticle pair. If those later annihilate, they produces two photons which therefore have less energy each.

And good luck trying to build a detector which can detect photons with a wavelength greater than the size of the observable universe.

TheMoonRover
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The maximum wavelength of an electromagnetic wave on Earth is

40,075 km.

We need at least a distance of one wavelength for a standing wave (I assume), and the longest distance we have is the equator, the longest circumference around the Earth (40,075 km), which is not exactly spherical.

The corresponding frequency of this wavelength is

7.48 Hz.

I see no reason such a wave can not exist, because we even have a water wave of this wavelength, seen as ebb and flow.

Volker Siegel
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