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In the context of EM radio frequency band, we have EM waves all over place which carry information with them.

I'm wondering, what happens when in the air there are EM waves from multiple different sources operating at same frequency and amplitude along with other sources operating at different frequency but in the same standard range (like 2.4ghz and 5ghz) ?

  • In my mind, having lot of different waves caring energy in the air will mix up/destroy the energy that is being carried. Hypothetically speaking, I imagine a wave in the ocean and in the same time a lot of other waves are coming from lot of directions thus mixing up everything...

Sorry for my basic logic, I don't have physics background (wish I could..) but I'm trying to learn...

Regards

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  • $\begingroup$ Possible duplicate on Engineering.SE: electronics.stackexchange.com/q/94388 $\endgroup$
    – Nihar Karve
    Commented Apr 15, 2021 at 11:08
  • $\begingroup$ @NiharKarve: I read it, it explains the different implementations to overcome interferences, however, it doesn't clearly describes how communication occurs (EM waves keep their structure) when the same frequencies are used by multiple EM emitting sources as well as what happens when random frequency waves travel through another wave, even on different frequency. $\endgroup$
    – Alex
    Commented Apr 15, 2021 at 11:29
  • $\begingroup$ Here's a question on Physics.SE too: physics.stackexchange.com/q/430175 $\endgroup$
    – Nihar Karve
    Commented Apr 15, 2021 at 11:30
  • $\begingroup$ While reading this Q, first imagination popped in my mind was intersecting rays of a laser [These are different frequency EM Waves], they just cross each being unaffected, so I suspect similar thing would happen here too. $\endgroup$
    – gobbledy-gook
    Commented Apr 15, 2021 at 11:48

3 Answers 3

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Electromagnetic waves in vacuum and air very accurately are subject to what is called the superposition principle, which in turn derives from the linearity of the equations of electrodynamics. The superposition principle says that

  1. waves that travel in different directions don't disturb each other
  2. waves with different frequencies/wavelengths don't disturb each other
  3. waves with different polarisation directions don't disturb each other

In short, waves don't get disturbed by other waves (and yes, that also includes non-interference of energy), unless their direction, frequency and polarisation are the same. Therefore, when designing radio applications, you can vary any one of these to make sure you are not disturbing someone elses radio.

Usually, EM radio waves are used for transmitting information. This requires that you are not using a single frequency, but actually a frequency band from this to that frequency. Then you will have to make sure that the bands don't overlap with another person's application (at least if polarisation and direction could be the same).

EM waves are different from waves in the ocean because the latter have often high amplitude, which makes them nonlinear, and thus, destroys the superposition principle. Water waves often disturb each other, producing strange effects like solitons. You can recognize when this happens if you see sharp "cuspy" structures moving on the water surface, which also includes the big ocean waves that break close to the beach with foam and all. All this is nonlinear stuff.

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"Each photon then interferes only with itself. Interference between two different photons never occurs." —Paul Dirac, The Principles of Quantum Mechanics, 1930, Chapter 1

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In vacuum em waves do not interact, as Maxwell's equations are linear. In medium like the atmosphere indirect interaction is possible. Effects such as four wave mixing can occur. See https://en.m.wikipedia.org/wiki/Four-wave_mixing.

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