: How come all the electromagnetic waves around does not affect eachother or get mixed up?



I was watching this Feynman’s talk where he said to the reporter that “I can see you because you’re in front of me, and person on my left can see person straight to him, on my right”,because light waves are always reflecting and bouncing off, it’s our perception.

That got me to question this about light and then all electromagnetic waves in general.

Apologies if this is a stupid question, just curious.

In: Physics

Photons don’t interact with each other. They interact with matter (actually the electrons on the outside of matter), but not other photons.

A photon is a particle, the flow of photons is a wave.

When two waves intersect, they do not collide, they simply add or subtract from each other for the brief time that they cross.

It took a long time for the great minds to come to this conclusion, see https://global.canon/en/technology/s_labo/light/001/11.html for the history.

They can kinda interact, but only where they overlap. For example, take a slinky and hold one end. Then have a friend take the other and move back until it’s fairly stretched out. Then both of shake it a few times randomly. You’ll see waves travel down the slinky towards one another. Eventually they’ll cross in the middle and interfere temporarily. The highs of your wave will cancel out the lows of his and vice versa, and where the highs overlap they’ll add together. It’ll only happen briefly when they’re on top of one another and then your waves will just keep moving on towards your friend like nothing happened, and his will keep moving towards you.

It’s the same with EM waves. Where they bump into each other they can add together or muffle each other, but then they’ll move on again like nothing happened.

Now, since they’re everywhere sometimes we do measure them where they overlap with other stuff. Usually though this isn’t an issue. For one we can separate different types of waves depending on what they need. Your wifi and TV remote are both EM waves, but they’re very different frequencies. So your computer only looks for WiFi’s frequencies, and your TV only looks for infrared. It doesn’t matter that they overlap since it’s easy to separate only what you want.

And when they do share similar frequencies, usually they’ll pick up all of the waves but only look for specific patterns that mean something to only your device. There might be other patterns coming in, but it’ll just ignore those until it seems one it needs.

They do interact, but they don’t get “mixed up”. This can be seen through things like diffraction (e.g. Young’s double-slit experiment), where the interaction between waves causes constructive and destructive interference, and that’s where you get your fringes from. This characteristic of light and all other EM waves is used in a whole bunch of things – from the way we use lasers to read certain media, to measuring distance or even the warping of space (e.g. LIGO). Indeed, the idea of constructive and destructive interference is what enables things like waveguides and cavities to work!

Why don’t they get mixed up? Well, because one photon doesn’t have an effect on another photon. They can add together (the magnetic and electric field can combine for an instant and interfere), but the total sum of the fields is still recognisable as two separate photons – their individual fields don’t change the direction of each other, and there’s no attractive or repulsive forces that cause any sort of collision.

If you drop two big rocks from opposites side of a pool the ripples will run over each other. They don’t interfere they just cross.

While photons don’t interact with each other, they do follow the law of superposition, meaning their waves add together. If the crest of one photon wave encounters the trough of another photon wave of equal wave length and energy, they will cancel out because -x + x = 0 so the net energy and thus net intensity at that point is 0. If two like troughs or like crests encounter each other the net energy and thus intensity is double. Crests and troughs can also be called positive and negative peaks.

This can be observed in the double slit experiment, which you can look up.

If you shine a laser on a surface, hold it still and look closely you will see a sparkling effect. This is from the superposition. The light loses coherence as it reflects and scatters off the imperfections in the surface. The bright points are when two positive peaks superimpose, or add together. The darkest areas are when they add to 0.

If we illuminated a room with a single wavelength of light, because of the short wavelength of the light relative to our scale and the loss of coherence, the effect of superposition becomes more difficult to see. White light contains all of the wavelengths of visible light and we end up seeing the average of the superimposed waves. Areas where they cancel completely become statistically scarce and too small and fleeting for use to detect with our naked eye.