Why is there interference if EM waves don't interact?

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Radio waves pass through everywhere without interacting with each other and that enables us to hear different phone calls and radios without disturbance. However, we do hear some noise sometimes because different signals interfere with each other. How are both of the last two statements true (if they are)? Also, why do two light waves produce interference patterns? They're EM waves and they shouldn't interact, right?
 

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  • #2
FactChecker
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Just because they don't interact doesn't mean that they are always easy to sort out. The interference patterns are what happens when there is no attempt to sort the individual waves out, but they have not interacted and they can, in fact, be separated completely with no change in them.
 
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Just because they don't interact doesn't mean that they are always easy to sort out. The interference patterns are what happens when there is no attempt to sort the individual waves out, but they have not interacted and they can, in fact, be separated completely with no change in them.
I see, so the issue is that our radios and phones are not always able to distinguish different waves. And why do we have interference patterns?
 
  • #4
Drakkith
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The different signals don't interfere with each other, they simply add together in the detector. By that I mean that each wave acts on the detector and their combined interaction results in a single detected signal that is the sum of each wave. If you and I were to beat a drum, our individual beatings would add together on the drum head without you and I having to hold hands or something. A microphone placed near the drum would pick up the sound waves generated by the drum head as a single fluctuating signal.

Note that if you look at the electric and magnetic field vectors (the things that are 'waving' in an EM wave) at a particular location in space, such as near the detector, these vectors would alternate back and forth at a rate that is the sum of each wave passing through that region at that time. The field vectors then act on the detector to produce the signal.
 
  • #5
Drakkith
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I see, so the issue is that our radios and phones are not always able to distinguish different waves. And why do we have interference patterns?
The interference pattern is generated by the EM field near the detector fluctuating in a particular manner. As I said in my previous post, two different waves simply add together to affect the field vectors of the EM field. The light and dark spots of an interference pattern are the result of these waves summing together at that location to either cancel each other out or enhance their amplitude.

Note that even though the EM field in a region of space with two or more EM waves passing through it is the sum of each wave, the waves themselves are not disturbed by passing through each other. They don't have their path altered or anything like that. We don't see two EM waves colliding with each other and simply disappearing or rebounding. That's what we mean when we say that they don't interact with each other.
 
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  • #7
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The interference patterns are just the pattern of summations of multiple waves. Suppose one wavefront is sent through two different slits so that the light from each slit reaches a plate behind the slits. The different positions of the slits will cause patterns to be formed on the plate because the waves will always add together differently at different plate positions.
 
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  • #8
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That explains it all, thank you all so much!
 
  • #10
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The first thing is to avoid a semantic error. The word interference does not mean the same in physics as in the context of radiocommunications.

In wave physics, interference is the passage of two or more waves through the same point, giving the point a resulting value by combining the individual values of the concurrent waves.

In radio communications, interference is all that is unwanted by the receiver, in addition to the desired signal. It captures unwanted signals because the receiver operates with a bandwidth, that is to say it captures a frequency and all the minor frequencies and mayored, between the limits fixed by the design.

In the combination of waves taken into account in physics, after the meeting zone each wave exhibits the same catacteristics that it exhibited before the encounter.
 
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