Photon-photon interaction, EM frequency overlap

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  • #1
artis
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Now from physics I read that photons don't interact with one another normally, at higher energies they might through pair production but that is besides this point.
So this means that for example if we have multiple sources of EM radiation like say multiple sub pixels within a screen then each sub pixel gives off it's own light and that light never interferes with the light coming from the very neighboring sub pixel of a different wavelength aka "color" much like two flashlights if put such that their beams cross the beams don't interfere.
So here is my question, if the wavelength aka "color" given off by one sub pixel doesn't interfere with the pixel next to it and so on for all pixels then at which point an image can be formed instead of just having a cross sectional area of repeating random colors?


I know the simple answer that different brightness of each of the RGB colors creates one final resulting color but when I think deeper I cannot understand why.
Could this be because typical sub pixels are very small and if viewed from sufficient distance then since the light given off by them is not coherent but incoherent it spreads in angle so instead of seeing different colors one sees a continuum of light and perceives and image or is the brain somehow also involved here?

Then a follow up question would be , what would change if the sub pixels were made from a source that radiates coherent light aka laser, would then we also perceive an image given the brightness would be kept as before?
 

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  • #2
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So this means that for example if we have multiple sources of EM radiation like say multiple sub pixels within a screen then each sub pixel gives off it's own light and that light never interferes with the light coming from the very neighboring sub pixel of a different wavelength aka "color" much like two flashlights if put such that their beams cross the beams don't interfere.
They do interfere. They do not interact. Interference is not an interaction.
 
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  • #3
sophiecentaur
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if the wavelength aka "color" given off by one sub pixel doesn't interfere with the pixel next to it and so on for all pixels then at which point an image can be formed instead of just having a cross sectional area of repeating random colors?
In order that interference should take place, the sources have to be coherent. The light that comes from two different pixels is totally non-coherent so the 'vector sum' of light arriving from two such sources will, as you describe it, be random colours. Your eye is not capable of separating out the precise wavelengths (it is a very broad-band detector) it will just integrate the power arriving from a small area and 'see' a resultant colour, according to the wavelengths from the sources, and no interference pattern.
To get optical interference, you need to split light from a single source and take it through different paths. It is actually possible to get the outputs from two lasers to be coherent between themselves but that's a very modern bit of technology and pixels are nothing like that.
 
  • #4
artis
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@Dale right my bad, misuse of words.
@sophiecentaur ok, thanks, well that's what I thought. So it is thanks to our inability to discriminate precisely that we can see an image , otherwise we would just see rows of random colored dots?


Could it also be that it has to do with size, because if the pixels were larger then the image would look like viewing an older CRT low resolution mask from very close.


But purely theoretically if I had a transmitter that could transmit all across the EM spectrum at once ,from VLF to RF to optical up to x rays , would it be true then that all these frequencies could travel from the transmitter outwards simultaneously without them destructively affecting one another?
Because from what I know about the so called "radio jamming" it's actually not destroying the original signal or altering it in any way but merely causing a stronger "noise" that floods the receiver , but in theory the original lower strength signal is still there unaffected?
 
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  • #6
Drakkith
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So it is thanks to our inability to discriminate precisely that we can see an image , otherwise we would just see rows of random colored dots?
If the subpixels are smaller than your eyes resolving power, then you can't see them as distinct objects and they appear as a single object whose color is the result of the sum of their individual colors. This has less to do with the light interacting with other light and more to do with image formation and diffraction.
 
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  • #7
sophiecentaur
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, but in theory the original lower strength signal is still there unaffected
Yes - in principle but there are systems for getting some wanted signals that are 'buried' in noise or interference. They are still there, though. It involves fancy filtering and other signal processing algorithms based on detailed knowledge of the signal that you are looking for.
Just blasting the district with high power signals can drive receivers into non-linearity and you can only hear the jamming waveform. Interestingly, it is sometimes possible to attenuate the feed from the antenna and lower the non linear effects. The wanted signal can be made 'quieter' but less degraded.
A few decades ago, I remember hearing jamming signals on the HF bands; nasty buzzing motorboat noises. It was an 'arms race' between the broadcasters and the jammers.
 
  • #8
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@Dale by destructively I meant in such a way that some of the transmitted wavelengths cause destructive interference with other ones, although i guess that doesn't happen between waves of different wavelengths that are incoherent ?
By the way are radio waves considered coherent EM waves like those from a laser in specific transmitters like the parabolic antenna? Or are all transmitters coherent to some degree but the percentage changes depending on the type of transmitter ?

I am still wondering what would be the answer to the question I posed about the theoretical transmitter that could transmit all possible EM spectrum, as much as I know it should be possible (not technically) but theoretically because if photons don't interfere with one another then multiple different frequencies can overlap simultaneously?

And as @sophiecentaur agreed a different frequency with a higher signal strength doesn't necessarily diminish the strength of another frequency with a lower signal strength it only makes the reception of that lower frequency more problematic.
Could we say that even though different wavelength and amplitude EM waves don't interfere in vacuum they do always interfere within a conducting medium like that of a copper wire or antenna, so the interference of the EM waves happens the moment they cause induction as electrons/current do interfere within the same medium ?

thanks.
 
  • #9
sophiecentaur
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Or are all transmitters coherent to some degree but the percentage changes depending on the type of transmitter ?
'Coherent' means having a constant phase relationship. The degree of coherence involves specifying a tolerance. The only sets of transmitters that can be called coherent are those which follow a master frequency source. The old Decca Navigator system used a master and a number of slave transmitters but that is a very special case. High stability independent sources can be installed locally and can produce interference patterns that are fairly well fixed. In the normal run of things, two transmitters with nominally the same frequency will usually beat together (the interference pattern is moving across the country at fairly high speed (and the modulation will also be messing up things by the two programmes beating with each other. The 'mush area' between two unrelated transmitters is not considered to be in a service area.
And as @sophiecentaur agreed a different frequency with a higher signal strength doesn't necessarily diminish the strength of another frequency with a lower signal strength
Will never - in a linear system. Except in rare, extreme events in the non-linear ionosphere and in normal Receiver input stages when over-loaded there is cross modulation and intermodulation. None of that is Field Interaction, though.
I get the impression that you just don't want to be wrong about this. Our radio communications do not assume that the metal parts of a properly engineered system will cause intermodulation. And even when there is intermodulation, the effect is not to do with the waves.
Show me an example that proves otherwise under normal conditions.
 
  • #10
artis
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I get the impression that you just don't want to be wrong about this. Our radio communications do not assume that the metal parts of a properly engineered system will cause intermodulation. And even when there is intermodulation, the effect is not to do with the waves.
Show me an example that proves otherwise under normal conditions.
Well maybe you have misunderstood me , because that is exactly what I was thinking, namely, that EM waves of different wavelength's don't interfere "mid air" but can do that once induction happens within a conductor/antenna and that is to do with electrons and currents.
Would you like such phrasing?
 
  • #11
sophiecentaur
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Non linearity is a well known phenomenon. Google Cross Modulation, Intermodulation, Receiver Design. Nothing magic and nothing to do with free space propagation of EM waves.

fillings in peoples’ teeth cause it too.
 
  • #12
italicus
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[...]

fillings in peoples’ teeth cause it too.
Excuse me , Sophiecentaur : what is the clear meaning of this way of saying ? I am Italian , and want to improve my English:biggrin:
 
  • #13
sophiecentaur
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Excuse me , Sophiecentaur : what is the clear meaning of this way of saying ? I am Italian , and want to improve my English:biggrin:
Sorry! Metal dental fillings (repairs) have been known to make people hear the local MF broadcast stations due to non linearities. At least that is the only explanation of hearing music etc. ( in some cases)
 
  • #14
sophiecentaur
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EM waves of different wavelength's
It's actually best to consider the frequency of the signals because the non linear products occur at combinations of the frequencies. The wavelengths will not be easy to determine for the signals in wires and metal structures and will vary. The non linearity is more of a microscopic effect than a macroscopic one, although the size a a structure can affect the signal strength (Volts) where the non linearity is generated.
 
  • #15
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Sorry! Metal dental fillings (repairs) have been known to make people hear the local MF broadcast stations due to non linearities. At least that is the only explanation of hearing music etc. ( in some cases)
Is this a joke, or what? If I understand well, metal dental repairs are considered a kind of antenna ?
 
  • #16
sophiecentaur
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Is this a joke, or what? If I understand well, metal dental repairs are considered a kind of antenna ?
The body acts as the antenna. The dental filling (a combination of metals) just acts as the detector of tiny currents flowing round the body ( is the general idea).
Note - I don't think this is an everyday occurrence because it relies on something wrong with the amalgam mix, I guess.
PS I found a few hit for my google search. This link suggests a possible mechanism for the effect. Good fun but not particularly good Science, I think.
 
  • #17
italicus
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The body acts as the antenna. The dental filling (a combination of metals) just acts as the detector of tiny currents flowing round the body ( is the general idea).
Cool! So when I have a toothache, I could say the music received by a “detector” was too heavy !:cool:

Note - I don't think this is an everyday occurrence because it relies on something wrong with the amalgam mix, I guess.
PS I found a few hit for my google search. This link suggests a possible mechanism for the effect. Good fun but not particularly good Science, I think.
So do I ! Thanks.
 
  • #18
jartsa
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that is exactly what I was thinking, namely, that EM waves of different wavelength's don't interfere "mid air" but can do that once induction happens within a conductor/antenna and that is to do with electrons and currents.

Let's say we have a speaker with straight frequency response, and we feed square wave shaped voltage pulses into the speaker.

Fourier analysis tells us that we are feeding bunch of continuous sine waves into the speaker.

Then we have one microphone with straight frequency response, and one microphone with not so straight frequency response.

Now we notice that the bad microphone detects a sound when the good microphone detects a silence. I mean between the pulses the good mic produces no voltage, but the bad mic does.

Now the aforementioned silence is a result of destructive interference of all the sine waves.


That was a nice little story : ) Now the same story with EM-waves:


Let's say we have an antenna with straight frequency response, and we feed square wave shaped voltage pulses into the antenna.

Fourier analysis tells us that we are feeding bunch of continuous sine waves into the antenna.

Then we have one antenna with straight frequency response, and one antenna with not so straight frequency response.

Now we notice that the bad antenna detects EM-waves when the good antenna detects an absence of EM-waves. I mean between the pulses the good antenna produces no voltage, but the bad antenna does.

Now the aforementioned absence of EM-waves is a result of destructive interference of all the sine EM-waves.


And now the same story with photons: The two antennas are different kind of observers, that's why they make different observations.

The last story was quite short, because I don't know much quantum mechanics. : )
 
  • #19
sophiecentaur
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Now the aforementioned silence (?) is a result of destructive interference of all the sine waves.
You are using the word 'interference' inappropriately, I'd say. The Fourier transform merely swaps between a time domain description of a signal and the frequency domain description of the same signal; neither is the 'correct one'. In addition to the familiar frequency domain filtering, filtering can be achieved with a time domain filter (temporal) which adds contributions of signals with various delays to enhance or cancel various frequency components. That has some of the properties of interference, if you like but I haven't come across that interpretation of what goes on.
Now we notice that the bad antenna detects EM-waves when the good antenna detects an absence of EM-waves. I mean between the pulses the good antenna produces no voltage, but the bad antenna does.
How would you propose to identify these 'silences' during the maxes and mins of the square wave? Any measurement system would need to be looking at the square wave over a period of time so that implies knowledge of the values of voltage over the whole time in order to know you are at a constant (flat) value of max or min. Drawing a graph on a piece of paper is not sufficient.

Stick with the regular theory before you branch out on your own attempt at an alternative. I'm not being smug or elitist - I'm just telling it like it is.
 
  • #20
dmerrett
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This started out talking photons, then moved to EM Waves. Its simpler to 'pick-one' for any particular problem. Personally, I don't believe photons exist at least until matter gets in the way. Once a wave is present, you can't reduce or destroy its energy with another wave.. but you can re-distribute it spatially to other directions. ( EG send its inverted-wave in original direction, so it splits into new ones)
 
  • #21
sophiecentaur
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I don't believe photons exist at least until matter gets in the way.
That's neither here nor there in the context of Physics. But, if there is evidence that they are not detectable without interaction with matter then it's an assumption which allows one to predict that they'll behave the same way tomorrow.
 

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