Physics question

This is a discussion on Physics question within the A Brief History of Cprogramming.com forums, part of the Community Boards category; This has been on my mind quite a while now (many years, since I was a child in primary school) ...

  1. #1
    Back after 2 years Panopticon's Avatar
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    Physics question

    This has been on my mind quite a while now (many years, since I was a child in primary school) and no one could answer me, but imo its kinda a simple question.

    My question is: according to the wave model for electromagnetic radiation, EMR should display characteristics of waves right?

    If this is true then I ask what will happen if one beam of a set certain frequency is shone at one direction, and another beam of the same frequency, but 180 degrees out of phase, shone at the first beam, from the opposite direction? Assume this occurs in total vacuum and both waves have equal peak amplitudes.

    If its like any mechanical waves, they should superimpose and simply cancel out. But won't this mean the conservation law be broken?
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  2. #2
    Just because ygfperson's Avatar
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    AFAIK, Electromagnetic radiation isn't waves. It isn't particles, either. It's sort of a mix between the two. Which is probably why you haven't gotten a straight answer.

  3. #3
    The Earth is not flat. Clyde's Avatar
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    "If this is true then I ask what will happen if one beam of a set certain frequency is shone at one direction, and another beam of the same frequency, but 180 degrees out of phase, shone at the first beam, from the opposite direction? Assume this occurs in total vacuum and both waves have equal peak amplitudes.

    If its like any mechanical waves, they should superimpose and simply cancel out. But won't this mean the conservation law be broken?"

    Ah but it will only cancel out in parts, in other parts you will get constructive interference. Essentially you will set up an interference pattern, imagine a rectagular pool of water, at either end you drop a stone, where the two waves meet you get interference, constructive in some places, destructive in others. So no net change in energy.

  4. #4
    Toaster Zach L.'s Avatar
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    I think the confusion comes from the fact that once you are considering the wave nature of EM radiation, you cannot cosider it to be a beam any more (since that is a manifestation of its particle-like behavior). Therefore, Clyde's pond analogy is correct since waves cannot be beams.
    The word rap as it applies to music is the result of a peculiar phonological rule which has stripped the word of its initial voiceless velar stop.

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    Back after 2 years Panopticon's Avatar
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    Yeah the pond-rock example explains the thing 3 dimensionally
    i forgot to mention that i'm thinking in 2d terms.

    e.g. if 2 transformers were connected with active to active and neutral to neutral, and having an oscilloscope in series with that circuit, you would be able to monitor the net waveform resulting from the out of phase interactions. in this example of transformer 1 (T1) were set to output 4v and T2 to 2v then the net waveform wud be 2v (cos they will always superimpose destructively). but i know that when this happens, no energy is lost because opposing electromotive force from T1 compensates T2's magnetic field, so in total only 2v is drawn from both Ts, which all adds up.

    Now apply this to electromagnetic waves. i forgot to mention my question involves 2d EMR. Uno how maxwell's original wave model for light looked like? a series of self inducing self propagating electric and magnetic fields. one induces the other and they go on indefinitely at speed c. i was asking if one of those was met with another, 180 degrees out of phase, opposite direction, occurring in vacuum.

    I figured that if it occurred in air or something, it -may- cause the air around it to heat up a lil, so energy is again transformed. but vacuo cannot contain heat, so.. once again im screwed.
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  6. #6
    Registered User major_small's Avatar
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    they just cancel eachother out or supplement eachother... no energy is gained or lost... it's just two waves being in teh same space at the same time...
    Code:
    these waves
        >   <
    ---/\--\/---
    
    become:
         ><
    ------------
    
    then seperate:
        <   >
    ---\/--/\---
    the model is the same, even for EMR waves... if you're looking at them as a wave... you just have to take the electric and magnetic waves as two seperate cases...

    just in case you're wondering - the electricity wave is at a right angle to the magnetic wave...
    Last edited by major_small; 06-08-2003 at 07:32 PM.
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    Back after 2 years Panopticon's Avatar
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    Ohhh so what ur saying is that they pass thru each other, and while passing they cancel out completely but still exist after that.. hmm interesting.
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  8. #8
    Registered User major_small's Avatar
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    exactly... only if they're 100% out of phase... if they're perfectly in phase, the amlitude doubles for the time they are combined, then they seperate and continue... if they are inbetween, you have to do some math to figure it out...

    Code:
    these waves
       >   <
    ---/\--/\---
    
    become:
         ><
         /\
    ----/  \-----
    
    then seperate:
       <   >
    ---/\--/\---
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  9. #9
    The Earth is not flat. Clyde's Avatar
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    "Now apply this to electromagnetic waves. i forgot to mention my question involves 2d EMR. Uno how maxwell's original wave model for light looked like? a series of self inducing self propagating electric and magnetic fields. one induces the other and they go on indefinitely at speed c. i was asking if one of those was met with another, 180 degrees out of phase, opposite direction, occurring in vacuum."

    The problem you have is that you are merely considering photons to be waves, they aren't waves, though they can behave like waves.

    When they do behave like waves you end up with interence patterns.

    If light were a 2 dimensional wave and if you caused two photons to meet that were entirely out of phase or in phase then you would seemingly violate conservation of energy.

    Presumeably it can therefore be inferred that in your scenario photons do not act like 2d dimensional waves. (Infact i'm not sure that light ever acts like a 2d waveform)

    I suspect you will get some form of local interference pattern, and then your two photons will go on their separate ways, possibly with an alteration in direction.

    The bigger question is what happens when you cause photons to overlap:

    Take two lasers point them both towards a lense such that the photons are made to overlap through diffraction. Then you can no longer claim that the photons pass through each other, which offers an easy way out.

    I don't know the answer, but i suspect that if you consider the photons as 3-dimesional wavepackets, and add in the rules of quantum uncertainty you have enough lee-way to give you another interference pattern, which solves the problem.
    Last edited by Clyde; 06-09-2003 at 04:43 AM.

  10. #10
    Back after 2 years Panopticon's Avatar
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    I think I understand now. This is one of the facets of EMR that can only be explained (afaik) by the particle model. Thanks to everyone who replied.
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