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What happens when two light beams collide ?

  1. Mar 4, 2008 #1
    Iam very new to this forum. I just had this doubt for the last week.
    pardon me if this is a silly one. the doubt is :

    what happens when two light beams travelling in perpendicular directions
    collide ? collide in the sense i mean what happens at their intersection point ?
    what i have thought is that there would the effect of superposition or something else.
  2. jcsd
  3. Mar 4, 2008 #2
    Uhm, stupid question in return: Why do you anticipate anything at all? Do you have two flashlights at your disposal? Take them and carry out the experiment. Maybe a black hole will emerge from the intersection, who knows. Just kidding - as regards the last sentence. Will you please specify why you'd expect an "effect on superposition"?

    €: Rude me. Welcome to the forums, first off :-)
    Last edited: Mar 4, 2008
  4. Mar 4, 2008 #3

    Andy Resnick

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    The photon-photon scattering cross-section (i.e. the probability that one photon will collide with another and both scatter) is vanishingly small (but not zero).

    Interference effects between two beams can occur if the beams are related to each other somehow: perhaps the originate from the same source, as occurs in an interferometer.

    So, the short answer is, unless you carefully construct the beams, nothing will happen and the beams will continue on their merry way, blissfully unaware of each other.
  5. Mar 4, 2008 #4
    Hey Andy,

    I didn't even know that photons have a scattering c.s. for other photons. What will such a scattering look like, if it occurst at all? Will one of the photons really change trajectory if it's scattered? Or will this have other effects so the scattered photon will not really stay alive?

    Do you have something to read on this?
  6. Mar 4, 2008 #5
    I think superposition would occur...
  7. Mar 4, 2008 #6
    i remember in my childhood physics classes, when waves are generated
    at two opposite ends of a water beaker, we can observe the points at which
    something happens( i dont know what exactly those points are called ? ).
    good example for this is http://www.kleinbottle.com/SpoutingBowls.htm" [Broken] .
    Even I have seen in geographic channel ancient chinese kings used these
    to wash their faces :) (its really true).

    so, will the same happen in case of light ?
    if so what will we see at those points ?
    Last edited by a moderator: May 3, 2017
  8. Mar 4, 2008 #7
    Phanidee, that's nothing else but plain interference. And yes: That also happes to light. You may have heard of the double-slit experiment where to coherent beams, emmitted from one laser or lamp, are shot through the slits, interfere with eachother, forming an interference picture on a screen behind the slits.
  9. Mar 4, 2008 #8


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    Something like this, I think, via virtual electrons and positrons:

    Attached Files:

  10. Mar 4, 2008 #9
    Virtual electrons? Tho I know a bit of Feynman diagrams I've never heard of virtual electrons. What is the probability of existance/emission for a virtual electron by a photon?

    Thanks for the nice drawing, by the way :-) t goes horizontally, right?

    Well at least, as electrons are far heavier than photons the moderating virtual electron would explain the very small c.s. because of its mass.
    Last edited: Mar 4, 2008
  11. Mar 4, 2008 #10


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    It's not a stupid question at all. I would love to know.

    I have a couple of lasers, and I shone a 5mW red laser (in line projection mode, like a barcode scanner) at itself using a mirror. It does interfere with itself, on the mirror itself (due to partial reflection) there is a series of concentric circles with the centre located exactly where light directly from the diode hits the mirror - like these:

    http://www.chemie.de/content/images...Figure 2 (right) fine particle scattering.jpg

    Of course, this is a head on "collision".
    Last edited: Mar 4, 2008
  12. Mar 4, 2008 #11


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    That picture is the Feynman diagram for scattering of photons by photons, mediated by virtual electrons. It was first derived over 50 years ago by Max Delbreuk (my spelling might be wrong.), and consequently the process bears his name, although it seems unknown to forumers. Since no one took him seriously, he became a famous biologist.
    As a virtual process, it is even a hot experimental process today and for the future.
  13. Mar 4, 2008 #12


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    The name is spelled Delbruck.
    Put "scattering of photons by photons" into Google to get many references.
  14. Mar 4, 2008 #13


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  15. Mar 5, 2008 #14


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    Delbrück. Don't forget the umlaut! :smile:

    In high school, I once missed acing a German test by forgetting an umlaut, and ended up with a score of 99.75%. The teacher wrote, "Ah ha! Ich habe einen Fehler gefunden!"
  16. Mar 16, 2008 #15
    No, small coupling constant of EM field to fermions is the reason that cross section for such process is very small. Moreover photon-photon scattering is the process of 4th order.
  17. Mar 21, 2008 #16
    to have this phenomenon you need energy> 1TeV so with a simple flash light, don't worry
    no sparticles will appear
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