Re Wikipedia article on photon-photon collisions

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					                          Re: Wikipedia article on photon−photon collisions

Re: Wikipedia article on photon−photon collisions

Source: http://sci.tech−archive.net/Archive/sci.physics/2007−04/msg01126.html



      • From: mmeron@xxxxxxxxxxxxxxxxxx
      • Date: Wed, 11 Apr 2007 06:17:34 GMT

In article <1176271008.767967.42760@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "nuny@xxxxxxxx"
<Alien8752@xxxxxxxxx> writes:

       On Apr 10, 10:18 pm, mme...@xxxxxxxxxxxxxxxxxx wrote:

               In article <1176255299.074875.43...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
               "n...@xxxxxxxx" <Alien8...@xxxxxxxxx> writes:


                       On Apr 10, 5:08 pm, "Matt Menge"
                       <mspme...@xxxxxxxxxxx> wrote:

                               On Apr 10, 5:44 pm,
                               mme...@xxxxxxxxxxxxxxxxxx wrote:



                                              The article
                                              in
                                              Wikipedia
                                              seems to
                                              suggest, to
                                              the
                                              contrary,
                                              that in a
                                              head−on
                                              photon
                                              collision (in
                                              the gamma
                                              frequency)
                                              at least one
                                              of the
                                              photons will
                                              act like a
                                              particle.



                                      Not "will", just "may". With


Re: Wikipedia article on photon−photon collisions                                    1
                         Re: Wikipedia article on photon−photon collisions

                                      extremely low probability.



                              Thanks. Will the probability of coupling
                              change with the frequency of
                              the photons (assuming they are all the same
                              frequency)?



                      Generally yes, but there are some caveats.



                      From your original cite:



                      "A photon can, within the bounds of the uncertainty
                      principle,
                      fluctuate into a charged fermion−
                      antifermion pair..."



                      The mention of the uncertainty principle is a clue that the
                      higher
                      the energy of the photon, the more likely you are to have
                      such
                      fluctuations that last long enough for the other photon to
                      notice and
                      interact with them; you need the photon to have nearly the
                      energy
                      equivalent of the rest masses of both particles.



                      In one sense you can say that all photons spend a certain
                      (small)
                      amount of time as particle−antiparticle pairs which
                      immediately
                      annihilate back to a photon*. That's one meaning of the
                      phrase
                      "virtual particles". But the energy restriction of the
                      uncertainty
                      principle basically means that the longer you want the pair to
                      exist
                      so that another photon can have time to interact with them,
                      the more
                      energy the photon that does the fluctuating must have. In

Re: Wikipedia article on photon−photon collisions                                      2
                         Re: Wikipedia article on photon−photon collisions
                      case you're
                      wondering if the pair can become non−virtual and continue
                      existing
                      independently if the original photon has more than their total
                      rest−
                      mass−equivalent energy, the answer is yes.



                      That last bit and Mati's (unintentionally, I'm sure) cryptic
                      comment
                      about probability you might understand better by reading
                      this:



                      http://en.wikipedia.org/wiki/Mass_shell



                      In the diagrams on the page you cited the circular loop is
                      what the
                      second page calls "internal propagators".



                      The above is complicated by the fact that you can't be certain
                      which
                      _kinds_ of particle−antiparticle you'll get other than the
                      minimum
                      match−the−rest−masses requirement and even that isn't
                      absolute. You're
                      more likely to get the lightest particles first, specifically an
                      electron−positron pair, but you also have respectively
                      smaller
                      probabilities of getting a muon−antimuon, tauon−antitauon,
                      quark−
                      antiquark pair and so on up the rest−mass−equivalent scale.



                      * The particles don't _have_ to annihilate back into a photon;
                      they
                      can combine to produce anything that neutralizes their
                      quantum numbers
                      and the energy difference, though they can do some "trading"
                      with the
                      other photon. The cited page mentions them recombining to
                      form vector
                      mesons.


Re: Wikipedia article on photon−photon collisions                                        3
                             Re: Wikipedia article on photon−photon collisions



                         Mark L. Fergerson



                         PS Any of this that I screwed up Mati will hopefully notice
                         and
                         correct.


                 Oh, it is all fine, I'll just elaborate on one point, the last. In
                 the case ofr a single photon in vacuum, the particles do have to
                 annihilate back into a photon since anything else would violate
                 momentum conservation. If it is a matter of two photons interacting,
                 or if there is another body around (say, an atomic nucleus) which can
                 act as momentum sink, then non−photon final states are possible
                 (provided the energy for this is there). Thus a gamma photon cannot
                 generate an end state of electron−positron pair on its own, but it can
                 do it in the field of an atomic nucleus.


        Thanks. I thought I should have made that part clearer.


        Mark L. Fergerson

        PS Dang, I'm getting better at this!


I ain't complaining, for sure:−)

Mati Meron | "When you argue with a fool,
meron@xxxxxxxxxxxxxxxxx | chances are he is doing just the same"
.




Re: Wikipedia article on photon−photon collisions                                         4