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View Full Version : Photon-photon collisions - observed, or not?



Jean Tate
2010-Apr-17, 08:16 AM
In my Universe Today article Magnetic Fields in Inter-cluster Space: Measured at Last (http://www.universetoday.com/2010/04/14/magnetic-fields-in-inter-cluster-space-measured-at-last/), I said:
A very neat [trick], one that relies on physics not directly tested in any laboratory, here on Earth, and unlikely to be so tested during the lifetime of anyone reading this today the production of positron-electron pairs when a high energy gamma ray photon collides with an infrared or microwave one (this can't be tested in any laboratory, today, because we can't make gamma rays of sufficiently high energy, and even if we could, they'd collide so rarely with infrared light or microwaves we'd have to wait centuries to see such a pair produced).
Leaving aside the TeV gamma-infrared or microwave part, for the moment, have any photon-photon collisions resulting in pair-production been observed (in a particle collider instrument perhaps)?

I've found references to reports of such, using LEP, involving GeV photons, but the collisions seem to be "quasi-real". How do these results relate to the real collisions between TeV gammas (from blazars) and microwave (the CMB) or infrared (EBL) photons? And what does "quasi-real" mean anyway??

And what would you say are the prospects of observing pair-production from TeV-IR or TeV-microwave photon collisions, in a laboratory here on Earth?

Jeff Root
2010-Apr-17, 08:40 AM
Note that photon energy is relative. A collision between two photons of say,
1 MeV is identical to a collision between a 1 TeV photon and a 1 eV photon.

-- Jeff, in Minneapolis

astromark
2010-Apr-17, 09:05 AM
What does 'Quasi-real' mean... ? It means its not. That in theories it should. That it could or even should... but that it does not. It does not behave as thought it would. Quasi = Not. Zip, zilch, Nada, zero. ...:eh: and does a photon ever actually collide ? or just get absorbed... refract, deflect..
It might be a question for a laser technician., or some one from CERN. I do not understand how we can test this. Can we ?

cosmocrazy
2010-Apr-17, 10:00 AM
A good point Astro and I was under the assumption that photons interfered with each other in wave forms rather than actually collide per-say?

Tensor
2010-Apr-18, 03:12 AM
Quasi-real means that pair production has been observed when a high enough energy (whether it's gamma ray or high energy x-rays as long as it has 1.02 MeV to create a electron-positron pair)photon collides with the electric field of an atom. The quasi part is in there because the electric field can be thought of as photons, but usually isn't in the case of a field.

astromark
2010-Apr-18, 04:00 AM
Quasi-real means that pair production has been observed when a high enough energy (whether it's gamma ray or high energy x-rays as long as it has 1.02 MeV to create a electron-positron pair)photon collides with the electric field of an atom. The quasi part is in there because the electric field can be thought of as photons, but usually isn't in the case of a field.

You might attach that meaning to those word groups... That does not look anything like the meaning of the words, to me...
Quasi-real., and pretending to be real. Might have some place in this. What ever it is that Tensor has just said... of which I have know idea.
I will look for a little more logic :o:sorry.

Shaula
2010-Apr-18, 02:56 PM
You might attach that meaning to those word groups... That does not look anything like the meaning of the words, to me...
Quasi-real., and pretending to be real. Might have some place in this. What ever it is that Tensor has just said... of which I have know idea.
I will look for a little more logic :o:sorry.
Quasi-real is a property of quasi-objects in objective philosophy IIRC. Essentially it means treating an object as real for the purposes of reasoning before you have evidence that it truly is real. So a population of nanognomes is quasi-real in the thought experiment "How many nanognomes can dance on the head of a match". That way you can reason and think on something not proven and, if evidence comes in refuting the evidence of nanognomes you know which bits of the theory to throw out.

In physics terms it is used to refer to the virtual particle of a field that interacts with an external particle (such as high energy electron scattering off an electric field belonging to another electron - see here (http://cdsweb.cern.ch/record/831709/files/)). So whether it seems likely or not I am afraid that Tensor is right!

DrRocket
2010-Apr-18, 05:38 PM
A good point Astro and I was under the assumption that photons interfered with each other in wave forms rather than actually collide per-say?

Photons can interact, but not directly. The coupling involves intermediate particles created from a photon and quantum chromodynamics.
http://www.hep.ucl.ac.uk/~opal/gammagamma/gg-tutorial.html

DrRocket
2010-Apr-18, 05:50 PM
In my Universe Today article Magnetic Fields in Inter-cluster Space: Measured at Last (http://www.universetoday.com/2010/04/14/magnetic-fields-in-inter-cluster-space-measured-at-last/), I said:
Leaving aside the TeV gamma-infrared or microwave part, for the moment, have any photon-photon collisions resulting in pair-production been observed (in a particle collider instrument perhaps)?

I've found references to reports of such, using LEP, involving GeV photons, but the collisions seem to be "quasi-real". How do these results relate to the real collisions between TeV gammas (from blazars) and microwave (the CMB) or infrared (EBL) photons? And what does "quasi-real" mean anyway??



http://arxiv.org/abs/hep-ex/9909023

Tensor
2010-Apr-18, 06:05 PM
http://arxiv.org/abs/hep-ex/9909023

I kinda figured you would know where a paper was.

EigenState
2010-Apr-18, 07:19 PM
Greetings,

Stanley J. Brodsky, "Photon-Photon Collisions Past and Future" SLAC-PUB-11581 (November 2005) might also be of interest.

Direct PDF download: http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-11581.pdf

Best regards,
EigenState

cosmocrazy
2010-Apr-19, 09:01 AM
Photons can interact, but not directly. The coupling involves intermediate particles created from a photon and quantum chromodynamics.
http://www.hep.ucl.ac.uk/~opal/gammagamma/gg-tutorial.html

ok, thanks for the link

astromark
2010-Apr-19, 10:46 AM
Its good here, a ? :o: Questions and well explained answers, great.

Jean Tate
2010-Apr-20, 05:14 PM
Thanks everyone for the thoughtful, and helpful, answers.

I don't really understand much (OK, to be honest, most) of the material, but if I may present this simple summary, in my own words?

No one has done an experiment, in a lab, with GeV (or MeV or TeV) photons colliding with other photons, such as an especially intense beam from some synchrotron light source or other.

In particle colliders, detectors around collision points record events which can be (are?) interpreted in terms of QCD (and QED?); these events include photon-photon collisions.

In such experiments, no GeV (or MeV or TeV) photons are observed directly.

The term 'quasi-real' is a technical one; to understand it well requires understanding quite a bit of the physics of particle collisions; in particular, it is not something you could explain well, in a thousand words or less, to someone who has little understanding of QCD (not to mention quantum mechanics).

Is that about right?