Upper Limit of Energy for Gamma Ray Bursts?

[selfAdjoint]

It's embarassing to have to ask this, but I have never seen this issue discussed. Is there an upper limit on the energy of the gamma rays from the gamma ray bursts? Namely 1.022 MeV? Because any gamma ray of that energy or greater can and will produce pairs of electrons and positrons by supplying its energy to the quantum vacuum. In accelerator experiments a high energy photon can't get more than a few centimeters without doing this.

 

[zoobyshoe]

Originally posted by selfAdjoint
It's embarassing to have to ask this, but I have never seen this issue discussed.

If you think that's embarrasing wait till you start getting hair growing in "funny" places and your voice changes.

 

[selfAdjoint]

A photon in pure vacuum can't pair-produce real particles (this violates conservation of energy-momentum).

Uh, no. The photon energy and momentum go away and are replaced by the particles' energy (mass and kinetic) and momentum.All conservation laws obeyed. And those experimental chambers are evacuated; who wants uncontrolled interactions? So what is the explanation of the TeV gammas' success in crossing astronomical distances?

And zooby, at my age I worry more about losing hair than gaining it.

 

[selfAdjoint]

Consider the physics in the center-of-momentum frame of the produced pair: the net momentum will be zero, and the net energy will be 2ãmc2, where m is the mass of one of the produced particles.

I'm not going to give up yet. Take the rest frame to be that of the point where production occurs. In THIS frame the sum of momenta has to be zero. The incoming photon has a momentum. The outgoing particles aren't standing still in this frame they are moving away. Their momentum relative to their own center of mass is indeed zero, But what balances the incoming momentum of the photon? The two particles center of mass has a momentum relative to the point of production and that balances the photon momentum. The track is not a tee but a vee.

I am just not convinced by your statement that pair produiction is always mediated by some other matter. Do you know the "Two photon" decays in QCD and their explanation?

 

[ranyart]

Originally posted by selfAdjoint
Uh, no. The photon energy and momentum go away and are replaced by the particles' energy (mass and kinetic) and momentum.All conservation laws obeyed. And those experimental chambers are evacuated; who wants uncontrolled interactions? So what is the explanation of the TeV gammas' success in crossing astronomical distances?

And zooby, at my age I worry more about losing hair than gaining it.

We have been busy DickT?..I see the connection, and we have discussed this elsewhere, in superstringtheory.com

In my 'original' question:Particle "Virtual" question at this location:http://www.superstringtheory.com/forum/partboard/index6.html

The answer was in the Question! I do see that Patricia is re-evaluating the superstringtheory site, it remains to be seen if there is going to be a worthwhile!

There has been an embarrassing amount of really interesting discussions, and Selfadjoint/DickT? you may wish to ponder the 'original' Question and how you responded to it..the process action-reaction in the responses you gave gives a good account of your current knowledge "in the context of this emmbarrassing question you raise here in PF"

 

[selfAdjoint]

Ambitwister, I concede, and I did learn something. I used a comoving frame that passes through the spacetime point of the supposed interaction and has a velocity equal to that of the center of motion of the output particles. Then in that frame we have the photon momentum pc, say along the x-axis, and the output momentum is zero since the frame coincides always with the output center of momentum. So momentum cannot be conserved and the interaction is impossible.

This of course is what you said, but I had to explain it to myself.

The "two photon decay" should have been the "two photon interaction", which I think is what you mentioned.

 

[zoobyshoe]

Originally posted by selfAdjoint And zooby, at my age I worry more about losing hair than gaining it.

Same here. Haven't you noticed your voice getting a little more gravelly, and hair growing in your ears?

 

[Nereid]

Mean-free path of TeV (+) gammas?

Inter-galactic space sure is a good vacuum, many orders of magnitude better than anything we can create here on Earth. However, it's not perfect.

Besides, there's the CMB everywhere.

Put all this together and some folk felt (feel?) that really high energy gammas can't possibly reach us from beyond ~100Mpc (there's even a name for this, which I can't remember just now). Trouble is, to the extent that space sources of such can be identified, blazers and their ilk seem to be prime suspects (along with supernovae shock fronts).

For sure, GLAST (and Auger?) will make things a lot clearer, but find lots new too.

Incidently, some models of GRBs suggest copious production of gammas with energies of 1,000 TeV and above. Enough energy in a single c-speed particle to produce observable GR effects on the curvature of space-time?