Upper Limit of Energy for Gamma Ray Bursts?

In summary: The net momentum of the two particles is still zero, but their total mass has increased by 2ãmc2.Originally posted by selfAdjoint In summary, the gamma rays from GRBs have enough energy to produce pairs of electrons and positrons. However, because space is a very good vacuum, they lose energy through higher-order interaction before reaching Earth.
  • #1
selfAdjoint
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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.
 
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  • #2
Originally posted by 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?

Gamma ray bursts have been observed up past the TeV range.


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.

I didn't realize the range in accelerators was so short, but if so, it's because it's interacting with other particles. A photon in pure vacuum can't pair-produce real particles (this violates conservation of energy-momentum). Space is a very good vacuum, better than anything we can produce.

Photons in GRBs do lose energy through higher-order interaction with microwave background photons, infrared photons in galaxies, etc. But they still have plenty of energy left when they get here.
 
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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.
 
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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.
 
  • #5
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.

Incorrect. This is well-known; it's the same reason why pair annihilation in vacuum can't produce a single photon, but must always produce two.

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 2γmc2, where m is the mass of one of the produced particles.

For energy and momentum to be conserved, the photon that produced them would have to have had energy 2γmc2, and zero momentum. A photon cannot have zero momentum, and in fact the momentum of a photon with energy 2γmc2 must exactly equal 2γmc (from E=pc).

(If you had two photons, then they could each have energy γmc2 and momentum γmc, but in opposite directions, so the net momentum would be zero, and the conservation laws would be obeyed.)

Anyway, just think about it: you're claiming that there is some energy cutoff for photons in vacuum. But the state of a single particle in vacuum must be Lorentz invariant: if the photon is above the cutoff in one frame, it's below the cutoff in another. From symmetry, it can't pair-produce in one frame but not another.

It's only when you have other particles around, that can define a preferred frame, that you can have real pair production: e.g., if the photon has energy above some cutoff as measured in the rest frame of another particle (or system of particles, like a photon gas) that it's exchanging energy and momentum with. (Or, instead of looking at it from a symmetry perspective, you can use the previous argument that there has to be another particle around for pair production to obey the conservation laws.)

And those experimental chambers are evacuated; who wants uncontrolled interactions?

The experimental chambers are not perfect vacuum, as I said.
 
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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?
 
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Originally posted by selfAdjoint
I'm not going to give up yet. Take the rest frame to be that of the point where production occurs.

A rest frame isn't defined by just a point, it's defined by a state of motion. We can take my frame to have an origin centered on the point where production occurs, too. Which frame are you talking about? A rest frame at rest with respect to what?


In THIS frame the sum of momenta has to be zero.

But that's the frame I used!


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 still don't know what frame you're working in. Above, you said that the sum of the momenta of the system was zero in this frame; now you say that the produced particles' center of mass has a nonzero momentum. But a frame in which the net momentum is zero is a frame in which the center of mass of the two particles has zero momentum.

Anyway, go ahead and just write down both the relativistic energy and momentum conservation equations in whatever frame you're working in, and you will find that you simply cannot satisfy them both.

I am just not convinced by your statement that pair produiction is always mediated by some other matter.

Fine. Then point out the error in either the conservation proof or the symmetry proof. You ignored them both and presented your own, non-mathematical handwaving argument. If you want to claim that everything balances, prove it. I can dig up some supporting references if you want, but they just say the same thing I already did.

And for the record, pair production from a photon doesn't have to be mediated by matter; it can be mediated by, say, another photon (in the inverse of pair annihilation). But it has to be mediated by some other real particle.

Note also that this argument applies specifically to pair production from a single massless particle. A massive particle can pair-produce in vacuum.

Do you know the "Two photon" decays in QCD and their explanation?

No. What is a "two photon decay" in QCD, and does it have anything to do with pair production from a single photon?
 
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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 [Broken]

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":wink:
 
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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.
 
  • #10
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?
 
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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?
 

1. What is the "Upper Limit of Energy" for Gamma Ray Bursts?

The "Upper Limit of Energy" for Gamma Ray Bursts (GRBs) refers to the maximum amount of energy that can be released during a GRB event. This limit is estimated to be around 1054 ergs, which is equivalent to the mass-energy of about 10,000 Suns.

2. How is the Upper Limit of Energy for GRBs determined?

The Upper Limit of Energy for GRBs is determined through observations and measurements of the luminosity and duration of GRBs. By analyzing the X-ray and gamma ray emissions from GRBs, scientists can estimate the amount of energy released during the event.

3. What causes the Upper Limit of Energy for GRBs to be so high?

The high Upper Limit of Energy for GRBs is believed to be caused by the collapse of massive stars, known as hypernovae, which release enormous amounts of energy during the formation of a black hole. This energy is then further amplified by processes such as magnetic reconnection and particle acceleration.

4. What are the implications of the Upper Limit of Energy for GRBs?

The Upper Limit of Energy for GRBs has significant implications for our understanding of astrophysics and the universe. It suggests that GRBs are among the most energetic events in the universe, and the immense amount of energy released can have a profound impact on their surroundings, such as shaping the evolution of galaxies.

5. Can the Upper Limit of Energy for GRBs ever be exceeded?

Based on current scientific understanding, it is unlikely that the Upper Limit of Energy for GRBs can be exceeded. However, new discoveries and advancements in technology may provide more insights and potentially lead to revisions of this limit in the future.

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