# Pair Production, electron and positron from isolated photon

by cooev769
Tags: electron, isolated, pair, positron, production
 P: 106 I don't completely understand why an electron positron pair cannot be created from an isolated photon. I understand it must have something to do with 4 momentum conservation, but I really can't see a problem given the photon has enough energy for the mass to energy vice versa conversion.\ The only seemingly sound argument I could seem to convince myself with is that if 4 momentum is conserved and we say pf=pi. And we take pi of the photon in the photons rest frame we have (0,0,0,E/c), but the electrons can never move at the speed of light and hence the 4 momentum of any two of the electrons will have a negative momentum value (-x,0,0,E/c) and hence it is therefore impossible. But I don't believe this is correct. Any suggestions appreciated thanks!
 P: 381 Pretty simple and trivial argument: equality of reference frames. You can always pick a reference frame where the isolated photon is Doppler shifted to have too little energy for pair production. Since the event must happen in all reference frames or in none of them, and there are frames where it is impossible, therefore it is impossible in all frames. Even simpler argument: an isolated photon has no time whatsoever and therefore no events whatsoever can happen to it. Splitting a photon into several photons of lower energy travelling in the same initial direction would not violate conservation of momentum (unlike production of particles with rest mass) but it is still impossible because of no time at the speed of light.
 P: 106 Cheers. What i'm really looking for though is a rigorous violation of the conservation of 4 momentum.
 Mentor P: 16,171 Pair Production, electron and positron from isolated photon You won't find a "rigorous violation", since it is conserved.
 P: 106 No it isn't as an isolated photon cannot produce and electron and an anti electron, therefore writing the 4 momentum before and after will produce a violation in conservation of 4 momentum, which I know occurs, I just don't know the proof and i've been trying for a while.
 P: 754 Can't photons of energy E>~1MeV lead to pair production?
 P: 106 Yep, but apparently in the presence of a nucleus or something, not an isolated photon which is why i'm trying to prove it mathematically.
 Mentor P: 16,171 No, momentum is conserved.
Thanks
P: 4,160
 Quote by cooev769 No it isn't as an isolated photon cannot produce and electron and an anti electron, therefore writing the 4 momentum before and after will produce a violation in conservation of 4 momentum, which I know occurs, I just don't know the proof and i've been trying for a while.
Take as your rest frame the center of mass frame of the electron-positron pair. Their total momentum in this frame is zero. Now what is the momentum of the photon? If momentum is conserved, the photon would also have to have momentum zero. But the momentum of a photon can't be zero, since it would have to be standing still.
P: 90
 No it isn't as an isolated photon cannot produce and electron and an anti electron, therefore writing the 4 momentum before and after will produce a violation in conservation of 4 momentum, which I know occurs, I just don't know the proof and i've been trying for a while
Just to follow up on Bill_K thread, it shows that momentum and Energy can't both be conserved. In the center of mass frame of the electron-positron pair their momentum is zero but there energy has to be at least twice the mass of the electron. A "zero momentum" photon also has zero energy and therefore can't have a four vector which is equal to the four vector of the electron-positron pair.

 Splitting a photon into several photons of lower energy travelling in the same initial direction would not violate conservation of momentum (unlike production of particles with rest mass) but it is still impossible because of no time at the speed of light.
I'm not sure this argument is true. I think that in general a massless scalar field can decay to two collinear scalar fields ( or more)
P: 90
 I'm not sure this argument is true. I think that in general a massless scalar field can decay to two collinear scalar fields (or more)
scalar particles not fields

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