Center of mass of a system of two photons

kthouz
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How can one calculate the center of mass of a system made of two photons? I know that they have zero rest mass so the first intuition is to say that they do not have any center of mass. However, I read about something called center of momentum which is defined as a frame in which a moving particle is at rest. So, if it is possible to have a center of momentum of a system of two photons, how can the center of mass of this system be defined?
 
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In their centre of mass frame they both have the same energies and fly in opposite directions, always equidistant from the centre.
Just solve appropriate Lorentz's transformation equations to find the centre of mass in laboratory frame if your photons have different energies.
 
"Center of momentum" simply means p=0, where p is the total momentum of both particles.
 
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