Wednesday, February 10, 2021

The problem with photon-photon scattering: the virtual pair has more degrees of freedom

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In pair production and annihilation, the initial state and the final state have the same dimension in the degrees of freedom. The pair side ...
Tuesday, February 9, 2021

What is the electron-positron annihilation rate at keV or eV energies?

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A brief Internet search did not return any measured data for low-energy annihilation cross sections. It looks like other processes dominate ...
Monday, February 8, 2021

Where do the momenta disappear in "classical" annihilation of a pair?

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Let us imagine that a classical electron and a classical positron collide head-on. Let the initial velocity of the particles be small.      ...
Saturday, February 6, 2021

Fermi's golden rule and "density" of states

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In our previous blog post we raised the question what exactly is "density of states". https://en.wikipedia.org/wiki/Fermi%27s_gold...
Friday, February 5, 2021

How to determine the weights of paths in a Feynman path integral?

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Our previous blog post brought up this well-known problem. There is no formal definition of how the weights should be assigned to paths in a...
Thursday, February 4, 2021

The photograph model strongly dislikes short-lived states: this is the reason why photon-photon scattering has a small cross section?

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In an the January 22, 2021 blog post we claimed that the Feynman diagram gives too low a probability amplitude for photon-photon scattering....
Wednesday, February 3, 2021

Cross section of photon absorption: hydrogen Lyman alpha

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In a blog post last week we asked what restricts the absorption cross section of a hydrogen atom. If the atom A outputs a plane wave in the ...