Why can't a photon transfer all its energy to a free electron?

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A photon cannot transfer all its energy to a free electron due to the need to conserve both energy and momentum, which requires the involvement of another particle. This principle is analogous to a scenario where two balls of different masses cannot completely transfer kinetic energy to each other. The primary interaction between a photon and a free electron occurs through Compton scattering, rather than complete energy transfer. High-energy interactions, such as photoproduction, are exceptions but are not typical. Understanding these interactions is crucial for grasping fundamental physics concepts.
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Why is it that a photon can't transfer all its energy to a free electron?
 
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It's imposible to conserve both energy and momentum in such a process. You need another particle or object in order to do this.
 
It's not the same thing, but there is a classical analogue of that fact: a ball of mass m cannot transfer all its kinetic energy to a ball of mass M=/=m.
 
The only way for a photon to interact with a free electron (other than photoproduction at very high energies) is via Compton scattering.
 

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