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## Homework Statement

Using energy and momentum conservation requirements, show that a free electron cannot absorb all the energy of a photon.

## Homework Equations

Einstein energy-momentum formula:

(E)² = (mc²)² - (c

**p**)², where m = mass, E = energy, c = speed of light, and p = momentum vector.

## The Attempt at a Solution

I expanded this by setting the initial energies of the photon and electron equal to the final. However, I am not entirely sure what significant expression I'm supposed to be looking for here in order to answer this question.

Thanks.