Relativity Questions: Impossible Processes Explained

[Annieo]

For some reason (most likely lack of coffee and sleep), I can't figure out these questions. I assume they all have to do with conservation of energy and/or momentum, but I'm lame and need help.

Explain clearly why the following processes are impossible.
1) A single photon strikes a stationary electron and gives up all its energy to the electron.

2)A single photon in free space is transformed into an electron and a positron.

3) A fast positron and a stationary electron annihilate, producing only one photon.

I also have a million other questions, but I'll keep my first post short.

 

[Dick]

You should keep the post about the same size, but ask only one question and give us an attempt at trying to solve it (as per the forum rules). Hint: yes, they have to do with conservation of energy/momentum. Try looking up 'four momentum'.

 

[borgwal]

Work out for each of those three what conservation of momentum AND conservation of energy [or equivalently, conservation of 4-momentum] would imply. You should find there is no way to satisfy both.