# Easy Relativity Question

1. Apr 6, 2005

### babtridge

Maybe its because my exams are close and I'm starting to panic but there is a question that has been bustin' my balls all day and was hoping any of you could help....

A particle with mass 10eV and a photon are produced simultaneously in a galaxy 150,000 light yrs away. The energy for both particles is 10MeV. Estimate the difference in arrival time between the particles.

My question really is: Is the energy for the photon a red herring because surely this travels at speed c and you don't need this info?!

Thanks in advance people cleverer than me

2. Apr 6, 2005

### dextercioby

So the photon has 10MeV (ignoring GR effects),and that is not a useful piece of information.It should arrive in exactly 150,000 years.

What about the electron...?The time that is needed is in the Earth's reference frame...

Daniel.

3. Apr 6, 2005

### marlon

you are gonna need the relativistic formula for energy. you know, the Einstein-formula(E² =p²c² + m²c^4). Also be sure to know wether the given mass is restmass or not...

For a photon E =pc since the restmass is ZERO

marlon

4. Apr 6, 2005

### dextercioby

You needn't momentum & stuff.

The gamma factor is 1000.That should give you the velocity,which is all u need.

Daniel.

5. Apr 7, 2005