system
Given the following post, respond as if you're commenting on Reddit:
human
Have photons a mass?

One time in Biology class my teacher asked what light is. I said it can be described as a particle without mass (a photon), or a wave. He then said that i am wrong and a photon has mass.

I researched alot...

I come to the conclusion that a Photon has no mass. My "Evidence" was:

E²=(mc²)² + (pc)²          because m(restmass) = 0 

its E²=(pc)² or E= pc      

because E=gamma*m*c² and p = gamma*m*v its:

v=c      so if m hits 0 v becomes c. a photon defines the speed of light...

My teacher then said following:

1. A Photon has to own a mass because its influenced by gravity.
2. A Photon has mass because it has momentum (p=mv)
3. A Photon has mass on its flight but not when it hits something.. the mass (magicaly) dissapears as soon as it hits an object... 
My Questions to you poeple are:

1. Can i Use p=mv on a Photon or do i Use p=E/c? and why?
2. Are Photons influenced by gravity because of their mass or because of the bended space?
3. Can a Photon bend space? (does mass bend space or does Energy bend it?)
4. Is it possible for a Photon to own a mass on its flight? (and only on that.)
5. and how is it able to move with the speed of light when it owns a mass?


If something isn't clear (wouldn't suprise me^^) i'll try to circumscribe it... 

I hope you guys can help me :)
gpt
As far as we know, photons are massless. If they do have a mass it must be less than 10^-54 kg, less than a trillionth that of an electron. Most of the misconceptions your teacher has comes from various inappropriate versions of energy, mass, and momentum.

1. This isn't the case, gravity affects the paths things travel on, including the paths of light.

2. p=mv is just an approximation for slow massive things.

3. This doesn't make sense.

4. You should use p=E/c=hf/c where h is Planck's constant and f is the frequency. 

5. The latter.

6. Yes it can, but we'll probably never detect this.

7. Generally no. You could probably construct a system (like a crystal that behaves differently as light goes through it) where there is an "effective mass" but it wouldn't be the same as a photon having mass.

8. It doesn't.