# Relativity and the mass of photons.

JDude13
[STRIKE]If we rearrange Einstein's mass-energy equivallency equation we get.

$$m=\frac{E\sqrt{1-\beta^{2}}}{c^{2}}$$

A photon moves at the speed of light making its beta=1. This would make its mass=0.
This would insinuate that in the original equation

$$E=\frac{mc^{2}}{\sqrt{1-\beta^{2}}}$$
$$E=\frac{0}{0}$$

$$\frac{0}{0}$$ equals every other number (if my research serves me correctly). Is this why photons can have many frequencys/energies?
However; a photon is only moving at c in a vacuum. What happens in other mediums? Does a photon gain mass? And, if so, what happens to its energy?

Please don't bite my head off for not grasping some seemingly-easy topics. I'm still only young.[/STRIKE]

What is the equation to figure out the energy-mass ratio of photons?

Last edited:

Staff Emeritus
Please start by reading one of the entries in the FAQ thread in the General Physics forum. The problem here is that you're starting off with the wrong premise/equation for photons.

Zz.

JDude13
How about, instead of "You're doing it wrong" *silence*, you actually help. My post was a question which you have failed to answer.

Staff Emeritus
I did answer it! You are starting off with the wrong equation that wasn't meant to be used for photons! Isn't that clearly stated in the FAQ? Or did you not even read it?

This is a VERY common question that we get on here. That is why it is in the FAQ! If you've read it, and still have something you didn't understand, then that's when a followup discussion on here will be appropriate!

Zz.

JDude13
Could you please tell me the correct equation, then?
Why is the equation different to other particles?
Also, I told you not to bite my head off.
I also know what it says in the FAQ about overly-speculative posts.

Staff Emeritus
Er... the FAQ contains NOTHING about "speculative post". You are confusing the FAQ with the PF Rules!

The FAQ thread, IN THE GENERAL PHYSICS FORUM, can be found here (in case you were lost in your search):

This link is also listed in one of the Sticky thread in the Relativity forum titled "IMPORTANT! Read before posting".

Zz.

There is rest mass, invariant mass, relativistic mass, gravitational mass, and inertial mass. Not all of these are equivalent to each other. First, figure out which one you are interested in.

Mentor
2021 Award
This would make its mass=0.
...
What is the equation to figure out the energy-mass ratio of photons?
Since you specify mass=0 then you are talking about the invariant mass (usually the best mass from K^2's list to talk about these days). Since the mass is zero and division by 0 is undefined the "X to mass" ratio is going to be undefined regardless of what X is.

Mueiz
Why not simply say:
Energy of Photon = h v (h is Plank's constant, v is the frequancy ,here we must use quantum mechanics because we can not find the energy of the photon using relativity because 0/0 is unknown)
Mass of Photon = 0
hv/0 = infinity (often symbolically represented by ∞) .
o/o is undefined but X/o is defined it equals infinity.
so energy-mass ratio of photons = ∞

Last edited: