# Infinite mass and light speed

1. Feb 3, 2005

### demosthenes_001

I recently heard something very interesting. For mass to go light speed, it must be infinite. It can be proved by breaking apart E=MC^2.

2. Feb 3, 2005

### Staff: Mentor

I highly doubt it.

3. Feb 3, 2005

### wlvanbesien

Even if it could, how could such a discovery in any way be significant?

4. Feb 3, 2005

### HallsofIvy

Staff Emeritus
On the contrary, for something to go light speed, its (rest) mass must be 0.

You may be thinking that, if an object with non-zero mass could go at light speed, according to the transformation formula, its mass would be infinite. What that does is prove, since there is no such thing as "infinite mass", that it is not possible for something with non-zero rest mass to go at light speed.

Last edited: Feb 3, 2005
5. Feb 4, 2005

Exactly!

6. Feb 5, 2005

### dextercioby

Simply because it's singular in the point v=c,the formula
$$m_{SR}=\frac{m_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}}$$

cannot be used
This one can and it yields the correct results within the framework of SR
$$E^{2}=m_{0}c^{4}+p^{2}c^{2}$$

Try to make rest mass infinite and see that the total energy is also infinite.Ergo,unphysical situation.

Daniel.

7. Feb 15, 2011

### rahuld.exe

$$m_{SR}=\frac{m_{0}}{\sqrt{1-\frac{v^{2}}{c^{2}}}}$$
according to this formula... anything that travels with speed of light will have infinite mass...

but light also travels like a matter.... and so light(matter) obviously travels at the speed of light.... doesnt it imply that light(as a matter) has infinite mass?

8. Feb 15, 2011

### Staff: Mentor

No. Anything traveling at light speed has zero mass (m0).

9. Feb 15, 2011

### rahuld.exe

yeah that is the rest mass.... and light is never at rest
so isnt talking about the other mass a little more practical?
and 0/0 still is infinite right? so the other mass is infinite....
doesnt that mean that when light falls on me... i should be blown (no pun intended)?

PS: as you can judge by my no. of posts, i am new to this world.... sorry if i sound stoopid or dumb....

Last edited: Feb 15, 2011
10. Feb 15, 2011

### Staff: Mentor

A better term than rest mass is invariant mass.
No, it isn't.
No, it isn't.

That equation you used does not apply where m0 = 0 or v = c. (As was pointed out in this thread over 6 years ago!)
No, the energy and momentum of light are related by the second equation in post #6, which gives you: E = pc. Neither the energy nor momentum of light is infinite.

If you wanted to define the 'relativistic' mass of a photon, it would be E/c2, which is also not infinite.

11. Feb 15, 2011

### rahuld.exe

hmm.... thank you for clearing it out!
what is 0/0 anyway? not defined?

12. Feb 15, 2011

### lightarrow

Not defined.

13. Feb 15, 2011

### rahuld.exe

thank u!

14. Feb 15, 2011

### dextercioby

It's better to think of and use only one formula which is valid for all <particles> from special relativity.

$$E^{2}=m_{0}c^{4}+p^{2}c^{2}$$

With operators instead of scalar functions, this formula holds valid in quantum specially-relativistic theories as well. That's where the photons come from.