# A question

1. Sep 29, 2007

### matteo16

for the equation m=m0/relativistic factor m is greater than m0 and about a photon wich has a speed that is equal to light's one the mass would be inifinite and so the gravity would be infinity too. or maybe does that increase convert in energy?
but if so in the sun as final effect(of nuclear fusion) it woulden't have a smaller mass because, if the greater one converts in energy, tha mass keeps itselph

2. Sep 29, 2007

### JesseM

For a photon, the rest mass m0 is zero, so the equation you mention just doesn't give a well-defined answer for the relativistic mass m, it's zero divided by zero. But you can use the quantum equation E = hf to find the energy of a photon, where h is planck's constant and f is the frequency...I suppose you could then define the relativistic mass m of the photon using the equation E = mc^2 if you wanted, in which case you'd have m = hf/c^2.

Anyway, you shouldn't assume any simple relation between relativistic mass and gravity--take a look at the question If you go too fast do you become a black hole? from the Usenet Physics FAQ.

3. Sep 30, 2007

### matteo16

i didn't understend very much
but i understood that the photon's m0 is equal to 0 but why can't i think the mass connecting with the gravity in a relativistic spacetime?
only because is relativistic?

4. Sep 30, 2007

### matteo16

ops sorry i was wrong