# Timon and Pumba on photons

ddr

## Main Question or Discussion Point

Timon, Timon, I wonder howcome photons have no mass at rest and yet mass is their only property?
Pumba, Pumba, you wonder - I know. There are no photons at rest!
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mathman
mass is their only property?
What do mean by that? Photons have energy, travel at the speed of light, have spin 1, and other properties.

ddr
it is because...

the energy of the photon is E=mcc
the momentum p=mc
...
---------------------------
if m=0 (there is no m) then
E=0;p=0... too(there is no E,p...).

neutroncount

Originally posted by ddr
the energy of the photon is E=mcc
the momentum p=mc
...
---------------------------
if m=0 (there is no m) then
E=0;p=0... too(there is no E,p...).
Doesn't work that way.

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/relmom.html

If mass is 0, than energy is equal to momentum times c.

Think relativistically!

Last edited by a moderator:
ddr

Originally posted by neutroncount
If mass is 0, than energy is equal to momentum times c.
Ok, how much is the momentum then?

Obviously then the photon must have mass ,right ? And if we follow this statement through to its conclusion it means that energy must have mass ?

neutroncount
Originally posted by McQueen
Obviously then the photon must have mass ,right ? And if we follow this statement through to its conclusion it means that energy must have mass ?
Why must a photon has mass? Look at the equations again. Energy approximately equates to momentum with a zero rest mass particle (mass disappears at c/c in the equation). So its MOMENTUM that relates to energy with the photon, not mass.