# Naive Mass of Photon Question

1. Mar 24, 2009

### SimonB

Hi

This is probably very naive!

What is wrong with the following

The (momentum of light) = E/c so it is then possible to write this as
(mass of light x speed of light) = E/c

and thus ... E = mc^2

........... this came up after I read the following with my students

http://www.davidbodanis.com/pages/promised_note_highschool2.html [Broken]

Which starts with Momentum = E/c but then takes a long route round to E = mc^2

Simon

Last edited by a moderator: May 4, 2017
2. Mar 24, 2009

### CompuChip

What's wrong with it, is that "mass of light" is zero, so you are saying 0 = E / c.
For light, the momentum is related to the wavelength, not the mass. The correct formula is
E^2 = (pc)^2 + (mc^2)^2
which for a particle at rest reduces to E = mc^2 and for a photon with momentum p to E / c = p.

In fact, you may want to read https://www.physicsforums.com/showpost.php?p=1285138&postcount=6 [Broken]).

Last edited by a moderator: May 4, 2017
3. Mar 24, 2009

### HallsofIvy

Staff Emeritus
"momentum= mass times velocity" does not apply to light.

4. Mar 24, 2009

### jnorman

"the momentum is related to the wavelength,"

and the wavelength is related to the energy. the total energy of any system is the critical element of consideration, and there is a reason why a system is treated mathematically via its total energy, rather than addressing mass and energy separately in a moving or accelerating system. it seems disingenuous to attempt to deal with mass and energy as f they were different entities - they are only different manifestations of the same thing.