Physics Help Forum Photons and Force

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 Jul 22nd 2017, 07:36 PM #11 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 519 If you're going to use Newton's formula then what are you going to plug in for the mass of the photon? I thought E=mc^2 didn't apply to photons and that you had to use E= p c for massless particles. Edit: sorry, missed the link first time, reading it now Edit: The images/diagrams won't load for me on that page Last edited by kiwiheretic; Jul 22nd 2017 at 07:39 PM.
Jul 22nd 2017, 08:31 PM   #12
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 Originally Posted by kiwiheretic If you're going to use Newton's formula ...
I never said that I used Newton's formula. In GR the expression is more complicated. While force is still defined as F = time rate of change of momentum the correct expression is derived here.

Gravitational Force

Notice that its P_k and not P^k that is in the expression for force, i.e.

Gravitational force = G_k = dP_k/dt

Note: I'd love to take credit for this. However for those of you who may wish to say that my derivation is wrong or that I don't know what I'm talking about, these things were defined and derived way before I was born. Blame Moller if you need to blame someone. Or Richard A. Mould since he also uses it in his text Basic Relativity (1994).

 Originally Posted by kiwiheretic ... then what are you going to plug in for the mass of the photon?
You use the time component of the photon's 4-momentum, P . If you use units for which c is not one and in the metric in which the spatial component of the 4 -momentum is the 3-momentum then m = P^ 0/c.

 Originally Posted by kiwiheretic I thought E=mc^2 didn't apply to photons and that you had to use E= p c for massless particles.
WARNING WILL ROBINSON!

In general E = mc^2 is wrong. That only holds in the absence of a gravitational field, i.e. in inertial frames.

"Light has no mass" is a common misunderstanding which resulted from people using the term "mass" to mean different things. See

http://www.newenglandphysics.org/com...an_Guth_01.mp4

If its the proper mass (aka "rest mass") then the correct expression is E_0 = mc^2 = proper energy (aka "rest energy"). If m stands for relativistic mass then E =mc^2 where m= p/c = momentum of photon/c

 Originally Posted by kiwiheretic Edit: sorry, missed the link first time, reading it now Edit: The images/diagrams won't load for me on that page
Sorry about that. The old version is online here: Gravitational Red Shift

Last edited by Pmb; Jul 22nd 2017 at 08:37 PM.

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