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

Feynman talks about this in

**The Feynman Lectures**
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