# Photon's disagree of the state of the universe?

1. Jan 25, 2012

### JPBenowitz

So in modern physics we were going through SR and I had a question: since gravitational fields effect the amount of time elapsed and speed of a particle; say we have photon A emitted at the origin (0,0) and photon B emitted at (2,2) such that there is a uniformly decreasing gravitational field in both (T,x) and (T',x'). Photon A and Photon B wold disagree on the state of the universe such that Photon B would be traveling slightly faster then Photon A given they both travel slightly less than c. Is this true? Do observers O and O' disagree on the gravitational field relative to each other?

2. Jan 25, 2012

### Staff: Mentor

The thing about photons is that they don't travel faster than one another. No matter what observer measures their speed they will get the same value c. Even if the observer were traveling at the speed of a photon, the other photons would pass by at the measured speed of c.

SR takes as an assumption that c is a constant for all inertial frames of reference.

Gravity affects light by bending it or changing its energy. Blue light escaping from the vicinity of a black hole will not appear blue to a far away observer as its energy is reduced and its wavelength lengthened. The blue light will be red-shifted.

3. Jan 25, 2012

### JPBenowitz

Instead of saying the photons "travel" slightly less than c can I say the gravitational field warps space-time in a manner that increases the distance between two points x and x_0 such that the photons arrive at x later than if ideally there was no gravitational field?