# Photon and gravity

1. May 12, 2006

### sruthisupriya

i read in my textbook that if a photon falls through gravity, the energy mgh manifests in an increase in frequency and that this result was verified using mossbauer effect.
my question is how can mossbauer effect be used to verify this?

2. May 12, 2006

### George Jones

Staff Emeritus
Consider a setup that consist of a source that emits photon vertically downwards and a photon counter that receives the photons. Just before the photons reach the counter, they go through an absorber tuned via the Mossbauer effect to the frequency of photons when they were emitted. Consequently, peak aborption by the absorber occurs for this frequency - for any other frequency, less absorption occurs. Now, at the bottom where the absorber is, the photons have a different frequency, so peak absorption does not occur unless the absorber is moved with a speed that introduces a Doppler (red) shift that exactly compensates for the frequency (blue) shift caused by the gravitational field.

The speed of the absorber is adjusted until the counter reads a minimum, i.e., the absorber has peak absorption. The speed is known, which allows the Doppler frequency shift to be calculated, which allows the frequency shift caused by gravity to calculated.

Regards,
George

3. May 12, 2006

### sruthisupriya

i didnt quite understand the 'absorber tuned via mossbauer effect' idea.

4. May 12, 2006

### clj4

Hi,

The whole effect is explained fairly nicely here:

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

In the experiment designd by Pound and Rebka, the receiver was put in the elevator and was moved at a precalculated speed. As Greg Jones (see above) is telling you, unless the receiver moved at the precalculated speed the receiver (absorber) will "see" less than peak absorbtion.

Last edited: May 12, 2006
5. May 12, 2006

### Garth

Note that in GR there is no force acting on the photon and therefore there is no exchange of energy, potential or otherwise.

So the change of the measurement of the energy state of the photon in gravitational red shift may be as much to do with the change of state of the apparatus measuring it as a change in the photon.

Garth