# Question about the GR gravitational redshift experiment

## Main Question or Discussion Point

GR predict that a photon climbing in the earths gravitational field will lose energy and will consequently be redshifted.

A test was done by Pound and Snider in 1965 using the Mossbauer effect
They measured the redshift experienced by a 14.4 Kev rays from the decay of Fe in climbing up a 20 m tower by determining the speed at which a detector at the top must be moved in order to maximize the detection rate i.e. the velocity blueshift balances the gravitational redshift.

My question is: how the frequency can be changed by gravitation?
Suppose sender was turned on for 1 month, it send N(very very huge number) waves, how many waves the receiver should receive? It's definitely N. No wave is lost or created in the 22.6 meter long road.
The sender keeps sending for 1 month, how long the receiver takes to receive the N waves? it's 1 month exactly.

Frequency v = N / T, how the receiver can get a different frequency? redshift or blueshift?

If gama ray device is not good for 1 month non-stop sending, we can use a short wave radio. It's the same but easier to think and understand.

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Jonathan Scott
Gold Member
GR predict that a photon climbing in the earths gravitational field will lose energy and will consequently be redshifted.

A test was done by Pound and Snider in 1965 using the Mossbauer effect
They measured the redshift experienced by a 14.4 Kev rays from the decay of Fe in climbing up a 20 m tower by determining the speed at which a detector at the top must be moved in order to maximize the detection rate i.e. the velocity blueshift balances the gravitational redshift.

My question is: how the frequency can be changed by gravitation?
Suppose sender was turned on for 1 month, it send N(very very huge number) waves, how many waves the receiver should receive? It's definitely N. No wave is lost or created in the 22.6 meter long road.
The sender keeps sending for 1 month, how long the receiver takes to receive the N waves? it's 1 month exactly.

Frequency v = N / T, how the receiver can get a different frequency? redshift or blueshift?

If gama ray device is not good for 1 month non-stop sending, we can use a short wave radio. It's the same but easier to think and understand.
The frequency of the photon isn't actually changed. The local time rate changes between the bottom and the top, so when a photon created at a different potential is compared with a similar resonance at the local location, it appears to have changed in comparison.

The frequency of the photon isn't actually changed. The local time rate changes between the bottom and the top, so when a photon created at a different potential is compared with a similar resonance at the local location, it appears to have changed in comparison.

I don't think the time rate is changed by gravitation.
There are many kinds of clocks, atomic, gravitational, mechanical, chemical, or even biological. Their time rate varies to bi-direction with gravitation change.

The test should proved light velocity change.
That is, from up to bottom, the light velocity gets higher.
Frequency v keep constant, wave length becomes longer, the speed get higher.

Jonathan Scott
Gold Member
I don't think the time rate is changed by gravitation.
There are many kinds of clocks, atomic, gravitational, mechanical, chemical, or even biological. Their time rate varies to bi-direction with gravitation change.

The test should proved light velocity change.
That is, from up to bottom, the light velocity gets higher.
Frequency v keep constant, wave length becomes longer, the speed get higher.
The relative time rate of a standard clock and any other local process is definitely changed by gravitational potential, which is what this experiment illustrates. However, this is not the only change.

If you try to create a flat coordinate system which covers both locations, then both the time rate and the size of a ruler vary with gravitational potential, and the coordinate speed of light varies with both, so it varies twice as much. In an isotropic coordinate system (one in which the scale factor between local and coordinate space is the same in all directions) the speed of light is approximately proportional to $1 + 2\phi/c^2$ where $\phi$ is the Newtonian potential, typically given by $-\,Gm/rc^2$, so light apparently speeds up as it rises within a potential.