Gravitational Red Shift

So...the first thing you'll have to know in order to understand this concept would be Doppler effect and its extension into light waves...

For those who are new to Doppler effect...

Whenever there is a relative motion between the source of sound and the observer, there is a change in the frequency of sound observed by the observer. If the relative motion is such that the source and observer approach each other, there is an increase in frequency, and if the relative motion is such that they recede away from each other, there is a decrease in frequency as observed by the observer.

Consider a similar case with light waves...

If the source moves towards the observer, we have an increase in frequency of light waves which we term as blue shift. If the source moves away from the observer, we have a decrease in frequency which is what we call as red shift. Practically, we don't feel this effect because the speed of the relative motion between the source and the observer is usually negligible compared to the velocity of light waves.

This redshift doesn't necessarily take place because of the Doppler effect.

Consider a photon travelling away from the earth's surface, or, let's say a gravity well. Its path would be similar to that taken up by a projectile which slows down as it gains altitude by transfering its kinetic energy to potential energy. But in the case of a photon, it cannot lose energy by slowing down as they are always considered to travel at 'c' . So we have to comprehend the loss in energy as a lowering in frequency of the photon waves, in other words, reddening of photons. This is termed as gravitational redshifting.

Here too, u have the gravitational blueshifting, which occurs when a photon falls into a gravity well.

If you are one of those techy guys looking for a valid derivation to support this theory, get some book relating to quantum physics.

- Maheshwar

Okay, now that is an interesting article from Mahesh. This article poses some unanswered questions.

1) In doppler effect (Sound) there is an apparent increase in frequency when the observer and source move towards each other. Frequency increase implies increase in energy. Where is this energy coming from?

2) Mahesh says that a photon can't lose or gain its kinetic energy and that is why the energy change happens through alteration of frequency. But a photon is always considered mass-less. Kinetic energy?

3) What is the difference between a normal redshift and a gravitational redshift?

4) If an observer is moving towards a source of light, the relative velocity between the light waves(or photons) and the observer is greater than the velocity of light. It is possible?

Take these questions to the forum or comment here.

-Premkumar