# Photon Energy Increase Under Gravity: Explained

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• calinvass
In summary, the energy of a photon increases when falling into a gravitational field. The potential energy decreases by the same amount the hf term increases.
calinvass
Why does the energy of a photon increase when falling into a gravitational field ?
If we use the equation E=hf, then the energy of the photon increases, but I understand that we also need to add the potential energy to find the total energy. Et=Ep+hf. The potential energy decreases by the same amount the hf term increases.
Is this correct?

Since we are in forum on relativistic physics, this is not true. In general, potential energy of gravitational field in general relativity cannot be defined and notion of conservation of energy doesn't work here. What is happening is simply the fact that time and space is "different in different places" so in one place you will measure different energy simply because your measurement tools are different. How different they are is given by spacetime metric.

However, thanks to the equivalence principle, the conservation of energy (and whole of physics) holds for "one point", i.e. in small enaugh region the physics works same as in universe without gravitation. But the "small enaugh" region is important, you cannot extend it too much.

Last edited:
calinvass
Umaxo said:
space is "different in different places" so in one place you will measure different energy simply because your measurement tools are different
This is not quite true. If we want to be rigorous about it, the tools should be assumed to be the same. What changes is the relation between the parallel transported 4-momentum and the 4-velocity of the observer. What is generally referred to as "gravitational redshift" presumes stationary observers in a stationary spacetime.

Umaxo
Let's say twins Bob and Jim are electrically charged. Bob descends in a gravity well, Jim stays up. When Bob observes Jim living his life, Bob sees Jim aging quickly, darting around quickly, and producing such EM-waves were a crest follows a through quickly.

Bob might think: There is some chemical energy in Jim, and that energy has some potential energy. When some of Jim's chemical energy is converted to EM-waves, the EM-wave energy has potential energy, so we might say that when energy is converted, the potential energy of the former energy becomes the potential energy of the latter energy.

Where was I going with this ... Let me ask: Does E=hf hold for potential energy?

calinvass

## 1. What is photon energy increase under gravity?

Photon energy increase under gravity is a phenomenon in which photons (particles of light) gain energy as they travel through a gravitational field. This is due to the curvature of space-time caused by the presence of a massive object, such as a planet or star.

## 2. How does gravity affect the energy of photons?

Gravity affects the energy of photons by stretching and compressing the wavelength of the photon as it travels through a gravitational field. This results in a change in the frequency and energy of the photon.

## 3. What is the relationship between gravitational potential energy and photon energy increase?

The relationship between gravitational potential energy and photon energy increase is that as an object falls towards a massive object, it gains kinetic energy and loses potential energy. This increase in kinetic energy leads to an increase in the energy of the photons in the object's vicinity.

## 4. How does the intensity of gravitational pull impact photon energy increase?

The intensity of gravitational pull impacts photon energy increase by determining the strength of the gravitational field. The stronger the gravitational field, the greater the curvature of space-time, and therefore, the greater the photon energy increase.

## 5. Is the effect of photon energy increase under gravity significant in everyday life?

No, the effect of photon energy increase under gravity is not significant in everyday life. It is only noticeable in extreme conditions, such as near black holes or in the presence of high-mass objects. In most cases, the effect is very small and cannot be observed without sensitive equipment.

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