# Where has the energy of the closed photon packet gone?

1. Jun 18, 2009

### quirck

A photon is emitted from a star in a far away galaxy.

Its energy is hv = 1000 keV
Its velocity is c.

When it arrives at the retina, the redshift/doppler caused the photon to have an energy less than 1000keV.

Where has the energy of the closed photon packet gone?

2. Jun 18, 2009

### bucher

Re: Photon

To answer this, imagine two vehicles traveling away from each other. As one vehicle emits some sort of sound (a siren maybe), the sound waves travel over to the other vehicle. However, since the vehicle is moving away, it will register the sound as having a lower frequency. This is the Doppler effect that many are familiar with.

Now expand this scenario to a galactic scale. Due to the expansion of the universe, most galaxies are moving away from us. If they are sending out photons at some frequency, then we in our galaxy would pick it up as a lower frequency (much like the vehicles). This lower frequency would then cause us to read the photon's energy as being less than when it left the other galaxy.

The reason this is is all because of reference. The energy of the photon was never lost, it was just adjusted for our galaxy's point of reference. Sort of like how kinetic energy is based on an object's speed. But if one observer is at rest and another is moving, then they will both have calculated different kinetic energies.

3. Jun 24, 2009

### jix

Re: Photon

Additionally, there is a gravitational shift. The photon, with an effective mass of hf/c^2, has a negative gravitational potenial, which is gained as photon energy as the photon moves away from the gravitational field. Since the gravitational potential increases (to zero), the total energy of the photon decreases, which shows itself as a red-shift, or decrease in energy.

The only problem with this is that the gravitational potenial may increase again upon entering our own galaxy. But the shift is a factor.