Imagine that at the start of the universe, a photon has a short wavelength x. Due to the universe expanding its wavelength decreases. A longer wavelength suggests that the photon has lost energy over time. (I've googled it and found out that the photon has lost over 99,9% of its energy over time).

 

What has happened to this energy? Can one of you come up with a reasonable understandable explanation (I'm a first year physics students, so my toolkit is very limited). It undermines the principle of the law of conservation of energy, if the energy is just lost.

The law of conservation energy applies only in a single frame of reference. If you measure the energy of a photon from a different frame of reference there will be a change of wavelength and frequency from the Doppler shift, thus the photon has a different energy in that frame, but there is no violation of conservation of energy because the reference frame has changed (things that are the same between frames are called invariant). Another example is a ball is moving with some speed v and thus energy mv²/2, but from the ball's frame of reference, the earth is moving at the same speed and has lots more energy because it has much more mass. Asking where the energy went (or came from) is an ill-posed question.

 

The expanding universe is not a single frame of reference, so asking where the photon's energy went is a similarly ill-posed question.

The law of conservation energy applies only in a single frame of reference. If you measure the energy of a photon from a different frame of reference there will be a change of wavelength and frequency from the Doppler shift, thus the photon has a different energy in that frame, but there is no violation of conservation of energy because the reference frame has changed (things that are the same between frames are called invariant). Another example is a ball is moving with some speed v and thus energy mv²/2, but from the ball's frame of reference, the earth is moving at the same speed and has lots more energy because it has much more mass. Asking where the energy went (or came from) is an ill-posed question.

 

The expanding universe is not a single frame of reference, so asking where the photon's energy went is a similarly ill-posed question.

That explains the whole thing, thank you

 

sorry read that wrong.

Edited September 18, 201312 yr by TrappedLight