Conservation of energy and red shift

[Joeltk]

If due to redshift, light shifts towards the red end of the spectrum and the energy of photons = hc/$$\lambda$$, then how does this reduction in energy of the photons not violate the conservation of energy law?

 

[Dale]

Hi Joeltk, welcome to PF.

Energy is frame variant. Conservation and frame invariance are completely different concepts. That means that the energy will be different in each frame (Doppler shift) but in each frame the energy will be conserved.

 

[Drakkith]

Imagine I throw a ball at you. If you are stationary to me, the ball hits you with an equal amount of energy that it left me with. (Assuming no friction and other forces.) However, if you are moving away, then it arrives at you with LESS energy than it did before. (From your point of view) The reverse is true if you are moving towards me. It's the same concept with light.

 

[Joeltk]

Ok, then the obvious question is where does this energy go?

Cheers.

 

[Dale]

What energy do you think has gone somewhere? In each frame the energy is conserved.

 

[Dmytry]

If due to redshift, light shifts towards the red end of the spectrum and the energy of photons = hc/$$\lambda$$, then how does this reduction in energy of the photons not violate the conservation of energy law?

the same way how reduction of kinetic energy of a spaceship as measured by spaceship going in same direction does not violate the conservation of energy. Energy isn't absolute but depends to velocity of observer.
edit: where does energy go:
If you are speaking of a case where e.g. you have a giant laser that is powering a spaceship that is moving away from the laser - the laser is sending say 1GW and spaceship is receiving 0.9GW , in addition to the dependence of energy on the frame of reference, the energy becomes stored in the growing column of light between laser and the spaceship. A column of light of power 1GW with length 1 light second stores 1 GJ, of course.

 

[cragar]

In gravitational redshift , the energy is converted into gravitational potential energy.