Conservation of energy question

  • #1
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Main Question or Discussion Point

Lets say we had a light source. It takes energy and turns it at some efficiency into light.

You put the light source on say, a white dwarf. The light source, still at the same efficiency, produces light that is gravitationally redshifted and therefore lost energy.

What happened to the energy in the light? All that happened was that we moved it around yet its energy changed?
 

Answers and Replies

  • #2
CWatters
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Where did the energy come from to escape the star?
 
  • #3
Bobbywhy
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The Wiki page on “Gravitational Redshift” begins with this:

“In astrophysics, gravitational redshift or Einstein shift is the process by which electromagnetic radiation originating from a source that is in gravitational field is reduced in frequency, or redshifted, when observed in a region of a weaker gravitational field. This is as a direct result of Gravitational time dilation; frequency of the electromagnetic radiation is reduced in an area of a higher gravitational potential. There is a corresponding reduction in energy when electromagnetic radiation is red-shifted, as given by Planck's relation, due to the electromagnetic radiation propagating in opposition to the gravitational gradient.”

Then there is a detailed description of the energy question further down the page.

http://en.wikipedia.org/wiki/Gravitational_redshift
 
  • #4
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thank you. what about universal expansion and associated redshift? where does the energy go?
 
  • #5
CWatters
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I don't pretend to understand it or even know if this is current thinking but google found..

http://arxiv.org/ftp/physics/papers/0407/0407077.pdf

One problematic aspect of the cosmological expansion is the apparent loss of energy
associated with the redshift. The effect is particularly bad with cosmological background
photons received in the current epoch – they are received with only about 0.1% of their
emission energy. Attempts to account for the missing energy within the framework of general
relativity have met with severe problems because of the difficulty in defining a local
gravitational energy density (gravitational energy cannot be expressed in tensor form). As a
result, it is widely accepted that energy is not locally conserved in general relativity3, although
claims are made that energy is globally conserved during expansion. ....continues...
He's also written other papers such as..

http://arxiv.org/ftp/physics/papers/0511/0511178.pdf
 

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