Conservation of energy and red shift

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Discussion Overview

The discussion revolves around the implications of redshift on the conservation of energy, particularly in the context of light and its energy as described by the equation E=hc/λ. Participants explore how energy conservation holds in different frames of reference and the nature of energy changes during redshift phenomena.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that energy is frame variant, meaning it can differ between observers due to effects like the Doppler shift, yet conservation of energy remains intact within each frame.
  • One participant uses an analogy involving a ball thrown between moving observers to illustrate how energy appears to change depending on the observer's motion relative to the source.
  • Another participant questions the fate of the energy that seems to be lost due to redshift, prompting further exploration of energy conservation principles.
  • A response suggests that energy does not disappear but is instead dependent on the observer's velocity, comparing it to kinetic energy changes in a moving spaceship.
  • One participant introduces the concept of energy being stored in the light column between a laser and a spaceship, providing a specific scenario to illustrate energy distribution during redshift.
  • Gravitational redshift is mentioned, with a participant noting that energy can be converted into gravitational potential energy in such contexts.

Areas of Agreement / Disagreement

Participants express differing views on the implications of redshift for energy conservation, with no consensus reached on the specifics of energy loss or transformation during these processes.

Contextual Notes

Participants discuss various scenarios and analogies, but there are unresolved assumptions regarding the nature of energy in different frames and the specifics of energy storage in light columns.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in physics related to energy conservation, redshift phenomena, and the implications of relativistic effects on energy measurements.

Joeltk
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If due to redshift, light shifts towards the red end of the spectrum and the energy of photons = hc/[tex]\lambda[/tex], then how does this reduction in energy of the photons not violate the conservation of energy law?
 
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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.
 
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.
 
Ok, then the obvious question is where does this energy go?

Cheers.
 
What energy do you think has gone somewhere? In each frame the energy is conserved.
 
Joeltk said:
If due to redshift, light shifts towards the red end of the spectrum and the energy of photons = hc/[tex]\lambda[/tex], 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.
 
Last edited:
In gravitational redshift , the energy is converted into gravitational potential energy.
 

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