Conservation of energy and red shift

In summary, the conversation discusses the concept of energy being frame variant and how it relates to the conservation of energy law. The example of a moving ball and a laser powering a spaceship are used to explain this concept. In the case of redshifted light, the energy is either stored in the growing column of light or converted into gravitational potential energy.
  • #1
Joeltk
2
0
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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  • #2
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.
 
  • #3
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.
 
  • #4
Ok, then the obvious question is where does this energy go?

Cheers.
 
  • #5
What energy do you think has gone somewhere? In each frame the energy is conserved.
 
  • #6
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.
 
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  • #7
In gravitational redshift , the energy is converted into gravitational potential energy.
 

1. What is the conservation of energy and how does it relate to red shift?

The conservation of energy is a fundamental scientific principle that states that energy cannot be created or destroyed, only transformed from one form to another. This principle is closely related to the phenomenon of red shift, which is the observed increase in the wavelength of light from distant objects in the universe. This increase in wavelength is due to the expansion of the universe, which causes the energy of the light to be stretched out, resulting in a shift towards the red end of the visible light spectrum.

2. How does the red shift provide evidence for the conservation of energy?

The red shift observed in light from distant objects in the universe is consistent with the conservation of energy, as it demonstrates that the energy of the light is not being lost or created, but rather transformed due to the expansion of the universe. This is supported by the fact that the total energy in the universe is believed to remain constant, despite the expansion.

3. Can the conservation of energy be violated in the presence of red shift?

No, the conservation of energy is a fundamental principle of physics that applies to all systems, including those affected by red shift. While the energy of light may appear to be increasing due to the red shift, it is simply being transformed from one form to another, and the total energy in the universe remains constant.

4. How does the concept of red shift impact our understanding of the universe?

The concept of red shift has greatly impacted our understanding of the universe, as it provides evidence for the expansion of the universe and the Big Bang theory. It has also allowed scientists to measure the distance and speed of objects in the universe, providing valuable information for studying the evolution and structure of the universe.

5. Are there any practical applications of the conservation of energy and red shift?

While the conservation of energy and red shift have primarily been studied in the context of astrophysics and cosmology, they also have practical applications in other fields. For example, the understanding of energy conservation has led to the development of renewable energy technologies, and the red shift of light can be used in medical imaging to detect changes in blood flow and oxygenation levels in the body.

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