Is the Universe Losing Energy Due to the Exchange of Photons?

In summary, the conservation of energy in the universe is a complex concept. While photons carry energy away from systems at the speed of light, most processes in the universe, such as star formations, involve the exchange of photons. This results in a decrease in the total energy of the universe, which cannot be recovered due to the vastness of space. However, energy is still conserved in a homogenous universe that is not expanding. In general relativity, energy-momentum is conserved instead of energy. This poses challenges in defining and measuring energy in a curved space-time, making it difficult to determine the conservation of energy in an expanding universe.
  • #1
waht
1,501
4
So when photons are created they carry the energy away from the system at the speed of light,

so basically can you say that since most processes in the universe like the star formations involve the exchange of photons, as a result the total energy of the universe is decreasing because recovering that energy is virtually impossible since the universe is so large,

Although, convervation of energy is still preserved, but it all turns into something you can't really recover.
 
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  • #2
In a homogenous universe that is not expanding, the energy density of any large region of space is constant. Some photons from distant places enter each such region, while others leave that region for other distant places. The net change in energy density is zero.

Since the universe is expanding, the energy density is constantly going down.

The actual sum of the universe's energy must, however, be constant, because energy cannot, by definition, enter or leave it.

- Warren
 
  • #3
waht said:
So when photons are created they carry the energy away from the system at the speed of light,

so basically can you say that since most processes in the universe like the star formations involve the exchange of photons, as a result the total energy of the universe is decreasing because recovering that energy is virtually impossible since the universe is so large,

Although, convervation of energy is still preserved, but it all turns into something you can't really recover.

We generally include these emitted photons in our definition of the universe, so this wouldn't really correspond to it losing energy. Rather, the concept you describe bears somewhat of a resemblance to the second law of thermodynamics and the ever-increasing entropy of the universe.

As for the conservation of total energy in general relativity, things can get a bit tricky. Here's a link that explains the problem in more detail:

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html"
 
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  • #4
SpaceTiger said:
As for the conservation of total energy in general relativity, things can get a bit tricky. Here's a link that explains the problem in more detail:

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html"
Indeed ST, that is a good link, as Weiss and Baez said:
it depends on what you mean by "energy", and what you mean by "conserved".

In general in GR it is energy-momentum that is conserved and not energy. Energy, as well as energy-momentum, is conserved if there is no space-time curvature, that is why in the spherically symmetric case energy conservation only works "at inifinity", and then only when gravitational waves have been eliminated from the equation, which is why it has to be a "null inifinity".

In the cosmological case the requirement for homogeneity and isotropy regains the conservation of the total energy of particles but cosmological expansion excludes the conservation of energy of photons, hence energy is simply 'lost' from the CMB.

The problem is that to define energy you have to specify the frame of reference in which it is measured, as energy is a frame dependent quantity. You cannot then translate that frame (parallel transport it) to a different part of curved space-time, such as a later time in an expanding universe, unless there is a time independent Killing Vector. These do not in general exist, although Robertson-Walker spacetimes admit a conformal Killing vector normal to the spacelike homogeneous hypersurfaces.

Garth
 
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  • #5
Only a handful of people will grasp the meaning of that discussion. Garth, I think, missed the point. Specifically, there is a tensor thing he might have overlooked.
 

FAQ: Is the Universe Losing Energy Due to the Exchange of Photons?

What is the "Universe losing energy"?

The concept of the "Universe losing energy" refers to the idea that the overall energy of the Universe is decreasing over time. This is known as the Second Law of Thermodynamics, which states that the total amount of energy in a closed system (such as the Universe) will always decrease over time.

How is the Universe losing energy?

The Universe is losing energy through various processes such as the expansion of space, which causes a decrease in the energy density of the Universe, and the conversion of energy into different forms such as heat and radiation. These processes are natural and inevitable due to the laws of thermodynamics.

What are the consequences of the Universe losing energy?

The consequences of the Universe losing energy are still being studied and debated by scientists. However, it is believed that as the Universe continues to lose energy, it will eventually reach a state of maximum entropy, where all energy will be evenly distributed and no work can be done. This could potentially lead to the end of the Universe as we know it.

Can we reverse the process of the Universe losing energy?

According to the laws of thermodynamics, it is not possible to reverse the process of the Universe losing energy. This is because the Second Law states that entropy (which is a measure of disorder or randomness) will always increase over time. Therefore, it is not possible to restore the Universe to a state of lower entropy.

How does the concept of the Universe losing energy affect our daily lives?

The concept of the Universe losing energy may seem abstract and unrelated to our daily lives. However, it has important implications for the future of the Universe and the fate of humanity. As the Universe continues to lose energy, it will eventually reach a state of maximum entropy, making it impossible for life to exist. This highlights the importance of preserving energy and finding sustainable ways to use it in our daily lives.

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