Energy not conserved
I studied the blog entry arXiv, which discusses the search for dark energy through atomic interferometry, and came across the following passage:
Even if I were not a professional cosmologist, it would be hard for me to believe that hundreds of cosmologists around the world accepted the idea of ​​violating one of the basic laws of physics, simply forgetting about it. If the idea of ​​dark energy would go against a more well-known and fundamental principle, it would not have received such popularity.
But for many people, it is this reaction that occurs. It is clear that cosmologists have not popularized enough about what has been known since 1920: in the general theory of relativity, energy is not conserved.
The essence is simple: when you thought that energy was conserved, this was the reason - invariance with respect to the translation of time. This concept means that the background against which particles and forces evolve, as well as the dynamic rules governing their movements, are fixed and do not change with time. But in GR, this is no longer the case. Einstein discovered that space and time are dynamic, and they can evolve over time. When the space through which particles move changes, the total energy of these particles is not conserved.
')
And it is not that after that all the gates of Hell open up - we simply consider physics in a more general context than was necessary for Newton's laws. And besides, there is one important equation, which is sometimes called "energy-momentum conservation."
i.imgur.com/Mkg2Y0k.png?1
The details here are not so important, but the meaning of the equation is quite simple: energy and momentum evolve according to a certain law, in response to the behavior of the space-time around them. If space-time remains motionless, then the total amount of energy is saved. If it evolves, energy changes in an unequivocal way.
In the case of dark energy, this evolution is simple: the vacuum energy density in empty space is constant, even when the volume of a region of space (moving together with galaxies and particles) increases with the expansion of the Universe. Therefore, the total energy, namely the density multiplied by the volume, increases.
This is annoying to many, but there is nothing new that would be presented to us in connection with the theory of dark energy. This is also true for radiation — particles like photons moving at near-light speed. Photons experience redshift and lose energy as space expands. If we follow a certain number of photons, their number will remain constant, and the energy of each of them will decrease - therefore their total energy will decrease. Reducing their energy also looks like a "violation of the laws of conservation," but for some reason it does not bother everyone so much. In the end, no matter how much this bothers you - after all, this is a completely clear consequence of GR.
And he was even experimentally checked! The success of the Big Bang nucleosynthesis depends on what we know how quickly the Universe expanded in the first three minutes of its existence - and this, in turn, depends on the rate of change in energy density. And this density is almost entirely composed of radiation; therefore, the concept of the absence of energy conservation simply must be taken into account in order to correctly fulfill the predictions of the ancient nucleosynthesis.
It would be superfluous to note that many cosmologists or GRT experts would explain all this differently. Science does not cause doubts in us - only the selection of words that need to be attached to it causes questions. Many would like to state that "energy in GRT is conserved, only you need to include in the equation the energy of the gravitational field, the energy of matter and radiation, and so on."
And this method is not wrong - it can be used only if the definitions are used correctly with it. It seems to me that it is easier to forget about the so-called. “The energy of the gravitational field”, and simply recognize that energy is not conserved. And there are two reasons for this.
First, unlike material fields, there is no such thing as gravitational energy density. The thing that you would like to define as the energy associated with the curvature of space is not uniquely defined at every point in space. Strictly speaking, you can accurately determine only the entire energy of the universe as a whole, instead of talking about the energy of its individual pieces.
Secondly, the meaning of this exercise is to explain what is happening in the world of GR to people unfamiliar with the mathematical details of the theory. All experts agree among themselves - this is a problem of translation, not physics. In my experience, explanations like “the energy of the gravitational field is negative and fully compensate for the energy acquired by the material fields” do not increase anybody’s understanding of the situation. It just drowns out their voices. And if you say that "in GTR space-time can transfer energy of matter, or absorb it from matter so that the total amount of energy is not saved," then people will be surprised, but at least they can at least understand something.
Energy is not saved - it changes due to changes in space-time. Here - not so difficult everything turned out, right?
The concept of dark energy is strange even in cosmological terms. Cosmologists have imposed on us this idea in order to explain the apparent acceleration of the expansion of the Universe. They claim that this acceleration is due to the energy that fills the space with a density of 10 -10 joules per cubic meter. The strangeness is that the space expands, which means that the amount of energy increases. If you notice a problem in these arguments, then you are not alone. Forget about the law of conservation of energy - this is not a joke.
Even if I were not a professional cosmologist, it would be hard for me to believe that hundreds of cosmologists around the world accepted the idea of ​​violating one of the basic laws of physics, simply forgetting about it. If the idea of ​​dark energy would go against a more well-known and fundamental principle, it would not have received such popularity.
But for many people, it is this reaction that occurs. It is clear that cosmologists have not popularized enough about what has been known since 1920: in the general theory of relativity, energy is not conserved.
The essence is simple: when you thought that energy was conserved, this was the reason - invariance with respect to the translation of time. This concept means that the background against which particles and forces evolve, as well as the dynamic rules governing their movements, are fixed and do not change with time. But in GR, this is no longer the case. Einstein discovered that space and time are dynamic, and they can evolve over time. When the space through which particles move changes, the total energy of these particles is not conserved.
')
And it is not that after that all the gates of Hell open up - we simply consider physics in a more general context than was necessary for Newton's laws. And besides, there is one important equation, which is sometimes called "energy-momentum conservation."
i.imgur.com/Mkg2Y0k.png?1
The details here are not so important, but the meaning of the equation is quite simple: energy and momentum evolve according to a certain law, in response to the behavior of the space-time around them. If space-time remains motionless, then the total amount of energy is saved. If it evolves, energy changes in an unequivocal way.
In the case of dark energy, this evolution is simple: the vacuum energy density in empty space is constant, even when the volume of a region of space (moving together with galaxies and particles) increases with the expansion of the Universe. Therefore, the total energy, namely the density multiplied by the volume, increases.
This is annoying to many, but there is nothing new that would be presented to us in connection with the theory of dark energy. This is also true for radiation — particles like photons moving at near-light speed. Photons experience redshift and lose energy as space expands. If we follow a certain number of photons, their number will remain constant, and the energy of each of them will decrease - therefore their total energy will decrease. Reducing their energy also looks like a "violation of the laws of conservation," but for some reason it does not bother everyone so much. In the end, no matter how much this bothers you - after all, this is a completely clear consequence of GR.
And he was even experimentally checked! The success of the Big Bang nucleosynthesis depends on what we know how quickly the Universe expanded in the first three minutes of its existence - and this, in turn, depends on the rate of change in energy density. And this density is almost entirely composed of radiation; therefore, the concept of the absence of energy conservation simply must be taken into account in order to correctly fulfill the predictions of the ancient nucleosynthesis.
It would be superfluous to note that many cosmologists or GRT experts would explain all this differently. Science does not cause doubts in us - only the selection of words that need to be attached to it causes questions. Many would like to state that "energy in GRT is conserved, only you need to include in the equation the energy of the gravitational field, the energy of matter and radiation, and so on."
And this method is not wrong - it can be used only if the definitions are used correctly with it. It seems to me that it is easier to forget about the so-called. “The energy of the gravitational field”, and simply recognize that energy is not conserved. And there are two reasons for this.
First, unlike material fields, there is no such thing as gravitational energy density. The thing that you would like to define as the energy associated with the curvature of space is not uniquely defined at every point in space. Strictly speaking, you can accurately determine only the entire energy of the universe as a whole, instead of talking about the energy of its individual pieces.
Secondly, the meaning of this exercise is to explain what is happening in the world of GR to people unfamiliar with the mathematical details of the theory. All experts agree among themselves - this is a problem of translation, not physics. In my experience, explanations like “the energy of the gravitational field is negative and fully compensate for the energy acquired by the material fields” do not increase anybody’s understanding of the situation. It just drowns out their voices. And if you say that "in GTR space-time can transfer energy of matter, or absorb it from matter so that the total amount of energy is not saved," then people will be surprised, but at least they can at least understand something.
Energy is not saved - it changes due to changes in space-time. Here - not so difficult everything turned out, right?
Source: https://habr.com/ru/post/369045/
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