Timeline for Is the total energy of the universe zero?
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17 events
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| Oct 10, 2023 at 10:52 | comment | added | FlatterMann | @greatscissors That is why energy is a system property. You have to specify the system you are talking about. If you want to talk about energy conservation, then it's the total of the emitter, the free field and the absorber. In that system energy is, indeed, a constant. Until we hit on the expansion of space, that is... because "space" is not a part of the system definition in physics... yet. | |
| Jul 11, 2023 at 18:33 | comment | added | greatscissors | @FlatterMann The reason that light appears to lose energy is because we are transitioning to frames moving away from the source with larger and larger speeds (Doppler shift) as a result of expansion. I assure you that if you blasted off in a rocket ship towards that galaxy, you would find that light capable of doing more work than it does here on Earth! But good luck getting to a speed where the difference is at all meaningful. | |
| Jul 11, 2023 at 18:28 | comment | added | greatscissors | @FlatterMann Energy is constant (in classical physics) only when viewed from a consistent inertial frame. The light coming from that remote galaxy has a color which can be anything depending on the inertial frame in which that light is observed (Doppler shift). No inertial frame is preferred (c.f. Einstein) and so there's no frame which observes light at it's "true" or "fundamental" color. E.g. the 10.2 eV photon which comes from the 2p->1s transition in H is 10.2 eV only in an inertial frame moving with the CM (typically coincident with lab frame---see Doppler broadening Wikipedia). | |
| Jun 17, 2023 at 18:49 | comment | added | FlatterMann | @safesphere Energy depends on the system. It is not a property of the car or the person being hit. It's a property of both. A frame is an abstract that pretends that you can move matter at any speed in any direction. The amount of work that can be done by the light coming from a remote galaxy is not an abstract. It's limited and decreasing, whether we like that or not. | |
| Jun 17, 2023 at 18:39 | comment | added | safesphere | @FlatterMann As an illustration, replace photons with little metal balls flying initially near the speed of light. After a while they will reach the areas of spacetime expanding with the same velocity. In this areas, these balls will appear no longer moving. This is the same as passengers moving with the same velocity as the car appear non-moving relative to the car. So your “facts on the ground” would look as if the balls have lost all their energy, but in reality, their energy has not changed at all, as properly measured in any particular frame. | |
| Jun 17, 2023 at 18:23 | comment | added | safesphere | @FlatterMann If you stand on the road, you can get hit by a moving car. In your terminology, the car can do work on you when it hits you. Yet when you sit in this car (different frame), you are in touch with it, but it cannot hit you or do work on you. So work directly depends on the frame. The “facts on the ground” you refer to relate to “the total energy in the universe”. On one hand, it does not depend on the frame, but on the other hand, it is not a definition of energy since energy depends on the frame. Strictly speaking, the total energy of the universe is undefined. | |
| Jun 17, 2023 at 17:31 | comment | added | FlatterMann | @safesphere Frames have nothing to do with the definition of energy. If you have energy, then you can perform work, if you don't, then you can't. Like I said, energy is probably the wrong observable, but you have to come up with a better one. Complaining about frames makes no difference about the facts on the ground. | |
| Jun 17, 2023 at 16:11 | comment | added | safesphere | @FlatterMann Energy is frame dependent. The energy conservation law holds when the energy before and after is measured in the same frame. When you properly do this, there is no Doppler shift. In cosmology, as well as in gravity, this shift is merely the effect of measuring in different frames. Since energy is frame dependent, you get different measurements as expected. This doesn’t violate energy conservation. It is like to say that a car moving at 100 mph has no energy to hit another car moving with the same speed in the same direction. True, but this doesn’t violate energy conservation. | |
| Jun 17, 2023 at 1:00 | comment | added | FlatterMann | @safesphere The definition of energy is the ability of one system to perform work on another system. If you want to keep that definition, then the Doppler shift does decrease the ability of distant systems to perform work on each other. There is no way around that. You could argue that energy is the wrong quantity to keep our eyes on in the cosmological context, but then please give us a new and better observable than energy, first. | |
| Apr 19, 2020 at 22:33 | comment | added | safesphere | (1) "every photon's wavelength increases" - It does not. In the frame of the emitter, the photon doesn't redshift. In the frame of the receiver, the photon is emitted already redshifted and doesn't redshift in flight. (2) "The cosmological constant has a constant energy density" - There is no cosmological constant. It is not required by the equivalence principle. The sole purpose of this constant is to save the Friedmann model, but this model has failed anyway. (3) "during inflation, the total energy grew exponentially" - There is no evidence for inflation. It is only a speculation. | |
| Aug 7, 2018 at 11:54 | history | edited | Nat | CC BY-SA 4.0 |
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| S Oct 12, 2017 at 13:58 | history | suggested | Janosh | CC BY-SA 3.0 |
Made self-promotion obvious, clarified abbreviation, added math delimiters, fixed typos
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| Oct 12, 2017 at 12:53 | review | Suggested edits | |||
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| S Feb 23, 2012 at 22:27 | history | suggested | CommunityBot | CC BY-SA 3.0 |
improved formatting, external links
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| Feb 23, 2012 at 22:19 | review | Suggested edits | |||
| S Feb 23, 2012 at 22:27 | |||||
| Jan 14, 2011 at 14:17 | history | edited | Luboš Motl | CC BY-SA 2.5 |
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| Jan 14, 2011 at 9:18 | history | answered | Luboš Motl | CC BY-SA 2.5 |