I know that it fluctuates, but, what would be a good estimate for how much kinetic energy is in the universe right now? I know that things cancel it out, but, how much is there?
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That's way way beyond what we can reasonably possibly quantify. I believe some folks around here might throw in some good guesses. – TechDroid Mar 10 '19 at 19:10
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2Possible duplicate of Is the total energy of the universe zero? – Mar 10 '19 at 19:25
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There is no well-defined answer to this question, for the reasons given in the answers to the question that this one duplicates. – Mar 10 '19 at 19:25
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@BenCrowell Kinetic energy is different from, although related to, the total energy. – GiorgioP-DoomsdayClockIsAt-90 Mar 10 '19 at 23:01
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@BenCrowell: Not a duplicate. We have a natural frame of reference (CMB) with which to define kinetic energy at least for visible matter. With further assumptions (such as ΛCDM) this question could also be answered for the rest of matter. – A.V.S. Mar 11 '19 at 04:02
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Shouldn't be even not well defined what total kinetic energy in the obs. U. is? Please comment. – Alchimista Mar 11 '19 at 09:32
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Is this tally supposed to include the kinetic energy of the gravitational field or is it just kinetic energy of matter? – Qmechanic Mar 12 '19 at 17:48
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The kinetic energy of matter. – George Cowley Mar 12 '19 at 18:01
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@A.V.S.: We don't have global frames of reference in GR, only local ones, so that doesn't work. – Mar 12 '19 at 18:54
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1@BenCrowell: I fail to see your point. “Observable universe” is a local-ish thing and is quite compatible with the “frame of CMB” at least for the purpose of kinetic energy definition. – A.V.S. Mar 12 '19 at 19:15
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First of all, I see the Tag is general relativity. I only have a very basic understanding of the subject. So take that into consideration with regard to my comments below.
I agree with Ben Crowell and TechDroid. It's interesting, however, that you are asking specifically about kinetic energy, as opposed to total energy (the subject of the link provided by Ben). I don't see how one can quantify the total amount of energy, kinetic or otherwise. But here are some preliminary considerations, albeit from a classical physics perspective.
Before we can even think about quantifying the amount of kinetic energy in the universe, we would first have to agree on what forms of energy in the universe are kinetic energy. Is dark energy kinetic? Is electromagnetic energy kinetic? Is mass energy kinetic or potential energy before it is released. I see many different opinions on what forms of energy should be considered kinetic. And I don't profess to have those answers.
Then we would need to differentiate between macroscopic kinetic energy (rotational and translational kinetic energy of extended bodies) versus microscopic kinetic energy (internal energy of mass, typically associated with temperature).
In the case of macroscopic kinetic energy, the amount would depend on the frame of reference in which it is measured. I'm sitting in a train moving along a straight track at constant velocity. I have an earth globe on a desk. I spin it. In my reference frame it has rotational kinetic energy only. In the reference frame of someone on the tracks watching the globe go by through the train window, it has rotational and translational kinetic energy. You get the idea. So there really isn't any single value of macroscopic kinetic energy in the universe just as there is no absolute frame of reference.
In the case of microscopic kinetic energy, I suppose we could estimate the total mass of the universe, its heat capacity, and average temperature, and come up with an estimate. Ah, but then again, is dark energy kinetic? Without an answer to that, our calculation could be considerably off!
All just food for thought.
Hope this helps.
Bob D
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That would be my opinion as well- it seems to me it is energy "locked up" as mass. I'm also not so sure I understand what the "kinetic energy of the gravitational field" is. I suppose it is a GR concept because in classical mechanics the gravitational field is potential energy. GR seems to undermine the idea that all energy is either kinetic or potential. In any case, it is issues like these which makes it so hard to single out kinetic energy. By the way, I thought someone else posted an answer, but it seems its been removed. – Bob D Mar 12 '19 at 18:10