The direction of entropy

Apr 28, 2021
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Would the direction of entropy run in reverse in a contracting universe? To me it seems that the whole reason entropy is increasing is because our universe is expanding. The reason for this is because particles have more places to go than previously. In a contracting universe the opposite would be true. Also a contracting universe would lead to a singularity which has zero entropy.
 
Would the direction of entropy run in reverse in a contracting universe? To me it seems that the whole reason entropy is increasing is because our universe is expanding. The reason for this is because particles have more places to go than previously. In a contracting universe the opposite would be true. Also a contracting universe would lead to a singularity which has zero entropy.
Only when the universe is crunched into 100% energy, or just before, is there much of an argument for regaining entropy. I think more cosmologists would suggest full restoration would be unlikely, which is one reason against a cyclic universe. But physics can’t reach into this near zero time period. Energies after the first trillionth second are studied in the LHC, however.
 
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Entropy is the amount of heat, in a closed system, that is not available to do work. Work can only be done by moving heat from a hotter area to a cooler area. Once thoroughly mixed, there is no heat available to do work thus the entropy is at 100%. Whether the universe is expanding, static or contracting is irrelevant.
When the universe is at a singularity, entropy is undefined since heat cannot move since a singularity has but one dimension. Once the universe expanded to a finite size, quantum fluctuations gave some areas more heat then others and the amount of entropy became defined. It has increased ever since and will reach 100% when all matter has finally disappeared, which happens somewhere around 10^100 years.
 
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It is hard to get a wholistic definition of "entropy" that can be used in all possible states of the universe for cosmology. In general, I don't see any reason to expect something like a gravitational collapse of the matter in the universe to make entropy decrease by some manner like "time running backwards" during the collapse. But, once you get inside an event horizon of a black hole, there can be some disputes about how to decide if some of the potential mathematical solutions are real or not, including the direction of time. So, I am not going to speculate about that. And, I also note that gravitational collapse in astronomy is one way to convert matter into energy via fusion. So, the diffuse matter, theorized to have been made diffuse by the expansion of space itself, doe not need much non-uniformity to eventually lead to gravitational collapse. Whether all matter can be converted to energy in some ultimate form of collapse, such as the state theorized for the initial moments of the Big Bang, is not clear either. Assuming that all matter can be converted to energy, and that it can then somehow rebound into space or make space itself expand, the issue is whether that could be so uniform that it would not create any differences in energy distribution that could then be used to "do work". I don't think that the collapse would necessarily be so uniform that the following rebound must be uniform, even if the minimum size is greater than the Plank size. We at least theorize about "isentropic" processes where there is no loss in process that turns useable forms of energy into unusable forms, such as friction turning motion energy into heat energy. It is not clear to me that the sum total of all the processes happening in the universe cannot be isentropic. But, somebody had probably claimed that it is not. However, with the BBT theorizing that what we are able to observe makes up only about 5% of all the energy and matter in the universe, it seems to me that any proof about how that is affected by the law of entropy could be missing a lot about the effects of dark matter and dark energy on total entropy.
 
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If the apparent expansion of the universe from any local point is never ending, toward infinity, the apparent energy expansion of the universe from any local point is never ending, toward infinity, and the equivalent mass expansion of the universe from any local point is never ending, toward infinity. The view is timelessly entropic to the maximum of entropy like infinite layers, levels, of chaos will reduce, flatten out, to being timelessly entropic to the maximum of entropy.
 
Apr 28, 2021
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But isn’t the increase in entropy in our universe linked to the expansion of our universe? At the beginning of the universe there was more energy in a smaller amount of space and therefore had lower entropy. If the universe continues on its current course we will get to the big freeze which would have minimal energy in a larger space and will have maximum entropy. Dark energy increases hand in hand with entropy. Almost as if energy is being converted into spacetime. I mean if energy is never created or destroyed then where is the increase in dark energy coming from?
 
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Entropy is the amount of heat in a closed system that is not available to do work.
Say you have a tank with two one kg glasses of water sitting in it. One is at zero degrees C and one is at 100 degrees C. Define zero degrees as the base and the cold glass has no heat in it. The entire amount of heat in the hot glass, 100 kilocalories, is available to do work. Entropy of the system is zero.
Now mix them together and you have two kg of water at 50 degrees C. There is still 100 kilocalories but none of it is available to do work. Entropy is thus 100 kilocalories.
Entropy went up. In a closed system, eventually the two glasses would reach the same temperature. This no matter how well you try to insulate them. It is unavoidable. Heat flows from hot to cold and there is no way to stop it. Thus entropy always increases. To make it decrease you would have to violate the second law of thermodynamics which says heat flows from hot to cold. Another way would be to make time go backwards but this is not possible.

The universe is no different, it started with a certain amount of heat difference from place to place due to quantum effects. Eventually the entire universe will be at the same temperature and entropy will be at a maximum. There is no way to stop this from happening.

Dark energy is created as new volume of space is created by the expansion of space. This positive energy is a property of space and continually appears. It is exactly balanced out by the negative potential energy of the expansion of gravitationally bound galaxies thus the conservation of energy is maintained.
 
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Thanks for that last paragraph. Very informative. Surprised that isn’t more talked about to the general public. Can you explain a bit more about the negative potential energy?
So our expanding universe is going from hot to cold. So then a contracting universe it would go from cold to hot therefore entropy would be reversed?
 
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I don't understand the energy balance very well myself. I have read up on it and all I can say is that when two objects with mass move farther apart against their gravity this is considered to be gravitational potential energy and has a negative sign attached to it.

Going from hot to cold has nothing to do with whether the universe is expanding or contracting. Heat moves from hot regions to cold regions and this cannot be stopped. We can slow it down with insulation but not stop it. Over time, everything will be the same temperature. This is unavoidable. It is called the "Heat Death" of the universe.
 
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There is more to it than going from cold to hot. Entropy would not be reversed unless and until there was a larger difference between the hots and the almost as hots at some point in time. Not clear how that would happen.

On the other hand, Bill sort of glossed over the assumed reason that there was ever any difference in temperature (or other energy forms) in the materials that comprise the universe. Cosmologists like to think of the universe as being uniform (except for "small" differences like galaxies and voids), and assume that the universe started as a tiny uniform spec that "inflated" to billions of light years. The tiny spec was so small that "quantum fluctuation" created even tinier non-uniformities, which then got "inflated" to enormous dimensions, so that was how the energy got separated into different regions and could be used to "do work" as it tries to become uniformly distributed, again.

But, it really is much more difficult to think about when you realize that the BBT starts with pure energy and somehow there is spontaneous development of order in the form of 6 types of quarks and various other forms of "sub-atomic matter" coming into existence, which then organized themselves into atomic matter, which then underwent fusion in stars to produce more complex atoms, which then combined chemically to produce, ultimately, life and us.

Another way of describing entropy is to relate it to order and disorder. But, when you look at the spontaneous development of all of that order in matter, starting from pure energy, it certainly looks like entropy must have decreased with that definition.

So, once we get out of the physical conditions of our experience with glasses of hot and cold water, and start trying to use the theory that entropy must always increase in anything the universe ever did or could ever do in the future, it seems to me that we should not be ruling out anything on the basis that it would require a decrease in entropy.

In particular, I do not believe that entropy should be used to rule out the possibility of a cyclic universe. Clearly, if the universe could gravitationally "crunch" back into a tiny spec like the BBT theorizes its beginning, then quantum fluctuations should be able to restore whatever entropy condition that existed when it was a tiny spec experiencing those fluctuations in the Big Bang, and then bang back to a new universe with nothing by hydrogen and energy (plus whatever dark matter and dark energy are, if they really exist).
 
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Right, it is not clear how you could increase the difference between the hots and the colds once things have evened out. It would require us to violate the second law of thermodynamics or run time backwards.
I am not well versed on the beginning of the universe other than to know that quantum fluctuations were calculated to be so great as compared to the differences found today that inflation was introduced in order to even them out to match what we see.
A cyclic universe is at odds with our current observation of increased acceleration at the far reaches of the universe.
 
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I agree that we don't see any indications of a cycle in what we can observe. But, that doesn't mean there cannot be a cycle in progress, with only a tiny fraction of the cycle period being observable from here, for now.

One of the things that amuses me about the BBT is when people claim that the conservation of energy is not applicable, but then insist on never decreasing entropy and even insist on conserving "information". Conservation of energy is a much more fundamental law than conservation of information, and is actually implied in the law requiring the increase of entropy.
 
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The problem is that the outer edges of the universe are accelerating faster and faster away from us. What would make those objects start slowing down and eventually causing a Big Crunch?
 
Bill, I don't know what would make the universe's expansion stop and go into contraction.

But, we don't understand why it is expanding in the first place, so I am not going to just assume it must keep doing so. Just naming some unknown mechanism "dark energy" doesn't really tell us anything about how and why. So, I am just not willing to accept "always" without understanding the "how" and "why".

That is my main problem with the BBT. It just keeps extrapolating current observations, backward to a singularity, or at least to the dimension that we say we cannot understand anything smaller. Just because theorists think they have a description of the early events that seems to fit theories derive from quantum mechanics experiments, that is really no guarantee that the universe actually did that. I am open to other possibilities, even though many of the BBT proponents are not.
 
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It’s frustrating how often the extrapolations of BBT occur.

Perhaps there should be a BBTc, where the “c” stands for “core”. The “core” limits BBT to objective-based science. No wild event suppositions at t=0, or to any time less than where particle colliders can go. And it would limit, or ignore all together, the suppositions extending the cosmos’ conditions much beyond our current time. It’s clear no one knows the future behavior of DE.
 

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