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14 of 16 people found the following review helpful
4.0 out of 5 stars Time, Entropy, and all that, May 3, 2011
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This review is from: From Eternity to Here: The Quest for the Ultimate Theory of Time (Hardcover)
Roughly the first 3/4 of the book is quite standard physics, and a few related fields, e.g. information theory. The last few chapters, where the author, Sean Carroll, suggests a possible answer to the puzzle, are much more speculative, something he makes very clear.

To me the book was quite interesting. A few equations are displayed, but there is no actual use of mathematics. I have an M.S. in Applied Physics, so I cannot really say how a reader with no technical background would cope with it. Carroll goes through a lot of material, and the sheer quantity of it might be overwhelming. Unfortunately, that is just the way things are. Nobody is going to cope with this without the willingness to do some hard thinking. Carroll does include a lot of pop culture references that readers can relate to, although one of those may not be in any future edition of the book.

A couple interesting (to me) notes:

The complexity of the universe is different from the entropy. Just after the Big Bang the universe was very simple--the same high energy subatomic soup every where. Right now the universe is very complicated: There are lots of galaxies, stars, planets, black holes, people, etc. However, the entropy of the universe has increased: The formation of all those objects is mostly due to gravitation, as matter coalesces together. This gravitational process increases the total entropy, more than offsetting the order in all the structure. Eventually all of this structure will fade away. Even black holes will decay by the Hawking process, leaving a very thin, cold, dark, and simple universe. So while the universe started in a simple state, evolved into a complex state, and will eventually decay into another simple state, the entropy is always increasing. See pages 199-201.

Long ago, as an undergraduate at Carleton, one of my professors talked about the total energy of the universe. The gravitational potential energy V between two bodies decreases as they approach, because gravitation is attractive. For computational purposes we usally set V = 0 when the distance between them is infinite, and he argued that this is the natural thing to do. Then the gravitational energy is always negative. Assuming a finite universe, you can add up all the positive energy of mass, kinetic energy, etc. and then offset it by the negative gravitational energy. Professor Titus suggested the total energy of the universe would be zero. Carroll mentions in passing that you can prove this in general relativity. See p. 358.

Now the universe appears to be infinite (this was not so clear back in 1972), so strictly speaking you cannot speak about its total energy. But the general concept still applies: Gravitational energy is still negative. Something can be created from nothing, if the something is offset by sufficient gravitational energy.

The whole book reminded me of what Sir Arthur Eddington wrote:

"The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations--then so much the worse for Maxwell's equations. If it is found to be contradicted by observation--well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."

On a lighter note, all of this talk about time and the universe also made me think of Severn Darden's
_Metaphysics Lecture_, which begins:

"Now, why, you will ask me, have I chosen to speak on the Universe rather than some other topic. Well, it's very simple, heh. There isn't anything else!"
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Showing 1-2 of 2 posts in this discussion
Initial post: Nov 20, 2011 3:00:22 AM PST
Feisty says:
I've not read the book, and I've no degrees in anything. But as an arm-chair scientist I think your review was top-notch! (corniness intended!)

Posted on Jun 13, 2015 6:06:28 AM PDT
If the expansion of space-time creates the mass we see (Stephen Hawking also said in his popular book that negative gravitational energy cancels all the positive mass energy that creates the gravity), then maybe the entropy per volume, adjusted for Hubble volume expansion, is not changing. The high energy particles were simpler in the past but in a smaller volume. This "intensive entropy" rather than the traditional "extensive entropy" makes even more sense if the expanding universe is infinite. So the development of time could be based more on the dual creation of mass and gravity at the same time. But if all energy cancels, was there zero energy in the big bang? By the way, if you use Einstein's (App 2 in "Relativity") meters=i*c*seconds conversion factor to express all lengths as i*time units or vice versa (time=length/i) then E=m*c^2 becomes E= -m*c^2. Energy is the negative of mass, and would have units of -1/seconds^2. Mass would have units of 1/seconds^2. So simply taking relativity seriously in your units gets rid of meters, Joules, and kilograms as distinct concepts. I believe it's squared because we have 6 layers of neurons in the brain which allows us to compress reality into 6 degrees of freedom which gives us the impression that reality has 3 dimensions of space, the only source of integers in physics. This in turn gives rise to perceiving integers in spin concepts the way we do. If we had 10 layers in the cortex, we would not think about energy and mass but quantities consisting of 1/seconds^3 or Einstein's equation would be E'=-i*m'*c^3. The "-i" because c^3 does not "get rid" of "i" like i^2 results in -1. The E' and m' are not the type of mass and energy that we currently see. This would make all mass at different velocities appear as distinct stationary objects, and all accelerations appears as velocities. 3D space is real to a 6-layered brain. 2-layered brains processing the information for the eyes of insects living on the ground is sufficient. The eyes are in a 2D world. But the antennae feelers and 2 front legs of ants are used to grab and turn objects and use 6 layered brains, keeping only 2 layers for their eyes. heir very close cousins the wasps have 6 layers for their eyes because they can fly. I do not know about ant drones, which should be good fodder for a PhD. The "Holographic Universe" goes the opposite way of 3D space, showing you that reality can be expressed equally well or better than 3D equations by using a projection on a 2D surface that surrounds a volume, rather than worrying about the details inside the volume. It seems like reducing the compression of reality down to 1D space and 1D time is the most logical choice. Quantum probabilities creating multiverses seems to another dimension. As I mentioned above, these would not need any creation of net energy or entropy.
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