Is Gravity Time’s Archer?

http://fqxi.org/community/articles/display/200

by dnetesn • 11 years ago

64 24 11 years ago

Is there a physicist here that can provide a better description of what's going on? How can you have shape "dynamics" without time evolution?

russdill• 11 years ago

I'm really not that well versed, but disorder and complexity seem to be used interchangeably which is incorrect:

"According to this fundamental law, entropy - and with it disorder - should always increase with time. This is manifestly not happening in the universe"

I'm completely baffled that they'd put that in their abstract or what they could possibly be thinking. In what way is entropy and disorder not increasing? And I think here is where they are confusing disorder and complexity. For a given system, as entropy initially increases, so does complexity, and then at some point, complexity peaks and falls.

A system with very low entropy is not complex. It is very easy to describe. Additionally, a system with really high entropy is also not complex, it is also very easy to describe.

http://www.scottaaronson.com/blog/?p=762

z-cam• 11 years ago

Both Becker and Aaronson arrive at an interesting puzzle

So the real question is not why the entropy is increasing, but why it was ever low to begin with

With regards to SapphireSun's question I am not a physicist either, but this seems to be a good place to start down the shape dynamics rabbit hole: https://www.youtube.com/watch?v=kwqkdvKHTlg

mromanuk• 11 years ago

Imagine that you could see your life in rewind: You would slide into bed each morning with the ring of your alarm clock. At bedtime, you would gently drift awake. In the evening, you would un-cook your dinner, transforming a plate of Tuesday spaghetti into raw noodles and cold water. Your morning coffee would spontaneously warm in its mug as it sat on the countertop, the steam drawing out of the air and back into your cup. Another day begins, and you’re younger than the day before.

Needless to say, this never happens in our universe.

Thank you.

Udo6• 11 years ago

The problem with this view is causality. Since things happening as a result of other things is pretty much the definition of "time moving forward", there is no reasonable definition of time moving backwards that still includes causality.

In other words, the "backwards" world he described is exactly the world we live in, since there would be no measurable way to tell the difference. A world where you're getting younger but can only remember the things that happened in the future is pretty much the same as a world where you're getting older but can only remember things that happened in the past.

meatysnapper• 11 years ago

We can now enjoy hangovers, because we know it guarantees that a tremendously good evening will ensue!

cshimmin• 11 years ago

things happening as a result of other things is pretty much the definition of "time moving forward"

No, that's the definition of causality. Just because you are used to thinking of casual relationships in the context of forward-moving time, doesn't mean that time's arrow has anything to do with it. That's kind of a meta correlation/causation argument.

In physics causality is more appropriately defined in terms of the propagation of forces/information. E.g. if you are farther than a distance c*t away from me, we cannot be causally connected because no signal moving at the speed of light could propagate (either forwards, or backwards in time) to connect our worldlines.

mkempe4• 11 years ago

Time, like space, is a relation. Causality indicates that things act according to their nature. This is how relations change. Change, what we commonly think of as the passage of time, is the inevitable unfolding of an entity's potential to actuality, acting according to its identity.

How would change switch direction, actuality revert to potential? would change still be change? I suggest reading Heraclitus, Parmenides, and Aristotle.

With eternal, metaphysical love.

postnihilism• 11 years ago

There is an entertaining book by PKD called Counter-Clock world with exactly this premise. The mores around eating and digestion shift drastically...

[https://en.wikipedia.org/wiki/Counter-Clock_World]

mmorris• 11 years ago

This reminds me a lot of one of my favorite parts in any book:

It was a movie about American bombers in the Second World War and the gallant men who flew them. Seen backwards by Billy, the story went like this: American planes, full of holes and wounded men and corpses took off backwards from an airfield in England. Over France a few German fighter planes flew at them backwards, sucked bullets and shell fragments from some of the planes and crewmen. They did the same for wrecked American bombers on the ground, and those planes flew up backwards to join the formation.

The formation flew backwards over a German city that was in flames. The bombers opened their bomb bay doors, exerted a miraculous magnetism which shrunk the fires, gathered them into cylindrical steel containers, and lifted the containers into the bellies of the planes. The containers were stored neatly in racks. The Germans below had miraculous devices of their own, which were long steel tubes. They used them to suck more fragments from the crewmen and planes. But there were still a few wounded Americans, though, and some of the bombers were in bad repair. Over France, though, German fighters came up again, made everything and everybody good as new. When the bombers got back to their base, the steel cylinders were taken from the racks and shipped back to the United States of America, where factories were operating night and day, dismantling the cylinders, separating the dangerous contents into minerals. Touchingly, it was mainly women who did this work. The minerals were then shipped to specialists in remote areas. It was their business to put them into the ground, to hide them cleverly so they would never hurt anybody ever again.

The American fliers turned in their uniforms, became high school kids. And Hitler turned into a baby, Billy Pilgrim supposed. That wasn't in the movie. Billy was extrapolating. Everybody turned into a baby, and all humanity, without exception, conspired biologically to produce two perfect people named Adam and Eve, he supposed.

(Slaughterhouse Five by Kurt Vonnegut)

snowwrestler• 11 years ago

From this article:

In this view, gravity drives complexity, and complexity draws the bow on the arrow of time.

Or, does complexity cause gravity?

http://www.technologyreview.com/view/418192/gravity-emerges-...

One of the hottest new ideas in physics is that gravity is an emergent phenomena; that it somehow arises from the complex interaction of simpler things.

trhway• 11 years ago

well, giving that gravity, Lagrangian "minimum action", Schrodinger, etc... all basically describe the evolution of a system exactly along the local gradient of entropy increase one can say that all those are just emergent phenomena :)

dpflan• 11 years ago

The article title reminded me of some Star Trek: The Next Generation episodes, so for fun: 1. Time's Arrow: Part 1 - http://www.imdb.com/title/tt0708828/ 2. Time's Arrow: Part 2 - http://www.imdb.com/title/tt0708829/

Strilanc• 11 years ago

But there is a problem with identifying entropy as the driving force behind the arrow of time. For this to be true, the universe must have started out in a very low entropy state. The puzzle is: why?

It's interesting that simplicity is the goal everywhere else in science, but it's bad when it comes to the initial state of the universe. After all, having a lot of neg-entropy is just another way of saying "takes very little information to describe". The fact that the initial state is surprisingly simple to us is oxymoronic in a way.

Is Gravity Time’s Archer?

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