Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Time’s Almost Reversible Arrow

quantamagazine.org

Post reply on HN
Print view View original

41–50 of 82 posts

Re: Time’s Almost Reversible Arrow

#41

Post by Strilanc »

mrfusion wrote: post #8
koverstreet wrote: post #6

Earlier quoted context omitted.

No. Consider what it would look like if you ran the solar system in reverse - all the planets and moons would go backwards in their orbits, but gravity still works exactly the same. The equations of mechanics don't care about time. And yes, information being lost in black holes would be an issue, if true - that's why that problem has been studied so much recently.

But if gravity isn't reversed how would planet formation be reversed? What would allow the matter in a planet to fly apart?

Gravity gets double-negated when you reverse time. Its units are over seconds squared instead of just seconds.

A simple way to see this is to imagine taking a movie of a heavy back arcing upward through the air then back down. What do you see when you run the movie backwards? A ball arcing upward them downward. The acceleration still points in the same direction: down.

Re: Time’s Almost Reversible Arrow

#42

Post by charlieflowers »

blaze33 wrote: post #12
maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

As I understand it, the universe actually started in a heat-death state (very small temperature differences, homogeneous and isotropic), it's just that it is a very hot heat death state, not the usual cold one where everything freezes. Then it is the universe expansion that made the entropy per volume decrease, actually allowing complex systems to evolve. The expansion still has to occur faster than the entropy incre…

Or whole universe was in a hot dense state. Then, nearly 14 million years ago, expansion started. Wait ... The earth began to cool.

Re: Time’s Almost Reversible Arrow

#43

Post by nickpeterson »

This is an honest question, how are we so certain time exists? Does anyone have a few quick references to books or articles that would explain it to the layman?

I always found the concept of time completely artificial, created by observation of cyclical systems.

Re: Time’s Almost Reversible Arrow

#44

Post by vardump »

TeMPOraL wrote: post #31
roberto wrote: post #19

Earlier quoted context omitted.

> Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic. This one thing I don't understand about the 2nd law of thermodynamics: given enough time won't the system go back to a more organized state simply by chance? And in that case, wouldn't the total entropy be reduced?

As others said, your understanding is fine - you just need to gain the appreciation to how much more unordered states are there for a system to be in than the ordered ones. My physics professor used to underscore it by drawing diagrams that show a line that suddenly goes from almost 0 to "the tip of this line is somewhere in the next galaxy". Or to give you a somewhat similar problem - consider a 32x32px 8bit image (…

> There are 6.02 x 10²³ atoms in 12 grams of carbon. I.e. in a tip of your pencil.

Pretty big pencil.

Re: Time’s Almost Reversible Arrow

#45

Post by guscost »

tbabb wrote: post #39
Houshalter wrote: post #38

Earlier quoted context omitted.

How do you know it's truly random though? Couldn't it be the result of a pseudo random process? Or universe splitting, where two universes are created, one where a coin lands head, and one where it lands tails. To the people inside, it seems random, but it's actually deterministic.

Pseudorandom means the choice is deterministically chosen by according to the state of hidden variables. Bell's theorem says there are no hidden variables that completely explain the predictions of quantum mechanics. A many-worlds interpretation (which is consistent with QM) is basically the same as saying the result is random, because you can't predict exactly which branch any particular interaction will follow.

No local hidden variables. Hugely important distinction.

Re: Time’s Almost Reversible Arrow

#46

Post by trhway »

maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

>The real mystery, for me, is: Why was the initial state of our universe so completely non-chaotic? Why did the universe not start out in a state that is essentially equal to the heat-death [1]?

taken as a closed system, our Universe's initial state was a heat-death and it would stay here, nothing would happen. Fortunately or unfortunately our Universe is embedded into previous very old and cool Universe, and this gradient is what drives the expansion of our Universe and thus provides for all the things we see happening to happen, including life (which is just entropy maximizing process in some conditions).

100-200+B year later our Universe will too be very cool and expanded very "thin". The matter will be collected into several hyper-super-black holes with Schwarzschild radii of tens or even hundreds o billions of light years - some probably bigger than our visible Universe (which has 10B light years Schwartzild radius). The matter inside a regular black hole is neutrons being driven into higher and higher states as result of mass being added into the black hole. At some point the neutrons get to split into gluons/quarks and those would continue to be driven into more energetic states as more mass is added, and may be split as a result into something else too. At the same time the space around becomes "thinner" and cooler. At some point, when even more mass is added, and thus this "soup" becomes even more energetic, beyond currently known physics unfortunately, something happens to the gravitational force what has been keeping it together and it explodes and start expanding (as there is extremely high gradient between the "soup" and the space of the old Universe - thus that will be a birth of new Universe inside our old Universe, inside the parent Universe of our Universe ... ad infinitum ... with non-zero gradient, though smaller and smaller as parent-child Universe pair is getting older...

Re: Time’s Almost Reversible Arrow

#47

Post by Nadya »

prewett wrote: post #36
TeMPOraL wrote: post #31

Earlier quoted context omitted.

As others said, your understanding is fine - you just need to gain the appreciation to how much more unordered states are there for a system to be in than the ordered ones. My physics professor used to underscore it by drawing diagrams that show a line that suddenly goes from almost 0 to "the tip of this line is somewhere in the next galaxy". Or to give you a somewhat similar problem - consider a 32x32px 8bit image (…

The Digital Library of Babel ( https://libraryofbabel.info/ ) was posted here a while ago, which is a tangible way to search through this space. The library comes from a book by Jorge Luis Borges, which has all possible books, meaningful and not. The people in the book are searching for books with meaning, but the chance of finding one is really small. The digital library lets you do the search electronically. Kind o…

It also has a link to the Universal Slideshow. Which let's one browse every image (of a given size) that represents what TeMPOral was talking about.

Ever since I discovered that site I've been slightly obsessed with it. In the "This is really interesting" sort of way. It makes one think how everything is noise. We just happen to find meaning in some of it.

Re: Time’s Almost Reversible Arrow

#48

Post by pdkl95 »

roberto wrote: post #19
maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

> Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic. This one thing I don't understand about the 2nd law of thermodynamics: given enough time won't the system go back to a more organized state simply by chance? And in that case, wouldn't the total entropy be reduced?

As entropy is usually a probabilistic concept, a "more organized" future is very very unlikely, but not impossible. Numberphile has a video[1] that discusses the Poincaré recurrence[2] time for the universe. So while it may be possible, you probably have top wait about 10^10^10^10^2.8 "Plank times, millenia, or whatever"[3] for a state to occur.

[1] http://www.numberphile.com/videos/longest_time.html

[2] https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theor...

[3] The units used in the associated paper. With a number that large, it doesn't really matter what unit of time you use.

Re: Time’s Almost Reversible Arrow

#49

Post by javajosh »

vardump wrote: post #44
TeMPOraL wrote: post #31

Earlier quoted context omitted.

As others said, your understanding is fine - you just need to gain the appreciation to how much more unordered states are there for a system to be in than the ordered ones. My physics professor used to underscore it by drawing diagrams that show a line that suddenly goes from almost 0 to "the tip of this line is somewhere in the next galaxy". Or to give you a somewhat similar problem - consider a 32x32px 8bit image (…

> There are 6.02 x 10²³ atoms in 12 grams of carbon. I.e. in a tip of your pencil. Pretty big pencil.

we're dealing with big numbers so an order of magnitude or two doesn't matter much.

Re: Time’s Almost Reversible Arrow

#50

Post by d4rkph1b3r »

darkmighty wrote: post #29
maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

Those questions are very old. You may think the anthropic principle is sufficient here: complex systems seem to rely on an entropy gradient (no life exists in an uniformly chaotic or static state). Ludwig Boltzmann made this argument in the 19th century [1]. However, the argument can apparently be extended to the conclusion that, in a probabilistic sense, your consciousness is just a sliver of a random fluctuation of…

>Feynman argued[2] that this shows we do not arise from a random fluctuation, because (even though we're conscious), when we look far away the universe looks highly organized.

Oh that is an interesting and compelling argument I have not heard! Thank you for sharing.

Post reply on HN