
Time’s Almost Reversible Arrow - alberto_ol
https://www.quantamagazine.org/20160107-arrow-of-time-axions/
======
maho
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 result unmix itself again, even though the interactions
between the individual molecules are perfectly reversible.

In fact, you can build a toy-universe in your head, where all interactions are
time-reversible, and you would still observe that the chaotic-ness increases
with time, unless you start with a completely random initial state (in which
the chaoticness stays roughly constant). This increase in chaoticness is
comletely unsurprising: It's a simple consequence of the fact that there are
much, much more "states" in which a system looks chaotic, than states in which
it looks ordered. 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.

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]? The microscopic processes that are
not time-reversible might have something to do with this, but it's not clear
at all how they can solve this mystery.

[1]
[https://en.wikipedia.org/wiki/Heat_death_of_the_universe](https://en.wikipedia.org/wiki/Heat_death_of_the_universe)

~~~
roberto
> 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?

~~~
TeMPOraL
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 (a
standard Windows icon since version 3.0). The image consists of 1024 pixels,
each capable of representing a different color from the set of 256 colors. How
many possible pictures like this are there? Or, as you'd ask in
thermodynamics, in how many states such a system can be? It's 256 states per
pixel raised to 1024th power (just like 6-digit binary number has 2^6 possible
values). It's 256^1024. You know how much is it?

    
    
      10907481356194159294629842447337828624482641619962326924318327861897213318491192
      95216264234525201987223957291796157025273109870820177184063610979765077554799078
      90629884219298953860982522804820515969685161359163819677188654260932456012129055
      39018863010179002525357999172000100796000265358368009052978058809523505016301954
      75653911005312364560014847426035293551245843928918752768696279344088055617515694
      34994540667782514081490061610592025643850457801332649356583604724240738244281224
      51315177575191648992263657437224322773680750276278830452065017927617009456991684
      97257879683851737049996900961120515655050115561271491492515342105748966629547032
      78632150573082843022166497032439613863525162640951616800542762343599630892169144
      61811874063953106654048857394348328774281674074953709935118687563599703901170218
      23616749458620969857006263612082706715408157066575137281027022310927564910276759
      16052087830463241104936456875492096732298245918476342738379027244843801852697776
      49410727156115804346908274593399919614142427414105991174260605564837637563145276
      11362658628383368621157993638020878537675545336789915694234433955666315070087213
      53547025567031200413072549583450835743965382893607708097855057891296790735278005
      49356215610907958451729541159729274798775277385600082041185589300047777487277618
      53813510493840581861598652211605960308356405941821189714037868726219481498727603
      65361629885617482241303348543878532402475141941718301228107820972930353737280457
      43720952287036227763639452908698062584223551485075710396193874496298668081887696
      62815778153079393179093143648340761738581819563002994422790754955061288818308430
      07964869323217915876591803556521615711540299212027615560787310793747746684152836
      29877086994501520312318625942030856938389446570613462367042340268211029589549511
      97087076546186622796294536451620756509351018906023773821539532776208676978589731
      96633030889330466516943618507835064156833694453005143749131129883436726523859540
      49042734559287239495252271846174043678547546104743770197680255766058810380772707
      07717942221977090385438585844095492116099852538903974655703943973086090930596963
      36076752996493841459818570596375456149735582781362383328890630900428801732142480
      86639626713335280092327583508730596141187237814221014601986157473868550968960891
      89180441339558524822867541113212638793675567650340362970031930023397828465318547
      23824423202801518968966041882297600081543761065225427016359565087543385114712321
      4227266605403581781469090806576468950587661997186505665475715792896
    

This much. (Source: [0]).

Now ask yourself, how many of those pictures represent a letter "A". Quite a
lot probably, but _nowhere near that much_. In all those 256^1024 pictures,
you have every possible representable letter A, as well as any other possible
Unicode character. In those images are all your most cherished private photos
(or at least their thumbnails), and also the photos of all things that you'd
wish happened but didn't. A thumbnail of every possible photo of the universe
is there as well.

There's also something else. Something much more frequent than all other image
I've just mentioned taken together. It's the _noise_. The things we don't
recognize, the things we consider uninteresting. I ask you, use your intuition
- if you were to create a random Windows icon every second, how soon would you
expect to get one depicting a letter "A"?

And now realize we were talking about silly icons that are probably barely
visible on your screen.

There are 6.02 x 10²³ atoms in 12 grams of carbon. I.e. in a tip of your
pencil. 602000000000000000000000 atoms. Those are your pixels. And if you want
to compute the number of states this pile of atoms can be in, that is the
number that goes _in your exponent_. The base is some combination of possible
positions, orientations and velocities, and then probably something else I'm
forgetting right now. And _almost all_ of those states are noise. That's how
the Second Rule works.

[0] -
[http://www.wolframalpha.com/input/?i=256%5E1024](http://www.wolframalpha.com/input/?i=256%5E1024)

~~~
prewett
The Digital Library of Babel
([https://libraryofbabel.info/](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 of fun to play around with.

~~~
Nadya
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.

------
brudgers
_The modern history of T invariance begins in 1956_.

Or 1781, when _Critique of Pure Reason_ was published. Kant proposed that time
was an empirical fact based on experiencing events in time being a
precondition of all human experience. The tradeoff for a well reasoned non-
skeptical position was that we give up claims to "really really" know how
things really are independent of human experience...i.e we give up claims to
potential omniscience.

That we normally take "knowledge" to mean human knowledge, doesn't change that
this is what we mean. Or to put it another way, what would time be for a
computer with knowledge or as the article mentions for creatures that
experience time backward?

------
rubidium
Great read on some of the history and current efforts in particle physics by a
leader of the field (Frank Wilczek).

I was surprised to read such a well written article on quanta magazine (I've
often been disappointed there) but it made sense when I saw who the author
was. It read more like what might be published in a general audience Nature or
Science article.

------
djfm
I'm very surprised no one posted this yet:

[http://xkcd.com/1621/](http://xkcd.com/1621/)

------
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.

~~~
jberryman
"created by observation of cyclical systems". Can you elaborate on that? I'm
not sure what you mean, but it sounds interesting. It seems to me we believe
time "exists" because of causality; things that happen at "this moment" affect
things that happen "in the future" but not things that happen "in the past".
We've also predicted and measured certain weird things about time, such as how
it can go faster or slower.

~~~
nickpeterson
So, you could condense my knowledge of physics to an index card, but here we
go...time seems like it's always based on the measurement of a regular thing.
Say the oscillation of a quartz crystal. So we have no proof there of time,
just that certain phenomena can be very regular in intervals/cycles.

I've heard things like, "if you go the speed of light, or at least really
fast, time will move differently." But that seems like it would be a side
effect of that local system moving at a rate that interferes with its physical
interaction relative to what we consider normal.

There are probably tons of holes in that. But my point is that we tend to
think of time almost as an invisible "thing", and I really have trouble
wrapping my brain around that. It just feels like things happen and we call it
time, but really its ordered intervals of effects.

------
mrfusion
Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean
time can't run in reverse.

Also information might be lost in black holes. Wouldn't that also prohibit it
reversing time?

~~~
koverstreet
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.

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

~~~
danielsju6
You'd see radiation hitting the planet warming the mantel, complex chemicals
breaking down & releasing heat, all warming the planet significantly to the
point where it was a molten ball, and finally lots of instability where large
explosions were throwing pieces of it into space.

Basically take the movie and play it backwards.

~~~
bronson
I find it very hard to picture a molten ball throwing a giant chunk of cold
comet deep into space.

~~~
tbabb
That is what is so unsettling about T-symmetry. What you describe is merely
_improbable_ , not impossible. With appropriate starting conditions, it is
certain. Basically we'd require all the particles in the comet to bounce into
each other in such a way that their heat motion cancels and organizes into
momentum in one direction, away from the planet. This is physically permitted
in either time-direction, but we only see it in one.

------
mrfusion
What s the current thinking on being able to run Schrödinger's cat backwards?
How can time be reversed on something in an indeterminate state?

~~~
jsprogrammer
The indeterminacy may not be real. It's just that _you_ don't know the state.
I don't believe there is a rule that says you get to know everything.

~~~
tbabb
Are you proposing a "hidden state" which is unobservable? Such a formulation
is not consistent with quantum mechanics, as proven by Bell's theorem, unless
you permit instantaneous influence from arbitrarily distant parts of the
universe.

~~~
jsprogrammer
I'm saying that the construction "indeterminate" is only a statement about the
knowledge of the person making the statement. Are you proposing that the
universe doesn't act unless _you_ are considering it?

~~~
tbabb
[https://en.wikipedia.org/wiki/Bell's_theorem](https://en.wikipedia.org/wiki/Bell's_theorem)

Certain observations are probabilistic, and the correlations in them predicted
by quantum mechanics cannot be reproduced by ascribing them to the traits of
particles.

It means that at the lowest level, the universe is (barring faster-than-light
communication, generally regarded to be false) "truly random". (No concept of
an "observer" needs to be devised in order to arrive at this conclusion).

~~~
Houshalter
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.

~~~
tbabb
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.

~~~
guscost
No _local_ hidden variables. Hugely important distinction.

~~~
tbabb
Yes, see the bit in my original post and the one that follows about
instantaneous communication.

------
mrfusion
Wouldn't the expending of space prevent particles from being able to run
backwards in time? Unless you also ran the expansion backwards?

------
Houshalter
There is a hypothesis that universes with reversible time can't exist. Or time
travel of any kind. Such universes, the hypothesis goes, can't have any
meaningful cause and effect, like we know it. Causality is an extremely
important property of our universe. And it's really weird that our universe
has it, if the hypothesis isn't true. The space of possible universes without
causality is much larger than restricted subset that obeys that rule. This is
difficult to explain, so see here:
[http://lesswrong.com/lw/fok/causal_universes/](http://lesswrong.com/lw/fok/causal_universes/)

------
ilostmykeys
With React/Redux you can reverse time! :D

