
Gravity Might Explain Why Time Never Runs Backward - ghosh
http://www.wired.com/2014/11/time-gravity/
======
FiatLuxDave
I have read the original PRL paper, and I think that the significant issue
with it is that it conflates the complexity (C5 in the paper, a measure of the
clustering of the gravitational objects) with entropy. I don't think that this
leap is warranted.

Complexity, as defined by the authors, is primarily a spatial dispersion
measure without a corresponding measure of energy dispersion. As far as I can
tell, this system should be subject to Liouville's theorem, which means that
the apparent entropy decrease (the decrease in C5) that occurs when the system
contracts is just hiding the entropy in the phase space of the particle
velocities. So, as the particles get closer together, their velocities get
farther apart.

This same kind of effect occurs in particle beams when you try to squeeze the
particle beam tighter. Assuming you aren't using some kind of beam cooling
(like adding cold electrons, or using stochastic cooling), every time the beam
gets squeezed, the beam gets tighter but the phase spread in the particles
increases. When the beam spreads out again, the phase spread usually goes back
down. It can get as low as its original value, but no lower. Just like entropy
(cause that's what it really is).

That's just my thoughts after a brief perusal. Feel free to point out any
obvious errors in my logic.

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beefman
A better article[1] including a link to the paper[2]

[1]
[http://physics.aps.org/articles/v7/111](http://physics.aps.org/articles/v7/111)

[2] [http://physics.aps.org/featured-article-
pdf/10.1103/PhysRevL...](http://physics.aps.org/featured-article-
pdf/10.1103/PhysRevLett.113.181101)

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Strilanc
The article is poor.

I think the research is another take on how finite reversible systems end up
cycling. Their entropy _has_ to eventually decrease because as a path through
the phase space runs out of places to go it must return to the initial state
to avoid getting stuck (i.e. violating reversibility).

I'm guessing that the interesting new thing here is that the researchers
showed systems following gravity have this entropy-goes-up-and-down property,
despite being an infinite continuous phase space where the bits could just get
more and more dispersed?

Of course in the actual universe there's other forces at work and they need to
be accounted for. You can't just say "gravity creates low entropy times
therefore we have explained the low entropy", because things like the
accelerating expansion of space have to be accounted for. You also need to
account for _how incredibly low_ entropy was near the big bang: it wasn't just
a single galaxy's worth of stuff converging, which would have been sufficient
and far far more likely, it was billions of times more.

Sean Carroll frequently talks about this issue in has talks and on his blog.
Hopefully I didn't get the details completely wrong.

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kazinator
This is completely wrongheaded nonsense. Time is only an abstraction related
to change. Even for unrelated events which happen in the same point in space
(such as the arrival of signals from different sources), we can tell whether
they happen at the same time, or else that one happens before the other. This
is because we have some clock running: a moving machine, and positions of its
hands on its dial are interpreted as "sooner" or "later". As to why the clock
doesn't run backwards, that can easily be explained using classical mechanics.

Some time-keeping machines do in fact reverse their motion. For instance an
ideal pendulum, which can serve as a clock, traces a path that perfectly
reverses itself every period. Looking at just the pendulum, we cannot tell
whether the pendulum has reversed direction due to the restoring force, or
whether suddenly time has started flowing backwards: the motion is symmetric.
This is irrelevant, because we interpret the pendulum's motion as a parametric
curve. The abstract t parameter of that parametric curve marches forward.

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gtrubetskoy
I seem to remember from watching a Sean Carroll talk (can't remember which one
now - could be this:
[https://www.youtube.com/watch?v=ZbPFrzliZJY](https://www.youtube.com/watch?v=ZbPFrzliZJY)
\- very entertaining), but he's sort of an expert in this subject, that the
ever increasing entropy is a phenomenon not a law of physics, i.e. there is no
underlying reason why the entropy couldn't be ever decreasing, and that
possibly there was a "time" when it was the case when the universe was
collapsing, which led to the big bang. If that's true, then in that period
time was essentially running backwards.

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dustingetz
" _simulated the universe as a collection of 1,000 particles that interact
with one another only by gravity, representing the galaxies and stars that
float around the cosmos. The researchers found that regardless of starting
positions and velocities, at some point the particles inevitably find
themselves clustered together in a ball before dispersing again_ "

Wait, what? That conclusion doesn't make any sense to me.

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PeterWhittaker
The most interesting notion is that of using complexity, a dimensionless
metric that can be measured from within, instead of entropy, which can only be
estimated from within, can only be measured from without.

Since our universe doesn't have a "without", we can estimate entropy. But we
can measure complexity, as it used in this article.

Intriguing. Reformulating the Laws of Thermodynamics could make for a
promising avenue of research.

------
OrwellianChild
I fail to understand how this article (and the research that underlies it)
have shown anything more sophisticated than "interacting particles
interact"... Can someone help me understand the novelty?

~~~
Manishearth
As far as I can tell _, it 's not a novelty, there has been other similar
research. Gravity -> certain types of interaction theories -> restrictions of
entropy -> time. That connection has been condensed into "gravity causes
time". Bleh.

The article is making sweeping statements, as usual.

_Physics undergrad here, done some advanced courses but not an expert on this
stuff

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nsxwolf
I thought the non-existence of time explained why time never runs backward.

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swframe
Why wouldn't time move backwards for the entire universe? If it does, there
can be no record of any future event and there is no way to determine that
time is moving backwards. If time moved backwards in one spot, what about all
the information connected to that spot. Why wouldn't that information rollback
too?

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trhway
>Instead of using entropy, the researchers describe their system with a
quantity they call complexity, which they define as roughly the ratio of the
distance between the two particles farthest from each other to the distance
between the two particles closest to each other. When the particles are
clumped together, complexity is at its lowest.

When the particles are clumped together, the entropy is highest. Gravitation
pulls in the direction of entropy increase. For a system of many objects
spread around some volume their "complexity" is just kind of "parallel" to
entropy, not really "instead". Thus it isn't surprising that they get the
first half - clumping - somewhat right. The second part - bouncing back - is
pointless to discuss in the framework of their model because such important
factors as, for example, space inflation (vacuum "thinning") were omitted from
the model while that inflation is the key for enabling new Big Bangs in the
old/inflated/cooled down Universe which thus becomes proto-Universe for the
new ones.

------
randallsquared
I am pretty confused by this article. In order to simulate gravity, you have
to have already decided on a direction of time, right? You'd know if time was
running backwards because gravity would push things apart rather than pull
them together.

~~~
Chinjut
Although counter-intuitive, reversing time doesn't change the rules of
gravity: the basic law of gravity just says that objects accelerate in a
certain way dependent on their masses and distances. Because acceleration is
the second derivative of position with respect to time, it is doubly negated,
and thus unaffected, by reversing time.

Reversing a planet's orbit produces another orbit just as valid under the law
of gravity. And even at the scale of ordinary experience, gravity doesn't
prohibit objects from rising (after all, in the canonical example of tossing a
ball through a parabolic arc, there is both a rise and a fall); footage of a
ball falling under the influence of gravity reverses into footage of a ball
rising under the influence of gravity and vice versa, but never does footage
compatible with the law of gravity reverse into footage incompatible with the
law of gravity.

~~~
Roboprog
True enough in free fall in a vacuum. (if one could find such a perfect
vacuum)

Apples don't usually leap off the ground into trees, though. Of course, that
would be more of a heat to kinetic energy conversion to start that process,
but I think that's more along the lines of what the original question really
meant: why don't things spontaneously launch, rather than having a one way
tendency to convert potential energy to kinetic energy to heat energy.

It's hard to see gravity act in isolation: trace particles eventually slow an
orbit in the same way that air friction deforms an idealized parabola/elipse
by dragging on a ballistic projectile; tidal forces between earth and moon
alter the moon's orbit.

~~~
Chinjut
Sure. As you note, an apple would leap into the trees if the ground under it
conspired to push it upwards. That the ground rarely does so (but the reverse,
an apple falling and dispersing energy into the ground, happens frequently) is
certainly an observation which merits explanation, but the explanation doesn't
come from an asymmetry in the laws of gravity, as such. It's often said the
explanation is found in something like the Second Law of Thermodynamics, which
in turn is explained by the fact that at some moment, there was a state of
extremely low entropy (the Big Bang); the "arrow of time" is then a
manifestation of being to one or the other side of this particular low entropy
moment. And the work discussed in the link (which I have no direct knowledge
of) purports to show why some such low entropy moment should be expected to
exist at all given the action of gravity over history.

But my point was simply to note that you don't have to pre-incorporate an
arrow of time to make sense of the laws of gravity, in response to the poster
who found this a sticking point and expressed confusion over it.

------
rawland
Actually there are several phenomena and theories which give the arrow of time
a certain direction. See:

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

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AnimalMuppet
I know it's Wired's headline, not HN's, but let's re-write it for accuracy,
shall we?

"How Gravity _Might_ Explain Why Time Never Runs Backwards (On This Side of
the Big Bang)".

~~~
dang
Ok, we added your "might" and took out the baity "how".

~~~
AnimalMuppet
Oh, wow. Cool. I wasn't actually asking for moderators to change the headline;
I was just whining...

~~~
dang
That was high-quality whining. I keep trying to tell people that the best way
to complain about a title is to suggest a better one.

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yegor256a
I wonder how much tax-payers' dollars they've spent to do this research :)

~~~
drjesusphd
It's fairly easy to estimate. Assuming an average salary of about 50k between
a grad student, a postoc, and a PI (a typical mix for a scientific
publication), one only needs to ask how long it took them to do the work.
Estimating about 6 months with a 3x infrastructure multiplier, I get about
$225k.

This is of course giving you the benefit of the doubt that you were actually
wondering, and not passive-aggressively implying the government should not
fund basic scientific research. Which is understandable, because that's a
ridiculous position for an educated person to articulate explicitly.

