
Is Gravity Time’s Archer? - dnetesn
http://fqxi.org/community/articles/display/200
======
mromanuk
> 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.

~~~
Udo
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.

~~~
mkempe
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.

~~~
Udo
Those are not statements about physics though. They just employ some of the
lingo.

~~~
mkempe
Physics depends on metaphysics.

~~~
Udo
I really do hate to start a reply with a flat-out "no", but in this case it's
warranted, sorry to say. There are two ways of looking at this, and none of
them permit this conclusion:

Physics as a science does not depend on metaphysics in any way. When a
physicist does his/her work, it does not require the invocation of
supernatural principles. While speculation and hypotheses are essential to the
scientific process because they drive further exploration, metaphysical
arguments on the other hand are just _postulates_ about the nature of reality
- and they are in most cases either in direct conflict with observational
data, or designed to be un-disprovable.

Now for physics, as in the actual universe. This doesn't depend on metaphysics
either. In fact, it's the other way around. As the universe develops,
intelligent life emerges, humans in our case. These humans can then invent
religion and the supernatural. The physical world came first.

~~~
mkempe
Proper metaphysics is very limited and has nothing to do with the
supernatural. When physicists do their work they rely on such concepts as
existence, identity, causality, the relation of consciousness to these. That's
metaphysics.

To state that "the physical world came first" is to assert the primacy of
existence in opposition to the primacy of consciousness (e.g. Descartes) --
that's correct and that's metaphysics.

~~~
Udo
We went off on a tangent there, I suspect we've already bored everyone else in
this thread to death ;)

I want to close with the observation that whether you're aware of it or not,
you're talking about supernatural claims that have been postulated without any
scientific evidence.

To make this distinction between the words metaphysical and supernatural feels
a bit similar to fundamentalist Christians who assert that they're not
religious - a phenomenon which I suspect happens for the same reasons: because
they believe their faith represents the "truth" about the universe, it doesn't
count as a religion. But to everyone else, it still does.

~~~
mkempe
Are you claiming there is no evidence for the concept of causality? and that
any discussion of existence or identity is the province of faith, not reason?
If some people replace metaphysics with religous idiocy, that does not
invalidate the field as such -- the same applies to all fields of rational
inquiry, from physics to psychology.

I subscribe to the evidence of the senses, logic, and science. If you think
any discourse about such topics belongs to the supernatural and religious,
that's your problem, not mine.

------
snowwrestler
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-...](http://www.technologyreview.com/view/418192/gravity-emerges-from-
quantum-information-say-physicists/)

> 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
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 :)

------
Strilanc
> _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.

------
dpflan
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/](http://www.imdb.com/title/tt0708828/)
2\. Time's Arrow: Part 2 -
[http://www.imdb.com/title/tt0708829/](http://www.imdb.com/title/tt0708829/)

------
SapphireSun
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
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](http://www.scottaaronson.com/blog/?p=762)

~~~
z-cam
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](https://www.youtube.com/watch?v=kwqkdvKHTlg)

