
Thermodynamic Asymmetry in Time (2016) - lainon
https://plato.stanford.edu/entries/time-thermo/
======
SilasX
IMHO, Julian Barbour and Gary Drescher have satisfactorily resolved this:

Memories[1] can only form in the increasing entropy direction.

Or, to be more precise, higher entropy states can contain memories of lower
entropy states, but not vice versa. This remains true regardless of how the
memory is represented.

So the arrow of time (and second law) are not facts about the universe, but
about observers' ability to represent it.

Even if you wound back time past t=0 and kept the simulation going, you would
see the same thing in the reverse direction, such that the negative time
direction should now be considered futureward.

IOW, it's not that "time goes forward, entropy goes up, such coincidence".
It's "if entropy were not higher, we could not regard it as the future."

[1] what Barbour calls "time capsules" and Drescher calls "[metaphorical]
wakes"

~~~
hadsed
I don't know about this because you can create memories using only reversible
circuits, which will not increase the entropy of the system. you're more
resource-limited if forced to make it reversible (you have to store additional
information to make it so) but you can still do interesting things. I'm not
sure i buy that only irreversible processes constitute "the future".

~~~
SilasX
The "write readably to a storage medium" part of a(n otherwise) reversible
circuit will still be reversible.

~~~
SilasX
Oops, that should be "will still be irreversible".

------
timmaxw
Here's a web app that gives a concrete example of an asymmetry in time arising
from time-symmetric rules:

[http://timmaxwell.org/pages/cellular-
automaton/index.html](http://timmaxwell.org/pages/cellular-
automaton/index.html)

Each row of the grid represents the state of the universe at a certain "time".
Given any two adjacent rows, we can compute the row above them or the row
below them according to a predefined rule, which is symmetric in "time". So we
randomly generate the two initial rows and then from there derive the rest of
the pattern. However, by default the two initial rows are mostly empty,
corresponding to a low-entropy initial state. From this initial state,
applying the rule either forward or backward in time leads to states of higher
and higher entropy. So the system exhibits "arrows of time" flowing from the
low-entropy initial state forward and backwards in time to the high-entropy
equilibrium states.

------
remcob
> Using Popper 1956’s famous mechanical wave example as an analogy, throwing a
> rock into a pond so that waves on the surface spread out into the future
> requires every bit the conspiracy that is needed for waves to converge on a
> point in order to eject a rock from the bottom. […] The main interesting
> difference is that Popper’s time-reversed pond seems approximately
> attainable […]

Indeed it is:

[https://www.youtube.com/watch?v=WffR6HrEqTA#t=40](https://www.youtube.com/watch?v=WffR6HrEqTA#t=40)

I wonder if the FloWave people ever considered Popper's argument and tried to
eject an object from the bottom.

~~~
jon_richards
I imagine the turbulence caused by a rock falling through water is much more
difficult to reverse than the waves caused by a rock impacting the surface of
water.

------
jonsen
>Does it account, for instance, for the fact that we know more about the past
than the future?

Reminded me of a side remark from another context:

The past only exists in our memory, therefore in some sense we can't recall
incorrectly.

~~~
lisper
That depends on what you mean. The past state of the universe is faithfully
encoded in the current state of the universe. But that does not mean that a
subset of that state (a single human, say, or even the entire population of a
single planet) cannot have false memories.

~~~
semi-extrinsic
What do you mean "faithfully"?

For sure we can all agree that this encoding is irreversible - i.e. we can't
reconstruct the past state from the present.

And due to the combination of chaos in macroscopic systems and quantum
uncertainty, I think it's highly likely that we can conceive of two slightly
different past states that give present states indistinguishable even to
$deity_of_unlimited_power.

So how is it "faithful"?

~~~
lisper
> What do you mean "faithfully"?

A good question! I mean that future states are produced by past states
according to physical laws.

> For sure we can all agree that this encoding is irreversible - i.e. we can't
> reconstruct the past state from the present.

Not with full accuracy, no. But past states are _constrained_ by present
states in useful and interesting ways.

~~~
kmill
Usually "encoded faithfully" is used to mean that the encoded thing can be
recovered perfectly (mathematically, not necessarily physically). I thought
you were saying that both the past and the future are functions of the
present, but _we_ can't calculate those functions ourselves with full
accuracy.

~~~
lisper
> I thought you were saying that both the past and the future are functions of
> the present

Yes, that is exactly what I'm saying.

> but we can't calculate those functions ourselves with full accuracy.

Yes, that is also true. The universe as a whole is not constrained by the
limits of what we humans can compute.

~~~
kmill
> Yes, that is exactly what I'm saying.

Sorry to be a stickler for detail, but earlier you defined "faithful" to mean
the future is a function of the present, and now you're saying that "the past
and future are functions of the present" is exactly what you are saying.
Perhaps you mean that it's exactly what you intended to say?

I'm not intending to undermine anything with "gotchas." It just seems to me
that more precision will help your arguments.

One thing I do not see, in your essay, is how reducing "memory" to
"entanglement" means time is asymmetric. What is it about entanglement that is
asymmetric that isn't just "that's how entanglement is defined"?

I also do not understand how the thought experiment implies time travel cannot
happen. Has anyone proved that the wave equation has no solutions when there
are closed timelike curves? (I assume not since there are papers studying
quantum computations possible in the presence of closed timelike curves, for
instance.)

~~~
lisper
All of those statements are consistent with the concept that I am attempting
to communicate through the imprecise medium of natural language.

> What is it about entanglement that is asymmetric that isn't just "that's how
> entanglement is defined"?

Entanglement is symmetric. You can reverse an entanglement. But _only_ by
returning the entangled particles to the same physical location.

The reason that this symmetric quantum process gives rise to (what appears to
us to be) a time-asymmetric classical universe is that what we classical
entities actually _are_ is massively entangled systems of vast numbers of
quantum particles (or, to be as precise as I can, we are quantum systems with
a vast number of mutually entangled degrees of freedom). All those
entanglements _can_ be reversed, but if you reverse them, the result is not
"moving backwards in time" as humans commonly conceptualize it. The result is
rewinding the whole universe to a previous state. That is actually possible in
principle. As I note at the end of the essay, there is no way to distinguish a
universe that is constantly being rewound and replayed from the universe we
live in. But what most people think of as going backwards in time is rewinding
the whole universe _with the exception of themselves_ to a previous state.
That's not possible.

I don't actually know what the consequences are of applying this idea to CTCs.
I don't understand CTCs well enough to do it myself. There's probably a thesis
or two in there somewhere. Mixing QM and GR is a rare skill.

------
lisper
The arrow of time can be explained by information-theoretical interpretation
of quantum mechanics:

[http://blog.rongarret.info/2014/10/parallel-universes-and-
ar...](http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-
time.html)

TL;DR: the decoherence process by which the classical world emerges from the
quantum is necessarily asymmetric with respect to time.

~~~
Nokinside
I don't think your theory holds water.

Quantum mechanics can be formulated to be time symmetric. With time symmetric
quantum mechanics decoherence happens symmetrically in both directions in
time.

If you use many-worlds interpretation as illustrative as metaphor for
decoherence, reality splits also backwards from the current moment in time
symmetric quantum mechanics. In fact I think Stephen hawking uses QM backwards
in his quantum cosmology theories. Calculating every possible history of the
that leads to next event might tell us something about early universe.

As a final point. Quantum decoherence cant' explain existing CP-violations
that are not time symmetric.

~~~
lisper
Did you actually read the essay? Because the whole point was to address this
exact issue. In fact, the _second sentence_ of the piece is, "All measurements
are in principle reversible."

~~~
Nokinside
You did not adress the issue, you mentioned it without solving the
complications it brings to your theory.

Quantum mechanics and entanglements have nothing to do with the thing you are
trying to explain. It's all thermodynamics and entropy. Memories can formed
only by increasing entropy. Thus personal time flows from lower entropy to
higher entropy.

~~~
lisper
> the complications it brings to your theory

Like what?

> Memories can formed only by increasing entropy

That's right. So? Once again you seem not to have read what I wrote at all (or
you did not read the background material that I link to).

~~~
Nokinside
Sorry if I can't understand what you are writing. Your writing is extremely
difficult to read. You should concentrate to the point you are making and not
try to lecture other things you know on the side.

Answer to this question:

If you can explain whole stuff using thermodynamics, why you bring QM and
entanglement into the picture at all? Time would work exactly same in
classical universe.

~~~
aisofteng
Nota bene: it appears you're talking to a crackpot.

~~~
lisper
Who are you talking to here, me or nokinside?

