
Quantum Entanglement Drives the Arrow of Time - jonbaer
https://www.simonsfoundation.org/quanta/20140416-times-arrow-traced-to-quantum-source/
======
tim333
The reasoning sounds a bit iffy as in:

“Finally, we can understand why a cup of coffee equilibrates in a room,” said
Tony Short, a quantum physicist at Bristol. “Entanglement builds up between
the state of the coffee cup and the state of the room.”

I think you can understand coffee cooling quite well without any quantum stuff
- the atoms in the coffee are moving faster than those in the room. There will
be a tendency when one impacts with an atom of the air in the room for that to
speed up and the coffee atom to be slowed.

Actual quantum entanglement is a strange and interesting thing. It's a shame
people tag the term on things it is not really relevant to try to sound
impressive for the most part.

~~~
arh68
I think the author pretty clearly explained the connection:

    
    
        the arrow of time does not seem to follow from the underlying
        laws of physics, which work the same going forward in time as in
        reverse. By those laws, it seemed that if someone knew the paths
        of all the particles in the universe and flipped them around,
        energy would accumulate rather than disperse: Tepid coffee would
        spontaneously heat up
    

What he's saying is a bit unclear: we know the _how_ , which is even a kind of
plausible explanation as to _why_. What the author says we couldn't explain
before is the converse: why does the opposite _never_ happen? So I interpret
that quote more like " _Finally, we can understand why a cup of coffee_ only
_equilibriates_..."

~~~
TTPrograms
Statistical mechanics doesn't say why a coffee cup never heats up
spontaneously - rather, it says that the probability of that happening is
incredibly small. You end up with probabilities like 10^-100000. This doesn't
require quantum at all, and is effectively the same.

------
cromwellian
This Google Talk
[https://www.youtube.com/watch?v=dEaecUuEqfc](https://www.youtube.com/watch?v=dEaecUuEqfc)
uses entanglement and quantum information theory in a clear and understandable
way to explain 'spooky' quantum phenomena, like the quantum eraser, de-
coherence, the aspect experiment, and the measurement problem. Even if you
don't know any QM, just basic algebra and calculus, it's really approachable.

I used to be a fan of the Many Worlds interpretation, but after seeing this,
I'm now a big fan of the Quantum Information Theory explanation. Starting
about 43 minutes in, he goes into the QM Information Theory explanation, but
I'd recommend watching the entire prezo.

Link to my original post on the subject:
[https://plus.google.com/110412141990454266397/posts/HC49S9ip...](https://plus.google.com/110412141990454266397/posts/HC49S9ipwgn)

~~~
loup-vaillant
I must say, I'm not sure what he even means by this "zero universe" business.
Sure, the classical illusion is bust, but the wave function itself, as far as
we know, is real, is it not?

While we're at throwing quantum explanation links, here is the Quantum Physics
Sequence on Lesswrong:
[http://lesswrong.com/lw/r5/the_quantum_physics_sequence/](http://lesswrong.com/lw/r5/the_quantum_physics_sequence/)
I'd say the math is even simpler there, yet the explanation go deeper. It's a
long read however.

~~~
Confusion
Whether the wave function is real depends on your answer to the question: is
momentum real? You can't directly measure or observe momentum, you can't point
to it in the world, but you can definitely feel its impact and it's a
convenient measure for certain properties of objects. In that sense the wave
function is just as real.

But you can argue both those things are not as real as e.g. extension, force
and energy, which are much more readily available to our senses.

~~~
stillsut
Can't I just weigh Galileo's cannon ball, then based on T from when I dropped
it know its momentum?

I think the difference comes on moving past the macroscopic world: say I drop
a uranium atom into Shroedinger's box - I can't calculate at time T whether
the cat is alive.

~~~
Confusion
Yes, you can calculate the momentum it had anywhere along the path while it
dropped. Does being able to calculate some property make it real? If I take
the square root of the momentum and call it the 'squish' of an object, is my
'squish' property then real?

------
pygy_
It reminds me of another paper that was discussed here earlier (I can't find
the submission, though):

 _A quantum solution to the arrow-of-time dilemma_ —Lorenzo Maccone

    
    
        The arrow of time dilemma: the laws of physics are
        invariant for time inversion, whereas the familiar
        phenomena we see everyday are not (i.e. entropy
        increases). I show that, within a quantum mechanical
        framework, all phenomena which leave a trail of
        information behind (and hence can be studied by
        physics) are those where entropy necessarily increases
        or remains constant. All phenomena where the entropy
        decreases must not leave any information of their
        having happened. This situation is completely
        indistinguishable from their not having happened at
        all. In the light of this observation, the second law
        of thermodynamics is reduced to a mere tautology:
        physics cannot study those processes where entropy has
        decreased, even if they were commonplace.
    

Phys. Rev. Lett. 103, 080401 – Published 17 August 2009

[http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.103...](http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.103.080401)
– [http://arxiv.org/abs/0802.0438](http://arxiv.org/abs/0802.0438)

------
elzr
This was surprisingly beautiful. As a geek in
programming/computers/information/mathematics, but only a physics admirer from
afar, it is very suggestive, even natural, to explain the deepest physical
reality in terms of information:

"It was as though particles gradually lost their individual autonomy and
became pawns of the collective state. Eventually, the correlations contained
all the information, and the individual particles contained none. At that
point, Lloyd discovered, particles arrived at a state of equilibrium, and
their states stopped changing, like coffee that has cooled to room
temperature."

“What’s really going on is things are becoming more correlated with each
other,” Lloyd recalls realizing. “The arrow of time is an arrow of increasing
correlations.”

“The present can be defined by the process of becoming correlated with our
surroundings.”

~~~
alxndr
Makes me imagine anthropomorphized variables in a program having discovered
the file system and the class definitions that they're instantiated from, but
are still trying to figure out what RAM is...

~~~
goldenkey
I laughed at how clever the analogy is. Time is relative to us as observers,
the arrow of time is fundamental I would think.

------
dalek_cannes
Do we need entanglement to explain the Arrow of Time? Even though in classical
mechanics, the past and the future are both equally observable, we remember
the past and not the future because the future does not contain certain
information yet -- the information to be introduced into the universe in the
form of quantum fluctuations. One could even argue that _all_ information in
the universe was created at some point in time due to one quantum event or
other.

I may have misunderstood though (I'm not a physicist). Entanglement _does_
however, explain why systems tend to equilibrium rather than any other type of
state as it evolves _forward_ in time.

On a related note, I found this quote interesting. It reminds me of how HN
comments about quantum information theory has a tendency to get downvoted:

> The idea, presented in his 1988 doctoral thesis, fell on deaf ears. When he
> submitted it to a journal, he was told that there was “no physics in this
> paper.” Quantum information theory “was profoundly unpopular” at the time,
> Lloyd said, and questions about time’s arrow “were for crackpots and Nobel
> laureates who have gone soft in the head.” he remembers one physicist
> telling him.

~~~
joe_the_user
Well,

Information is produced in the course of a system evolving and information is
destroyed (the past is forgotten).

The tendency of a system to move towards greater entropy could be said to give
an explanation for the difference between past and future. But how does that
work in an open system like the planet earth, where entropy hasn't increased,
where the system has self-organized over time.

The ability of a system to store information in only one direction of movement
could be the explanation - if we could define that more exactly. But since
information is constantly being "created", destroyed and transformed, defining
this is a difficult task.

Which is to say the problem as a whole is hard.

~~~
mtrimpe
I always like the crazily simplified explanation: to truly undo something you
need to undo your memory of it as well.

~~~
agos
this exmplanation reminds me of the “mu” word from zen culture (as seen on
Gödel, Escher, Bach).

------
mbq
This is nonsense; entropy and the arrow of time are essentially a many-body
effects and require no quantum effects to occur. A simplest way to see it is
to make small simulation of a, say, 1000 gas particles with only classical
bouncing in a one side of a box partitioned in half with a barrier, obviously
with a time-reversible numerical method -- after the removal of the barrier
the gas will evenly spread over the box without any entanglement.

~~~
loup-vaillant
yet, once the particles are spread, if you somehow reverse their velocities,
they will concentrate back to one half.

Yet we can never do that. Why do you think?

The obvious answer in a classical universe is that we simply don't know the
velocity and position of each particle, so we can't just reverse them. To pull
such a feat, we'd have to be _incredibly_ lucky, as in "winning every lottery
for a century" lucky.

With entanglement and de-coherence however, such reversal becomes impossible
even in principle: see, when the universe splits through de-coherence, you no
longer have access to the other half. Even if you manage to reverse your half
of the universe, you need the other half to be reversed too, or they'll never
merge back together.

And not just the other half, since de-coherence happens all the time. You need
_all_ the Everett branches to be reversed. No. Way. So it does look like a
better candidate for the arrow of time.

(Of course, a better candidate still would be collapse interpretation, since
that one is _not_ time reversible in the first place. But this interpretation
is ridiculous to begin with, so let's ignore it.)

\---

Read this to have an idea of how time could work in a timeless universe:
[http://lesswrong.com/lw/qr/timeless_causality/](http://lesswrong.com/lw/qr/timeless_causality/)

~~~
shasta
> The obvious answer in a classical universe is ...

Since there is an obvious answer in classical physics, it's a bit disingenuous
to claim that this solves a long-standing problem in classical physics, no?

> With entanglement and de-coherence however, such reversal becomes impossible
> even in principle [...] So it does look like a better candidate for the
> arrow of time.

That would be true if you could demonstrate by experiment that a broken egg
springing back up onto the table and reforming is physically impossible
instead of just unfathomably unlikely. Can you demonstrate that?

~~~
loup-vaillant
You do not understand. Let me rephrase.

You go on a space ship, on your way to the edges of the universe. Your buddy
goes on a space ship, on his way to the _other_ side of the universe. You will
soon be outside each other's observable universe.

Now, if you drop something in your ship (it spins, so you have gravity), you
can "reverse time", and pull it back up. Can you do the same to you buddy's
ship, should something ever fall there?

The simple answer is no. You can't. He's on his own.

Now there _is_ a way I haven't spoken of: non-causal interaction. You and your
buddy could agree on some things before you depart. For instance, you could
agree to pull back up whatever falls.

With Everett branches, it's even easier: you pre-commit to reversing your own
Everett branch, whatever it is, so all your selves do it. If successful, the
worlds should merge back together, at least locally. Just one catch: _all_
your other selves must successfully reverse time locally. It only takes one
failure for the plan to fail.

But if you want to reverse time _after the fact_ , say because you happen to
be in an Everett branch you don't like (you lost a bet about which way the
photon will go), you won't be able to reverse time here, because your other
self certainly will not (he won the bet, so…). Maybe, just maybe, you could
use the vanishingly small entanglement left with the other Everett Branch to
directly communicate with your other self. I'm not even sure it can be done in
principle. For practical purposes, it should be forever beyond reach, even if
you have a super-intelligent AI to help you.

~~~
shasta
> Maybe, just maybe, you could use the vanishingly small entanglement left
> with the other Everett Branch to directly communicate with your other self.
> I'm not even sure it can be done in principle.

My understanding is that this would violate a physical law that we still
believe to hold.

You're right though, that I don't understand. At least I thought I understood
your last post. With this one, I don't follow the connection to the OP.

~~~
loup-vaillant
Oops, I didn't intent to obscure my meaning.

I was replying mainly to mbq:
[https://news.ycombinator.com/item?id=7602812](https://news.ycombinator.com/item?id=7602812)
My understanding is, with QM, is is even harder or even impossible to reverse
time, even with perfect knowledge of your reachable surroundings.

In a classical universe however, it looks much easier, so in such a universe,
the explanation for time is less satisfactory.

------
throwaway7548
I have a question. I just went to a source of physical (quantum) randomness
[http://www.randomnumbers.info/](http://www.randomnumbers.info/) and I'm
giving you a random number between 0 and 10,000 which I've just generated
there. Here it goes: 6296.

Ok. Now that light cone had finally reached you. And you (neurons in your
brain to be precise) are thoughtfully entangled with that random event
(outcome), now in your past.

Now imagine the following. A few days passes. And you forget that number. A
few years passes. Connections between the neurons which were storing this
information are now gone. Molecules and atoms which were part of these neurons
are gone from your body. There are no entanglements any more which link you to
that event. Is that event in your future now? Again?

~~~
aaren
> There are no entanglements any more which link you to that event.

Not directly, but the information has spread out from your neurons into the
surrounding matter ad nauseum. It's just we can't interpret the information
anymore.

The event still happened in your past, you just can't see it through your
limited human view of reality.

~~~
throwaway7548
And what if you would move away from that surrounding matter? Or, say launch
it away with near light speed, so it would get behind the horizon at some
point. How is that situation different from the one in which I've just
generated the number and the light cone haven't reached you yet?

~~~
aaren
> And what if you would move away from that surrounding matter?

You can't :)

By surrounding matter I meant the rest of your body that isn't neurons, as
well as the environment outside your body.

~~~
throwaway7548
Can't I? What if the state of these poor remaining neurons and the body is
scanned, encoded as polarization of a bunch of photons and sent to a receiver
far far away? In that case good old environment would definitely end up behind
the cosmic horizon.

~~~
aaren
Then you have implemented teleportation and we have new problems to think
about!

~~~
throwaway7548
Oh. Common. No need for any new inventions.

Just replace a person [that gets entangled with a particular outcome of a
random event (have measured it)] with a simpler organism, say a dog. Or a
hamster. Or with a roomba vacuum cleaner ;). Or even with a computer. And we
definitely know that a state of a computer can be represented as a bit string
encoded on any media. Including polarization of a bunch of photons.

------
sheerun
I love following article:
[http://www.flownet.com/ron/QM.pdf](http://www.flownet.com/ron/QM.pdf)

It basically shows that observation (measurement) and entanglement are the
same things.

Think about it: particles are not magically going out of superposition as we
observe (measure) them. We (our atoms) become entangled with those particles,
we become superposition. It's just propagation of entangled state.

Why we don't perceive ourselves as in superposition? "It turns out that this
result generalizes to any number of mutually entangled particles. If we ignore
any one particle, the entropy diagram of the remaining particles looks like a
system of N-1 particles in a classically correlated state with a non-zero
entropy.". That means each atom of our bodies _perceives_ other atoms
entangled with it as they were not in any superposition (though as a whole,
the system is still in superposition). We (atoms) are constantly entangled and
in superposition with our environment, but we perceive it as classical state.

In what state each atom "sees" every other? According to probability. That's
why in double slit experiment we see only _one_ of most probable outcomes, not
a random one.

Time could be rate of entanglement propagation. Entanglement propagates with
speed of light (speed of particles), so we seem live in same timeline. But if
something moves away from us with speed of light, the time for this object
goes slower, but only relative to us.

Until two particles interact with any way, they live in totally different
timelines. After they "observe" each other (entangle with each other), also
their _time_ becomes entangled. That's why after we see a cup begin dropped,
it becomes part of our reality, and the cup becomes broken in our time.

We live in _spacetime_. As mentioned in article "“Spooky action at a distance”
ought to be no more and no less) mysterious than the “spooky action across
time” which makes the universe consistent with itself from one moment to the
next.".

Why arrow of time? The article says: "Under QIT, a measurement is just the
propagation of a mutually entangled state to a large number of particles. To
reverse this process we would have to "disentagngle" these quantum states. In
principle this is possible. In practice it is not.". I think differently
though.

That are my thoughts. Please don't judge :)

~~~
3rd3
I have a question: What does being in a superposition mean exactly here? I
only know the term from classical mechanics, e.g. that an acceleration vector
in R^3 can be seen as a superposition of three accelerations along the base
vectors (a linear combination).

~~~
nzp
It's kind of like that. The state of a particle in classical mechanics is a
linear combination of some basis vectors which are not it's states as such.
But the superposition in QM is the superpositions of _states_. The phase space
in classical mechanics is not a vector space, but in QM it _is_. This is a
fundamental difference because in QM you can take any number of states, make a
linear combination a it will result in another perfectly valid state. Not so
in classical mechanics. For example, consider a particle orbiting a point. If
you take some position vectors as it moves and make a linear combination, the
resulting vector will not be a possible position of a particle in this system.
In QM, it would necessary be another possible state. This is a slightly sloppy
example, but I hope you get the gist.

------
spcoll
The question of whether time is in fact directional is far from being closed,
at least for quantum physicists. In fact, one of the physicists cited in the
article is known for proposing a time-symmetric formulation of Quantum
Mechanics [1].

[1]
[http://www.phy.bris.ac.uk/people/Popescu_S/papers/sandu_othe...](http://www.phy.bris.ac.uk/people/Popescu_S/papers/sandu_others5.pdf)

~~~
fraserharris
Interesting overview of the Arrow of Time on wikipedia:
[https://en.wikipedia.org/wiki/Arrow_of_time](https://en.wikipedia.org/wiki/Arrow_of_time)

------
jostylr
From a Bohmian perspective, quantum mechanics consists of a wave function
psi(q) that guides all the particles Q. The wave function is distinct from the
particles. The particles are in equilibrium, relative to the wave function. It
is the wave function that is not in equilibrium in its realm of states.

As it turns out, the usual psi^2 probability distribution of the particles is
a reflection that the particles are in quantum equilibrium, that is, psi^2 is
the natural measure in quantum mechanics for what equilibrium ought to be
since it is the only measure preserved by the dynamics. And so if the
particles start that way, they stay that way. And they are likely to start
that way using psi^2 as the distribution.

There is actually a lot of subtlety involved in accepting that argument; I
recommend [http://plato.stanford.edu/entries/qm-
bohm/#qr](http://plato.stanford.edu/entries/qm-bohm/#qr) and an actual paper:
[http://www.ge.infn.it/~zanghi/BMQE.pdf](http://www.ge.infn.it/~zanghi/BMQE.pdf)

But what it implies is that the wave function is responsible for the arrow of
time. It is a special state that evolves into a less special state. Presumably
this is what their research is pointing at.

I would also comment that their description is exactly the classical
explanation transferred to the quantum world (which it needs to be since our
world is quantum). That is, we start in a special state and it evolves into a
less special state because the less special states are more numerous and so
more likely to be, all things being equal. And by more likely, we are talking
10^100 kind of more likely.

They still have the problem that the fundamental evolution of the wave
function is time reversible. So if that bothered someone (it shouldn't), then
their argument does not actually resolve that problem.

So I take from their work that what they are doing is getting the classical
thermodynamic explanation (which is about volumes in phase space, not human
ignorance) and translating it to the quantum theory. Neither wrong nor
revolutionary.

------
fspeech
The title of the article is unfortunate. Classical physics adequately
addresses the arrow of time through thermodynamics (the second law in
particular). What is missing is so called "decoherence". In other words,
quantum physics is supposed to explain everything but when we interpret
results we divide the world into classic (the observer) and quantum (the
observed) parts. The answer, from reading the article, seems to be that even
if the world is in a pure state (quantum) a large part of it could behave like
a mixture state (classic observer/environment) through entanglement. This
makes it easier to have a coherent mental picture of quantum physics.

------
millstone
> After some time, most of the particles in the coffee are correlated with air
> particles; the coffee has reached thermal equilibrium.

No doubt this is some way oversimplified explanation, but it still makes no
sense.

Say I have hot coffee and lukewarm coffee. The lukewarm coffee will
equilibrate faster. Does it interact with the air faster? What if I bring in
coffee that's the same temperature as the air, so that it's instantly at
equilibrium. Does it interact with the air instantly?

~~~
nzp
> Say I have hot coffee and lukewarm coffee. The lukewarm coffee will
> equilibrate faster.

It won't (all else being equal). :) Because of Newton's law of cooling, hot
coffee will cool down faster (precisely, the rate of change of temperature of
a solid body is proportional to the difference in temperature between the body
and the environment).

~~~
judk
Parent meant "sooner" not "faster". The derivative of temperature was not
intended, the dervative of entanglement was intended.

------
TeMPOraL
Is this really new? IANAP, but I clearly remember being taught about the Arrow
of Time as a probabilistic/thermodynamical phenomenon even in high school and
I also read similar explanations that involved causality and probability
theory _without refering to quantum entanglement_. Is the "quantum" bit even
needed there for anything?

~~~
baddox
Probability theory is how we _model_ the arrow of time, but it's not a
physical mechanism by which the arrow of time occurs. The article covers this
distinction.

~~~
x3c
Isn't the classical definition of Arrow of time is from the system with less
entropy to a system with more entropy? What is lacking in this definition of
Arrow of time that we need to take it to Quantum level?

~~~
baddox
Again, probability is just a model that can predict something, but not explain
how it works. Take a pseudorandom number generator and do rand() > 0.5. Your
probability is 50%, and that is a pretty good model to predict how the
subroutine will behave, but it may also be useful to understand what algorithm
the PRNG is using.

~~~
judk
In physics, a "model" is the same as "how something works", unless and until
you can uncover details (like the precise order of generated numbers) that the
old model can't express.

------
neolefty
From the article

    
    
      One aspect of time’s arrow remains unsolved. 
      “There is nothing in these works to say why you
      started at the gate,” Popescu said, referring to
      the park analogy. “In other words, they don’t
      explain why the initial state of the universe was
      far from equilibrium.” He said this is a question
      about the nature of the Big Bang.
    

Could it be that expansion, which proceeded much faster than light, therefore
didn't allow entanglement to take place, delaying the heat death of the
universe until everything is fully entangled?

If expansion had been slower, would entropy maybe have kept up with it,
leaving us as just a single black hole instead of a dispersed, interesting,
unentangled, things-are-still-happening universe 13 billion years later?

------
rturben
A lot of people in this thread are questioning how this adds any new
information about time or how it is different/better than a classical
explanation of systems (coffee cup reaching equilibrium based on thermodynamic
laws).

One way that this result makes sense to me is by considering the properties of
light speed and "spooky action at a distance." Particles become entangled with
one another at the speed of light -- photons or fields carrying the
information between the two. Looking at this from the perspective of light
speed, there has been an instantaneous change between the two particles. State
A has led directly to a more complicated, entangled State B. Still looking at
this from light speed, there is no time between the transition from one state
to the next and from that one to the next and so on. The universe has already
worked itself out from the initial disentangled state to all the states that
are increasingly more entangled.

Thanks to Einstein, we know that all objects try to move at light speed, but
that the more massive they are the slower they become. Because we are massive
objects, we don't experience time instantaneously like the photons do. We see
the propagation of entanglement and see the state transitions. Our massiveness
has given rise to a direction of time, the order that we understand the states
of the universe to be proceeding in. Unlike light, we have to experience all
the intermediate states in the order of less entangled -> more entangled. Thus
an arrow of time.

This is already subtly bundled up in the classical explanations. Coffee cools
off because it reaches equilibrium. Classical physics says this is because the
particles in the coffee are hotter than the surrounding air, so it is more
likely for those particles to break free of the coffee, thereby reducing its
average kinetic motion. Consider though how those particles are interacting
with one another. They don't just "know" the direction they're supposed to go,
they bump into each other's fields and communicate at light speed. Each
particle informs the next and as they become more entangled and learn more
about where they are, they progress from state to state.

------
SoftwareMaven
I wonder how much knowledge is lost because the research isn't "popular". What
is the opportunity cost of so many researchers doing string theory research,
not necessarily because they believe they'll find a breakthrough (obviously,
this doesn't describe most), but because they won't be able to get published
or find a research position if they aren't doing the "in" thing.

This doesn't just apply to physics, but the history of physics makes it easy
to find case studies in this.

------
EGreg
Wow, just today I read this:

[http://physics.stackexchange.com/questions/10068/on-the-
natu...](http://physics.stackexchange.com/questions/10068/on-the-nature-of-
the-collapse-of-the-wave-function)

and I thought it was all explained quite simply and now this?

------
yati
I've always wanted to study quantum mechanics because of this very
"entanglement". Can people please post recommendations on good resources/books
on the topic for a person like me having no solid experience with
physics(except college level courses)?

~~~
sinesha
The bible is Nielsen and Chuang's Quantum Computation and Quantum Information:
[http://www.amazon.com/Quantum-Computation-Information-
Cambri...](http://www.amazon.com/Quantum-Computation-Information-Cambridge-
Sciences/dp/0521635039) See also these resources in Nielsen's blog:
[http://michaelnielsen.org/blog/writing/](http://michaelnielsen.org/blog/writing/)

~~~
yati
That looks interesting. Does one need to have a background in QM before
reading this? If yes, what would you suggest as a good read on basics of QM?

~~~
sinesha
Let me double-check it today, but I think that it introduces the basics of QM.
You need to know some linear algebra though.

~~~
yati
I started reading this, and boy am I loving it - thank you :)

------
Tarrosion
I found the article rather confusing: it starts out by saying look, the laws
of physics make sense forwards or backwards. But we only see one kind (entropy
increasing) of process. Why is that? Entanglement.

Is there something about entanglement that is irreversible? As the article
says "it is the loss of information through quantum entanglement, rather than
a subjective lack of human knowledge, that drives a cup of coffee into
equilibrium with the surrounding room." Okay, but then why don't we ever see
the reverse making coffee depart from equilibrium? Something like the
acquisition of information through breaking entanglement drives a cup of
coffee away from equilibrium.

~~~
jaekwon
Perhaps they got lost in their math.

[http://arxiv.org/abs/gr-qc/0507094](http://arxiv.org/abs/gr-qc/0507094)

------
one-more-minute
This is an interesting step, but doesn't actually explain why time is
asymmetrical. Ok, so things equilibrate as time moves forwards because they
entangle as time moves forwards. But this just shifts the question – why is
entanglement asymmetrical when time, when the underlying laws are not?

You still have the same problem: if you reverse time, the states become
untangled and the coffee heats up.

It's nice to be able to model this from a quantum perspective, but make no
mistake – no philosophical issues have been resolved here, and we don't
"finally" understand anything we didn't before.

------
Houshalter
Just a thought that I've been thinking about. Time has a direction because of
causation. State1 causes state2 which causes state3 and so on. You get weird
paradoxes if you allow causation to work in both directions. The universe
would also have to magically align everything perfectly so that everything is
consistent.

Another observation is that even with reversible laws of physics that can work
in both directions, if you have a single starting state, all other states will
causally propagate from it. In a single dimension of time/causation.

~~~
jpeterson
Causation relies on a particular direction of time, not the other way around.

For example, suppose you have two billiard balls, A and B. In our world, A
travels from left to right, strikes B, and then B flies off to the right.

We would say A striking B caused it to fly off.

But an observer moving through time in the opposite direction would be no less
justified to claim:

B travelled from right to left, struck A, and caused A to fly off to the left!

~~~
Houshalter
Yes, reversible laws of physics are possible. State1 can cause state2, and
state2 can cause state1.

Here is the question: Do you pick a state for state1 and then figure out what
state2 should be from it (that is, cause state2)? Or do you pick a state for
state2 and figure out what state1 should be (causing state1)?

If you have a "starting state", it doesn't matter if the time is reversible.
All states will be caused by the starting state propagating forward in one
direction. This is _exactly_ what we seem to observe in the real universe. The
big bang starts the universe and everything appears to be a chain of cause and
effect from it. We never observe events that are _caused_ by things in the
future. Glasses do not spontaneously assemble themselves out of shards and fly
on top of tables. Photons do not just spontaneously fly from all directions in
space to form sensible images on Earth.

Now you might say "what if the universe somehow decided on all the states at
once". Well that isn't what appears to be true in our universe for one (or
you'd have things spontaneously happening in the future and then rippling
effects back in time, rather than the other way around.) Second it might not
even be possible. In order to do that you'd have to try every possible
combination of states and see which ones are valid. Does writing down every
possible combination of bits create universes? Even if you apply some rule to
them to check if they are "valid" universes?

I think that "real" universes like ours have to have a chain of causation like
that.

Here is a better description of this line of thought than mine:
[http://lesswrong.com/lw/fok/causal_universes/](http://lesswrong.com/lw/fok/causal_universes/)

------
denom
In the article the author describes the notion of a "pure state" which is
something that has independently evolving probability. Individual 'units' lose
their pure state and become part of an entangled ensemble--move to
equilibrium.

How is the evolution of biological organisms and technological systems
explained in this sense? Played backwards, evolution would fit this and
traditional notions of thermodynamic entropy. Is evolution a kind of de-
entangling?

~~~
eli_gottlieb
>How is the evolution of biological organisms and technological systems
explained in this sense? Played backwards, evolution would fit this and
traditional notions of thermodynamic entropy. Is evolution a kind of de-
entangling?

Evolution and technological change are completely different from the physical
arrow of time. Optimization processes (evolution, technology) _cannot_ , as
far as we know, actually disentangle themselves on-net. What they can do is
move the waste-heat/entropy/entanglement into concepts they don't care about,
or entangle themselves with some radiating source of "fuel".

(Shout-outs to everyone who thinks the concept of an "optimization process" is
total hokum, as I'd like to hear alternate explanations for the apparently
similar behavior of so many things that seem to share no purely physical
properties at all, and yet all seem to function to shift entropy from some
things into other things according to a computable ordering.)

Or in other words, yes, all Earthly life actually lives by converting sunlight
into a combination of life and waste, with the "waste entropy" often being
radiated off as waste-photons into space, which we don't care about.

An interesting question, I conjecture, is how this conception of
entropy/entanglement ties into _energy_ , which apparently remains necessary
for the whole process to occur, and yet is conserved in all physical
processes.

------
wmnwmn
It's never been clear to me how deeply the thermodynamic quantities are really
connected to time. For example there could be a state in which entropy (or
entanglement) increased from left to right in space, yet it wouldn't mean that
time flows from left to right.

------
officialjunk
i recall learning that time "flows" both ways at the quantum scale, but i
admit is has been a while since i've attended any lectures. has there been any
new discoveries to say otherwise? i think i've read about research of both
time reversal violations and time-invariance at the quantum scale.

also, what are peoples' thoughts on time being an emergent property at the
macro scale and that down at the quantum level, everything is described by
time independent equations, like the Wheeler-DeWitt equation?
[http://en.wikipedia.org/wiki/Wheeler%E2%80%93DeWitt_equation](http://en.wikipedia.org/wiki/Wheeler%E2%80%93DeWitt_equation)

------
spikels
Quantum mechanics is where physics became more like mathematics: common sense
no longer provides much guidance. It is really cool that it provides the
missing explanation for one of the most common sense ideas in classical
physics: the arrow of time.

------
dominotw
I love this[1] 'arrow of time documentary' if you are looking for something
fun to watch.
[https://www.youtube.com/watch?v=4BjGWLJNPcA](https://www.youtube.com/watch?v=4BjGWLJNPcA)

------
clavalle
Does it seem to anyone else that quantum entanglement and decoherance is the
universes way of doing the least amount of computation possible? Like the
universe is lazily loaded?

~~~
andy112
Can you give a concrete example of what you mean?

~~~
clavalle
Sure (as far as my layman's understanding of quantum physics goes).

So, you have independent physical system (say, a photon). Before it interacts
it exists in quantum superposition. It has the potential to exist in any
possible configuration and, in fact, seems to exist in all of them
simultaneously. Almost like a variable of a certain type but this type is kind
of special in that some possible values are more likely than others.

You can combine these 'probability types' systems with others and get useful
information out of them in aggregate (example, wave interference patterns)
very easily. They seem very well suited to treating them in aggregate in bulk
calculations -- IOW, it is computationally cheaper to keep them in this state
until the individual values of the variables must be extracted and used. Then
the universe accesses the actual value of that individual variable that, so
far, has only been needed as part of the aggregate if at all, through an
interaction. Then, and only then, is the variable loaded with a concrete value
chosen from its probability type through entanglement and it becomes useless
as part of these aggregate calculations since it must be treated as an
individual entity with a value.

(I am ignorant of quantum physics outside of pop science articles, so if I
have something egregiously wrong, please let me know).

Let's say you've made a video game. It is a MMO set in the Stone Age based
around massive battles with slings and swords. There are hundreds of thousands
of players and computer controlled NPCs in the same world simultaneously. To
conserve server resources many calculations are done on the client computer.
The clients cannot be expected to do a full calculation for every projectile
in the world so there is an algorithm that chooses to load state of only those
players that are near enough to actually act on that particular client.

Let's say a player has come onto the battlefield out of range of everyone else
but has a scrying spell that allows that player to see an overhead map showing
dots of troop formations. This is the aggregate calculation. This player
doesn't need to know every other player's full state, just their general
density (because he only gets a dot for every ten or so players). As this
player moves toward the battle, some NPC comes in range. Suddenly the player
needs to know much more state so it is chosen from the possible values and now
that NPC must act in a certain way -- only have certain state changes, because
only certain transitions make sense now that he's been interacted with. The
NPC can't go from fully armored with a spear to in a loin cloth with a sling,
for example. And any NPCs in range of _that_ NPC must also de-cohere into a
fully defined state and so on. They all become entangled.

Does any of that make any sense whatsoever? I know these are imperfect
analogies.

~~~
andy112
Very cool way of thinking about it! Thanks for that elaboration.

Re: SoftwareMaven's "why is the 'player character' so important?", I don't
think the player character we follow is necessarily any more important than
the other player characters. You can say every individual character (or
particle in the real-world side of the analogy) has an importance only within
its own reference frame and I think it still works.

Just like you can have many players playing the MMO game and "collapsing the
state" of different things from their own reference frames at different times,
you can have the same be true for the particles in the real-world analogy. No
one player of the game is more "important" than any others. The only
requirement is that it all stays self-consistent in the backend and across
everyone's individual points of view at all times.

But (for better or worse) I don't think what we're talking about now is
science, really, unless there's some way to test it.

Maybe you could try to detect a "lag" by doing something that causes an
especially large number of states to collapse across an especially large
number of frames of references all at once, but I'm not sure if you could do
anything that would detect this lag because all of our ways of detecting would
be lagging too. (This goes along the same lines as trying to tell if the
computer you're using is running within a VM or running natively.)

Any ideas?

~~~
clavalle
I really don't know.

I need to research more into what it means to have a collapse of state. Also
whether a state of superposition is one that a system can return to or if once
a system has collapsed if it stays that way. And if a system can return to
superposition, what makes that happen?

------
thibauts
So, if I get it right, states become more and more coupled, thus entropy tends
to decrease in an open system ? I'm confused.

------
homerowilson
David Ruelle elegantly discusses similar ideas in a great nontechnical book
called Chance and Chaos (1995). Worth reading.

------
EGreg
I thought it was the second law of thermodynamics that already explained the
arrow of time? Well, that and friction?

~~~
fununclebob
The second law is an empirical statement. This entanglement based approach is
an attempt to understand the underlying physical mechanism that controls the
spread of entropy. If this picture is right, the next question is what is the
physical process behind entanglement.

~~~
EGreg
The second law was proven as a statistical outcome. Ordered states are vastly
outnumbered by disordered ones.

And isn't the whole description of entanglement based on empirical
observations anyway?

------
EGreg
The real question is, given the second law of thermodynamics, where did all
the order come from in the first place?

------
analog31
Should I be looking for Planck's constant in the equations of thermodynamics?

------
softatlas

        The rate of information increases.
    

Hence why

    
    
        Information wants to be free.
    

Parasitic on

    
    
        Only information explains its own existence.
    

Which all, I think, intuitively follows from Spinozist/Cartesian "Conatus"
principle. That is to say:

    
    
        The order and connection of ideas is the same as the order and connection of things.
    

Some of us rave about this or that: "well, how many folk use X today" or
"qualify as X" or "subscribe to X". But these expressions are all within the
scope of multiply converging nexuses of increasing correlative potentia. The
coffee cup _is_ a simple example — so like Wittgenstein's point: "if a lion
could speak, we could not understand him". The lion, like the cup, has
restricted correlative powers: these laws apply, these others do not.

The laws of information are laws about the dimensions of proportionality,
which give the arrow of time an aspect of curvature (needing to exhaust a
universe for exponentially narrowing arrows, so the onion-skinning of
properties of a thing "come way may" at "frozen" temporal localities — what
happens when we "bend" time at certain family resemblance (physical)
properties?).

~~~
cgio
I have the sense there is something you are trying to say, but I cannot know
what the dimensions of proportionality can be. I can see why you assign frozen
temporal localities to property assignment but I think there is some confusion
in your argument between information as perceived and information as signaled.

------
suprgeek
At or very near the Big Bang, the Universe was in a state of minimum Entropy
i.e. minimum entanglement i.e. maximum order (in some sense).

Post Big bang the cosmological arrow of time is in the direction of increasing
disorder i.e. increasing Entanglement i.e. decreasing order

On a smaller closed system, Before is when the system is more pure, less
entangled, more ordered After is when it has become less ordered, more
entangled.

Obvious really...

------
salimmadjd
Seeing this article is rather bittersweet. I came to a similar conclusion in
my college years but I never pursuit it further.

Taking Quantum Physics in college was a life changing experience and it
reshaped how I viewed the world. I was always obsessed by time and one
afternoon it became clear.

I explained my variation not as a cup of coffee but a handful of dice.
Essentially every tick of time is rolling these dice. And the variation of
dice from one combination to the next is the arrow of time.

Like one of the authors in this article, I got the most amount of resistance
from physics major. For most part they had a dogmatic view of anything that
they had not studied yet. If it wasn't in their books then it didn't exist.

I also came to the conclusion time travel as depicted in the movies will never
happen. It can happen randomly in a smaller body but for anything large the
arrow of time is almost impossible to reverse.

~~~
andbberger
There's good reason you got the most amount of resistance from physics majors.
Intuition won't help you with theoretical physics. The only understanding is
that achieved through studying the math in depth. Unless you've developed the
formalism (in which case you'd be the subject of this article), you haven't
reached any conclusions.

Hate to break it to you. There's no such thing as pedagogical theoretical
physics- no shortcut to understanding physics.

~~~
salimmadjd
How did you assume I took quantum chemistry in college and was asked by my
prof to join his lab without having any math background, or to be the #1
student in my physics class of 300, or was a member of 3 person team that won
our state math championship. Not to mention scoring high enough to make it to
the chemistry Olympiad, but ultimately being rejected once realizing US
citizenship was required....

