
A Debate Over the Physics of Time - azuajef
https://www.quantamagazine.org/20160719-time-and-cosmology/
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mathgenius
"The laws that underlie these theories are time-symmetric".

I don't get how people can say this. Quantum measurement (collapse of the
statevector) is not at all time-symmetric. People seem to always leave this
out in such discussions. They either view it is a temporary "problem" with the
theory that we will one day fix, or just as something that is plainly not a
theory. Or who knows what. The wikipedia article on interpretations of quantum
physics has a giant table that illustrates the lack of consensus:

[https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...](https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Comparison_of_interpretations)

~~~
wyager
Only if you believe in non-unitary collapse, which is probably wrong IMO.

Einselection is my favorite potential solution. Straightforward, clean,
unitary, and the math is promising. The idea is that apparent collapse is just
emergent behavior of the schroedinger equation, via induced superselection
rules between incompatible measurement eigenstates.

~~~
mathgenius
> which is probably wrong IMO.

There are so many IMO's floating around when it comes to foundations of QM. I
think that is fascinating in and of itself.

> Straightforward, clean, unitary, and the math is promising.

My IMO definitely does not coincide with your IMO :-)

~~~
wyager
My "IMO" in this case means "by Occam's razor". We don't have enough physical
evidence to strongly prefer one theory on the other, so all we have to go by
is simplicity.

~~~
mathgenius
To me these arguments involving decoherence only show that the classical world
is _consistent_ with the quantum world, and indeed place constraints on the
two theories in order to achieve this consistency. But in no way does
decoherence (exponential suppression of off-diagonal terms in a density
matrix) imply collapse of the state vector. In other words, we don't know why
anything happens!

It's such a beautiful perplexing quandry, and I just don't understand this
attitude of "nothing to see here, move along".

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whiddershins
What if whatever force makes time asymmetrical also is at the root of volition
and consciousness. So by definition we would only be able to experience time
in one direction.

~~~
dmreedy
Scott Aaronson touches on this idea a bit during a discourse with Roger
Penrose[1], commenting on the unification of entropy and information:

"So on the picture that this suggests, to be conscious, a physical entity
would have to do more than carry out the right sorts of computations. It would
have to, as it were, fully participate in the thermodynamic arrow of time:
that is, repeatedly take microscopic degrees of freedom that have been
unmeasured and unrecorded since the very early universe, and amplify them to
macroscopic scale."

\---

[1]
[http://www.scottaaronson.com/blog/?p=2756](http://www.scottaaronson.com/blog/?p=2756)

~~~
eli_gottlieb
Well, any kind of computation that erases information (for instance, taking
two bits and using them to produce one bit in an AND gate) _does_ have to
increase entropy, so those two things are already one and the same.

------
Houshalter
I really like this theory:
[http://lesswrong.com/lw/fok/causal_universes/](http://lesswrong.com/lw/fok/causal_universes/)

But regardless, what would it even mean for there to not be an arrow of time?
Such a universe would have egg shells on the floor, and then waves of energy
that just happen to hit the eggshells, at just the right velocity to propel
each piece to assemble an unbroken egg. What a remarkable coincidence that
would be. It would be very difficult, perhaps impossible, to make a universe
like that.

IMO it's just like cellular automata. You start with a "start state" of
something simple or random. Perhaps just a single cell. And then it evolves
into complex patterns as each time step _causes_ the next time step.

There are reversible cellular automata, that in theory don't have an arrow of
time either. You could start at any state, and reverse it, just as easily as
advance it. But unless you are remarkably clever, or lucky, at arranging the
start state, it will tend to increase in entropy over time. Or rather, as the
step counter increases.

~~~
gizmo686
>what would it even mean for there to not be an arrow of time?

Thermaldynamic soup? Our universe has an entropy based arrow of time because
it started in a low entropy state. If a hypothetical universe started in a
high entropy state, than forward time would appear no more probable than
backward time. Of course, such a universe would also not have eggshells; it
would just be a thermodynamical soup.

~~~
Houshalter
Well humans wouldn't exist in such a universe, and couldn't observe it.

But even if you started with a completely random cellular automata, it's often
common for order and structure to emerge. You could even say that entropy is
going in reverse. But causation and the arrow of time still only flows one
direction. Perhaps given enough time, life could evolve in such a universe,
and ask why time only goes one direction.

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neom
I've been watching quantum for the last 5 years, let's just say the next 15
are going to be a fucking trip. ;)

<3

~~~
imglorp
It's been a trip since at least 1905 when we started looking into the
photoelectric effect. But yeah, I can't wait until we get some more
understanding of the current questions. That will reveal more questions, of
course!

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

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kordless
Upgrade the block universe to a _blockchain_ universe and you should see where
time's arrow comes from.

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empath75
Smolin's career seems to basically be entirely comprised of telling the only
people who are creating successful theories that their theories can't be
right, because he doesn't like the way they make him feel.

~~~
dmreedy
A hunch can be a powerful thing; sometimes, it's the symptom of a nascent
theory that hasn't hit the requisite critical model mass to attain that self-
sustaining "eureka" moment in the brain yet.

Of course, this validates truthers as well...

~~~
mark_edward
Or it's just nonsense like most hunches. I hope Smolin keeps on trucking but
hunch based contrarianism isn't actually a good indicator of truth.

~~~
maxharris
Have you read Smolin's _Time Reborn_? I've just finished it, and while I don't
agree with certain aspects of some of his ideas, I found it quite interesting
and valuable.

