
The Quantum Origin of Time - dboreham
http://www.bbc.com/earth/story/20160708-the-past-is-not-set-in-stone-so-we-may-be-able-to-change-it
======
fernly
> If you watched a video of two billiard balls colliding and bouncing away,
> you would be unable to tell if it was being run forwards or backwards.

I've seen this image used more than once to make the point, but now I think
about it, I don't think it works. It is only acceptable if we think of
billiard balls as ideal elastic spheres. They are good illustrations of the
concept of "ideal elastic sphere" in our material life -- but they aren't,
really, and that destroys the thought experiment.

Let's run the tape at super-duper slo-mo, shall we? The two balls slowly
approach, collide, rebound. But now at super-duper slow-mo we can see that,
being actual collections of atoms, they behave somewhat like water-balloons.
At the point of contact, there is deformation, flattening, and rings of
compression waves radiate away, to meet at the other side of sphere,
interfere, and come back. (Something of the sort happened to Saturn's moon
Mimas.)

So if we play the video in reverse, it is quite easy to tell the "before" from
the "after". In reverse, we have balls that, as they roll toward each other,
develop increasing concentric ripples of deformation that start from the
opposite sides and shrink rapidly to the contact faces, disappearing
completely at the moment of contact, and two unperturbed balls separate.

The only way the billiard balls image works is if we instead use ideal perfect
spheres of adamantium. Even balls of titanium or diamond would have some kind
of internal elastic shape change that would exist only "after" and not
"before" the collision.

Which is simply to admit that in the real world, there are no collisions that
are actually reversible even in principle.

~~~
s_q_b
It's the old,"A farmer has three chickens that won't lay eggs. So he calls a
vetrinarian, and he can't help. So he calls a biologist, and she can't help.
So finally he calls a physicist.

The physicist takes copious notes, precise measurements, and spends the rest
of the day furiously scribbling in his notebook. Finally the physicist shouts
excitedly, and rushes into the farmhouse.

The farmer says, "Did you figure it out?" And the physicist says, "I did! But
it only works for spherical chickens in a vacuum."

~~~
philipov
I thought the canonical version of the joke was about a spherical cow[0].

0:
[https://en.wikipedia.org/wiki/Spherical_cow](https://en.wikipedia.org/wiki/Spherical_cow)

~~~
s_q_b
Idealized spherical cows in a vacuum reduce to idealized spherical chickens in
a vacuum if you work out the math ;)

------
chris_va
If you want to really blow your mind with regards to time, look at QED. I
highly recommend the Feynman lectures.

The net result of quantum interference is that light tends to move at the
speed of light, particles tend to move forward through time, but only because
everything else mostly cancels itself out.

(The rest of this is slightly rambling, but so was the article so it's hard to
comment without matching it, so dusting off my very rusted physics degree...)

With Multiple-Worlds, I think the delayed-choice experiments more show that
each universal snapshot is inherently self-consistent, rather than retro-
casual. Any non-consistent snapshot is just canceled out. Meaning, there was
probably a universe in which the 2-slit delayed-choice experiment didn't show
interference, but it didn't survive past the delayed choice boundary. Now, the
really big question (to which people keep arguing no, but I have doubts) is
can you establish faster-than-light communication using a delayed-choice style
effect by essentially canceling out any casualty that doesn't agree with your
communication.

My personal favorite for explaining the directionality of time is looking at
the boundary conditions. If you assume that at time T0 around the big bang,
the universe looked like X, then it requires that particles move away from it
in spacetime (otherwise it wouldn't look like X anymore). Thus, there is
probably an antimatter version going the other direction in time away from us.
Away from the boundary, entropy-like effects would prevent the odd positron
coming back in time at just the right moment to produce a non-local causality
violation.

~~~
andrewflnr
That sounds like it might be testable by looking at the CMB for evidence of
interference with the antimatter universe. It lso looks a bit like a solution
to the whole "why is there more matter than antimatter" question. That's a
very interesting hypothesis you've got there. :)

------
electrograv
It seems that if we speak of information traveling "backwards in time", we
must make an assumption of a global time (against which to travel) -- but
isn't that (global synchronized time) known to be a myth?

What if each particle/object has its own relative time frame? Doesn't that
solve the problem without needing to invoke "retro-temporal causality"?

For example, in the double slit experiment, perhaps the wave/particle and the
slits form a local time frame group, within which no further waveform collapse
is necessary. When this group interacts with the "environment", it may or may
not have to collapse a large set of possibilities into a smaller set. If that
environment includes a system which requires particle-like behavior, the
interaction resolves to particle like behavior (as opposed to wave
interference). The interaction of the "slit-particle/wave group" with the
"environment group" would be a "synchronization event" of sorts which actually
determines "causality". No backward time travel necessary if the "arrowed"
forward motion of time _is_ the act of waveform collapse. Causality of this
sort would affect trajectories we perceive as spanning into the "past"
(because we tend to want to view time as globally synchronized), even if in
reality that whole past-present trajectory was just a superposition of all
possibilities, until resolved via interaction of other "quantum timeframes".

Does that make sense to anyone else? Apologies if this is obviously true or
false -- I'm not a physicist, but do love thinking/learning about this stuff
:)

~~~
danbruc
Not a physicist either, but I will give it a shot. Relativity tells us that
their is no universal time ordering of arbitrary events, you can find
reference frames where A occurs before B and where B occurs before A if the
events occur at different positions in space. There is nonetheless still a
global causal ordering of events - events can only be influenced by events in
their past light cone and they can only influence events in their future light
cone and light cones do not change when you switch reference frames so
everybody agrees on this causal ordering.

I also think your view of quantum effects has some misconceptions. A photon is
what it is, it does not switch between particle like and wave like behavior.
It goes neither through one slit, nor through the other, nor through both or
neither. It is really different from a classical wave or a classical particle.
But that is just the way the universe works, the strange thing is not what the
photon does, but that macroscopic objects do not behave like quantum objects.

I am not so sure about that one, but I think there are not so many people
believing that the wave function collapse is a real fundamental thing. It is
just incompatible with the unitary evolution of the wave function and causes
you all the trouble figuring out what constitutes a measurement and what not.
This is obviously an open problem but the collapse postulate is probably
viewed as an approximation at best by most.

And the problem of the arrow of time kind of sits on top of the problem what
time is in the first place. One view I picked up recently makes a lot of sense
to me but I did not have much time yet to really look at it in some depth.
Anyway, here we go. Fundamentally all fundamental particles are massless and
travel at the speed of light. This kind of removes the need for time, massless
particles travelling at the speed of light experience no time and if
everything just moves at the speed of light there is no real need to use time
to describe velocities, changes of position over time, because it is implied
due to everything moving at the speed of light.

But then fundamental particles interact with each other, they attract and
repel one and another. This opens the possibility that the composite system of
several fundamental particles moves at a speed slower than the speed of light
while all the constituents still just move at the speed of light. It are only
the composite systems that are not following lightlike trajectories which
experience time. One just compares the rate at which processes occur in
composite systems to establish a time base.

Take a photon clock, a photon bouncing back and forth between two mirrors a
fixed distance apart. All the electrons, quarks and gluons in the mirrors
travel at the speed of light but the composite system is at rest from your
point of view. The events of the photon hitting one mirror and then the other
establish a time base for you. If you then boost the photon clock away from
you with a very high speed, maybe close to the speed of light, the photon
takes longer to bounce back and forth between the mirrors because it moves
still at the speed of light but the mirror kind of runs away from the photon,
that is time dilation. And if you boost another person along with the mirror
and look at the cells of that person you see the same thing, molecules take
longer to move around and reach their destination because all the parts of the
cell move with a high speed and velocities do not simply add linearly, the
person ages slower.

As said, I had not yet the opportunity to really look into this way of
thinking. Especially I am not sure if really all mass stems from dynamic
processes or if there are some fundamentally massive things out there. Maybe
someone can provide some further insides whether this is a valid or helpful
way of thinking about it.

~~~
lisper
You might enjoy this:

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

------
Florin_Andrei
> _In other words, a putative "future memory" is fine-tuned to a particular
> outcome, and must be readjusted if any slight change occurs between now and
> the appearance of the "remembered" future state. That means it is too
> fragile to count as a genuine memory._

Or perhaps it's enough if "future memories" just become more and more fuzzy as
they refer to events further and further away into the future.

------
dahart
"Yet none of this one-way flow of time is apparent when you look at the
fundamental laws of physics: the laws, say, that describe how atoms bounce off
each other. Those laws of motion make no distinction about the direction of
time."

I'm not certain precisely what laws we're talking about, but I'm having a
reaction that this argument may be specious due to the fact that the known
fundamental laws of physics are human constructions, not the entirety of all
that is possible. Most of the physical laws I know were specifically designed
to factor out time; conservation of mass & energy equate state before and
after an event, and prove nothing else. This article seems to be suggesting
that the known "laws" of physics do encompass everything possible and thus the
lack of a law proving the existence of time somehow proves that time doesn't
exist (or is bi-directional).

------
lisper
If you like thinking about this sort of thing you might want to read this:

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

------
Aelinsaar
I wanted to like this article, but it seems very confused. The essential
premise, that the arrow of time is a QM process rather than a classical
thermodynamic one, isn't presented as much as reams of background on the
general topic are.

~~~
dboreham
I posted it more for its interest (to me) as a long scientific article from a
popular news outlet on a topic I'd never heard about before. It looks like it
may be a re-working of this older article :

[https://www.quantamagazine.org/20140416-times-arrow-
traced-t...](https://www.quantamagazine.org/20140416-times-arrow-traced-to-
quantum-source/) and of course the underlying scientific papers:

[http://iopscience.iop.org/article/10.1088/1367-2630/14/1/013...](http://iopscience.iop.org/article/10.1088/1367-2630/14/1/013063)

[http://math.rutgers.edu/~oldstein/papers/TypicalDecayLett03....](http://math.rutgers.edu/~oldstein/papers/TypicalDecayLett03.pdf)

~~~
Aelinsaar
I appreciate the additional reading very much.

------
EGreg
Actually isn't time's arrow pointing in the direction of information being
increased? Entropy as defined in Claude Shannon's information-theoretic sense
is actually correlated with information. More entropy means more information.
And indeed, intuitively, we have more information as time goes on (about the
past) and not the other way around. If you assume that information does not
travel backwards in time - then you once again get that information increases
in the direction of time. The article stated it wrong.

------
maxidog
Am I unusual in being constantly surprised when people assume time's arrow in
everyday conversation? I can never quite accept the sheer oddness and
asymmetry of it.

~~~
hanoz
It does sound very unusual to be surprised by that, yes. Although it is
understandable that your surprise is constant.

~~~
jessaustin
Well done! I'm reminded of an Ani Difranco song that was playing back in my
college days: perhaps parent is a goldfish?

------
robobro
The grammar is atrocious. And why are all of the paragraphs only a sentence or
two long? Is this the ELI5 quantum physics/philosophy column?

------
skc
God, the double slit experiment drives me absolutely crazy

------
dschiptsov
Axaxaxaxa

