
The Quantum Mechanics of Fate (2014) - jeremynixon
http://nautil.us/issue/9/time/the-quantum-mechanics-of-fate?utm_source=frontpage&utm_medium=mview&utm_campaign=the-quantum-mechanics-of-fate
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chakademus
"The standard interpretation of entanglement is that there is some kind of
instant communication happening between the two particles."

This is flat-out not true. The standard interpretation is that the entangled
objects are part of a single quantum state. Measurement/observation of the
state collapses the wave-function, and does not necessarily imply
communication between the constituent parts.

~~~
mahranch
> This is flat-out not true. The standard interpretation is

Unrelated Query: How can something be "flat out not true" if it's only an
interpretation? Seems to be that what you're describing is a consensus or
popular view opposed to a hard fact or universal truth.

~~~
JadeNB
> How can something be "flat out not true" if it's only an interpretation?

I think that chakademus meant that it is flat-out not true that "the standard
interpretation … is that there is some kind of instant communication happening
between the two particles" (notice the scope of the quotation marks); that is,
that this is not the standard interpretation, not that this interpretation is
'wrong' (whatever that means).

------
mjrpes
"An atom that normally emits light will cease emitting when its surroundings
become incapable of absorbing that light. Thus one event (emission) depends on
something that does or doesn't happen in the future (absorption)."

What role does relativity play in this discussion?

From the point of view of a photon, there is no lapse in time between emission
and absorption. Emission and absorption happens instantaneously, since time
never "ticks" for the photon as it traverses space. So, at least from the
photon's point of view, there is no need to evoke the past/future causality in
this situation.

------
lisper
> the universe as a whole is skewed in the forward direction, because its past
> endpoint was highly ordered, and its future endpoint is highly disordered.

This is highly misleading. The arrow of time is not thermodynamic, it's
quantum. It's defined by increased entanglement, not increased entropy.

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

~~~
jeffwass
Sorry, your grand statement matter-of-factly dismissing thermodynamic arrow of
time needs a much better source than some computer scientist's armchair blog
about QM.

I know it's been recently trendy for armchair physicists to post grand
philosophical claims about the universe from some vague understandings of
Bell's Inequality and/or entanglement, and blurring the line between
philosophy of physics with physics. But your casual dismissal of entropic
thermodynamic arrow-of-time time in favor of purely increasing entanglement
ranks way high on the BS meter.

For context - I have a PhD in experimental condensed-matter physics, and TA'd
thermodynamics and statistical mechanics for several years. I left the field
of physics many years ago so maybe there were some notable recent developments
in quantum information theory that I missed. But I skimmed your linked article
and can't take it seriously. Would love a better source of you can provide
one, eg peer-reviewed (in a respectable physics journal) or written by an
actual career physicist at a reputable institution.

~~~
lisper
[http://arxiv.org/abs/quant-ph/9605002](http://arxiv.org/abs/quant-ph/9605002)

This doesn't discuss the arrow-of-time directly, but it's a pretty
straightforward corollary.

~~~
jeffwass
Cheers for the article, quite interesting.

I'd still maintain the original comment in the original article is correct,
that the arrow of time is still driven by 'thermodynamic' processes, where I'm
allowing thermodynamic really means statistical mechanical process, and
statistical mechanics includes (as it always has) quantum statistics of
quantum processes.

Ie, your arxiv link gives further quantum detail of the underlying macroscopic
measurements (eg, the schrodinger cat example).

But the irreversibility and arrow-of-time come from the enanglement of the
particle with the 10^23 atoms of the macroscopic observer system. And in that
sense the irreversibility is more due to the large size of the system, just
like a classical example (such as shaking a bottle of originally separated
black/white marbles) than the fact that there is quantum entanglement between
the particle and the measurement system.

So while Cerf and Adami identified interesting quantum processes to describe
underlying interactions, the irreversibility still comes from a statistical
consideration of the interaction with a macroscopically large system. Just
like the traditional arrow-of-time statement that you had originally refuted.

~~~
lisper
I respectfully disagree. I maintain there is a fundamental difference between
the Cerf & Adami process (which is really just decoherence) and classical
thermodynamic processes. That difference is: for ordinary thermodynamic
processes, the odds of observing a decrease in entropy with increasing time is
very small but nonetheless non-zero. In the decoherence model, the odds of
observing a reversed entanglement is exactly zero because measurement _is_
entanglement. As a practical matter it amounts to the same thing, but in
principle it is as different as GR is from Newtonian mechanics.

~~~
jeffwass
Your arxiv link disagrees with you. From the paragraph after equation 6.6 :

>This irreversibility is completely equivalent to the irreversibility in
classical mechanics. Indeed, classically, to reverse the microscopic time
evolution, it is necessary to invert the velocity of all the particles, the
practical impossibility of which gives a macroscopic irreversible aspect to
time evolution. In quantum mechanics, it is necessary to undo any unitary
evolution associated with all interactions that particles have undergone, so
that reversibility is practically impossible if a macroscopic number of
particles have been involved. We are led to conclude that irreversibility is
not an inherent feature of quantum mechanics.

~~~
lisper
I know that looks like a disagreement, but it's actually not. Note that I said
the odds of _observing_ a reversed entanglement is exactly zero, not that the
odds of it actually _happening_ were exactly zero. The odds of it _happening_
are comparable to the odds of thermodynamic reversal. The difference is that
if a thermodynamic reversal _were_ to happen, it could be observed. If a
quantum reversal were to "happen" (I put "happen" in scare quotes because the
whole concept of something "happening" becomes a little murky here) it could
not possibly be observed, because any attempt to observe it would "prevent" it
from "happening."

~~~
jeffwass
lisper, you're obviously a pretty smart guy or gal. But your nit-picking here
over 'observing' vs 'happening' is so far from your original very bold claim
that the arrow of time is not thermodynamic but quantum. You even went so far
to say the original article was 'misleading' because of this.

I had to call you on this. Bold claims require strong confirmation. And even
the arxiv paper you linked to makes the same statement that I defended - that
irreversibility of purely quantum processes is still due to the macroscopic
(ie large) system size, which immediately implies the same broad thermodynamic
(and statistical ensemble) considerations we've been assuming for the past
century.

~~~
lisper
We may just have to agree to disagree on this. IMHO the difference between the
arrow of time being due to entanglement and the arrow of time being due to the
2nd law is analogous to the difference between gravity being a force and it
being an artifact of the curvature of spacetime. Not a big difference as a
practical matter but it's a huge difference conceptually. And in an article
entitled "The quantum mechanics of fate" this difference matters.

Also, this is HN comments, not Physics Review A. Not everything that gets said
here needs to pass the highest standards of peer review.

------
increment_i
Wouldn't messages passing from the future to the past imply that the future
can then influence the past? Which of course introduces a slew of paradoxes. I
realize the concept is concerned with single particles, but could there exist
a series of messages that alters the original particles so much so as to
introduce a paradox? (e.g. in the vein of "the grandfather paradox" in human
time travel to the past, where a man kills his own grandfather.)

edit: clarity.

~~~
DennisP
Time travel doesn't necessarily introduce paradox.

Imagine you have a billiard table, on which there's a wormhole. If you roll a
billiard ball into the wormhole on the right trajectory, it will emerge three
seconds prior to entering the wormhole, hit its earlier self a solid blow, and
its earlier self will never enter the wormhole, creating a paradox.

But when you try, it emerges from the wormhole on an altered trajectory, and
strikes itself only a glancing blow. And why did it emerge with an altered
trajectory? Because it was struck a glancing blow.

This is the Novikov self-consistency principle [1]. Physicists have been
unable to find initial conditions that don't allow a consistent solution. On
the other hand, nobody has _proven_ that consistent solutions always exist.
And sometimes there are lots of consistent solutions. (I wonder whether that's
why quantum physics isn't deterministic, but I'm no physicist.)

[1] [http://en.wikipedia.org/wiki/Novikov_self-
consistency_princi...](http://en.wikipedia.org/wiki/Novikov_self-
consistency_principle)

------
lordnacho
>To think about this problem, consider the most prosaic of objects: a popsicle
stick. The stick will bend or buckle, depending on the pressure you apply to
both ends. Now imagine a popsicle stick whose ends are separated in time,
rather than in space.

I have to admit I don't know what that means. Anyone got a way to describe
this?

~~~
weinzierl
"to buckle" probably is meant in its technical sense referring to Eulers
formula where the boundary conditions play an important role.

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

In short:

If a structure fails by buckling _only_ depends on the following properties:

load on the structure, modulus of elasticity, moment of inertia, length of the
structure and a _factor, whose value depends on the conditions of end support
of the structure_.

The factor is a number between 0.5 and 2 depending only on the condition of
both ends. For example if both ends are free to rotate it is 1.0, if both are
fixed, it is 0.50, and so on.

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timinman
If the future or present influence the past, it undermines scientific study,
including, ironically, the hypothesis of the article, because disruption from
the future would be an unknown variable in every experiment. It's pretty hard
to understand what can happen outside the dimensions we are constrained by.

If the past is malleable, we could keep using the scientific method, but with
humility, like: "The experiment's data support my claim so long as there was
not unknown interference from actors unbound by linear time, space, and
matter."

That sentiment is not too far from the attitude of many of us, rational
people, who believe that in addition to humanly measurable causes, there are
also supernatural or spiritual realities to consider.

------
Fjolsvith
This is the scientific explanation of how occult magical systems work.

~~~
Desloges
I'm currently reading Cosmic Trigger (1977!) with an open skeptical mind and
this was the first thing I thought.

------
amelius
Why don't we just define time such that no communication to the past is
possible.

~~~
ChrisClark
Isn't that just the same thing as covering our ears and saying, "Na na nana
na, I can't hear you!"

If some causal effects might be travelling back in time, we should investigate
it instead of just saying, "I don't like it, so let's just define it the way
we want."

~~~
amelius
No it is not the same thing. It provides a different way of looking at the
same problem. Because if we force the direction of time to be always forward,
we would have to modify our understanding of physics to make that assumption
hold, leading to a different view of the same results, which could be
extremely useful.

