"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.
I read that to be more like "the naive interpretation of entanglement". The author even spends the remainder of the paragraph reminding (or educating) the reader that faster-than-light communication is not possible, and then the rest of the article is exploring retrocausality as another mechanism to explain entanglement with violating FTL restrictions.
> 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.
> 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).
"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.
> 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.
This is highly misleading. The arrow of time is not thermodynamic, it's quantum. It's defined by increased entanglement, not increased entropy.
That's not at all what I remember from physics class when I was at university, and would seem to imply (incorrectly) that non-QM models can't produce an arrow of time.
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.
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.
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.
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.
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."
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.
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.
I actually tried checking, to see if 'retro-casually', at around 1 us timescale, a more 'entropic' outcome of otherwise perfectly balanced source of physical randomness would show up more often than it should.
Used a 2-mbit source of randomness, been running the setup continuously to get a few terabits of data. Fixed a bunch of imbalances in the state-of-the-art commercially available source of physical randomness. Nope. Source code/results are available here: https://code.google.com/p/reasonable-deviations/source/brows...
What's the difference? I'm familiar with (and endorse) the Julian Barbour "time capsule" interpretation of time, and I just read the link. It cites the exact same facts, and provides the same explanations, that I do when demonstrating that the arrow of time is thermodynamic (ie time slices can only contain memories of "pastward" states as Gary Drescher puts it).
The QM explanation is not contrary; it's just a special case of the thermodynamic explanation; entanglement is one way entropy can increase, but it happens in the classical world too, which also has time symmetric laws.
So in what sense is someone erring when they call it a thermodynamic arrow of time?
I was reading about computation the other day and stumbled upon a concept where something can't be computable if it requires more operations than there is energy in the universe to do the operations. While reading the post you linked to, it struck me that the speed of light is a similar barrier to us. We can't travel the speed of light. We always like to talk about it and what we would 'observe', but it's theoretically impossible for us to do.
Why be so dogmatic? You can define an arrow of time going away from some initial ordered state even in a purely classical universe. No QM needed. Although the relationship between entanglement and entropy is worth considering for sure.
Is this an accepted viewpoint? I've never heard this before, and though I'm not a physicist I've certainly been taught that the arrow of time is thermodynamic.
Accepted by who? It's probably not the mainstream view (yet), but that's because there's a ton of misinformation out there about QM. For example, many people (even many physicists) think that the "measurement problem" (http://en.wikipedia.org/wiki/Measurement_problem) is still a problem. It isn't, and hasn't been for a very long time (decades). The reason people think it's still a problem is that the solution requires accepting the fact (and it is a fact) that classical reality isn't really real, it's just a very good approximation, and a lot of people are (understandably) resistant to that idea.
AFAIK this is an open question. My guess would be that yes, it does. I'm also guessing that the first person who actually does the math to prove it will win the Nobel prize in physics.
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.)
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.)
> 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). “That’s one of the examples of a particle probing the future and seeing what’s there, and then making a decision based on it, and just not decaying
Using the above analogy the message sent from the future (the man exists) to the past will prevent the grandfather to be killed just like the decaying atom, maybe the messages send back and forth from the future rewrites the events to prevent the paradox, something like backtracking until it works.
Outside of physics, the future influences the past all the time, though.
Imagine that you are in Stalinist Russia and win a minor scientific prize for a paper you wrote. There is a purge, and while you manage to not get deported to Siberia, the prize is reassigned to a less brilliant but more politically orthodox colleague, to the point that he is listed as first author of your paper.
He goes on to have a better career because of this paper, and by the end of the affair, nobody remembers that you wrote the paper to begin with.
If all observable effects of an events have been retconned, can that event be said to have occured?
Can a similar situation happen in physics, and if so, does it "count" if only macroscopic effects are reversed? (To keep with the analogy, in this case, everything but your memory has been changed, but your memory hasn't been - although it might over time, due to you wanting to avoid cognitive dissonance).
>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?
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.
It's just comparing time and space by analogy. Instead of two related events separated in time, we have the two ends of the popsicle stick separated in space. Obviously if an event A causes an event B to happen, then changing A will have an effect on B. In the same way, pushing on end A of the popsicle stick will affect end B. But with the Popsicle stick things also work the other way around; pushing on end B affects end A too. The jump is made here: if pushing on end B of the popsicle stick affects end A, then couldn't we imagine changing event B to have an effect on event A (even though A occurs before B)?
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.
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."
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.
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.