Quantum Entanglement Drives the Arrow of Time • Hacker News Board

Re: Quantum Entanglement Drives the Arrow of Time

#81

Post by jules » Thu, Apr 17, 2014, 1:00 PM UTC

loup-vaillant wrote: post #79

mbq wrote: post #64

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 bo…

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 e…

> So it does look like a better candidate for the arrow of time.

I don't fully agree. You seem to agree that even in a fully classical world, you would experience physics as irreversible on a macro level. So we cannot dismiss subjective uncertainty (lack of knowledge that could in principle be known) a priori as an explanation for the apparent irreversibility of physics. Certainly quantum uncertainty is an additional component of uncertainty, but most likely it's not only quantum uncertainty that causes our experience of irreversibility. For instance if we create a gas of very heavy particles, we would still experience irreversibility, but the quantum effects would be negligible. It seems to me therefore that it would be a good idea to try to investigate to which extent our experience of irreversibility is due to quantum uncertainty and to which extent it is due to subjective uncertainty.

Re: Quantum Entanglement Drives the Arrow of Time

#82

Post by mbq » Thu, Apr 17, 2014, 1:10 PM UTC

loup-vaillant wrote: post #79

mbq wrote: post #64

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 bo…

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 e…

Assuming that we can breaks nothing; particles will concentrate, but without the barrier they will spread again and this would be a disturbance that simply averages out.

Re: Quantum Entanglement Drives the Arrow of Time

#83

Post by shasta » Thu, Apr 17, 2014, 1:14 PM UTC

loup-vaillant wrote: post #79

mbq wrote: post #64

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 bo…

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 e…

> 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?

Re: Quantum Entanglement Drives the Arrow of Time

#84

Post by agos » Thu, Apr 17, 2014, 1:22 PM UTC

mtrimpe wrote: post #65

joe_the_user wrote: post #55

Earlier quoted context omitted.

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 sys…

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

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

Re: Quantum Entanglement Drives the Arrow of Time

#85

Post by Confusion » Thu, Apr 17, 2014, 1:30 PM UTC

loup-vaillant wrote: post #57

cromwellian wrote: post #3

This Google Talk 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, bu…

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/ I'd say the math is even simpler there, yet the explanation go deeper. It's…

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.

Re: Quantum Entanglement Drives the Arrow of Time

#86

Post by gameshot911 » Thu, Apr 17, 2014, 1:50 PM UTC

shasta wrote: post #83

loup-vaillant wrote: post #79

Earlier quoted context omitted.

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 e…

> 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…

I'm on it. I've placed a broken egg on my table. I'll let you know the results.

Re: Quantum Entanglement Drives the Arrow of Time

#87

Post by thibauts » Thu, Apr 17, 2014, 2:08 PM UTC

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

Re: Quantum Entanglement Drives the Arrow of Time

#88

Post by jerf » Thu, Apr 17, 2014, 2:14 PM UTC

mbq wrote: post #64

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 bo…

Here's a crazy thought... the really stonking smart physics PhDs who have spent their whole lives working on this problem are perfectly aware of the classical 18th-century physics you are referencing, and they've found it an unsatisfactory explanation. Perhaps you might like to dig in a bit further before being so dismissive.

(As a bit of a hint, your argument circularly assumes the existence of a forward arrow of time to demonstrate the forward arrow of time. Your "simulation" snuck a forward arrow of time into its definition, then proceeded to prove it exists. This is not satisfactory.)

Re: Quantum Entanglement Drives the Arrow of Time

#89

Post by denom » Thu, Apr 17, 2014, 2:20 PM UTC

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?

Re: Quantum Entanglement Drives the Arrow of Time

#90

Post by snarfy » Thu, Apr 17, 2014, 2:48 PM UTC

mbq wrote: post #64

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 bo…

It depends which point of view. If I made a movie of the simulation and then played it backwards, the gas would go into the box.

There is nothing in your experiment that says this is wrong. In your experiment, you must wait, and therein lies the problem. You've implicitly put a forward arrow in there.