Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Quantum Entanglement Drives the Arrow of Time

simonsfoundation.org

Post reply on HN
Print view View original

131–140 of 151 posts

Re: Quantum Entanglement Drives the Arrow of Time

#131

Post by clavalle »

andy112 wrote: post #124
clavalle wrote: post #101

Earlier quoted context omitted.

Sure (as far as my layman's understanding of quantum physics goes). So, you have independent physical system (say, a photon). Before it interacts it exists in quantum superposition. It has the potential to exist in any possible configuration and, in fact, seems to exist in all of them simultaneously. Almost like a variable of a certain type but this type is kind of special in that some possible values are more likely…

Very cool way of thinking about it! Thanks for that elaboration. Re: SoftwareMaven's "why is the 'player character' so important?", I don't think the player character we follow is necessarily any more important than the other player characters. You can say every individual character (or particle in the real-world side of the analogy) has an importance only within its own reference frame and I think it still works. Ju…

I really don't know.

I need to research more into what it means to have a collapse of state. Also whether a state of superposition is one that a system can return to or if once a system has collapsed if it stays that way. And if a system can return to superposition, what makes that happen?

Re: Quantum Entanglement Drives the Arrow of Time

#132

Post by coldtea »

cousin_it wrote: post #130
im3w1l wrote: post #129

Earlier quoted context omitted.

I think of the arrow of time like this. We started in an extremely exceptional state of very low entropy. The state space is very high dimensional. Therefore the entropy is much higher, pretty much everywhere we go. In this space we do a random walk. Now, the mystery that remains to be solved is: Why did we start with such an exceptional (low entropy) state?

One possible answer is based on complexity: we were more likely to start in a low entropy state because it had a description with low Kolmogorov complexity, or something like that. (K-complexity induces a probability distribution on the state space that is very different from the uniform distribution.) If that reasoning works, it seems to reduce the question "why did we start in a low entropy state?" to the question…

>Another possible answer is anthropic: you see an ordered universe because otherwise it wouldn't create an ordered mind like you.

Well, DUH.

This, so called, "anthropic" answer is still a naive non-answer. It's as if those that use the anthropic priniple in this sense have a very shallow understanding of Epistemology and/or Philosophy of Science.

It's obvious that the question concerns at least one level deeper than what the "anthropic principle" attempts to answer. Now just "how come" (possibillity of it being observed by an ordered entity) but "why" (actual mechanism causing this).

Re: Quantum Entanglement Drives the Arrow of Time

#133

Post by Tarrosion »

I found the article rather confusing: it starts out by saying look, the laws of physics make sense forwards or backwards. But we only see one kind (entropy increasing) of process. Why is that? Entanglement.

Is there something about entanglement that is irreversible? As the article says "it is the loss of information through quantum entanglement, rather than a subjective lack of human knowledge, that drives a cup of coffee into equilibrium with the surrounding room." Okay, but then why don't we ever see the reverse making coffee depart from equilibrium? Something like the acquisition of information through breaking entanglement drives a cup of coffee away from equilibrium.

Re: Quantum Entanglement Drives the Arrow of Time

#134

Post by [deleted] »

Confusion wrote: post #85
loup-vaillant wrote: post #57

Earlier quoted context omitted.

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, wh…

[deleted]

Re: Quantum Entanglement Drives the Arrow of Time

#135

Post by judk »

arh68 wrote: post #118
tim333 wrote: post #8

The reasoning sounds a bit iffy as in: “Finally, we can understand why a cup of coffee equilibrates in a room,” said Tony Short, a quantum physicist at Bristol. “Entanglement builds up between the state of the coffee cup and the state of the room.” I think you can understand coffee cooling quite well without any quantum stuff - the atoms in the coffee are moving faster than those in the room. There will be a tendency…

I think the author pretty clearly explained the connection: the arrow of time does not seem to follow from the underlying laws of physics, which work the same going forward in time as in reverse. By those laws, it seemed that if someone knew the paths of all the particles in the universe and flipped them around, energy would accumulate rather than disperse: Tepid coffee would spontaneously heat up What he's saying is…

Two objects always attract under gravity, never repel under gravity. Why is this less mysterious than thermodynamic equilibrium, which is some sense just an opposite of gravity (repulsive instead of attractive)?

Re: Quantum Entanglement Drives the Arrow of Time

#136

Post by judk »

DonGateley wrote: post #24
dkarapetyan wrote: post #10

Earlier quoted context omitted.

The physical implications of entanglement might be strange but on a mathematical level it's all just vector addition so it's not really that strange.

The strangeness is the fact that they add and that the sum is as physically meaningful as the components. In a nutshell, so to speak.

Gleason's theorem speaks to that. Mathematically, any "well-behaved" density function can be expressed as linear combination of quantum states

Re: Quantum Entanglement Drives the Arrow of Time

#137

Post by chm »

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…

It's important to capitalize the A in Aspect, as it refers to the name of the physicist.

http://en.wikipedia.org/wiki/Alain_Aspect

Re: Quantum Entanglement Drives the Arrow of Time

#138

Post by judk »

baddox wrote: post #63
x3c wrote: post #46

Earlier quoted context omitted.

Isn't the classical definition of Arrow of time is from the system with less entropy to a system with more entropy? What is lacking in this definition of Arrow of time that we need to take it to Quantum level?

Again, probability is just a model that can predict something, but not explain how it works. Take a pseudorandom number generator and do rand() > 0.5. Your probability is 50%, and that is a pretty good model to predict how the subroutine will behave, but it may also be useful to understand what algorithm the PRNG is using.

In physics, a "model" is the same as "how something works", unless and until you can uncover details (like the precise order of generated numbers) that the old model can't express.

Re: Quantum Entanglement Drives the Arrow of Time

#139

Post by stillsut »

Confusion wrote: post #85
loup-vaillant wrote: post #57

Earlier quoted context omitted.

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, wh…

Can't I just weigh Galileo's cannon ball, then based on T from when I dropped it know its momentum?

I think the difference comes on moving past the macroscopic world: say I drop a uranium atom into Shroedinger's box - I can't calculate at time T whether the cat is alive.

Re: Quantum Entanglement Drives the Arrow of Time

#140

Post by judk »

nzp wrote: post #104
millstone wrote: post #31

> After some time, most of the particles in the coffee are correlated with air particles; the coffee has reached thermal equilibrium. No doubt this is some way oversimplified explanation, but it still makes no sense. Say I have hot coffee and lukewarm coffee. The lukewarm coffee will equilibrate faster. Does it interact with the air faster? What if I bring in coffee that's the same temperature as the air, so that it'…

> Say I have hot coffee and lukewarm coffee. The lukewarm coffee will equilibrate faster. It won't (all else being equal). :) Because of Newton's law of cooling, hot coffee will cool down faster (precisely, the rate of change of temperature of a solid body is proportional to the difference in temperature between the body and the environment).

Parent meant "sooner" not "faster". The derivative of temperature was not intended, the dervative of entanglement was intended.
Post reply on HN