The Quantum Thermodynamics Revolution (2017)

https://www.quantamagazine.org/the-quantum-thermodynamics-revolution-20170502/

by uberdru • 8 years ago

105 49 8 years ago

sagebird• 8 years ago

The article states:

"A central pillar of quantum theory is that the information — the probabilistic 1s and 0s representing particles’ states — is never lost. (The present state of the universe preserves all information about the past.)"

What if the current state of the universe could have been arrived at by two different historical paths? That would imply that neither one could be a preferred history, that we would have to consider the present state to be -- arrived at through both histories?

Do photons have their probability distributions with interference patterns on screens to resolve a some sort of preferential history information storage? Like there is not enough bits available to say which screen the photons went through so they must say both?

Retra4• 8 years ago

In order for us to meaningfully assert that two things are different, there must be some information difference between them. If the same universe results from two paths, then they are the same path. Ultimately because your brain is in that universe, and it's current state is the thing responsible for distinguishing histories.

What you suppose is logically inconsistent with that fact, and the whole idea is going to be unfalsifiable. The human brains' ability to distinguish possibilities is contingent upon the presence of information.

Do photons have their probability distributions with interference patterns on screens to resolve a some sort of preferential history information storage? Like there is not enough bits available to say which screen the photons went through so they must say both?

Yes, photons are said to go through both slits because there does not exist any information which would distinguish the paths. As soon as you arrange an experiment which provides such information, the chosen path becomes clear.

foob• 8 years ago

Yes, photons are said to go through both slits because there does not exist any information which would distinguish the paths. As soon as you arrange an experiment which provides such information, the chosen path becomes clear.

This isn't correct. Photons are said to go through both slits because they travel like waves and actually go through both slits. How would your interpretation account for the fact that a single photon at a time fired through a double slit still produces an interference pattern?

Koshkin3• 8 years ago

because your brain

Can't we just agree that "brain" has no place in objective science (unless it's the brain science, of course).

jessriedel3• 8 years ago

If the same universe results from two paths, then they are the same path.

No, one could easily imagine fundamental laws of physics which map multiple states at one time step to the same state at the next. Indeed, the emergent laws of thermodynamics, which govern the accessible physical quantities of macroscopic systems, have this feature. It turns out in our universe that this apparent irreversibility is due to a special initial state and information being dumped into inaccessible microscopic degrees of freedom, so that statistical mechanics and atomic theory can be used to derive the appearance of thermodynamics. But there's nothing internally inconsistent about a fundamentally irreversible theory.

kgwgk10• 8 years ago

If the same universe results from two paths, then they are the same path.

And if from the current universe two paths may result then they are the same path. So there is a single path?

memebox3v2• 8 years ago

I thought that two paths leading to a single path was equivalent to wave function collapse. When we percieve things as acting clasically, what has happened is that from our point of view, all those universes that are compatible with our current observations have collapsed into a single thing.

posterboy3• 8 years ago

I have a loose rule to read parentheses as brain-farts (I think, this applies here, two).

The quoted remark assumes the universe was a state-machine. Which is a tall order. On the other hand, it's just a reformulation of the "energy is never lost" hypothesis (axiom), or "entropy always increases". But that's really just pop-sci. As was pointed out up-thread, it's rather describing the human way of thinking. Which to me also implies that we like to exaggerate a little: "Of course I could figure out where you were last month, in principle, it's just ... I don't have time for that right now!"

Edited: I always mix up increase and decrease of Entropy.

krastanov2• 8 years ago

The quote is actually a great way to phrase in simple terms an important property of quantum mechanics. It is called "unitarity" or equivalently "time evolution is described by a unitary linear operator". Obviously, the technical terms would have meant nothing to the general reader, so they were simplified, but they are most definitely not (to quote you) "brain-farts".

PS It is more general than a classical state machine.

PPS https://en.wikipedia.org/wiki/Unitarity_(physics)

whatshisface7• 8 years ago

One property of the laws of physics is that paths through history do not cross. If it is possible for two situation to time-evolve in to one, then we've got the wrong laws of physics.

krastanov2• 8 years ago

For the sake of completeness, another way to say the same thing is: The laws of nature are the same if we reverse the direction of time.

Until you start talking about entropy and chaos and statistics (which is a fascinating can of worms), the laws that you use to predict the next state of a system are the same that you would use to calculate the previous state of a system.

To restate it in another way once again: The function mapping "state of the universe at time t" to "state of the universe at time t+dt" is a one-to-one map, hence two possible pasts could not have created the same present.

luk322• 8 years ago

Ok imagine you are seeing a screen and from behind you is coming a photon. How do you know whether there was a double slit behind you or not for a single photon?

Those are two situations that will lead to the same outcome and you don't know which happened.

Also reverse is possible. You have the same situation evolving into two different situations with double slit and interference.

Ari_Rahikkala• 8 years ago

Just to make sure I'm not completely confused here: "Information can't be lost" and "you can't arrive to the same state of the universe through two different paths" are two ways to state exactly the same thing, right? Regardless of the the details of the rest of your physics (though you do need various notions to build up that far - time, with at least a past and a present, the ability to call states the "same" or not, etc.)

For instance, in cellular automata, the way that you would state that same concept is "the update function is injective". In our universe's physics, AIUI injectivity is involved somewhere in the definition of unitarity.

hvidgaard• 8 years ago

It depends how you define information and state. If the state of the universe includes all history of previous states, then you cannot arrive to the same state by two different paths because that would imply a different history.

If information cannot be lost, it must mean that past states are included in the current state, and then the two statements are equivalent.

splittingTimes• 8 years ago

Strictly speaking there is no "single" historical path in the quantum world. Looking at Feynmans Path Integral method, you come to realize that the probaboility of a system to transition from state A to state B is the (weighted) sum of the probabilities of all possible trajectories the system could have taken to get from A to B.

d--b• 8 years ago

As cool as it sounds, this is way beyond intelligibility for non-physics people. I don't understand the need for trying to vulgarizing anything quantum related.

I understand the Maxwell demon, and Bennet's argument, but everything after that is simply too hard.

knowThySelfx• 8 years ago

Both papers published simultaneously... How does that happen? But the probability of that happening seems to be more since the last 100 years ;)

And no, I don't have any research papers or statistics to back up my statements. Just saying from observing this pattern on reading upon many of the stories about inventions/discoveries etc.

BTW, what is the probability of others feeling the same about this "observation"?

memebox3v2• 8 years ago

Its because new ideas are built from old ideas. And old ideas travel around the world from mind to mind. When it is time for a new idea to be born, the ideas from which it is constructed are readily available. That is to say: When the last piece of a new idea is distributed rapidly, lots of minds can simultaniously arrive at the same insight. The same new idea. That is why in the modern age, as information travels faster and further, people are having the same idea at the same time, more often.

The Quantum Thermodynamics Revolution (2017)

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