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The Quantum Thermodynamics Revolution (2017)
- sagebird 9y agoThe 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?
- Retra 9y agoIn 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 9y 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?
- BoiledCabbage 9y agoThere is DeBroglie-Bohm which says the photon only goes through on slit, the waveform goes through both. The singlr photon interacts with the waveform which influences its valid locations on the detector/screen producing the interference.
- Koshkin 9y ago> How would your interpretation account for the fact... Thing is, an interpretation cannot account for anything, it is a theory that does...
- posterboy 9y agoThe parent that said it's all subjective applies here, too. You still have to interpret the theory.
- trhway 9y ago>Photons are said to go through both slits because they travel like waves and actually go through both slits. not really. See mentioned below Hitachi electron double slit experiment. > 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? Because position is quantized, ie the position probability has that wave form, and thus position probability of hitting the screen has that interference looking pattern. There is no real physical interference though between any real waves here. It is just a position probability pattern formed as direct sum of 2 other patterns - the 2 cutouts made by the slits from the original wave pattern of quantized position probability. The Hitachi electron double slit experiment (https://www.youtube.com/watch?v=PanqoHa_B6c https://www.youtube.com/watch?v=PanqoHa_B6c) is much more illustrative because electron position quantization is different from the electron's DeBrogile - unlike photons. In Hitachi you can see that each electron hits the screen as a singular point and only the statistical aggregation of these hits - such aggregation naturally visualizes the position probability density - forms the "interference" pattern.
- Retra 9y agoWhat's not correct? Photons are said to go through both slits because they go through both slits? It is true that they go through both slits. That's why we say that they go through both slits. Because there's no information available to contradict that fact. I really don't know what you're disagreeing with.
- Koshkin 9y 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).
- danharaj 9y agoWhomst do you think performs the experiments?
- Retra 9y ago"Brain" is just a stand-in for any system which can correlate its state with another system.
- jessriedel 9y 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.
- Retra 9y agoI'm not talking about the reversibility of a theory. You can conjecture that our current reality could have been arrived at from two different paths. What you cannot do is assert the truth of a theory which selects one path in favor of the other, because the existence of any information which would make that determination would contradict that these both histories led to the same present. Thus there is no physical way to distinguish these histories.
- jessriedel 9y ago> I'm not talking about the reversibility of a theory. You asserted that there was something wrong in principle with many-to-one dynamics. I pointed out that we have consistent irreversible theories that are considered in-principle acceptable theories, and in particular are strictly preferred over one-to-one theories in their respective domains. > You can conjecture that our current reality could have been arrived at from two different paths. What you cannot do is assert the truth of a theory which selects one path in favor of the other, because the existence of any information which would make that determination would contradict that these both histories led to the same present. Just because two different pasts would have led to identical presents does not mean we don't have criteria (such as simplicity) to favor one past over another. Indeed, all of science is based on this. Ultimately all we can use to check our theories are our observations, and the actual microscopic state of the universe is vastly under-determined from these observations alone. Assumptions about simplicity and regularity must be deployed. As an extreme example: it's logically possible a cheesecake materialized in the center of the sun 1 second ago. (You can either think of this as a thermodynamic fluke, or as a new proposed fundamental theory where the standard model of physics is temporarily suspended 13.8 billion years after the big bang and exactly 1 cheesecake appears in the center of each star.) The cheesecake would be instantly destroyed, and it would not influence my observations, and thus there are two possible pasts (cheesecake vs. no cheesecake) which give the same present state and observations and we cannot categorically rule one out. Nevertheless, we assign astronomically low probability to the cheesecake past.
- kgwgk 9y 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?
- memebox3v 9y agoI 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.
- posterboy 9y agoI 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.
- krastanov 9y agoThe 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) https://en.wikipedia.org/wiki/Unitarity_(physics)
- posterboy 9y agoOK but is there a reason entropy is said to increase? I mean linguistically, if you will. What with "heat death" it doesn't sound too positive.
- whatshisface 9y agoOne 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.
- krastanov 9y agoFor 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.
- jessriedel 9y agoThis > The laws of nature are the same if we reverse the direction of time. and this > 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 are not the same. Even for deterministic theories, time-reversal invariance implies one-to-one mappings but the reverse is not true. And for a stochastic theory it breaks down completely; you can have time-reversal symmetry but still lose information about the past (e.g., each time step the system changes into a completely random state form the state space). Whether quantum mechanics is considered fundamentally stochastic or deterministic depends on interpretation. (Roughly, Copenhagen is stochastic and Many Worlds is deterministic.)
- luk32 9y agoOk 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 9y agoJust 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 9y agoIt 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 9y agoStrictly 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 9y agoAs 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.
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- knowThySelfx 9y agoBoth 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"?
- memebox3v 9y agoIts 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.