The Quantum Thermodynamics Revolution (2017)
quantamagazine.org
quantamagazine.org
"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?
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.
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?
Thing is, an interpretation cannot account for anything, it is a theory that does...
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) 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.
I really don't know what you're disagreeing with.
Can't we just agree that "brain" has no place in objective science (unless it's the brain science, of course).
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.
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.
And if from the current universe two paths may result then they are the same path. So there is a single path?
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.
PS It is more general than a classical state machine.
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.
> 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.)
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.
However, this mechanism is not necessarily a law of physics: we can explain all of the same results without it. So, deciding whether or not wavefunctions "actually" collapse is 100% philosophy.
If you are going to tell me that it is according to the eigenvalues of the observable operator it's not that different from saying that there is a collapse on one of the eigenvalues of the measurement.
And the question remains for your "isolated particles in a box doing multiverse". How is the partition of the wavefunction done if there is no preferred basis?
Edit: Maybe in your interpretation the only "physical" thing is the universe described by its wave function and those infinite multiverses are just mathematical "projections" of that wavefunction. But then how can a mathematical operation without any physical substrate explain anything about the physical world?
In Multiverse, the projection-onto-eigenbases statistical rule (which guides the partition of the wavefunction into conceptual universes), is seen as being like thermodynamics: statistical, and motivated to compress vast microscopic information into variables that are nice for humans. Someone who thought MV was the right idea would say that projections were a way to calculate the fraction of universes in which something was true, and thereby your probability of ending up in one where it was. In that view, it's emergent, instead of fundamental - like temperature. This reduces the number of fundamental ideas necessary by allowing projection to emerge instead of being asserted.
I don't see the point of the analogy. In statistical mechanics we have to consider all the possible microstates because we don't know which one is real. In the multiverse approach we know what universe is real, so what is the point in keeping all the universes that "could have been but are not" around? We know they are not real! Deriving thermodynamics from statistical mechanics we get a useful theory. What does the multiverse bring us?
The wave function can be interpreted epistemologically, as the expression of our lack of knowledge of the precise state of the universe (v.g. pilot wave theory). But there is no need for parallel universes that we know are not real, if you want to have virtual parallel universes they will be just those that we could be in as far as we know (and one of them will be the true one). The wave function collapse is in that interpretation the fact of narrowing the set of potential universes compatible with the actual one, as we learn more about the universe we live in.
In a nutshell the idea of multiverse is that the entire universe evolves as an isolated system, without any wavefunction collapse.
"Branching," to the extent that branching is a good word for what happens, already is known to be a behavior of wavefunctions: as a Gaussian pulse moves, it spreads out (due to dispersion inherent to the Schrodinger equation), and we as humans can arbitrarily call that branching. (But, like I said, it's continuous instead of discrete like the word branching would imply.)
So, what remains is to explain why we find the universe in a definite state, if it time-evolves into something other than specific eigenvalues. But, first, I'll ask you: what happens if you put a manned capsule inside of the isolated particle box, so that the person inside the capsule starts dispersing too?
As for your question: I don't know. Let's say the Schroedinger equation is actually not perfectly linear and there is spontaneous collapse which happens quite rapidly for a system with N~10^30 particles. Now what?
0) quantum mechanics is great, but it can lead to quantum systems described as a superposition of states and then we need to explain why we find the universe in a definite state.
1a) one option is to say that the wave function collapses to a definite state.
1b) another option is to say that there is a multiverse... and what remains is to explain why we find the universe in a definite state.
Solution (a) may be ugly, but “solution” (b) sets you back to the starting point!
Seriously, what do you mean by "will lead to the same outcome"?
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.
If information cannot be lost, it must mean that past states are included in the current state, and then the two statements are equivalent.
I understand the Maxwell demon, and Bennet's argument, but everything after that is simply too hard.
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"?