A Debate Over the Physics of Time
quantamagazine.org
quantamagazine.org
I don't get how people can say this. Quantum measurement (collapse of the statevector) is not at all time-symmetric. People seem to always leave this out in such discussions. They either view it is a temporary "problem" with the theory that we will one day fix, or just as something that is plainly not a theory. Or who knows what. The wikipedia article on interpretations of quantum physics has a giant table that illustrates the lack of consensus:
https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...
The equations are all time symmetric, yes. But the theory is not.
You might argue that the same is true for the collapse postulate and the Born rule but I have to disagree. Yes, it gives the correct answer in as many experiments, but those are parts of the experiments that we don't really understand. We have classical system interact with quantum systems and our measurement devices are just such gigantic quantum systems that we can't treat them quantum mechanically.
The collapse postulate is obviously true in a kind of classical limit, but it is really just an emergent law and nothing fundamental. If you could show that a wave function collapse can occurs in quantum system before we measure it, that would change the game. But that is not the case, the wave function collapse only occurs when we throw a gigantic classical system at the quantum system which we can not treat properly due to its complexity.
So I argue people are rightly ignoring wave function collapse when they talk about time symmetry because it is just an emergent phenomenon. And there are several ideas that can explain why it looks like a wave function collapse occurs, so we are not simply throwing a corner stone of our theory out of the window.
I would love to find out the answer to this! There needs to be some more surveys of physicists...
> And there are several ideas that can explain why it looks like a wave function collapse occurs
I don't think it is at all as simple as you are implying. This is still very much an open question, even if most physicists don't care to think about it.
Given that a reversible linear mechanics is so much more useful than one that isn't, I think it's way more likely that the universe isn't in fact linear, but the best way people got to mapping it was to shove all the non-linearity at the border of the mechanics, right before they get something from the experiment.
The result of a measurement is not dealt with by the equation. Schrödinger only lets you calculate the probabilities of measurement outcomes, but not which one it will be. That's why Einstein proposed we just didn't knew the whole picture. He thought we needed more information.
On the case of QM, that's the set of equations, observable operators, and the instructions of hoe to interpret them. None of those predict anything alone.
Besides, the Schrodinger equation does not predict how the state will evolve after a measurement is done. That's the entire problem.
And that state is a completely made up mathematical artifact.
And yes, the Schrödinger equation predicts exactly how a state evolves, even after a measurement. It does not tell you what the outcome of a measurement is - you give it the outcome, and it gives you the time evolution.
There is a problem, and it lies in what happens during measurement - or what is the exact nature of measurement. But given any state, the SE tells you how it evolves.
Einselection is my favorite potential solution. Straightforward, clean, unitary, and the math is promising. The idea is that apparent collapse is just emergent behavior of the schroedinger equation, via induced superselection rules between incompatible measurement eigenstates.
There are so many IMO's floating around when it comes to foundations of QM. I think that is fascinating in and of itself.
> Straightforward, clean, unitary, and the math is promising.
My IMO definitely does not coincide with your IMO :-)
It's such a beautiful perplexing quandry, and I just don't understand this attitude of "nothing to see here, move along".
The central idea behind many worlds is that the concept of "measurement" is extraneous and can be removed. When to particles interact on a quantum level, they become entangled. The behaviour of this entanglement is well understood and accurately described by pretty simple (but potentially counter-intuitive) math. Our best description of these systems involves them being in a superposition of many states. We have also observed in experiments these superimposed states turn back into pure states, in exactly the way we would expect from the time reversible equations governing quantom mechanics.
The idea behind many-worlds is that "measurement" is just a bunch of these quantom interactions. In theory, we could reverse these interactions in the same way we could reverse them at a two particle level. In practice, this never happens because it is the same idea as the Earth shooting out a giant asteroid. Theoretically possible as the time reverse of an asteriod hitting Earth, but massively improbable.
Well obviously I need to read more about MWI because this just looks like standard quantum physics to me. Or perhaps "Church of the larger Hilbert space". Is it just saying superposition of states looks like "many worlds"? This looks to me like a non-interpretation interpretation.
TIL the many-worlds interpretation is actually the many-worlds many-interpretations. That's so meta!
But I don't understand why do they talk about time reversability of physical systems and don't mention CPT symmetry[1]? Standard model is not actually time symmetrical, but you one should also reverse charge and reflect spatial coordinates to obtain symmetry after time reversal because there are violations of CP symmetry [2].
But physics can also be thought of as rules of the from "the joint probabilty distribution of preparations S_j and measurment results R_k is ..."
This in turn will depend on prior-probabilities for the S's and R's, and also the rule which tells us
P_jk \propto |<S_j|U|R_k>|^2 (where U is some unitary).
Now the S's and R's are different. The prior for S can have any probability distribution we can engineer, and can even be deterministic. But the prior for the R's must be completely uniform unless we post-select results.
So there is a real asymmetry, but it exists in the past and future boundary conditions that we work with, and not in the mechanics (quantum or classical) of what happens in between.
"So on the picture that this suggests, to be conscious, a physical entity would have to do more than carry out the right sorts of computations. It would have to, as it were, fully participate in the thermodynamic arrow of time: that is, repeatedly take microscopic degrees of freedom that have been unmeasured and unrecorded since the very early universe, and amplify them to macroscopic scale."
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But regardless, what would it even mean for there to not be an arrow of time? Such a universe would have egg shells on the floor, and then waves of energy that just happen to hit the eggshells, at just the right velocity to propel each piece to assemble an unbroken egg. What a remarkable coincidence that would be. It would be very difficult, perhaps impossible, to make a universe like that.
IMO it's just like cellular automata. You start with a "start state" of something simple or random. Perhaps just a single cell. And then it evolves into complex patterns as each time step causes the next time step.
There are reversible cellular automata, that in theory don't have an arrow of time either. You could start at any state, and reverse it, just as easily as advance it. But unless you are remarkably clever, or lucky, at arranging the start state, it will tend to increase in entropy over time. Or rather, as the step counter increases.
Thermaldynamic soup? Our universe has an entropy based arrow of time because it started in a low entropy state. If a hypothetical universe started in a high entropy state, than forward time would appear no more probable than backward time. Of course, such a universe would also not have eggshells; it would just be a thermodynamical soup.
But even if you started with a completely random cellular automata, it's often common for order and structure to emerge. You could even say that entropy is going in reverse. But causation and the arrow of time still only flows one direction. Perhaps given enough time, life could evolve in such a universe, and ask why time only goes one direction.
<3
Of course, this validates truthers as well...
Just like pop physics tends to distort people's perception of the field as a whole (e.g. giving the impression that 90% of theoretical physics is string theory) it also makes researchers' body of work look very different than it is.