> we think classically of the past as definite and the future in terms of possibilities- but in reality both the future and the past are indeterminate in exactly the same way
This has nothing to do with quantum theory. It's from classical physics already, from chaotic systems. For conservative systems, both predicting and retrodicting them is equally hard, due to high sensitivity of both to initial condition which we do not know exactly.
> it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy
This holds only in special cases, like when entropy of an isolated system increases (e.g. evolution towards equilibrium). When the system gets into equilibrium, entropy then remains constant, and both past macrostates and future macrostates have the same number of microstates, so for any state after that, "the number of futures" is the same as "the number of pasts".
> In fact you could define the past as being the direction in configuration space with the smallest uncertainty
In other words, you suggest to define future as that direction in which entropy increases. This is consistent with the usual meaning of future for an isolated thermodynamic system in a non-equilibrium process, evolving towards equilibrium; 2nd law implies entropy cannot decrease when comparing to the past macrostate, and entropy often increases when the new macrostate is reached.
But this is not usable definition in general. Earth, Solar System, Galaxy interact with their neighborhood; they are not such isolated systems in a macrostate, moving towards equilibrium. We do not know how to measure their entropy, to find out whether they move towards past or future. And if we somehow found entropy of Earth decreases, nobody would seriously suggest Earth is moving towards the past.
Instead, everything moves towards the future, just by definition, because that is how we think for ages. If spilled milk somewhere is observed to jump back into a cup, nobody will think the milk went back in time; instead, we will say that a very improbable thing just happened, violating prior experience and 2nd law, but that process would happen in time just as everything else, from the past towards the future.
Redefining "future" as that direction of a process in which "entropy is increasing" is really an unnecessary and flawed idea of future. We knew what future was before people invented kinetic theory and entropy; it's those events which we approach, anticipate, and when they come, we label them with a number for date and time on the clock, which (if it is running correctly) never decreases, only stays the same or increases. What entropy did in the meantime does not matter at all; a good clock does not care.
“ This has nothing to do with quantum theory. It's from classical physics already, from chaotic systems”
Not exactly. What’s important here is that every past is real. In the sense that different paths a particle can take can interfere with each other. It’s not indeterminate in a some fuzzy intuitive sense of “something we don’t know.” every past that matches the current known state of a system is real. That’s something that is included in the package of “quantum weirdness.”
And, if you make the assumption that every future is real in the same way, that’s the many worlds/decoherence interpretation.
You are using "real" in a way I don't recognize. In my view, the past that is real is that which corresponds to and is consistent with records of past events. This allows for many different pasts, and many can have some plausibility. But if my diary on a given date says I popped my knee, then any past which is inconsistent with this record is not real.
In orthodox quantum theory, past events, even those that happened in experiments showing quantum effects, such as double-slit experiment with single particle, are determinate in the sense they can sometimes be retrodicted from the present knowledge, even when they could not have been predicted before they happened (e.g. which hole the particle went); only future events are not determined by psi.
No, that’s not correct- there are no hidden variables such that a particle really took only one path to a point and we just don’t know which one it was. It really did go through all of them. All the ones that are consistent with the current state of the system. (So, no I’m not talking about pasts that are not consistent like your popped knee).
And that’s super cool, and something not many people understand! It’s the basis of the schrodingers cat experiment- you’ve heard of it. The cat is really, actually both alive and dead before the system is opened to the world, assuming that no information leaves or enters the box. It’s not that we just don’t know. It’s actually in both states. And although it’s practically speaking impossible to do the experiment, you could throw different versions of the cat through some diffraction grating a zillion times and prove that yes, the live and dead version interfere with each other. They’re both real.
I hope I’m blowing your mind a little bit with this, or if not at least being entertaining :). It’s super counterintuitive!
> there are no hidden variables such that a particle really took only one path to a point and we just don’t know which one it was.
We don't know that. It is a statement of your belief in such interpretation of quantum theory.
But I wasn't suggesting hidden variables predict which path will be taken - that is a different idea. I admit my description was somewhat confusing. My point is that retrodiction of trajectory is much easier than prediction of trajectory, because of records of the past. In other words, past is constrained by existing records of the past, while future is not constrained by records of the future, because those do not exist yet.
This retrodicted past is still loaded with uncertainty stemming from the records having uncertainty, and is speculative and not experimentally testable, of course.
> It really did go through all of them. All the ones that are consistent with the current state of the system.
This is also a speculative statement that is experimentally untestable. And I agree it is consistent with quantum theory, if we add "...and consistent with records of the past". So both stances are just an interpretation of what is going on, there is no experimentally testable idea here.
>We don't know that. It is a statement of your belief in such interpretation of quantum theory.
No, Bell’s Theorem conclusively proved it.
From Wikipedia: “To date, Bell tests have consistently found that physical systems obey quantum mechanics and violate Bell inequalities; which is to say that the results of these experiments are incompatible with any local hidden-variable theory.”
Bell's Theorem does not prove that. It is a theorem itself, so this theorem has been proven using quantum theory and other assumptions in Bell's paper.
In the quote, notice "Bell tests...have consistently found" and the word local. So not the Theorem, but the "Bell test" experiments' results, when interpreted using theorems like the Bell theorem and similar, show nature manifests non-local behaviour. They do not prove your belief that hidden variables do not exist.
Also, I recommend using more reliables sources than Wikipedia to argue a point about physics. It is not a reliable source, even though it is useful for discovery and occasionaly is correct.
Oh ok. So bells theorem is wrong and I have to cite scientific sources to convince some random person on the internet.
Look: i have a degree in physics, and I also don’t have time to argue with you. You don’t want to learn? Your loss.
You’re one of those people for whom “winning” is more important than the truth, I think.
You know what? You win. You’ve outlasted me. You’ve successfully learned nothing, and nobody will ever care about your statements about bells inequality because they’re laughably wrong and nobody will read this anyway. Victory is yours!
I didn't say that, and I think that statement is wrong.
> i have a degree in physics
Oh my. You lose credibility in an argument about physics when you fall back on authority, and even more, when that authority is supposed to be you.
> I also don’t have time to argue with you. You don’t want to learn? Your loss.
But if you don't have time to argue your point, why did you post it and defended it, and only pull this lack of time now? I would like to learn something from your posts, but so far, you regurgigated the usual incorrect/misleading claims about quantum theory and what the Bell theorem and related experiments imply. So then I thought it is you who may learn something new - please check the article I gave you above. If you do not want to take it from me, take it from people in academia who are better experts on this topic.
> You win. You’ve outlasted me. You’ve successfully learned nothing, and nobody will ever care about your statements about bells inequality because they’re laughably wrong and nobody will read this anyway. Victory is yours!
I disagree, because my aim was to learn or make you learn something I know, and so far I think I didn't succeed in any of those. So, please check the article in the link, if you have genuine interest in this topic.
This has nothing to do with quantum theory. It's from classical physics already, from chaotic systems. For conservative systems, both predicting and retrodicting them is equally hard, due to high sensitivity of both to initial condition which we do not know exactly.
> it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy
This holds only in special cases, like when entropy of an isolated system increases (e.g. evolution towards equilibrium). When the system gets into equilibrium, entropy then remains constant, and both past macrostates and future macrostates have the same number of microstates, so for any state after that, "the number of futures" is the same as "the number of pasts".
> In fact you could define the past as being the direction in configuration space with the smallest uncertainty
In other words, you suggest to define future as that direction in which entropy increases. This is consistent with the usual meaning of future for an isolated thermodynamic system in a non-equilibrium process, evolving towards equilibrium; 2nd law implies entropy cannot decrease when comparing to the past macrostate, and entropy often increases when the new macrostate is reached.
But this is not usable definition in general. Earth, Solar System, Galaxy interact with their neighborhood; they are not such isolated systems in a macrostate, moving towards equilibrium. We do not know how to measure their entropy, to find out whether they move towards past or future. And if we somehow found entropy of Earth decreases, nobody would seriously suggest Earth is moving towards the past.
Instead, everything moves towards the future, just by definition, because that is how we think for ages. If spilled milk somewhere is observed to jump back into a cup, nobody will think the milk went back in time; instead, we will say that a very improbable thing just happened, violating prior experience and 2nd law, but that process would happen in time just as everything else, from the past towards the future.
Redefining "future" as that direction of a process in which "entropy is increasing" is really an unnecessary and flawed idea of future. We knew what future was before people invented kinetic theory and entropy; it's those events which we approach, anticipate, and when they come, we label them with a number for date and time on the clock, which (if it is running correctly) never decreases, only stays the same or increases. What entropy did in the meantime does not matter at all; a good clock does not care.