Quantum Question

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Tchebu
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Re: Quantum Question

Postby Tchebu » Mon Dec 11, 2017 4:15 am UTC

Yeah, I suppose I ignored your addition of the timer/camera. Those should be treated as additional parts of the system. Having the camera take a picture at some time, basically does the same as having Alice look at the cat. It entangles the states of the camera and the cat, leaving the combination of them in superposition, but each of them individually as being in a density matrix indicating a classical distribution of non-superposition states.

Since Alice knows about the presence of the camera, she will also agree that this entanglement will happen and also conclude that the cat by itself won't be in a superposition as of 2:15 (although the cat+camera system will). Bob, also knowing the experimental protocol, will similarly conclude the same thing. Nobody will conclude that the cat will be in superposition right after 2:15 strikes.

What is different between all these POV's is not whether they think a given system is in a superposition or not, but "why" it's not in a superposition anymore. Some observers will say it's because their observation collapsed the wavefunction and others (typically higher-level observers) will say that it's because the system got entangled with whatever lower-level measurement apparatus interacted with the system. This disagreement does indeed indicate that this is a matter of interpretation. This is the main difference between Relational and Many-Worlds interpretations. Relational just lives with the disagreement, while Many-Worlds invites you to "think bigger" every time you're tempted to invoke "collapse" and just think in terms of a larger system, where the destruction of superposition is due to entanglement with some part of a larger system.
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Re: Quantum Question

Postby ucim » Mon Dec 11, 2017 5:25 am UTC

Tchebu wrote:Having the camera take a picture at some time, basically does the same as having Alice look at the cat. It entangles the states of the camera and the cat, leaving the combination of them in superposition, but each of them individually as being in a density matrix indicating a classical distribution of non-superposition states.
Ok, let's ditch the cat. Experiment is now a hammer over the clock, triggered by a nucleus decay (and carefully arranged so we can see the time it was destroyed). Alternatively, Alice is a very good forensic technologist, and can tell when the (original) cat died even without a camera. Either way, the experiment is Occamed a bit more (no need to consider the "cat+camera" system). In the first case, the (sole) clock is taken out of superposition, and in the second case, the cat is (but we can still determine the time of death).

I looked up the Relational interpretation; it was new to me, but seems to encompass what I'm getting at. Neither MW nor Copenhagen is very satisfactory; they seem like kludges to me. Relational at least hints at our looking at the wrong thing, in an analogous sense to relativity (in connection with time, space, and simultaneity) said that (referring to Newtonian physics) we were looking at the wrong thing.

At this point (in my thinking), while 4D spacetime is not philosophy, the interpretations of QM still seem to be. But at least RQM hints that there might be science behind it to look for (if the universe doesn't actually consist of particles, but of interactions, for example, assuming it "consists of" anything).

Now I'm going to have to look again at the math behind it all. (Never was my strong point.)

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Re: Quantum Question

Postby Pfhorrest » Mon Dec 11, 2017 6:17 am UTC

ucim wrote:(if the universe doesn't actually consist of particles, but of interactions, for example, assuming it "consists of" anything).

That was AN Whitehead's view.

Although there's really not much of a difference between ''made of particles" and "made of interaction" when your particle model gets down to the point where everything is fundamentally massless and moves at c, and slower particles are actually just the net effect of these massless particles interacting with each other, some of them constantly interacting via the omnipresent Higgs field and so seeming to have 'intrinsic' mass. Because from the frame of reference of a massless particle, there is no time or distance between the event that emits it and the event that absorbs it; its entire existence just is the interaction between creation and destruction. From a photon's point of view, your eye is presently touching the distant star that emitted it millions of years ago, and the photon itself just is that contact. (That is, that would be the case if it weren't for all the air between you and the star, which results in it not really being the same photon hitting you eye as travelled all that distance, but only a similar one re-emitted by the air nearest your eye at the end of a chain of absorbtions and emissions instigated by the original photon hitting the outer atmosphere).
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Re: Quantum Question

Postby Tchebu » Mon Dec 11, 2017 6:23 am UTC

Oh yeah, interpretations of QM are definitely philosophy. My only point is that superposition is a very real feature in almost all of them and is not observer dependent in any of them. (I say "almost", only to not leave out non-local hidden variables... but given your fondness for relativity, I doubt it's a serious contender in this discussion)

As for your setup with the clock recording the time of its demise, if the clock has a minimal time resolution, say just a minute hand that ticks, rather than moving slowly, then after T minutes, its state would be
|still ticking> + |broken at 1 minute> + |broken at 2 minutes> + ... |broken at T minutes>
suitably normalized, of course. If the outputs of the clock are continuous, the sum becomes an integral...

When Alice opens the box, she and the clock will form an entangled state that looks something like
|alice saw ticking clock>|still ticking> + |alice saw broken clock showing 1 minute>|broken at 1 minute> + ...
Just as before, simply being in this state already means the clock by itself is not in a superposition, although Alice might say that the superposition was removed by "wavefunction collapse". Again, no disagreement on whether superposition happens, only on how to think about why it disappeared.
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Re: Quantum Question

Postby tomandlu » Fri Dec 22, 2017 4:02 pm UTC

It might be worth mentioning that there are alternatives to 'classical' QM. De Broglie and Bohm, and their work on the pilot-wave model, are worth reading up on. There's a decent article on this in the 8 April 2017 issue of New Scientist, as well as the wikipedia articles below.

https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory
https://en.wikipedia.org/wiki/Pilot_wave
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Re: Quantum Question

Postby doogly » Fri Dec 22, 2017 4:17 pm UTC

De Broglie / Bohm is particularly rubbish
New Scientist, generally rubbish
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Re: Quantum Question

Postby tomandlu » Fri Dec 22, 2017 4:39 pm UTC

doogly wrote:De Broglie / Bohm is particularly rubbish
New Scientist, generally rubbish


Can you justify either of those statements?
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Re: Quantum Question

Postby doogly » Fri Dec 22, 2017 5:11 pm UTC

yes, but I'm working, and they're not novel observations so you can probably find a good explanation of their faults more quickly than I could write one.
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Re: Quantum Question

Postby tomandlu » Fri Dec 22, 2017 6:10 pm UTC

doogly wrote:yes, but I'm working, and they're not novel observations so you can probably find a good explanation of their faults more quickly than I could write one.


I found two criticisms, both from several years ago. One was based around a cover that had been co-opted by the creationist movement, but, seriously, that's the creationists' problem. The other was on some blurred physics, where even the critic acknowledged that NS do a great job "about 70% of the time".

As for DB/B, IANAS but it seems hasty to rule anything reasonable out until some fairly fundamental problems and questions about QM and co. are resolved.
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Re: Quantum Question

Postby doogly » Fri Dec 22, 2017 7:20 pm UTC

What problems do you have with QM? DB/B breaks locality, which is astonishing violence to our notion of reality, to no explanatory advantage. Also probably violates relativity, but people wiggle on that.

There's lots of irresponsible reporting. Here's a lil one.
http://backreaction.blogspot.com/2015/0 ... s-cat.html
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Re: Quantum Question

Postby tomandlu » Sat Dec 23, 2017 2:11 pm UTC

doogly wrote:What problems do you have with QM?


Well, they're not my problems... ;)

Grand unification (or lack thereof), black-holes and information, gravity, dark matter - all the things that, one way or another, indicate that our conceptual model is very incomplete. My reasonable (imho) gut-instinct is that there is something fundamental that has yet to be discovered that will cast what we know in a very different light. Then we get to see the whole elephant.
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Re: Quantum Question

Postby Tchebu » Sun Dec 24, 2017 3:53 am UTC

Grand unification (or lack thereof), black-holes and information, gravity, dark matter


None of those are problems with the conceptual foundations of quantum mechanics.

Grand unification and dark matter are both problems of finding specific quantum mechanical models that have these features and confirming them experimentally.

All recent (i.e. 20+ years) progress on black holes and information seems to indicate that it's our thinking about gravity that's at fault, not our way of thinking about quantum mechanics.
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Re: Quantum Question

Postby tomandlu » Sun Dec 24, 2017 3:02 pm UTC

Tchebu wrote:
Grand unification (or lack thereof), black-holes and information, gravity, dark matter


None of those are problems with the conceptual foundations of quantum mechanics.

Grand unification and dark matter are both problems of finding specific quantum mechanical models that have these features and confirming them experimentally.

All recent (i.e. 20+ years) progress on black holes and information seems to indicate that it's our thinking about gravity that's at fault, not our way of thinking about quantum mechanics.


I can't really justify it beyond a gut-feeling, and I should have said TOE rather than GUT, but I remain convinced that there is a fundamental missing piece, a connection, as significant as the discovery of atoms or relativity - if we don't understand gravity, how can we claim to understand QM? I'm no doubt an idiot, but I'd happily bet that one day, QM, and consequently physics in general, will undergo a huge revolution to arrive at TOE, rather than a progression.

That said, my silliest secret theory is that the speed of light is not constant, but is related to the size of the universe.
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Re: Quantum Question

Postby Eebster the Great » Sun Dec 24, 2017 9:04 pm UTC

doogly wrote:New Scientist, generally rubbish

You mean Darwin wasn't wrong?

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Re: Quantum Question

Postby Tchebu » Mon Dec 25, 2017 5:16 pm UTC

tomandlu wrote:
I can't really justify it beyond a gut-feeling, and I should have said TOE rather than GUT, but I remain convinced that there is a fundamental missing piece, a connection, as significant as the discovery of atoms or relativity


I mean... sure, there is. But there's also been progress towards it, and like I said, it strongly seems to lean in the direction of "it's GR's fault, not QM's".

if we don't understand gravity, how can we claim to understand QM?


Because QM is theoretically sound and can make accurate predictions to a dozen decimal places, and GR explicitly spells out its own demise on a theoretical level (via singularity theorems) before you even start comparing it to experiment.

I'd happily bet that one day, QM, and consequently physics in general, will undergo a huge revolution to arrive at TOE, rather than a progression.


This is tough to judge, but in some sense, the mere existence of string theory as a theoretical construct, kinda seems to speak against this. It's a fundamentally quantum mechanical theory, in terms of its mathematical formalism, and it explicitly contains quantum gravity. So on the theoretical level there doesn't seem to be any fundamental need for a revolution in the formalism of quantum mechanics. New perspectives on it, definitely, but not a full-blown overhaul like the transition from classical to quantum mechanics was.
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Re: Quantum Question

Postby tomandlu » Wed Dec 27, 2017 4:07 pm UTC

Tchebu wrote:New perspectives on it, definitely, but not a full-blown overhaul like the transition from classical to quantum mechanics was.


Well, yes. I certainly don't dispute the strong possibility - after all the blind men really could have been dealing with a snake, a fan, and a tree-trunk...
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Re: Quantum Question

Postby tomandlu » Thu Mar 15, 2018 7:59 am UTC

I read an article that suggested that quantum entanglement could be explained without spooky action if causality breaks down at the quantum level - i.e. a particle sends a message back in time to its twin about how it should spin, etc.

If true, IYHO would this qualify as a major revision of QM or just a refinement?
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Re: Quantum Question

Postby gmalivuk » Thu Mar 15, 2018 11:54 am UTC

I think sending a message back in time qualifies as spooky in the same sense.
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Re: Quantum Question

Postby doogly » Thu Mar 15, 2018 12:46 pm UTC

Entanglement is not actually spooky. Einstein described it that way, but Einstein is not correct. There are no violations of special relativity or causality in quantum mechanics.

If you instead wanted to model what is happening with a backwards propagating super determining, the violence you have done to our model of the universe is intense.

Not so intense that someone like 'tHooft wouldn't do it, and I respect him, but it still only slightly less nuts than the nonsense Penrose is up to.
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Re: Quantum Question

Postby p1t1o » Thu Mar 15, 2018 2:05 pm UTC

doogly wrote:Entanglement is not actually spooky. Einstein described it that way, but Einstein is not correct. There are no violations of special relativity or causality in quantum mechanics.

If you instead wanted to model what is happening with a backwards propagating super determining, the violence you have done to our model of the universe is intense.

Not so intense that someone like 'tHooft wouldn't do it, and I respect him, but it still only slightly less nuts than the nonsense Penrose is up to.


There's an actual definition for "spooky"?

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Re: Quantum Question

Postby doogly » Thu Mar 15, 2018 2:37 pm UTC

Violations of special relativity are spooky.
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Re: Quantum Question

Postby ucim » Thu Mar 15, 2018 3:30 pm UTC

doogly wrote:There are no violations of special relativity or causality in quantum mechanics.
No. Just a violation of the sense of what "is" is.

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Re: Quantum Question

Postby doogly » Thu Mar 15, 2018 3:57 pm UTC

Only if you've been temporarily confused by Schrodinger into thinking that this psi(x) business is meaningful. If you live your life classically caring about observables, and then quantum mechanically caring about observable operators, then your life is seamless and good. We just seem to think that the best way to rear up young physicists is to expose them to staggering confusion that gripped the field between flinty sparks of Planck and Bohr and the illuminating lamp of Dirac.
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Re: Quantum Question

Postby Tchebu » Thu Mar 15, 2018 6:36 pm UTC

A complete non-local hidden variables model that reproduces the predictions of QM and plays nicely with special relativity would count as a major revision of the theory in my book, because it introduces another layer of formalism from which standard QM follows. But as it stands, it's also completely unnecessary, unless this new layer also has additional consequences.

doogly wrote:Only if you've been temporarily confused by Schrodinger into thinking that this psi(x) business is meaningful. If you live your life classically caring about observables, and then quantum mechanically caring about observable operators, then your life is seamless and good. We just seem to think that the best way to rear up young physicists is to expose them to staggering confusion that gripped the field between flinty sparks of Planck and Bohr and the illuminating lamp of Dirac.


Wait.. what? It seems to me that only caring about observables is precisely what makes entanglement "spooky"... A good catchphrase for entanglement is "correlation without correlata", and that correlation is precisely what's being encoded in the total wavefunction of the system. Do you have some way to account for the correlations of measurement outcomes of an entangled pair without referencing some notion of their combined state?
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Re: Quantum Question

Postby doogly » Thu Mar 15, 2018 6:47 pm UTC

But states are not localized, so there isn't any way for anything to be nonlocal about what happens. The operators, which are localized, do not behave weirdly at all. That is, they all obey [A,B]=0 when A and B are acausal.
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Re: Quantum Question

Postby Soupspoon » Fri Mar 16, 2018 5:36 pm UTC

doogly wrote:staggering confusion that gripped the field between flinty sparks of Planck and Bohr and the illuminating lamp of Dirac.
If you don't go head on into the subject and deal with it Diractly, it all either sounds Bohring or leaves you looking as thick as two short Plancks.

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Re: Quantum Question

Postby PM 2Ring » Sat Mar 17, 2018 4:38 am UTC

tomandlu wrote:I read an article that suggested that quantum entanglement could be explained without spooky action if causality breaks down at the quantum level - i.e. a particle sends a message back in time to its twin about how it should spin, etc.

If true, IYHO would this qualify as a major revision of QM or just a refinement?

That sounds like Cramer's Transactional Interpretation of QM
https://en.wikipedia.org/wiki/Transacti ... rpretation which has always been one of my favourites.

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Re: Quantum Question

Postby tomandlu » Sat Mar 17, 2018 10:27 am UTC

PM 2Ring wrote:That sounds like Cramer's Transactional Interpretation of QM
https://en.wikipedia.org/wiki/Transacti ... rpretation which has always been one of my favourites.


HIs SF book sounds like it might be fun.

https://en.wikipedia.org/wiki/Einstein%27s_Bridge_(novel)
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Re: Quantum Question

Postby tomandlu » Sun Mar 18, 2018 9:07 am UTC

BTW can you tell, looking at an individual quantum particle, whether it exists in a universe of maximum or minimum entropy? Basically, is there a difference between the quantum state* of a young universe, and a universe that's reached full entropy?

* a somewhat vague description, but hopefully yswim. I'm basically wondering if we assume that time's arrow relies on entropy, then whether time's arrow might not apply to something immune to entropy. This is more SF than S, so apologies for the brain-fart.
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Re: Quantum Question

Postby doogly » Mon Mar 19, 2018 2:15 am UTC

You are asking a question about a single particle quantum state in the first sentence, and the state of the universe in the second, so could you connect some dots?
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Re: Quantum Question

Postby tomandlu » Mon Mar 19, 2018 8:11 am UTC

doogly wrote:You are asking a question about a single particle quantum state in the first sentence, and the state of the universe in the second, so could you connect some dots?


For simplicity's sake, single particles, but I'm also thinking of ratios - i.e. any statement about the whole that can be inferred from studying the singular. To coin a phrase, not seeing the wood for the trees is a truism, but if you count enough trees in a specific area, you can still conclude you're in a wood. Does that make sense?
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Re: Quantum Question

Postby p1t1o » Mon Mar 19, 2018 11:27 am UTC

Someone correct me if Im wrong, but:

If there is an entity present capable of observing a particle, you are not in a maximum-entropy-state universe.

If you're in a minimum-entropy-state universe, there are no particles to look at...yet.

So the answer is either: yes, you can tell, or; the question doesnt make sense.

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Re: Quantum Question

Postby tomandlu » Mon Mar 19, 2018 3:27 pm UTC

p1t1o wrote:Someone correct me if Im wrong, but:

If there is an entity present capable of observing a particle, you are not in a maximum-entropy-state universe.

If you're in a minimum-entropy-state universe, there are no particles to look at...yet.

So the answer is either: yes, you can tell, or; the question doesnt make sense.


I'm not so much interested in an observer (or even maximum-entropic-states), but whether entropy, and therefore by inference*, time's arrow, does not apply to quantum particles or, I suppose, quantum-level phenomena.

* according to some hypothesis
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Re: Quantum Question

Postby Eebster the Great » Mon Mar 19, 2018 4:03 pm UTC

No. Imagine a universe in a box which (ignoring gravity) is at maximum entropy. It's basically a big, homogeneous gas at thermal equilibrium. If you pick out a particle at random, it will have random properties. That doesn't give you any information at all.

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Re: Quantum Question

Postby p1t1o » Mon Mar 19, 2018 4:35 pm UTC

Eebster the Great wrote:No. Imagine a universe in a box which (ignoring gravity) is at maximum entropy. It's basically a big, homogeneous gas at thermal equilibrium. If you pick out a particle at random, it will have random properties. That doesn't give you any information at all.


Except that if the universe is a homogenous gas, *I'm* a homogenous gas and therefore cannot make any observations.
Ergo, if I *can* make an observation, I know that whatever particle I am looking at is not in a max-entropy universe.

I know that not exactly the question - its not a property of a particle giving you the information, but Im finding it hard to parse the problem in any other way. It is literally impossible to observe a particle in a max-entropy universe. If there *was* information to be gleaned, it would be impossible to extract.

Sorry if that sounds obnoxious, I dont mean it to, I know Im at a bit of an angle to the scope of the original question here.

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Re: Quantum Question

Postby gmalivuk » Mon Mar 19, 2018 4:49 pm UTC

The language might have been sloppy, but it seems pretty clear that tomandlu was asking whether the properties of one particle can carry information about the entropy of the whole universe.
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Re: Quantum Question

Postby ucim » Mon Mar 19, 2018 5:35 pm UTC

Naively, it seems to me that time's arrow is a result of the third law and the uncertainty principle, and as such, is a macroscopic thing. Consider an explosion; a classical case of increasing entropy. During the explosion (like at most any other time), the future is likely to be more disordered. Any given particle, when it interacts, will do so randomly (according to QM), and this (summed over all particles) is what drives the universe towards maximum entropy.

If we reverse all the momentums of each of the particles ("reversing time"), classically the explosion will "un-happen", but when QM sticks its nose into it, any given particle, when it interacts, will do so randomly. So, the particles will not retrace their steps - every interaction is an opportunity to "un-re-undo" what would have been the past. It is highly unlikely that all the random interactions will just happen to happen the reverse of the way they happened before. So, the un-explosion will most likely be a dud, and entropy again will increase. This is time's arrow.

Entropy is a property of a collection, not an individual particle.

So, my answer to what gmalivuk thinks tomandu is asking is "no".

Jose
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Re: Quantum Question

Postby elasto » Mon Mar 19, 2018 5:58 pm UTC

tomandlu wrote:HIs SF book sounds like it might be fun.

https://en.wikipedia.org/wiki/Einstein%27s_Bridge_(novel)

I read it many years ago and, while I don't remember the details, I did enjoy it.

As a reference point I am a big fan of Stephen Baxter for SF.

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Re: Quantum Question

Postby Soupspoon » Mon Mar 19, 2018 11:10 pm UTC

gmalivuk wrote:The language might have been sloppy, but
"…that's the basis for quite a lot of how the What-If questions are answered." :P

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Re: Quantum Question

Postby tomandlu » Wed Mar 21, 2018 3:57 pm UTC

ucim wrote:Naively, it seems to me that time's arrow is a result of the third law and the uncertainty principle, and as such, is a macroscopic thing. Consider an explosion; a classical case of increasing entropy. During the explosion (like at most any other time), the future is likely to be more disordered.

Jose


Is "more disordered" the right description? Disorder could be regarded as the domain of minimal entropy. Aren't maximal entitled systems incredibly uniform? They're only really disordered from an anthromorphic pov.

I like your "why we can't unexplode" hypothesis, but it's slightly irrelevant to my burbling, since I'm pondering whether the uncertainty might disappear if quantum particles are time agnostic.
How can I think my way out of the problem when the problem is the way I think?


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