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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Tchebu
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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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tomandlu
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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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tomandlu
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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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doogly
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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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tomandlu
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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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tomandlu
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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?

Tchebu
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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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