
A Quantum Theory That Peels Away the Mystery of Measurement - dnetesn
http://nautil.us/blog/the-quantum-theory-that-peels-away-the-mystery-of-measurement
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gexaha
There was a nice thread about these discoveries on physics stack exchange
[https://physics.stackexchange.com/questions/484675/does-
the-...](https://physics.stackexchange.com/questions/484675/does-the-new-
finding-on-reversing-a-quantum-jump-mid-flight-rule-out-any-inter/)

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ainar-g
Very interesting. Funnily enough, Wikipedia doesn't seem to know anything
about quantum trajectory theory. It's always amusing to find something that
_isn 't_ on Wikipedia.

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inciampati
It would appear to be very similar to the Bohmian interpretation.
[https://en.m.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theo...](https://en.m.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory)

There have been some nice papers on this. See "Experimental nonlocal and
surreal Bohmian trajectories"
[https://advances.sciencemag.org/content/2/2/e1501466.abstrac...](https://advances.sciencemag.org/content/2/2/e1501466.abstract).
They use weak measurements to make inferences about particle trajectories (ala
QTT).

Maybe the authors feel that they are working from a different theoretical
foundation. The foundational argument, that quantum systems have specific but
unobservable states, is the same in QTT and DeBroglie/Bohm models.

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pdonis
The article notes: "Achieving this degree of control and information capture
is very challenging."

This is an extreme understatement: for anything but the simplest quantum
systems in the simplest, most tightly controlled configurations, it's
practically impossible. That's not to say that the QTT experiments described
aren't impressive achievements; they are. But they should not be oversold
either.

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AnimalMuppet
I think it's more than that. If I understand correctly (and I may well not),
this kind of thing isn't possible within classical QM, _even in principle_.
The fact that we can do it at all, even in perfect circumstances, is enough.

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pdonis
_> classical QM_

There is no such thing as "classical QM". If you mean pre-quantum classical
physics, this kind of "information capture" certainly _is_ possible in
classical physics, regardless of how complicated the system is. It's only in
quantum physics that it becomes an issue.

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AnimalMuppet
Yes, I know that QM is not "classical". By "classical QM", I meant regular,
Schrodinger-equation QM, not "quantum trajectory theory" QM.

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pdonis
_> By "classical QM", I meant regular, Schrodinger-equation QM, not "quantum
trajectory theory" QM._

"Regular" QM is not limited to the Schrodinger Equation. You also have to use
the collapse postulate and the Born Rule where appropriate, i.e., whenever
measurements are made. QTT is basically just a reformulation that makes it
easier to see how to make use of the information revealed by measurements.

The "Schrodinger" viewpoint referred to in the article is not the Schrodinger
Equation, but Schrodinger's personal take on QM.

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AnimalMuppet
Are you saying that the Schrodinger Equation, the collapse postulate, and the
Born Rule are mathematically equivalent to QTT?

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pdonis
_> Are you saying that the Schrodinger Equation, the collapse postulate, and
the Born Rule are mathematically equivalent to QTT?_

Yes.

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miej
wait, but doesn't typical qm operate off of integrating over every possible
quantum trajectory, including relativity-prohibited ones where particles take
superluminal velocities? so if qtt is valid and otherwise compatible with
vanilla qm, how does it address that discrepancy?

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dangirsh
You're confusing quantum trajectories for classical ones. The latter are what
we use in path-integral formulations, and constitute a single path through
spacetime. No such path exists for quantum states, since that would be a local
hidden-variable theory (which has been experimentally ruled-out).

Trajectories of quantum states are well described here:
[https://arxiv.org/abs/quant-ph/0302080](https://arxiv.org/abs/quant-
ph/0302080)

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miej
ah right, thanks. been a few years since I did any qm work last

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jmole
This is just reversible computation, isn’t it?

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ryanthedev
This is very similar to my algorithm I have been working on. It's an
connection of past and future states. It provides a translational symmetry
between the events. That symmetry creates a unique pattern of the connected
events. It's principal is on the banach Tarski paradox and specifically the
axiom of choice. That coupled with unique constraints solves the information
entropy issue. Essentially using 4/pi. ++,+-,--,-+.

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ryanthedev
If you think I'm crazy.

Phys.Org: A connection between quantum correlations and spacetime geometry.
[https://phys.org/news/2019-07-quantum-spacetime-
geometry.htm...](https://phys.org/news/2019-07-quantum-spacetime-
geometry.html)

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danbmil99
I don't have time to waste wading through what is inevitably going to be a
watered down and screwed up lay person's interpretation of some paper they
don't understand that got some hype. I scanned the article but didn't see any
scholarly citations. What's the real research behind this?

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l33tman
I directly found a completely wrong description of the Heisenberg uncertainty
relation and stopped reading after that. But yes, the underlying research
probably has some interesting stuff!

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dangirsh
You probably mean:

"A quantum measurement influences the system being observed: The act of
observation injects a kind of random noise into the system. This is ultimately
the source of Heisenberg’s famous uncertainty principle."

I'd agree that's an inaccurate description of the uncertainty principle, but
then the following sentences are:

"The uncertainty in a measurement is not, as Heisenberg initially thought, an
effect of clumsy intervention in a delicate quantum system—a photon striking a
particle and pushing it off course, say. Rather, it’s an unavoidable outcome
of the intrinsically randomizing effect of observation itself."

I'm not sure I could explain it better at this level...

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Koshkin
Well, it has to be reminded that the uncertainty principle and the randomness
of the outcome of a measurement are two different things. The former involves
_two observables_ which cannot be measured _simultaneously_ with an arbitrary
precision whereas the latter is the result of an observable having more than
one eigenvalue.

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kgwgk
I have not thought this through, but it seems to me that if all observables
commuted there would be no randomness and if there was no randomness all
observables would commute. Does anyone see an obvious flaw on this?

