kicking the observer out of QM

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kicking the observer out of QM

#1  Postby cavarka9 » Oct 28, 2011 6:14 am

Quantum upgrade removes need for spooky observer


http://www.newscientist.com/article/mg2 ... erver.html


QUANTUM mechanics - simultaneously physics's most successful and most baffling theory - may be in for an upgrade.

It faces a challenge from a modified version that would solve a largely ignored puzzle at the heart of the theory: why do subatomic particles never let us catch them in the act of being in many places at once but instead "collapse" into a single position as soon as we observe them?

Unlike most attempts at modification, this latest upgrade is generating special excitement as it meshes with another pillar of physics, Einstein's special relativity.

It also comes at the same time as a proposed test for such modifications - and a recent plea from a Nobel laureate to stop ignoring the collapse problem.

"Like many physicists, I have used quantum mechanics throughout my working life, cheerfully ignoring the deep questions about its meaning, but with a nagging feeling that this is something I ought to understand," says Steven Weinberg of the University of Texas at Austin, who recently posted a blueprint for devising such modifications online.

If the recent refinement holds true, much of the "spookiness" that still surrounds quantum theory would melt away.

When subatomic particles aren't being measured, they behave very strangely indeed, occupying many positions all at once. These superpositions are represented by a wave function, which can extend out into space. When a measurement is made, however, the wave function collapses and we only ever see the particle in a definite spot, never the blurry wave itself.

How can a particle possibly "know" when it is and isn't being watched? And why should observation change its behaviour, anyway? The majority of physicists aren't at all bothered by these unknowns, content in the knowledge that the theory has passed every single experimental test thrown at it. But a small band of rebels, including Weinberg, are very bothered indeed.

Since the 1980s they have wanted to modify quantum mechanics so that wave function collapse doesn't require an observer. Instead, they argue that collapse happens at random and is simply more likely when a measurement is made. The sticking point was that all attempts to mesh these theories with special relativity had failed.

Recently though, Daniel Bedingham, who splits his time between work in the financial industry and physics research at Imperial College London, has come up with a way to do just that. His theory has its roots in an approach called GRW, named after its 1986 inventors GianCarlo Ghirardi, Alberto Rimini and Tullio Weber.

GRW says that collapses are extremely rare for an individual particle, but that making an actual measurement on a particle forces it to interact with the measuring equipment. The particle becomes intimately linked, or entangled, with the many atoms that make up the measuring equipment...





http://pra.aps.org/abstract/PRA/v39/i5/p2277_1
http://pra.aps.org/abstract/PRA/v83/i4/e043621
http://arxiv.org/abs/1103.3974
http://arxiv.org/abs/1003.2774
Last edited by cavarka9 on Oct 28, 2011 11:14 am, edited 1 time in total.
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Re: kicking the observer out of QM

#2  Postby james1v » Oct 28, 2011 6:23 am

Sounds reasonable.
"When humans yield up the privilege of thinking, the last shadow of liberty quits the horizon". Thomas Paine.
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Re: kicking the observer out of QM

#3  Postby cavarka9 » Oct 28, 2011 6:27 am

its not complete article, go to new scientist
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Re: kicking the observer out of QM

#4  Postby twistor59 » Oct 28, 2011 7:03 am

cavarka9 wrote:its not complete article, go to new scientist


I think you have to subscribe to get the full article ?

Unless he's done something more recent, I would imagine they're talking about the stuff treated in this preprint. The problem is that the Schroedinger equation is linear, so the sum of a pair of solutions is still a solution. One solution might be "particle at A" another might be "particle at B". Linearity then implies "particle is at A and at B" is a solution. And at C and at D etc...... But when we check to see where the particle is, we find it's only in one of those places (exactly which one of those places is given by a probability obtained from the solution of the Schroedinger equation). But how exactly did it choose one of those places ?

The solution of the Schroedinger eqn which represents the situation where the particle is at A and B and C......etc is called the wavefunction, and the process by which nature "chooses" one of those possible outcomes, during an experiment/observation is called "wavefunction collapse" or "state vector reduction". Bedingham is proposing a mechanism by which this occurs - a so called "dynamical state vector reduction" mechanism. He is one of many to have tried such things - adding nonlinear terms to the Schroedinger equation typically.

If I had to guess, though, I would say that mainstream opinion on this "problem" these days is that the apparent state vector reduction is explained by decoherence. In this picture, there is no need to change quantum mechanics - everything is governed by the Schroedinger equation, but the information in the "interference terms", which give the weird quantum superposition behaviour, sort of "leaks away" into the environment, and the thing being measured (say a particle position) and the measuring apparatus appear to behave classically.
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Re: kicking the observer out of QM

#5  Postby Richard46 » Oct 28, 2011 10:57 am

I thought it had long been accepted that it is not observing something that affects it (unless you are Dean Radin perhaps) but some interaction with the measurement equipment.
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Re: kicking the observer out of QM

#6  Postby Evolving » Oct 28, 2011 11:20 am

...making an actual measurement on a particle forces it to interact with the measuring equipment...


That's what I always thought QM meant anyway - and not that there had never been a collapse of a wave function anywhere in the universe before humans started observing particles.
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Re: kicking the observer out of QM

#7  Postby cavarka9 » Oct 28, 2011 11:22 am

also, cool expt which perhaps I mentioned before, maybe not here.
Quantum probes that won't kill Schrödinger's cat
http://www.newscientist.com/article/mg2 ... s-cat.html
"take a wire put charge on top of it , put it into superposition, put that into a box, now you could observe from the outside the electromagnetic field with sensors"
http://www.sciencemag.org/content/332/6027/330
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Re: kicking the observer out of QM

#8  Postby cavarka9 » Oct 28, 2011 11:24 am

Evolving wrote:
...making an actual measurement on a particle forces it to interact with the measuring equipment...


That's what I always thought QM meant anyway - and not that there had never been a collapse of a wave function anywhere in the universe before humans started observing particles.

but we did not go further and asked how, and what exactly is a measurement ? and how is that different from some other particle interacting with each other rather than with the particles of the measuring equipment.
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Re: kicking the observer out of QM

#9  Postby cavarka9 » Oct 28, 2011 11:29 am

GRW says that collapses are extremely rare for an individual particle, but that making an actual measurement on a particle forces it to interact with the measuring equipment. The particle becomes intimately linked, or entangled, with the many atoms that make up the measuring equipment.

Because these atoms are numerous, one of their wave functions is bound to collapse during the measuring process. Thanks to entanglement, that triggers the collapse of the rest - including that of the particle being measured. So the particle's wave function collapses on measurement, without needing any spooky reason for a change dependent on the observer.
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Re: kicking the observer out of QM

#10  Postby twistor59 » Oct 28, 2011 11:35 am

Evolving wrote:
...making an actual measurement on a particle forces it to interact with the measuring equipment...


That's what I always thought QM meant anyway - and not that there had never been a collapse of a wave function anywhere in the universe before humans started observing particles.


I would go further and ask - if you make the wavefunction sufficiently universal (i.e. to encompass the observed system and enough of the enivironmnent), then is it necessary to even talk about collapse of the wavefunction ?
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Re: kicking the observer out of QM

#11  Postby Evolving » Oct 28, 2011 11:40 am

Hmm...

Good luck writing that function down!
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Re: kicking the observer out of QM

#12  Postby cavarka9 » Oct 28, 2011 11:40 am

Well, maybe randomness is inherent in the universe.
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Re: kicking the observer out of QM

#13  Postby twistor59 » Oct 28, 2011 11:49 am

Evolving wrote:Hmm...

Good luck writing that function down!


I have written it down, but it is a secret. I'm not telling anybody what it is ... :shifty:
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Re: kicking the observer out of QM

#14  Postby cavarka9 » Oct 28, 2011 12:08 pm

only registration reqd for this article, for a brief period of time few articles can be accessed.
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Re: kicking the observer out of QM

#15  Postby Evolving » Oct 28, 2011 12:10 pm

twistor59 wrote:
Evolving wrote:Hmm...

Good luck writing that function down!


I have written it down, but it is a secret. I'm not telling anybody what it is ... :shifty:


I imagine the margin's not big enough to contain it?
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