# Weak quantum measurements might not actually tell us anything



## twilyth (Oct 15, 2014)

Honestly, I've never quite gotten the hang of what a 'weak' measurement is, but since these are widely used in a variety of experiments involving quantum mechanics, the idea that they don't provide any actual information about the quantum world would be pretty important.

An example of one recent experiment that uses weak measurements is the quantum pigeon hole paradox.  

The idea behind weak measurements is to avoid collapsing the wave function with a direct observation.  In the pigeon hole paradox experiment, this is done by seeing if a magnetic field along one of the paths causes a change that indicates an electron passed along that path.  So you don't actually measure if an electron did or didn't pass, instead, you create a disturbance in the motion of any electrons that do happen to pass.  Later, you measure which electrons were actually perturbed - at least this is my best reading of the thought experiment.

But using weak measurements seems to create a number of results that are very weird even by quantum mechanics' standards.  The quantum cheshire cat experiment is a good example of this.  There, the researchers seemed to be able to send neutrons down one path but their spins down another.  The implication is that any property of a particle can be separated from the particle itself.

But both experiments relied on weak measurements.  So if these turn out to represent something like classical probabilities rather than the type that exist in the quantum world, it could well invalidate some of these new 'discoveries.'

Article.


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## twilyth (Jan 9, 2015)

It turns out now that there isn't any actual paradox in the pigeonhole experiment.



> *Possible paths*
> Rae and Forgan have now analysed the outcome of such a hypothetical experiment, and have shown that applying quantum mechanics and the classical pigeonhole principle can explain the apparent paradox. Their calculations involve constructing the quantum-mechanical wavefunction of the three electrons in terms of the possible paths that the electrons can take through the experiment – all three electrons going through one arm, for example, or two electrons going through one arm and one through the other.
> 
> They find that if there is a relatively strong interaction between the electrons, you would see 12 spatially separated peaks at the detector, which would be a sign of the classical pigeon principle at work (see figure at the top of the article). In other words, there would be four peaks for each electron, corresponding to the four possible histories of the electron motion: travelling alone, with one or other of the other two electrons, or with both. If the interaction strength were zero, on the other hand, then only three peaks – one for each electron – would be measured in the detector. As expected, such a measurement will yield no information about how many electrons travelled through each arm of the interferometer.
> ...


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## Arjai (Jan 9, 2015)

Hmmm...Much ado, about nothing.

I can prove, in three separate experiments, that I know what shit is. I can also explain it in three languages. Does that make me smart?

Quantum Physics, I only understand it in another state.

Maybe I should move to Montana...


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## CAPSLOCKSTUCK (Jan 9, 2015)

It comes to something  ( or not ) when it takes a paradox to explain a paradox paradoxically.


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## twilyth (Jan 10, 2015)

I think the real issue is whether or not weak measurements are useful or if they even tell us anything.  As I understand it, in quantum mechanics, certain properties are paired with other properties and measuring one of them precisely precludes your being able to measure the other.  Most people will know this phenomenon by the name 'uncertainty principle.'

So for example you can either know the position of a particle precisely or its momentum, but not both at the same time.  What weak measurements try to do is get around this limitation by making, as the name implies, weak measurements of many, many particles.  These don't give you precise information but just a statistical probability - again, as best as I can understand it.

Weak measurement experiments have, as the OP notes, resulted in many curious results like the pigeonhole paradox.  But it may just be that such measurements aren't really telling us anything useful and that could mean that these "results" aren't meaningful.


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## CAPSLOCKSTUCK (Jan 10, 2015)

Useful and meaningful are two completely different things.

Having established a cause and effect we then say " so what"

Afterall this isnt theory it is demonstratable and repeatable.


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## qubit (Jan 10, 2015)

@twilyth don'tcha just love the way quantum mechanics totally does your head in trying to figure it out? 


@CAPSLOCKSTUCK 

"Afterall this isnt theory it is demonstratable and repeatable."

A theory is actually demonstrable and repeatable and has lots of facts to back it up. I think you may have a misconception of what a theory is.


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## CAPSLOCKSTUCK (Jan 10, 2015)

One of the many definitions of theory is supposition.

Facts may reinforce a theory but until that theory is proven the theory is not a fact.

Look up the word theory then tell me im wrong.


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## twilyth (Jan 10, 2015)

If you're talking about quantum mechanics, that's very thoroughly established by countless experiments. In fact, it's so well established that it's simply referred to as 'the standard model.'  If you're talking about weak measurements, that's definitely still up for grabs.  The uncertainty principle is an integral part of the std model but that doesn't necessarily mean there might not be ways around it for specific purposes.


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## qubit (Jan 10, 2015)

CAPSLOCKSTUCK said:


> One of the many definitions of theory is supposition.
> 
> Facts may reinforce a theory but until that theory is proven the theory is not a fact.
> 
> Look up the word theory then tell me im wrong.


You're mixed up about it as I suspected, so yes, you're wrong.

The true definition of a theory is the way the word is used by scientists, which is as I described above. A theory is actually greater than a fact, since it uses many facts to support it.

In the ignorant everyday layman's language it's used to mean something that's speculation or opinion with a big chance of being incorrect. This is wrong and always will be wrong. This is the way that you are using the word.

I'm not making any wild claims here which put the onus on me to prove to you that I'm right. I'm simply repeating a very well established scientific principle, so if you really want to know the correct definition of the word theory, 10 seconds with a google search will show it to you and you'll stop arguing with me and twilyth about it.


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## CAPSLOCKSTUCK (Jan 10, 2015)

I didnt realize i was arguing with anyone.

In theory i wasnt


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## qubit (Jan 10, 2015)

Yes, you were and still are. Look it up.

I suspect that you're intentionally remaining ignorant of what it means, making you incapable of intelligent debate on this.

Whatever, that's your problem, not mine, lol.


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## Arjai (Jan 14, 2015)

Relax guys, I really hate arguments about Theory and/ or Quantum Physics.

Either you think you know, you know or you should ask and learn. In any one of these cases, an open mind should be required.

So, once again, this Thread re-hashes, Much ado about nothing. 

Granted, Weak Measurements may be made sense of, at some point. However, let's continue to experiment in other directions, in hopes of making sense of the already found. Paradoxes, are made to be solved. So, move on. 

Find another paradox then combine the two, or three, and see if anything makes sense?


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## CAPSLOCKSTUCK (Jan 14, 2015)

Quantum Physics in a pint glass.


www.youtube.com/watch?v=JKGZDhQoR9E

   cheers boys


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## Arjai (Jan 15, 2015)

CAPSLOCKSTUCK said:


> Quantum Physics in a pint glass.
> 
> 
> www.youtube.com/watch?v=JKGZDhQoR9E
> ...


Nice. Dude has some crazy big eyes!! Aside from that, warmed up my think bowl.  Glad I was here, at the bar, with only one in me when I started this video.


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## CAPSLOCKSTUCK (Jan 15, 2015)

Is Muon detection as demonstrated in that lecture a useful application of "Weak Measurement"  then ?


Try this link.  I know its a radio show but worth a listen if you like atomic stuff.

( when i was a kid people still called radios "wirelesses")


http://www.bbc.co.uk/programmes/b04xxvtb



..a former herion addict having a guided tour of CERN ...........  i kid you not.... this guy took heroin on a former British Prime Ministers' aircraft on an election campaign tour.

(*remember kids, take drugs seriously* or not at all)

I think he asks some pretty good questions as to why this stuff matters.


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## twilyth (Jan 15, 2015)

I skipped through the video so I may not have followed everything but the beer glass was an improvised cloud chamber.  So that would be a direct measurement.  A weak measurement would be something like, you apply a magnetic field to a path where you know certain particles will pass.  That will influence any charged particles that pass through the field.  Then when you measure or observe the particles, their properties will tell you which ones passed through the field and which ones took a different path.  This is one of the methods I think they used in the pigeonhole experiment.  Only it turns out that the initial interpretation of the results was incorrect - at least that what I think happened.

I don't know if he talked about Bell's theorem but all that business about realism relates to the theorem.  Bell proved that if you have a system like our universe that obeys the rules of quantum mechanics, then either locality or realism has to be violated.  Locality means no 'spooky action at a distance.'  So you can't have something that happens in one place creating an effect in some unrelated, unconnected place.  Measurements of entangled particles are an example of that.  If 2 electrons are entangled and you measure the spin of one, the other will have the opposite spin.  The particles can be on opposite sides of the universe, but as soon as you determine the spin of one, the spin of the other is immediately fixed.

Realism is technically counterfactual definiteness.  All that really means is that things continue to exist even when not observed.  So if you measure the location of a particle, you can't measure moment, but you assume that the particle has momentum even if it wasn't measured.  Because you know that when you independently measure one property or the other for many, many particles, you find that both agree with what the equations predict.  The alternative is that things that aren't observed, don't exist.

Bell proved that you have to give up either locality or realism.  As it turns out, locality is the attribute that we give up.  Many quantum phenomena violate locality.


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## CAPSLOCKSTUCK (Jan 15, 2015)

Fantastic.  All that you said ctually made sense.
Reassure my faith in humanity and tell me you didnt cut and paste all that.


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## Arjai (Jan 15, 2015)

You really should watch the Whole thing. I know, it's an hour+. I would not give up that hour, now that I have spent it.

Very well thought out point of view.

I wanted to violate realism. Damn! Here I thought QM was my Savior!!


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## twilyth (Jan 15, 2015)

CAPSLOCKSTUCK said:


> Fantastic.  All that you said ctually made sense.
> Reassure my faith in humanity and tell me you didnt cut and paste all that.


No cut and paste.  I did have to check to make sure my description of counterfactual definiteness was basically correct, but that was it.


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## CAPSLOCKSTUCK (Jan 16, 2015)

I watched something recently about how observing or measuring these particles affects them  and how in order to measure a particle you use 2, one which is observed and another one which is not observed .

Have you any idea what this theory is called so that i can look it up.

It really twisted my melon man, i couldnt make any sense of it.


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## xvi (Jan 16, 2015)

Arjai said:


> Quantum Physics, I only understand it in another state.


Quantom physics is like trying to plug in a USB flash drive by feel only. No matter how many times you flip it around and try again, it will never go in until you look at it. Until observed, it exists in a superposition of being simultaneously up and down.


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## twilyth (Jan 17, 2015)

CAPSLOCKSTUCK said:


> I watched something recently about how observing or measuring these particles affects them  and how in order to measure a particle you use 2, one which is observed and another one which is not observed .
> 
> Have you any idea what this theory is called so that i can look it up.
> 
> It really twisted my melon man, i couldnt make any sense of it.


That sounds like a weak measurement but I'm not sure if that's really what you mean.  Maybe 'quantum discord?'


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## CAPSLOCKSTUCK (Jan 17, 2015)

Just found it in my history. 

Quantum Entanglement Documentary

On my phone  so i cant easily send a link its on youtube with that title.


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## qubit (Jan 17, 2015)

CAPSLOCKSTUCK said:


> I watched something recently about how observing or measuring these particles affects them  and how in order to measure a particle you use 2, one which is observed and another one which is not observed .
> 
> Have you any idea what this theory is called so that i can look it up.
> 
> *It really twisted my melon man, i couldnt make any sense of it.*


You're not the only one it does that to.


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## twilyth (Jan 17, 2015)

CAPSLOCKSTUCK said:


> Just found it in my history.
> 
> Quantum Entanglement Documentary
> 
> On my phone  so i cant easily send a link its on youtube with that title.


Entanglement is an example of non-locality and what Einstein was referring to by the 'spooky action at a distance' quote.


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## twilyth (Jan 30, 2015)

Everyone knows that subatomic particles can exist in multiple places at once.  But what about larger objects like atoms?  A macro-realistic approach claims that they can't.  They should behave more in accordance with classical laws.  But according to a recent experiment, it seems that even atoms can be in 2 places at once.





> Can a penalty kick simultaneously score a goal and miss? For very small objects, at least, this is possible: according to the predictions of quantum mechanics, microscopic objects can take different paths at the same time.  The world of macroscopic objects follows other rules: the football always moves in a definite direction. But is this always correct? Physicists of the University of Bonn have constructed an experiment designed to possibly falsify this thesis. Their first experiment shows that Caesium atoms can indeed take two paths at the same time.
> .
> .
> .
> ...


Note:  I put this here instead of the misc. science thread since it involves indirect measurements.  Except here the measurements seem to have destroyed the superposition - at least if I understand what they were attempting to do.


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