High School Bohr vs Einstein: is the Moon there when we are not looking?

[naima]

Dr Chinese,
Your remark is illuminating!
Bhobba write for years that proper and improper mixtures are the same thing. So there is no difference between apparent collapse and collapse.
In this experiment particles are entangled and left particles give no interference pattern. I do not think that it is because the particles in the other direction could be measured. they are not. In the simple Young experiment we do not say that , as the way could be measured it would suppress interferences!
So tracing out on the degrees of the other particle is like the particles was measured at each slit.
When an apparatus measures some property of a particle, there is a unitary process (premeasurement) in which the particle is entangled with the apparatus. As we do not know the details of the apparatus, we have to trace out on it. we have no more superposition and we get an output.
I think that it has something to do with the no-hiding theorem and finiteness of information but it is a personal belief.

 

[bhobba]

Bhobba write for years that proper and improper mixtures are the same thing.

I most definitely did NOT say that. They are NOT the same thing. Their QM state is the same thing - but they are prepared entirely differently. This crucial difference is needed to understand apparent collapse and actual collapse.

,Your remark is illuminating!

That I agree with entirely. As usual an excellent post.

Thanks
Bill

 

[quarknsoul]

Realities depend just on five senses and apparatuses. No one can know the true nature of anything in the universe.

 

[bhobba]

Realities depend just on five senses and apparatuses. No one can know the true nature of anything in the universe.

QM says nothing one way or the other about statements like the above which is really philosophy - not science.

We have interpretations where its close to that - and many totally opposite. Learn to live with it and avoid pedantic statements.

Thanks
Bill

 

[Hornbein]

So, what do you think: is the moon there when you are not looking?
(Do you think the experiments of Bell's tests alone confirmed Bohr was right?)

Yes, Bohr was right. Of course they weren't really talking about the moon. That quotation comes from a private letter of Heisenberg, by the way. He wasn't entirely serious. It was an informal communication.

We simply cannot observe a photon directly. All we can observe is the effects when it whacks into something, like our retina. So, what is it doing when we can't see it? Is it in an undefined state, or in a defined state that we do not know? Answer: Undefined state.

I don't see why people have such a problem with this. Why should teeny tiny things behave like humongous things like baseballs? They don't. Get used to it.

I'd recommend you look at the Conway-Kochen free will theorem. It takes the Bell thing a step or two further. My vote for the Theorem Deserving of Wider Recognition. I don't know why it hasn't garnered any fame. But considering the botch jobs popular science is prone too, maybe it is just as well.

 

[bhobba]

Answer: Undefined state.

No - the answer is interpretation dependant.

Who was right? Bohr or Einstein. Their debates were magnificent - but actually both were wrong:
http://www.fisica.ufmg.br/~dsoares/cosmos/10/weinberg-einsteinsmistakes.pdf

Thanks
Bill

 

[Hornbein]

No - the answer is interpretation dependant.

Who was right? Bohr or Einstein. Their debates were magnificent - but actually both were wrong:
http://www.fisica.ufmg.br/~dsoares/cosmos/10/weinberg-einsteinsmistakes.pdf

Thanks
Bill

That is so. They were both wrong. But Einstein was more wrong.

 

[bhobba]

That is so. They were both wrong. But Einstein was more wrong.

I don't look at it that way - they both had issues. I think Dirac was right with his basically shut-up an calculate view - but that's just me. Note - although often attributed to Dirac and/or Feynman they didn't really say it but it does reflect both their views.

Thanks
Bill

 

[Hornbein]

I don't look at it that way - they both had issues. I think Dirac was right with his basically shut-up an calculate view - but that's just me.

Thanks
Bill

Well, the "I-have-no-model" view is logically unassailable. But it ain't no fun.

 

[adfreeman]

There's something I don't understand regarding the subject we've been discussing.

I heard quite a few times that if instant communication was possible using entangled particles, then this would also mean that we would able to send messages backwards in time. I don't understand how, so imagine for a moment that we could use entangled particles for communications; how would this messages to the past work?

 

[bhobba]

I heard quite a few times that if instant communication was possible using entangled particles, then this would also mean that we would able to send messages backwards in time.

This is way off topic - but - yes - you would be able to violate causality eg you could create a device that sends a signal back in time to destroy it.

You can find the detail in Rindler:
https://www.amazon.com/dp/0198539525/?tag=pfamazon01-20

If you want to pursue it start a thread in the SR sub-form.

Thanks
Bill

 

[naima]

it smells like an upcoming closure?

 

[Nugatory]

I heard quite a few times that if instant communication was possible using entangled particles, then this would also mean that we would able to send messages backwards in time. I don't understand how,

Any faster than light signal transmission will do the trick, and "instantaneous" is of course faster than light. Google for "tachyonic anti-telephone" to see how it works, and if you want more start a thread over in the relativity forum - the people there are who you should be asking.

 

[naima]

I have a problem with DrChinese entangled particles.
If the system is in the state (|u>|d> + |d>|u>)/2 the density matrix of the left particle is Id/2 so there is no interference behind the slits.
If alice apply a local rotation of 45° in the u/d plane the systeme is no more entangled and interferences can be seen behind the slits. So Bob could know that Alice applied the rotation. There is obviouly a problem. Where is it?

 

[stevendaryl]

There's something I don't understand regarding the subject we've been discussing.

I heard quite a few times that if instant communication was possible using entangled particles, then this would also mean that we would able to send messages backwards in time. I don't understand how, so imagine for a moment that we could use entangled particles for communications; how would this messages to the past work?

That's got nothing to do with quantum mechanics. It's just pure relativity: if a signal is instantaneous in one frame, then it is back-in-time in a different frame. To use this fact to communicate with your own past requires that the instantaneous signal can be sent relative to any frame. If there is only one frame that allows instantaneous signals (that is, if there is a preferred rest frame), then back-in-time communication would not be possible; it would only seem back-in-time to some observers, but not to others.

 

[DrChinese]

I have a problem with DrChinese entangled particles.
If the system is in the state (|u>|d> + |d>|u>)/2 the density matrix of the left particle is Id/2 so there is no interference behind the slits.
If alice apply a local rotation of 45° in the u/d plane the systeme is no more entangled and interferences can be seen behind the slits. So Bob could know that Alice applied the rotation. There is obviouly a problem. Where is it?

I will repeat my comment (with reference) in #29: entangled particles do not produce interference. Breaking Alice's entanglement - by a measurement - does NOT suddenly cause any detectable change in Bob. And vice versa.

When Alice breaks entanglement with Bob's photon, further (non-commuting) measurements of her photon will not be correlated to any of Bob's measurements.

So the "problem" is that your view of entangled particle behavior needs a minor adjustment.

PS a rotation of Alice (say by a quarter wave plate) will not break entanglement.

 

[naima]

I know that alice cannot change what Bob sees. When she breaks the entanglement i think that the global state changes. Could you give an example where Alice breaks it to see interferences and Bob's density matrix does not change?

 

[entropy1]

I am wondering what the term "instantaneous" means in this context: simultaneity is relative in RT! You can only observe that the correlations are measured space-like separated by Alice and Bob. Information does not exist 'in between' two space-like separated events. The correlation can only be established by slower-than-light communication, as we all know. So IMHO "instantaneity" is not a term that covers the notion of FTL. If you do, you will have to refer to time 'between' the events, and that can only be done in terms of lightcones. The term 'space-like separated correlation' suits better, IMHO.

 

[stevendaryl]

I am wondering what the term "instantaneous" means in this context: simultaneity is relative in RT! You can only observe that the correlations are measured space-like separated by Alice and Bob. Information does not exist 'in between' two space-like separated events. The correlation can only be established by slower-than-light communication, as we all know. So IMHO "instantaneity" is not a term that covers the notion of FTL. If you do, you will have to refer to time 'between' the events, and that can only be done in terms of lightcones. The term 'space-like separated correlation' suits better, IMHO.

Well, the collapse interpretation of QM would seem to require a preferred rest frame. But the odd thing is that it doesn't actually matter which frame you pick--there are no detectable consequences of the choice.

 

[naima]

I read in the Zeilinger paper that Alice can observe interferences after erasement and coincidence countings. Is it always the same trick with intricated particles?

 

[DrChinese]

I read in the Zeilinger paper that Alice can observe interferences after erasement and coincidence countings.

Yes, that is true. Note that again, there is nothing specific changing that you see without sending classical communication.

 

[Robert100]

We need to be very careful about this question. The moon, as is currently understood and explored by modern science and lunar landers, absolutely most certainly is there, even when it is not being observed by any form of life on Earth.

To claim otherwise is equivalent anti-Science, postmodernist social deconstructionism.

No same physicists believes that recent human observations of the Moon called it into existence. If any part of these babbling philosophical physics papers were true, then the moon as explored by Chinese scientists would likely have vastly different properties than the moon has, as explored by Russian or American scientists.

Yet no matter who the observer is, American, Russian, or Chinese, and no matter the year in which we make the observation, the results about what the moon is, where it is, and what it is made of, are always the same.

Yes, there are some serious issues in understanding QM. We obviously have a lot to learn about the fundamental nature of reality itself. Perhaps are three dimensional universe, as is now fashionable to believe, maybe a three-dimensional projection of a different dimensional space. Maybe there are wonderful and weird explanations for the non-local connections between particles that we have measured in EPR experiments

But the planets and moons do exist independently of our observations. At this point one might ask what the nature of reality is, but not whether or not some form reality exists.

 

[DrChinese]

The moon, as is currently understood and explored by modern science and lunar landers, absolutely most certainly is there, even when it is not being observed by any form of life on Earth.

To claim otherwise is equivalent anti-Science, postmodernist social deconstructionism.

...

I don't know what "postmodernist social deconstructionism" is, but it sounds like fun.

The actual context of the "moon" reference in the OP makes it clear that there is no question as to whether the moon itself exists when not being observed. The quote is attributed to Einstein, and states:

"I think that a particle must have a separate reality independent of the measurements. That is: an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it."

So the actual question is whether or not quantum objects, such as an electron, have simultaneous non-commuting observables independent of the act of observation. Most physicists, but not all, would disagree with Einstein on this point. Of course, we have the great advantage of being aware of Bell's Theorem and the other no-go theorems and experiments.

So Einstein's comment really has nothing to do with the existence of the moon, and everything to do with his view of quantum reality. Referencing the existence of the moon is simply a shorthand for this.

 

[eltodesukane]

"is the Moon there when we are not looking?"
Just like an old Pokémon game I used to play.
When I get to some part of the map, nurse Joy is looking at me from the hospital's window.
If I walk just a bit so that the hospital is no longer visible on the 2D screen, is nurse Joy still looking at me?

 

[naima]

You discovered the displacement of the Heisenberg's cut. congratulations.

 

[naima]

A last question about the Zeilinger paper. If the first detector is in the focal plane of the lens, the which way information is lost and there is an interference pattern for photons correlated to those passing thru the slits.
Could you give me the state of the global system dor coinciding particles. By two tracing out, we should have a pure reduced density matrix in the focal plane and Id/2 at the slits?

 

[DrChinese]

A last question about the Zeilinger paper.

Which paper? It sounds like you are thinking of a quantum eraser setup but am not sure.

 

[mikeyork]

The moon is still there because the rest of the universe is "observing" (interacting with) it even when we are not looking. And those observations can be communicated to us so that, in effect, we are always observing it. This is simple logic based on the principle that our observations of nature (or at least our interpretations of those observations) must be self-consistent.. Now I grant that at the microscopic level things are more subtle, but the non-contradictory requirement of our observations still holds. If some "observer" in the universe knows that an electron is in a specific state and that electron remains isolated in that state, then any subsequent "observer" capable of communicating with the previous "observer" must record that same state.

 

[bhobba]

So Einstein's comment really has nothing to do with the existence of the moon, and everything to do with his view of quantum reality. Referencing the existence of the moon is simply a shorthand for this.

Very nice.



Einstein to his dying day believed QM incomplete. Ever since a famous attack Einstein bought against QM that Bohr defeated he believed it correct - but that there was something else behind it. That was his beef with Bohr and the Copenhagenists - they believed there view of QM was complete. He even had his own interpretation - the Ensemble interpretation. But his version was not as developed as the version now espoused by Ballentine - it was really a hidden variable theory in disguise. Indeed Ballentine's famous 1970 review article had exactly the same issue - but his book now has a more subtle view. We are really still having the same arguments today.

Thanks
Bill

 

[naima]

Dr chinese,
the question is about your link:
skip to the erasing in the focal plane of Heisenberg lens.
Bob sees no interference but he can get an interference pattern from coinciding particles. I wonder what is the pure state for these pairs in the global Hilbert space.

That is correct; there is no interference UNLESS you first make the light coherent by diffracting it through a pinhole or similar. Entangled photons are not coherent. It makes sense when you think about it, but I never did until someone pointed this out to me. See an enlightening article by Anton Zeilinger, p. 290, Figure 2.

Experiment and the foundations of quantum physics ...