Brian Green's Beam Splitter Experiments

[selfAdjoint]

Well then, here's a thought experiment / question:

What if we observe the which-path information, in which case we see no interference pattern, and then erase the which-path information. Would we then see the interference pattern? Probably not...

At this case "our knowledge" of the which-path information would affect the outcome of the experiment..?

No this fails as an explanation. It was our actions, not our knowledge, that caused the behavior. And it is easily possible to imagine our actions performed automatically, completely independent of our knowledge.

Let me add another thing that confuses people thinking about this. It is natural to think of the photons as lttle bullets that have a succession of well defined positions along the various paths as they "move" from source to target. But QM says no such thing! Whether you use the "wave function" view or the Feynman "sum over histories" view, there are no successive postions. Source, paths, target are all described by one extended "shape" in spacetime. And that shape constrains what happens, not "particle motion".

 

[kvantti]

Yes I realized that and therefore deleted my post... apoligies.

 

[Rake]

The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.

Ho-hum. Another experimental proof of QM. This is the way it works, folks.

The first sentence states that the position of the photon at detector D0 has been registered. And I think that this never changes.

What changes is the position of the photon ARRIVING at D0. MEANING that once the twin idler info has been gleaned, then the experimenter will be able to say something about the PATH that the twin signal took to get to the position on D0.

I think that the wording of the conclusion is poor and is what is misleading here. The past is not being changed at all. Only new information is being attained about a past event with the recording of the idler. That information has to do with the position of the photon as it arrives D0.

So the experimenters do not know which of the photons make up an interference pattern vs. which ones do not until the arrival and subsequent correlation of the idlers into D1-D4 has been completed.

 

[MojaveJoe]

The experiment is set up to seem to demonstrate that ("Delayed Choice"), but consider; if automatic software were designed to (pseudo-)randomly scan or not scan with no human the wiser (double-blind) as to which "choices" had been made and only long after did anyone look at the results. Do you think the same patterns of behavior would result, or not? I believe they would. I think the "our knowledge" issue is a red herring, an artifact of the experimental design.

It does seem like a red herring doesn't it? How can nothing tangible (the "quantum information"), have a tangible effect on the experimental outcome? But that's exactly what this experiment implies and that's why I asked...swore I misunderstood the experiment. I assumed there was some hidden variable or something...

The problem is how the heck do you set up software to monitor something but we can't "know" the answer or that screws up the experiment!

I take that back, quantum information isn't nothing...to me, its the 800 lb gorilla in the room!

Cheers!

 

[eep]

The first sentence states that the position of the photon at detector D0 has been registered. And I think that this never changes.

What changes is the position of the photon ARRIVING at D0. MEANING that once the twin idler info has been gleaned, then the experimenter will be able to say something about the PATH that the twin signal took to get to the position on D0.

I think that the wording of the conclusion is poor and is what is misleading here. The past is not being changed at all. Only new information is being attained about a past event with the recording of the idler. That information has to do with the position of the photon as it arrives D0.

So the experimenters do not know which of the photons make up an interference pattern vs. which ones do not until the arrival and subsequent correlation of the idlers into D1-D4 has been completed.

I don't see how this fixes anything. But I also don't understand how a registered position could "change". I assume it's not possible that the photon that hits D0 already "knows" what it's entangled partner is going to do - that would imply so-called hidden variables, correct?

 

[selfAdjoint]

David Lindley has written a book called https://www.amazon.com/gp/product/0465067867/?tag=pfamazon01-20 about quantum theory. Here are a couple of excerpts from his discussion of the delayed choice experiment:

If we want to know which way the photon went, we can't see the interference pattern. If we want to see an interference pattern, we can't ask to know which way the photon went.
...
The problems only arise when we surrender to the urge to start imagining, once we know what the outcome of an experiment is,that we can deduce accurately what must have happened along the way. That is, if we see an interference pattern, we think the photon must have divided itself into separate pieces, whereas if we know the photon was detected along one pathway, that it must have actually gone down that pathway and not the other. But then when we do a delayed choice experiment, we find that insisting on one or the other of these two mutually exclusive interpretations leads us into trouble, because it appears that the photon must know in advance what is going to happen, so that it can choose the appropriate behavior to follow. But, and this is the fundamental point, those two possible behaviors are not actual behaviors that we know the photon must in fact have followed, but inferred, deduced, or (more accurately) speculated behaviors...

The lesson of the delayed choice experiment is that any hypothesis that forces the photon to adopt one or the other of two distinct and contradictory behaviors is, in fact, not reasonable.

And we should have known this in advance. What was the first thing we learned about quantum measurements from the Stern-Gerlach experiment? That the spin of an electron is strictly an undetermined quantity until an experiment yields a measurable value for it. And, moreover, that any attempt to say, having found the electron to be in an "up" orientation, that it must have been that way all along, is demonstrably incorrect.
..
And the delayed choice experiment brings that point forcefully into the open. Seeing an interfrence pattern, or detecting a photon in one path or the other, is at bottom a simple measurement made on a complicated system. And any attempt to think that we know, once the measurement is done, what really went on inside the system (whether the photon went one way or the other, or both ways at once) is precisely an attempt to pin down the prior state of a system after a measurement has been made. It causes trouble and we musr resist the temptation to do it.

The minimal interpretation of quantum mechanics embedded in the first two sentences of this {quotation} is like a wise but stern parental injunction against certain kinds of teenage behavior: limiting, to be sure, but it keeps people out of danger.

Quantum mechanics says the state, whether particle or wave, doesn't exist until an interaction produces a value for it; and furthermore that value is for that interaction only and gives no warrant for saying the value existed going into the interaction.


.

 

[MojaveJoe]

Quantum mechanics says the state, whether particle or wave, doesn't exist until an interaction produces a value for it; and furthermore that value is for that interaction only and gives no warrant for saying the value existed going into the interaction.

That seems to answer the delayed choice past/present question - or at least state this is acceptable wierdness. However, it still doesn't seem to address the fact that the final data depends on whether or not we know the which path (a real empirical effect)...except that it is "dangerous" to draw this conclusion. That is unsatisfactory to me. Granted, I assumed I was wrong when I posted the initial question and I am content to simply know that I didn't misunderstand the experiment, but...

What exactly is the author afraid of?

Cheers!

Edit: I guess what I mean is I'm not asking whether there is an interference pattern or not or whether the photon went one way or both ways...what I am asking is why a particular type of knowledge has ANY effect at all. It shouldn't! Heck, it can't! Granted it could be, like you said, an experimental artifact...but to me that means we should dismiss an awful lot of experiments.

 

[MeJennifer]

Of course there is a completely different interpretation of all these "misteries".

So we have a photon, two slits, a screen, and a measuring device that is on or off and an experiment to follow.

What if the result is already determined classically before the experiment starts?
What if it is just us, experimenters, who don't know it yet.
And in our "infinite wisdom" we think that we are the ones who can influence the outcome of the experiment while in fact our actions are determined by the outcome.
Are we the owners taking the dog for a walk or is it perhaps the other way around?

As somebody once wrote "what if we are the hidden variables"?

Is anyone with me on this thought?

 

[Farsight]

Me.

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

If you look at page 16 and count the dots on figs 3 4 and 5, you see there's twice as many joint detections for R₀₁ and R₀₂ as for R₀₃₍₀₄₎.

In total there's four times as many, but I doubled the number of detections on fig 5 because they occur on both R3 and R4.

Does this mean there's something amiss with the 50:50 beamsplitters? Is there some simple geometric explanation for all this? Like the photon only goes through the beamsplitter when it's sideways on and therefore only goes through one of the slits rather than both.

 

[selfAdjoint]

What exactly is the author afraid of?

Mojave Joe, what he is afraid of is the kind of non-starter speculations that MeJennifer's post #44 expresses. If you purely imagine the photon moving somehow classically inside the equipment (which QM gives you NO reason to do!), you get into paradoxes, and in attempting to resolve those paradoxes in your mind you get into speculations like "We are the hidden variables" that are not only unwarranted, but don't work in experiments. The Bell theorem and the experiments like Aspect's that have confirmed it have ruled out hidden variables, in us or any other way.

In order to comprehend these experiments, EPR, double slit, Stern-Gerlach, delayed choice, and whatever the brilliant experimenters come up with next, you have to abandon thinking classically ("paths") about quantum systems.

And no, "our knowledge" isn't necessary for these things to happen. An unthinking quantum interaction can, and in fact does, drive the experiment.

 

[Farsight]

Stern Gerlach experiment:

http://plato.stanford.edu/entries/physics-experiment/app5.html

 

[Farsight]

SelfAdjoint: I was looking up EPR and came across this website:

http://freespace.virgin.net/ch.thompson1/

It looks reasonable to me. In your expert opinion is it trustworthy?

If not, please delete this post.

 

[selfAdjoint]

It is not VERY trustworthy. The late Carolyn Thompson was a fierce critic of the Aspect type experiments. Some of her initial critiques had some basis, and corrections were made in the areas she and others noted during later experiments. But she refused to give up criticizing, as she supported a classical view of physics that the EPR experiments contradicted. She posted here at PF quite a bit, and toward the end I think it is fair to say she was regarded as pretty much a crank.

Nobody I know of with enough training in the area to have an informed opinion now doubts that the EPR experiments do correctly eliminate classical and hidden variable explanations of microphysical interactions.

Rather than delete your post, I am leaving it up with this reply, since Ms. Thompson's views are out there and a warning reference on them is useful.

 

[moving finger]

Me.

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

If you look at page 16 and count the dots on figs 3 4 and 5, you see there's twice as many joint detections for R₀₁ and R₀₂ as for R₀₃₍₀₄₎.

In total there's four times as many, but I doubled the number of detections on fig 5 because they occur on both R3 and R4.

Does this mean there's something amiss with the 50:50 beamsplitters? Is there some simple geometric explanation for all this? Like the photon only goes through the beamsplitter when it's sideways on and therefore only goes through one of the slits rather than both.

The "number of dots" simply represents the measurement precision on the x-axis. For the D1 and D2 idlers (showing interference) the data are displayed at 0.1mm resolution (one data point each 0.1mm along the x-axis), whereas for the D3 and D4 idlers (which show no interference) the data are displayed at 0.2mm resolution.

All this means is that the authors possibly "threw away" (or merged) half of their data points from the D3 D4 idlers (becasue they did not need such high resolution data to show the overall shape of the distribution).

I have an interesting question - what does one get if one "adds together" the data from D1 and D2? (ie merge the two curves in figures 3 and 4?)

Best Regards

 

[moving finger]

Finger, you keep plugging Cramer. I used to like Cramer too, but how does it work now that the "future absorber" it depends on, once identified with the supposed future big collapse of the universe, has apparently been done away with by the accelerated expansion?

Cramer has attempted to address that issue in his 1983 paper here :

http://mist.npl.washington.edu/npl/int_rep/dtime/dtime.html

He postulates a boundary condition at T=0 (the Big Bang) which would in effect "reflect" advanced waves in time. Sounds weird I know.

One of the other interesting conclusions from his 1983 paper is that for a "Big Crunch" scenario :

such models are intrinsically time symmetric and cannot explain the dominance of retarded over advanced radiation (in the absence of additional special postulates).

In other words, an ever-expanding universe would be one way to explain the fundamental time asymmetry.

It's interesting to note that this paper was published when there was still some support for a Big Crunch scenario.

I accept that there are some cosmological issues to be addressed in the Transactional Interpretation. But right now I don't see many other candidates for explaining the ontology behind the weird results in delayed choice / quantum eraser experiments. Of course one can simply ignore the ontology and just accept the maths (the "shut up and calculate" approach), but for someone like me that's not a very satisfying philosophy

Best Regards

 

[selfAdjoint]

Hi mf, Yes I see the asymmetry argument but I still don't see the how Cramerite satz works in the asymmetric case. Originally the retarded quantum waves went into the future, and the advanced waves went into the past, and both were reflected by the supposed abosrbers, handwavingly (as far as I ever saw) identified with the big bang and the big crunch. Then the relected waves interfered, and here he did calculations in the simple cases at least to show that the famous quantum behavior resulted. But if there is no reflection for the retarded waves how does this work?

As far as "explaining reality" goes, I am of two minds currently. One is that quantum mechanics was never built for that purpose, it's complete in itself but it is a machine that gives accounts of behavior, not explanations, and to attempt to use it for that incorrect purpose results in science-fiction - Cramer's time travel or Everett's multiple universes.

My other mind leads me to look into the combination of decoherence and relational quantum mechanics, to see how much can be retrieved there. Decoherence can explain how the classical world automatically results from the quantum world, but it doesn't address the measurement problem. RQM is one way of addressing the measurement problem, but it seems to me very much a work in progress at this point. I don't really like Consistent Histories because it seems to be just SUAC in disguise.

 

[Farsight]

Thanks for that rather sad feedback, selfAdjoint. Could you recommend any papers/links on Aspect or EPR explaining why classical and hidden variables must be eliminated?

moving finger: I tried combining D1 and D2 via manual dotting and got myself a mess - there's a displacement that I missed. Maybe I'll try printing two copies and sticking through the top one with a pin. I have to say I don't like the sound of throwing away data points.

 

[selfAdjoint]

Thanks for that rather sad feedback, selfAdjoint. Could you recommend any papers/links on Aspect or EPR explaining why classical and hidden variables must be eliminated?

I don't think you can go wrong with https://www.amazon.com/gp/product/0521523389/?tag=pfamazon01-20 by Bell and Aspect themselves. It's a collection of papers at different levels of technical difficulty, including some that require no math at all (though they all require you to keep your wits carefully about you as if your were solving a difficult puzzle!).

Currently my favorite hard-nosed popular account of the issues with quantum mechanics is Where Does the Weirdness Go? by David Lindley, which I excerpted a biut earlier in this thread.

 

[physicsmasta]

I too read the book in question. But the answer to that question has evaded me also. I would love to if you find out anything surrounding the book.

 

[physicsmasta]

I just read, "Beyond the Quantum" and in it it suggest that light particles have mass and all we have to do measure speed, position and force simutamelously is to measure the mass of light in the presence of gravity on earth. Do you believe this would work please reply!

 

[Reagle]

I have read through this thread and I would respectfully like to verify a few conclusions I have reached regarding the whole

"delayed choice quantum eraser"

phenomenon.

My questions relate to Yoon-Ho Kim, R. Yu, S.P. Kulik, and Y.H. Shih fairly famous paper calledhttp://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf"

As noted in this thread this paper is the subject of comments made by Ross Rhoades in his http://www.bottomlayer.com/bottom/ki...scully-web.htm" at Wikipeadia.

Here are my conclusions about this paper:

1.

It makes no difference what is at D3.

If the idler photon was sent to D3, and D3 had been a coffee cup (instead of a detector), then the interference pattern would be still destroyed.

2. If conclusion 1 is true then the interference pattern at DO is dependent on the existance of which path information, even if which path information is impossible to recover (such as a photon hitting a coffee cup at D3).

3. If conclusion 1 and 2 are true then a lot of people like to make overreaching assumptions about this paper, leading to unsupportable conclusions. Especially people like the producers of the "[MEDIA=youtube[/URL]. These people are interpreting the interference effect as being determined by "what an observer knows", when it really has nothing to do with an observer at all and only has to do with the existence, or non existence, of which path information.

Am I right or wrong regarding these 3 observations?

 

[selfAdjoint]

Here are my conclusions about this paper:

1.
Quote:
It makes no difference what is at D3.

If the idler photon was sent to D3, and D3 had been a coffee cup (instead of a detector), then the interference pattern would be still destroyed.

2. If conclusion 1 is true then the interference pattern at DO is dependent on the existence of which path information, even if which path information is impossible to recover (such as a photon hitting a coffee cup at D3).

3. If conclusion 1 and 2 are true then a lot of people like to make overreaching assumptions about this paper, leading to unsupportable conclusions. Especially people like the producers of the "What the bleep clip here", and even Ross Rhoades in this video. These people are interpreting the interference effect as being determined by "what an observer knows", when it really has nothing to do with an observer at all and only has to do with the existence, or non existence, of which path information.

Am I right or wrong regarding these 3 observations?

Sure looks right to me. "What the bleep.." is a notoriouus new age commercial that has been complained about by just about every physicst with a blog. Would that newspaper journalists would pick up on that.

And yes a whole lot of people have egg on their faces, not just mystigogical cranks.

In my mind (and here I am treading on sacred professional ground) is that the whole personalizing thing in QM (as in using "observation" instead of "interaction" to describe whatever it is that generates real number observable values) should be abandoned. It was introduced by Bohr who was a famous mutterer and "guru" to three generations of physicsists including some of the great ones, but it's time to see it for what it is, a crutch, and a broken one that is droppiing us on our keisters at that.

 

[Hurkyl]

Maybe you've figured this out already, but this is something that wasn't clear to me when I was trying to figure out the delayed choice eraser experiment.

If you're just looking at D0, there is no interference pattern. The "interference pattern" only surfaces when split all the photons striking D0 into two groups, each group corresponding to having its pair strike D3 or D4.

 

[moving finger]

Thanks for that rather sad feedback, selfAdjoint. Could you recommend any papers/links on Aspect or EPR explaining why classical and hidden variables must be eliminated?

moving finger: I tried combining D1 and D2 via manual dotting and got myself a mess - there's a displacement that I missed. Maybe I'll try printing two copies and sticking through the top one with a pin. I have to say I don't like the sound of throwing away data points.

isn't that what destructive interference is all about? take two waves 180 degrees out of phase, and you end up with no wave.

the whole experiment is about throwing away (or deleting) information - delete the "which way" information (by randomising with a beamsplitter) and you see the interference - but retain the information and you don't see the interference. It just seems to me that the phase shift between the D1 and D2 data is just enough so that when combined these two signals destructively interfere. Is that significant, or irrelevant?

Best Regards

 

[Reagle]

Thank you selfAdjoint, for your feedback. I had an unresolved question in my mind regarding this paper. So let me respectfully ask one more question:

Q) The whole point of Yoon-Ho Kim, R. Yu, S.P. Kulik, and Y.H. Shih's fairly famous paper calledhttp://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf"
is that the position of the signal photon impact on detector D0 is belatedly dependent on if the idler photon goes to D3 (which establishes which path information) or goes to either D0 or D1 (which erases which path information).
This is a pretty standard delayed quantum eraser setup with entangled pairs.

But in my simple mind this is totally impossible, since once the location of the D0 signal photon impact has been recorded, it can not change.

What am I missing here?

 

[moving finger]

Hi mf, Yes I see the asymmetry argument but I still don't see the how Cramerite satz works in the asymmetric case. Originally the retarded quantum waves went into the future, and the advanced waves went into the past, and both were reflected by the supposed abosrbers, handwavingly (as far as I ever saw) identified with the big bang and the big crunch. Then the relected waves interfered, and here he did calculations in the simple cases at least to show that the famous quantum behavior resulted. But if there is no reflection for the retarded waves how does this work?

I agree it seems like handwaving. As far as I can see Cramer is suggesting the advanced waves get reflected at T0, and thereby constructively interfere with retarded waves (see figure 2 in his paper attached to this post as a GIF file – I can’t explain it any better than he shows it in his figures).

This explains why we see retarded waves. That there is no future singularity then (according to Cramer) means that there is no future point of reflection, hence why we see asymmetry (we see waves going from past to future, but not from future to past). In other words, an ever-expanding universe with no future singularity (rather than being a problem because of future transparency and hence lack of absorbers) is actually necessary for the emergence of the electromagnetic arrow of time.

As far as "explaining reality" goes, I am of two minds currently. One is that quantum mechanics was never built for that purpose, it's complete in itself but it is a machine that gives accounts of behavior, not explanations, and to attempt to use it for that incorrect purpose results in science-fiction - Cramer's time travel or Everett's multiple universes.

OK, I can go along with this as far as it goes – this is the Shut Up & Calculate approach. But to me not a very useful philosophy.

My other mind leads me to look into the combination of decoherence and relational quantum mechanics, to see how much can be retrieved there. Decoherence can explain how the classical world automatically results from the quantum world, but it doesn't address the measurement problem.

Neither, it seems to me, does it address the delayed choice problem.

RQM is one way of addressing the measurement problem, but it seems to me very much a work in progress at this point. I don't really like Consistent Histories because it seems to be just SUAC in disguise.

I’m not familiar with RQM – does this address the delayed choice problem?

Currently my favorite hard-nosed popular account of the issues with quantum mechanics is Where Does the Weirdness Go? by David Lindley, which I excerpted a biut earlier in this thread.

Yes, this is a good book – from memory though doesn’t it say “decoherence is the answer”? How does decoherence address the delayed choice issue?

I just read, "Beyond the Quantum" and in it it suggest that light particles have mass and all we have to do measure speed, position and force simutamelously is to measure the mass of light in the presence of gravity on earth. Do you believe this would work please reply!

Light does have a “mass-equivalent” in the sense of E=mc^2 where E = hf in the case of light (f = frequency), but light does not have rest mass. Is this what you mean?

1. It makes no difference what is at D3.

If the idler photon was sent to D3, and D3 had been a coffee cup (instead of a detector), then the interference pattern would be still destroyed.

Agreed. That’s an important point (but we would have no way of detecting and displaying that destroyed pattern any more, because we’ve now lost our detector and substituted a cup of coffee instead).

2. If conclusion 1 is true then the interference pattern at DO is dependent on the existence of which path information, even if which path information is impossible to recover (such as a photon hitting a coffee cup at D3).

The interference pattern is not “at D0”, it is in the coincidence between detections at D0 and D1 (or D0 and D2). There is no interference pattern in the D0 data alone. But the detections at D0, D1 and D2 do not contain any which-path information – this information is only in the coincidence between detections at D0, D3 and D4 (which do not show interference fringes). If we replace D3 / D4 with a cup of coffee then we cannot recover any coincidence detections between these cups of coffee and D0, hence the which path information would be lost. But the interference at D0/D1 and D0/D2 would still exist. The presence of interference fringes at D0/D1 and D0/D2 has nothing whatsoever to do with what happens at D3 and D4 – different idler (and signal) photons are involved.

If you're just looking at D0, there is no interference pattern. The "interference pattern" only surfaces when split all the photons striking D0 into two groups, each group corresponding to having its pair strike D3 or D4.

That’s correct.

For each signal photon hitting D0, there is an idler photon which either goes to D1, D2, D3 or D4. The interference fringes emerge only when we look at the coincidence detections between D0 and D1, or between D0 and D2. The which way information only emerges when we look at the coincidence detections between D0 and D3, or between D0 and D4.

Q) The whole point of Yoon-Ho Kim, R. Yu, S.P. Kulik, and Y.H. Shih's fairly famous paper called "A Delayed Choice Quantum Eraser"
is that the position of the signal photon impact on detector D0 is belatedly dependent on if the idler photon goes to D3 (which establishes which path information) or goes to either D0 or D1 (which erases which path information).

This is a pretty standard delayed quantum eraser setup with entangled pairs.

But in my simple mind this is totally impossible, since once the location of the D0 signal photon impact has been recorded, it can not change.

Imho your “simple mind” interpretation is correct – this is impossible. I do not believe we can understand what is going on by saying the position on D0 is “belatedly dependent” on what happens later at D1, D2, D3 or D4. In terms of temporal sequence, the signal photon hits D0 BEFORE the idler photon has “decided” whether to go for D1, D2, D3 or D4. Whatever happens to that idler cannot (imho) then retrospectively cause the detection position on D0 to change – EXCEPT in the Transactional Interpretation sense that an advanced wave may be sent out by D1/2/3/4 which (travelling backwards in time) somehow then “causes” the signal photon to land at a certain point on D0.

It seems to me that the only way to explain what is going on is either that there is some backwards-in-time correlation between entangled states, or that the world is super-deterministic via hidden variables (whatever it is that causes the signal photon to land at a particular position on D0 also cause the twin idler to hit either D1/2/3 or 4).

If anyone has another explanation I’m all ears….

Best Regards

Below for selfAdjoint is attached figure 2 from Cramer's paper on The Arrow of Electromagnetic Time and Generalized Absorber Theory, showing advanced wave reflection at the T0 singularity. The caption to this figure is :

Figure 2: Minkowski diagram showing an open-ended emission transaction. The conventions used here are the same as those used in Fig. 1. The advanced waves propagate backward in time to the T=0 origin, where they are subject to a reflection boundary condition. The reflected wave arising from the boundary condition cancels the advanced wave up to the emission event, and at times after emission it reinforces the retarded wave from the emitter.

 

[selfAdjoint]

Hi mf, Yes I see the asymmetry argument but I still don't see the how Cramerite satz works in the asymmetric case. Originally the retarded quantum waves went into the future, and the advanced waves went into the past, and both were reflected by the supposed abosrbers, handwavingly (as far as I ever saw) identified with the big bang and the big crunch. Then the relected waves interfered, and here he did calculations in the simple cases at least to show that the famous quantum behavior resulted. But if there is no reflection for the retarded waves how does this work?

I agree it seems like handwaving. As far as I can see Cramer is suggesting the advanced waves get reflected at T0, and thereby constructively interfere with retarded waves (see figure 2 in his paper attached to this post as a GIF file – I can’t explain it any better than he shows it in his figures).

This explains why we see retarded waves. That there is no future singularity then (according to Cramer) means that there is no future point of reflection, hence why we see asymmetry (we see waves going from past to future, but not from future to past). In other words, an ever-expanding universe with no future singularity (rather than being a problem because of future transparency and hence lack of absorbers) is actually necessary for the emergence of the electromagnetic arrow of time.

And does Cramer have a detailed account of his approach dealing with the delayed choice quantum eraser?

As far as "explaining reality" goes, I am of two minds currently. One is that quantum mechanics was never built for that purpose, it's complete in itself but it is a machine that gives accounts of behavior, not explanations, and to attempt to use it for that incorrect purpose results in science-fiction - Cramer's time travel or Everett's multiple universes.

OK, I can go along with this as far as it goes – this is the Shut Up & Calculate approach. But to me not a very useful philosophy.

You and vanesch both complain about that. Who says nature has to be fully explained in 2006? What justification does "needing a useful philosophy" give for forcing the QM formalism into doing things it was never intended to?

My other mind leads me to look into the combination of decoherence and relational quantum mechanics, to see how much can be retrieved there. Decoherence can explain how the classical world automatically results from the quantum world, but it doesn't address the measurement problem.

Neither, it seems to me, does it address the delayed choice problem.

Since I think the delayed choice quantum eraser (DCQE) is a sub-problem of the measurement problem (being a combination of double slit with entanglement), it would follow that decoherence doesn't address this. Decoherence is not a complete explanation for quantum effects but a useful part of any complete explanation.

RQM is one way of addressing the measurement problem, but it seems to me very much a work in progress at this point. I don't really like Consistent Histories because it seems to be just SUAC in disguise.

I’m not familiar with RQM – does this address the delayed choice problem?

On the Beyond the Standard Model board, vanesch and I have discussed a paper detailing the RQM account of the entanglement problem. I am sure they can do double slit too, so it seems to me that if someone wanted to work it out, they could do DCQE as well. The basic principle is that quantum systems are only real in the process of interaction; that's where their observables take on real number values. And each interacting system sees the other's values relative to itself. This gets into the point that the photon doesn't have any spacetime real position between interactions, hence asking "which path did it go down" is a meaningless noise.

Currently my favorite hard-nosed popular account of the issues with quantum mechanics is Where Does the Weirdness Go? by David Lindley, which I excerpted a biut earlier in this thread.

Yes, this is a good book – from memory though doesn’t it say “decoherence is the answer”? How does decoherence address the delayed choice issue?

Lindley likes decoherence and uses it to show that some old chestnuts like Schroedinger's cat and whether the moon is there when nobody is looking are bunk. He doesn't use it for the quantum eraser. For his account of that see the long excerpts I copied earlier in this thread. His explanation there is basically what you might call "highly motivated SUAC" One possible caveat; I think Lindley's discussion is pre-delayed choice, but I don't think that changes the argument much.

 

[jjalexand]

Alternate Universes interpretation

I'm a non-physicist, but it seems to me that making an observation could be seen as akin to locking oneself into a sub-set or an otherwise larger set of previously possible universes. I.e., by making an observation (or even by having been able to make that observation perhaps) the observer becomes "racheted-in" to a new smaller (but diverging?) subset of alternate universes?

In other words, all possible universes co-exist but you chose to increment your path through them every time you make an observation.

Alternatively you could be seen as being in the middle of a small personal subset that swung in a different direction every time your attention focussed on something! :)

 

[Cane_Toad]

I'm still trying to get a handle on this sort of thing. My 2 cents worth:

It is a serious mistake to take the descriptions given by quantum functions literally. Quantum theory describes *HOW* things work, not *WHY*. When people start adding *WHY* they are engaging in philosophical speculation. As far as I know, the why of the quantum world is still unknown.

Popular science writers have been doing a disservice to the field by selling their books based on the sensationalist views of the various "whys", i.e. many universes, human conscious interaction with wave collapse, etc., which are all great fun to think about, but are meaningless in the scientific sense of testability The process of trying to understand the "why" of quantum mechanics by the experts, which can be fantastical, has been translated to the public (often) without the realistic counter-point that these are only attempts at putting a framework around all the odd mathematical behavior.

So with the interference experiment, you can choose to *believe* that the experimenter's knowledge of an event affects correlated/entangled/etc. events. You can also choose to believe anything you want, but that's not science. Again, the mistake is to confuse a mathematical description of a behavior with reality.

People will always gravitate towards the mysterious, and promoting a book, paper, theory, etc. based on this, is taking advantage of non-experts who don't have the background frame their own skepticism (even if the author *believes* it him/herself).

My guess is that some day if we ever do understand what is going on at the sub-atomic level (my use of "why" above), we will find that quantum/string/brane/etc theories were good *models* of the physical world, but flawed in the long run just as Newton/Einstein/etc. have broken down at deeper/lower levels.

As one who once thought quantum mechanics was connecting the spiritual to the physical world, I have been (contentedly) disillusioned since then. It was akin to the disappointment/fascination at discovering that Santa Claus was really daddy in a red suit. The problem is that quantum theory is a much more difficult puzzle than Santa Claus.

Anyway, that's my world view, though I'm always ready to change it (via the school of hard [mental] knocks :-)

 

[MojaveJoe]

3. If conclusion 1 and 2 are true then a lot of people like to make overreaching assumptions about this paper, leading to unsupportable conclusions. Especially people like the producers of the "[MEDIA=youtube[/URL]. These people are interpreting the interference effect as being determined by "what an observer knows", when it really has nothing to do with an observer at all and only has to do with the existence, or non existence, of which path information.[/QUOTE]

That sounds good, but only if you don't think about it too closely. What is which path information? It is when the data has "meaning" aka...knowledge. There are plenty of points in the experiment where the data has no meaning and we see the interference pattern...it is only when we are able to analyze the data and arrive at a conclusion about the data that the pattern changes.

It was posited earlier in the thread that software that could "analyze" the which path data without any human interaction would also destroy the interference pattern. Even if this is the case, it seems to me that we still have a hugely significant problem in that knowledge (whether using a human mind or a "pseudo" mind like a computer) would appear to have an effect on our physical universe.

Edit:
Something similar to an artifact is what I initially thought was happening in these experiments...and I still think something similar to this is happening. However, I don't see how this can be the case at least looking at the initial Scully and Druhl experiments (which is where I got this question from originally). This is a quote from wikipedia as I don't have the book in front of me:

[Quote=wikipedia]In the double-slit experiment, the common wisdom is that the Heisenberg Uncertainty Principle makes it impossible to determine which slit the photon passes through without at the same time disturbing it enough to destroy the interference pattern. However, in 1982, Scully and Druhl found a way around the position-momentum uncertainty obstacle and proposed a quantum eraser to obtain which-path or particle-like information without introducing large uncontrolled phase factors to disturb the interference. They found that the interference pattern disappears when which-path information is obtained, even if this information was obtained [B]without directly observing[/B] the original photon. Even more surprising was that, if you somehow "erase" the which-path information, the interference pattern reappears! And, perhaps most provocative of all, you can delay the "choice" to "erase" or "observe" the which-path information and still restore the interference pattern, even after the original photon has been "observed" at the primary detector!
[/Quote]

As a non-physicist (wish I had made some different choices at university - granted I'd properly be a lot poorer!) I can imagine that these experiments are exceedingly delicate - we are dealing with virtually massless particles after all, and it seems to me that an artifact is very possible. But that is simply not what the experiment says...the experiment says otherwise.

[Quote=selfAdjoint]Sure looks right to me. "What the bleep.." is a notoriouus new age commercial that has been complained about by just about every physicst with a blog. Would that newspaper journalists would pick up on that.

And yes a whole lot of people have egg on their faces, not just mystigogical cranks.
[/Quote]

I'm at work so I cannot look at those "commercials". However, it seems to me that a physicist who posits an esoteric theory and then faces some (probably not all unjustified!) criticism in an attempt find an answer to this quandry is not entirely unadmirable.

Edit#2: Provided they understand the data and don't knowingly misrepresent it of course!

Cheers!

 

[MojaveJoe]

Popular science writers have been doing a disservice to the field by selling their books based on the sensationalist views of the various "whys", i.e. many universes, human conscious interaction with wave collapse, etc., which are all great fun to think about, but are meaningless in the scientific sense of testability The process of trying to understand the "why" of quantum mechanics by the experts, which can be fantastical, has been translated to the public (often) without the realistic counter-point that these are only attempts at putting a framework around all the odd mathematical behavior.

I disagree with this as far as it pertains to Brian Greene's book Fabric of the Cosmos. Mr. Greene made no claims about the original question in the book, it was simply my own thought derived from his explanation of the Scully/Druhl's experiments. It was a rather obvious thought to be honest and I simply asked for some clarification. Whether Mr. Greene agrees/disagrees with this position is never clarified - indeed that's why I came to this board to ask!

Cheers!

 

[Cane_Toad]

I disagree with this as far as it pertains to Brian Greene's book Fabric of the Cosmos. Mr. Greene made no claims about the original question in the book, it was simply my own thought derived from his explanation of the Scully/Druhl's experiments. It was a rather obvious thought to be honest and I simply asked for some clarification. Whether Mr. Greene agrees/disagrees with this position is never clarified - indeed that's why I came to this board to ask!

Cheers!

Interesting. Does he put forth any ramifications?


Also, in trying to understand this experiment, I'm wondering why there was any surprise when they found that getting which-path information killed the interference pattern ... after all, they were measuring an entangled photon, whose function interacts with the other photon. It was already known that measurements, by definition in the small world, affect the item measured, and thus break the interference pattern experiments.

It seems like the Scully/Druhl was a *failed* attempt to get information without interacting. They simply found that you can't cheat by using entangled photons. It isn't presented that way though.

Now, if somebody would just explain what the hell entangled photons are, I'd be happy. They imply that at the quantum level, distance is meaningless, and the only thing that holds up the universe is that God continues to care to throw the dice (i.e. nothing is real but probability functions). It does bring to mind the idea of these entangled photons being a single N-dimensional object projected into 3D space. Is this discussed somewhere?

 

[Cane_Toad]

Another question...

In that silly cartoon link posted earlier, it explained the experiment using electrons instead of photons, and using some (what?) kind of detector in the path of the electron before the double slit. I can only guess that it must measure some wiggle in the field of a passing electron, since the electron isn't consumed?

Anyway, if this is true (and the cartoon isn't completely bogus), then I have no clue when a probability function becomes un-collapsed(term??) for something. It suggests that any material close enough to the path of the electron would collapse it, acting like the detector. When the electron is first emitted, it is immediately in the vacinity of the emitter and other surrounding EMF. Why isn't its probability function collapsed just after it starts moving?

When does the electron regain its un-collapsed state? Presumably when it enters a shell in an atom, but why? What's special about that? Is an atom a probability generator? Has anybody tried to un-collapse anything?

 

[MojaveJoe]

Interesting. Does he put forth any ramifications?

Nope, its pretty much a survey book. He's partial to certain theories (like String) and does try to give attention to possible theories where they are noteworthy...he didn't give any attention to this particular theory though - maybe he didn't think it noteworthy.

Also, in trying to understand this experiment, I'm wondering why there was any surprise when they found that getting which-path information killed the interference pattern ... after all, they were measuring an entangled photon, whose function interacts with the other photon. It was already known that measurements, by definition in the small world, affect the item measured, and thus break the interference pattern experiments.

I would imagine the surprise comes because it was assumed that there is some "unknown" physical interaction (see the wikipedia quote I included) on the part of the observer and this experiment made that position very difficult to maintain. Indeed, if you leave out a physical interaction the only explanation of what is causing the wave to collapse is the knowledge of the which-path which is the question of this thread.

It seems like the Scully/Druhl was a *failed* attempt to get information without interacting. They simply found that you can't cheat by using entangled photons. It isn't presented that way though.

Yup. Cheaters never prosper

The nature of that interaction is the $64,000 question.

Cheers!

 

[MojaveJoe]

Another question...

In that silly cartoon link posted earlier, it explained the experiment using electrons instead of photons, and using some (what?) kind of detector in the path of the electron before the double slit. I can only guess that it must measure some wiggle in the field of a passing electron, since the electron isn't consumed?

Anyway, if this is true (and the cartoon isn't completely bogus), then I have no clue when a probability function becomes un-collapsed(term??) for something. It suggests that any material close enough to the path of the electron would collapse it, acting like the detector. When the electron is first emitted, it is immediately in the vacinity of the emitter and other surrounding EMF. Why isn't its probability function collapsed just after it starts moving?

When does the electron regain its un-collapsed state? Presumably when it enters a shell in an atom, but why? What's special about that? Is an atom a probability generator? Has anybody tried to un-collapse anything?

I think it may not be a good idea to think of "collapse" or "uncollapse" as pertaining to an individual electron cloud but rather look at it as a large collection of particles and their distribution on a detector screen. Collapse would be seen simply as the disappearance of an interference pattern (wavelike behavior) on the detection screen and the appearance of a standard curve (particle behavior). Uncollapse would be the opposite - and in fact does happen...that's the eraser part of the experiment.

Cheers!