Quantum Eraser and Its Implications

[DevilsAvocado]

self-styled quantum physics experts

That’s me!

(:rofl:)

In other words, classical wave mechanics has no difficulty with DCQE

Now you’re dreaming again Ken G.

 

[San K]

Of course!

And this data at D0 is recorded 8ns earlier than that of D2?? We could easily extend this to seconds, hours...

I don’t get it... P.S. Wouldn’t we be able to tell 'which path' from the phase differences in the set up in 'my' picture in post #27?

hi devilsavocado

thanks for editing my other post.

Not sure what you are asking, however if I guessed correctly (as to what you are asking) then the below information might help.

When the photon at Do is recorded then the probabilities of its entangled twin photon (hitting specific locations/detector also gets fixed (the wave function collapses and the idler photon has a definite state).

Thus we can probabilistic-ally say:
given that the photon at Do is (recorded)at position x,y ...the probability of its twin photon arriving at a detector d2 is z etc.

On a separate note: can we say that an avocado is much higher on the morality hierarchy and closer to god/heaven than an advocate?...:)

 

[DevilsAvocado]

hi devilsavocado

thanks for editing my other post.

You’re welcome, hope it helped!

Not sure what you are asking, however if I guessed correctly (as to what you are asking) then the below information might help.

When the photon at Do is recorded then the probabilities of its entangled twin photon (hitting specific locations/detector also gets fixed (the wave function collapses and the idler photon has a definite state).

I see a problem right there; the idler twin photons don’t hit any "specific locations" (on the x-axis). It’s only a count of photon hits and timing, and depending on path you could tell/or not tell which slit it went thru. That’s it.

Thus we can probabilistic-ally say:
given that the photon at Do is (recorded)at position x,y ...the probability of its twin photon arriving at a detector d2 is z etc.

I don’t get it. There is no "z" for D2, only registration of the hit + time.

My confusion is due to the fact that no interference pattern is ever seen in the total pattern of signal photons at D0, meaning that if you "throw away" the other idler detectors you got nothing but 'noise' at D0.

This picture shows data for D0+D2 and D0+D3:

Now imagine you would add the data for D1 & D4 also, and remove any colors and 'guiding curves' = there’s your "noise" at D0.

So my question is: I understand that we could get a mix of interference/non-interference pattern in D0, what I don’t understand is how this could be set before any of the idler detectors has recorded anything at all. In this paper the time difference is 8ns, but this could easily be extended into 'absurdum'...

Unless I’m missing something substantial – something 'weird' is happening here.

On a separate note: can we say that an avocado is much higher on the morality hierarchy and closer to god/heaven than an advocate?...:)

Definitely, case closed! ()

 

[Ken G]

So, do so-called quantum erasure experiments inform wrt the reality underlying instrumental behavior? Or, are we still left with the fundamental conundrum illustrated by quantum double-slit experiments?

I think we are left with the fundamental conundrum, but we get insights into how to make it not a conundrum: by working on our expectations rather than our physics. In some sense, the discovery of DCQE moves us one step farther away from understanding the double slit-- it is a more sophisticated way to explore the double-slit behavior, but it results in even more sophisticated questions that we cannot answer, rather than answering the original ones! But in another way it moves us closer to not being concerned about our lack of answers, because it actually teaches us something about what an answer is, specifically, it teaches us the difference between an answer to a question that an experiment can give meaning to, and an answer to a question that we imagine has meaning but probably doesn't (because we can't find an experiment to answer it, without changing the question we are answering).

I'd say the main lesson is that what we think happened in some experiment depends on how analyze and test what happened, because many roads can lead to the same destination-- many types of empirical augmentation can be relevant to the same originally stripped-down empirical investigation, but they all might be consistent with something different happening because the original stripped-down version doesn't distinguish them.

Where all this is most relevant is in regard to the "next theory" after quantum mechanics. The key question is, is there really something wrong with quantum mechanics that needs fixing, or is there something wrong with our expectations for physics that need fixing? If we work on the latter hard enough, we might be able to get quantum mechanics to seem like a "perfect theory," in that it does everything we can expect a physics theory to do. But that doesn't mean there aren't really problems with quantum mechanics, that might make some future generation look back on us, with their new improved theory, and say "I can't believe you were really satisfied with that state of affairs." Just as we look back on those before us.

 

[Ken G]

I understand that we could get a mix of interference/non-interference pattern in D0, what I don’t understand is how this could be set before any of the idler detectors has recorded anything at all. In this paper the time difference is 8ns, but this could easily be extended into 'absurdum'...

This is the point I'm making, that when you call something "noise", you have no idea what "information" went into it. One man's noise is another man's information, the only difference is how they are slicing that information. You see "no interference pattern", and conclude that no interference occurred at all. But you can't conclude that-- you can only conclude that no interference occured in the net. It seems to me that the main message of DCQE is that if our experiment cannot tell us why or how we get the "noise" we get, then we cannot conclude it is "really noise", it might be layers and layers of highly structured information (including interference patterns) that we are simply not extracting in our experiment, whether it be a stripped-down double slit with which-way information, or a more sophisticated DCQE experiment with a delayed choice to extract which-way information. Other experiments (like a delayed choice to "leave in" the which-way information rather than extract it by destroying the necessary coherences) might be able to extract that information, but not by changing anything that happened in the first detection-- but simply by looking at it in a different way. There is no problem with looking at something in a different way any amount of time later-- what actually happened is never changing. (That's what I meant by the causes of WWI, an analogy that doesn't seem to resonate for you. Too classical for your taste, probably.)

But what gets very subtle is when "what actually happened" is itself a construct of our intelligence, based on what information we have about the apparatus, and what expectations we have for physics. So looking underneath the lesson of DCQE, I think we see a deeper message: what actually happened is what the detector showed, period-- our desire to deconstruct further what happened requires different apparatus to disentangle, but if the apparatus is different then something different happened-- even if it is something that must be consistent with the stripped-down version, it doesn't have to be the same thing as happened in the stripped-down version. Gone are the days when we could imagine our measuring apparatus was a "fly on the wall" to what is really happening-- instead, we find that our measuring apparatus is a defining element of what is really happening-- regardless of when in time those defining choices are made. Thus we should not talk so much about what actually happened, or changing what actually happened, we should simply talk about what we can say about what happened, and when we can say it!

 

[Joncon]

Assuming the setup as shown in DevilsAvocado's post, you always have two fields arriving at the final BS. One from the red path and one from the blue path. As you cannot say that the photon that will be detected later has taken one of these paths, you need to take both of them into account which then gives the interference effect mentioned earlier. If you just send single photons down one of these paths, you will instead get a pattern like the last one you posted.

Ah OK, I think I've got it now. So just to confirm I've understood - the idler photon "travels both paths" in the same way the signal photon does, and so interferes with itself?

 

[Cthugha]

P.S. Wouldn’t we be able to tell 'which path' from the phase differences in the set up in 'my' picture in post #27?

No. You start with an unknown phase. So you end up with a phase of unknown +x for going one way and unknown+y for the other which is basically still unknown. The difference y-x has some influence on the result, but gives no which way info.

I understand that we could get a mix of interference/non-interference pattern in D0, what I don’t understand is how this could be set before any of the idler detectors has recorded anything at all. In this paper the time difference is 8ns, but this could easily be extended into 'absurdum'...

I am not quite sure I get your question right, so forgive me if my answer is way off target. What you get at D0 is basically a superposition of many interference patterns which then add up to noise. As a maybe easier to grasp example imagine a sine wave. You have a device that gives out two identical sine waves as a signal. You switch it on and off and you get sine waves out every time, but the initial phase differs every time you switch it on. So the sine waves sometimes start at the highest point, sometimes at zero, sometimes at the lowest point, sometimes in between and so on, but both of the waves coming out at the same time are exactly equal. Now you switch it on and off many times and perform two different measurements on the two sine waves coming out:

1) For one of them you just add up every sine wave that comes out after each time you switch your device on. If you integrate long enough, all you will get is a straight line as for each sine wave coming out having its highest value at some point there will on average also be one sine wave coming out which is exactly out of phase so it has its lowest value at that very same point. The sum of all of them just gives the straight line.

2) For the other sine wave coming out, you do not measure the shape of the sine wave, but have some measuring device that just gives you the initial phase. Nothing more.

So now you can sort. The sum at 1) is a straight line, but you can filter using the information from 2. For example you could take all runs with the initial phase being zero. If you now take all switch on processes at 1) which correspond to the subset you chose at 2), you will get a sine wave back. If you pick the subset with a phase of pi, you will get a different sine wave. And so on and so forth.

The DCQE experiment of course adds some extras, for example non-local effects enter as you have entangled photons.

I see a problem right there; the idler twin photons don’t hit any "specific locations" (on the x-axis). It’s only a count of photon hits and timing, and depending on path you could tell/or not tell which slit it went thru. That’s it.

That's almost it. One important point is missing. If you cannot tell which slit it went through, this means that the photon will either arrive at D1 or D2. However, the probability is not 50/50. Depending on which position of D0 the corresponding signal photon is detected, it is either more likely that the idler will end up at D1 or at D2.

the idler photon "travels both paths" in the same way the signal photon does, and so interferes with itself?

Yes!

 

[Joncon]

Thanks Cthugha, that one's been bugging me for a while

Although this doesn't seem too much more mysterious than the standard double slit experiment now ...

 

[Cthugha]

Although this doesn't seem too much more mysterious than the standard double slit experiment now ...

Well, there are several implementations of DCQE out there. Some of them are more complicated, some can be broken down to be explained in rather simple terms. That depends. By the way in my opinion the standard double slit can already be pretty mysterious...

 

[Joncon]

By the way in my opinion the standard double slit can already be pretty mysterious...

Oh yeah, I agree. What I mean is that the DCQE (at least the one mentioned here) doesn't seem to add as much mystery as it appears to initally.

 

[DevilsAvocado]

I am not quite sure I get your question right, so forgive me if my answer is way off target.

I have a feeling it’s a great answer... if there’s any 'problem' it most probably is located between my ears...

I think I’m almost there, but I’m slightly 'blinded' by years of discussing EPR-Bell, rotating polarizers, and stuff, so I must ask you this, before going any further:

• Has the 8ns delay, mentioned in the paper, anything to do with anything?
  
  
• What kind of pattern would one get at D0 if we completely remove everything on the idler side (from the PS and on)?
  
  
• Is the entanglement just a 'tool' to get everything "in phase" like your "two identical sine waves"?

 

[DevilsAvocado]

This is the point I'm making, that when you call something "noise", you have no idea what "information" went into it. One man's noise is another man's information, the only difference is how they are slicing that information.

That sounds like politics to me...

Seriously, I think my way of expressing myself has caused some 'misunderstanding'. When I say "noise" I mean "pattern noise", or the lack of a "meaningful pattern". The actual measurements (counts/hits) of single photons are not noise to me (okay, there are of course real noise, but this is reduce by the coincidence counting). In other threads we have had 1,500+ comments on EPR-Bell and measurement loopholes, I don’t think I can take another round on this...

You see "no interference pattern", and conclude that no interference occurred at all.

No, that’s not what I’m saying. I see the interference pattern, clearly. However, I don’t understand how this works and how it got there. (Until Cthugha is about to save my soul ;)

Other experiments (like a delayed choice to "leave in" the which-way information rather than extract it by destroying the necessary coherences) might be able to extract that information, but not by changing anything that happened in the first detection-- but simply by looking at it in a different way.

I agree that we are not changing anything in already performed measurements, but to me this is not "the problem". Maybe I’m "looking at it" in the wrong way...

There is no problem with looking at something in a different way any amount of time later-- what actually happened is never changing.

Agree 100%

(That's what I meant by the causes of WWI, an analogy that doesn't seem to resonate for you. Too classical for your taste, probably.)

Well, here we disagree. To me there’s a HUGE difference between historical human events and physics, and that is repeatable empirical data that fits the theory, again and again and again and again... until someone come up with a brighter idea.

But the data never change, and Newton’s apple does not suspend itself in mid-air just because of Einstein, it will continue to fall the same way it always has.

Philosophy, psychology, economy, history, metaphysics, etc, don’t have this luxury and this makes it very different (to me).

But what gets very subtle is when "what actually happened" is itself a construct of our intelligence, based on what information we have about the apparatus, and what expectations we have for physics. So looking underneath the lesson of DCQE, I think we see a deeper message: what actually happened is what the detector showed, period

Yes, the detector is all we have to hold on to...

Gone are the days when we could imagine our measuring apparatus was a "fly on the wall" to what is really happening-- instead, we find that our measuring apparatus is a defining element of what is really happening-- regardless of when in time those defining choices are made. Thus we should not talk so much about what actually happened, or changing what actually happened, we should simply talk about what we can say about what happened, and when we can say it!

To us the apparatus must be real (even if theory eventually says otherwise on a more fundamental level). The value the apparatus shows must be real, and we must be able to agree on this value.

If it shows 120 photons, then it is 120 photons to everyone. Not with respect to this and that. This is what we got, period.

If we give this up, we have nothing, absolutely nothing, and science becomes 'philosophical gibberish' – "Please define 120!"


(I’m not saying every measurement is 'perfect', but I think we have to assume that they are 'reasonable' and as good as it gets, in case of real science.)

 

[Joncon]

I think I’m almost there, but I’m slightly 'blinded' by years of discussing EPR-Bell, rotating polarizers, and stuff, so I must ask you this, before going any further:

OK I'll take a punt, and treat it as a test until someone with more knowledge can come back with the proper answers

[*]Has the 8ns delay, mentioned in the paper, anything to do with anything?

I think this is just the bit which puts the "Delayed" in DCQE. So we send our photons through the double slit, but then can choose to "erase" the which path information 8ns after the actual detection.

[*]What kind of pattern would one get at D0 if we completely remove everything on the idler side (from the PS and on)?

I would imagine the pattern would be completely unchanged as there is still the possibility, in principle, to obtain which path information.

[*]Is the entanglement just a 'tool' to get everything "in phase" like your "two identical sine waves"?

Again, this just enables us to retrieve or "erase" which path information *after* the signal photons have been detected, which wouldn't be possible just using single photons.

 

[Ken G]

No, that’s not what I’m saying. I see the interference pattern, clearly. However, I don’t understand how this works and how it got there. (Until Cthugha is about to save my soul ;)

That's what I have been talking about, the answer to the "how it got there." The answer is, it was always there, it just wasn't discernable-- it looks like noise when the experiment is not able to separate it. It's a bit like a code-- if you see a coded message, it might look like complete gibberish, no rhyme or reason there and certainly not a message. But if you have the decoder, the message pops right out. You don't ask "where did this message come from, it was complete gibberish a moment ago-- have I done something that propagated a signal into the past and turned gibberish into a message being sent?" The time later that you decode the message is completely irrelevant, it could be 100 years later, because the message was always there. You didn't change it, you decoded it. That's what correlating the entanglements does. But you can "decode" it in several ways, based on what you choose to do with the entanglements. Do one thing, and the message is still gibberish-- you haven't extracted that information (you extracted some other information instead, perhaps some other message that now makes sense to you). Do something different with the entanglements, and it is like using a different cypher. The message pops right out, any amount of time later.

Well, here we disagree. To me there’s a HUGE difference between historical human events and physics, and that is repeatable empirical data that fits the theory, again and again and again and again... until someone come up with a brighter idea.

It's just an analogy, but I think it is a good one. The point is, if we stick to a "just the facts ma'am" approach, we have a bunch of events that led up to WWI, and we have a bunch of photons hitting detectors at various times. That's it, nothing more. But we are not satisfied, we want to seek reasons for why these events transpired, what was the "cause" of the presence or absence of patterns. Right away we are telling a story-- we've left the dry narrative of photons hitting detectors and people knocking off Archdukes, and we are saying "this led to that." It isn't history any more, it isn't physics any more-- yet we still call it history, and physics, because in fact this is what we want to know about history and physics. But our means of analysis has entered the picture-- we no longer are dealing in irrefutable empirical data, we have invoked a process of description, and it need not be unique.

That's the key point, the different things we do with the entangled pairs, long afterward, are like choosing different processes for describing what happened in the original data. No matter which process we choose, we still have to explain the same initial data, but the way we explain it can be very different. That's quantum erasure, and it's also historical analysis-- at least, that is the similar features to them. There are of course also differences!

But the data never change, and Newton’s apple does not suspend itself in mid-air just because of Einstein, it will continue to fall the same way it always has.

Ah, but the first half of your sentence has nothing to do with the second! The data never change, true, but whether or not the apple continues to fall is a description of what happened to the apple, it isn't data! Newton says the apple fell, Einstein says the apple ceased to be accelerated by the branch. A totally different story about what happened to the apple, both consistent with the data. So the data did not change when Einstein came along, but what "happened to the apple" certainly did change with Einstein! Because what happened is a construct, and changes in information, centuries later, can change that construct dramatically.

Philosophy, psychology, economy, history, metaphysics, etc, don’t have this luxury and this makes it very different (to me).

But physics is actually not so different-- it doesn't have that luxury either. All that is different is the precision that is possible, and the scale where we encounter just where that "luxury" breaks down.

To us the apparatus must be real (even if theory eventually says otherwise on a more fundamental level). The value the apparatus shows must be real, and we must be able to agree on this value.

Yes, the value the apparatus shows-- but not why it shows it. Not whether or not interference occured, not which slit the particle went through. Those are not part of the data until we make the choice to make them part of the data-- at which point our description of what happened also changes, even long after the original experiment in which the happening happened.

If it shows 120 photons, then it is 120 photons to everyone.

Certainly, but that's not "what happened". We don't say "120 photons hit a detector in this here pattern", we say "no two-slit interference". The latter is not 120 photons, it is a kind of judgement about what happened, and that's what quantum erasure shows is not a unique thing, and can change a century later without actually changing anything at all but our mode of analysis of the original happening. This is no minor point-- quantum mechanics is extending to physics the much more general rule that our descriptions of what happened are dependent on our means of establishing what happened.

 

[DevilsAvocado]

OK I'll take a punt, and treat it as a test until someone with more knowledge can come back with the proper answers

Thanks, much appreciated!

I think this is just the bit which puts the "Delayed" in DCQE. So we send our photons through the double slit, but then can choose to "erase" the which path information 8ns after the actual detection.

Okay... interesting... but if I got this right; the entanglement does not affect the outcome at D0 one bit, right? So what’s in fact is 'delayed' is the choice to measure "which path", or not, in "cloned twin beam", right?

If I understand this right, non-locality is not the crucial thing here, but a "clone copy" of the signal beam, right?

I would imagine the pattern would be completely unchanged as there is still the possibility, in principle, to obtain which path information.

This is, I think, the 'Gordian Knot' to me... Why do we get a mixture of interference/non-interference pattern in D0? What causes it? There’s no "flip-flopping gate" at the double slit, is it? I don’t get it? In a normal experiment we would get an interference pattern or no interference pattern, not both, right??

Again, this just enables us to retrieve or "erase" which path information *after* the signal photons have been detected, which wouldn't be possible just using single photons.

This I get, but as you see in Cthugha answer we could also do it with another mechanism of two "cloned waves"... I think I’m 'over-interpreting' the part of entanglement in this experiment... I don’t know...


(Ken G, get back later...)

 

[Cthugha]

Okay... interesting... but if I got this right; the entanglement does not affect the outcome at D0 one bit, right? So what’s in fact is 'delayed' is the choice to measure "which path", or not, in "cloned twin beam", right?

If I understand this right, non-locality is not the crucial thing here, but a "clone copy" of the signal beam, right?

Well, entanglement has two main effects. One is that that the two entangled particles do indeed bahave like cloned copies (or shifted copies or whatever one likes to call it) with the main point being that you can tell what the entangled partner will do if you know what the first particle does. The second point in entanglement is that these properties are not imprinted from the beginning in a hidden variable-like fashion, but the state of both is fixed when the first measurement occurs which implies nonlocality. The latter is what is tested in Bell tests.

So for the experimental outcome it is in fact only the first property which matters. There is information that can only be accessed when detecting both entangled particles and "matching up". Non-locality does not really matter in terms of the outcome, but in terms of the interpretation of the results. For example the standard DCQE experiment could be changed such that the delay between detections of signal and idler becomes large and one could perform the measurements in such a fashion that Bell inequalities are tested.

Why do we get a mixture of interference/non-interference pattern in D0? What causes it? There’s no "flip-flopping gate" at the double slit, is it? I don’t get it? In a normal experiment we would get an interference pattern or no interference pattern, not both, right??

In a normal double slit experiment the pattern you will see depends on the geometry of your experiment. If your light source is for example not exactly centered between the two slits, you will get a slightly different pattern as the distances between the source and the two slits are now different. As you move the source around, you will get different patterns. So if you now place several light sources at different positions you will now get a superposition of all of these patterns. If you have enough sources the superposition will be no pattern at all. This is foe example the same reason why the interference pattern disappears in a common double slit experiment if you place your light source too close to the slits. It will then reappear as you increase the distance between the slits and the source.

Now in the DCQE you have a similar setting (I am referring to figure one in the paper by Kim, Kulik, Shih and Scully). You have two atoms A and B placed at the slits which could emit entangled photon pairs. This process is completely random and one cannot distinguish from which atom some certain phton pair comes (unless you have detections at D3/D4). However, the two atoms are not synchronized. The phase of the light fields emitted from the two atoms is pretty much random with respect to each other and it also fluctuates randomly. This is pretty much like having a like source emit a single photon in the common double slit experiment and then moving it somewhere else, emitting another photon, etc which will add up to no interference pattern at all. However, if you note the position of the source for each photon and afterwards just pick a subset of detections corresponding to the source being at the same position, you will find some pattern. If you pick a different subset corresponding to a different position, you will find some different pattern.

In DCQE the position of the peaks in the double slit pattern will depend on the relative phase between the fields emitted by the atoms, so you do not see a single interference pattern, but a superposition of many which add up to no interference pattern. However, you can find any of these interference patterns by just picking those detections that correspond to some phase difference between the fields. That picking is now done using the other entangled partner. Depending on the relative phase that partner is more likely to be detected at D1 or D2, so you get a fringe/antifringe pattern when the coincidence counts D0/D1 or D0/D2 are considered.

 

[DevilsAvocado]

One is that that the two entangled particles do indeed bahave like cloned copies
...
So for the experimental outcome it is in fact only the first property which matters.
...
Now in the DCQE you have a similar setting (I am referring to figure one in the paper by Kim, Kulik, Shih and Scully). You have two atoms A and B placed at the slits which could emit entangled photon pairs. This process is completely random and one cannot distinguish from which atom some certain phton pair comes (unless you have detections at D3/D4). However, the two atoms are not synchronized. The phase of the light fields emitted from the two atoms is pretty much random with respect to each other and it also fluctuates randomly. This is pretty much like having a like source emit a single photon in the common double slit experiment and then moving it somewhere else, emitting another photon, etc which will add up to no interference pattern at all. However, if you note the position of the source for each photon and afterwards just pick a subset of detections corresponding to the source being at the same position, you will find some pattern. If you pick a different subset corresponding to a different position, you will find some different pattern.

In DCQE the position of the peaks in the double slit pattern will depend on the relative phase between the fields emitted by the atoms, so you do not see a single interference pattern, but a superposition of many which add up to no interference pattern. However, you can find any of these interference patterns by just picking those detections that correspond to some phase difference between the fields. That picking is now done using the other entangled partner. Depending on the relative phase that partner is more likely to be detected at D1 or D2, so you get a fringe/antifringe pattern when the coincidence counts D0/D1 or D0/D2 are considered.

Yay! I think I’ve got it! Many THANKS!

I’ve been completely lost... (as you may have noticed). All the time I have assumed that an interference pattern is created already directly after the slits, that is "picked up" by the BBO to then be (randomly) 'transformed' into two entangled pair... god I’m stupid...

The distance is obviously too short for this to happen... the 'interference' is instead created by the random and relative phase (shift) in the photons emitted from the BBO. Please, tell me I got this right?? :uhh:

I was thinking... is it ever possible to move the BBO to be placed before the slits? And then use a (P)BS after the slits to separate signal & idler, and thereafter do "fancy measurements" on the two entangled twins?

Or is it already a dead end when hitting the BS??

 

[San K]

You’re welcome, hope it helped!

I see a problem right there; the idler twin photons don’t hit any "specific locations" (on the x-axis). It’s only a count of photon hits and timing, and depending on path you could tell/or not tell which slit it went thru. That’s it.

Sorry replace the concept "specific location" with detector. Once
Signal photon has stuck the detector the probability of idler hitting a particular detector is calculable (based on location of signal photon on the screen)

 

[Quaman]

Hi!
I have not finished reading through the whole thread, so please excuse me if I am asking something that was already covered. Here are my questions related to a DCQE setup:
1/a) If which-path information is observed (but not recorded!) by a non-human observer at D3 and D4, will or will not the interference pattern collapse at D0 when D0 is observed by a human?
1/b) If which-path information is observed and recorded by a non-human observer at D3 and D4, will the interference collapse at D0 when observed by a human? Let's assume that no human ever sees that record. It is clear that if the record later gets observed by a human, D0 will not show interference pattern.
1/c) What if the record made by a non-human observer at D3 and D4 is irrevocably deleted before any human can observe it? Will the interference collapse at D0 when D0 is observed by a human? If yes, why? I think it should not, because the entangled pair (idler) will never be observed.
2) In the same dcqe setup if which-path information is observed by Alice at D3 and D4, but does not have the possibility to tell Bob about it (eg. dies before she can do so), will Bob at D0 see interfence? This is somewhat similar to 1/c, but the difference is that the record was made by a human in this case and that record unfortunately also got 'irrevocably erased'.
I am pretty sure no-one has ever carried out experiment no. 2), as it is apparently against federal laws :) But I expect it should give the same results as 1/c, right?

If you know that an experiment was carried out before with any of the above or similar setup, I would be really glad if you could share the link to the paper (or at least to its abstract) here :) Thank you!

 

[Cthugha]

Same answer to all questions: The presence or absence of humans has absolutely nothing to do with the outcome. The meaning of observing in physics does not require any consciousness. Any irreversible interaction is sufficient.

 

[Ken G]

The meaning of observing in physics does not require any consciousness.

And I'd like to interject one caveat that is seemingly picky but I think is actually quite important: meaning always requires consciousness. But your point, I believe, is that consciousness does not enter at the same physical level as the other elements of the experiment, the detector, the slits, etc., nor does it enter at the level of the theory, the Hilbert space or the operators, etc. So there is no "piece of the apparatus" we need to call conscious, and there is no term in the equation we might want to call the "effect of consciousness." All the same, I think quantum mechanics is one of the places where we are forced to come to terms with the fact that the "fingerprints" of our consciousness are all over what we are doing there. A universe without consciousness has no quantum mechanics in it at all, nor anything recognizable as physics, because as Bohr put it, physics is about what we can say about nature. And by "we", he means our consciousness/perceptions/intelligence, etc.. Not our measuring apparatus, which is quite mute.

. Any irreversible interaction is sufficient.

This provides me with a more concrete example of my point-- the fundamental interactions in physics are generally time reversible, so "irreversibility" is already a concept that appears, not in the physical interactions, but at the level of our mental processing of those interactions. Hence, there is no such thing as "irreversible" in the first place, without consciousness (and here I make no effort to parse between what it means to be conscious versus intelligent or able to perceive, as I don't think we have any kind of precise language to use to parse those notions).

 

[DevilsAvocado]

... A universe without consciousness has no quantum mechanics in it at all, nor anything recognizable as physics, because as Bohr put it, physics is about what we can say about nature. And by "we", he means our consciousness/perceptions/intelligence, etc.. Not our measuring apparatus, which is quite mute.

I don’t recall Bohr or the Copenhagen interpretation ever saying anything about consciousness... this sounds like the von Neumann/Wigner interpretation.

But I’m curious; exactly how do you explain the evolution of the universe and ΛCDM if consciousness has any crucial role in this evolution? For example red-shift of the CMB, in relation to "our consciousness", is something that you could describe in detail. Thanks.

I’m all ears!

 

[DevilsAvocado]

It's just an analogy, but I think it is a good one. The point is, if we stick to a "just the facts ma'am" approach, we have a bunch of events that led up to WWI, and we have a bunch of photons hitting detectors at various times. That's it, nothing more.

I don’t agree that "events that led up to WWI" could be compared to physics and the scientific model. If this analogy was ever to operate as a theory that is refutable, you need two things:

1. A mathematical description of the human mind, which we don’t have.
   
   
2. The ability to experimentally repeat "events that led up to WWI" as many times you like, which is impossible.

Ah, but the first half of your sentence has nothing to do with the second! The data never change, true, but whether or not the apple continues to fall is a description of what happened to the apple, it isn't data! Newton says the apple fell, Einstein says the apple ceased to be accelerated by the branch. A totally different story about what happened to the apple, both consistent with the data. So the data did not change when Einstein came along, but what "happened to the apple" certainly did change with Einstein! Because what happened is a construct, and changes in information, centuries later, can change that construct dramatically.

You are mixing apples and oranges. As I said Newton’s apple does not suspend itself in mid-air just because of Einstein, it will continue to fall the same way it always has, i.e. to the ground.

This is the empirical fact that no new theory could ever change - apples will not suspend in mid-air, period.

The explanation on why and how apples fall could of course change in the future.

 

[Cthugha]

And I'd like to interject one caveat that is seemingly picky but I think is actually quite important: meaning always requires consciousness. But your point, I believe, is that consciousness does not enter at the same physical level as the other elements of the experiment, the detector, the slits, etc., nor does it enter at the level of the theory, the Hilbert space or the operators, etc. So there is no "piece of the apparatus" we need to call conscious, and there is no term in the equation we might want to call the "effect of consciousness."

I have already stated elsewhere (was it inside this thread? I do not know) that one cannot rule out the influence of a human looking at some experiment with absolute certainty as there is no possibility to test this experimentally. You cannot find out what happens if you never find out what happens. However, this also makes it a non-scientific question for exactly the same reason. It is interesting from a philosophical point of view, though.

This provides me with a more concrete example of my point-- the fundamental interactions in physics are generally time reversible, so "irreversibility" is already a concept that appears, not in the physical interactions, but at the level of our mental processing of those interactions.

I doubt that. Irreversible interactions occur in every measurement. Whenever some superposition of states ends up in an eigenstate. Whenever entropy changes. In the context of my post you quoted I just wanted to point out that there is some difference between processes like inserting a wave plate in a light beam and rotating its polarization on the one hand and processes like absorbing a photon at one certain position. The first is reversible and does not constitute a measurement. The second is usually irreversible, collapses a wave function and constitutes a measurement.

Irreversibility shows up in entropy and such things as the arrow of time. While it is true that many underlying fundamental processes may be reversible, there is nevertheless a statistical prevalence for things to happen in a certain way (see Feynman's famous broken cup example) in the sense of statistical mechanics. I do not think that mental processing is a necessity for that concept from a physics point of view. Philosophical discussions are of course a different topic, bit from my point of view rather distracting when discussing real experiments.

 

[Maui]

But I’m curious; exactly how do you explain the evolution of the universe and ΛCDM if consciousness has any crucial role in this evolution? For example red-shift of the CMB, in relation to "our consciousness", is something that you could describe in detail. Thanks.

I’m all ears!

Something tells me a non-realist would not consider the CMB and the Big Bang the ultimate explanation of existence and reality. I am not sure they believe in reality at all(and that's not the craziest I've seen in the interpretations war)

 

[DevilsAvocado]

 

[San K]

I have already stated elsewhere (was it inside this thread? I do not know) that one cannot rule out the influence of a human looking at some experiment with absolute certainty as there is no possibility to test this experimentally. You cannot find out what happens if you never find out what happens. However, this also makes it a non-scientific question for exactly the same reason. It is interesting from a philosophical point of view, though.

I think there are (many simple) ways to rule out the influence of a human looking at an experiment.

By setting a series of processes, a cascade/chain of events (...like a domino effect).

One could still argue that the whole process happens when a human looks at it.

one of the arguments (and there are more) refuting that would be:

if you were to repeat the experiment say a hundred/thousand times...you could predict with exact certainty at what any particular point in time where the cascade of events would have reached...for each, and every, instance/run of the experiment.

another refutation would be:

that the series of events cannot happen in an instant, the moment a human looked at the other end of (or anywhere in-between) the series of events

another refutation would be:

even if you assumed multiple universes...you end up with infinite universes...for the more complex setups/experiment...

 

[Ken G]

I... this sounds like the von Neumann/Wigner interpretation. don’t recall Bohr or the Copenhagen interpretation ever saying anything about consciousness

Bohr said that physics is not about nature, it is about what we can say about nature. Last I checked, "we" were conscious.

But I’m curious; exactly how do you explain the evolution of the universe and ΛCDM if consciousness has any crucial role in this evolution?

How do "I" explain it? You mean using my consciousness, or not using my consciousness?

 

[Ken G]

I have already stated elsewhere (was it inside this thread? I do not know) that one cannot rule out the influence of a human looking at some experiment with absolute certainty as there is no possibility to test this experimentally. You cannot find out what happens if you never find out what happens. However, this also makes it a non-scientific question for exactly the same reason. It is interesting from a philosophical point of view, though.

There might not be such a "firewall" between what is scientific and what is philosophical. The point being, consciousnesses do both science and philosophy.

I doubt that. Irreversible interactions occur in every measurement.

Not so, irreversibility is imposed by the analyst, as all the primitive happenings as described by classical physics are reversible. What's more, nothing ever actually reverses, it's just a mode of thinking that they could or could not. We judge the event to be irreversible based on assumptions we make about the constraints on the system. These judgements are useful, they are not mistaken or illusory, but they do come from our analysis. Nature never reverses itself, so nothing is actually reversible in nature. What's more, everything in nature happens only once, or at least that is a natural assumption to make that no experiment has ever refuted. Thus, the whole notion of "reversibility" comes from us, yet it has value in our physics, like so many of the other notions of physics that come from us. It's a little off topic though to get into thermodynamics!

Whenever some superposition of states ends up in an eigenstate.

I think you mean, whenever we choose to treat a system as changing from a superposition of states to an eigenstate. And we have good reason to do that, I don't dispute that, I'm just pointing out that it is we who are making that choice-- not nature. Such happenings do not actually occur in nature, none of those things actually exist. Quantum mechanics certainly doesn't claim they exist, the theory is perfectly clear on the fact that idealizations uphold that kind of language.

In the context of my post you quoted I just wanted to point out that there is some difference between processes like inserting a wave plate in a light beam and rotating its polarization on the one hand and processes like absorbing a photon at one certain position. The first is reversible and does not constitute a measurement. The second is usually

Yes, that is certainly true, I'm not trying to contradict the validity of that point. I'm saying something different-- as the thread is about the role of consciousness in physics.

While it is true that many underlying fundamental processes may be reversible, there is nevertheless a statistical prevalence for things to happen in a certain way (see Feynman's famous broken cup example) in the sense of statistical mechanics.

Not for things to happen that way, but there is certainly value in organizing our experiences around that way of thinking. In our consciousness.

I do not think that mental processing is a necessity for that concept from a physics point of view.

But it is, and just look at your own language: there is no "physics point of view", because physics doesn't have a point of view, that is the prerogative of consciousness.

Philosophical discussions are of course a different topic, bit from my point of view rather distracting when discussing real experiments.

In many situations in physics, yes, but not when the question is fundamentally about the role of consciousness in physics. In that situation, these other issues need to be raised, not in contradiction to what you were saying, but in addition to it.

 

[DevilsAvocado]

Bohr said that physics is not about nature, it is about what we can say about nature. Last I checked, "we" were conscious.

Okay, so how did you check this, with the help of "God"? Or, did you call 9-1-1?

I mean, according to you, everything is a wobbly fairy tale of "philosophical talk" and "(un)consciousness". So how could you ever check/prove that you are conscious?

Crucial, since most of your claims seem to be compatible to cranky unconsciousness.

[I know it’s hard for you to talk about anything that makes sense. Therefore, I’m going to help you with the most basic question:

– If nothing is real besides your (un)consciousness, how are you ever going to prove that your (un)consciousness is real, that it mean anything, except mumbo-jumbo?]

Please elaborate.

A universe without consciousness has no quantum mechanics in it at all, nor anything recognizable as physics

Unscientific nonsense and distortions of history... if you are claiming that Niels Bohr ever said anything like this stupidity, quote please.

This is what Niels Bohr actually said, as quoted in "The philosophy of Niels Bohr" by Aage Petersen (September 1963):

"There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature..."​

And of course, the key word is quantum. Bohr never doubted the ontological reality itself, only the extent of our epistemic access to the real universe.*

You are distorting the words by Bohr as to "if we don’t talk about the universe, it will cease to exist" which is hilarious and completely absurd, and something that Bohr never said.

Niels Bohr is known as "The Man Who Talked".
Ken G is known as "The Man Who Spoke in Tongues".

How do "I" explain it? You mean using my consciousness, or not using my consciousness?

Well, it doesn’t seem to make much difference what "mode" you are in – nothing you say makes sense anyhow. But let’s not get stuck in tribulations between your ears; how about "fingerprints"?

All the same, I think quantum mechanics is one of the places where we are forced to come to terms with the fact that the "fingerprints" of our consciousness are all over what we are doing there.

Again, I will make it simple for you: – Are there, or are there not "fingerprints" of your (un)consciousness in the Cosmic Microwave Background, that has been red-shifted for ~13 billion years?

[If Yes, how was life at time of last scattering? Any "scattering parties"?]

So there is no "piece of the apparatus" we need to call conscious

Wow! I’m stunned! You are sure about this?? The apparatus need not to be conscious!?
If this will cause you any trouble, just send me a PM and I will talk to Harry Potter!


*Bringing the Human Actors Back On Stage; The Personal Context of the Einstein-Bohr Debate, David Kaiser, British Journal for the History of Science 27 (1994): 136-137.

 

[DevilsAvocado]

There might not be such a "firewall" between what is scientific and what is philosophical. The point being, consciousnesses do both science and philosophy.

Don’t know about the "firewalls" in your world, but on PF there are rules, and if you want to discuss philosophy or fuzzy personal speculations, you are in the wrong forum.

It's a little off topic though to get into thermodynamics!

No worries mate! We’re already as off topic one could get, wonderland here we come!

I'm just pointing out that it is we who are making that choice-- not nature.

Right, your brain has been controlling the universe from the beginning.

as the thread is about the role of consciousness in physics

Gosh, there must be something wrong with my browser... I have been searching OP for "consciousness"... and I just can’t find it...

In our consciousness

Go man, go!

that is the prerogative of consciousness

Yes!

fundamentally about the role of consciousness in physics

Yeah! YAY!

 

[Ken G]

Okay, so how did you check this, with the help of "God"?

Um, now you are disputing that we are conscious? I don't think you are interested in informative exchange. We'll just have to leave it that you are not understanding anything I'm saying, and not bother to try and place the blame. You are welcome to continue to imagine that the universe is just the universe, and our conception of it has nothing to do with how we conceive. I wouldn't try to tax you, my comments are for those willing to get past that.

Gosh, there must be something wrong with my browser... I have been searching OP for "consciousness"... and I just can’t find it...

Try post #55, or get a new browser. It's a long thread-- perhaps it has not occurred to you the thread has taken some twists and turns since the OP five pages ago. But as I said, you are not interesting in informative exchange, so that's fine, I'm willing to discuss these issues with those who are.

 

[DevilsAvocado]

Um, now you are disputing that we are conscious?

Of course not, I’m in the mainstream camp, consciousness is what it is, and as far as I know no one has yet defined it mathematically or proved that it can create universes, etc.

You are the one disputing reality, that nothing except your conscious is real. And the question is really simple: How do you prove that you are conscious if you live in a "bubble of self-creation", i.e. nothing outside your conscious is real?

By your "bubble-version-of-reality": It could all be a dream. [And I am your worst nightmare]

And there’s no way for you to prove or disprove this statement, right?

If there is; please let us know.

I don't think you are interested in informative exchange.

Sure I am, very much so. The problem is that when you face a question you can’t answer, you start to play games, and I think you just have to accept that I "play a little" in return. Fair enough, huh?

So, I’m going to give you one more chance to answer a serious question in a serious way. You seem not to like the Cosmic Microwave Background, so let’s talk stars instead (like our sun).

Could you please explain to me (and the readers): How intelligent life and consciousness could arise *before* any stars ignited in the universe? According to you, no evolution of gas clouds and formation of stars can ever take place, without a consciousness "making them real"?

[If you are refuting biological evolution as well, you’re definitely in the wrong place.]

Try post #55, or get a new browser.

More games:

Same answer to all questions: The presence or absence of humans has absolutely nothing to do with the outcome. The meaning of observing in physics does not require any consciousness. Any irreversible interaction is sufficient.

And you take this as a justification to start a long harangue about consciousness and physics??

Ken G, some post of yours are very nice and very deep, and you know I have praised you for it, but then some post doesn’t follow any logic at all... it’s just n-u-t-s...

 

[Ken G]

Of course not, I’m in the mainstream camp, consciousness is what it is, and as far as I know no one has yet defined it mathematically or proved that it can create universes, etc.

Nor did I ever say it could. More straw men? Don't you ever get tired of misquoting me?

You are the one disputing reality, that nothing except your conscious is real.

More misquotes. Please don't refer to anything I say without quoting me, you never interpret it correctly.

And the question is really simple: How do you prove that you are conscious if you live in a "bubble of self-creation", i.e. nothing outside your conscious is real?

None of that has anything to do with anything I said. I'm am utterly uninterested in proving that I am conscious, this can be taken as an axiom or go have a discussion with someone who has some definition of the word where I am not it. What "conscious" means is like what a "point" means in geometry-- it must be held as axiomatic that we understand this, because defining it is fruitless for those who don't understand it already.

And there’s no way for you to prove or disprove this statement, right?

Correct, there is no way, and no need, to prove that life is not a dream. Science certainly does not require that we prove this, nor do I. Science does not care if you think life is a dream, or if you think there is absolute truth, or if you think our most naive notions of reality are absolutely true. Science doesn't need any of that baggage, nor do I-- all we do is make predictions, and form a sense of understanding, which together give us power over our environment and a sense of aesthetic order and beauty. That's it, that's what science does-- it is totally unimportant if you imagine it is a dream or if you imagine it isn't a dream, science just goes," huh? What difference does it make to me? I never had anything to do with your belief system."

The problem is that when you face a question you can’t answer, you start to play games, and I think you just have to accept that I "play a little" in return.

You are mistaken. When I face a question I can't answer, I look at why I can't answer it. You just pretend that you can, and when I point out the pretense, you start misquoting me. If you could really make a logical argument, you would not need to replace what I really say with caricatures about dream worlds that are not even wrong, they are simply irrelevant.

Could you please explain to me (and the readers): How intelligent life and consciousness could arise *before* any stars ignited in the universe? According to you, no evolution of gas clouds and formation of stars can ever take place, without a consciousness "making them real"?

More misquoting. I already answered this, but you didn't understand. Let me try again. First of all, I "like" the CMB just fine. But if you insist on repeating your same question for stars, I'll give you the same answer for stars. The word "star" is an invention of human intelligence. This is quite demonstrable, just pick up a dictionary or astronomy text, and look where it says "author". So, your question is, how could this word that we invented, to go with a concept that we formulated, not have existed before there were humans. Well, I'm sorry, but the only thing I can say is, how could either the word or the concept have arised before there where humans? Now, of course I know what you will say, you will say you are not talking about the word or the concept, you are talking about the actual thing. Um, just think about that for two seconds, please.

[If you are refuting biological evolution as well, you’re definitely in the wrong place.]

And the endless litany of misquotes go on and on. Maybe if I explain this one more time, you'll get it: there is nothing wrong with theory of evolution. It's fine, it's a wonderful theory, which means it is a wonderful way that human intelligence uses to organize and make sense of our sensory perceptions. That also means it is very good science. It also means it is a construct of human intelligence. That also means there is a role of consciousness in the theory of evolution. These are all just plain facts, I'm sorry that you cannot accept these facts, and feel the need to replace them with preposterous claims I never made in order to refute them. Maybe you just want to have a simplistic view of what science is, and don't like being asked to think in a more sophisticated way. I don't know why you feel the need to replace what I say with something else.

 

[DevilsAvocado]

You are the one disputing reality, that nothing except your conscious is real.

More misquotes. Please don't refer to anything I say without quoting me, you never interpret it correctly.

Feel free to blame me for anything, that’s totally okay. The real problem though, is the fluffy way you are expressing yourself, like the in following comment below, where one could only come to the conclusion that biological evolution "is a construct of human intelligence" solely, while you ignore raw data like bones and DNA. And if I would bring the that up, you would go on with something like "well, human [consciousness] got those bones out of the ground, no?", which is some sort of "kindergarten logic".

There’s only one "little" problem with these homemade personal cranky speculations of yours; you’re in a never ending recursive loop that leads to a catastrophic contradiction:

To discover [the theory of] evolution, one needs human intelligence and consciousness, and to get human intelligence and consciousness, one needs [biological] evolution.​

But you don’t have the capacity to separate facts from theory, and that’s why you end up in an intellectual Black hole, with no odds of ever getting out.

As you are clearly ignorant of terminology, process and the facts, here’s quote that should make everything clear (for any normal person):

Evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts do not go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered. — Stephen Jay Gould​

[my bolding]

And the endless litany of misquotes go on and on. Maybe if I explain this one more time, you'll get it: there is nothing wrong with theory of evolution. It's fine, it's a wonderful theory, which means it is a wonderful way that human intelligence uses to organize and make sense of our sensory perceptions. That also means it is very good science. It also means it is a construct of human intelligence. That also means there is a role of consciousness in the theory of evolution. These are all just plain facts, I'm sorry that you cannot accept these facts, and feel the need to replace them with preposterous claims I never made in order to refute them. Maybe you just want to have a simplistic view of what science is, and don't like being asked to think in a more sophisticated way. I don't know why you feel the need to replace what I say with something else.

These are all just plain facts... Well, that goes straight down the intellectual Black hole...

I think you have to come up with a quote from On the Origin of Species where Charles Darwin clearly state; "Without my intelligent consciousness our ape-like ancestors would never have climbed down from the trees".

Or apologize for the extensive disinformation you are posting on PF.

[Please note: you added the words "theory of", where I originally used the phrase "biological evolution", in case you want to run the "misquote stunt" again.]


[my bolding]

More misquoting. I already answered this, but you didn't understand. Let me try again. First of all, I "like" the CMB just fine. But if you insist on repeating your same question for stars, I'll give you the same answer for stars. The word "star" is an invention of human intelligence. This is quite demonstrable, just pick up a dictionary or astronomy text, and look where it says "author". So, your question is, how could this word that we invented, to go with a concept that we formulated, not have existed before there were humans. Well, I'm sorry, but the only thing I can say is, how could either the word or the concept have arised before there where humans? Now, of course I know what you will say, you will say you are not talking about the word or the concept, you are talking about the actual thing. Um, just think about that for two seconds, please.

Thank you very much Ken, I understand if you are sorry, but I can’t remember when I laughed this hard. I have one last simple little question for you, which will end this remarkable discussion:

What came first, humans or our sun*?

*I don’t care how you define "sun", anything goes; word/concept/actual thing. I just want to hear you spell this out. If you need three alternative answers, that’s okay too.

Gosh! This is so exciting! :!)