This is the name of an experiment first proposed in 1982, and is the one that I have had in mind when talking about God and quantum mechanics. I realized that most of the comments I have received seem to be taking me to be talking only about the standard double-slit experiment; this is of course my own fault since I haven’t done a very good job of indicating what I had in mind. So, let me describe these results and then reformulate the argument.
We have to build up to this, so let’s start with the quantum eraser experiment. Here is how Brian Greene describes the experiment in his recent book The Fabric of the Universe.
A simple version of the quantum eraser experiment makes use of the double-slit set up, modified in the following way. A tagging device is placed in front of each slit; it marks any passing photon so that when the photon is examined later, you can tell through which slit it passed…when this double-slit-tagging experiment is run, the photons do not build up an interference pattern.
As he goes on to point out, this is what we would expect. Since we measure the photon’s path, we get the photons acting like particles. But then it gets weirder. As Green continues, the quantum eraser asks,
What if just before the photon hits the detection screen, you eliminate the possibility of determining through which slit it passed by erasing the mark imprinted by the tagging device?
The answer, as it turns out, is that the interference pattern shows up again. Which, is , uh, weird. But again it gets weirder with the delayed-choice quantum eraser. Greene describes it thus,
It begins with [the set-up of the quantum eraser], modified by inserting two so-called down-converters, one on each pathway. Down-converters are devices that take one photon as input and produce two photons as output, each with half the energy (“down converted”) of the signal. One of the photons (called the signal photon) is directed along the path that the original would have followed toward the detector screen. The other photon produced by the down-converter (called the idler photon) is sent in a different direction altogether. On each run of the experiment we can determine which oath a signal photon takes to the screen by observing which down-converter spits out the idler photon partner. And once again, the ability to gleen which-path information about the signal photons– even though it is totally indirect, since we are not interacting with any signal photons at all– has the effect of preventing an interference pattern from forming.
OK, so far so good. This is just a fancier version of what we have already talked about, with the exception that we are now no longer causally interacting with the signal photon. Everything we know about the signal photon we learn by observing the idler photon. But even so, we get the photons acting like particles. But we aren’t done yet. Again Greene
Now for the weirder part. What if we manipulate the experiment so as to make it impossible to determine from which down-converter a given idler photon emerged? What if, that is, we erase the which-path information embodied by the idler photon? Well, something amazing happens: even though we’ve done nothing directly to the signal photons, by erasing which-path information carried by their idler partners we can recover an interference pattern from the signal photons[!!!!!!]
OK, so what this seems to show is that it is not anything that we do to the photon that determines which way it will behave. Rather what determines this is whether or not we are able to know which path the photon takes to the detector. Nothing changes here except our ability to know which path the photon took.
We can hammer home this point with one further modification of the experiment. Suppose that we set it up so that we could only get which-path information from some of the photons (and further that which ones we get this information about is random). Again Greene.
Does this erasure of some of the which-path information– even though we have done nothing directly to the signal photons– mean that the interference effects are recovered? Indeed it does– but only for those signal photons whose idler photons [had their which-path information erased]…If we hook up equipment so that the screen displays a red dot for the position of each photon whose idler photons [had their which-path information erased] and a green dot for all others, someone who was color-blind would see no interference pattern, but everyone else would see that the red dots we arranded with bright and dark bands– an interference pattern.
So, it is the knowledge of which-path information that determines which way the photons behave. Since God always has which-path information, whether he obtains it in such a way as to effect the physical world or not, He will never see the interference pattern. Or in other words, the wave like nature of reality will be hidden from Him.
Sheez! That took longer than I thought!!
Richard Brown 
So we can’t see interference patterns because God could show us the which-path information by revalation.
Anthropomorphizing God is not going to help you figure out why you are incorrect. God is possibility. The origin of ALL things. This information is ever existent and transcends time, space and matter. The which path information is information. IN FORMATION.
God is not an observer. God is the keeper OF the information AS all possibility.
Hi Richard,
I wonder if we can say that there is specific path that the photon takes if the whole system configuration is such that which-path-information isn’t obtained.
If there is no specific path than nothing of this creates problem for God. Because IF there is no specific path unless the system is such that we obtain which-path-information, there is no which-path to be known.
(Again, God can know that given that the configuration was different [the cases where we determine which path information], the photon would take that and not that path)
Hi again Chris,
Yeah that would be the idea. In fact I have toyed with the idea that our ability to see the interference pattern is evidence that there is no God….
Tanasije,
I am not sure I am followng your proposal. If God knows everything then He know which-path information. So what you suggest would have to impossible, should God exist…
Sir,
You’re misunderstanding the concept of God.
Knowledge isn’t the key to collapsing the wave function. If it was, then by simply deleting the data, we could see a wave pattern reererge. But, it doesn’t.
The cause of the collapsing wave comes from interaction with the Quantum system. If we interact with the system and negate the interaction, then there is a wave pattern. If we interact enough and refuse to restore the system to it’s original state, then we lose the wave pattern.
Since God isn’t part of this universe, any interaction with the Quantum system would have no effect whatsoever. God’s existence isn’t relevant.
Hi Mike234, thanks for the comment but I do not think I am misunderstanding anything. There is of course a lot of debate about quantum mechanics but there is some good evidence that interaction isn’t what collapses the wave function and that what does do so is *knowledge*. If you do not like the delayed choice quantum eraser that I talk about here check out the no-interaction paradigms. You might start with this recent video from Inspiring Philosophy: An Interaction-Free Quantum Experiment: https://www.youtube.com/watch?v=vOv8zYla1wY
Hi Richard,
I’m saying there are no paths in any of the cases the interference pattern shows up, so there is no which-path-information to know.
That is, when we have a system where we ‘tag’ the photons, we have system where there are specific paths photons take.
When we have system where we tag them, and then erase them, we have a system where there are no specific paths photons take, so there is no which-path-information to know.
To explain more… In classical thinking, we think that tagging happens before erasing the mark, so it must be that there is specific path between tagging and erasing the mark, and if there is God can know it which would produce the collapse. But, I’m not sure if in QM that is the right way to think about it, and not discuss the whole configuration instead, in which there are no specific paths at all.
Sorry, still not getting it. If there is no which-path info then God will not see the particle like nature of reality…either way there is an aspect of reality that is hidden from Him…
In those specific cases there is no specific path, but in other cases where there is specific path God knows it.
Even if there is no specific path, he can affect it in such way that there is specific path, or/and he can know given that it is affected in certain way there will be specific path.
So, he is aware of the both aspects of the reality, knows phenomena in the nature in whatever aspect they appear, knows how phenomena changes from one to another aspect, can change it etc…
I don’t see anything which is outside of God’s knowledge there.
What reason is there to think that there is ‘no specific path’ taken when we aren’t measuring the path? As far as I can see none. The issue here involves our ability to determine which path it takes…but presumably whether we determine that or not there is some specific path thatr is taken and if so God knows about it and the argument goes through…
presumably whether we determine that or not there is some specific path thatr is taken and if so God knows about it and the argument goes through
Isn’t the whole weirdness with QM that it lets you deny this? That is, it doesn’t take one path. Mathematically, the particle is spread out over multiple paths and there is no more to say about it (and if you want to say otherwise you’d have to say that special relativity is wrong because of Bell’s inequality). That’s the usual spin anyway.
Hi again Eric,
Yeah I think that that is the idea that Tanasije was trying to get at…I may have been misunderstanding it.
But even so I don’t think that this can stop the argument. The problem is, how is God NOT supposed to have access to which-path information? The experimental results above suggest that it is our being able to find out which path the photon took that determines how the photon behaves, and I take it that God is always able to access that information. Hence the problem.
@ Richard Brown: If you’re a Christian like me and believe that Jesus is the Word (information) of God through whom everything was made and if you believe John Wheeler’s ideas that everything in the universe is, at its most basic level, just digital information, then it becomes apparent that our physical experience is in a sense virtual reality and that we access the multi-layered informational universe on a need-to-know basis, simply by having this information automatically communicated to us when we switch to a different level or acquire a special ability in the video-game, so to speak. The universe is much more complex because the information contained in it is much more complex. We only access parts of that information, according to our pre-programmed decoding system (senses and intellect).
The interference pattern is just an expression of that informational universe interacting with our particular decoding system but not confined by it. The physical experience of an interference pattern is therefore contained in the bigger informational package, hence quite accessible to the originator of that information, who is God.
Think of it as a video game. The programmer has access to all the functions and special extras but a particular video game character’s experience is limited to its programming.
To summarize, God can see both the clump pattern and the interference pattern of a particle at the same time, and that’s because they are not really different things, but only different expressions of the same thing, and the difference is caused simply by programming restrictions set on the human subject, of which God is fully aware (therefore acquainted to the way humans experience it) but also completely free (therefore able to experience all possible ways in which something can manifest itself, simultaneously – because information is timeless and space-less).
Tom Campbell talks about this in length (not the God part, the part about reality being a simulation). I recommend this lecture series… https://www.youtube.com/watch?v=0AihD2__gKE
I am way out of my specialty here, so I have no idea. These quantum results always blow my mind. Maybe God is there fucking with us, using these results to keep us humble, and knows what’s going on the whole time. 🙂
Yeah, that was what I was trying to get at.
But I feel same as Eric about discussing those more complex situations. It is my intuition (in the ‘I’m guessing’ sense of intuition, ‘I understand’ variety 🙂 ), that in those situations where “we are not able to figure-out which-path information”, in fact there is no which-path information. The whole thing is waves (which actually eventually shows up on the screen through interference pattern).
But, as I said, I feel like Eric here, having just layman knowledge of QM, I just want to shut up before saying something stupid 🙂
What’s wrong with saying something stupid? I do it all the time!! 🙂
Say what you want about those situations, it will never be the case that God couldn’t figure it out though; hence the problem…
Yeah, like Erick I think God knows how that works.
That made remember of a story I heard long ago, about St. Augustine walking on the beach, trying to understand the holy Trinity. He stumbles upon a boy you is running back and forth to the ocean bringing water in a shell and dumping it in a hole in the sand. St. A. asked what he was doing, and the boy said he was transfering all the ocean to the hole. St.A said that’s impossible. And the boy said he could transfer the ocean to the hole sooner than him understanding the mystery of God, and then disappeared. [Twilight zone music here]
Funny story 🙂
But I am not sure what you take yourself to be agreeing with Eric about. If God knows this stuff, then, according to the argument, there is still something that he doesn’t know.
Tanasije Gjorgoski got it right.
The problem with saying the experiment disproves God (or, in particular, God’s omniscience) is that it assumes that the path exists. Clearly the path does not exist, because if it did exist then your disproof of God would also be a disproof of reality itself. That is, if you substitute the concept of “exists in reality” for “exists as a matter of God’s knowledge”, then your proof works against reality just as easily as it works against God. It isn’t just that God cannot know the path, reality itself cannot “know” it.
But this isn’t the way things work in the theory, in the theory there is no actual path of the photon when it isn’t measured. It is less confusing if you look at it from a many-worlds interpretation. In this interpretation there are three possibilities (a) photon is measured as having taken path A, (b) photon is measured as having taken path B, or (C) photon is not measured. We always view the world from one of these perspectives and the quantum phenomenon is a matter of selection. Within any of these worlds, the results are always consistent, even with a quantum eraser (which is simply a matter of delaying the revelation about which of those three worlds we are in).
The Copenhagen interpretation of waveform collapse is equivalent, if you equate the process of collapse with the process of selection in the many worlds interpretation, but the “delayed choice” aspect becomes more confusing.
In any of the three cases, however, there is no problem with God’s knowing what is going on.
The real problem is, in fact, with the “exists in reality” interpretation. The experiment is actually strong evidence that objective reality does not exist, and in favor of a model of reality that is based upon mind rather than on matter. In other words, the experiment shows that results are determined by what is known and knowable (i.e. by information) and contradict a model which is based on matter that exists independent of knowledge. In the “matter” view, it is hard to explain how the reality of no path versus path A versus path B can depend, after the fact, upon information. The experiment is yet another demonstration (alongside General Relativity) of Berkeleyan immaterialism, which is an idealistic form of panentheism.
Under the hypothesis that God is a unity without parts, and that reality exists as a manifestation within the awareness of the divine mind, then any moment of reality seemingly would have to logically cohere according to a single principle. In other words, monotheism may imply that there is a single mathematical law (a theory of everything) whose application creates the entirety of reality. In any case, it is not surprising that we can work out physical laws in a model based on an underlying unity.
But what is harder to explain is how the interacting parts of reality can interact without an underlying implementing unity. It is like assuming that two parts of a dream can interact without an underlying dreamer. Or that Mario and Luigi can interact without an underlying game system. This kind of thing never happens in real life, and the assumption that it happens in the one system we call “reality” which we cannot see “from the outside” seems to be special pleading for an otherwise irreproducible result.
But if you turn your mind-matter duality upside down, and assume that the foundation of reality is a unitary mind, then experiments like this one make sense. Matter becomes nothing more than a name for the fact that we can form mathematical models of the reality we observe, but it doesn’t really exist as a substance. Under this immaterialist view, the path actually does not exist when it isn’t being observed. In other words, this kind of experiment is manifestly consistent with an information-based model of reality, but not with any known matter-based model.
As Berkeley said, “esse est percipi”.
Hi Shack Toms, thanks for the comment!
It isn’t just that God cannot know the path, reality itself cannot “know” it.
This is a bit confusing…reality can’t know anything…
But this isn’t the way things work in the theory, in the theory there is no actual path of the photon when it isn’t measured.
Right, and the problem I am pointing out is that God is always, in effect, measuring it.
In other words, the experiment shows that results are determined by what is known and knowable
Again right, and the problem is that God always knows.
As for the Berkeley stuff, I am sympathetic to this (see here, and here) but it doesn’t help the case you are making since if God exists he is always observing and so the path exists as well…
Hi Shack Toms, thanks for the comment!
I was pleased to find your post. I have been very interested in what the quantum eraser experiment says about reality and about God (which explains why Google led me here).
This is a bit confusing…reality can’t know anything…
That’s why I put it in quotes. However, if you adopt a completely materialist model, then reality can know things in precisely the same sense that we can know them (which is yet another reason for rejecting materialism).
But OK, I concede that the correct contrast is between “exists in reality” versus “exists in God’s knowledge”.
Your proof that “God’s knowledge” didn’t contain the path also shows that “reality” didn’t contain the path. If reality didn’t contain the path, then its exclusion from God’s knowledge doesn’t disprove God’s omniscience.
Right, and the problem I am pointing out is that God is always, in effect, measuring it.
Why do you think this? Do you think that God is measuring other things that don’t exist?
You seem to be asserting that God would have to manifest a path, even though God were not manifesting any of the physical apparatus that performed the measurement. In other words, that God’s knowledge of reality would have the same function, within the quantum-mechanical model, as a measurement.
But if God’s knowledge were less limited than ours, so that God’s awareness could encompass many worlds (whereas we are limited to one) then (equivalently) God’s knowledge would not collapse the wave function.
Furthermore, if God has foreknowledge of the results of the experiment, then there is not problem with God knowing the path (or knowing that the results were inconsistent with a path) before the measurement is made.
Finally, if God can alter the past (much as an author updating an earlier chapter in a book), then there is no problem.
What the experiment actually disproves is the assumption that, with respect to any given point in time, the past is deterministic but the future is not.
It is only within a materialistic model of objective reality, in which the universe evolves in time in such a way that the past is fixed and the present is not completely determined, that there is a problem.
In the immaterialist view, what exists are the objects of awareness within the minds of the experimenters, and these are consistent across experimenters because nothing appears in the mind of any experimenter without also appearing in the mind of God. But, of the two approaches, only in materialism is anything else required to exist. In immaterialism, the whole notion of photons is just an interpretation of a mathematical model that describes the logical coherence of different aspects of our experience.
Again right, and the problem is that God always knows.
The idea that effects can be “non-local” is a problem for physicists. It is really just a suitably non-theistic sounding term for “decided by God”. A materialistic model requires that physical effects arise from local causes only. No action at a distance. Maybe an exception to that would be a materialistic many-worlds model in which each possible world had material existence.
I concede that most Physicists are materialists, but it is a matter of hope and belief only. There is no known “reasonable” materialistic model of reality that is consistent with quantum mechanics, and Bell-Aspect shows that any materialistic model would have to be very strange. I don’t think anybody knows how to make a materialistic model that is strange enough. It seems impossible.
You also need to consider why it is that knowledge collapses the wave function. The reason it does is that the physical effects of that knowledge have to be consistent in the evolution of the universe. So the wave function only collapses in the sense that there is, within the materialistic model, a materialistic change. But there is no materialistic “brain of God” that we can observe, and therefore the knowledge does not collapse the wave function.
Consider Schrödinger’s cat. Suppose you and I are next to the unopened box. The box contains a superposition of states–the wave function describes it. Suppose that I look in the box, but you do not. Now, from your perspective, I am a superposition of states.
In each of those states there is a “me” that thinks he has collapsed the wave function. But the wave function hasn’t collapsed for you. Now you ask me what I saw. You will now, in the Copenhagen model, collapse the wave function. But there might be someone else, for whom you and I would both exist in a superposition of states–but there would still be just the two states in the superposition. Within each of those states we would find a progression of people thinking that they had collapsed the wave function, but what we call “wave function collapse” is really just a requirement that the physical effects within each state are consistent with each other. The measurement imposes a physical constraint on that consistency.
Since God’s knowledge is not a result of a physical effect (rather the situation is the reverse) it doesn’t show up within any of the states. In that sense, the physical world exists in a superposition of states, each of which has the consistency requirements imposed by the physical measurements, but it may actually be God who collapses the wave function. Note that wave function collapse is not a physical process (e.g. it is non-local). There is no objective way to decide if the wave function has collapsed (as evidenced by the Schrödinger’s cat situation with multiple, sequential observers). Wave function collapse is subjective.
Put yourself in God’s position. A lot of people think that God created laws of nature in order to hide from us, in order that belief in God would be a matter of unsupportable faith. But maybe the opposite is true. Maybe laws of nature are a necessary consequence of some attribute of God but God is going to great lengths to reveal himself despite that.
Suppose that you are a mind that exists as a unity without parts, and thus you can only manifest, at any one time, an idea that is entirely self-consistent according to a single principle. So everything within your mind at any time has to obey consistent laws, in fact a single consistent law.
In this case, you might be able to create a moment of reality, but everything within that moment would appear to hang together as the result of a single principle, so it would perhaps appear to be the result of a universe that evolved from an origin in a single, symmetric point source. There would be the appearance, in that moment, of history, and also of a future, but all consistent with what appeared to be “physical laws”.
Given this restriction, how would you make yourself manifest? One way you might do it would be to manifest objects such as avatars, prophets, and scriptures within the reality that pointed to you and which raised the possibility. But these objects would necessarily always exist within a world that seemed to be (and which was, within any given moment) completely bound by physical law. And it would hard to manifest these without also manifesting false avatars, false prophets, and false scriptures (if all of reality had to follow a consistent physical principle).
Another way you might do it is to base reality on a principle such as quantum mechanics that was logically incompatible with a matter-based explanation. You might base it on principles such as Relativity which were logically incompatible with absolute time and space. If the principle included that the future was not completely determined by the past, yet the past could be determined retroactively by a quantum eraser, then this would show that even a materialist interpretation of the objective concept of time was an illusion.
In other words, far from this experiment showing that God does not exist, it very well could be revelatory.
“There is a dream dreaming us, a vast dream, dreamed by a single being in such a way that all the dream characters dream, too.” — said by Little Frieda in Meatball Fulton’s “The Incredible Adventures of Jack Flanders”
That’s why I put it in quotes. However, if you adopt a completely materialist model, then reality can know things in precisely the same sense that we can know them (which is yet another reason for rejecting materialism).
No. That’s not what maerialism says at all…that is unless reality has sensory systems…
The rest is a bit long. I may try to respond to it at some point, but the major points I have already addressed in previous comments. The data shows that it is simply whether the path information is knowable that matters, and if that’s the case the argument goes through…
That’s not what maerialism says at all…that is unless reality has sensory systems…
Sensory systems are just thermodynamic machines.
The data shows that it is simply whether the path information is knowable that matters, and if that’s the case the argument goes through…
No, that’s not quite right. It has to be objectively knowable. That is, there has to be some experiment whose outcome can reveal the knowledge. Subjective knowledge doesn’t have an impact.
For example, consider the question of simultaneity in relativity. The ladder paradox shows that simultaneity is not an objective concept. And yet people have a subjective feeling that a particular time is “now”. If that feeling about a particular time were objective, then it would be possible to have a strict ordering between events. The fact that we subjectively consider a particular time to be “now” doesn’t violate relativity because our knowledge of “now” has no objective implications.
To remove the tag just before one wave packet hits the screen (for one photon only) we would require a switch of huge speed. There is NO EVIDENCE of this in the experiments. This means that wave packets arriving at the screen have already travelled the distance from slits to screen. Anything being done is done ‘along the way’ and invalidates the conclusion.
If wave packets have arrived at the screen – without being observed at all – then they give the normal results to be expected.
I think the experiment is obfuscated to try to prove nonsense. QM is not that bad!
Hi Shack, sorry for the delay…
Sensory systems are just thermodynamic machines.
So? The universe ain’t got ’em…
It has to be objectively knowable.
Right, and to God everything is objectively knowable.
Hi Debbie, thanor the comment…hough I am not really sure what it is that you are trying to say. What has been obfusccated?
So? The universe ain’t got ‘em…What is it about a sensory system, as a material object, that allows it to have subjective knowledge without the usual objective implications? You seem to be saying that a sensory system has an ineffable configuration.
Right, and to God everything is objectively knowable.Why do you think that? It seems like a surprising position. Not even everything that I know is objectively knowable, so why should God have the limitation of having no private knowledge?
However, I still think you are making a mistake in insisting that there is a single photon path, simply because of God’s knowledge of the physical situation, even when there is no physical apparatus to measure it. That is a misinterpretation of the physical model.
The way I interpret your argument is that the experiment shows there cannot be local knowledge of the path, but if God has all knowledge then God must have local knowledge of the path, thus God cannot be omniscient. The problems I find with your argument, which I don’t think you have addressed, are:
1) a sufficiently subtle measurement does not necessarily collapse the quantum ensemble,
2) you are confusing knowledge and measurement,
3) God’s knowledge might be global,
4) the reality you assert God is measuring might not actually exist.
Consider, for example, quantum teleportation of a photon. In that kind of experiment the quantum state of the source photon is transferred to the target photon, and is thus measured, but that measurement doesn’t collapse the wave function because it isn’t measured in a way that reduces the coupling between alternatives in the quantum ensemble. That experiment refutes your notion that measurement necessarily must be done in such a way as to collapse the wave function.
You are correct that what is important is information, but the meaning of the word “information” in this involves physical effects. A measurement that doesn’t have a physical effect on the couplings between states in the quantum ensemble doesn’t enter the math in the same way as a measurement that has a physical effect.
There are three possibilities in the delayed choice quantum eraser: 1) Photon takes path A, 2) Photon takes path B, 3) Photon path exists as a superposition of path A and path B. In this third possibility it is simply unfaithful to the physical model to say that there is a path to measure. In the original article by Kim, Scullly, et al, they refer to information about (1) and (2) as “which-path” information, and to information about case (3) as “both-path” information. The “both-path” case is a physical phenomenon, on a par with the “which-path” cases.
The quantum eraser involves a particular measurement that can destroy the both-path case, but you incorrectly infer from this that it is impossible to have complete knowledge of the physical situation without destroying the both-path case.
The way you write the equations down is as a superposition of states. The importance of the physical apparatus that measures “which-path” is that it can affect the coupling between different states in the superposition. When that device exists, the physical situation changes so that the both-path situation doesn’t arise. But other kinds of measurements could be done which wouldn’t have that effect, for example, the photons could be teleported. That kind of measurement doesn’t decouple the quantum ensemble.
The change that leads to a different result is not simply a change in knowledge, it is a change in the physical apparatus. To see this, suppose that the physical apparatus is in a state where the which-path information has been measured, but you choose not to look at it. The interference pattern will still be missing, despite that you lack knowledge of the which-path information and that the which-path information has been presented to nothing we would consider a “sensory system”. So “sensory systems” are not what creates the effect.
Consider a process that produces pairs of photons with a total spin of zero. You can use the statistical arguments of Bell-Aspect to show that, until the spin of one of these photons is measured, that neither of the photons has a definite value for its spin, and that a measurement of the spin of either photon immediately creates a definite value for the spin of the other photon–quickly enough that any physical mechanism that might be involved would violate the principle of causality. And yet one could do a quantum teleportation experiment on one of these photons without destroying the indeterminacy of its spin or its correlation with the other photon. So it is possible to have knowledge of the situation, without destroying the situation.
The arguments against “hidden variables” (and the path information, if known to God, would qualify) only apply to local models of hidden variables. There is no proof against divine knowledge of the entire quantum state. Things like EPR, Bell-Aspect, and the delayed choice quantum eraser show that such knowledge cannot be local, if it exists it has to be global. There are (very strange) hidden variable models that work, but they require non-local effects. There is no workable, materialistic model of reality that involves reality having a well-defined state when it is not being measured. So these experiments are much more problematic for material interpretations of reality than for ideal interpretations.
“Can an event which has just taken place be determined by another event that hasn’t taken place yet?” This is the central question arising from experiments involving delayed choice. If the future event is the result of a choice made by a conscious being then clearly he can have no knowledge of what has already happened or he could change his mind creating a contradiction. To illustrate this idea consider the notion that God knows everything we are going to choose. This can only happen if we don’t know what he knows otherwise we could change our mind, again resulting in a contradiction. Relativity Theory contains a similar paradox. Suppose an alien is heading toward Earth from vary far away at near the speed of light. Then for him I’m already doing things I haven’t chose to do yet. This is the principle of simultaneity. If he could instantaneously know whats happening on earth and instantaneously send me a message telling me what I’m doing I could just not do it creating a contradiction again, but since information can not travel faster then the speed of light this is not possible. The same is true of delayed choice. I’m not forcing the future choice maker because I know what he is going to do before he does it. I’m just able to be like god for a moment, if you will. Its still his decision. I don’t think this concept is violating any laws I am waiting for your opinion.
“Can an event which has just taken place be determined by another event that hasn’t taken place yet?”
That’s a good question, Walt Bates, and I agree that it is central in this class of experiments. The current laws of physics (as most widely accepted) say that the answer is no,
That answer is known as the principle of causality, it states that causes must precede effects in every reference frame. Note that events don’t necessarily occur in the same order in every reference frame, but causation between events can only happen for events that are spaced such that the time order is unambiguous. So if A causes B then A must precede B in every reference frame.
But the quantum state can be revealed (i.e. wave function collapse) in a way that involves faster-than-light correlation (or even delayed choice correlation), but this (according to current theory) cannot be used to send other information faster than light or back in time.
This implies that wave function collapse (in the Copenhagen interpretation) cannot involve a physical mechanism (otherwise it would violate causality). This collapse is sometimes known as “spooky action at a distance”. But spooky action at a distance does not carry information of a kind that can involve causation.
To really see that causation isn’t what is going on, note that in the Yoon-Ho Kim, et al, delayed choice quantum eraser experiment the “interference pattern” and “no interference pattern” measurements are done at the same time within the same apparatus. So the interference pattern is both there and not there.
That is, there is a photon detector (call it D0) that can move in one direction, and the events from that detector (which records photons from either path) are correlated to events at detectors that detect whether a photon came from path A (the experimenters called it detector D3) or from path A and/or B (the experimenters called one such detector D1). So an event at detector D3 means there was a photon on path A, and an event at detector D1 means that there was a photon on path A or path B, but you don’t know which. Then you move detector D0 and look (simultaneously) for correlations with D1 and D3, counting how many correlations you get within a given time at each position of D0. When you draw the graphs, the correlations with D1 show the interference pattern over the range of motion of D0, but the correlations with D3 do not.
This reveals that the disappearance of the interference pattern isn’t exactly an exclusive alternative but a matter of interpretation of the data from the perspective of an additional condition.
There is no question that the statistical properties of entangled pairs of particles have a non-local correlation. But (so far) there is no way to use this to send external information back in time, so that a later event causes an earlier one. Instead what is seen that it is provable that the quantum state is locally indefinite and also provable that the quantum state of entangled pairs is correlated.
So far, the way out of this mess is a rejection (as positively falsified) of the notion of “local realism”. There isn’t a local, material model that really works. Note that most physicists are materialists, so they hold out hope that a workable material model will eventually be discovered. But if one is found, it provably won’t be anything that corresponds to intuitive ideas of matter. Or as Einstein once said (I am paraphrasing), “Everyone knows what a photon is, but is mistaken”.
Note that the principle of causality is axiomatic, it doesn’t arise from any other known principles, the other fundamental laws of physics are symmetric with respect to time (strictly, with respect to simultaneous reversal of time, charge, and parity). There is a lot that rides on the principle of causality, even though there is no model for how the principle operates or comes to exist. So if there were an experiment that violated the principle of causality it would be a big deal. Nevertheless, it is a topic of active research and there is some evidence (e.g. the Afshar experiment) that causation works both ways and that the way things are is a kind of standing wave that has to work in both directions.
A lot of people have the intuition that the universe unfolds much like a strata-cut animation, in which there is a hyperplane of events that occur at the same time and some mechanism that moves this hyperplane forward in time, creating the unfolding of events one after another. But the existence of objective consequences of such a hyperplane would violate the laws of physics, which require (in some cases, such as the ladder paradox) that the time order of events is ambiguous. So the physical model doesn’t actually include such hyperplanes as physical objects. In physics, there is no such thing as “now”, apart from the event of making such an utterance at some point in space-time. The events at each point in space-time exist without reference to a universal “now” and thus it is hard to understand why the arrow of causation works in one direction only.
I do know that when deriving the laws of motion (these terms are known as “propagators”) in quantum field theory you naturally get terms for particles that go forward in time and terms for particles that go backward in time. The terms that go backward in time don’t yield agreement with experiment, and are simply dropped as inconsistent with the principle of causality.
…………….The change that leads to a different result is not simply a change in knowledge, it is a change in the physical apparatus. To see this, suppose that the physical apparatus is in a state where the which-path information has been measured, but you choose not to look at it. The interference pattern will still be missing, despite that you lack knowledge of the which-path information and that the which-path information has been presented to nothing we would consider a “sensory system”. So “sensory systems” are not what creates the effect………
So if the following experiments took place would you see which path or wave information where appropriate?
Suppose several double slit experiments were performed where the detector at the slit was able to switch on or off randomly and without the knowledge of the experimenter at the commencement of each experiment, and furthermore that the build-up on the screen was not witnessed by the experimenter either, say by using an unseen photographic plate.
Assume this experiment was performed several times and that the detector recorded sequentially whether it was deployed at each run of the experiment, and also assume that the photographic plates were similarly numbered, and taken into a dark room to be developed.
At this point, no conscious observer would have witnessed the experiments, except to know that a certain number of double slit experiments had taken place.
Now having developed the plates what would the experimenter see on the plates?
1. First looking at the numbered photographic plates, without peeking to see whether the slit detector had or had not been deployed.
2. Subsequently checking to see whether the slit detector had been active or not for each run of the experiment. ( This being the first time that a conscious being had become aware as to whether the slit detector had been active or not for each run. )
When I have read about the double slit experiment I believe it is performed where the experimenters are aware as to whether the detector is switched on or not and so it may seem that this awareness causes the wave result when the detector is switched off, and the two paths result when the detector is switched on. ( As the experimenters look to see which path the electron or photon takes, it is suggested that the probability wave collapses as I understand it )
But if a random ( subsequently check-able ) on or off detector were to be used and the screen result not viewed until photographically developed and then later still, on/off detection confirmed for each plate, consciousness surely cannot account for QM probability wave collapse as the result on screen would be a fait accompli.
And similarly if entangement experiments could be performed in like manner and also where the apparatus cannot itself have disturbed the photons pass through the apparatus would the results still be expected to show which path and/or wave result as appropriate ? What would an experiment performed this way tell us about, either probability wave collapse, or multiple worlds theory?
Thanks, Thomas
Thomas, I think the experiment can be done.
In your last paragraph you mention that the apparatus disturbs the photons. This is not the source of uncertainty in measurement. You can tell that it isn’t the source of the uncertainty by noting that there is no uncertainty at all in the exact conservation laws (such as conservation of energy, conservation of momentum, conservation of angular momentum, etc). So we may make a number of measurements, each of which entails quantum uncertainty, but the total of those measurements might have zero uncertainty because of the constraint of an exact conservation law.
For example, if a process in which a decay with no change in spin (intrinsic angular momentum) produces two photons, then there may be uncertainty in the measurement of the spins of the individual photons, but the sum of those two photon spins will always be exactly zero. If it were a matter of the measurement imparting the uncertainty, then the sum of the measurements would also be uncertain.
The measurement clearly makes a difference, but it isn’t a difference in the sense of imparting an uncertain amount of energy or momentum from the apparatus to a state that was well-determined before the measurement took place.
With respect to your slit detector, I think it is a mistake to treat the awareness of a conscious being as a special event in the way the physical laws unfold. Physicists don’t consider that in their calculations, they consider the apparatus.
Consider Schroedinger’s cat. Suppose that the cat in the box has a 50% chance of being alive, and I look in the box and write down what I see. Then you look in the box and read my note.
Classically, it is very simple. The cat is either alive or dead. I look into the box and write down what I see, and when you look into the box you see the same thing.
In quantum mechanics, what happens is that there are two possible states, the live cat state and the dead cat state. Within either of these states all of these measurements will be consistent with each other. It isn’t that the wave function collapses, but that there is a conditional probability associated with correlations between the measurements within any given state.
In the double-slit experiment, from the perspective of any particular state, seeing the interference pattern is related to the conditional probability that the photon took no particular path. (Note that it isn’t that the path is unknown.)
Complete information, therefore is a matter of knowing this state of affairs, not of knowing which path the photon took in a situation where that is a physically meaningless concept.
The way you have presented questions 1 and 2, it wouldn’t matter which one you looked at first, you would either see the interference pattern or not and this would be correlated with non-deployment of the slit detector. Consciousness really doesn’t come into it, from the standpoint of Physics. You might make any machine that would evaluate the plates and the deployment of the slit detector, and it would find the same correlation.
The quantum eraser experiment was wonderful because it allowed a single run of the experiment to produce both the interference pattern result and the lack of interference pattern result from the same inteferometer looking at the same photons, with the difference being solely whether the measurement was correlated with which-path information. So clearly it isn’t consciousness that distinguishes the two cases, but correlation.
Thank you very much for your detailed and kind reply, the contents of which, as I am a non- scientific layman, will no doubt take me some considerable time to ponder over.
I just wanted for now to make a quick reply to say that I had read your post with interest.
I’m sorry but what makes you think God is limited by laws that are found within the universe? It seems to me that you’re assuming something about God, i.e. a limitation, that is just not there.
Hi Iain, I am only assuming that God know everything and then I argue that this is inconsistent with QM as we know it…that is not a limitation on God so I am confused by your comment…
[…] It would not bring out the collapse of the wave-function and so His knowledge is not mysterious. I disagreed with this objection, but then I started thinking that even if I grant the objection there is still a problem here. So […]
I think all inference, observance, and even thought, must adhere to a notion of time. When someone stops asking you what time it is, then you’ll know time no longer has meaning…
All of the events in what I’ve read here, are based on the precept of time (existing in the mind of God). However, time is a concept not known to God, because of the definition of eternity (~).
If we all agree the infinite is plausible, then the arguments set forth, render time unto itself meaningless, and therefore any argument subject to time breaks down for all human perception except for God.
Why waste time (on earth) wondering how God thinks, when man does not think that way?
Chris
Considering the QM notion, of the live cat, dead cat, if we assume the 50/50 chance of one condition or the other, who’s perception is reliable, if only two people are left on the earth? Who wants to be wrong then?
Or if you accept the notion of infinity, then the process of consideration (with no bound in time), is impossible.
What is the relevance in either case?
Hello Richard Brown,
I recently did the double slit experiment with erasure. The entire time I knew that the photon was traveling through both slits. The description that it travels through both slits contains all the information there is. If you guessed it went through the left slit, you would be wrong every time because it always goes through both.
Now I knew this information the entire time and got the expected results. Now if I can know this without wrecking the results I expect God could manage that to.
Joseph
A related experiment, to help show that full knowledge of the state of affairs does not entail knowledge of the path is the Elitzur-Vaidman bomb detector.
In that experiment, there are a number of bombs with detonators that are each designed to be set off by a single photon. But some of the bombs are believed to be duds. So the question is how to reliably distinguish the duds, without exploding all the non-duds.
The photon beam is split so that any photon may take an A path or a B path. A photon that takes the B path explodes a bomb (but only if the detonator is not a dud). So if the bomb is not a dud then we can tell whether the photon took the A path or B path, and thus there is never any interference. If the bomb is a dud, then we can’t tell with path the photon took and the two paths interfere. So there are three possible outcomes…
Explosion (photon took B path, not a dud)
Interference (bomb is a dud)
No interference (bomb is not a dud, photon took path A)
This reliably tests whether the detonator is a dud, but only exploding 1/2 the non-dud bombs. But using more complex arrangements, that ratio (of exploded non-dud bombs) can be made arbitrarily small.
But the neat thing is that the photon contains information about the bomb on path B *even when the photon is known to have taken path A and not path B*. And no human knows whether the bomb is a dud at the beginning of the run, so it isn’t a matter of human sensory systems affecting the outcome.
Another neat thing is that the experiment has actually been done, with the predicted results (though not with actual explosive bombs).
This demonstrates the problem in thinking that the state of the system is independent of the experimental apparatus, in other words that the photon has a well-defined state but some experiments withhold knowledge about that state and other systems do not. But there is no withholding of knowledge. In the systems that don’t define the path of the photon it simply doesn’t have a well-defined path–and perfect knowledge (e.g. God’s presumed knowledge) would include the knowledge of that fact.
The original argument seems to be of the form: “Suppose we invent some facts about reality, with no physical evidence to support these facts, and then demand that God knows these facts. We then use God’s knowledge of these facts to assert that these facts are physical, and thus that they will have measurable effects. Since we don’t observe these effects, we conclude that God does not exist.”
But I don’t understand why we should accept the premise that the facts in question existed in the first place, particularly when the existence of these facts (and not merely their knowledge) would violate the laws of physics.
Well, I messed up on the description of the bomb detector, but you can read a good description on wikipedia. The conclusion, that the detector figures out if the bomb is live even when the photon is known to have taken the other path, holds up.
Considering we all have free will, how can God possibly know everything?
The reality is that a measurement of the signal photon on detector D0 alters the probabilities for which detectors (D1 through D4) where the idler photon will be detected.
This is in fact not a delayed quantum eraser experiment. The proposers of the experiment have confused causality and reversed it in their minds. Quantum decoherence is a much better way of looking at such experiments. With quantum decoherence, you treat all particle paths as waves. Where two waves meet, you have to break the system into two decohered realities:
Reality 1: The two waves are collapsed, particles are represented at the interaction sites and new waves propagate from their particle sites.
Reality 2: The waves traverse each other at site of overlap. The probability of the overlapped portion of the wavefront must be calculated with a sine wave conjugation and can potentially go to zero.
Because of possible interference patterns, the calculation is difficult and certainly not possible to imagine in one’s head.
This is why it is necessary to have a suitable mathematical model to compute what is happening. Something as simple as the delayed choice quantum eraser has already been confused. Only computers can figure this out.
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