Q: If God were all-seeing and all-knowing, the double-slit experiment wouldn’t work, would it? Wouldn’t God’s observation of the location of the photon collapse its probability wave function?

Physicist: Deep…

It depends on how entangled God is with the rest of us. If He can make the observation, and then keep a completely straight face forever (never leak any information about the result in any way), then His mighty observation would not collapse the wave, from our perspective.

That’s why sparrows can be in so many places! (Matthew 10:29)

When you hear about “observations”, in the context of quantum mechanics, it usually takes the form of some smart-looking, glasses-wearing, dude saying things like “a particle is in a ‘super-position’ of states until someone observes it, and then suddenly it’s in only one state”.  The misconception here is that the universe as a whole (which includes people and Gods and whatnot) is in one state, and only tiny things can be in many states.

The traditional thought experiment for talking about this is “Schrödinger’s Cat”.  Schrödinger puts his cat into a purfectly information-proof box, so that there’s no way to see what’s going on inside.  Also, he rigs up something deadly that has some chance of killing the cat in the next day or so.  Before the box is opened, the cat is both alive and dead, but afterward the cat is either alive or dead.  The cat undergoes “wave function collapse”.

This is a metaphor for, for example, the photons in the double slit experiment.  Each can demonstrably go through both slits, but if you determine which slit a photon goes through, then it only goes through one.

Unfortunately, this has been taken as a rallying cry for a lot of bad physics.  A better thought experiment (and one which gets closer to explaining the answer to the question) is “Schrödinger’s Graduate Student”.

Repeat the cat in a box experiment, but this time Schrödinger finds that, overcome by quantum grief, he cannot bring himself to open the box.  Instead, he goes out for tea or something and has a graduate student complete the experiment for him.  Once done, the student then finds Schrödinger and reports the results.

But, since Prof. Schrödinger hasn’t observed the graduate student yet, from his perspective the cat and student are still in a super-position of states.  The only difference is that now they’re entangled.  They’re in a combination of the “dead-cat/horrified-student” and “living-cat/cautiously-optimistic-student” states.  When the student finally reports the results to Schrödinger, then the cat/student wave function collapses (from his perspective).

In practice of course, there’s information leaking all over the place.  Considering all of the different paths that information can take: chemicals, air movement, stray photons, gravity, etc. it’s really hard to construct “information-proof-anythings”.  If someone on the far side of the planet sneezes you’ll be entangled with that event within moments.  That’s not saying as much as it sounds like it is.  Essentially, you don’t have to worry much about “big and nearby” events (like sneezing on Earth) being in multiple states.

So, say there’s a God, and say that he/she/it/they/we observes every quantum event.  As far as each of us are concerned, unless God somehow conveys the information (burning bush, cheeky glance) he’ll/she’ll/it’ll/they’ll/we’ll be in the same position as the graduate student was before he delivered the results to Schrödinger.

So, in order to keep a quantum event many-stated and quantumy (from our perspective), all that an all-seeing God needs to do is play it cool, and not reveal any clues, whatsoever, about that event.

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17 Responses to Q: If God were all-seeing and all-knowing, the double-slit experiment wouldn’t work, would it? Wouldn’t God’s observation of the location of the photon collapse its probability wave function?

  1. Russ Abbott says:

    I think it was in an article by Paul Davies that said that one of the confusions about quantum mechanics is that it mixes information with physical stuff. That seems similar to what you are saying–although I’m not sure if you agree. Davies says that the wave function is information. It is not the case that the world is composed of wave functions. One can’t go out and measure a wave function. All one can measure is the collapsed result, i.e., the actual stuff.

    If I’m understanding Davies correctly and if I’ve explained it properly, that seems like a nice way of by-passing the QM paradoxes. It isn’t that the cat is both dead and alive. It’s that the cat is possibly both dead and alive and that there is no way of knowing until one looks.

    I’m concerned, though, that this explication sounds too easy. If it were that easy why does QM seem so paradoxical?

    How would this apply to the double slit experiment. It would require that the wave, which we want to think of as information must physically pass through both slits, thereby interfering with itself. Is that a problem? If one propagates the wave in the context of two slits and computes its transition in time, it would go through both slits and interfere with itself. Again, though, that seems too easy. So what am I missing?

  2. The Physicist The Physicist says:

    You’re right, it isn’t that easy.
    There’s a very powerful statement called “Bell’s Theorem” (proven) which says that the universe cannot be “locally real”.
    “Locality” basically means that no effects travel faster than light.
    “Reality” means that the universe, and everything in it, exists in only one state.
    This is extremely hand-wavy and incomplete, but:
    If relativity (which implies locality) holds, then things can exist in multiple states. Quantum mechanical randomness stems from the many-stateness of things (there’s no way to predict what state something is in when it’s in several). Most physicists who’ve spent some time looking at the problem choose locality.
    What you’re describing (with “information” scouting ahead) is a good summary of Bohm’s pilot-wave theory. It abandons the locality assumption, and embraces the reality assumption. Unfortunately, the best, paradox free, but unfortunately mind-bending, choice is to abandon the reality assumption, which puts you right in the middle of the “Many Worlds” camp.

  3. Peter Davis says:

    OK, omniscient God is entangled with all events. So when Schrödinger sees the burning bush, God has collapsed the wave function of the universe! Neat trick, being both all-knowing and all-powerful 😉

    If God defines reality and exists outside of time (locality), then is Bell’s theorem conserved?

  4. The Physicist The Physicist says:

    By looking at the bush, Schrödinger (on his own) collapses the bush’s wave function from his perspective. From a larger perspective, he and the bush are still in many states, but entangled (like the grad student).
    You don’t need a God to look at stuff on your behalf.
    In order to approach a problem like the Almighty Corolary to Bell’s Theorem from a theoretical perspective, I’d need to know what “outside of time” means, the nature of the interaction, and how the reality-defining is done.
    But, considering that the Bell inequality can be experimentally verified, you can chuck all that and say, yes; Bell’s theorem holds.

  5. Jake says:

    How quickly do things become entangled? To continue with the grad student example, I’m thinking of a scenario where the grad student takes a measurement while the Professor is on the other side of the planet. Is there a sphere of entanglement that spreads out at the speed of light as soon as the grad student makes a measurement? Or could the grad student somehow selectively entangle the Professor by some other means while leaving his roommate across town unentangled?

  6. The Physicist The Physicist says:

    They become entangled as fast as they can interact, so the speed of light is the upper bound. Any means that Schodinger’s Grad Student can use to interact with Schrodinger’s Professor, and not with Schrodinger’s roommate will work.
    But, in general it’s very difficult to not interact at all with something as big as people, as close as a planet or less away.

  7. Andrew says:

    I accept that the wave nature of a photon is statistical –a probability wave rather than a physical wave. Max Born re-interpreted the “matter-wave” ideas of de Broglie and Schrodinger as mathematical wave functions, but certainly there must also be a physical wave nature to light, as electromagnetic waves are dealing with physical forces, i.e., charge, and do not propagate merely probabilistically. (I am guessing.) I have searched through answers to the same question in the Physics Forum, but the varied answers only seem to confirm confusion. So, what is the relation of a probability wave to a physical force wave? And can this be explained for a non-scientist with limited math skills?

  8. Pablo Exequiel Lena says:

    i am a musician and a philosopher and i love maths but all i know is how to do triangles, but as a philosopher i could say that only when we liberate the consciousness we can be, we can fully be, then we can define but we can play the game endlessly without defining, and this in spirituality would be never pointing at anyone because then like that you set them free, it can be that one who redeems you, either a witness who said saw nothing or by the grace of god, so as to say, they both act as intercessors, and then you become a free person, instead of being defined by ego, your granted favor to be forgiven turns you into a spiritual being part of the quantum ocean,

  9. john says:

    Shroedingers Cat. Try this.

    Lets not forget that inside the box the cat is either dead or not regardless of Shroedigers later observation. Can it really only become dead from Shroedigers frame of reference whan the observation is made?

    Lets go back into the box. The cat dies and begins to decay. Shroediger opends the box and the cat ‘dies’ upon his observation is the classic interpretation. Since the cat is already decayed then causality has been breached with the cat decaying before it died!

    The Shroediger cat example therefore confuses uncertainty with relativity for most people.
    There must be some valid debate about precisely when the cat died but not about whether it was dead before the box was opened as it clearly was.
    This argument reapplied to the original example means that the only valid observation to be made is that the cat is dead and it died while it was in the box.
    The principle of being both alive and dead violates causality as life must precede death even if only be a vanishingly small degree.

  10. The Physicist The Physicist says:

    Sounds like you’re landing on exactly why Schrodinger’s cat is so strange. The cat really is both alive and dead, with both of the histories necessary to back that up.
    Of course, Schrodinger was originally talking about particles being in super-positions of states (several states at the same time), but he used a clearly impossible cat example because it’s easy to write off particles as strange, but harder to write of more intuitive objects (like cats).
    Actual super-position requires the thing in question to be completely isolated from the observer in question. Not a single stray photon (or at least, not many). If you were to try this experiment in reality, you’d find that it’s impossible (with current technology) to isolate a cat-sized-thing enough for any weird quantumy things to happen.

  11. Abe says:

    God is without place or time he doesn’t interact with his creation. He set rules for the universe and the creation abide by those rule (physics). Now the real question is that how would the photon know it’s being observed and then forced to act as a single proton? Unless it has a brain or some sort of consciousness which we know it doesn’t leads me to believe that God gave it a distinct attribute to act normal whilst being observed by creations. Because only things of this universe could interact with one another.

  12. NotThisWhat? says:

    I may be going off track a little here, but I figured the subject of the question begged a little more discussion with regards to the “concept” of God.

    The answer given in the main article says the probability wave function doesn’t collapse, if God keeps quiet! In other words, in a universe where God is not entangled with us (his creation) his all seeing eye wouldn’t collapse the probability wave function. This seems to be based on a fundamentally dualistic view of the existence.

    So, what about non-dualism? In Advaita (of Indian origin) philosophy dualism is an illusion. So from the Vedic perspective God must be entangled, which would seem to lead to the opposite conclusion. A similar story comes about when considering the Hebrew God. However, maybe it doesn’t. It really depends on whether we are applying “God” correctly within our model.

    Now I don’t know if I’m anywhere near applying “God” correctly, but here are my thoughts.

    It appears to be paradoxical for a universe of non-duality to contain an all seeing God and a sustained probability wave function. However, if God is entangled with us, and all we (as in ALL beings in the universe) ever see is the totality of all that can be seen (over all time), then his all seeing eye is nothing but a reeduction of the multiplicity of individual acts of seeing to one universal act of seeing. Bear with me on this, there is a point!!!

    In a non-dual philosophical system, the universe and God are entangled, the individual observers and God are entangled, and whenever the probability wave function is seen to collapse it is impossible to differentiate whether this is a product of the universal (God’s) act of observation or an individual act of observation, due to their entanglement.

    Wherever we find philosophies (or mythologies – which are often used as vehicles for philosophical exposition) that speak of one God (in a sense of a non-dual universal unconditional being) we are bound to be entering subtle metaphysical territory. The Indian systems speaks of Brahman, the Hebrew of Yahweh, yet they seem share one common theme, God is one, and is one with the creation, including all things and all beings – even when appearances seem at odds with it.

    However, some interpretations of the God concept seem to ignore this. When not ignored, however, it seems to me that to speak of an omniscient being is to speak of a being that sees all that is see-able. There is nothing un-see-able that can be seen. This is no different to saying God sees the totality of what we have ever and will ever see. In this sense there is nothing that God sees that we won’t see, the difference is that as individuals we get an infinitely limited piece (a bit like an abridged proposition about the whole, through a lens), whereas the universal gets it all, the totality of all the individuals seeing (a bit like all the propositions that form the whole).

    It can, therefore, be logically inferred that God’s act of seeing is present in everything that is seen by us. In this sense, every time the probability wave function collapses due to an act of individual observation, the universal observer (God) also (though in some systems is considered to be the only true observer, the individual being like a wave on the ocean) witnesses the event. Since there is nothing to be seen that can’t be seen, there is no necessary collapse of the probability wave function implied by the existence of an omniscient God, rather there is the implication that the all seeing God is present in every act of seeing, which means that the collapse is contingent on God’s entanglement with the individual. So as stated previously, the universal observer is entangled with the individual observer – at least that’s what is implied by the notion of an omniscient God.

    To take this further:

    It is possible to interpret The Book of Genesis as giving a philosophical grounding for the entanglement of the omniscient God with the Heaven and the Earth. I’ll leave out the concept of “creation”, since even though this is commonly thought to be an explicit statement of the first line in Genesis, it depends on the interpretation, and is more likely to be a dubious transliteration. Essentially it boils down to the discourse of Genesis involving premeditation and observation.

    I’m not about to point out sentences in the book (and it is a book, written by human philosophers), it’s not necessary, the point is that the picture painted in Genesis is one of an emergent universe in transition through a sequence of phases (seven), each phase marked by a threshold (day) where the universal being observes the result of a particular premeditation. If (and only if) the premeditations/thoughts such as “let there be light” are considered to be analogies for logical processes of pure reason (albeit with the details left unexplained) that underpin each phase, then the first chapter of Genesis paints a very rudimentary picture of the function of probability wave collapse being present at the very core/beginning of everything – the first thing to emerge being light, or energy. Of course, it’s all in a mythological discourse, no maths, no proof – just faith.

    However, logically, the progression of the myth dictates each phase acts as a substratum of the next. Interestingly, that which it all emerges from initially (the Earth, which is not our planet, it can’t be, light/energy hasn’t even appeared at this point, never mind atoms, rocks, planets etc) is considered to be “void and without form” – which sounds very similar to the state of quantum mechanical randomness prior to probability wave collapse.

    So, in short, even in a universe where God and Man are entangled, the omniscience of the universal being doesn’t require it/he/she/them to hide it. Rather, the supposed existence of God allows for an explanation of the beginnings of a universe facilitated by observation and probability wave collapse in the wake of it. Of course, the terminology of probability wave collapse came a lot later than the term God/Heaven/Earth, but they are not inherently non-conducive with each other – even if most religious theologies don’t like to let such serendipity between ideas to mature.

  13. The Cool Dude says:

    I can’t believe nobody caught the cat pun.

  14. Prof. Rajesh Kumar says:

    I DO UNDERSTAND THIS POST IS NOT ABOUT GOD vs SCIENCE but pls read ahead as its important i beleive that we understand such deep things as well.
    MOST IMPORTANTLY, mankind gave “God” the property of being all-knowing, we kinda came up with the whole religion thing and ‘omni properties’, we know that man can be wrong, so are we sure we are right on this one…? We came up with the defination of god, he never came to us telling us the quantum states and everything else about everything??
    Being an acamedecian( am a fore-runner in quantum mechanics) “violate my believes” in my religion and faith. 🙂 God condradicts quantum mechanics – the basis of modern and future science under which newtonion; even einstenian mechanics are just special cases, so an omnipresent entity, or one which knows the quantum states of every spinnig electron in the world+heaven+hell 🙂 or for the entire universe for that matter and knows its precise momentum and position simply violates Heisenberg, along with the rest of Q.M……

  15. Super says:

    I’ve always had some basic questions about the double slit experiment that follow from each other (depending on the answers, the subsequent line of questions may be rendered moot).

    The “detector” we set up at one slit – what is it? I’ve never heard that explained. Does it cause the photon to become entangled merely by being there, through some sort of physical interaction, or does entanglement require that some person or device observe/note/record that it has detected the photon?

    What if we put the detector in place but failed to turn it on/didn’t hook it up to any sort of recording device/didn’t set it to transmit any results it detects/etc. (phrased that way due to my ignorance of what the detector actually is)? Would entanglement happen?

    Same as above except we set up unattached detectors at both slits?

    What if immediately after signaling to Schrodinger’s cat either the presence or lack thereof of a passing photon, our detector delivers a fatal electrical charge, thoroughly frying its brain in the process (i.e. dead cats tell no tales)?

  16. Derek Potter says:

    Well, the question assumes wavefunction collapse. This cannot be demonstrated, indeed in MWI the appearence of wavefunction collapse is emergent on pure non-collapsing wave mechanics. It doesn’t really happen. Technically, interactions, such as observation, create improper mixed states which look like probabalistic collapse but really represent a superposition of possible outcomes. Even God would not collapse the wavefunction by observation if observation does not collapse the wavefunction! However He might very well impose superselection – getting rid of unwanted branches – but that is a considerable deviation from known physics. Physicist is correct, if God’s observation creates entanglement then wavefunction collapse, real or apparent, will happen. The apparent case is interesting, it means that God’s observational knowledge is divided. To humans this seems bizarre, we assume our experience of the world supervenes on the state of our brain and thus on the outcome of an observation. God may not work that way, He may have arranged things so He combines His knowledge of all outcomes. Of course the question is a little confused theologically too – the omniscience of God does not depend on observing everything, he just knows. This kind of knowledge cannot possibly collapse the wavefunction. There are, of course problems with free will and omniscience: opinions are divided between compatibilists and incompatibilists. It could be that a free-will event is not knowable without looking – even to God. I mention this because as soon as you ask “What if God?” you introduce a whole load of possibilities that physics cannot answer.

  17. Angel says:

    @Prof. Rajesh Kumar:

    I suggest that next time you read closely before you comment. Notice that the question says “If God were all-seeing and all-knowing, the double-slit experiment wouldn’t work, would it? Wouldn’t God’s observation of the location of the photon collapse its probability wave function?”. The initial IF means we are assuming it, it does not mean we know it. We don’t even know if he exists or not, let alone whether he is all knowing and all seeing. It is a mere ASSUMPTION. It doesn’t say “Because God is all seeing”, no, it only says “If God were…”.

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