Field of Science

Quantum Reality and the Measurement Paradox

I may be primarily an evolutionary theorist nowadays, but I have many interests, and this summer is proving to be a good time to explore some areas not directly connected to my need to publish. Lately I've been doing some reading on quantum mechanics, and what it tells us about reality.

QM is astonishing in both its mathematical elegance and its fundamental counter-intuitiveness. Unfortunately, I think many (including mathematicians) are discouraged from learning about quantum because it is typically presented assuming a deep knowledge of classical mechanics. But in my view, QM isn't just a theory about physics. It's a theory about reality and truth, and many of its implications can be understood with no knowledge of physics at all.

The essential feature of quantum reality, and what makes it different from the way we naturally think, is the superposition principle. It says that if A and B are two possible states of something (a photon, a cat, the whole world...), these states can be added to get another possible state, A+B. For example, if a light switch can exist in ON and OFF positions, there must also be a possible state ON+OFF. Subtraction works too: the state ON-OFF must is a valid state as well. To my mathematician friends: we are moving from the set of possibilities {ON, OFF} to the two-dimensional vector space generated by the basis vectors ON and OFF.

It's important to delineate what is not happening here. ON+OFF does not mean that the switch is stuck somewhere between on and off. It also does not mean that it might be either on or off and we just don't know which. ON+OFF is a fully-determined state which is neither ON nor OFF, but a superposition of the two.

Of course, no one has ever observed a light switch being ON+OFF. Something happens when we observe these superimposed states, such that we can only ever see the "classical" states ON or OFF.

In the standard (a.k.a. Copenhagen) interpretation of quantum mechanics, when a superimposed state is observed, it "collapses" into one of the classically observable states. In the case of ON+OFF, whenever we look at the switch, it collapses into either an ON or and OFF state, with equal probability. But until we look at it, in remains in the state ON+OFF, which has unique properties making it distinct from either the ON or OFF state.

This interpretation poses a host of logical difficulties. What exactly constitutes an "observation", and how would a light switch "know" that it is being observed and should therefore jump into an observable state? Many of the best minds in physics believe that observation has something to do with consciousness, but this raises several obvious questions: How is consciousness is defined? What gives it this unique power to induce jumps in physical states?

I've recently come across a new interpretation, proposed in 1997 by Cerf and Adami. They suggest that superimposed states do not collapse when observed, but rather the observer becomes entangled with the observed, forming a larger superimposed state.

To illustrate this, let's turn to Schrodinger's cat paradox. An atom is prepared in a superposition of two states: one in which the atom will emit a photon and one in which it won't. This atom is placed in a box with a cat and an apparatus which will release poisonous gas if the photon is emitted (the details of the setup are unimportant). According to the Copenhagen interpretation, the system exists in the superimposed state


until such point as the box is opened by a conscious observer, whereupon the system "collapses" and the cat becomes either just alive or just dead. (This raises some questions of whether cats count as conscious, but such objections only deepen the underlying paradox).

In the Cerf and Adami interpretation, there is no collapse, only entanglement. When we observe the contents of the box, we ourselves become entangled with this system. We become part of the resulting superimposed state:


Of course, we still only see the cat as being either dead or alive, not both. But according to Cerf and Adami, this is only because the state EMIT+NOT_EMIT of the atom is unobservable to us. Of the full superimposed state, we can only see the parts pertaining to the cat and to the observer. Observing only part of the system, it appears to us that the cat is either alive or dead. Anyone else observing the cat would see it to be in the same state that we do, but this is only because the second observer is just as entangled as we are. The cat is still superimposed between alive and dead, and if we could see the whole system, we'd realize that we ourselves are superimposed between seeing it alive and seeing it dead.

From a mathematical point of view, Cerf and Adami's proposal neatly resolves the paradox of observation and state collapse. However, it raises far more troubling questions of its own, which the authors do not begin to explore.

Think of a decision you made today. It's not unreasonable to think that there are quantum processes in our brain whose outcomes affect our decisions (this view is advanced by my friend Bob Doyle). Let's say that there was a certain quantum state in your brain whose collapse into one of two states (in the Copenhagen interpretation) tilted your decision one way or the other.

If this is true, then in Cerf and Adami's interpretation, we actually exist in a superposition of realities: one in which your decision went one way and one in which it went the other. You can only see one of these realities, and everyone you've encountered since has become entangled with you and therefore sees the same reality that you do. But the alternate reality is playing itself out, Sliding Doors-style, superimposed on top of our own.

Furthermore, due to quantum interference, any actions taken in this reality can affect any of the superimposed other realities. And conversely, anything your alternate-reality twin does in his or her reality can affect the reality you and I see.

I tend to believe Cerf and Adami's idea, because millenia of physics research have shown us that the mathematically elegant solution is usually the right one. But this means our universe is weirder than we can possibly imagine.


  1. The basic idea is probably more fairly attributed to Everett, rather than Cerf and Adami; also, typically, branches of the wavefunction aren't taken to be interacting.

    (I find the relational interpretation interesting, because it's as parsimonious of hypotheses as the many-worlds idea — no extra "collapse of the wavefunction" postulate — while being set up from the beginning to work with special relativity.)

  2. Also, if you have an hour to spare, Sidney Coleman's "Quantum Mechanics in Your Face" is well worth watching.

  3. Thanks for the connection--I had heard of relational quantum mechanics, and was planning to look into whether it amounts to the same thing as Cerf and Adami's proposal.

    It looks like they are formally the same, but with different focuses. RQM focuses on the information available to each observer and asserts that no single observer's vantage point should be privileged, while Cerf and Adami focus more on the complete quantum state of the world.

    There's a theological principle here: If you suppose that there is a God-like observer who is able to see the entire quantum state of the universe, then that observer will be able to see all the various superimposed realities playing out at once, but will have no way of knowing which of these realities is playing out in the consciousness of any of the universe's beings. If He/She were to gain this information, He/She would become entangled in the system and would no longer have complete knowledge.

    A final point: I see no reason to assume that the superimposed realities do not interfere with each other. Interference is the norm for quantum systems; there would have to be some special exception to this rule in order for alternate realities to be non-interacting.

  4. The amplitudes for the two paths in a double-slit experiment "interfere with one another", in the sense we get a cross term when we figure out the probability in the end, but branches of the "universal wavefunction" which have decohered do not.

  5. I had thought that decoherence was offered as a physical alternative to having to rely on some mystical conscious observation. I am by no means an expert though and am not familiar with the literature, which is why I'm going to reference this post with a wikipedia link:


  6. To phrase it a bit better, maybe: "interference" is not the same as "interaction". It's not that an electron going one way feels the Coulomb repulsion from an electron which might be going the other way; a Lagrangian doesn't have to have an interaction term for an interference pattern to appear.

  7. Aha. So what I'm proposing is that there is interference between the realities but no interaction. So we will never physically "feel" any effects from other realities, but what happens there can influence the probabilities of quantum events that we see.

  8. I realize the post is about the Cerf/Adami/Everett idea, but the line that caught me was:

    "Many of the best minds in physics believe that observation has something to do with consciousness..."

    I'm told that centuries ago, a common theory of vision was that our eyes emitted some invisible beam that shot out into the world and, landed on things, and told us what they were. I think many people have this intuition (I certainly did as a child), and it's encoded in our language ("her eyes fell on the stranger"). Part of this intuition is that looking at something affects it in some way.

    But that's not how, of course, vision works. Nothing is affected by your viewing it, since as best as we can tell, your eyes don't shoot out beams of anything. The view espoused by "many of the best minds in physics" sounds like the same theory -- your mind reaches out and touches things in the world. Either that, or it's "consciousness is magic" hypothesis.

    As a cognitive scientist, I'm committed to the propositions that a) consciousness is not magic, and b) it's fully contained in our corporeal bodies and cannot escape to wreak havoc on the external world. This doesn't have to be true, but it's certainly the theory that best fits the extant data (quantum physics notwithstanding).

    Have I completely misunderestood the proposal? And do Cerf/Adami/Everett also require consciousness to be magic? Or is it a proposal that we can have two conscious states simultaneously but only be aware of one of them? I admit I didn't follow all the details.

  9. The standard Copenhagen interpretation, at least in some of its variants, posits that consciousness somehow has the power to collapse superimposed states. (I should qualify that probably no one is really happy with this idea, but some scientists, including von Neumann, viewed it as the best way of dealing with the paradox.)

    Relational quantum mechanics (Everett/Rovelli/Cerf/Adami/...) does not see this as the case. In RQM, the collapse of superimposed states happens only in the viewpoint of the observer.

    For example, suppose an experimenter prepares a light switch in the state ON+OFF. He then looks at the light switch, thereby entangling himself with it. To an outsider who has no interaction or entanglement with the system, the system's state is


    But to the experimenter, the light switch is either on or off, not both.

    But the issue of consciousness seems to be rearing its head again here. What exactly does it mean for something to be true in the viewpoint of the experimenter? Does an entity need to be conscious in order to have a viewpoint?

    The conclusion I'm coming to is:

    1. All entities in the universe exist in multiple superimposed realities

    2. Those entities that are conscious, are conscious of only one reality.

    3. If two conscious entities interact, then the realities they are conscious of are consistent with each other.

  10. I'm a little confused, because it seems like collapsing actually has measurable effects. Looking at the two-slit experiments, it seems like when superimposed realities have been collapsed, they lose the capacity to interfere with one another. How does the relational explanation take this into account? Is it that in each of the superimposed realities there is no interference pattern, but if you can perceive the superposition -- i.e., if you haven't committed to one of the two -- the interference pattern becomes visible? If so, then the capacity for the different realities to interfere with each other becomes considerably less interesting.

    I'm also perhaps not familiar enough with the literature to get how consciousness works for explaining the collapsing of realities. The observer in the two-slit experiment is mechanical, right? So presumably I have the option to place the "observer" there and just not look at it, right? If I do so, does the interference pattern come back? And if I then look at the data from the observer, does the interference pattern disappear? Because if that's true, this is way trippier than I thought.

    My impression was that whether or not anyone who we'd normally consider conscious looks at the data, the interference pattern disappears. It seems like that rules out the idea that this has anything to do with consciousness. Am I wrong?


    I disagree with this, as a physicist doing research in quantum mechanics, I really don't think there is anything special about consciousness acting as an observer for a quantum mechanical measurement (such as in the Schrödinger cat experiment), and I don't know many physicists (if any) who have this point of view. Some physicists in the past (even many prominent ones) have held this view, but I don't think this notion is so popular today.

    I should note that although consciousness is sufficient as a observer, it's not the only thing that can "observe" in order to collapse states.

    To see why consciousness is not special, consider the following example. If we redo the cat experiment again, but instead have a computer open the box and measure the pulse of the cat instead of a person looking in. Then from the point of view of the computer, the cat is either alive or dead, so we might have the state:


    The computer has some extra information about the system, namely the pulse of the cat in the box. From an information theory point of view, this is no different from a person observing the cat. The person would see that the cat is alive, which would store this information in the person's memory, and from this, we say that the state of the system has collapsed with respect to this person (as an observer). For the computer, the same thing happens. It takes the pulse of the cat, and therefore it stores the information about whether the cat is alive or dead. Therefore, there is no difference between what the computer and the person do, they both have enough information that is sufficient to "collapse" the state of the system. So the computer works as a perfectly good observer without being conscious.

    @Ben: "But to the experimenter, the light switch is either on or off, not both.


    This is a good point. The information about whether the light switch is on or off depends on which observer you use to represent the state of the system. Depending on the information you have about the system, the way you describe it will be different.

    As a simple example from everyday ("classical") life: say that I flip a coin and hide it. I look at it to see what side is up, say it's tails. From my point of view I describe the state of the coin as (TAILS), but from your point of view, since you haven't seen which side is up, would be (Probability 1/2 TAILS, Probability 1/2 HEADS). This is the same principle in use for your light switch example between someone who observes the state of the light switch and an outside observer. However, we could repeat the light switch example, or coin flip example with computers, and the states would be the same. Again, there's nothing special about consciousness.

    @Per: I don't quite understand what you're trying to say about the double slit experiment, but it seems you're confused about how it works.

    If you run the double slit experiment, with a film at the end to record output of the two slits, you will record an interference pattern. Whether you look at this film during, or after the experiment, you will still have the same pattern. However, if you record the path that the particles take (i.e. which slit each particle takes) then the interference pattern disappears, and you get just the addition of the two patterns you would get from two single slits.

    But you're right that whether the "observer" used in the experiment is conscious or not is not important, the same interference pattern still occurs.

  12. But millions of decisions are taken per second, by billions of people all at once. Does that mean there are INFINITE realities being created all the time that conscious decisions are being made?

  13. I subscribe to theories put forth by Dr. Amit Goswami.
    The theory begins with Downward Causition. Consciousness is the foundation of reality, not matter.
    There is a greater non-local consciousness with which we are all connected. Through this non-local consciousness, such decisions are made, (switch on/off, cat alive/dead), which then becomes an agreed upon reality.
    Through this theory, there is no paradox, as everything in the universe or multiverse, is always in contact with consciousness and Quantum Wave Collapses in to Quantum Particle as a result.


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