It’s not surprising that quantum mechanics, the branch of physics that studies the smallest level of reality, has many conflicting interpretations to what’s really happening and what actually exists. A book I read recently, The Anthropic Cosmological Principle, discusses many areas of science that relate to the anthropic principle, some of the most interesting of which are the interpretations of quantum mechanics (QM), in particular, the Many Worlds Interpretation (MWI). This is what we’ll look at today:

From The Anthropic Cosmological Principle by John Barrow and Frank Tipler (1988):
“The question of why does this universe rather than that universe exist is answered by saying that all logically possible universes do exist. What else could there possibly be? The MWI cosmology enlarges the ontology in order to economize on physical laws.”

By the way, I have a lot of “stars” in the text below, which correspond to notes at the bottom that give a bit of physics details for those who are interested (it’s not necessary to read though).

One of the many problems in QM we still need to solve is how to make sense of observers. Namely, us, the ones who perform experiments. We seem to have a peculiar influence over what happens to particles at the subatomic level just by observing them. Indeed, before a measurement of a particle is taken (usually something like its position or speed. I’ll just talk about position though), the usual interpretation, called the Copenhagen Interpretation, says that the particle exists in a “superposition” of states: not at any particular location. It is spread out over a certain area and is described by a wavefunction (usually denoted by ψ). Although the particles is more likely to be found in a particular area, it can still be found outside this region.*1

The interpretation favoured by the Copenhagenists is that when an observation is made, the wavefunction “collapses” to a particular value. When we weren’t looking, it was spread out over a large area, existing, in a sense, in many places at once, but now, it is nicely settled at a single point in space. In the picture below, the graph on the left shows the probability of observing a particle in a particular location (in x and y coordinates, so two dimensionally). This is represented by the particle's wavefunction (ψ) on the vertical axis.*The particle isn’t “in” any one spot, but spread out over an area, though it is most likely to be found at the peak.


However, when an observation or measurement is made, the wavefunction collapses so that the particle is now in a particular spot (the spike on the graph to the right). So, as the physicist Niels Bohr said, “an independent reality in the ordinary physical sense can neither be ascribed to the phenomena nor the agencies of observations.” Observation, in a sense, brings the properties of particles into being—they never really “existed” beforehand.

This interpretation seems ludicrous, and there are some serious issues with it as well.*3 This might just be how nature works, but fortunately, there are also other options available to us. First, the uncertainty in our description of QM (for example, of locating a particle) is probably just that: it is only an uncertainty in our description, which is tailor-made for human experience. It describes our uncertain knowledge of the world, not an uncertainty in the world itself. There is an inherent limitation to what we can know (from our limited senses and the fact that we have to interact with the world when we observe it) but that doesn’t mean that the world is actually “uncertain” and existing in many states before it’s observed. It’s naïve to assume that we can know everything through our senses and the mathematics we have created; it’s more reasonable to assume that as creatures living in three spatial dimensions, able to travel one direction in time and being mostly limited to our five senses, if we dig deep enough, there may be some things we can only understand probabilistically—but that’s not to say that this is the way they are, just the way they are to our perceptions. There must be a physical reality independent of our observations even if we can’t observe it, because otherwise, what would our probabilistic descriptions be describing? Thus, it’s important to distinguish between the mathematics we use to describe the world, and the world itself, which need not be constrained by our theories and human limitations.*4

This ties in with a different interpretation of QM called the Many Worlds Interpretation.*5 In this theory, each quantum mechanical possibility occurs in a different world. Introduced by Hugh Everett and endorsed by many physicists today (for example, Max Tegmark, said, “Accepting quantum mechanics to be universally true means that you should also believe in parallel universes.”), in the MW interpretation, every quantum mechanical possibility happens, but each occurs in a different world.*6 So everything happens, and there are nearly infinite possibilities! So think about every possible universe, those that are only slightly different to ours (e.g., one in which your favourite colour is red instead of purple) and ones that have entirely different planets, solar systems, and galaxies. In this scenario, the observer has no magical ability to make wavefunctions collapse, but rather, when they make a measurement, they only measure the particle’s position in their particular universe. Everett said that “it is not so much the system which is affected by an observation as the observer,” so the MW approach is a realist view of QM (meaning, in this context, that each world exists whether we observe it or not).
So when a measurement is made on a system, all the possibilities that could happen will “split” into different worlds. If one views the universe as a whole, however, (though here, it might be better to say multiverse) there is no real splitting and no change in the particle’s wavefunction: the general wavefunction represents all the different particles that exist in every universe, even though in our particular universe, it only follows one option. The real difference is in the measuring device (or ourselves, since we perceive the particle to be in one state or another), because we measure different values depending which universe we’re in. Indeed, it’s more apt to say that the measuring device splits rather than the particle and the universe around it, because it’s the measuring device that is registering different values depending upon the state of the particle. Barrow and Tipler say that, “There is only one Universe, but small parts of it—measuring apparata—split into several pieces…upon the act of measurement.”

I find this hard to imagine physically, since, if anything in the universe splits into multiple states, where do these copies exist? They have to be somehow distinct from each other or else it wouldn’t make sense to talk about a “split” in the first place. And it isn’t as though there is “the” universe, and then off in some other plane of existence, there are various measuring devices with different readings existing in and of themselves. I can’t explain why there wouldn’t be as many universes as there are splits, even if, at the moment of the split, they all start out the identically except for the reading on the measuring device and the particle’s state. So the view of having many worlds seems to make sense only if there are many fully-fledged universes distinct from ours.*Yet later on, they mention multiple “worlds,” each with different states for the particle that is measured, which makes more sense. They say that “each measurement splits the apparatus (or equivalently, the universe).” There is only one “Universe” (capital “U” to denote all the worlds together), but this Universe represents a collection of “universes” (each with the different possible outcomes). It is the measuring device that causes the world to split, but it is not the only thing that exists in these other worlds.

To return to the quote at the start: “enlarging the ontology” simply means that there are many universes rather than one, which might seem to be superfluous. The ontology is just our description of the universe at large, and to have all possibilities happen rather than just one is certainly enlarging it. To “economize on physical laws” means that we need not evoke a strange quantum collapse that deviates from the equations of physics to describe how a particle’s wavefunction evolve: you have a wavefunction to describe the Universe, and it never splits. And if every possible universe exists, there is no need for us to specify why the universe exists the way it does: we happen to live in this universe because it is habitable to life. Of course, we wouldn’t expect to find ourselves in a universe so vastly different to ours, or else it wouldn’t harbour life.*8

This ties in with a similar explanation of the MWI described by Max Tegmark in various papers (using Everett’s original ideas). There is no “collapse” of the wavefunction: it proceeds according to physical equations.*9 It isn’t that the wavefunction “splits” into many worlds, but that all possibilities exist, even though we only perceive one of them in a given scenario (so it looks like a split). There is “apparent randomness from the inside viewpoint…[but] strict causality from the outside viewpoint,” which means that although we can’t know which state of a particle will be in when we measure it in a given experiment, we know with 100% certainty that all outcomes will happen. From an “outside” point of view, we would be able to see all the possibilities occurring, which is where the many worlds come in, since each happens in a different “world.” This is called decoherence, when you go from a quantum (small-scale, one world) description to a classical (large-scale) description within one of the worlds. So it “is essentially indistinguishable from the effect of a postulated Copenhagen wavefunction collapse from an observational (inside) point of view.”

Thus, there is no real randomness to the universe: it only appears so to us because we cannot predict which outcome we will end up with in our universe. Tegmark says “whenever a quantum event appears to have a random outcome, all outcomes in fact occur, one in each branch [universe].” There is nothing random or uncertain about the wavefunction itself: it follows certain physical laws just like the objects in the macroscopic world, but “observers subjectively experience this splitting merely as a slight randomness.”


The MW theory thus has two descriptions: the inside view, and the outside view. Tegmark describes this as the bird’s point of view looking down on the world below (or in this case, worlds), and the point of view of a frog within one of the worlds. From the bird’s point of view, there is only universe described by a single wavefunction, and it does not split into many worlds. All these “worlds” and possibilities occur simultaneously, splitting and merging according to fixed physical laws. It is only on a smaller scale, with the frog, that ideas about randomness and probability make sense. The frog experiences a small portion of the full reality, perceiving only a single world within the entire multiverse.*10 At every instant, when there are multiple outcomes that can arise in a given situation, the bird can see each of them playing out, all in different worlds, whereas the frog only experiences the different worlds “as a slight randomness.” There is no real randomness, but since the frog doesn’t have access to any other worlds, it can only understand the world in terms of probabilities rather than certainties. This is why mathematics is so powerful: it allows us to temporarily rise out of the frog-level of existence and understand what’s really going.*11

This is the basic idea of how you can explain QM mechanics with the MW theory. It does, however, introduce many philosophical puzzling questions, such as:
  • If there are many copies of “you” in different worlds, each slightly different, which is the “real” you? Are they all you, are they all different people, or is it meaningless to talk about a unified self at all?
  • If the soul exists (and we have good reason to believe that it does), then is it spread out over the different copies of you, or are all copies but one “soulless”? This, however, depends on the answer to the first bullet point.
  • Is there a theory of everything to describe the many worlds as a whole, or, even from the bird’s eye view, is probability the best we could ever get?

In a future post, I’ll talk about how this can be understood within the framework of Neoplatonic philosophy, but for now, that’s another story 

*This can be seen clearly in Heisenberg’s Uncertainty Principle: ΔxΔp ³ ħ/2, where Δx is the uncertainty in the particle’s position, and Δp the uncertainty in its momentum. So the more accurately you know the position, the less accurately you know its momentum/speed, and vice versa, so you can never localize a particle precisely.

*To be more accurate, it’s actually the particle’s probability density, |ψ|2

*For instance, how can our knowledge, something that exists in our minds and is, in a sense, nonphysical, affect particles outside our bodies? If there are no observers, will the world exist? Does, say, a fruit fly count as an observer? What about an unconscious measuring device operating without any human influence? And, as in Schrodinger’s cat scenario, how can a cat be both dead and alive until we observe it?

*Some physicists don’t like this idea, saying that the unknowable properties of particles are “hidden variables” and that the underlying reality of the world involves statistics rather than certainties. But philosophically, this makes little sense. For a related topic, check out my previous blog post Platonic Realism in Physics.

*Another interpretation that could also be philosophically sound is having an “Ultimate Observer,” but I won’t get into that.

*A simple example is a system with a particle that could be spin up or spin down. Although we describe the particle as being in a superposition of the two spin states, we only measure one of these two possibilities when the spin of the particle is measured: one in which the particle is spin up, and the other spin down, each corresponding to two different universe.

*It may make sense to mathematically describe only the apparatus splitting, but when you think about what physically happens, you can see that the rest of the universe will be “copied” in the other world.

*This is the weak anthropic principle: “weak” not because it is lacking in some sense, but because it doesn’t make any grand statement about the universe. In fact, it really just states the obvious, since everything we perceive in the universe is restricted by the fact that we exist and are able to observe it, so it must be conductive to life, otherwise, we wouldn’t be here to observe anything.

*The system evolves according to the Schrodinger equation at all times. If there was a collapse, the Schrodinger equation would be violated.

*10 Decoherence “prevents them from seeing…parallel copies of themselves.” (Tegmark)

*11 I don’t mean that we can get out of probabilistic interpretations, because that is embedded in our mathematics as well, but rather, we can conceptually understand the general structure of reality and what this means for the existence of other worlds, probability, and determinism.


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