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.
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.*7 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.
- 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?
*1 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.
*2 To be more accurate, it’s actually the particle’s probability density, |ψ|2
*3 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?
*4 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.
*5 Another interpretation that could also be philosophically sound is having an “Ultimate Observer,” but I won’t get into that.
*6 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.
*7 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.
*8 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.
*9 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.