Quantum Mechanics poses the following idea: an atom or photon can exist in multiple states at the same time in superposition, until it is observed or interacts with the external world. It then collapses into one of its states.
Erwin Schrödinger formulated a thought experiment to highlight a problem with quantum theory: if a radioactive source (in which atoms may or may not decay, at random), a sensor, a flask of potion and a cat are put in a sealed box, set up so that the flask of poison shatters when there is radioactive activity, then quantum theory would have to hold that the cat is both alive and dead at the same time, in superposition, until an observer opens the box. Bottom line: if it is ridiculous to think of a cat as being alive and dead at the same time, then it is also ridiculous to think of atoms as being in two states at the same time. Schrödinger’s Cat is a paradox.
The Many World Interpretation of Quantum Mechanics is meant to offer a solution to this paradox. Suppose that whenever a Schrödinger’s Cat type situation arises – when there are more than one possible alternative states for a particle to be in – the particle is not in two states at the same time, but instead two worlds are generated: one world in which one state obtains, and one world in which the other state obtains. In the case of Schrödinger’s Cat: whenever Schrödinger does this experiment, the world splits into two separate worlds: one in which the cat is dead in the box and one in which the cat is alive.
The idea is that these worlds are truly parallel. They exist in superposition: they exist in the same space at the same time, but cannot interact with one another. Since there is one universe – since the universe is all that exists, so there can’t be more than one of it – with multiple, indeed very many, worlds in it (some philosophers speak of a multiverse with parallel universes in it, which can be confusing, but the idea is similar).
If this picture is true, then this has implications for how we think about ourselves and our personal history and future. Imagine you’re flipping a coin. How often have you done that in your life? Each time you flip a coin, it seems as though it’s landing on only one of both sides. But now imagine that in fact, as you flip, the world splits in two. In one world your coin lands on heads, in another world your coin lands on tails. This means that there are now two versions, two exact copies, of you: one in each world. Of course you never notice anything of this world-splitting. After all, parallel worlds cannot interact with one another and you are always experiencing only one of them. Yet, if the world splits every time you perform a quantum experiment, there should now be countless worlds with you in it, and every copy of you will walk a different path after the each t-crossing in the path of your personal time line. For instance: if you flipped the coin to help you take a decision – between asking your beloved out on a date or not asking your beloved out on a date – then the copy in one world did ask your beloved out on a date and the copy in the other world didn’t. You can see how this can quickly lead to very different personal histories from that moment onward.
Quantum mechanics and its Many World Interpretation have a solid mathematical side to it. In theoretical physics it is a credible theory. What if it is true? Then many philosophical questions are cast in a new light. For instance: what is a world? The sum total of all particles in a definite state? This is one popular interpretation, with the advantage of observer independence. If this is the world, then we can easily explain that there was a world when there was nobody to observe it. An alternative conception of the world is observer-centered: the world is everything within the limits of an observer’s awareness. My world, then, is slightly different from your world. The advantage of this conception is that our relation to a universe with multiple worlds becomes easier to conceive, since there are already as many worlds as there are observers anyway. The disadvantage is, of course, that in this view there were no worlds before there were observers, and the galaxies in the universe that are not within the limits of anyone’s awareness are not part of any world. For many, that is counter-intuitive, or at least too much of a stretch of the word ‘world’ as we normally use it.
Another philosophical question that asks for consideration when we accept the Many Worlds Interpretation is the question of personal identity (the continuity question, as we’ve discussed before). Who am I? Do I still exist as the same person after I flip the coin? If there are two versions of me in two different worlds after I flip the coin, which of them is me? This is not the same question as “who is Marthe?”. It is easy to see that in a universe with many worlds there are also many Marthes. But which of them am I? The grammatical first person, “I”, indicates a subject with a single point of view that persists through time. Duplication is contradictory to this, because it forces one to lose the singularity and the subjectivity. If I flip a coin I get one result, and not two. Who is that I?
A related question is: should I care about what happens to me in a parallel world? Perhaps my parallel version died (in fact, if we accept this many world picture, then lots of worlds exist in which I died. In some of them in a horrible way. Tough luck, that’s what happens if everything that’s possible must exist in a parallel world). Should I be sad about that? In parallel worlds, copies of my friends and family members exist. Should I care about them? On the one hand, one might say that I shouldn’t, because my world can’t interact with their world anyway, so it’d be pointless. On the other hand, we do care about the future of our world after we cease to exist even though we don’t have access to that future once we’re gone (we invest in our children, in the environment, in our legacy, we want to be remembered…). Why should we care about this world beyond our reach but not about parallel worlds beyond our reach?
But there is a plus side: if you don’t win the lottery in this world, you definitely will have won it in another world.
Hilary Putnam doesn’t believe in the Many World Interpretation of Quantum Mechanics, because, he says, it renders the idea of probability meaningless. Read his argument here.
Lev Vaidman, however, has an answer. Read his entry in the Stanford Encyclopedia of Philosophy here.