Parallel Worlds – 25/04/2016

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.




Hilary Putnam’s Semantic Externalism – 11/04/2016

Philosopher Hilary Putnam (1926-2016) passed away last month. Not only did he contribute to many debates in analytical philosophy, but he was also known to exhibit a great philosophical virtue: he was his own fiercest critic. He subjected his own positions to thorough scrutiny and was never too proud to reject earlier positions, when he had exposed their flaws, and change his mind in favour of another view.

In his honour, we will discuss one of Putnam’s arguments: his argument for semantic externalism. Crudely put, semantic externalism is the view that words and sentences only have meaning when there is an external world they can refer to. Putnam’s argument in favour of this view also serves to dismiss Cartesian scepticism. I will start from this vantage point to explain Putnam’s view.

Scepticism: Are we a brain in a vat?

Imagine you are, without knowing it, no more than a brain floating around in a vat of nutrient fluids. The brain is connected to a computer which stimulates the brain in such a way that it gives you the experience of an external world – you see trees, apples, other people, you smell flowers, you feel the fur of the cat and everything else you normally experience. These computer generated experiences are, to you, indistinguishable from experiences of real trees, apples, people, flowers and cats. You have been a brain in a vat from the beginning of your existence, and you have never known that you are a brain in a vat, because the computer has generated a whole life full of experiences for you. Therefore, you think that you see apples and feel the cat, but you are mistaken, because there are no apples and cats; there are only brains, vats, and computers (of which you have no experience). A bit like in the film The Matrix. If you’ve attended the class on Descartes, you’ll recognise his idea of a mind that is deceived by an evil demon. The brain-in-vat thought experiment is based on that idea.

This thought experiment aims to show that scepticism, the view that we can’t know anything about the world, is true. The argument goes as follows: if we know something about the world, then we can thereby also know that we are not brains in vats; but there is no way of knowing whether we are brains in vats, so we don’t know that we are not brains in vats; since we don’t know that we are not brains in vats, we must conclude that we don’t know anything about the world. (The logic applied here is a modus tollens: 1. if p then q, 2. not q, therefore not p.)

Putnam’s refutation

A friend of my housemate says that his worst fear is that we are indeed brains in vats, without ever knowing it. Luckily, Putnam has good news for him: we can know that we are not brains in vats. That’s a relief! But how does Putnam know? Putnam’s argument runs as follows:

Imagine a world in which the only existing things are brains, vats with nutrient liquids, and computers that stimulate the brains. Nothing else. Quite like the scenario outlined above. In this scenario, you are a brain in a vat (henceforth BIV) who has experiences as if you see trees, apples and cats, even though there aren’t any. You think you speak English, but this too is an illusion; you don’t speak at all (because you have no mouth or vocal cords, only a brain), it’s just the computer giving you the experience as if you speak English, you don’t actually speak. Hold that image for a minute. Let’s call this world the BIV-world.

Now imagine another world where you are not a brain in a vat. You are a human being in a natural environment, exactly as you experience it. You see trees, apples and cats and they are indeed really there. There are also other people who experience these same things, and such day-to-day experiences of the environment are not illusions. In this world, you also think you speak English, but this time, that is not a mistake: you do indeed speak English. In this world, when you use the word “tree”, the meaning of that word is, according to Putnam, caused by real trees in the world. The word “tree” refers to a tree, and that’s why it is a meaningful word in the English language. Let’s call this world the natural world.

Back to the BIV-world. In your mind, you’re speaking English. You use words like “tree”, “apple” and “cat”. But what do these words mean? There are no trees, apples or cats that can give these words meaning, because there are only brains, vats and computers. What the word “tree” actually refers to, in this case, is merely a set of electrical impulses generated by the computer. So the word “tree” in the BIV-world in BIV-English means something else than the word “tree” in English in the natural world.

So in the BIV-world, the words “brain” and “vat” refer, just like “tree” and “apple” to particular sets of electrical impulses, and not to the actual brain nor the vat it’s floating in in the BIV-world. So, paradoxically, even if you were a brain in a vat, your sentence “I am a brain in a vat” would be false, since “brain” and “vat” in that sentence don’t refer to your BIV-situation, but to illusory brains and illusory vats in your illusory environment.

I understand if your brain hurts a bit after reading this. Here’s the argument again, but this time in a schematic form. Concepts, references and words referring to objects in the BIV-world are indicated with an *:

  1. Either I am a BIV (speaking vat-English*) or I am a non-BIV (speaking English).
  2. If I am a BIV (speaking vat-English*), then my utterances of ‘I am a BIV’ are true if and only if (iff) I am a brain* in a vat*.
  3. If I am a BIV (speaking vat-English*), then I am not a brain* in a vat*.
  4. If I am a BIV (speaking vat-English*), then my utterances of ‘I am a BIV*’ are false.
  5. If I am a non-BIV (speaking English), then my utterances of ‘I am a BIV’ are true iff I am a BIV.
  6. If I am a non-BIV (speaking English), then my utterances of ‘I am a BIV’ are false.
  7. My utterances of ‘I am a BIV’ are false in either case.
  8. My utterances of ‘I am not a BIV’ are true.
  9. My utterances of ‘I am not a BIV’ are true iff I am not a BIV.
  10. Conclusion: I am not a BIV

If this hurts your brain even more, I understand. It’s all very technical. That’s the thing with analytical philosophers. They tend to do that. But the bottom line of Putnam’s argument is understandable enough: the brain-in-vat hypothesis mixes two worlds of reference, which messes with the meaning of what we talk about when we say “I am a brain in a vat”. If it were true that I am a brain in a vat, it would be meaningless because the concepts I use to talk about that situation don’t take their meaning from the BIV-world, but from the world I experience. Hence, the world I experience is the only world I can talk about, and in that world I am not a brain in a vat.

The Twin Earth thought experiment

Putnam makes the same point using a different thought experiment.

Imagine that somewhere far, far away in the universe, there is a planet that is an exact copy of Earth. Let’s call it Twin Earth. On Twin Earth, there is a twin of every person on Earth. So let’s say that Tony lives on Earth, and Twin Tony lives on Twin Earth. There is only one difference between Earth and Twin Earth: there is no water on Twin Earth. Instead, there is another substance with the exact same qualities as water, except that its chemical composition is not H2O, but XYZ (elements that don’t exist on Earth) Twin Tony, who speaks Twin English, calls this substance “water”. Now let’s go back a few hundred years in time, when nobody knew about the chemical elements and the composition of water. Would we say that Tony and Twin Tony mean the same thing when they say “water”? Putnam would say “no”, because Tony means H2O when he says “water” and Twin Tony means XYZ. The stuff they refer to is different stuff, even if it has the same function and Tony and Twin Tony have the same concept in mind. It is the actual things in the world that words refer to that give those words their meaning; language on its own, isolated from a world of reference, is meaningless, according to Putnam. In his own words: “‘Meanings’ just ain’t in the head”.

Of course, Putnam wouldn’t be Putnam if he hadn’t revised his position since. He later agreed with Tyler Burge that Tony and Twin Tony wouldn’t in fact have the same mental state when they talk about “water” (Tony has the concept of H2O and Twin Tony the concept of XYZ).