In this blog post I will introduce a core philosophical issue at the heart of quantum mechanics, and some of the most popular resolutions to this problem. This will be a non-technical introduction – I will try to avoid jargon and unnecessary details. But for those who like the gritty details, a second blog post will soon follow in which I will give a more technical and thorough (but also slightly biased) discussion of the same issues.

__Schrödinger’s cat__

One of the founders of quantum mechanics, Erwin Schrödinger, proposed the following thought experiment: A cat is placed in a sealed box with a device that contains a radioactive atom and some poison gas. If the radioactive atom decays, then the device is designed so that it detects the decay of the atom and subsequently releases the poison gas into the box, and this tragically kills the cat. Our intuition says that there are two options here. Either the atom decays and the cat is dead, or the atom does not decay and the cat remains alive. But quantum mechanics tells a different story. In quantum mechanics objects can have more than one property simultaneously, and in particular it is possible to put the atom into a state where it has both decayed, and not decayed, at the same time. But it doesn’t stop there: quantum mechanics also predicts that if the atom has both decayed and not decayed, then this leads to the poison being released, and not released, at the same time. In turn, quantum mechanics predicts that the cat will be dead, and alive, simultaneously!

What do you think would happen if you were to open the box and look at the cat? Would you see the cat as being both dead and alive simultaneously? The answer is of course no – a large object such as a cat has never been seen in such a bizarre state. But why not? Quantum mechanics predicts that the cat can be dead and alive, and quantum mechanics has never been proved wrong. There seems to be a paradox here! But we need not fear, because there are a range of different theories that solve this riddle, and I will introduce some of the main theories below. Bear in mind that none of these theories have yet been proved wrong, and so you are free to choose whichever theory you like…

__Collapse theories__

Perhaps the most intuitive explanation is to say that the description above is not quite correct, and we must add an additional rule that prevents objects such as cats from having multiple properties, such as being dead and alive, simultaneously. In other words, quantum mechanics must be modified slightly, and once this is done it will better fit with our view of reality. So how exactly should we modify quantum mechanics? What should this new rule look like? There are different theories of precisely what this rule is, but they all involve the idea of the quantum state “collapsing”. Using the example above, they say the cat cannot be dead and alive simultaneously, and therefore the state of the cat must “collapse” into being either dead **or** alive, but not both. Now, however the collapse works, we know from experiments that small objects such as atoms *can* have multiple properties simultaneously, so the collapse does not happen at this scale. So what are the main differences between cats and atoms that mean that the cat collapses but the atom doesn’t?

**Gravity causes collapse.** Cats are vastly more massive than atoms. One collapse theory, developed by Roger Penrose and others, exploits this to say that gravity causes collapse. Specifically, the more massive an object is, the more likely it will collapse. This theory says that atoms are small enough so that they can have multiple properties, for example having decayed and not decayed, simultaneously. This is precisely what we see in experiments. However, the cat is so large that, with near certainty, its state will collapse into being either dead or alive.

**Complexity causes collapse.** The main theory of this sort is known as the Ghirardi–Rimini–Weber theory, and it is actually quite similar to gravity causing collapse. It basically says that the more particles an object is made of, the more likely it will collapse. A cat is made of many many particles, and therefore, again with near certainty, its state will collapse into being either dead or alive.

**Consciousness causes collapse.** Now, we don’t actually have a universally agreed upon definition of what consciousness is, and so the theory that consciousness causes collapse is far from being precisely formulated. It of course depends on precisely which creatures (or artificial intelligences!) are said to be conscious. Many would agree that a cat is conscious, and this theory would roughly then say that such a conscious creature cannot have multiple properties simultaneously, and therefore its state will collapse into being either dead or alive. However, if you think that a cat is not conscious, or you replace Schrödinger’s cat with Schrödinger’s microbe (or something else you deem to be not conscious), then this theory would predict that whatever is in the box *is* dead and alive simultaneously. Only when you open the box, and your consciousness interacts with its contents, would the state collapse, and you would be left in either an elated state of seeing an alive creature, or a devastated state of seeing a dead one.

The theory that consciousness causes collapse is perhaps the most compelling, and to some people the most intuitive, explanation of why we never see the cat as being dead and alive simultaneously. However, as I argue here, the picture of reality that this theory predicts is fantastically bizarre and obscure, and far from intuitive!

__Many worlds theory__

The collapse theories introduced above all modify quantum mechanics in some way, and by doing this they can explain why we never see a cat that is simultaneously dead and alive. But is it really necessary to modify quantum mechanics? According to many worlds theory the answer to this is no. However, as explained above, unmodified quantum mechanics predicts that the cat is dead *and* alive, so there is clearly some explaining to do to unify this prediction with our view of reality.

Before opening the box, the cat is dead *and* alive. Technically there is only one cat, which is simultaneously dead and alive. But the great insight of Hugh Everett, who first proposed this theory, was that we should actually treat it as *two cats*, one dead and one alive. Can we really do this? To show that we can, some calculations need to be done, in particular using a framework known as decoherence, but this is too technical to introduce now; see __here__ for an introduction to decoherence in the context of many worlds. The important conclusion from these calculations is that the dead and alive cats can never interact with each other: the alive cat cannot see the dead one, and it can’t smell it nor touch it; as far as it is concerned the dead cat need not exist. For this reason, the usual terminology is that there are two “worlds”, one containing a dead cat and one containing a living cat. This is the idea of “many worlds”. Another way to put it is that there are two parallel universes, with one cat occupying each. But whatever terminology you like to use, the important point is that it is completely consistent within quantum mechanics to say that both cats are equally real, and for all intents and purposes they exist isolated from one another.

What then happens when you open the box? The answer is that you split into two versions of “you”, one that sees the alive cat, and one that sees the dead cat. Again these two versions of you can never interact, and have no way of measuring each other’s existence. They are, for all practical purposes, in separate parallel universes.

Many worlds theory in fact predicts that our reality is almost continuously splitting into multiple parallel universes. In each parallel universe there will be a different version of you. There will be almost infinite versions of you, each going about their day oblivious of all the others. This may seem completely far-fetched, but just because something is not at all intuitive does this mean that it is wrong? It used to be considered absurd that the world is round, or that the universe is vastly larger than our solar system, or that our bodies contain billions of microscopic organisms without which we couldn’t survive.

__QBism – what do our quantum mechanical equations really tell us?__

Are we looking at all this in completely the wrong way? Imagine the state of the cat before the box is opened. Using quantum mechanics, it is in principle possible to write down an equation representing the state of the cat. What would this equation really tell us? In many worlds theory, and indeed in most ways of thinking about quantum mechanics, this equation tells us *what state the cat is in*. Specifically, we are assuming that the cat *does exist*, and that our equation tells us something about it.

But we can take a different perspective of what this equation represents. To see this, we can ask the question: what do we normally *use* this equation for? The answer is that we use this equation to tell us the *probability* that, when we open the box, we will see an alive cat. We cannot use the equation to tell us with certainty whether the cat will be alive or dead – it only ever tells us the probability. For example, it will be possible to set up the thought experiment so that there is a 50% chance of seeing an alive cat once the box is opened, and a 50% chance of seeing a dead cat. Now, over 100 years of experiments have shown that quantum mechanics is extremely good at predicting the probabilities of different events happening in experiments. In fact, as quantum mechanics has never been proved wrong, it is so far perfect at predicting probabilities of outcomes to experiments. Therefore, we know the probability of what will happen when we open the box, and repeating the experiment many times would indeed show that half the time the cat was alive, and half the time the cat was dead.

But what makes us think we know what is happening inside the box before we open it? One way of looking at quantum mechanics, which is often called “QBism”, is to say that our equations do not directly tell us what happens inside the box before we open it. The equations just tell us the probabilities of different events happening. In particular, our equations *don’t* directly tell us that the cat does exist, and that it is both dead and alive simultaneously. The same can be said for all other quantum experiments. For example, when we measure a radioactive atom we can use quantum mechanics to calculate the probability that it will decay. And with today’s simple quantum computers we can calculate the probability that, given a certain input, we will measure a certain output. But our equations do not tell us the state of the atom or the quantum computer before the measurement.

This way of thinking about quantum mechanics has similarities to the question* if a tree falls in the woods with no one around, does it still make a noise?* If we replace the tree with the cat, and the woods with the box, then the QBism answer is that we cannot know anything about the cat before we open the box! Normally, we think of science as telling us something about a real world independent of us, that still exists regardless of our presence in it. QBism takes a different view: quantum mechanics is just a toolbox for predicting probabilities of events.

To many this will seem like a limited view, or perhaps a pessimistic view of the capabilities of science. But how do we really know what happens before we observe/measure anything? The extreme version of this viewpoint says that we can never truly know anything other than our own conscious thoughts. How do we know we aren’t in the matrix? How do we know that the signals entering our brains aren’t just fed into us? The much more conservative version of this view is that a real world *does* exist independent of us, but quantum mechanics doesn’t tell as anything about it. Either way, the riddle of Schrödinger’s cat is no longer a problem: Is the cat really dead and alive before we open the box? The answer is that we do not, and cannot, know. It is a meaningless question!

__“Shut up and calculate”__

Still unsatisfied? Are you not willing to modify a theory that has never been proved wrong? Or believe in almost infinite parallel universes containing almost infinite versions of you? Or is it unsatisfactory to reject the *existence* of things before we measure them? There are some other ways of looking at quantum mechanics which I haven’t mentioned, such as pilot wave theory or relational quantum mechanics, but in my view each of these has significant overlaps with some of those introduced above. Therefore, if you completely reject *all* of the above viewpoints, then maybe you are destined to never be satisfied!

But is this really a problem? Quantum mechanics works, and it works extremely well. It is often stated as being “our most successful theory ever”, owing to the extremely precise predictions of quantum mechanics that have been vindicated, and the vast number of successful experiments over the past 100 or so years. One further viewpoint, then, is that we shouldn’t care whether the cat is dead, or alive, or both. Instead of being distracted by parallel universes and bizarre thought experiments, we should focus on using quantum mechanics better. This is particularly relevant at the moment: the “quantum technology revolution” is making great headways towards fulfilling its promise of transforming future technologies. Quantum cryptography is said to make communication 100% secure; __quantum metrology__ promises to make ultra-precise measurements allowing us to investigate previously-inaccessible phenomena; and quantum computers have the potential to exponentially speed up our computations, thereby revolutionising the whole computing industry. Should people like me therefore stop quibbling about philosophical obscurities, and knuckle down to the real business. Indeed, should we *shut up and calculate*?

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The question arises in my mind: who decides, what decides which individual is going to see the alive cat/ the dead cat? Could we somehow influence this choice?

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I guess that depends on which interpretation you are considering. In collapse theories, it is random whether you see an alive or dead cat. But in many worlds, before opening the box there is one version of you, but after opening there are two versions of you. Both versions share the same history and have the same memories prior to opening the box. One of these versions will be thinking “I wonder why it is ME that sees the dead cat. Why did I not see the dead cat? What influenced this?” and the other will be thinking the opposite. But these are meaningless questions in many ways, and have somewhat trivial answers: The version that sees the alive cat did not see the dead cat because they saw the alive cat! And nothing influenced the fact that they saw the alive cat — there was always going to be one version of you that saw the alive cat. It’s a bit like asking “Why do I experience my experiences, rather than someone else’s?” If you experienced someone else’s experiences then you wouldn’t be experiencing your experiences any more, so you would no longer be YOU anyway!

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