Escaping from quantum weirdness by playing cards

Quantum physics can be seen as a victory for common sense. Surely it is worth discarding the old ideas that make people think the opposite. 

A quantum card trick 

Many card games, if played fairly, are simply games of chance, of probabilities. Quantum physics usually gives results in terms of probabilities, but these are given exotic names like wavefunctions and superpositions, and are presented as things beyond our everyday experience. Are the probabilities we find in quantum physics anything like the ones we find playing cards? The answer is yes, but only after a small trick has been played. 

Consider the top card in a shuffled pack – it has a 1 in 52 chance of being the ace of spades. Before the card is dealt we don’t know what it is, we just know the probability.

We need to be clear with what we mean here by shuffling, in case you worry later on that it is open to sleight of hand. Shuffling here just means reordering the pack in a way that, for practical purposes, is unpredictable. If you like, you can imagine a machine that blows the cards around in turbulent air for a few minutes before collecting them together again as a pack.

Common sense tells us that the identity of the first card in a shuffled pack is already settled before the card is dealt, as a result of the shuffle – it is just that we don’t know what it is until we see it. This is a classical or “local hidden variables” view of the probability.

An alternative view could be that the first card in the shuffled pack, before it is dealt, is in some way a mixture of all 52 cards, and that it only turns into a specific card when it is first seen. This seems like a “quantum” view of the situation, where each card in the shuffled pack is in a “superposition” of 52 possible states. It is undoubtedly a very strange view to take, which we would nearly all agree is wrong. So far, then, our example of a pack of cards supports the idea that the probabilities we meet in quantum theory represent something weird compared to those we encounter in everyday life.

At this point some explanations try to argue that a shuffled pack of cards is a very large collection of atoms, and this somehow makes it fundamentally different from a simple quantum situation with a few elementary particles, so that different rules apply. But instead we will perform our small trick.

Suppose we now consider the top card in a shuffled pack before the shuffling has taken place. We have a pack of cards that is to be shuffled in the future, but we need to describe it, as it will be after the shuffle, now. The top card will still have a 1 in 52 chance of being the ace of spades, but this time, in a sense that we can all understand, it will be no particular card, but 52 possibilities all at the same time. Or to put it another way, our best description for the location of the ace of spades is in 52 different places at the same time. It is a very simple illustration of the type of probability we encounter in quantum physics. Of course no one has ever seen a pack of cards after it has been shuffled in the future, but that does not mean it is a weird or even a particularly difficult concept to grasp. If we describe the measured-but-not-yet spin of an electron as pointing in two directions at the same time, it is supposed to be weird, but if we say that the ace of spades in a shuffled-but-not-yet pack of cards is in 52 places at the same time, it is no big deal. 

A common sense view of the future 

This brings us to a very simple idea, which is that there are many possible versions of the future, but only one version of the past. This idea is not a proven fact, and is the subject of ongoing philosophical debate, but the point I wish to make is just that it is both a valid and a desirable idea, and that it feels like common sense. If it is true, there must be a fundamentally unpredictable process that sorts out the one version of the future that happens from the ones that don’t. If this process was predictable, even in principle, there would not be many possible versions of the future, just one possible one and some other impossible ones. 

Perhaps the most persuasive arguments come from personal experience, but we can also look to the non-human world for supporting evidence. The existence of many possible futures is closely related to the idea that there is some randomness in the processes that determine the course of events. There are many examples from physics at the atomic and subatomic level, such as radioactive decay or the motion of a particle as it passes through a narrow slit (known as diffraction), where the results appear to have a genuinely random element. 

As another example, in round numbers there are 350,000 known species of beetle in the world, with no doubt many more unknown ones. Why this many? To me this is very plausible in a world where the process of evolution contains a haphazard element which has thrown up variations on the beetle theme at random. It is much less plausible that every detail of each species (and indeed each individual beetle) followed inexorably from how matter was set in motion at the beginning of time, and could not have been different in even the slightest way. And, incidentally, if the process has instead been guided by an intelligence, it must be an intelligence, as J B S Haldane put it, with “an inordinate fondness for beetles”.  

At the human level, the “many futures” idea seems to this human to be virtually unassailable. We feel instinctively that we make choices that affect the future, that we ought to behave in a certain way, that events could have turned out differently, that sports fixtures could have had different results. If someone says they are compelled to act the way they do by forces beyond their control, it is not a healthy sign. Societies set up moral and legal codes on the basis that people’s future behaviour can be influenced, and would not have been influenced otherwise. It is possible to argue that everything that has ever happened and everything that will happen is inevitable but nearly all of us live our lives on the basis that this is wrong. 

Classical (pre-quantum) physics only ever predicted one version of the future, which should have been seen as a major obstacle to any claim for it to describe the real world. Quantum physics, on the other hand, gives us many possible outcomes, which seem very like many versions of the future. This should be a cause for rejoicing, but curiously it is usually seen as part of its weirdness. It seems weird because we have no experience of many possibilities existing at the same time, but perhaps the reason for this is almost too obvious to put in words – it is because none of us has direct experience of the future.  

Another common sense 

Physicists have been very reluctant to pursue the “many futures” idea. In my view this is because a rival, contradictory and ultimately wrong version of common sense has dominated people’s thinking. This common sense is based on the metaphysical idea that nothing happens without being caused, that nothing happens for arbitrary reasons.

This seems very natural, and it provided common ground for a kind of unholy alliance between physics, philosophy and theology. In physics, the idea had been enormously successful in guiding classical theories, so that it seemed heretical to deny it, like advocating a return to the dark ages before Newton. Elsewhere, it seemed a necessary idea if some sort of order was to reign in the world – divine or otherwise – and, after all, didn’t it show how in touch you were with the successes of physics. The fact that the idea means there is only one possible future, and is therefore contrary to our previous line of common sense, was either ignored, seen as a secondary issue to be sorted out later, or seen as too bad for common sense. 

One major figure in the interpretation of quantum theory was Albert Einstein. Einstein was undoubtedly a scientific genius, with several other outstanding qualities including a rare ability and willingness to express his thoughts clearly. Being a genius did not of course mean that he was always right, so it is only to be expected that there were times when he said things that were clearly wrong. He was firmly in the “no arbitrary events” camp, which he expressed by his belief that “God does not play dice”. He gave another clear example of his view in a letter to the relatives of a close friend who had just died, a few weeks before Einstein’s own death in 1955. He wrote that “people like us, who believe in physics, know that the separation between past, present and future is only a stubbornly persistent illusion.” I would like to suggest that as well as reflecting his personal feelings at a difficult time, this was the view of a man deeply attached to “classical” common sense when quantum physics was showing the way to a much better version of common sense – that the future is quite different from the past because there are many futures but only one past. With hindsight, he was confusing the playing field (space-time), which has no distinction between past and future, with the events taking place on it. 

“Creating our own reality” 

There is a vague but popular view that quantum physics “lets us create our own reality”, and it is worth following this line of thought a little. If there are many futures but only one past, something very strange must happen in the present, when only one of the possible futures is adopted. In that sense, reality is continually being created where it was absent or at least blurred beforehand. And if we believe, as most people do, that we have some freedom of choice over our own actions in the present, we believe we can, to some extent, influence reality as it is created. So at least to that extent, “creating our own reality” is a remarkable but everyday phenomenon that was around long before anyone thought of quantum physics. Perhaps this popular view of quantum physics should be expressed in a subtly different but less pretentious way, like “quantum physics lets us choose what we have for breakfast.”  

Quantum physics seems unacceptably weird only when we stick with a “common sense” idea that worked well for classical physics, but which another “common sense” idea, closer to our instincts and experience, tells us is wrong anyway. The idea that quantum physics in fundamentally incomprehensible is attractive to a wide range of people, but this should not become an excuse for failing to move on.

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