Why do we never see the weird world that underpins the everyday world? Because we never observe it – we only ever observe ourselves!
We have found a strange footprint on the shores of the unknown. We have
devised profound theories, one after another, to account for its origins. At last,
we have succeeded in reconstructing the creature that made the footprint.
And lo! It is our own.
Arthur Eddington, Space, Time and Gravitation, 1920
We don’t see things as they are, we see things the way we are.
David Mitchell, Number9dream, 2001
The football sits on the penalty spot. The crowd in the stadium is hushed. The player with the hopes of his team mates on his shoulders looks up, takes a deep breath and runs towards the spot. His boot connects with the football. It loops to the left. It loops to the right. It scythes along the ground. In fact, it flies along a thousand different trajectories as if a thousand different penalty takers have struck a thousand separate footballs. The kicker jumps with joy, the kicker drops to his knees in despair; the ball thwacks into the back of the net, the goalkeeper palms it safely away; the crowd roars, the crowd groans.
What is going on? This is not reality. No, it is not. But the question is, why not? It sounds like a totally ridiculous thing to ask. But, actually, it is entirely sensible. In fact, it is just about the most profound question that can be asked of reality.
You see, everything, including a football, is made of atoms. They are the Lego bricks from which the world is assembled. And it has long been known that an atom flying through space really does fly along multiple trajectories simultaneously (it sounds unbelievable – but there you are). So, when a football is kicked, it ought to follow multiple paths through the air like the atoms from which it is made. But it does not. Why? Or, to put it another way, why is the world of atoms – the world that underpins our world – so different from the world of footballs and people and planets? In short, where does the everyday world come from?
Remarkably, it is only in the last decade or so that physicists have zeroed in on the answer to this fundamental question. And the answer, it turns out, is both surprising and deeply, deeply subtle.
To understand how it is that an atom can do many things at once – like fly along multiple trajectories through space – you first need to know some pertinent history. In the course of the past century, a multitude of laboratory experiments have shown that atoms and their ‘subatomic’ constituents, such as electrons, have a peculiar, schizophrenic nature. They behave not only as ‘particles’ – localised objects like microscopic billiard balls – but also as ‘waves’ – smeared-out entities like ripples on a pond. The twist is that the wave associated with an atom is not a tangible thing like a wave on the surface of a body of water; it is an abstract, mathematical thing. Nevertheless, it can still be imagined spreading through space just like a real wave. There is even an equation which predicts exactly how the wave propagates. It is called the Schrödinger equation, after the Austrian physicist Erwin Schrödinger.
Since the wave associated with an atom is an abstract thing, it might be expected that it is of no practical consequence. Nothing could be farther from the truth. The wave does indeed make contact with reality. It does it through its height, or ‘amplitude’, which the Schrödinger equation allows to be calculated at any location in space. In fact, the important thing is the ‘square’ of the wave’s amplitude. This quantity turns out to be the chance, or ‘probability’, that the atom will actually be found at the location if anyone cares to take a look. For instance, the probability of finding it at one location might be 25 per cent and 5 per cent somewhere else.
Already, this highlights a dramatic difference between the large-scale, everyday world and the small-scale world of atoms. Whereas you can know with 100 per cent certainty where, for instance, your car is parked – unless it has been towed away, it is around the corner where you parked it last night – prior to looking, we can never be exactly sure where an atom is located. All we can know is that it has a certain probability of being over here, a certain probability of being over there, and so on (the moment an atom is ‘observed’, however, it is pinned down to one place, and one place only, and all the possibilities that existed prior to the observation cease to exist).
Not knowing where an atom is with certainty turns out to be only one of the many peculiarities of the Alice-in-Wonderland world of atoms. Not only is it impossible to know with absolute certainty where an atom is located, it is impossible to know with certainty what it is doing. There is no way, for instance, to know which path an atom will take as it flies through space – only that it will take one path with a particular probability, another path with another probability, and so on. The fact that atoms can behave like waves as well as particles also has other profound consequences for the microscopic world. These follow unavoidably from the fact that everything that real waves can do, so too can the abstract waves associated with atoms. Take the tendency for waves on water to spread out with time. The abstract waves associated with atoms do the same thing. It means that, in the microscopic world, the longer you wait, the greater is your uncertainty about where an atom actually is. But the property of real waves which has the most shocking ramifications in the microscopic world is a less well-known one – their ability to combine with each other to form composite waves.
Anyone who has watched the sea knows that you can get big, rolling waves, and you can get small ripples, caused by the breeze. They will also know that you can get a combination of the two – a big, rolling, wave with small ripples superimposed on it. This turns out to be a general property of waves of all kinds. If two or more types of wave are possible, then a combination, or ‘superposition’, of the waves is also possible.
In the everyday world, this is a mundane observation. However, in the world of atoms and their constituents – the quantum world – it has consequences which are … well … earth-shattering. Consider, for instance, a quantum wave that is big in one particular location – that is, there is a high chance of finding the atom there if you look. Now, consider a quantum wave that is big in some other place. In other words, the atom has a high probability of being found in the other place. Well, if these two waves are possible, then it follows that a combination is also possible. Nothing too remarkable about this, you might think. Until it dawns on you what such a superposition in fact represents: the atom literally being in two places at once – the equivalent of you being in New York and London at same time!
This is no theoretical fantasy. It is actually possible to observe an atom being in two places at once – or, to be more precise, the consequences of it being in two places at once. There is a famous physics experiment in which atoms, or other microscopic particles, are fired like tiny bullets at an opaque screen with two close-together vertical slits cut in it. In the ‘double slit’ experiment, the quantum waves representing the atoms go through the slits and mingle with each other on the far side of the screen. Where the crests of the two waves coincide, the waves are boosted, and where the crests of one coincide with troughs of the other, the waves cancel each other out. This reinforcing and cancelling phenomenon is known as ‘interference’ and is common to all types of waves.
Because of interference, when a second screen is placed in the path of the mingling atoms, there will be places on the screen where the quantum wave associated with them is big and places where the quantum wave is non-existent. These will correspond to places where lots of atoms hit the second screen and places where no atoms at all hit the screen. If an atom makes some kind of black mark where it hits the screen, the result will be a pattern of vertical black-and-white stripes not unlike a supermarket bar code.
The crucial prerequisite for such an ‘interference’ pattern is that two things mingle – in this case, the waves emerging from one slit with the waves emerging from the other. Remarkably, however, the pattern on the second screen still builds up even if the atoms are fired at the first screen one at a time, with long intervals of time in between. The unavoidable conclusion is that each atom mingles with itself. In other words, it goes through both slits simultaneously – it is in two places at once.
And, if being in two places at once is not bad enough, there is worse. The quantum wave associated with an atom represents more than simply the probable location of the atom. It represents everything that nature permits us to know about the atom such as its speed, its energy, how it might be spinning and so on. Consequently, the fact that a superposition of quantum waves is possible does not simply mean that an atom can be in two places at once. It also means it can do two things at once – the equivalent of you walking the dog and washing the car at the same time.
If things in the everyday, large-scale world behaved like microscopic, quantum particles, a skier, faced with a tree blocking their path, would go both ways round it at once. And, as pointed out, a football struck from the penalty spot would fly through the air along all conceivable paths.
So, how is it that atoms and their like can be in many places at once and do many things at once whereas skiers and footballs – which are merely large assemblages of atoms – cannot? Why do we not ever see a person walking through two doors simultaneously? Or witness a superposition between a giraffe and a zebra?
For a long time, the standard explanation of why we do not see weird superpositions was provided by the so-called Copenhagen interpretation of quantum theory. This says that, when an atom is not being ‘observed’, its associated quantum wave spreads through space according to the edict of the Schrödinger equation. The atom has a certain probability of being over here, a certain probability of being over there, and so on. As soon as the atom is observed, however, things change abruptly. Somehow – and nobody knows quite how – the ‘act of observation’ forces the atom to stop misbehaving and plump for being in one location with 100 per cent probability. In the jargon, it ‘collapses’ the quantum wave down to a single possibility, in the process destroying all other possibilities.
In essence, the Copenhagen interpretation says that quantum theory applies except where it does not. And where it does not is when an ‘observation’ is made.
To say the Copenhagen interpretation is unsatisfactory is a bit of an understatement. For a start, the collapse of the quantum wave is a phenomenon tantamount to magic. Physicists put it into the theory in an ad hoc way and it appears to happen instantaneously – that is, in no time at all. This is of course ridiculous. Nothing in the real world happens in no time. As quantum physicist Anton Zeilinger of the University of Vienna says: ‘The speed of collapse is bull.’
Even worse, the Copenhagen interpretation does not even specify what exactly constitutes an ‘observation’. Does an atom have to be observed by a mindless particle detector or by a conscious human being? If the latter is the case, then it could be said that the everyday world of certainty does not exist without conscious observers to observe it. ‘Does the Moon exist if nobody looks at it?’ asked Einstein. If you believe the most extreme form of the Copenhagen interpretation, the answer is no! Which is why Einstein asked the question – he hated the Copenhagen interpretation of quantum theory and missed no opportunity to highlight its inadequacies.
A less extreme form of the Copenhagen interpretation says the everyday world of certainty comes about when a big thing observes a small thing. The everyday world of big things like canon balls flying through the air and planets circling the Sun is said to obey the laws of ‘classical’ physics, which are basically laws like Newton’s that predict what will happen in any given circumstances with 100 per cent certainty. According to the Copenhagen interpretation, then, the everyday world is created when a classical object observes a quantum object.
But what exactly defines a classical object? Does it have to be a collection of billions upon billions of atoms or just a few hundred? The Copenhagen interpretation is tight-lipped on this, which leaves physicists decidedly uncomfortable.
One thing it is impossible to deny is that quantum theory is successful. In fact, it is arguably the most successful scientific theory ever devised. It has predicted the outcome of all atomic and subatomic experiments to a stunning level of precision. And it has literally made our modern world possible, not only giving us lasers, computers and nuclear reactors but an explanation of why the Sun shines and why the ground beneath our feet is solid. This has left physicists in little doubt that quantum theory is a deep and fundamental theory.
But, surely, if a theory is really deep and fundamental, it should apply to everything in Creation – not only to the world of small things like atoms but also to the world of big things like footballs? Why then is the microscopic world, which dances to the tune of quantum theory, so different from the everyday world of trees and planets and people?*
The answer, it turns out, is very subtle. It stems from a crucial observation about the world: we never actually directly see a quantum thing like an atom or a photon of light. What we see instead is its ‘effect’ on some kind of detector or even the retina of our eye. What we observe, in other words, is not the quantum object itself but the ‘record’ that the quantum object leaves on a large number of other atoms.
‘A detector does not measure an exterior system directly, but rather, through an act of observation, changes the state of its own system,’ says the Belgian physicist Sven Aerts. In the case of the eye, for instance, light falls on the cells of the retina and changes them – and it is these changed cells that the brain senses, not the light itself. We think we are directly observing light but we are in fact only observing it indirectly. It is ourselves that we are observing directly. ‘All observation is self-observation,’ says Aerts.
The American physicist Wojciech Zurek puts it more lyrically. ‘What the observer knows is inseparable from what the observer is,’ he says.
In the light of this – no pun intended – it is now possible to further sharpen the original question, ‘Why is the everyday world so different from the microscopic world of atoms?’ We can now say ‘Why does the “record” of a quantum object like an atom not show any signs of weird schizophrenic quantum behaviour?’
The American physicist Larry Schulman of Clarkson University likes to answer this question with the example of the ‘cloud chamber’. This is an ingenious device that creates a record of the passage of a microscopic particle such as an atom. Crucially, the record is substantial enough to be visible to the eye, so the question of how the record of a quantum event loses its quantumness is no longer academic. It is a nuts-and-bolts question which requires a nuts-and-bolts answer.
Essentially, a cloud chamber is a sealed box containing water vapour with an observation window in its side. Now, water vapour, if cooled, condenses to form visible droplets of water. This is of course what happens when a cloud – a very large volume of water vapour – is forced to rise over a hill or mountain in its path. Because the air is cooler at higher altitude, water vapour condenses into droplets, which fall as rain. However, the formation of a water droplet requires more than simply the cooling of water vapour. There must also be a ‘seed’ around which the water droplet can grow.* In the atmosphere, the necessary seeds are provided by tiny motes of dust floating in the air. If no such seeds are around, it follows that it is extremely difficult for droplets to form. This observation, it turns out, is the key to the operation of the cloud chamber.
The cloud chamber is filled with water vapour that is ultra-pure. It is so ultra-pure, in fact, that there are no seeds around which droplets can form. This means that, when the water vapour is cooled, water droplets are absolutely desperate to form but they cannot. They are utterly frustrated.
Think of the water vapour in a cloud chamber as like a minefield in which all the mines are on the very brink of exploding. In such circumstances, the tiniest breath of air or vibration of the ground will be enough to set the mines off. Similarly, in the cloud chamber, the tiniest thing will be enough to act as the necessary seed for the explosive formation of a water droplet. And ‘tiny’is the operative word here. For, remarkably, the seed can be something as small as a single quantum event – for instance, the collision of a high-speed atomic particle with a water ‘molecule’.*
Imagine, then, a high-speed atomic or subatomic particle passing through the cloud chamber. It could be a particle fired deliberately into the chamber by an experimenter or perhaps a ‘cosmic ray’ particle which has come down from space. As it zips through the chamber, the particle will smash into water molecules in its path, kicking free their orbiting electrons. A molecule bereft of some of its electrons is said to be ‘ionised’ and an ionised molecule, it turns out, is sufficient to act as the necessary seed for the formation of a water droplet. Consequently, a stream of tiny water beads will mark the track of the particle through the cloud chamber. If illuminated properly, the stream will be visible to the naked eye through the window in the cloud chamber.
The cloud chamber was in fact the first device ever built that could reveal the ‘tracks’ of subatomic particles such as electrons. It was invented by the English physicist Charles Thomson Rees Wilson in 1911. Intrigued by the peculiar clouds that formed above the peak of Ben Nevis in Scotland, he had attempted to create artificial clouds in his laboratory in Cambridge. The fruit of his efforts was the cloud chamber, for which he shared the 1927 Nobel Prize for Physics.
For Wilson it was sufficient that he had miraculously made the atomic realm visible. This was in the days before there was a full appreciation of the bizarre nature of the quantum world so he did not get hung up on the details of precisely how the cloud chamber worked. Nowadays, however, understanding the way in which the droplets form is the key to understanding how the familiar, everyday world emerges from the bizarre, schizophrenic world that underpins it.
Imagine a water-vapour molecule sitting in the path of the high-speed particle flying through the cloud chamber. The particle can either ionise it or not ionise it. This is obvious and uncontroversial. But we are dealing with an event in the quantum world. And this means that, in addition to there being a quantum wave representing the molecule being ionised and another quantum wave representing the molecule not being ionised, there is a third possibility – a superposition of the two waves. In other words, the water molecule can be simultaneously ionised and un-ionised.
By rights, a water molecule that is simultaneously ionised and unionised ought to trigger a curious water droplet – one that hovers half in existence and half out of existence. A water droplet that, like the smile on the face of the Cheshire cat, is only half there.
This is pretty much a real-life enactment of the famous ‘Schrödinger’s cat’ thought experiment. The Austrian physicist Erwin Schrödinger imagined an unfortunate cat sealed in a box with a vial of poison which could be broken by a hammer. The hammer was triggered by the disintegration, or ‘decay’, of an unstable atom which had a 50 per cent chance of decaying during the experiment. Since quantum theory permits an atom to simultaneously decay and not decay, Schrödinger’s question was: ‘Before the box is opened and the cat is observed to be either dead or alive, was it simultaneously dead and alive?’
Schrödinger’s thought experiment elevated an event in the quantum world into the everyday, large-scale world, forcing physicists to seriously address the crazy implications of their theories. However, it was hardly an experiment that could actually be done (at least, without upsetting legions of cat lovers). Schulman’s cloud chamber, though not as striking as Schrödinger’s cat, at least has the merit of being a real, nuts-and-bolts, practical device.
So, what of a water droplet that hovers half in existence and half out of existence? It goes without saying that nobody has ever seen such a schizophrenic water droplet while looking through the window in a cloud chamber. But why? Where does the quantum weirdness go?
Well, in order to see weird quantum effects it is fundamental that the individual waves of a quantum superposition overlap each other. If they do not overlap, then they cannot interfere with each other and it is through interference that quantum weirdness occurs.
In the case of the double-slit experiment, for instance, it is never possible actually to observe an atom going through both slits at the same time. Nature, it turns out, is extremely careful to conceal its hand. All we can ever see is the ‘consequence’ of the atom going through both slits simultaneously – that is, the black-and-white-striped interference pattern on the second screen. In the double-slit experiment, this is the quantum weirdness – and it happens only if the quantum wave representing the atom going through one slit and the quantum wave representing the atom going through the other slit interfere with each other. Which they can do only if they physically mingle – that is, pass through the same region of space, or overlap.
So, does the quantum wave that represents a water droplet that exists and the quantum wave that represents a water droplet that does not exist overlap? This is the key question. If there is indeed an overlap, we should see that a water droplet hovering half in existence and half out of existence can exist. If not, we should never see such a bizarre vision.
The only way to answer the question is to imagine the individual water molecules that make up a water droplet. Say, for the sake of argument, that there is a water-vapour molecule that is painted red – forgetting for a moment that it is impossible to paint a molecule red! – and that it is ionised by the particle flying through the cloud chamber. Now focus on a nearby water-vapour molecule. Water is denser than water vapour, which means the molecules are closer together than they would be in the vapour. So, if the red molecule is ionised and a water droplet forms, the nearby molecule will be closer to the red molecule than it would be if the red molecule is not ionised and no droplet forms. Say the overlap between its quantum wave in the first and in the second case is 50 per cent. This is still enough for the waves to interfere.
Now consider a second molecule near the red one. Say, once again, the overlap between its wave in the case when the red molecule is ionised and in the case where the red molecule is not ionised is 50 per cent. What this means is that, if we consider the two molecules together, the overlap between their combined waves is ½ × ½ = ¼. If we consider three molecules, it will be ½ × ½ × ½ = ⅛.
Now – and this is crucial – a water droplet big enough to be visible to the naked eye might contain anywhere from millions to billions of molecules. In other words, the overlap between the wave representing the water droplet existing and not existing is ½ × ½ × ½ … millions or billions of times over. Clearly, this will be a number within a whisker of zero. Essentially, therefore, there is no overlap between the quantum waves. And, if there is no overlap there can be no interference.*It means that, when we look through the window of a cloud chamber, we will always see the formation of a water droplet or the formation of no water droplet. We will never see a ghostly water droplet, hovering half in existence and half out of existence.
Here then is the super-subtle explanation for why the schizophrenic events of the quantum world never manifest themselves in the everyday world. To do so, they would have to create a visible record, which in practice means affecting millions or billions of molecules. However, as the cloud chamber example illustrates, it is immensely difficult to have a quantum superposition which involves millions or billions of molecules. By the time large numbers of molecules are involved, the quantum waves in the superposition simply will not overlap, the necessary prerequisite for interference and all weird quantum behaviour.
Physicists have a special word for quantum waves which overlap and which are therefore capable of interfering with each other. They say they are ‘coherent’. When a quantum object is recorded by a large number of atoms in a cloud chamber or in the environment, this coherence gets lost. The process, not surprisingly, is called ‘decoherence’ and it is the ultimate reason why we never see chairs or people in two places at once. It is decoherence that creates the familiar world around us from the nonsensical microscopic world that lies beneath.
The example of the cloud chamber no doubt seems esoteric and far removed from everyday experience. However, in essence, the cloud chamber does something very similar to an electronic particle detector or the human eye. For instance, the eye boosts, or ‘amplifies’, a quantum event – the arrival of a photon of light.* And it does so by letting the event impress itself on a large number of atoms. Inevitably, by the time large numbers of atoms are involved, the crucial overlap between the individual waves of the quantum superposition is lost. Hey presto. Even in the eye, the weird, schizophrenic quantum world is replaced by the well-behaved, everyday classical world.
It is a common belief that quantum theory applies only to the microscopic world of atoms and their constituents and not to the everyday world of big things. But decoherence shows that this is a complete fallacy. In reality, quantum theory is a fundamental theory that applies to all aspects of the Universe. It is simply that its most peculiar aspects are never apparent in the everyday world. They are hidden by the obscuring hand of decoherence.
The instant a microscopic object impresses itself on its environment – be it a cloud chamber droplet or the human eye – the quantum weirdness gets irretrievably lost.† The environment simply fails to record its schizophrenic nature. Think of an individual at a football game whose voice leaves no discernible impression on the roar of the crowd. That is kind of the way it is with quantum events.
It is often said that a quantum superposition is a terribly fragile thing and that, when it interacts with the environment, the environment destroys it. In fact, this is back to front. A quantum superposition is a pretty robust thing. However, when it interacts with its environment, the environment has enormous trouble recording the superposition among its vast number of ‘degrees of freedom’. It is actually the environment’s ability to record the superposition that is a fragile thing.
The environment need not be anything as esoteric as a cloud chamber or even as biological as an eye. It can simply be the surrounding Universe. The Moon, for instance, is continually impressing itself on its environment because photons – particles of sunlight – are constantly bouncing off its face and flying away into the night. Sooner or later, they impress themselves on everything in the environment from the Earth to the distant stars to the human eye. And it is in these bodies – large collections of atoms – that any weird quantumness of the Moon is lost.
Einstein asked: ‘Is the Moon there when nobody looks?’ The answer is yes – because the presence of the Moon is continually being recorded by the Universe itself.
Because coherence – which leads to quantum weirdness – is lost when an object impresses itself on its surroundings, it follows that quantum weirdness is a property of objects that are isolated from their surroundings. It is the isolation that is the key. Contrary to the popular view, quantum weirdness turns out to have nothing to do with an object being big or small. If it were possible to take a big object and successfully isolate it from its environment, it would continue to behave like a weird quantum thing. In fact, this is exactly what a team led by Zeilinger at the University of Vienna has been trying to show with increasingly large objects. So far, they have succeeded in making a ‘buckyball’, a football-shaped molecule containing 60 carbon atoms, ‘be in two places at once’ by going through two slits in a screen simultaneously. For their next trick, they aim to do something similar with a virus, a bundle of protein molecules which straddles the border between life and non-life.
In principle, it is possible to arrange for a human being to walk through two doors simultaneously. What makes it impossible in practice is the severe difficulty of isolating a big thing from its surroundings. This is the reason – a solely practical one – that weird quantum behaviour is almost exclusively associated with small things like atoms. Better experiments will reveal bigger and bigger objects behaving in quantum ways. As Zeilinger puts it: ‘The border between classical and quantum phenomena is just a matter of money.’
The difficulty in isolating objects from their surroundings creates a tremendous problem for those who would like to exploit the ability of atoms to do many things at once. If this could be done, it would, for instance, be possible to make a computer that could do many calculations at once. So far, in the quest for such a ‘quantum computer’, physicists have managed to harness no more than ten atoms – each recording a binary ‘bit’.* The huge problem they face is in maintaining coherence within the computer – that is, an overlap between the individual waves of a quantum superposition when that superposition impresses itself on the large number of atoms of the computer. This means keeping the quantum superposition totally isolated from its surroundings, which is extremely hard.
The Herculean struggle faced by physicists trying to build quantum computers is a perfect illustration of how difficult it is for quantum superpositions to survive once large numbers of atoms become involved. And this explains why schizophrenic superpositions can be a central feature of the small-scale world of atoms yet can never be seen in the large-scale everyday world. But does it really explain why the large-scale world of trees and people and planets and stars is so very different from the microscopic world of atoms and their constituents – where the everyday world comes from? The answer is – not completely.
All that decoherence really explains is why, in the world around us, we do not see weird quantum superpositions – for instance, a water droplet that both exists and does not exist at the same time. Being able to rule out this extraordinary possibility is undoubtedly important. But there remain two other, ordinary possibilities. Namely, a droplet that exists and a droplet that does not exist. What determines which of these possibilities actually occurs?
Here we come to a place where science is still all at sea. There is no universally agreed explanation for what rules out one possibility contained in the quantum wave rather than the other possibility. Arguably the most extraordinary explanation, however, was proposed by Princeton graduate student Hugh Everett in 1957. According to his ‘Many Worlds’ idea, mentioned earlier, all the possibilities encapsulated in the quantum wave in fact occur. At first sight, this seems to fly in the face of our experience. After all, when we look at a particular location in a cloud chamber, we either see a droplet or we see no droplet. If, for instance, we see a droplet, then surely the possibility of a droplet not forming has ceased to exist? No, claimed Everett.
In the Many Worlds, reality, like a forked road, splits into two. In one reality, there is a version of you who sees a droplet form and in the other reality a version of you that sees no droplet form. According to Everett, the quantum wave is not merely an abstract mathematical convenience, useful for calculating things like the probability of something happening. It is completely and utterly real and every possibility encapsulated in it in fact occurs in some reality. Of course, we experience one reality only, and are utterly unaware of all the other realities. And it goes without saying that no one has the slightest idea where all the alternative realities are!
The Many Worlds may sounds like science fiction. However, it may shed light on the operation of quantum computers. Remember, quantum computers exploit the ability of particles such as atoms to be in many places at once to do many calculations at once. The field is still in its infancy and the capabilities of quantum computers are puny. However, there is no reason to believe that, within the next twenty or thirty years or so, more powerful quantum computers will not be built. Such computers could solve in seconds certain problems that would take a conventional computer longer than the age of the Universe.
And herein lies one of the central puzzles posed by quantum computers. Extrapolating into the far future, it is easy to imagine a quantum computer so powerful that it can carry out more calculations at any one time than there are particles in the Universe. An interesting question will then arise. Where will such calculations actually be carried out? After all, if a quantum computer is doing more calculations at any instant than there are particles in the Universe, the Universe simply does not have the physical resources at its disposal to do what the computer is doing.
The Many Worlds provides a natural, but mind-boggling, answer to the conundrum. A quantum computer is never short of the resources it needs to carry out its calculations because it does not have to rely on a single universe. Different parts of its calculations are performed in different realities. Bizarre as it seems, quantum computers may achieve what they achieve by exploiting huge numbers of versions of themselves in other neighbouring realities!
What is unique about the Many Worlds is its view of the quantum wave as a totally concrete thing, with all of the possibilities contained within it equally real. This is not, however, the view of the majority of physicists. They believe that the quantum wave is merely an abstract mathematical thing, useful only in calculating what is likely to happen in any given real-world situation. Consequently, they do not worry about the quantum possibilities that are not actualised. They are not anything anyway, they say. Quantum theory provides physicists with the probability of a particular outcome – the probability of seeing a droplet or no droplet. When one possibility is actualised, there is no need to puzzle over where the other possibility has gone to – as Everett did when he conceived the idea of alternative realities. The other possibility was never anything more than a mathematical possibility anyway.
Whatever your view, it appears that two steps are involved in creating the everyday world from the quantum world beneath. There is the process that stops the individual waves of a quantum superposition from overlapping and interfering. This suppresses quantum weirdness and prevents the existence of, among other things, cats that are both alive and dead. And there is the process which selects one component of the quantum wave. This selects one possibility from all the possibilities present in the quantum wave.
So much for why we do not see big things like people and planets in two or more places at once, but why do we not see big things like trees or footballs becoming more smeared out as time passes? After all, the quantum wave associated with them ought inevitably to spread out with time.
Take that football being struck from the penalty spot again. If it smears out with time, this is equivalent to the football taking several paths through the air simultaneously. Say, for simplicity it goes along just two paths, each with a 50 per cent probability.
Now – and this is the key – the football taking two paths through the air has consequences which are visible to a spectator if and only if the waves representing the two possibilities produce a record on their eyeball with sufficient overlap that there can be interference. Interference, after all, is the prerequisite of seeing quantum weirdness. In practice, however, this would require there being coherence between the waves representing billions upon billions of atoms in the spectator’s eyeball. This is impossible. And so the spectator sees the football fly along one and only one trajectory with 100 per cent certainty, obeying Newton’s laws of motion and not some averaged form of the Schrödinger equation. Thus is the everyday world saved, once again, from the madness of the quantum world.
* The success of quantum theory creates another big headache for physicists. Einstein’s theory of gravity – the general theory of relativity – has passed every observational test like quantum theory. But it is fundamentally a ‘classical’ theory, which predicts the future with 100 per cent certainty – for instance, where the Earth will be in its orbit next week. At first sight, this does not seem a problem since there appears to be no overlap between the two theories: quantum theory describes the world of small things such as atoms, while general relativity describes the world of big things like stars and the Universe itself. However, when the Universe was young – in the Big Bang – it was smaller than an atom. In order to understand the origin of the Universe it is therefore necessary to mesh quantum theory and general relativity – to create a ‘quantum theory of gravity’. Since one theory is founded on uncertainty and the other on certainty, this is a formidable challenge, to say the least.
* Actually, this is not strictly true. It is possible for a water droplet to grow spontaneously – that is, without a seed. But this is extremely rare.
* Water is made of ‘molecules’, each of which contains one atom of oxygen bound to two atoms of hydrogen.
* Implicit in this remark is something very peculiar and subtle about quantum theory. The quantum wave representing a particle is a mathematical thing which can be imagined spread throughout ordinary three-dimensional space (after all, the square of the wave height at any point in space is the probability of finding the particle at that point). However, if there are two or more particles, each particle has, in a sense, its own copy of ‘coordinate space’. In other words, the wave representing five particles lives in a space of 5 × 3 = 15 dimensions. And there can be interference only if there is a non-zero overlap between the waves in all the spaces. This is why the overlap in the example is given by ½ × ½ × ½ …
* Actually, the light-detecting cells in the retina are not up to detecting a single photon. But, remarkably, they are sensitive enough to respond to a mere handful.
† Decoherence is one of the fastest processes known. It is not unusual for it to happen within a 10 million trillionth of a second.
* Actually, a quantum computer stores and manipulates quantum bits, or ‘qubits’. Whereas a normal bit can represent only a ‘0’ or a ‘1’, a qubit can exist in a superposition of the two states, representing a ‘0’ and a ‘1’ simultaneously. Because strings of qubits can represent a large number of numbers simultaneously, they can be used to do a large number of calculations simultaneously.