Hot things glow red, hotter things yellow, and really hot things white. When you heat glass, it does not shine forth with an encouraging green or pale chestnut, it glows red, then yellow, then white. And if you heat iron to the same temperatures, it will do exactly the same thing. Take any solid element that doesn’t vaporize, and heat it to, say, 700 degrees and you will find it emits exactly the same spectrum of light as any other solid element also at 700 degrees. The reason this happens is because we live in a quantum world. Energy can only be lost or gained in small discrete units, called quanta, and it is because of this restriction that only some, and not all, spectra appear when you are blowing glass, or making toast.
The quantum nature of the world has vast implications. It is the reason, for one, that light can be said to behave like a stream of particles. As John Gribbin tells us in his slim yet concise and comprehensible introduction to quantum physics, Six Impossible Things, experiments conducted to study the photoelectric effect in the early twentieth century knocked electrons out of a metal surface with a beam of light:
When the energy of the ejected electrons was measured, it turned out that for a given colour of light the energy of each electron was always the same. For a bright light there are more electrons ejected, but they still all have the same energy as each other … It was Albert Einstein who explained this in terms of particles of light, what we now call photons – or in his language, quanta of light.
Photons have the same discrete energy, which depends on the colour of the light. The brighter the light the more photons, but their energy stays the same. “After a hundred years of thinking of light as a wave, physicists had to start thinking of it as a particle.” It was for this work, and not for the theory of relativity, that Einstein was awarded the Nobel Prize.
It’s no secret, however, that light does act like a wave as well – which is very much where the questions start. And, in fact, wave-particle duality exists not just as a phenomenon of light, but, seemingly, of all matter as well. one of the most famous experiments in all quantum theory is the double slit experiment. It relies on diffraction, which most of us are familiar with from school. Pass a water wave through two slits close to each other and ripples will propagate from them both; as the two sets of ripples encounter one another you will see a pattern of constructive and destructive interference between them, as they either enhance or attenuate the oscillations. The same is true of other at least nominally more corpuscular things. Fire a beam of electrons towards two slits and you will also see peaks and troughs spread out on the wall behind them.
Moreover, if you fire one electron at a time through the slits, so that they cannot interfere with any other particle, you still get peaks and troughs at the other end. An interference pattern emerges over time as you fire electrons one by one through the slits, though individually they arrive on the other side of the slits in a defined place. We are forced to conclude that electrons can interfere with themselves like waves. They pass through both slits at once.
But this isn’t the strangest thing about the experiment. Every attempt – every attempt – to measure an electron passing through the slits has destroyed the interference pattern, and produced a definitive answer to which path the electron took. Without a measurement there is concrete evidence for a wave-like nature; but as soon as one is made, however it is made, all evidence of it disappears and we are just left with particles.
This seems baffling – or at least it should. Niels Bohr, one of the fathers, and for a good time high priests, of quantum mechanics, is said to have given a talk on the subject to a group of philosophers, and was disappointed when they simply accepted what he told them about it, instead of protesting vehemently: “If a man does not feel dizzy”, he said, “when he first learns about the quantum of action, then he has not understood a word”. While it may seem complicated, it is perfectly possible to explain what goes on in the subatomic world, what happens when you do this or that, and in fact, once you have the knack, it can feel quite intuitive – but nobody knows why things behave the way they do. For this reason, in 1964, the same year he was awarded the Nobel Prize in Physics for his work on quantum mechanics, Richard Feynman could tell a lecture hall: “I think I can safely say that nobody understands quantum mechanics”.
Not to say people haven’t tried very hard. As it stands today, depending on how you want to interpret the results from a litany of physical and mathematical experiments all validating each other, you are left, basically speaking, with only so many possibilities of how you might understand the world. Gribbin chooses six of the more scientifically realized and commonly endorsed. As he summarizes them:
One. The world does not exist unless you look at it.
Two. Particles are pushed around by an invisible wave. But the particles have no influence on the wave.
Three. Everything that could possibly happen does, in an array of parallel realities.
Four. Everything that could possibly happen has already happened and we only noticed part of it.
Five. Everything influences everything else instantly, as if space does not exist.
Six. The future influences the past.
Leonard Susskind seems to have been underplaying it when he said that “in accepting quantum mechanics, we are buying into a view of reality that is radically different from the classical view”.
Gribbin’s book walks the reader through these six “solaces”, as he calls them (mainly so he can make the bad joke “quanta of solace”, but also to point out that the interpretations offer us comfort in the face of the mighty mysteriousness of our world). None of them is “correct”, they all have flaws and inconsistencies, but they are the best we have and help us try to make sense of things.
Initially, one can see the attraction of the first option on Gribbin’s list – “the world does not exist unless you look at it” – as it directly addresses the strange importance of “measurement” to the subatomic world. This version is known as the Copenhagen interpretation, because, when quantum physics was a suckling babe in the 1920s, it was developed by Bohr and his colleagues at the University of Copenhagen. It is sometimes said to be the “orthodox” version of quantum mechanics, mainly because Bohr in particular fought a hard PR battle for his theory, and succeeded in making most people believe, for at least twenty years, that it was the only legitimate way of interpreting the results. This is nonsense, but the theory is useful because, as Philip Ball says in his excellent book Beyond Weird, “it is an interpretation that doesn’t so much tell us ‘what is happening’, but rather, proscribes what we can legitimately ask about it”. It is good at pointing out where the problems lie.
Bohr said that the world revealed by measurements is the only reality worthy of the name, that the act of measurement actively constructs the reality that is being measured. Put an electron in a box. According to the Copenhagen interpretation – as Jim Holt describes in When Einstein Walked with Gödel – it “does not have a definitive location until we look inside to see just where it is. Prior to that act of observation, the electron is in a mixture of potential locations spread throughout the box”. This mixture is “mathematically represented by a ‘wave function,’ which expresses the different probabilities of detecting the electron at the various locations inside the box”. In French the wave function is poetically called the densité de présence, which is a helpful way of thinking about it. The wave function “collapses” to a single point when an observation – a measurement – is made; potentiality becomes actuality and we know the electron’s definite position. Put pithily, in the words of John Wheeler, “No phenomenon is a phenomenon until it is an observed phenomenon”.
Famously, Einstein once remarked, concerned at quantum mechanics’ reliance on probability, that “God does not play dice”. Less well known is Bohr’s rejoinder: “Einstein, stop telling God what to do”.
Before quantum mechanics was formulated, there was a determinist belief that physics would be able to predict the future, that if we could learn the laws that govern the fundamental particles we would be able to explain everything that hap- pened in the universe. As Tom Stoppard’s character Thomasina says, in Arcadia:
If you could stop every atom in its position and direction, and if your mind could comprehend all actions thus suspended, then if you were really, really, good at algebra you could write the formula for all the future; and although nobody can be so clever as to do it, the formula must exist just as if one could.
But it doesn’t, and you can’t. And this is down to what is described by Heisenberg’s Uncertainty Principle. It so happens that there are some quantum properties that are “conjugate variables” (position and momentum, energy and time, for instance) – which means the more precisely you know one of them, the less precisely you know the other. If you know a particle’s absolutely precise location then you won’t have a clue about where it’s going or how fast it’s getting there. This isn’t a problem about being able to measure one and not the other because our apparatus isn’t good enough. Not at all. As Ball tells us, “Quantum objects in principle have a number of observable properties, but we can’t gather them all … in a single go, because they can’t always exist at once”.
So we cannot know everything; the subatomic world does indeed prevent us from acting as God. But that is no reason why it should prevent us from trying to guess what game he is playing. Bohr and the Copenhagenists were, broadly speaking, happy to stop asking questions as long as they got results. “Shut up and calculate” is a phrase now commonly associated with their approach. The other interpretations on Gribbin’s list (respectively the Pilot Wave, Many Worlds, Decoherence, Ensemble and Transactional interpretations) are all attempts to ask why reality is the way it is. And as the summaries of them show, if you are to explain the collapse of the wave function and the prominence of the observer, or even to do away with them, you are forced to produce some fairly unorthodox ideas.
But then, the subatomic particles themselves are wacky. Another cornerstone of the quantum world is “entanglement”, a process in which two particles become paired with each other, often by being created at the same time. In certain ways, a pair of entangled particles act as if they are one system. The easiest way of explaining the effects of entanglement, the one used in all popular science books, is to talk about a particle’s “spin”. Spin is a property of a particle, and one, it is useful to stress, which has no relation at all to what we call spinning. Familiar words are never your friends in quantum mechanics – as Ball has it: “In quantum theory words are blunt tools. We give names to things and processes, but those are just labels for concepts that cannot be properly, accurately expressed in any terms but their own”. In thinking about the electron, say, it is worth bearing in mind what the physicist Arthur Eddington wrote in his book The Nature of the Physical World (1929):
No familiar conceptions can be woven around the electron … something unknown is doing we don’t know what. [This] does not sound a particularly illuminating theory. I have read something like it elsewhere –
The slithy toves
Did gyre and gimble in the wabe.
An electron’s spin can exist in two states, either “up” or “down”, but one could just as well say “slithy” or “uffish”. In an entangled pair of particles, one will have spin up, and the other spin down. These properties are not built in to the pairing; it is not that, as they go bumbling through the universe, one electron has the properties of an orange, and the other an apple, but that they share the properties of both fluidly, in what is known as a superposition. This only changes when one of the electrons is measured, when, as Gribbin says, it “interacts with something else”. At that point, the measured electron will be wholly and only an apple. Instantaneously, the other, unmeasured electron will be wholly and only an orange.
The reason this is important is because of that instantaneousness: the fact that an entangled particle has suddenly fixed its spin is, at least in appearance, conveyed to its sister faster than the speed of light, no matter what distance separates them – even if they are at opposite ends of the observable universe. In the world of relativity, this “non-locality” is not ok. Indeed, it is incompatible with it.
Understandably, then, there have been many efforts to try to prove that, as seems logical, the spins are fixed from the start, that one electron is always an orange and one an apple, and there is no fruity mixing going on. But they have not succeeded. Quite the contrary; entanglement is a proven phenomenon of nature – quantum computers work because of it, and last July a team of scientists from Glasgow managed to take the first-ever photograph of entanglement at work. And so this seemingly faster-than-light action must be accounted for – you are forced to conclude that either time or space is not fundamental to the universe.
In other words, it looks like we might need a new theory.
Enter Lee Smolin, a significant theoretical physicist who has made important contributions to the search for quantum gravity. He has bravely attempted to supply such a theory in his new book Einstein’s Unfinished Revolution. His argument is framed in terms of the divide between “realist” and “anti-realist” interpretations of nature. Realists argue that “matter [has] a stable set of properties in and of itself, without regard to our perceptions and knowledge”, and that “those properties can be comprehended and described by us”. Anti-realists, like the Copenhagenists, do not believe this, as quantum mechanics has features that “preclude realism” (such as the measurement problem). “Einstein was a realist. I am also a realist”, says Smolin, and this, he admits, rather backs him into a corner. If you are to go about asserting such madness as “there is a real world and we can understand it”, then you are forced to believe that quantum mechanics as it stands is false. “It may be temporarily successful, but it cannot be the fully correct description of nature at an atomic scale.”
The world, Smolin says, is a relational one – that is, location and time are defined with reference to something else (“Three blocks south of the supermarket is a relative location”). And he also claims that time is fundamental to the universe; the future is produced by present events, in a process that is irreversible. Despite our everyday experiences corroborating this – the trundling of the sun across the sky, the fact that “the sword outwears its sheath, / And the soul wears out the breast” – there is in fact a reasonable argument which suggests that time is not linear at a microscopic level. But let’s roll with it – Smolin tells us he has argued this point many times elsewhere and doesn’t want to do so here. Space, then, is emergent and “non-locality” can be explained as a remnant of the primordial spaceless world.
What is born out of these principles is “the causal theory of views”, central to which is what Smolin, somewhat unfortunately, insists on calling “nads”: he designates them as the elements of a relational model of the universe. They are seemingly anything – a hydrogen molecule, a quark, a bunny rabbit – although most particularly they are events in time. Each nad has a “view” of the universe, which summarizes its relations with the rest of the nads – how similar are they? What makes them dissimilar? What are their relational properties? It is the relationship between things, the differing “views”, that Smolin says is real, and from which everything falls out.
This might sound hard to wrap your head around. But the causal theory of views is still an achievement, for it is a working, coherent “realist” explanation of the universe, agreeing with both quantum mechanics and relativity. If nothing else, it shows that quantum mechanics is not the only scientifically convincing way of understanding the foundations of nature. After spending a good deal of time working through the theory’s many intricacies and trying to convince you of its strengths, Smolin concludes by saying that it is “still only part of the story, and there is still much to learn about it, but it is a way the world might be”, which is hardly a brazen trumpet blast of confidence. But then, the quantum world has humbled many scientists, some of them very great. To claim to have seen through its mysteries would be arrogant and foolish, and Smolin is neither. He is confident that ultimately “nature is comprehensible”, but that for now we can only hope to make small progress, trying to find “less arbitrariness and more reason” in the counterintuitive building blocks of the universe. Perhaps it is worth bearing in mind what Einstein is quoted as saying:
The most beautiful emotion we can experience is the mysterious. It is the fundamental emotion that stands at the cradle of all true art and science. He to whom this emotion is a stranger, who can no longer wonder and stand rapt in awe, is as good as dead, a snuffed-out candle.
Let it not be said, then, that our world does not give us fuel for the light that is our wonder. We are blessed to live in such a strange place.
Samuel Graydon works at the TLS