The Big Questions: Physics (11 page)

It is important to make the distinction between chaotic and truly random systems, however. A dice throw is not predictable to us, but neither is it random: we know it follows discernible laws, just not ones whose consequences we can accurately compute given our limited knowledge of the initial circumstances. We can
say the same about the weather: it is our limitations – our ignorance, in Quetelet’s words – that make it seem random. So is anything truly random? This is a question that lies at the centre of one of the greatest, and most fundamental, debates in science.

At the beginning of the 20th century, Lord Kelvin expressed his delight at the way physics was progressing. Newton had done the groundwork, and his laws of motion could be used to underpin the emerging understanding of the nature of light and heat. Yes, there were a couple of small issues – ‘two clouds’, as he put it – but essentially physicists were now doing little more than dotting the ‘i’s and crossing the ‘t’s on our understanding of the universe. Coincidentally, the great German mathematician David Hilbert was feeling similarly optimistic. In 1900, at a mathematical congress in Paris, Hilbert set out 23 open mathematical problems that, when solved, would close the book of mathematics.

Both Hilbert and Lord Kelvin were guilty of misplaced optimism. Within a few years, relativity and quantum theory had blown apart the idea of using Newton to formulate the future of physics. What’s more, the Austrian mathematician Kurt Gödel had pulled the rug from under Hilbert’s feet, answering a mathematical question that Hilbert had not even asked – and taking away all certainty that any of Hilbert’s questions could be answered.

Gödel had formulated what he called an incompleteness theorem. It says, essentially, that there are some mathematical problems that can never be answered. Because of the way we formulate mathematical ideas, some things can never be proved. Mathematics is destined to be eternally incomplete. This has deep relevance for the question of randomness. If some things are unknowable, their behaviour may be, for all we know, random. Randomness might not actually be an inherent property of the system, but we can never prove that it is not. Gödel published his incompleteness theorem in 1931. By this time, the notion of limits to what we can know was hardly even a surprise. If you
were familiar with the newly birthed quantum theory, you were already resigned to your ignorance of the ultimate answers.

First, quantum theory gave us the problem of inherent uncertainty. Werner Heisenberg was the first to notice that, when dealing with the equations of quantum theory, you could ask questions about the characteristics of the system under scrutiny, but there were certain combinations of questions that couldn’t be asked simultaneously. The equations will give you the precise momentum or position of a particle, for instance. But they won’t give you both at the same time. If you want to know the precise momentum of a particle at a particular moment, you can say literally nothing about the position of the particle at the same moment. This, the Heisenberg uncertainty principle, is a fundamental characteristic of quantum theory.

Heisenberg used the analogy of a microscope to justify this. If we want to look at the position of a particle, he said, we have to bounce something off it – a photon of light, in this case. But by so doing the photon imparts momentum to the particle. In other words, by measuring the position, we have introduced a change to a separate characteristic – we cannot simultaneously know the position and the momentum with any accuracy. Any measurement – to determine momentum, or energy, or spin – will have concomitant effects on other characteristics. Certainty about every characteristic of a system at one moment in time can never be achieved.

The second even more fundamental problem is not so much one of practical limitations, but straightforward inherent predictability. The classic example for this is a piece of radioactive rock, such as the lump of radium that Marie Curie carried around with her. Physicists can tell you that, if it is composed of the quickest-decaying isotope of radium, the radioactivity of the lump will be halved every three and a half days. After a week, then, it has a quarter of its original radioactivity.

This, however, is a statistical average. It tells you nothing about whether any particular atom of radium will decay in a given
time. After 1,000 years, some of the atoms in that lump will still not have decayed. Some will decay in minutes of you starting your clock. And there is no way to predict which is which. Nothing in quantum theory tells us what prompts the decay. It is, to all intents and purposes, random, as if the Almighty rolls ten dice for each atom and only a set of ten sixes causes decay. Einstein took this as proof that quantum theory is incomplete. There must, he said, be some ‘hidden variables’ that substitute for this divine dice game.

The idea that the ‘Great One’ does not play dice is perhaps Albert Einstein’s most documented concern. It is worth pointing out that it was not religiously motivated. Einstein often used ‘God’ as a metaphor for nature or the universe. His point is simple and materialistic. Surely the universe runs by deterministic laws? Surely every effect has a cause? Niels Bohr, widely seen as the founding father of quantum theory responded to this with scorn. Quantum theory, he told Einstein repeatedly, is founded on randomness. Some effects have no cause. ‘Einstein, stop telling God what to do,’ he said.

As with Heisenberg’s uncertainty principle, this randomness does seem to be written right into the equations of quantum theory. The central equation, the only way to make sense of experiments carried out on quantum systems, is the Schrödinger wave equation. This assigns quantum objects the characteristics of waves. If we want to know something about the quantum world, we solve this wave equation. All we get out of it, though, is a probability.

This is really what sets quantum theory apart. By the time quantum theory was born, in the 1920s, statistics was a firmly established discipline of science. Thermodynamics, the study of heat that had partnered the Industrial Revolution, relied upon it.
Many other branches of science used statistics to verify the results of experiments. Quantum theory, though, seemed unique – and, to Einstein, disturbing – in its assertions that its results could
only
be expressed as probabilities.

The results of quantum experiments, according to the orthodox interpretation of quantum theory, were down to pure chance. Einstein’s refusal to accept this is largely to do with the profundity of its implications. Quantum theory describes the world at the scale of its most fundamental particles. If quantum processes are random, then
everything
is ultimately random.

Bohr had no problem with this because he believed that, ultimately, nothing has any properties at all. Our experiments and measurements, he believed, will produce certain changes in our experimental equipment, and we interpret those in terms of the momentum of an atom, or the spin of an electron. But, ultimately, he said, those qualities are not a reflection of something that exists independently of the measurement. Thus, to Bohr, there was no reason why the results of experiments should not appear randomly distributed; there was no ordered objective reality from which some non-random result could arise. To his mind, it would be odd if it were any other way.

It seems an extraordinary point of view: radical and shocking. An electron only exists as some quirk of our measuring apparatus. Small wonder that the infinitely more ‘common sense’ oriented Einstein debated this with Bohr for decades. Einstein said he ‘felt something like love’ for Bohr when it began, such was the intensity and pleasure of their intellectual jousting. However, by the end, it had reached the point where the pair had nothing to say to one another. One dinner given in Einstein’s honour saw Einstein and his friends huddled at one end of the hall, while Bohr and his admirers stood at the other.

Ultimately, history has decided that Bohr was right. Perhaps that is inevitable, given the force of Bohr’s personality – he did once
reduce Werner Heisenberg to tears, for instance. Whatever the truth, while Einstein’s notion of a set of hidden variables that are waiting to be discovered remains scientifically respectable, the mainstream view is that objective reality does not have any independent existence. All we can say about the reality that manifests in quantum experiments is that we can predict the spectrum of possibilities, and how likely each one is to be seen. So, is that the last word? Is the universe ultimately random? Are we, as creatures composed of quantum molecules, doomed to find ourselves at the mercy of capricious forces? Yes – but the question is loaded like a Roman die.

We naturally seem to phrase randomness in negative terms, talking about suffering ‘the slings and arrows of outrageous fortune’. But, as Shakespeare well knew, luck is often kind too. He has Pisanio declare in
Cymbeline
, for example, that ‘fortune brings in some boats that are not steered’. The problem is, millennia of religious thought has imposed a sense that everything that happens in the world around us happens for a reason. Science has reinforced this: we appreciate predictability. But randomness can be useful too (see box:
More Than Noise
).

What’s more, it may even be at the root of our very existence. The Heisenberg uncertainty principle is, as we have seen, fundamental to the universe. One of the consequences of this is that even regions of empty space cannot have zero energy; instead, all of space is populated by a frothing of ‘virtual’ particles that pop in and out of existence at random. These quantum fluctuations in the ‘vacuum’ of space are thought to be the source of the ‘dark energy’ that is driving the accelerating expansion of the universe. A similar kind of fluctuation that came ‘out of nothing’, but grew rather than disappeared again, is the best explanation we have for the cause of the Big Bang that gave rise to our universe. You might think randomness is a bad thing, but without it you wouldn’t be here to think at all.

The Higgs boson, the LHC and the search for the meaning of mass

You may not be surprised to learn that it has nothing to do with God. Except, perhaps, in the sense that no one has ever proved it exists. Nobel Prize-winning physicist Leon Lederman coined the phrase. Partly it was a humorous dig at physicists who thought that a particle would answer all their questions about the universe, partly it was a dig at the idea that science’s discoveries might have anything to say about the meaning of life.

Unfortunately, the God particle does neither: it won’t tell us everything about the universe, and it won’t tell us the meaning of life. But that doesn’t mean the Higgs boson is not worth searching for. It is the final piece of the puzzle in the standard theory of particle physics. If it exists, we can rest assured that we have uncovered much of the essential nature of the universe and found what gives materials their mass. If it doesn’t, we might just have to go back to the drawing board.

The stage upon which this drama will play out is in Switzerland. At the European Organisation for Nuclear Research (CERN) in Geneva, the world’s most powerful particle accelerator will be the arbiter of the truth in what physicists call the ‘standard model’ of physics. As the Large Hadron Collider (LHC) smashes together protons with the force of two high-speed trains colliding,
the God particle may spill out. Around the world, physicists are on tenterhooks, waiting to see if, way back in 1964, Peter Higgs was right.

Peter Higgs’s suggestion was fairly straightforward. Responding to various attempts to work out the origin of mass, he wrote a paper that described how theoretical physics allowed for the existence of a new kind of field. It would be an addition to the known fields, such as the gravitational or electromagnetic field. This new field would have appeared as the universe cooled down from the Big Bang fireball, and could provide a source of drag on certain types of particles, perhaps endowing them with the property we know as mass.

The paper was initially rejected by the editors of the journal
Physics Letters
as being ‘of no obvious relevance to physics’. Higgs rewrote it to give the idea a concrete application: it might arise, he said, in the force that holds the particles in the nucleus together, but still no one made much of it. Until, that is, Steven Weinberg, Sheldon Glashow and Abdus Salaam set about trying to unify the electromagnetic and weak nuclear forces.