The Big Questions: Physics (4 page)

Let’s start with something that is just about conceivable. Take a lump of polonium, a radioactive material discovered by Pierre and Marie Curie around 100 years ago. One form of polonium, polonium-209, has a half-life of about 100 years; that is, after a century, half of its atoms will have emitted a burst of radiation and transmuted into more stable atoms.

If the Curies had taken two identical lumps of this material when they discovered it, and left one in their Paris laboratory while shooting the other one on a round trip into space at 0.99 of the speed of light, returning to Earth today, we would notice something remarkable about the amount of radiation they were giving off. The lump that stayed in Paris would lose half of its radioactive polonium atoms during that century. The thing is, its twin, the lump that had rocketed into space and back while 100 years passed on Earth, would only have lost 10 per cent of its radioactive polonium atoms.

That is because the motion relative to Earth at 0.99 the speed of light (setting aside practical issues such as acceleration, deceleration and turning round) slows time for this lump. Its ‘clock’, as measured by the rate at which its atoms experience radioactive decay, is running at only 14 per cent of the speed of its twin that never left the planet. That is why so many of its radioactive atoms remain intact. This, perhaps, is hard enough to swallow. But now for something truly inconceivable.

Let’s allow Pierre and Marie Curie to guard the two lumps of polonium. Pierre will accompany one lump on that same return trip into space, while Marie remains in Paris with her lump. The scientists’ bodies have internal clocks, too: as with the polonium, their atoms change with the passage of time, creating a heartbeat, for instance, and cells that shut down after performing a certain number of divisions – a phenomenon that biologists believe to be the root of ageing and death.

Turning a blind eye to the likely catastrophic effects of the radiation, the atoms – and thus the cells and the heartbeat – in Pierre’s body will run slow compared to Marie’s, just as the polonium’s radioactive decay runs slower than on Earth. When Pierre returns, 100 Earth years later, Marie is long dead, but Pierre’s body has aged only 14 years. One immediately obvious conclusion from this is that, given the right resources, time travel into the future is entirely possible. But it is a short step from this point to the astonishing revelation that Einstein’s special theory of relativity does away with the notion of some common future anyway. And neither is there a common present or past.

You might claim, as you stand looking at the traffic lights, that you saw two events happen simultaneously. But as we have seen, the passenger in the car has a clock that runs at a different speed. The information they gain about the timing of those two events could well be different. Worse, you might see two events, A and B, happening at distinct times, with B following A. Depending on how your relative friend is moving, however, they could see A
follow B. That is potentially catastrophic: if you think A caused B, how is that explicable to someone who saw B happen first?

Past, present, future, simultaneity, cause and effect – nothing is universal. When it comes to time and the processes it governs, you and your striatum really are on your own. There is a simple answer to all this confusion, however, and it is an answer that is appealing to many physicists and philosophers. We could do away with the very notion that time exists.

It is an argument that harks back to the 17th century. Newton, whose Christian faith required that space and time reflect the character of God, considered time to be a real entity, an absolute that moves on independently of everything in the universe. But his great rival Gottfried Leibniz believed time to be a human construct. All we can do, Leibniz said, is describe how the positions of things in space relate to each other, and how that relation evolves. It is useful that a clock’s pendulum swings back and forth and the clock’s hands circulate around the dial in response, for example, but that doesn’t mean the clock is measuring something that actually exists. Time, in this view, comes out of our desire to make sense of the world, but it is no more than a useful means of orientation. It is a shorthand, like the spatial concept of ‘up’. ‘Up’ means a certain direction when I am stood in London, but the same direction is actually ‘down’ in Sydney.

This link is slightly more than a convenient illustration. When Einstein published his general theory of relativity (the ‘special’ in ‘special theory’ refers to a special, i.e. particular case, not a special significance), he postulated a bond between time and space. Time, he said, is just one of four dimensions to the universe. The other three are the familiar ones in which you move your physical body: up and down, across, forwards and backwards. The only difference is that, while we conscious creatures can choose how we move through the spatial dimensions, we have no control over our movement through time.

Einstein’s four dimensions of space and time – together known as space–time – can be thought of rather like a piece of fabric that can be distorted, bent, folded, twisted and even torn by anything within them that has mass or energy. From this foundation, general relativity has equipped us with equations that describe the features of the cosmos with unprecedented accuracy, allowing us to find out how the universe works, send spacecraft to distant destinations and create the array of global positioning satellites that tell us where on Earth we are. But perhaps most intriguingly of all, the pliable nature of Einstein’s four-dimensional fabric hints at the origin of time.

Your mass distorts space–time very little. The mass of the sun distorts it much more – according to general relativity, this distortion is the root of the gravitational attraction that keeps our planet in orbit. Even more powerful is the distortion that is brought about by a collapsed giant star: a black hole. And it is here that we glimpse the true power of Einstein’s work.

The enormously strong gravitational field of a black hole means that there is a spherical region close to its centre where the velocity required to move away from the black hole is greater than the speed of light – an impossible velocity to achieve. Nothing, including light, can get out of this region, and so we cannot gain any information about anything that goes on beyond its boundary. Hence its name: the event horizon.

At the event horizon, time dilation is infinite. Somebody watching from a safe distance as you fall towards the event horizon would see your movements slow down then freeze as time runs infinitely slowly for you compared to the observer. Only in the observer’s infinite future would you reach the event horizon, so you never actually disappear from view. Your experience, on the other hand, would be hugely dramatic. Your body is extremely unlikely to survive the enormous gravitational forces, but if you did survive you would eventually encounter what, according to relativity, is a breakdown in the very fabric of
space–time. This ‘singularity’ at the centre of a black hole occurs as the distortion becomes infinite. Here, we reach the limit of the known laws of physics – beyond this point, they no longer apply.

Though it is commonly associated with destruction, the singularity is also thought to be the key to creation. In the early 1970s, Roger Penrose and Stephen Hawking adapted the mathematical notion of the black hole singularity to explain the origin of the universe. In a black hole everything disappears into the singularity. Reverse the mathematics of the process, though, and the singularity could give birth to the very fabric of space–time. For more than three decades this has been seen as our best description of the Big Bang, the origin of time itself.

If general relativity sheds some light on where time comes from, it still does not tell us a great deal about what time is. What’s more, impressive as Einstein’s formulations of the character of space and time are, we know that special and general relativity are not the final answer.

If the singularity shows us anything, it is that, while general relativity works remarkably well in many scenarios, it offers no satisfactory explanation for the most extreme phenomena of our universe. A more complete description of the cosmos and how all its contents (including the centres of black holes) behave – a theory often referred to as ‘quantum gravity’ – still eludes us. And, as it turns out, the nature of time is right at the heart of the problem.

Quantum gravity has to work relativity’s notions of time into quantum theory, our best description of how the microworld of molecules, atoms and subatomic particles behaves. But quantum theory takes little note of time. In the standard formulation of the theory, you can’t ask questions about how long a process takes, for example. Then there’s the problem that quantum theory tells us that most of the subatomic particles exist independently of the direction of time. Just as they can spin clockwise and anticlockwise at the
same time, their quantum states can evolve forward and backward in time. Researchers are even learning to do quantum experiments where information seems to come from the particles’ futures. What’s more, special relativity tells us that massless particles, such as photons and the gluons that bind nuclei together, travel at the speed of light and do not even experience the passage of time.

The great physicist John Wheeler once said, ‘Time is nature’s way to keep everything from happening at once.’ He would have said it with a twinkle in his eye, knowing full well that the apparent simplicity of time belies its true nature. Saint Augustine was more honest when he said, ‘What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.’

Despite all our scientific achievements since Augustine, time remains an enigma, possibly the biggest question facing physicists today. But if time is an illusion, it is at least a useful one. Our interpretation of its consequences – our memories of the past, our existence in the present and our hopes for the future – lie at the core of the human experience. Or that’s what your striatum wants you to believe.

Quantum physics and the nature of reality

It was 1925, the heyday of Buster Keaton and Charlie Chaplin. The world was getting excited about
The Gold Rush,
hailed as Chaplin’s finest film to date, coming out next month. And poor Wolfgang Pauli, a physics student based in Hamburg, Germany, was depressed. ‘Physics at the moment is again very muddled; in any case, for me it is too complicated,’ he wrote to a colleague. ‘I wish I were a film comedian or something of that sort and had never heard about physics.’

Pauli was right: physics was muddled. No one understood what the newly formed quantum theory was all about. Experiments dictated that energy must be split into indivisible packets or quanta, but no one could say why. Then, just a few months later, the Austrian physicist Erwin Schrödinger cleared the confusion. It happened during a trip into the Swiss mountains with a woman who was not his wife, and ended with his questioning the fate of an imaginary cat. The creature quickly became the most famous animal in science. The story of Schrödinger’s cat has the weirdness of quantum theory running right through it, and its appropriately enigmatic nature remains intact to this day.

The source of Schrödinger’s breakthrough lay in the work of a French physicist called Louis de Broglie. In 1923, de Broglie put together relativity, generally the physics of the very largest
scales of distance and speed, and the nascent quantum theory, the physics of the very small. The outcome was a simple equation. Every moving particle, de Broglie said, could equally well be described as a wave. Every wave could be described as a moving particle. Einstein, when presented with the work, pronounced it ‘quite interesting’. Two years later, however, Schrödinger showed it was much, much more than that.

Erwin Schrödinger worked out the mathematical implications of de Broglie’s formula during a Christmas holiday in 1925. Leaving his wife in Zürich, Schrödinger took his mistress off to a chalet in the Swiss mountains. It was not unusual behaviour for him – he and his wife seemed to come to several ‘arrangements’ through their marriage. Whatever went on, the trip was obviously inspiring. Schrödinger came back from the mountains with what is now known as the Schrödinger wave equation. This describes how a quantum particle behaves when it is considered as a wave.

The Schrödinger equation provides a way of understanding where quantum states come from. Take the Bohr model of the atom, for example, where an electron circling the nucleus can only have particular energy states. Schrödinger’s equation gives a way of working out what those ‘quantized’ energies are: the electron is stable only when its wave completes a whole number of oscillations during its orbit.

This was a revelation to physicists, who had no proper justification of the quantized energies. But the equation also gives a way of working out how the energy, say, of an electron will evolve over time in a particular situation. It can equally well give us the particle’s position, or its momentum, or how the quantum states of two interacting particles will end up. It was hailed as a masterstroke. There was only one problem.

No one could agree on what the wave equation actually meant. Did it mean the particles were really waves? Schrödinger believed – or rather hoped – so. Einstein stood with him. But
others disagreed. The University of Göttingen physicist Max Born, for instance, showed that solutions of the wave equation might give nothing more than probabilities. The probability of finding a particle in a particular space, say, or the probability that a particle will have a certain momentum.