The Big Questions: Physics (3 page)

There is no end of questions in sight. Physicists used to be fond of saying their work was done. In 1894, the American physicist Albert Michelson announced that, ‘The most important
fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplemented by new discoveries is exceedingly remote.’ Within a decade, we had the twin revolutions of relativity and quantum theory.

In 1888, the astronomer Simon Newcomb had announced the end of astronomy: there was little left in the heavens to discover, he suggested. Newcomb was wrong too. Our view of the cosmos has probably changed more radically since Newcomb’s time than it did in the thousands of years of scientific discovery that took place before he was born. Although the major breakthroughs of the last century showed us where we came from, outlining the entire history of the universe, the hubris is gone from our world view; with the discovery that most of the universe is in a form unknown to science, physicists now appreciate that they have got to grips with only a tiny percentage of the universe.

There is, it has to be said, one end in sight: the theory of everything. If physics began with the Milesian quest for the laws governing natural phenomena, it will (theoretically) end with the discovery of just one law: the ultimate description of the universe. This ‘theory of everything’ will reduce all the particles, the forces that govern their interactions and the space and time in which their existence plays out, to a single unified description (see
Is String Theory Really About Strings?
).

At the moment, we are far from achieving that goal, but here, perhaps, we have found the true point and the essence of physics: to discover the span of our ignorance, and to do what we can to reduce it. Sometimes, as with the atomic bomb, there is a price to be paid for this journey of discovery. Sometimes, as with the development of quantum mechanics, we reap great practical rewards from it. But most of the time, physicists will tell you, physics is simply about the thrill of discovery – and then discovering that our discoveries have made the world more interesting, not less. As the poet John Dryden said, ‘Joy in looking and comprehending is nature’s most beautiful gift.’

Progress, disorder and Einstein’s elastic clocks

Deep in your brain there lies a lump of tissue called the striatum. This assortment of neurons is, to the best of our knowledge, the only dwelling place of time. It accumulates the first record of the moments of your life, and provokes your sense that your childhood was a tumbling assortment of significant and fascinating moments, while adult life hurtles by too fast to be properly appreciated.

You shouldn’t set too much store by these sentiments, though. The striatum’s gift is actually to create an impression – perhaps even an illusion – of time passing. The problem is, its measure of time depends on what is going on in your conscious mind. Every time you perform a conscious task such as putting the kettle on, the various electrical circuits in your brain spike in unison. The striatum records this simultaneous signalling and starts to note the subsequent patterns of electrical signalling from areas such as the frontal cortex. Your notion of how much time has passed before the kettle boils is nothing more than a measure of the accumulated electrical signals.

That’s not so bad at home, where you can calibrate it with a glance at the kitchen clock. But as soon as you are denied access to clocks, things go awry. When, in the early 1960s, the French geologist Michel Siffre took off his watch and lowered himself into a dark cave for 60 days, his perception of passing time unravelled.
By the end of the experiment, what Siffre thought was an hour was often four or five. Drugs such as valium, caffeine or LSD will send your sense of time similarly awry. As will your memory.

We often think busy times make life flash by, but experiments show that’s only true
while
you’re busy. Afterwards, when you reflect on your existence, your busy periods will seem much longer. That’s why your childhood now seems to have been a series of long, golden summers – life was exciting when you still had so much to experience, and your brain thinks that those heightened signalling levels must correspond to huge stretches of time. Your grip on the passage of time, then, is as precarious as you may always have suspected. But it turns out that our problems with the perception of time are as nothing compared with our problems with the notion of time itself.

You might think we ought to have a handle on time by now. After all, time is a universally understood concept – every human culture knows about it, talks about it, feels it. And we have been thinking about what it means for millennia. In 350
BC
, Aristotle, for instance, wrote a work called
Physics
, which included one of the first attempts to grapple with the notion of time.

‘First, does it belong to the class of things that exist or to that of things that do not exist?’

ARISTOTLE

Aristotle’s work on time begins with a question. ‘First,’ it says, ‘does it belong to the class of things that exist or to that of things that do not exist?’ Here in the second millennium
AD
, that is still an open question. If our minds are fooled by the passage of time, that may be because time itself is an illusion. From the Greeks to modern-day physics, the main conclusion about time has remained constant: it is, at the very least, about change. Through time, one thing changes into another.

But while Aristotle’s Greek peers were obsessed with the circle as the most fundamental concept in the universe, meaning that time must flow in cycles, modern physics is focused on linear
processes: beginning to end, Big Bang to cosmic shutdown. With time, that translates into an overwhelming sense of time’s arrow: in our modern view of the universe, time moves irreversibly forward. Eggs break, and cannot be unbroken. Clocks wind down, and do not spontaneously wind up.

This process of change, in which systems move irreversibly into disorder, is known as the thermodynamic arrow of time. It arises from one of the most fundamental laws of physics: the second law of thermodynamics. This states that, as a whole, the universe is caught in a process of unravelling order. Entropy, a measure of the disorder in a system, is always increasing.

The arrow of time might arise from a variety of sources. The ‘cosmological arrow of time’, for example, cites the creation of the universe as a move away from a special, low entropy state where everything was neatly ordered. It is rather like handing a fully solved Rubik’s Cube to a curious child; as time progresses, the universe moves to an ever-more disordered state, just as the neat order on the faces of the Rubik’s Cube will give way to a messy jumble of colours. While some things, such as galaxies, appear ordered, with structures that are often intricately beautiful, the order of the universe as a whole is decreasing. The end will come when there is no more disorder to be created; or, as Lord Kelvin put it, when the universe has reached ‘a state of universal rest and death’.

Our familiar arrow of time could equally result from quantum theory. In one (probably the most popular) school of thought, quantum systems undergo an irreversible ‘collapse’ when they are measured. This originates from the remarkable ability of a quantum object such as an atom to exist in two entirely different states at once. It might, for example, be spinning clockwise and anticlockwise at the same time. When the measurement is made, however, that double state is forced to become one or the other: the measured atom will be found to be spinning clockwise or anticlockwise, and will not spontaneously revert to the state of doing both.

There is a problem with these descriptions of time’s arrow, however. They get us nowhere because they require the concept of change. And change, as Aristotle noted, is a marker of time passing. Through considerations of the arrow of time we are really no further forward in defining time. All we have is a putative explanation for the direction it appears to take. And even that has been undermined. Time’s arrow might be part of our individual experience, but we have no reason to believe that makes it real. Worse still, we have good reason to believe it isn’t.

We have Albert Einstein to thank for this alarming insight: it lies at the heart of his special theory of relativity. Einstein was relatively unknown when he published his ideas in 1905. Special relativity was a revolutionary work, dismissing in a single stroke the popular and long-lived concept of the ether, a kind of ghostly fluid that fills all of space and provides a background through which electromagnetic fields such as light could move.

It is worth mentioning at this point that while, as the late Carl Sagan once said, extraordinary theories require extraordinary evidence, special relativity is one of the few such theories where extraordinary evidence has been found to back it up. What you are about to read may seem absurd, but there is every reason to take it seriously.

The central point of special relativity is that the laws of physics work the same for everyone, regardless of how they are moving through the universe. The most important consequence of this is that the speed of light is a constant, universally known as c. If you were to measure the speed of the light emitted from the headlights of a vehicle travelling towards you at 100 kilometres per hour, the speed of the light would be c, not c plus 100 kilometres per hour (62 mph). The speed of light does not change depending on the relative motion of the emitter and observer. The extraordinary upshot of the constancy of c is that, when conditions require it, everything else does change – and
that includes time. The passage of time is as flexible an affair in the real, physical world as it is inside your mind.

Let’s imagine a scene where you are standing 100 metres from an intersection controlled by traffic lights. You are equipped with a stunningly accurate stopwatch, a metre rule and lightning reflexes. The light changes to red, and you are able to measure the time it takes for the first pulse of red light to travel the length of your metre rule. At that moment, a car passes you, travelling towards the intersection at 100 kilometres per hour. The passenger in the front seat has the same skills and equipment as you, and makes the same measurement: the time taken for the light to travel the length of the ruler.

You have both measured the speed of light, and Einstein insists that you must both get the same result. But as the car moved past you towards the traffic light, the metre rule within it also moved past you. By the time the light reached the end of the ruler in the car, the far end of the ruler was closer to the traffic lights, and so the light had to travel less distance compared with yours. The passenger in the car should measure light as faster, completing a metre in less time. How then, can you both get the same result? The answer has to do with the passage of time in different situations. Compared with your clock, the clock in the moving car runs slow. So, although the light apparently had less
distance to travel, the time measurement was larger than yours, cancelling out the effect.

This is not a sleight of hand where a combination of illusions leads to you getting the right result. The effect, known as time dilation, only becomes markedly noticeable when the clock moves at speeds close to the speed of light, but it remains true that a clock that is moving relative to you really will run slower than a clock held in your hand. And the word ‘clock’ refers to anything that can mark the passage of time. Dissect that statement, and you’ll find that all kinds of disturbing implications emerge.