The Big Questions: Physics (6 page)

So, information does not have to enter a conscious mind to constitute a measurement: it just has to leak away from the system under scrutiny. It appears that a flow of information about the health of Schrödinger’s cat is enough to force it into one of the two states available. Where humans and cats are concerned, that information leaks away because our bodies interact with our environment in myriad ways, radiating heat and knocking air molecules about. Information about the position of our bodies is available, which means we can’t be in two places at once. This spilling of information is known to scientists as ‘decoherence’. Decoherence is not a trivial issue: it might just show us the very nature of the universe.

‘Anyone who is not shocked by quantum theory has not understood it.’

NIELS BOHR

The physicists looking into the enigma of Schrödinger’s cat are now wondering whether it points to the notion that information is the most fundamental element of reality. Quantum theory, in the form of Schrödinger’s unfortunate cat, suggests that the universe can be described as one giant information processing machine. And this leads to potential applications too. The role of information in quantum theory has led us to one of our most ambitious technological projects: the quest to build a super-powerful processor called a quantum computer (see box:
Computing with Cats
).

However powerful it turns out to be, the quantum computer is unlikely to be able to help us comprehend how a cat really can be alive and dead at the same time. The idea that this is part of the nature of physical reality remains truly outrageous to the human mind. Wolfgang Pauli, who didn’t give up on physics, and became one of the most brilliant physicists in the history of science, was right. It’s too complicated to grasp. As Niels Bohr once said, ‘Anyone who is not shocked by quantum theory has not understood it.’

Gravity, mass and the enigma of relativity

Because of gravity, of course. Everybody knows that. However, what is the fundamental nature of gravity? That is a much harder to question to answer, despite the fact that gravity is the first of nature’s fundamental forces to penetrate your consciousness.

Here’s an experiment you can try at home. You’ll need a six-month-old baby (you could borrow one). Tie a piece of fishing line to one of the baby’s toys – a rattle, say. Now suspend it from the ceiling at a height where it will rest lightly on a chair with the line taut and invisible. Get the baby to watch as you whip the chair away. Keep your eyes on the baby’s: when, for no obvious reason, the rattle doesn’t fall to the ground, the baby will stare at it for much longer than is normal.

This, according to psychologists, is how babies express astonishment. It seems that we know from a surprisingly young age that things are meant to fall downwards when unsupported, and we are mystified if they don’t. No wonder the levitation tricks of the Victorian illusionists entranced an entire generation. When things cheat gravity, our very core takes delighted offence.

Gravity, you see, is a tyrant. It cannot be cheated. We cannot, as we can with an electric or magnetic field, block it out. Neither can we counter it with an opposing force – there seems to be nothing in physics that equips us with antigravity. The rule of
gravity is so central to human experience that we have become, essentially, oblivious to gravity’s presence. It is only in its absence – or, rather, its apparent absence – that we remember it is always there.

Perhaps that is why the earliest ventures into science largely ignored gravity. As we understand it now, one type of action governs the fall of a tripping human, the arc of an arrow’s flight and the motion of the planets, but Aristotle’s textbook
Physics
makes no mention of any universal force orchestrating the universe. He did suggest that objects did not fall off the Earth because of the Earth’s ‘heaviness’, but his reasoning was askew. He suggested that the strength of the Earth’s pull depended on how big an object was and what it was made of.

In Aristotle’s view, heavy objects fall more quickly than light objects. That is because of the Greek obsession with the elements: Earth, Air, Fire and Water; most of the heavy objects Aristotle knew about were made from materials found in the Earth, and the strong pull, he said, was because they were compelled to return there. Our understanding didn’t really move on from this flawed idea for almost 2,000 years. Eventually, though, the Italian scientist Galileo Galilei established that Aristotle was wrong: heavy objects are not more strongly attracted by the Earth. As long as air resistance is not a factor, a heavy and a light object will fall at the same rate.

Sadly, the romantic stories about Galileo’s proof of this – by dropping cannonballs from the leaning tower of Pisa – are not true (the myth was started by Galileo’s student Vincenzo Viviani), but it has nevertheless been proved in an even more spectacular fashion. In 1971,
Apollo 15
astronaut David Scott paid tribute to the discovery’s profound consequences by dropping a hammer and a falcon feather onto the moon’s surface. ‘One of the reasons we got here today, was because of a gentleman called Galileo,’ Scott said as he let them fall. The hammer and the feather landed, of course, at the same time.

Scott’s appraisal was almost correct: astonishingly, it really didn’t take much more than Galileo’s 17th-century insights to get us to the moon. The gaps were filled by a man born just one year after Galileo died: Isaac Newton. Unimpressive as he was at birth – his mother said he could be ‘put in a quart mug’ – Newton took just a couple of dozen years to gather all the information it would take to plot a course for the
Apollo
astronauts four centuries later. And here, of course, is where the apple comes in.

Unlike the stories of Galileo’s experiments on the leaning tower of Pisa, accounts of Newton’s gravitational epiphany at the sight of a falling apple are almost certainly true. It was late summer, 1666, and Newton was sat in his garden at Woolsthorpe Manor in Lincolnshire. The apple tree is still there, and still bearing fruit every autumn.

An apple falls because it has a property called mass, and so does the Earth. Newton’s great leap forward was to spell out how everything with mass attracts everything else with mass. His universal law of gravitation, constructed at the tender age of 23, said that the attractive force is dependent on those two masses, the distance between them, and a constant known as G.

Actually, physicists are often over-familiar with the gravitational constant and call it ‘Big G’ to distinguish it from (little) g, the acceleration due to the Earth’s gravitational pull. However, despite the familiarity, G is actually the least well defined of all the fundamental constants.

The size of G, like that of all the other fundamental constants, is known not through some theoretical argument, but through measurement. The English physicist Henry Cavendish was the first to measure it, in 1798, by analysing the gravitational attraction between two known masses that were a known distance apart. His answer for G was 6.754 × 10
^–11
metres cubed per kilogram per second squared. Today, G is officially 6.67428 × 10
^–11
m3/kg/s
²
. The uncertainty on this measurement is about one part in 10,000. Compare that to the precision with which we know the
other fundamental numbers, such as the Planck constant used in quantum theory: that is known to 2.5 parts in 100 million.

There are two reasons G is so difficult to measure accurately. The first is that it is impossible to screen out gravitational fields using any known physics. That means any measurements have to take into account the influence of any and all objects in the vicinity. This makes the measurements unreasonably sensitive to external influence; there are stories of researchers having to recalibrate their apparatus after someone two laboratories away has moved a large pile of books into their office. For this reason, gravity measurements have to be done in isolated laboratories using extraordinarily sensitive instruments.

The second difficulty with measuring the gravitational constant is the fact that gravity is the weakest of the fundamental forces. When that apple falls to the ground, it does so with relatively little acceleration, despite the fact that the mass of the entire planet is tugging it downward.

If you’re not convinced that gravity is weak – maybe you’ve done a parachute jump or been on a rollercoaster and experienced a terrifying acceleration – think about the magnets sitting happily on the door of your refrigerator. The mass of the entire planet is working to pull them towards the ground too – and yet a buttonsized dot of magnetized iron can resist the planet’s pull. Magnetism results from the electromagnetic interaction between charged particles inside a magnet. And that force is around 10
⁴²
– that is, around 1 million trillion trillion trillion – times larger than the gravitational force between them. So gravity is weak: G is astonishingly small. But why? Though the weakness of gravity is one of the central mysteries of physics, we do have some ideas that might account for it. The best is that gravity ‘leaks’ into or out of our universe.

Various branches of modern physics suggest that there are many more dimensions of space than the three (up and down, side to
side and backward and forward) that we are familiar with. One of the consequences of this is that certain forces can become ‘diluted’ through spreading into these extra dimensions. If the gravitational force is weak, that may be because it is spread more thinly than the others.

The ‘extra’ dimensions are thought to be ‘compactified’ – rolled up, essentially – so small we don’t experience them in day-to-day life. It’s just a theory at the moment, but a few researchers are trying to find evidence for this. One route is through examinations of the way the gravitational attraction between two objects changes with the distance between them.

‘One of the reasons we got here today, was because of a gentleman called Galileo.’

DAVID SCOTT

Newton showed that gravity follows an ‘inverse square law’. That means that the gravitational force one object exerts on another decreases in proportion to the square of the distance between them. Separate two objects by a metre, and measure the gravitational force. Then separate them by a further two metres and measure the attraction again. It will be nine times weaker because they are three times further away.

The hidden dimensions enter our world at submillimetre scales. If gravity behaves differently from normal on these very small scales – if the inverse square law doesn’t hold when masses are separated by only a few thousandths of a millimetre – that may be because these dimensions are interfering with things. Spot some disturbance here, then, and we might have evidence to support our most daring theories.

This is why physicists are carrying out the most exquisite experiments to probe gravity at microscopic scales. So far, however, they have found no evidence of violations of the inverse square law. That’s a great shame, because one of the roles of these advanced, multidimensional theories is to improve our best theory of gravity, Einstein’s relativity.

Einstein’s theory of relativity cast space and time as a four-dimensional fabric and said the presence of mass or energy distorted this fabric. Where Newton had declared that bodies in motion will move in a straight line unless acted on by a force, Einstein added a twist. Yes, they moved in a straight line through space, but would have to follow any distortions in that space.

The distortion that the sun’s mass creates, for instance, means that a nearby planet in motion will be pulled into a curved trajectory. Balance the masses and the speed of motion, and you have an orbit. Hence, in Einstein’s view, gravity is a kind of illusion. Though it looks like a force that acts across space and time, it is actually more like topographical features – hills and valleys – added to the landscape, features that make it hard to travel in certain directions, and easier to travel in others.

Neat though this is, and supported by numerous experimental findings, we know it is not the final answer. In a way, Einstein has only given us a clever description of
how
gravity works. The
why
is still wide open. There is hope, though. Relativity, in its current form, is not compatible with quantum theory. We will have to wait for some future ‘quantum gravity’ theory to unite the two. And that theory, presumably, will give us the
why
of gravity, just as we have recently got to grips with the
why
of mass.