What a Wonderful World (25 page)

In recent years, the technology of choice has been the laser interferometer which attempts to measure the deformation of space with rulers made out of laser light. Gravitational waves have the peculiar property that, as they pass, they simultaneously stretch matter in one direction while squeezing it in a
perpendicular
direction. Giant gravitational-wave detectors, each with two perpendicular arms to detect this effect, have been built in Europe and the US. The Laser Interferometer Gravitational Observatory (LIGO), for instance, consists of detectors in two different US states, and has perpendicular arms 4 kilometres long.

Physicists operating such detectors face apparently
insurmountable
difficulties. By the time gravitational ripples from even the most powerful astrophysical sources reach the Earth, they are enormously attenuated by distance. Experimenters are faced with detecting a deformation of space so tiny that it would change the
distance between the Earth and the Sun by less than a tenth of an atomic diameter.

Likely sources of a strong enough pulse are not only a pair of neutron stars or black holes spiralling together but the birth of a black hole in the catastrophic collapse of the core of a star. The latter process is believed to occur when an extremely massive star detonates as a supernova.

Although there is an enormous amount of indirect evidence that black holes exist, there is no direct evidence because, by their very nature, they are very small and very black. However, in the formation of a black hole, the membrane, or event horizon, that surrounds it is expected to vibrate violently, generating copious gravitational waves. Crucially, just as the sound from a bell is unique to the bell, revealing its shape and size, the ripples in space–time spreading outwards from the birth of a black hole are expected to be an unmistakable signature of the event. The detection of the birth cry of a black hole will not only confirm Einstein’s theory of gravity but at long last will provide the definitive proof of the existence of black holes.

The comparison of gravitational waves with sound waves is apt. For all of human history, we have obtained our knowledge of the Universe essentially from light – our sense of sight.
14
As far as the Universe is concerned, we have been stone deaf.
Gravitational
waves are the
sound of gravity
. Once we detect them, we shall at last be able to ‘hear’ the Universe.

Richard Feynman was arguably the most important American physicist of the post-war era. He won the Nobel Prize for
devising
the theory of quantum electrodynamics, which describes how light interacts with matter and, in doing so, explains pretty much every aspect of the everyday world. In
The Feynman Lectures on Physics
, Feynman asks, ‘If in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words?’ Feynman answers, ‘All things are made of atoms.’
1

The idea of the atom has an ancient history. Around 440
BC
, the Greek philosopher Democritus picked up a stick or rock or it might have been a vase, and asked himself, ‘If I could cut this object in half, then in half again, could I go on subdividing like this for ever?’ To Democritus it was inconceivable that he could. Sooner or later, he reasoned, he would come to a grain of matter that could not be cut in half any more. Since the Greek for uncuttable was
a tomos
, Democritus called such an indivisible grain an ‘atom’.

Democritus further postulated that atoms come in just a
handful
of different types. And by combining them in different ways it is possible to make a flower or a cloud or a newborn baby. ‘By convention there is colour, by convention sweetness, by
convention
bitterness, but in reality there are atoms and the void,’ said Democritus.

It was a remarkable leap of the imagination. The world around us looks bewilderingly complex. But this is an illusion, according to Democritus. Beneath the skin of reality things are simple.
2
Everything is made of a limited number of types of atom.
Everything
is in the combinations. Atoms, in short, are the alphabet of nature.

Democritus was led to his idea by the power of thought. But atoms, if they existed, were far too small to see with the naked eye. It took more than two millennia and the rise of science before indirect evidence was found for Democritus’ idea. In a steam engine, for instance, steam pushes with a pressure on its
container
. If the container is fitted with a movable wall – a piston – this can then drive machinery such as a spinning machine or a train. The movement of the piston can be explained, scientists discovered, if steam consists of countless tiny atoms
3
flying about randomly through space. Their ceaseless drumming on a piston like raindrops on a tin roof creates a jittery force, which, smoothed out, we observe as pressure.
4

‘So many of the properties of matter, especially when in the gaseous form, can be deduced from the hypothesis that their minute parts are in rapid motion, the velocity increasing with the temperature,’ said the nineteenth-century Scottish physicist James Clerk Maxwell. ‘The relations between pressure, temperature and density in a perfect gas can be explained by supposing the particles move with uniform velocity in straight lines, striking against the sides of the containing vessel and thus producing pressure.’
5

The behaviour of gases such as steam provides evidence of Democritus’ idea that reality is composed of tiny grains of matter. But what about his idea that those grains also come in different types? Proof of this came from an unexpected direction.

For a long time, alchemists hoped it might be possible to turn base materials such as lead into precious stuff such as gold. Not only did they fail but they proved the opposite of what they set out to show. Some substances cannot, by any means, be broken down into simpler ones. In the late eighteenth century, the French man Antoine Lavoisier guessed that such
elemental
materials are large collections of a single type of atom. Gold was an obvious element. But, over the years, chemists – the successors of alchemists – discovered many more. Today, we know of 92 naturally occurring elements, ranging from the lightest,
hydrogen
, all the way up to the heaviest, uranium – and we have even made heavier, artificial, elements such as plutonium.

By 1815, the English physician William Prout had noticed that most atoms appear to have a mass that is a whole-number multiple of the mass of a hydrogen atom. This led him to
propose
that atoms are actually made of smaller things – hydrogen atoms. Actually, as the atom was systematically broken apart in the late nineteenth and early twentieth century, it became clear that it was made of not one smaller thing but
three
smaller things. Protons and neutrons are close in mass to Prout’s hydrogen building block while electrons are about 2,000 times lighter.

The picture that emerged gradually was of an atom as a
miniature
Solar System. At the centre, like a Sun, is a tiny nucleus,
containing
pretty much all the mass of the atom in the form of protons and neutrons (the exception being the lightest atom, hydrogen, whose nucleus contains only a proton). Around the nucleus, like planets around the Sun, there orbit electrons. The protons in a nucleus each have a positive electric charge and they are matched by an equal number of electrons with a negative
charge. In fact, it is the force of attraction between opposite charges that keeps the electrons bound to the nucleus.

The planetary picture of the atom was deduced by the New Zealand physicist Ernest Rutherford in 1911. Rutherford’s protégés, Hans Geiger and Ernest Marsden, had in 1909 fired
subatomic
bullets from the world’s smallest machine-gun – a sample of radioactive radium – at a thin foil of gold. The picture of the atom at the time was of a Christmas pudding, with electrons studded like raisins in a sphere of positive charge. This predicted that Geiger’s and Marsden’s subatomic bullets – alpha particles – would fly through the gold atoms as surely as real bullets would fly though a cloud of gnats. To the astonishment of the two young experimenters, however, 1 in 8,000
bounced back
. It took Rutherford two years to deduce that, contrary to the
plum-pudding
model, 99.9 per cent of the mass of the atom must be concentrated in a tiny nucleus, which 1 in 8,000 alpha particles had hit and bounced off.

One of the shocks of the planetary picture was of the
incredible
emptiness of an atom. A whopping 99.9999999999999 per cent is nothingness. If you could squeeze all the empty space out of all the atoms in all the people in the world, you could fit the human race in the volume of a sugar cube. The best image of the atom comes from the English playwright Tom Stoppard: ‘Now make a fist, and if your fist is as big as the nucleus of an atom, then the atom is as big as St Paul’s, and if it happens to be a hydrogen atom then it has a single electron flitting about like a moth in an empty cathedral, now by the dome, now by the altar.’
6

The electrons, whirling far from the nucleus, represent the surface of the atom, where it makes contact with the world of other atoms. Their number – which is matched by the number
of protons – therefore determines how an atom behaves; for instance, how it links with other atoms to make molecules. The lightest atom, hydrogen, has one proton in its nucleus and one electron circling; the second lightest, helium, two protons and two electrons; the third lightest, lithium, three protons and three electrons; and so on.

The neutrons in a nucleus carry no electric charge and play no role in determining how an atom presents itself to the world. Instead, they act as nuclear peacemakers, gluing the nucleus together via the strong nuclear force. Without their stabilising presence, the enormous electrical repulsion between protons would blast the nucleus apart.

It may seem that, with the atom turning out to be made of even smaller building blocks – protons, neutrons and electrons – Democritus’ idea has gone out of the window. However, his proposal was merely that matter, ultimately, is made of
indivisible
grains. And it turns out that Democritus is right. The world
is
ultimately made of indivisible grains. It is just that they are not what we have chosen to call atoms. That is our mistake. The
ultimate
elemental building blocks turn out instead to be
subatomic
particles known as leptons and quarks.

In fact, normal matter appears to be made of just four basic building blocks: two leptons and two quarks. The two leptons are the electron and the electron-neutrino. The electron is well known because most commonly it orbits in atoms, but the neutrino is less familiar, mainly because it is so amazingly
unsociable
. Although neutrinos are generated in prodigious quantities
by the sunlight-generating nuclear reactions at the heart of the Sun, they interact with normal matter so rarely that they fly through the Earth as if it were transparent.
7
In fact, about 100 billion solar neutrinos are streaming through your thumbnail every second without you ever noticing. Eight and a half minutes ago, they were in the heart of the Sun.
8

In addition to the two leptons, there are two quarks – the
up-quark
and the down-quark. These clump together in threes to make the proton and the neutron, with the proton consisting of two up-quarks and one down-quark, and the neutron two
down-quarks
and one up-quark. The existence of quarks was proved by essentially repeating Geiger’s and Marsden’s experiment of 1909 in which they fired alpha particles into atoms and saw that they were deflected by the atomic nucleus deep inside. Physicists
instead
fired electrons into protons. In experiments carried out in the late 1960s and early 1970s, the ricocheting electrons revealed the existence of three point-like particles deep inside: the quarks.

Although physicists are now certain that protons and neutrons are made of quarks, bizarrely it is impossible to knock one out and create a free quark. This is because of the peculiar behaviour of the strong nuclear force that glues together quarks. Not only is it super strong – Newton was right to say, ‘The smallest
particles
may cohere by the strongest attractions’ – it gets stronger the further apart are two quarks. It is as if they are joined by elastic that resists more the more it is stretched. Long before two quarks are free of each other, the energy put into stretching the ‘elastic’ is transformed into the mass energy of new particles, as permitted by the law of conservation of energy. Specifically, the laws of particle physics cause a quark–antiquark pair to be
conjured
into existence.
9
Experimenters must now separate two more
quarks. But, in attempting to do that, they will create two more quarks, and so on.

But how do we know that the electron, neutrino, up-quark and down-quark are really nature ’s ultimate indivisible grains? The answer is because of the Pauli Exclusion Principle.
10
This quantum edict states that certain subatomic particles cannot share the same quantum numbers. In the case of the electron, this means that two electrons in an atom cannot share the same orbit (and spin). This ensures that electrons do not pile on top of each other, which would effectively make possible only one kind of atom rather than the 92 whose combinations create the variety of our world.

The Pauli Principle is a consequence of three things, one of which is that particles such as electrons are indistinguishable.
11
If two things are indistinguishable, it implies they have no substructure – otherwise the arrangement of their components could be used to tell them apart. The point is that leptons and quarks both obey the Pauli Principle. And the only way they can do this is if they are indistinguishable – that is, if they have no
substructure
and are truly nature ’s indivisible grains of matter.

So is that it? Ultimately, the world is made of just four building blocks – the electron, neutrino, up-quark and down-quark? Not quite. There is a twist. Isn’t there always? For some mysterious reason, nature has decided to
triplicate
its building blocks!
Instead
of one quartet of particles, there are
three
quartets, each containing successively more massive versions of essentially the
same particles
. So, in addition to generation 1, which consists of the electron, electron-neutrino, up-quark and down-quark, there is generation 2, which consists of the
heavier
muon,
muon-neutrino
, strange-quark and charm-quark, and generation 3,
which consists of the
even heavier
tau, tau-neutrino, bottom-quark and top-quark.

Bizarrely, neither of the two heavier families plays any role in the everyday world. In fact, since it takes a large amount of energy to create them, they were common only in the
super-energetic
fireball of the big bang in the first split second of the Universe ’s existence. When the muon – essentially a heavier version of the electron – was discovered in 1936, the American physicist ‘I. I.’ Rabi said, ‘Who ordered
that
?’ The same could be said of the all the duplicates of nature ’s four basic building blocks: ‘Who ordered
them
?’

But how do we know there are not many more than three generations of fundamental building blocks? The answer comes from a surprising place: cosmology. Between 1 and 10 minutes after the birth of the Universe, the big-bang fireball was hot enough and dense enough for protons and neutrons to run into each other and stick together to make nuclei of the second heaviest element, helium. Remarkably, this primordial helium has survived until today and it can be observed throughout the Universe. Astronomers find that it accounts for about 10 per cent of all atoms. However, it turns out that, if there are many more generations of neutrinos, the gravity of their extra mass would have braked the expansion of the big-bang fireball,
causing
the Universe to stay denser and hotter for longer so that it cooked up a different amount of helium. According to
calculations
, a Universe with 10 per cent helium atoms is possible only if there are at most three or four generations of neutrinos. So there may be a fourth, even heavier, generation of fundamental building blocks still to be found. However, most physicists would bet against it.
12