The Big Questions: Physics (8 page)

Three near-simultaneous developments changed that. The investigations of English physicist Joseph J. Thomson revealed the existence of particles, which he called ‘corpuscles’, that were negatively charged and were 2,000 times lighter than even the lightest of the atoms. With this discovery, we had at last found something – we now call it the electron – that was smaller than an atom.

By 1904 Thomson, was suggesting that atoms were composed of positive and negative parts mixed together to give a ‘plum pudding’ kind of structure. Around the same time, in Paris, Pierre and Marie Curie and Henri Becquerel discovered radioactivity. Their subsequent investigations showed that at least some of the activity resulted from the emission of charged particles from atoms. Back in England, meanwhile, the brash New Zealander Ernest Rutherford had arrived. In just a few decades of research, Rutherford was to make the greatest inroads into the atom for thousands of years.

Perhaps the most significant discovery was the revelation that Thomson’s ‘plum pudding’ model of the atom was entirely wrong. Rutherford fired positively-charged alpha radiation particles – helium atoms stripped of their electrons – at a thin piece of gold foil. Almost all of the alpha particles passed straight through. Some, however, were strongly deflected. A few even bounced back at the emitter. This shocked Rutherford. ‘It was as if you had fired a fifteen-inch shell at a piece of tissue paper and it had come back and hit you,’ he later wrote.

To Rutherford, there was only one interpretation of this extraordinary result. A few of the positively charged helium atoms had happened to be fired directly at a concentration of positive charge, and been strongly repelled. Most of the volume of the atom was empty space. But at the centre lay all the positive charge – and almost all the mass. Rutherford had discovered the atomic nucleus.

The emptiness of an atom is hard to grasp, and provides our first clue to the illusion of solidity. The nucleus in the atom,
Rutherford said, is ‘like a gnat in the Albert Hall’. Others around him called it ‘the fly in the cathedral’. Either way, it’s a monstrous emptiness. If the nucleus was the size of a small apple, the edge of the atom, defined by the outer orbit of its negatively charged electrons, would be 3 kilometres (2.5 miles) in diameter. Each electron, meanwhile, would be smaller than the full stop at the end of this sentence. We can look at the emptiness another way. If you could remove the empty space in atoms, and pack hydrogen nuclei into the volume of a penny with no space between them, you would have a penny-sized object that weighed more than 30 million tons.

The nucleus in the atom, Rutherford said, is ‘like a gnat in the Albert Hall’

As the lightest element, hydrogen has the simplest possible nucleus: a single positive nuclear charge, or proton. But, generally, there is more to nuclei than just the proton. The carbon atoms we have been examining, for example, have a much more complex nucleus, containing half a dozen uncharged particles called neutrons. All atoms (apart from hydrogen) contain neutrons. The neutron, which is very slightly heavier than the proton, was discovered by James Chadwick at the University of Liverpool in the early 1930s. Carbon has a nucleus composed of six protons and, depending upon the exact ‘isotope’ we are dealing with, six, seven or eight neutrons.

So, is there any solidity here? Rutherford found the proton to be around 10
^–15
metres in diameter. The neutron is of approximately the same size. And atomic nuclei don’t mirror the emptiness of the atom. The carbon nucleus is no bigger than one would expect if the particles within it were packed tightly together. Larger nuclei make the tight packing of the nucleus even clearer. A uranium nucleus, which contains 238 particles, is only 14 proton-widths in diameter – it is rather like a basketball stuffed with 238 ping-pong balls.

With this discovery, physicists had a notion of solidity at the core of matter. But only for a while: things soon got slippery again, taking us on a downward spiral that, today, tells us there is nothing solid in the entire universe. The problem is that, being packed with positive charges, the nucleus should not hold together. The protons in a carbon nucleus should, by rights, repel each other.

That means another force must be at work. Physicists call this the ‘strong’ nuclear force simply because it has to be strong enough to overcome the repulsive electromagnetic force. To investigate the strong force required physicists to delve into the characteristics of the proton and neutron, or nucleon, as they are collectively known. What they discovered was that the nucleons were not fundamental, indivisible particles, but composed of three ‘quarks’.

The name ‘quark’ was chosen by physicist Murray Gell-Mann in 1964, who picked it out after reading the phrase ‘Three quarks for Muster Mark’ in James Joyce’s
Finnegan’s Wake
. The quark started out as a hypothetical particle, whose existence was also suggested, independently, by the Russian-American physicist George Zweig (who wanted to call it an ‘ace’). Both men’s guess turned out to be a good one – though it took a good while to prove it.

Physicists can only see matter on this scale by smashing together subatomic particles in accelerators. The collisions create smaller particles, whose fleeting existence has to be inferred by the trails left behind in detectors that line the walls of the accelerator at the collision site. The first quarks were identified from collisions at the Stanford Linear Accelerator Centre (SLAC) in 1968. Two more decades passed before all the hypothesized quark particles had been seen. But we now know that quarks come in six ‘flavours’, exotically named: strange, charm, top, bottom, and the much more common up and down.

Protons are composed of two up quarks and one down quark; neutrons are two down quarks and one up quark. But it is the top quark that may be the undoing of solidity. The top quark is
unaccountably heavy. It weighs almost the same as a gold atom, which is why it took until 1995 for our particle accelerators to be able to make one. Particle accelerators are governed by
E = mc
²
, and it takes a great deal of energy to make so much mass.

A gold atom contains 79 protons and 118 neutrons. That is a total of nearly 600 up and down quarks. How can just one top quark weigh nearly the same? Something in the nature of quarks, and how they come together, suggests there is a mystery in the nature of mass. A theory called quantum chromodynamics (QCD) makes that clear. It has shown that the up and down quarks that make up protons and neutrons account for only 1 per cent of the mass of these particles. The rest is, as provided by
E = mc
²
, held in the energy that binds the quarks together. This is the ‘strong’ nuclear force.

According to QCD, the strong force has its roots in the uncertainty principle of quantum mechanics (see
Is Everything Ultimately Random?
). This principle says that nothing that can be measured actually has a precisely defined value. That even applies to empty space: it can’t have exactly zero energy. As a result, empty space has a fluctuating but finite amount of energy.

This fluctuating energy manifests as particles called gluons, and it is gluons that create the strong force that binds the quarks. So when you hold a diamond in your hand, you feel its weight. But what you sense as the mass of the diamond is actually the result of a shifting, shimmering energy field that creates the weight of the quarks that make up the protons and neutrons in the nucleus of each carbon atom. In a sense, that diamond, that most solid of objects, doesn’t have a permanent existence at all. As it rests on your hand, all that is happening is that a continuum of energy fluctuations are manifesting as solidity.

Perhaps we shouldn’t be surprised that the rules of solidity are turning out to be flexible. Solids, after all, are only solid under certain
conditions. Heat up an ice cube, and it will create a pool of water. The molecules haven’t changed their essential nature; it is simply that the environmental conditions have altered the strength of the bonds between them. The same is true when we heat the water and it turns into steam. Now the bonds between the molecules have disappeared – but still the molecules themselves haven’t changed.

We can create a new kind of matter, at the other end of the temperature scale, too. As we cool some kinds of materials down, we can create a new kind of matter. To solid, liquid and gas, we can add the phase known as the ‘Bose-Einstein condensate’. The BEC is the result of a radical transformation that only happens at extremely low temperatures. Temperature is, in essence, a measure of how much energy an object has to ‘jiggle about’. At very low temperatures, a material is stripped of all energy, and so hardly moves at all. But quantum theory dictates that the more precisely you pin down an object’s momentum – in this case to near-zero – the bigger the uncertainty in its position. So every particle in the BEC has an uncertain position. In effect, all the particles overlap each other, merging into one big quantum object, like a giant atom.

In this state, all kinds of strange behaviours arise. When niobium metal turns into a BEC, the quantum laws turn it into a ‘superconductor’ that carries electrical current without any of the resistance associated with currents in normal metals. When helium atoms form a BEC, for example, a similar thing happens: stir a cup of this ‘superfluid’ helium, and the swirl goes on swirling for ever. Even more bizarrely, superfluid helium can defy gravity, flowing up the sides of a container. Turn helium into a solid, where its atoms are held together in a crystal, and the weirdness gets worse.

Not that it’s easy to make solid helium. To get it to a liquid requires cooling it to within 4 degrees of absolute zero. To turn that liquid into a solid requires crushing the atoms together: the liquid has to be cooled to within 1 degree of absolute zero and compressed with 25 times normal atmospheric pressure. Once you’re there, though, you can see the strangest solid in the universe.

The bonds between the atoms in solid helium are extremely weak. So weak, in fact, that atoms can break off. This leaves what is known as a ‘vacancy’ in the crystal. Physicists have long known that these vacancies can be treated like particles in their own right. They are really like an atom with slightly different properties. They affect the way a material conducts electricity, for example; it is only because of vacancies that semiconductors have the properties they do. The entire multi-billion dollar business of electronics relies on the properties of vacancies.

In an ultra-cold helium crystal, the laws of quantum mechanics lock all the vacancies in the structure together to form a vacancy-based BEC. With the atoms locked together too, the helium crystal becomes two ‘supersolids’. And, if you get the experimental conditions right, they can pass right through each other. In theory, any solid crystal will behave in this way under the right conditions.

It might not even require the formation of vacancies: in some materials it ought to be possible to make all the freed atoms lock together and move around the crystal as one, meaning that the solid will pass through itself. It’s not unlike the strange conjuring tricks where two solid rings are made to pass through each other, lock together and then, with a flourish of the magician’s hand, come apart again. In this case, though, it is the solidity that is the illusion.