The Big Questions: Physics (7 page)

So far, we have been blithely talking about mass, while avoiding the obvious question. What does it mean that something has mass?

Physicists categorize mass in two distinct ways. One is ‘gravitational mass’, which is what produces and responds to gravitational fields. This is what makes the apple fall. The other is ‘inertial mass’, which is a measure of how hard it is to move something out of its current state of motion or rest. When you try to push a broken-down car, its inertial mass stands against you.

As far as we know, inertial and gravitational mass are entirely equivalent. Imagine standing on Earth in a sealed box like a stationary elevator. You feel the push of the floor as your gravitational mass responds to the influence of gravity. Now imagine taking that elevator box into space, away from gravitational fields, and sticking a rocket engine on it that accelerates it at 9.81 metres per second per second, the acceleration due to gravity at the Earth’s surface.

There will be no difference in what you feel, Einstein says. This ‘equivalence principle’, which says there is no distinction between your gravitational mass and your inertial mass, is part of the bedrock of Einstein’s general relativity. Although we don’t have a definitive proof that it is absolutely correct, experiments have shown this is certainly true to at least one part in 10
¹²
. A decade before he created general relativity, though, Einstein asked another question about mass. In 1905, his ‘miracle year’ when he also published his special theory of relativity, Einstein came up with an interesting line of enquiry. He asked, in a landmark paper, whether the inertia of a body depends upon its energy content.

This was the origin of the world’s most famous equation:
E = mc
²
(see
Why Does E = mc
²
?
). Energy and mass, in Einstein’s view, could be interchanged. It took almost a century, but we now know through this equation that energy is indeed the root of mass. Take that apple, for instance. Its mass resides in its constituent components. Going down in scale, these are molecules, which are composed of atoms, which are composed of electrons, protons and neutrons.

The origin of the mass of the electron (which is only one-thousandth the mass of the proton and neutron) remains a mystery. But physicists are at least getting to grips with the mass of protons and neutrons. These particles are each composed of three particles called quarks. However, the masses of the quarks account for only around 1 per cent of the proton or neutron mass. The rest comes from a shadowy, quantum world of energy-stealing ‘virtual particles’.

Down at the quantum scale, the rules are very different from those that we encounter in our day-to-day lives. Here, a phenomenon called the ‘Heisenberg uncertainty principle’ holds sway, and issues strange declarations. One is that nothing has a definite amount of energy, even when that energy is zero. Instead the energy fluctuates around zero, allowing seemingly empty space – something physicists call the ‘vacuum’ – to fizz with appearing and disappearing particles.

These particles appear in pairs: a particle and its antiparticle, spontaneously created as the energy of the vacuum of empty space fluctuates around zero. According to a Nobel Prize-winning branch of physics called ‘quantum chromodynamics’ (QCD), the particles can appear with various amounts of energy, giving a spectrum of characteristics. Sometimes they take a form where they are known to physicists as ‘gluons’. Gluons create a force known as the strong nuclear force, which holds quarks together to create a proton or a neutron. And it is gluons – or rather their energy – that give the apple most of its mass. Working out exactly
how much mass comes from the energy of all these virtual particles has not been easy, involving crunching combinations of around 10,000 trillion numbers. When the results came out, though, they were within a couple of per cent of the experimentally recorded masses of these particles.

The energy associated with the gluons, converted via Einstein’s
E = mc
²
formula, accounts for almost all the mass in a proton or neutron. There is a little missing: the mysterious electron mass, and a contribution from some more virtual particles, such as pairs of virtual quarks and antiquarks, and the Higgs boson (see
What is the God Particle?
). Essentially, though, the mass of the apple – and of the Earth – is a manifestation of the energy contained in the vacuum of empty space.

The success of quantum chromodynamics in establishing the origin of mass has given physicists hope that similar ideas will eventually lead us to the final
why
of gravity: the graviton. The electric and magnetic forces are manifest through atoms exchanging of packets of energy called photons. The strong nuclear force comes via gluons, as we have seen. The weak nuclear force is known to result from the exchange of energy-laden particles known as the W and Z bosons. All of these have been seen in experiments. Gravity is thought to rely on the exchange of particles known as ‘gravitons’. These, however, remain hypothetical. Despite all our advances in understanding, we still haven’t seen a graviton.

That is not our only remaining problem with gravity, however – a much more embarrassing and basic issue remains unsolved. Bizarre as it may seem, although we have worked out the origin of mass using the most ingenious minds, the biggest computers and the greatest theories of physics, we still don’t have a good way to measure the very thing that gravity acts upon: mass. Every other standard unit of measurement has a precise, atomic foundation. The second is based on a certain number of oscillations of a caesium atom. The metre is the distance light travels in a particular fraction of that second. The kilogram,
though, is the mass of a lump of metal kept locked inside a Paris vault.

It’s not any old vault, of course: it is contained within the hallowed walls of the International Bureau of Weights and Measures (BIPM) near Paris. And it’s not any old metal, either: it is a cylinder of platinum, chosen as the most stable, incorruptible material available. The mass of this platinum cylinder is the kilogram against which all other kilograms are calibrated. The problem is, its mass is changing. Metrologists have made dozens of copies, and the original no longer weighs the same. There is about 100 micrograms of difference, roughly equivalent to the mass of a couple of grains of salt. Researchers are planning ways to bring the kilogram into line with other standards, by using atomic measurements. One hope is to create a polished sphere of silicon containing a determinable number of atoms. The kilogram will then be defined as the mass of a certain number of silicon atoms.

Another possibility is to use something called a Watt balance to measure mass in terms of energy. Einstein told us that mass and energy are interchangeable; the Watt balance would invoke this by measuring mass against the energy contained in a carefully configured electromagnetic field. Until these plans come to fruition, though, we are stuck with plugging slightly inaccurate numbers into Newton’s formula.

Gravity is everything to us – it pulled particles together to create the Earth, it holds us in orbit around our life-giving sun, it creates the tides that allowed that life to form and move on to land. And now we return the favour and use our gravity-given minds to make extraordinary discoveries about the very nature of this attraction. At the same time, though, we have only primitive tools to get the measure of it. While we can talk about gluons inside the nuclear structure of the atoms within the apple, we cannot be precise about how much the apple weighs. The essence of gravity remains deliciously difficult to tame.

Atoms, quarks and solids that slip through your fingers

If all the world was made of gas, we could not exist. The way our bodies are organized, the way information is stored in the structure of DNA, the way our brains process and hold information, all requires that atoms are fixed in place, not floating freely around. Life, at least life as we know it, requires solidity. But what is a solid?

A gas is a collection of atoms or molecules that have no or extremely weak bonds between them. A liquid has weak bonds between the particles, allowing them to slip over and past each other. A solid, though, has its particles held together by strong electrostatic bonds. But that doesn’t make a solid solid. Hold your hand up in front of your face. It looks solid enough, doesn’t it? But to neutrinos, the tiny subatomic particles that flood the universe, your body is far from solid. Every second, trillions of neutrinos pass right through you, failing to interact with a single atom of your body. Scientific progress has made it clear that most of our solid matter is empty. We have even devised solid materials with the ghostly power to pass through each other. Experimental science is teaching us that the concept of ‘solid’ is a slippery one at best.

Our brains, another mass of solid atomic matter, have been able to probe this at a level even deeper than our experiments. Though there is no certainty here yet, our best understanding leads us to a remarkable conclusion: that there is no such thing as a solid.
Every piece of matter is, essentially, the result of a random fluctuation in the energy of space and time. Solidity is, at its root, an illusion.

To explore this, let’s start with a familiar solid. Something dependable, something robust. Diamond seems a good solid to test. It is the hardest naturally occurring material, and used as a tool to cut through the toughest of metals. How solid is diamond? It is diamond’s molecular structure that makes it particularly hard. Its carbon atoms are bonded in a rigid tetrahedral arrangement, sitting about 10
^–10
metres apart from each other. Since it is the outermost electrons in the atom that form these bonds, it will come as no surprise to hear that this is roughly the size of the atom. But that doesn’t make it truly solid. It is time for us to explore the strange world of atomic structure.

The first scientist to look into this question is generally considered to be Democritus. He was actually a Greek philosopher rather than a scientist, but he made a scientific conjecture about the nature of matter. All matter, he suggested, can be split so far, but no further. At the most fundamental level was the concept of
atomos
, from which we get our word atom. In Democritus’s view,
atomos
were the particles that could not be split, destroyed or changed in any way.

And, until the earliest moments of the Industrial Revolution, that was essentially that. The age of the telescope came, and we learned to probe the heavens, but we made no progress in getting to the root of matter. That’s because we needed tools that could influence matter on the atomic scale.

It was the English schoolteacher John Dalton who kicked off the investigation of the atom. Towards the end of the 18th century, Dalton proposed that any single element was an assembly of identical atoms. These all had the same properties. Chemical reactions, he suggested, joined two different kinds of atoms together to form a chemical molecule. Dalton backed up his ideas with chemical experiments that determined the ratio of elements within certain substances, such as carbon dioxide: one part carbon to two parts oxygen.

‘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.’

ERNEST RUTHERFORD

The concept of atoms lent itself to the processes of the Industrial Revolution, enabling the pioneers of thermodynamics to work out the gas pressures and heat transfer rates that powered the rise of the machine. But we were no wiser about whether it might be possible to get inside an atom. In the age of the British Empire, the steam train and massive industrialization, the science of the atom had hardly moved on from the Greek idea of an indivisible substance.