The Big Questions: Physics (18 page)

If
E = mc
²
rules the modern world, thermodynamics created it. The discovery that heat was a form of energy, and thus could be converted to kinetic energy that would perform work, was revolutionary in every sense. Heat a bath of water sufficiently, and its conversion to steam – when under pressure – could move a piston. And moving a piston could change the fate of nations. The discovery of machines such as the engine and the refrigerator, powered by mechanical work or heat, created the Industrial Revolution, the foundation of the modern, technological age.

This conversion of heat energy into kinetic energy is just one example of how energy is conserved, moving between multiple forms but never disappearing from the universe. ’s Gravesande’s weights, for example, had ‘gravitational potential energy’ before he dropped them. That potential energy came from the energy, stored in his muscles, used to lift them to the drop height. That energy came from the food he ate, and that, in turn, came from his food’s ultimate energy source: sunlight. When the weights hit the clay, their potential energy, ultimately derived from sunlight, was converted to kinetic energy (or movement) in the clay, some heat energy (due to friction), and sound energy. The energy did not disappear from the universe.

Similarly, a paraffin lamp contains potential energy. When the paraffin burns, the stored chemical potential energy is released as heat and light. The heat energy will be imparted to the molecules in the air around the lamp, and will manifest as kinetic energy: the molecules will move faster.

What seems surprising, though, is that energy can take the form of mass. Mass is surely very different to energy: while mass is associated with solidity, energy seems transient and ephemeral. But there is a link – and it is found in James Clerk Maxwell’s equations of electromagnetism.

In the 1830s, Michael Faraday showed how electricity and magnetism are inter-related: electricity produces magnetism, and vice-versa. Shortly afterwards, Maxwell came up with a series of equations that detailed exactly how this process worked. Many physicists looking into Maxwell’s equations saw that they contained the essence of mass. It was well known, for example, that a box containing electromagnetic fields weighed more than one containing none. The question was, what did that mean?

The mainstream view was that inertial mass – the resistance of a body to movement – lies in the fact that charged particles would be difficult to move in the vicinity of their own
electromagnetic fields. True to his character, Einstein did not follow the mainstream. Instead, he found the answer in one of the flaws of the equations.

Maxwell once said that his equations describe a ‘mutual embrace’ between electricity and magnetism. However, it is actually a three-way embrace: electricity and magnetism do not exist without movement; the motion of charged particles creates electricity and magnetism. And herein lay a deep problem. Analysis of experiments showed that motion could invalidate the equations. If the emitter of electromagnetic radiation was moving relative to the observer, the equations no longer predicted the correct values for the electromagnetic field.

This was what motivated Einstein’s 1905 paper ‘On the Electrodynamics of Moving Bodies’, in which he introduced special relativity. Einstein’s genius was to insist that the laws of physics be consistent however you are moving through space. To do this, he modified Maxwell’s equations so that you couldn’t move in any way that changed the speed of light from its absolute value, c. The speed of light is an unassailable constant. Move towards a stationary source of light, and the light will always come at you with speed c. Move away, and you will measure it passing you at c. And here is where we find the link between energy and mass.

Einstein suggested that the existence of energy – any kind of energy – brings with it an associated mass. As he stated in a letter to his close friend Conrad Habicht, shortly after publication of the
E = mc
²
paper, ‘The relativity principle, in association with Maxwell’s fundamental equations, requires that the mass be a direct measure of the energy contained in a body; light carries mass with it.’

The first implication Einstein noted was for radioactivity: if radium was giving out energy, it ought also to lose some mass. The German physicist Max Planck saw a more prosaic (but in
some ways more profound) implication. A hot object – a frying pan, say, will weigh more than a cold one. This was a revolutionary idea – even today it still seems strange. Nevertheless, it is absolutely correct. We now have good evidence that mass is just one way of carrying energy. You can move, and carry kinetic energy, but you can also lock your energy in by simply existing. To see why, we need to explore the origin of mass.

You are made of particles that, at their root, have no independent mass. They get their mass from a quantum phenomenon known as the ‘Heisenberg uncertainty principle’. At root, this says that every quantity in nature has a fuzziness; it doesn’t have a fixed value. That’s true even of the energy of empty space: while we think of it as having zero energy, it is actually fizzing with energy that manifests as pairs of ‘virtual’ particles that disappear as quickly as they appear. These fleeting, ghostly particles, it turns out, give the frying pan its mass.

When you shrink down in scale from frying pan, to iron atom to iron nucleus, you end up looking at particles called quarks, which make up the protons and neutrons in the iron nucleus. When physicists work out the mass of quarks, there is nowhere near enough to account for the heaviness of the pan. The mass actually exists in virtual particles that manifest from the fuzzy energy of empty space. Experiments involving high energy particle collisions and the crunching of millions of numbers have confirmed that these ‘gluons’ act to hold the quarks together in the proton and neutron, and that the energy involved is what we see as most of the mass of the pan.

Hence the hot pan weighing more. Given that almost the entire mass of a frying pan comes from the fizzing energy of
empty space, it doesn’t seem quite so hard to believe that adding a bit more energy, in the form of heat, also adds to the mass. The ability of high-energy processes to release this energy is what lies at the heart of our existence. When hydrogen atoms fuse in the sun, eventually forming a helium atom, the process releases some of their gluon energy (energy that we call mass) as heat and light – the very heat and light that created life on Earth.

Thanks to the colossal size of the speed of light, coupled with the fact that
E = mc
²
, there is a surprising amount of energy locked up in ordinary matter. A single walnut, for example, has enough potential energy locked within it to power a city. We have released something like this, of course – not with walnuts, but with atoms of uranium. Suitably prepared, their gluon energy can be released to provide electrical power to cities – or to bomb them.

Whether in bombs or in power stations, we have measured the mass of the particles we begin with, the mass at the end of the process, and the amount of energy released. In every case it has been shown to be true:
E
really does equal
mc
²
. The most accurate proof we have of the validity of Einstein’s equation was carried out in 2005. Unsurprisingly, it involved painfully sensitive measurements. The energy measurement for the left hand side of the equation, for example, required a team of researchers to measure the energy of a gamma ray photon to around one part in 1 million.

For the mass side of the equation, meanwhile, the researchers had to measure how the mass of an ion changes when it gives off a gamma ray photon. That is a tiny change in mass, equivalent to seeing a hair’s breadth change in the distance from New York to Los Angeles. There were no nasty surprises: the researchers found a startling agreement between the two measurements. It seems that
E
really does equal
mc
²
, to better than one part in 2 million. You can rest easy: that one equation you know is pretty solid.

Spooky quantum links and the chance to rewrite history

Einstein put this question a slightly different way. In the early 1950s, he once turned to the young physicist Abraham Pais, raised his eyebrows, and asked, ‘Do you really believe that the moon only exists when you are looking at it?’

Einstein had spent the last two decades growing increasingly frustrated by the pioneers of quantum theory. Their ringleader, Niels Bohr, claimed that the weirdness inherent in the theory, such as atoms existing in two places at once or effects preceding their cause, could only be explained if nothing – not even the moon – really existed until it was measured or observed.

Einstein’s question to Pais was an exasperated appeal to common sense. The idea that something as big and as permanent as the moon could be at the mercy of a tiny human observer hundreds of thousands of miles away is nonsensical. But that doesn’t necessarily mean it is nonsense. With the advent of quantum theory at the beginning of the 20th century, the ridiculous had become the sublime. Pais remembers wondering why Einstein was so stuck in the past. ‘Why does this man, who contributed so incomparably much to the creation of modern physics, remain so attached to the 19th century view of causality?’ he wrote in
Subtle Is the Lord
, his biography of Einstein.

Even in the 18th century, people had been questioning the nature of reality. Bishop George Berkeley famously asserted that, if
no one was around to hear a tree falling in a forest, the tree would make no noise. What’s more, the tree doesn’t even exist unless someone is perceiving it. Fortunately, Berkeley suggested, our common sense is preserved because God is always present to act as observer.

‘Do you really believe that the moon only exists when you are looking at it?’

ALBERT EINSTEIN

Niels Bohr took the same approach to the quantum world: the only proper interpretation of the vagaries of quantum theory, Bohr said, is that nothing has any properties or existence until it is observed in some way. Einstein’s refusal to accept this idea isolated him from the development of quantum theory. What’s more, his best attempt to refute it ended with its confirmation. Every experiment we have carried out suggests that, yes, you can change the universe with a glance. The means of your power? A quantum phenomenon known as ‘entanglement’.

Erwin Schrödinger called entanglement the defining trait of quantum theory. He first spotted it in 1935, noting that the equations of quantum theory, applied to two interacting particles, impart an unusual quality. After their encounter, they can no longer be properly described as individuals. They are linked; the quantum description of particle A – its momentum or spin, for example – contains information about particle B, and vice-versa.

That has a very strange consequence. If you change the properties of particle B, you necessarily change the properties of particle A. This doesn’t require a physical link; the entanglement link changes properties whatever the separation between the two entangled particles. Two suitably prepared entangled particles can instantaneously change each other’s quantum state even when at opposite ends of the universe.

Einstein was having none of this, and dubbed it
spukhafte Fernwirkungen
: ‘spooky action at a distance’. It showed, he said,
that there were still gaps in quantum theory. And, with the help of two friends called Boris Podolsky and Nathan Rosen, he set out to prove it. The scenario the trio outlined is still the gold standard for proving the weirdness of the quantum world. It is known as the EPR paradox, and concerns the fate of two pairs of particles, with each pair separated from the other by an enormous distance.

The most rigorous experimental version of the EPR paradox was drawn up in 1964 by John Bell of the European Organisation for Nuclear Research (CERN), the particle physics laboratory based in Geneva, Switzerland. Bell imagined separating two entangled electrons, and sending them to experimenters on opposite sides of the Earth. The experimenters then simultaneously measure the electron spin. The details of the set-up are complex, but Bell’s challenge was that, if orthodox quantum theory was right and Einstein was wrong, particular kinds of measurements would show a correlation between the two spins.