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WHEN Albert Einstein formulated his general theory of relativity, the physical foundation of our standard cosmology, he began with a simple observation: that any object’s gravitational mass is exactly equal to its resistance to acceleration, or inertial mass. From this “equivalence principle”, he deduced equations that showed how space is warped by mass and motion, and how we see that bending as gravity. Apples fall to Earth because Earth’s mass bends space-time. In a relatively low-gravity environment such as Earth, general relativity’s effects look very like those predicted by Newton’s earlier theory, which treats gravity as a force that travels instantaneously between objects. With stronger gravitational fields, however, the predictions diverge considerably. One extra prediction of general relativity is that large accelerating masses send out tiny ripples in the weave…

It’s a weirdness exemplified by Schrödinger’s cat – a thought experiment invented by Austrian physicist Erwin Schrödinger in 1935 to demonstrate just how absurd quantum theory’s conclusions about reality seemed to be. SCHRÖDINGER’S thought experiment places a real cat in a box with a vial of poison that can be smashed, releasing gas that kills the cat, by a mechanism that depends on whether a radioactive atom decays or not. Radioactive decay is a quantum process, and according to the underlying mathematics of quantum theory, the radioactive atom exists in a quantum superposition state of decayed and not decayed before it is measured to ascertain which. When you open the box to find out what’s happened, you’ll find a cat that’s dead or alive. But what state is it in…

THE only way you know you exist as a conscious being is experience of your own consciousness. The nature – and existence – of other people’s consciousness is a closed book. For all you know, everyone else is a zombie. Let’s set philosophical solipsism to one side, however, and allow other people to have conscious experience. Do they all perceive the same events in the same way? The evidence suggests that they don’t. If you have ever watched a football match and felt incredulity at the referee’s decisions, take comfort from the fact that the opposing fans feel the same – although for the opposite reasons. Both sets will end up feeling that they were on the wrong end of all the dodgy calls. This, of course, isn’t objectively possible,…

Choosing your socks might not change the world much, but other decisions do – and our deliberative power to do just that is, in terms of the physical laws we have uncovered so far, a complete enigma. AT ITS simplest, agency is easy to define. “It is just the notion that certain systems in the world have intentional states, desires to bring stuff about,” says philosopher Eleanor Knox at King’s College London. “We’re clearly systems like that.” The arguments start with what else is too. “Quantum fields don’t have any agency. Atoms don’t, do bacteria?” asks physicist Sean Carroll at the California Institute of Technology. “I don’t know, but human beings do. Somewhere along that continuum it sneaked in.” Philosophers and theologians have been poring over that and agency’s relationships…

We humans have a bit of a problem with reality. We experience it all the time, but struggle to define it, let alone understand it. It seems so solid and yet, when we examine it closely, it melts away like a mirage. We don’t know when it began, how big it is, where it came from or where it is going, and we certainly don’t know why it exists. Finding answers was once exclusively the purview of philosophers and theologians. But over the past few centuries, scientists in many disciplines, from fundamental physics to neuroscience, have done ever more to peel back its many layers – even if we still aren’t entirely sure what we have revealed. In the chapters that follow, we will explore the various elements of reality,…

IF YOU were to list the imperfections of the standard model of particle physics, pretty high up would have to be its prediction that we don't exist. The culprit is one of the most unexpected apparitions in the theory. In 1928, British physicist Paul Dirac was attempting to marry quantum theory and Albert Einstein’s special relativity to describe how the electron worked - the very first effort to devise a relativistic quantum field theory of the sort that came to underlie the standard model. But the elegant equation he devised came with a nasty sting. For every positive-energy solution representing an electron, there was also a negative-energy one, representing… what? Dirac initially thought the "holes" in this theory might be protons, then the only other subatomic particle known. But it…

THE standard model of particle physics arguably fulfils at least one philosophical definition of reality: what exists and what does it do? Yet we know it is nowhere near a complete answer. Just the fact that it leaves out the substances that seem to make up 95 per cent of material reality, dark matter and dark energy, tells us that. Despite high hopes that the Large Hadron Collider would follow the discovery of the Higgs boson with at least some hints about a more complete theory, none has yet been forthcoming. Meanwhile the fact that the standard model, rooted in a picture of quantum particles and fields, describes three of four fundamental forces we know of in nature – but not the fourth – remains a fundamental mystery. Gravity remains…

It’s the sort of maths you might use to tell you the likely state of a rolled die that you can’t see. At first, this sort of description was seen as a mathematical convenience – but the reality, it seems, is far more mysterious. THE emergence of the wave function as a central element of quantum theory was by no means an overnight thing. Mind you, neither was the emergence of quantum theory itself. The theory was born in 1900 when German physicist Max Planck found that he could explain the baffling spectrum of light emerging from a hot furnace only if the vibrational energy of the atoms giving out the light came in discrete chunks, or quanta. Planck himself thought this was merely a convenient mathematical device. It was…

MANY experiments expose the sheer apparent strangeness of quantum reality. But quantum weirdness perhaps reaches its apogee at the half-silvered mirror: a mirror that reflects half the light that hits it, but lets the other half pass through. Let light hit the mirror in the right way, and it is not just the light beam that is split, but individual photons. They become in effect two photons. One of them passes through the mirror, and the other is reflected. Each of these photons has particular properties, for example of spin, a quantum-mechanical quantity that can be envisaged rather like a rotation in space. But something very odd happens when you decide to measure these spins one after the other. You can do this over and over again, each time measuring…

IT IS not easy having Cotard’s syndrome. People with this extremely rare neuropsychological condition are convinced they are dead or do not exist. They will often not see the need to do basic things to stay alive – eat, for example. But the really odd thing is that in even though people with Cotard’s syndrome feel they don’t exist, there is still an “I” experiencing that feeling. What is that “I”? The French philosopher René Descartes, of “I think, therefore I am” fame, was convinced that the body and conscious mind are two different substances: the first is made of matter, the latter is immaterial. These “dualist” ideas influenced neuroscience until a few decades ago, but things have moved on. Today, it is widely accepted that our brains give rise…

AT FIRST blush, to suggest that we create reality sounds like a combination of arrogance and absurdity. In what warped version of reality could reality only exist because of us? And yet if you spend any time pondering quantum theory, it is hard to escape the idea that the world becomes “real” only when we are looking at it. But does it? The starting point for this question is the way quantum wave functions predict that objects such as electrons exist in a cloud of possibilities before measurement – yet we only ever see a definite reality. On measurement, an electron always assumes a single, definite position or state – something we would recognise as real. ← Turn back to page 60 for more on the quantum wave function That…

“The eternal mystery of the world is its comprehensibility… the fact that it is comprehensible is a miracle.” The author of those words, Albert Einstein, was expressing a remarkable truth about our reality that we began to explore in the previous chapter: the existence of precise, mathematical laws that have predictive power, and so seem to express some form of objective truth about the world. This realisation brought about a revolution in human knowledge. It is a gift that has kept on giving, as over the past few centuries physicists have followed their noses – and mathematics – to an ever more accurate understanding of reality. That quest is far from over, and the deeper questions of why mathematics is so effective, and what mathematics itself consists of, remain mysterious.…

COSMOLOGY 1 Constant speed of light Nothing can exceed this cosmic speed limit Back in the 1860s, James Clerk Maxwell was melding electricity and magnetism into one unified theory. But however he sliced the equations, they only made sense if light travelled through space at the same constant speed, regardless of the speed of its source. This is odd. If someone fires a bullet from a moving car, to a bystander the bullet travels at the sum of its speed and the car’s speed. Yet when 20 years later US physicists Albert Michelson and Edward Morley were looking for the luminiferous ether, a medium supposed to carry light, they reached the same conclusion: however you look at it, the speed of light is a constant. Not only that, it is…

TO ISAAC NEWTON, space was the “sensorium of God”, the organ through which the deity surveyed His creation. It was absolute, unchanging, infinite. Flowing through it “equably without regard to anything external”, as Newton wrote in his great physical treatise Principia, was another similarly absolute heavenly creation: time. Not everyone bought that idea. Newton’s perennial antagonist Gottfried Leibniz was notably sniffy of a God who needed an organ to perceive things, and asked pointedly whether a clockmaker deity would need to “wind up his watch from time to time”. A few centuries on, God features less prominently in the debate, but arguments about the nature of space and time swirl on. Are both basic constituents of reality, or neither – or does one perhaps emerge from the other in some…

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THOMAS YOUNG’S original experiment was simple. He shone light on a screen with two tiny, parallel slits in it, and on another screen a distance behind the first saw alternating vertical fringes of light and dark. It seemed incontrovertible proof that the light was diffracting at the slits and interfering like a water wave. The real strangeness started with experiments in the 20th century that lowered the light intensity in the double-slit experiment to the point at which only a single photon entered the apparatus at any one time. Place a light-sensitive detector at one or other of the slits and you heard the beep, beep of single particles hitting it. But remove the particle detector and place a light-collecting screen – a kind of long-exposure camera – a distance…

Not so with a related quality that has been the subject of debate among philosophers since time immemorial: free will. To what extent are our decisions the product of our own desires, or just predetermined? Modern physics has given us some new ways of framing that question. “OH, I am fortune’s fool,” says Romeo in Romeo and Juliet, having killed Tybalt and realising he must leave Verona or risk death. He was expressing a view common in Shakespeare’s time: that we are all marionettes, with some higher cause pulling the strings of our destiny. Chance – let alone our own decision-making – plays little part in the unravelling of cosmic designs. In this way of thinking, even processes that inherently involve chance are pre-determined. Long before dice were used for…