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TO SAY Albert Einstein is a towering genius of modern physics is not to say anything anyone didn’t know. But it is hard to get your head around the full scope of the cosmic revolution his theories of relativity wrought in the early years of the 20th century. It is no exaggeration to say that today we live in Einstein’s universe. This latest New Scientist Essential Guide, the tenth in the series, aims to give an overview of that revolution, from the basics of Einstein’s special and general theories of relativity to all the strange and wondrous discoveries we have made off the back of them: the expanding big bang universe, the mysterious entities of dark matter and dark energy, black holes and the ripples in space and time known…

Albert Einstein’s name has become a byword for genius. He is arguably the most influential and recognisable scientist of the 20th century. Einstein was born into a family of Jewish heritage in the city of Ulm in what is now the south of Germany on 14 March 1879. He moved to Switzerland in 1895, becoming a Swiss citizen in 1901. Einstein studied physics and maths in Zurich, receiving a PhD from the university there in 1905, by which stage he was working at the Swiss Patent Office in Bern. It was in this year, at the age of 26, that Einstein published three papers which would alone have ranked him among the half-dozen great pioneers of 20th-century physics. Those papers dealt with the special theory of relativity, the way tiny…

For 380,000 years after the big bang, there was nothing but a ludicrously hot, dense soup of subatomic particles. Photons of light pinballed between them, unable to escape, trapped just as light is trapped in a dense fog. At some point in the universe’s expansion and cooling, however, things had cooled enough for hydrogen atoms to form. This cleared the subatomic obstacles from the way of the photons. It was as if someone had flicked a switch: light shot out in every direction and the cosmos became transparent. The cosmic microwave background is made up of those first liberated photons. Those photons have been travelling in all directions through the expanding cosmos ever since. You can’t see this light with the naked eye because, as the universe expanded, it stretched…

WIMPS The textbook solution to dark matter is that it is a thick, slow-moving soup of weakly interacting massive particles (WIMPs). That could explain the odd way galaxies rotate – yet no detector has yet found a WIMP. If they do exist, it seems they must be lighter than we thought. MACHOS This is the idea is that dark matter is just normal stuff hiding at the edges of galaxies, “massive astrophysical compact halo objects” that are so dim as to be invisible. Candidates include black holes or failed stars. Alas, MACHOs could only account for a tiny fraction of the universe’s missing mass. MACROS It could be that the dark stuff is made of dense clumps of quarks, the particles that, in pairs or triplets, form ordinary matter. These…

THE success of the lambda-CDM model in reproducing the features of the cosmic microwave background (CMB) suggest a definitive test of consistency. You could take precise measurements of the universe’s expansion rate when the radiation was released and use the model to wind forward and predict the current rate of expansion, known as the Hubble constant. “It’s the ultimate end-to-end test of the universe,” says Adam Riess, an astrophysicist at Johns Hopkins University in Maryland who led one of the teams that uncovered dark energy. “To go from the beginning to the end and have the two ends of the bridge that you are building meet up.” The trouble is that they don’t meet. When we extrapolate forwards from the big bang using lambda-CDM, we get a lower rate of…

Before the discovery of dark energy, the future of the universe depended on geometry. Either the cosmos was closed and would collapse in on itself in a “big crunch” or it was open and would expand forever. Now, however, cosmology’s standard model assumes that we live in a flat universe that, thanks to dark energy, will expand eternally. If dark energy is nothing more exotic than a cosmological constant, meaning it doesn’t fluctuate over time, then the heat death of the universe (or the “big freeze”) will be the outcome. But it isn’t the only possibility. An alternative is the “big rip”. Here, the dark energy keeps getting stronger and the expansion of the universe keeps accelerating. This is, in some ways, the most exciting option on the table. Even…

A RATHER glib distinction is often made between the two pillars of modern physics. Quantum mechanics is the physics of the very small, while general relativity is the physics of the very large. That’s not quite accurate – for example, quantum-mechanical effects have been observed spanning hundreds of kilometres. And at some scale, surely these two supremely accurate theories must come together. Yet wherever they do cross paths, the two theories fail to play nicely together – such as around black holes. Even things as basic as space and time highlight how badly these two theories get along. Relativity’s space-time is a smooth four-dimensional sheet; the quantum field theories that underlie the standard model suggest that space is pixelated into units with sizes of about 10-³⁵ metres, and don’t even…

A crucial step from the special to the general theory of relativity was treating time and the three dimensions of space as one unified entity: space-time. But this idea wasn’t Einstein’s. It was Russian mathematician Hermann Minkowski who showed that all the strange results of length contraction and time dilation in special relativity could be understood in terms of foreshortened views of an object with different orientations in a 4D space-time. This unification comes with a twist. Whereas the extent of an object in space can be given by the three-dimensional version of Pythagoras’s famous formula, s2 = x2 + y2 + z2, in Minkowski’s conception, the 4D extension of an object is given by s2 = x2 + y2 + z2 – c2 t2, where c is the speed…

WHY are we here? How did the universe begin? According to the Bushongo people of central Africa, before us there was only darkness, water and the god Bumba. One day Bumba, in pain from a stomach ache, vomited up the sun. The sun evaporated some of the water, leaving land. Still in discomfort, Bumba vomited up the moon, the stars and then the leopard, the crocodile, the turtle and, finally, humans. This creation myth, like many others, wrestles with the kinds of questions that we all still ask today. With the general theory of relativity, Albert Einstein provided the tools to supply, for the first time, some sort of scientifically grounded answer. Not that Einstein believed it when he first saw it. At the time he was formulating the general…

A CENTURY ago, if you asked a cosmologist the universe’s age, the answer may have been “infinite”. These days, most agree that it has a finite age of about 13.8 billion years, give or take. We can cross-check that against the oldest star we know of. HD 140283, aka the Methuselah star, is made almost entirely of hydrogen and helium, the predominant elements in existence in the big bang’s aftermath. Astronomers reckon it is 14.46 billion years old, give or take 0.8 billion years. That could make it slightly older than the universe, but the fact that the age of the oldest star is so close to our estimates of the universe’s age suggests we are on the right general track. Getting your head around what an expanding universe of…

THE ghosts first entered the cosmological machine back in the 1930s, although no one took much note back then. That was when astronomer Fritz Zwicky noticed that galaxies towards the edge of the Coma cluster of galaxies were rotating faster around the cluster’s centre than they should have been, given the amount of visible matter there was. But the idea only really gained traction in the late 1970s, when astronomers Vera Rubin and Kent Ford found a consistent effect in rotating spiral galaxies: stars were orbiting their galactic centres faster than we would expect based on how massive the galaxies are presumed to be, just counting stars and adding their masses together. The gravity of the visible matter would be too weak to hold these galaxies together according to general…

Despite dark matter’s long-standing refusal to reveal itself, most physicists remain confident that it exists – the evidence in its favour is just too great. A few, however, champion a very different possibility. Rather than explaining the motions of stars around galaxies with new forms of matter, they speculate that a different conception of gravity may be the answer. These ideas fall under the general umbrella of modified Newtonian dynamics, or MOND. This postulates that gravity works ordinarily here on Earth and in our solar system, but differently in the low-acceleration environments experienced by stars throughout the Milky Way and other galaxies. In these circumstances, the force of gravity is effectively stronger than Newton or Einstein thought. This strengthening of gravity creates the illusion that unseen dark matter must be…

OF THE simplifying assumptions used to make the fiendishly complex equations of general relativity tractable, one has a particularly distinguished pedigree. When Nicolaus Copernicus laid out the Copernican principle in the 16th century, it was to say that Earth isn’t the centre of the universe. In modern cosmology, it has morphed into the cosmological principle: that Earth is nowhere special at all. We see the universe from a representative standpoint, and draw conclusions that can apply everywhere else, too. The universe is homogeneous, looking roughly the same in all locations, and it is isotropic, looking roughly the same in all directions from any standpoint. For some, these are simplifications too far. In the universe today, galaxies exist in clusters and filaments of matter distributed around the boundaries of huge, bubble-shaped…

A BLACK hole’s defining feature is its event horizon, the boundary inside which gravity is so strong that light can’t escape. As nothing can go faster than light, this means anything crossing the event horizon is irreversibly lost to the outside universe. At least, that is the case on a simple reading of general relativity. This also says that hidden from view in the heart of a black hole is a “singularity”, an infinitely warped edge or boundary of space-time where the laws of physics break down. Any matter that hits a singularity – and, crucially, any information encoded in that matter, for example how the molecules in a cloud of gas are distributed – must disappear from space-time. This is a challenge to traditional views in physics about time…

MATTER causes space-time to curve, says general relativity. When a large mass accelerates, that curvature should change. The result is ripples in space-time that spread out at the speed of light, just as electromagnetic waves generated by accelerating electric charges spread. Gravitational waves are actually a subtle consequence of the special theory of relativity derived by Einstein in 1905, where they serve to prevent gravitational influences propagating across space-time instantaneously, faster than the speed of light. The analogy with electromagnetic waves, also bound to the speed of light, isn’t perfect: rather than propagating through space-time as electromagnetic waves do, gravitational waves are contractions and expansions of space-time itself. Because gravity is much weaker than electromagnetism, they are also minuscule by comparison. All of that makes calculations with gravitational waves rather…

Each of LIGO’s detectors, at two sites in Washington and Louisiana, consists of two 4-kilometre-long arms. They are identical in almost every respect. The arms each contain a steel pipe housing a vacuum through which one half of a split laser beam pings back and forth, bouncing off a mirror at the far end. Back at the main building, this reflected laser beam is reunited with its twin that has zipped up and down the other arm. Any change in the length of either arm caused by a passing gravitational wave distorting space-time would alter the patterns of constructive and destructive interference that form when the two light waves recombine. Gravitational waves squeeze space in one direction while stretching it in the other, making the LIGO detector with its arms…

STRING THEORY String theory predicts that space has hidden extra dimensions, invoking symmetries embedded in these 10 (mainly tiny) dimensions to “fold” energy into geometric shapes that look like certain fundamental particles, or mimic the way that space curves in the presence of mass. Different string vibrations produce particles with different characteristics: electrons, quarks, the Higgs boson, the hypothetical “graviton”. M-THEORY A more general 11-dimensional theory that ties together variants of string theory. A puzzling feature is that it predicts many (possibly infinitely many) ways of curling up these dimensions, leading to a multiverse of different universes. Some may look like ours, with three generations of quarks and leptons and four forces; many will not. But from a theoretical point of view, they all seem plausible. LOOP QUANTUM GRAVITY It…

THE basic scientific statement of relativity goes back to way before Einstein, to the time of Galileo Galilei and Isaac Newton. It holds simply that the same laws of physics apply for all “inertial” observers – ones moving in straight lines at constant speeds relative to one another. Galileo wrote of a passenger inside a ship who cannot tell if it is moving or standing still “so long as the motion is uniform and not fluctuating this way and that”. The analogy appeared in Galileo’s Dialogue Concerning the Two Chief World Systems, his treatise of 1632 that got him into hot water with the Catholic church for discussing Copernicus’s idea that Earth goes round the sun. It was aimed squarely at those sceptics who believed that Earth couldn’t be moving…

WE SEE its effects in how stars move within galaxies, and how galaxies move within galaxy clusters. Without it, we can’t explain how such large collections of matter came to exist, and certainly not how they hang together today. But what it is, we don’t know. Welcome to one of the biggest mysteries in the universe: what makes up most of it. Our best measurements indicate that some 85 per cent of all matter in our universe consists of “dark matter” made of something that isn’t atoms. Huge underground experiments built to catch glimpses of dark matter particles as they pass through Earth have seen nothing. We hoped that particle-smashing experiments at the Large Hadron Collider (LHC), buried underground near Geneva in Switzerland, would create dark matter – but they…

IT WAS while serving in the German army on the Russian front in the winter of 1915 to 1916 that physicist Karl Schwarzschild sent Albert Einstein some papers. He had solved Einstein’s equations of general relativity for the first time and shown what happens to space-time inside and outside a massive object – in this case, a perfectly spherical, non-spinning star. Einstein was thrilled. He wasn’t so thrilled with a prediction that eventually emerged from Schwarzschild’s work. Make a star compact enough and it could develop a gravitational pull so great, and warp space-time so much, that even light wouldn’t get away. In fact, he wasn’t the first person to toy with the idea of an object so massive that not even light can escape the pull of its gravity…

No one knows for certain what goes on beyond a black hole’s event horizon – but we can sketch out some of what would happen to you if you were to stray too close. For a start, a black hole’s gravitational pull is so massive that time itself would start to warp. You wouldn’t feel anything different as you fell in, but to anyone watching, you would appear to slow down. “That’s when the universe starts to go bizarre on you,” says Priyamvada Natarajan at Yale University. Circling down the drain of this cosmic plughole, all the photons being pulled alongside you would create a stream of blinding light orbiting a hole of total blackness. Two freaky effects would colour the final approach: a looming darkness would wash over your…

GENERAL relativity predicts that infalling matter ends up concentrating onto a single central point of infinite density, called a singularity. This is a sort of end to reality, a point where time itself stops and everything vanishes into nothingness. But this prediction isn’t reliable, because the centre of the hole is outside the domain of Einstein’s great theory. Here, gravity is so strong that quantum effects can’t be neglected. To understand what happens, we need a quantum theory of gravity. → Chapter 6 has more on quantum gravity Quantum theory has a habit of solving problems of this kind. At the beginning of the 20th century, classical theory predicted that the energy of an electron orbiting the atomic nucleus would spiral down infinitely. Quantum theory clarified why this doesn’t happen: it…

Everything in the universe is constantly being stretched and squeezed by gravitational waves. That suggests a way we can make detections of a background of waves permeating the universe using normal radio telescopes, rather than making single detections with ultra-sensitive detectors on Earth or in orbit. That is the guiding idea behind the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) consortium, which uses a so-called pulsar timing array to attempt to build a sort of map of gravitational waves. Pulsars are neutron stars that rotate extremely rapidly and regularly, sending out beams of light that act as “ticks” in extraordinarily precise cosmic clocks. When a gravitational wave passes through the same region of space-time that those beams of light are travelling through, it makes the light appear to take…

LET’S say you want to meet a friend for coffee. You have to tell them where you are going to be – your location in space – but you also need to let them know when. Both bits of information are necessary because we live in a four-dimensional continuum: three-dimensional space and everything within it, from steaming coffee machines to stars exploding in faraway galaxies, all happening at different moments of one-dimensional time. “Space-time” is simply the physical universe inside which we and everything else exists. And yet, even after millennia living in it, we still don’t know what space-time actually is. Physicists have strived to work it out for more than a century. In recent years, many of us have been trying to figure out what might be the…