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Pedro Ferreira

INSTANT EXPERT 1

100703_F_General Relativit.indd 1 23/6/10 11:03:26 Einstein’s insight In 1905, at the age of 26, proposed energy that it contains. Now known his special . The theory reconciled as the , and ”Space tells matter how the of moving bodies developed by Galileo published in 1916, they supplanted to move and matter tells Galilei and Newton with the laws of electromagnetic Newton’s law of universal gravitation radiation. It posits that the is always and are still used today, nearly a space how to curve” the same, irrespective of the motion of the person century later. who measures it. implies that Using general relativity, Einstein space and time are intertwined to a degree never made a series of predictions. He previously imagined. showed, for example, how his theory Starting in 1907, Einstein began trying to would lead to the observed drift in broaden special relativity to include . His first Mercury’s orbit. He also predicted breakthrough came when he was working in a patent that a massive object, such as the office in Bern, Switzerland. “Suddenly a thought struck , should distort the path taken by me,” he recalled. “If a man falls freely, he would not light passing close to it: in effect, the feel his weight… This simple thought experiment… geometry of space should act as a lens led me to the theory of gravity.” He realised that there and focus the light (see diagram). is a deep relationship between systems affected by Einstein also argued that the gravity and ones that are accelerating. wavelength of light emitted close to The next big step forward came when Einstein was a massive body should be stretched, introduced to the mathematics of geometry developed or red-shifted, as it climbs out of the by the 19th-century German mathematicians Carl warped space-time near the massive Friedrich Gauss and Bernhard Riemann. Einstein object. These three predictions are applied their work to write down the equations that now called the three classical tests relate the geometry of space-time to the amount of of general relativity.

Einstein suggested that light rays skimming past the sun CLUSTER, would be bent by its gravity. To test the idea, Arthur 150 LIGHT YEARS AWAY rst photographed the Hyades at night. APPARENT He then needed to photograph them when they were POSITION ACTUAL on the far side of the sun. For this picture, it only became POSITIONS possible to see the starlight when the glare of the sun APPARENT was eliminated by a total . His images POSITION con rmed Einstein’s prediction

According to general relativity, space-time can be viewed as a smooth, exible sheet that bends under the inuence of SUN massive objects

EARTH The mass of the sun bends space-time, so bright rays from the Hyades cluster bend too. Viewed from Earth, the stars appear to have shifted

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100703_F_General Relativit.indd 18 25/6/10 12:39:52 HISTORY OF GENERAL RELATIVITY

Albert Einstein’s general theory of relativity is one of the towering achievements of 20th-century physics. Published in 1916, it explains that what we perceive as the force of gravity in fact arises from the curvature of space and time. Einstein proposed that objects such as the sun and the Earth change this geometry. In the presence of matter and energy it can evolve,

ibrary stretch and warp, forming ridges, mountains and valleys that cause l bodies moving through it to zigzag and curve. So although Earth hoto p hoto appears to be pulled towards the sun by gravity, there is no such science science force. It is simply the geometry of space-time around the sun telling Gravity Earth how to move. before The general theory of relativity has far-reaching consequences. Einstein It not only explains the motion of the planets; it can also describe the history and expansion of the universe, the physics of black holes and In 1686, Isaac Newton proposed an incredibly powerful theory of motion. the bending of light from distant stars and galaxies. At its core was the law of universal gravitation, which states that the force of gravity between two objects is proportional to each of their masses and inversely proportional to the square of their distance apart. Newton’s law is universal because it can be applied to any situation where gravity is important: apples falling from trees, planets orbiting the sun, and many, many more. For more than 200 years, Newton’s theory of gravity was successfully used to predict the motions of celestial bodies and accurately describe the orbits of the planets in Physicist, superstar the solar system. Such was its power In 1919, the English astronomer Arthur Eddington there was some controversy that that in 1846 the French astronomer travelled to the island of Príncipe off the coast of west Eddington’s analysis had been biased Urbain Le Verrier was able to use it Africa to see if he could detect the lensing of light towards general relativity. Matters to predict the existence of Neptune. predicted by general relativity. His plan was to observe were put to rest in the late 1970s There was, however, one case By 1930, Albert a bright cluster of stars called the Hyades as the sun when the photographic plates were where Newton’s theory didn’t seem Einstein and passed in front of them, as seen from Earth. To see analysed again and Eddington’s to give the correct answer. Le Verrier Arthur Eddington the starlight, Eddington needed a total solar eclipse analysis was shown to be correct. measured Mercury’s orbit with were famous for to blot out the glare of the sun. Eddington’s result turned Einstein exquisite precision and found that it their work on If Einstein’s theory was correct, the positions into an international superstar: drifted by a tiny amount – less than general relativity of the stars in the Hyades would appear to shift “Einstein’s theory triumphs” was one-hundredth of a degree over by about 1/2000th of a degree. the headline of The Times of London. a century – relative to what would To pinpoint the position of the From then on, as more consequences be expected from Newton’s theory. Hyades in the sky, Eddington first of his theory have been discovered, The discrepancy between Newton’s took a picture at night from Oxford. general relativity has become theory and Mercury’s orbit was still Then, on 29 , he entrenched in the popular unresolved at the beginning of the photographed the Hyades as they imagination, with its descriptions of 20th century. lay almost directly behind the sun expanding universes and black holes.

pl

/s during the total eclipse that Príncipe In 1959, the American physicists experienced that day. Comparing Robert Pound and Glen Rebka SOCIETY

L L the two measurements, Eddington measured the gravitational red- ICA was able to show that the shift was shifting of light in their laboratory Images of the 1919 as Einstein had predicted and too large at Harvard University, thereby ASTRONO M

solar eclipse proved that L to be explained by Newton’s theory. confirming the last of the three

gravity bends starlight ROYA Following the eclipse expedition, classical tests of general relativity.

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100703_F_General Relativit.indd 19 25/6/10 13:05:40 ”No black holes have been seen directly yet, though there is overwhelming evidence that they exist”

Black holes Shortly after Einstein proposed his general theory of with normal stars. As a star orbits The evidence now points to there relativity, a German physicist called the , it slowly sheds some being a supermassive black hole at found one of the first and most important solutions of its material and emits X-rays. The the centre of every galaxy, including to Einstein’s field equations. Now known as the first such black hole to be observed our own. Indeed, observations of the Schwarzschild solution, it describes the geometry was Cygnus X-1, and there are now orbits of stars near the centre of the of space-time around extremely dense stars – and a number of well-measured X-ray Milky Way show that they are moving it has some very strange features. binaries with black holes of about in very tightly bound orbits. These can For a start, right at the centre of such bodies, the 10 times the mass of the sun. be understood if the space-time they curvature of space-time becomes infinite – forming a Evidence for much larger black live in is deeply distorted by the feature called a singularity. An even stranger feature holes came in the 1960s when a presence of a supermassive black hole is an invisible spherical surface, known as the number of very bright and distant with more than 4 million times the horizon, surrounding the singularity. Nothing, not objects were observed in the sky. mass of the sun. even light, can escape the . You can Known as quasars, they arise from Despite their names, British almost think of the Schwarzschild singularity as a the havoc black holes seem to create physicist has hole in the fabric of space-time. at the cores of galaxies. Gas at the pointed out that black holes may not In the 1960s, the New Zealand mathematician centre of a galaxy forms a swirling be completely black. He argues that, discovered a more general class of solutions disc as it is sucked into the black near the event horizon, the quantum to Einstein’s field equations. These describe dense hole. Such is the strength of the creation of particles and antiparticles objects that are spinning, and they are even more black hole’s pull that the swirling may lead to a very faint glow. This bizarre than Schwarzschild’s solution. gas emits copious amounts of glow, which has become known as The objects that Schwarzschild and Kerr’s solutions energy that can be seen many , has not been describe are known as black holes. Although no black billions of light years away. Current detected yet because it is so faint. holes have been seen directly, there is overwhelming estimates place these black holes Yet, over time, Hawking radiation evidence that they exist. They are normally detected at between a million and a billion would be enough to remove all the through the effect they have on nearby astrophysical times the mass of the sun. As a energy and mass from a black hole, bodies such as stars or gas. result, they are called supermassive causing all black holes to eventually The smallest black holes can be found paired up black holes. evaporate and disappear.

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100703_F_General Relativit.indd 20 23/6/10 11:04:59 HOW GENERAL RELATIVITY SHAPES OUR UNIVERSE

Einstein’s general theory of relativity has revealed that the universe is an extreme place. We now know it was hot and dense and has been expanding for the past 13.7 billion years. It is also populated with incredibly warped regions of space-time called black holes that trap anything falling within their clutches.

The expanding universe

One of general relativity’s most striking predictions arises if we consider what happens to the universe as a whole. Shortly after Einstein published his theory, Russian meteorologist and mathematician and Belgian priest Georges Lemaître showed that it predicted that the universe should evolve in response to all the energy it contains. They argued that the universe should start off small and dense, and expand and dilute with time. As a result, galaxies should drift away from each other. Einstein was initially sceptical of Friedmann and Lemaître’s conclusion, favouring a static universe. But a

NASA discovery by the American astronomer Edwin Hubble changed his mind. Images from the Hubble Hubble analysed how galaxies Space Telescope (above) recede from the Milky Way. He found and Chandra X-ray that distant galaxies move away Observatory have firmed faster than those that are relatively up our relativity-based nearby. Hubble’s observations ideas about the universe showed that the universe was indeed expanding. This model of the cosmos later became known as the . Over the past 20 years, a plethora of powerful observations by satellites and large telescopes have further firmed up the evidence for an expanding and evolving universe. We have obtained an accurate measure of the expansion rate of the universe and of the temperature of the “relic radiation” left over from the big bang, and we have been able to observe young galaxies when the universe O was in its infancy. It is now accepted C/ SA X

/C that the universe is about 13.7 billion

NASA years old.

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100703_F_General Relativit.indd 21 23/6/10 11:05:13 Gravitational waves According to general relativity, even empty space- primordial gravitational waves are time, devoid of stars and galaxies, can have a life too faint to be detectable directly, of its own. Ripples known as gravitational waves but it should be possible to see their can propagate across space in much the same way imprint on the relic radiation from that ripples spread across the surface of a pond. the big bang – the cosmic microwave One of the remaining tests of general relativity background. Experiments are now is to measure gravitational waves directly. To this under way to search for these end, experimental physicists have built the Laser signatures. Interferometer Gravitational-Wave Observatory (LIGO) Gravitational waves should also at Hanford, Washington, and Livingston, Louisiana. Each be emitted when two black holes experiment consists of laser beams that are reflected collide. As they spiral in towards each between mirrors placed up to 4 kilometres apart. If a other, they should emit a burst of passes through, it will slightly distort gravitational waves with a particular space-time, leading to a shift in the laser beams. By signature. Provided the collision is monitoring time variations in the laser beams, it is sufficiently close and sufficiently possible to search for the effects of gravitational waves. violent, it may be possible to observe No one has yet detected a gravitational wave directly, them with instruments on Earth. but we do have indirect evidence that they exist. When A more ambitious project is the pulsars orbit very dense stars, we expect them to emit Laser Interferometer Space Antenna a steady stream of gravitational waves, losing energy (LISA), made up of a trio of satellites in the process so that their orbits gradually become that will follow the Earth in its orbit smaller. Measurement of the decay of binary pulsars’ around the sun. They will emit orbits has confirmed that they do indeed lose energy precisely calibrated laser beams and the best explanation is that these pulsars are towards each other, much like LIGO. losing energy in the form of gravitational waves. Any passing gravitational wave will Pulsars are not the only expected source of slightly distort space-time and lead to gravitational waves. The big bang should have created a detectable shift in the laser beams. gravitational waves that still propagate through the NASA and the European Space Agency cosmos as gentle ripples in space-time. These hope to launch LISA in the next decade.

The LIGO detectors (left) are looking for gravitational waves ringing through space

Time travel Einstein’s theory allows for the intriguing The possibility of time travel ”It is possible to build possibility of time travel. One suggested way of can lead to physical paradoxes, tunnels linking different achieving this involves the construction of tunnels such as the grandfather paradox in called that link different parts of which the time traveller goes back in parts of space and space at different times. It is possible to build time and kills her grandfather before different parts of time – wormholes – in theory. But unfortunately they he has met her grandmother. As a would require matter with negative energy, and result, one of her parents would not in theory, at least” other unnatural physical circumstances, not only have been conceived and the time to open them up but also to allow them to be traveller herself would not exist. It has traversed. Another possibility is to create a large been argued, however, that physical region of space that rotates, or use hypothetical paradoxes such as these are, in objects called cosmic strings. practice, impossible to create.

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100703_F_General Relativit.indd 22 25/6/10 13:14:33 FRONTIERS OF GENERAL RELATIVITY

General relativity predicts that the universe is full of exotic phenomena. Space-time can tremble like the surface of a pond and it seems to be full of a mysterious form of energy that is pushing it apart. It is also conceivable for space-time to be so warped that it becomes possible to travel backwards in time.

The dark universe The expanding universe predicted by general relativity observations, which emerged just over has become firmly entrenched in modern science. As a decade ago, is that the expansion of w I c

S our ability to observe distant galaxies and map out the the universe seems to be speeding up. T S / cosmos has improved, our picture of the universe has One explanation for this

; NASA revealed some even more exotic features. accelerating expansion is that the

WFI For a start, astronomers have been able to universe is permeated by an exotic measure how fast distant spiral galaxies spin, and this form of energy, known as dark energy. al.; ESO t shows that the outskirts of galaxies are rotating far Unlike ordinary matter and dark lowe e

C too quickly to be reined in by the mass of the stars and matter, which bend space-time in a . D gas at their centres. More matter is needed in galaxies way that draws masses together, dark

rizona/ to generate enough gravity to prevent galaxies from energy pushes space apart, making it A . U flying apart. expand ever more quickly over time. The popular explanation is that galaxies contain If we weigh up all the forms of large quantities of other forms of matter – known as matter and energy in the universe we al.; Magellan/

t “dark matter” because it does not emit or reflect light. end up with a striking conclusion: only ch e

t Dark matter is thought to clump around galaxies and 4 per cent of the universe is in the clusters of galaxies in gigantic balls known as halos. form of the matter we are familiar Dark matter halos can be dense enough to with. Around 24 per cent is dark /M.Markevi A f

C significantly distort space-time and bend the path matter and 72 per cent is dark energy. / C X of any light rays that pass close by. This gravitational This result emerged from the : C t

igh lensing has been observed in a number of clusters of marriage of the general theory of galaxies, and is one of the strongest pieces of relativity and modern astronomy evidence for the existence of dark matter. and it has become a prime focus of

ave bullock R But that’s not all. Cosmologists have been able to physics. Experimenters and theorists : D t figure out how fast the universe expanded at different are directing their efforts at trying times in its history. This is done by measuring the to answer the burning questions: far lef distance to exploding stars called supernovae, and what exactly are dark matter and NASA how quickly they are receding due to the expansion dark energy? And why do they have enze/

H of space-time. The ground-breaking results from these such strange properties?

The of galaxies known as the Bullet cluster provides good evidence for dark matter (added in blue)

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100703_F_General Relativit.indd 23 25/6/10 13:04:36 Pedro Ferreira Pedro Ferreira is professor of Next at the University of Oxford. He works on the origin INSTANT of large-scale structures in the EXPERT universe, on the general theory of relativity and on the nature of er dark matter and dark energy

Adl Ian Wilmut s è cloning M ouri e ll 7 August sabe I

The big unSOLVED problem

General relativity is only one of vibrating strings. Depending on RECOMMENDED reading the pillars of modern physics. The how they vibrate, the strings will be The State of the Universe by Pedro G. Ferreira (Phoenix) other is , which perceived as different particles – Black Holes and Time Warps: Einstein’s describes what happens at the atomic including the graviton, the particle Outrageous Legacy by and subatomic scale. Its modern thought to carry the gravitational force. (Papermac) Gravity: An Introduction to Einstein’s incarnation, quantum field theory, Another possibility is that space- General Theory of Relativity by James has been spectacularly successful time is not smooth but built up of B. Hartle (Addison-Wesley) at describing and predicting the discrete building blocks that interact Time Travel in Einstein’s Universe by Richard Gott (Phoenix) behaviour of fundamental particles with each other. As a result, if we Was Einstein Right? Putting General and forces. were able to peer at its fine structure, Relativity to the Test by Clifford Will The main challenge now is to it might look like a frothy space-time (Basic Books) combine the two ideas into one foam. In such theories, what we Cover image overarching theory, to be known as perceive as the space-time that bends XMM-Newton/ESA/NASA quantum gravity. Such a theory would and warps smoothly in the presence be crucial for explaining the first of matter is merely an emergent moments of the big bang, when the phenomenon masking more radical universe was dense, hot and small, behaviour on small scales. or what happens near the singularity The quest for the theory of at the cores of black holes, where quantum gravity is arguably the the effects of quantum physics biggest challenge facing modern may compete with those of physics. One of the difficulties is general relativity. that it only really manifests itself at Although there is as yet no final extremely high energies, well beyond theory of quantum gravity, there our experimental reach. Physicists are several candidate theories being now face the task of devising actively explored. One is string theory, experiments and astronomical which describes the fundamental observations that can test candidate constituents of matter not as point- theories of quantum gravity in the like particles but as microscopic real world.

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