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at r=0, the Schwarzschild metric has another surprise in store; the spacetime curvature becomes infinite. In other words, the gravitational field becomes infinite. This is known as a singularity. In physical theories, the existence of singularities signals the edge of the applicability of the theory; in simple language, there must be more to it! This has led many physicists to search for a new theory of gravity. Quantum theories of gravity such as string theory may be able to avoid the appearance of singularities, by effectively setting a minimum distance scale below which spacetime does not behave in the manner described by Einstein’s equations.

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Black holes are fascinating objects; we don’t understand them, and yet we know they exist. They are of immense importance…the physics that lies inside the event horizon is undoubtedly fundamental.

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As yet, we do not know whether any of these current theories are correct, or even if they are on the road to being correct, but what we do know is that black holes exist. At the centre of our galaxy, and possibly every galaxy in the Universe, there is believed to be a supermassive black hole. Astronomers believe this because of precise measurements of the orbit of a star known as S2. This star orbits around the intense source of radio waves known as Sagittarius A* that sits at the galactic centre. S2’s orbital period is just over 15 years, which makes it the fastest-known orbiting object, reaching speeds of up to 2 per cent of the speed of light. If the precise orbital path of an object is known, the mass of the thing it is orbiting can be calculated, and the mass of Sagittarius A* is enormous – 4.1 million times the mass of our Sun. Since the star S2 has a closest approach to the object of only 17 light hours, it is known that Saggitarus A* must be smaller than this, otherwise S2 would literally bump into it. The only known way of cramming 4.1 million times the mass of the Sun into a space less than 17 light hours across is as a black hole, which is why astronomers are so confident that a giant black hole sits at the centre of the Milky Way. These observations have recently been confirmed and refined by studying a further 27 stars, known as the S-stars, all with orbits taking them very close to Sagittarius A*.

This artist’s impression helps us to visualise the mysterious objects in space that are black holes.

Black holes are fascinating objects; we don’t understand them, and yet we know they exist. They are of immense importance, because despite the fact that we will never encounter one directly, the physics that lies inside the event horizon is undoubtedly fundamental. These are objects that will require a new theory of gravity, indeed a new theory of space and time, to describe. One of the holy grails of observational astronomy is to find a pulsar orbiting around a black hole. Such a system surely exists somewhere, and to be able to observe the behaviour of one of these massive cosmic clocks in the intensely curved spacetime close to a black hole would surely test Einstein’s Theory of General Relativity to its limit. It may even, if we are lucky, reveal flaws that point us towards a new theory

THE ANATOMY OF A BLACK HOLE

For all their mystery, we do know that black holes exist. The idea of a body so massive that even light could not escape its grip was first suggested in the eighteenth century, and today we now know that there is not only a black hole at the centre of our galaxy, but also possibly in the centre of every galaxy. We may never directly see one, but the secrets they contain may one day help us answer some of the most fundamental questions in the Universe.

Nathalie Lees © HarperCollins

NASA

CHAPTER 4

DESTINY

THE PASSAGE OF TIME

This is the story of something so fundamental that it’s impossible to imagine a universe without it, yet it is a property of the Universe that modern science still struggles to explain. Time is something that feels very human; it regulates our days and its relentless and unavoidable passing