'The Eye of Harmony. Exploding star in the act of becoming a black hole. Time Lord engineering. You rip the star from its...o...b..t, suspend it in a permanent state of decay.'

The Eleventh Doctor, Journey to the Centre of the TARDIS (2013)

On 28 November 1967, as the Second Doctor battled the Ice Warriors for the first time in Doctor Who, two Cambridge University radio astronomers Jocelyn Bell Burnell from Northern Ireland and Antony Hewish from Cornwall detected something strange in s.p.a.ce. Every 1.33 seconds there was a pulse of energy radio waves from the same part of the sky. Or, rather, the place it was coming from slowly moved, but perfectly in time with the movement of the stars as the Earth rotated under them (as we saw in Chapter 1). That meant it was very unlikely that the pulse had been created on Earth: it had to be coming from somewhere deep in s.p.a.ce.

Bell Burnell and Hewish called the pulse LGM-1, short for 'little green men', because the regularity of the pulse at least suggested that it was artificial in origin, as if some kind of alien life form had engineered it. Scientists soon worked out that the pulse was a natural phenomenon, produced by a kind of star, but the explanation turned out to be just as remarkable as if it really had been some alien broadcast.

Stars like the one Bell Burnell and Hewish found are now called pulsars short for 'pulsating radio star' because of the regular pulses of radio waves they create. The pulses are the result of two characteristics of a pulsar. First, they have strong magnetic fields. Earth also has its own, less strong magnetic fields which can interact with particles emitted from the Sun to create dazzling displays of light in the atmosphere, usually at high lat.i.tudes near the North and South Poles. These displays are called aurora, or the Northern and Southern Lights. Jupiter, Saturn, Ura.n.u.s and Neptune also have magnetic fields that create aurorae.

The Earth's relatively weak magnetic field is able to funnel electrically charged particles from the Sun down over our planet's poles, where they collide with molecules of air high above the ground and cause them to glow. But the magnetic field of a pulsar is around a trillion times stronger than the Earth's, blasting charged particles away from its magnetic poles and out into s.p.a.ce. As they accelerate, these particles emit radio waves along their direction of travel, creating a narrow beam of electromagnetic energy that shines out from each of the star's two magnetic poles. But the magnetic poles are not precisely lined up with the star's axis of spin (this is also true on Earth, which is why the magnetic north indicated by a compa.s.s needle is a few degrees different from the true, geographical north shown on maps). As the pulsar spins, these beams of radiation sweep around the sky, and each time one pa.s.ses over the Earth we detect a pulse of radio waves. Imagine a lighthouse with a revolving lantern at the top, casting out a beam of light. The beam sweeps round and round in the darkness but if you were out at sea, too far away to see the lighthouse, you'd see a regular flashing on and off of light. (With the pulsar, only one beam hits the Earth; the other is facing in the wrong direction.) Of course, the speed of the pulse that Bell Burnell and Hewish found showed that the pulsar had to be revolving very quickly every 1.33 seconds. This is dizzyingly faster than the rotation period of an ordinary star like our Sun, which spins once every 24.47 days (measured at its equator). Even the Earth's 24-hour rotation seems leisurely by comparison. But this incredible speed wasn't the strangest thing about the discovery of the first pulsar.

Astronomers knew that when a very large star reaches the end of its life, it explodes in what's called a supernova leaving behind a huge cloud, or nebula, of gas called a supernova remnant. The gas ejected in these violent stellar death throes can eventually condense and collapse to form a new generation of stars and planets but some physicists wondered whether supernova explosions might also leave something else behind. They calculated that, as the outer layers of the dying star were blasted into s.p.a.ce, the star's core would suffer the opposite fate, being crushed and squeezed by gravity into a tiny remnant made almost entirely of neutrons. They called this theoretical remnant a neutron star. It would contain almost as much matter as our Sun, but squeezed into a compact ball just ten kilometres across. A teaspoon full of this incredibly dense material would weigh as much as a thousand Egyptian pyramids. That might come in useful: a s.p.a.ceship made of such material would have so much ma.s.s it could warp s.p.a.ce, allowing it to cross long distances more quickly. That's what happens in the Doctor Who story Warriors' Gate (1981), with the ship made of 'dwarf star alloy'.

The pulsar discovered in 1967 was spinning so rapidly that astronomers knew it must also be very small just a few kilometres across. Yet its intense magnetic field implied that it contained a ma.s.s similar to that of the Sun. The pulsar was clearly a neutron star the first one ever found.

However, by the same calculations with which physicists had predicted the existence of neutron stars before finding one, some had suggested that the explosion of an even more enormous star would create a sort of funnel of increasing density which would have extraordinary properties. In 1964, an American science journalist, Ann E. Ewing, came up with a name for this amazing (and, some thought, ridiculous) idea: she called the funnel a black hole.

'A black hole's a dead star. It collapses in on itself, in and in and in until the matter's so dense and tight it starts to pull everything else in, too. Nothing in the universe can escape it. Light, gravity, time. Everything just gets pulled inside and crushed.'

The Tenth Doctor, The Impossible Planet (2006)

The gravity of an object depends on how much matter it contains, and the more densely its matter is squeezed together the more intense its gravity becomes. This gravity warps the s.p.a.ce-time around the object, causing nearby objects to fall towards it. We discussed in Chapter 1 how a very ma.s.sive object such as our Sun warps its surrounding s.p.a.ce-time so much that the planet Mercury doesn't appear where it ought to according to Newton's laws of gravity.

A black hole contains at least four times as much material as our Sun, but squeezed into a region far smaller than a neutron star. As we see in the Doctor Who episodes The Impossible Planet and The Satan Pit (2006), it's very difficult for anything such as a planet or s.p.a.ceship to escape the enormous gravitational pull close to a black hole. But the closer you get, the stronger the gravitational pull becomes. At a certain distance called the event horizon the pull is so strong that you would need to be moving faster than the speed of light to escape it. According to Einstein, nothing can move faster than the speed of light, which means that nothing not even light itself can escape the gravitational pull at this point. (According to Doctor Who, the TARDIS can, which is why it can rescue s.p.a.ceships from inside a black hole, as it does in The Satan Pit.) Once inside the spherical boundary of the event horizon your fate is sealed. All possible trajectories lead only to the centre of the black hole a point of zero size and infinite density which physicists call a singularity.

If light cannot escape from a black hole, we'll never be able to see one directly which is why they were given that name. But the discovery of a neutron star proved the theory that had predicted them, and suggested that black holes might really exist, too. That encouraged more physicists to puzzle out what they would be like and to look for them.

How can you find something that's effectively invisible because light can't escape from it? There are ways of detecting things that are otherwise invisible, by looking for the effects that they have on their surroundings. For example, in the Doctor Who story The Daleks' Master Plan (19651966), the Doctor, Steven and Sara are transported to another planet, but don't know where they are. We see they are not alone: an invisible something leaves a trail of footprints as it follows the Doctor. Later, when the invisible creature makes a noise and moves the branches of the foliage, the Doctor strikes out with his walking stick and makes contact with something he cannot see. He strikes again, confirming his suspicion that it's an invisible creature and forcing it to retreat. From this encounter, he deduces that he has just been attacked by a Visian, which means he's on the planet Mira.

In the same way, we can deduce the existence of invisible black holes from their effects on their surroundings. Rather than leaving footprints or moving foliage, material from nearby can spiral round the black hole in what's called an accretion disk. The strong gravitational field close to a black hole can pull nearby gas towards it, making it spiral inwards faster and faster and heating it to extremely high temperatures. Before it disappears for ever inside the black hole's event horizon, this doomed material emits a blaze of radiation, from infrared to X- and gamma rays, allowing our telescopes to detect it.

At slightly larger distances from a black hole, objects can just about orbit safely, but the hole's extreme gravity yanks them round at very high speeds. An object such as a star moving in such a tight orbit will still be visible and its rapid motion can betray the presence of an invisible companion the black hole. Astronomers can detect this motion by studying the spectrum of light which the star gives off. In 1972, astronomers Louise Webster and Paul Murdin at the Royal Greenwich Observatory and Charles Thomas Bolton at the University of Toronto published evidence of exactly this phenomenon, suggesting they had found the first black hole: Cygnus X-1. The discovery or all the talk about black holes generally inspired the people making Doctor Who at the time.

'Singularity is a point in s.p.a.ce-time which can exist only inside a black hole. We are in a black hole, in a world of antimatter very close to this point of singularity, where all the known physical laws cease to exist. Now, Omega has got control of singularity and has learned to use the vast forces locked up inside the black hole.'

'Now, that is how Omega is able to create the world we are now living in by a fantastic effort of his will.'

The Second and Third Doctors, The Three Doctors (19721973)

The Three Doctors was the first Doctor Who story to feature a black hole, and its writers also threw in another exotic (but unrelated) prediction of theoretical physics called antimatter (see here). It's a very important black hole, created by a Time Lord engineer called Omega as the power source that originally gave the Doctor's people their mastery over time. In fact, just as we refer to 'the' Sun when it's only one of millions of similar suns out in s.p.a.ce, the Time Lords refer to this as 'the' black hole.

In fact, we now know that at the centre of every galaxy is a 'superma.s.sive' black hole yes, that's what astronomers really call them. The superma.s.sive black hole at the heart of our galaxy, the Milky Way, is estimated to have a ma.s.s four million times that of our Sun (which is itself 330,000 times the ma.s.s of Earth). That's the material of four million stars squeezed into a tiny point at the centre of the galaxy. Don't worry, though: we're at a very comfortable distance of 26,000 light years from this monster black hole: in fact our solar system, and almost everything else in the galaxy, is safely orbiting round it.

How do we know this if, again, we can't see a black hole? The deduction work goes like this. It takes our solar system over 200 million years to make one complete orbit around the galaxy, but stars in the centre of the Milky Way zip around their orbits in just a few years. Moving at such rapid speeds, these stars would be flung out of the galactic centre unless there was a very powerful source of gravity and yet no object is visible to hold them in place. From their speeds and the size of their orbits scientists can calculate the ma.s.s of the central, invisible object. It turns out to be very small but very ma.s.sive indeed and invisible. A superma.s.sive black hole is the only answer that makes sense.

But how could a black hole help Doctor Who's Time Lords travel in time? We already know that an object with a large ma.s.s warps s.p.a.ce-time. If you were standing on the surface of a neutron star wearing a special s.p.a.cesuit so you were not squished by the intense gravity time would be slowed down by about thirty per cent compared to time on Earth. Time would seem to pa.s.s normally for you, but if somehow you could see the Earth from where you stood, things happening there would appear to be speeded up. Effectively, the neutron star's gravity would allow you to travel through time by fast-forwarding into the Earth's future.

Close to a black hole, the effect would be even greater. In fact, warping of s.p.a.ce-time caused by the black hole's extreme gravity can lead to some very strange effects indeed. As you fell towards a black hole an observer watching you at a safe distance from the event horizon would see time appear to slow down for you. On reaching the event horizon itself, the observer would see you apparently frozen in time. But from your point of view time would seem to pa.s.s normally, while events in the rest of the universe appeared to run at an ever increasing rate. If you could somehow pause at the event horizon and look out at the rest of the universe, you'd see all of eternity pa.s.sing. That's what seems to happen to Mother of Mine at the end of The Family of Blood (2007): we are told that the Tenth Doctor tricks her into the event horizon of a collapsing galaxy.

But if you journey past the event horizon, there is no force which could stop your fall and, as you plummeted ever closer to the singularity, the distorting effects of gravity would become even more p.r.o.nounced. Once inside the event horizon, the normal directions of s.p.a.ce and time become wrenched around. The s.p.a.ce direction which was 'down' towards the centre of the hole becomes the time direction 'into the future'. Effectively your destiny becomes the singularity at the centre of the black hole and the universe outside the event horizon is forever barred to you, because it's locked in your personal past. So travelling into a black hole inevitably involves travelling through time, faster and faster into the future and to escape it you'd need to be able to travel backwards in time into the past.

In effect, whatever the size of the black hole she was imprisoned in, Mother of Mine wouldn't feel she was there for very long: time outside would whizz by in an instant as she plummeted to her doom in the singularity. It's a boggling concept and not easy to comprehend. But if a ship such as the TARDIS could somehow move freely inside the black hole's gravitational field it would inevitably also travel through time. No wonder the Time Lords are interested in black holes.

'You know why this TARDIS is always rattling about the place? ... It's designed to have six pilots, and I have to do it single-handed. ... Now we can fly this thing ... Like it's meant to be flown. We've got the Torchwood Rift looped around the TARDIS by Mr Smith, and we're going to fly Planet Earth back home.'

The Tenth Doctor, Journey's End (2008)

In The Deadly a.s.sa.s.sin (1976), the Doctor reveals that all the power of the Time Lords devolves from the nucleus of a black hole called the Eye of Harmony though it's not clear if this is the same black hole as the one in The Three Doctors. According to the Time Lords' own histories, the Eye of Harmony was created by a Time Lord called Ra.s.silon not Omega. That might just be the official history leaving out Omega's role, but whereas we see the black hole in The Three Doctors somewhere out in s.p.a.ce, the Eye of Harmony is revealed to exist on the Time Lord home planet, Gallifrey. It's such an outlandish idea a black hole being kept on a planet that other Time Lords don't believe it.

Another of the Doctor's own people his companion, Romana is surprised when the Doctor suggests in The Horns of Nimon (19791980) that a black hole can be created artificially using a gravity beam to track matter to one point in s.p.a.ce, until there's enough ma.s.s pressed together that it collapses to a singularity. It turns out that two artificial black holes have been created, with a hyperspatial tunnel between them. The fact that this is a surprise in the story is important: it suggests that the Doctor and his people don't fully understand the physics of black holes.

Yet in the television movie Doctor Who (1996), we learn that the Doctor's TARDIS is powered by something also called the Eye of Harmony. When opened, this power source has the ability to warp matter so that the Doctor can step through a pane of gla.s.s, while later it endangers the whole planet. It's not stated in the TV movie that this Eye of Harmony is a black hole, but the Eleventh Doctor confirms that in Journey to the Centre of the TARDIS (2013).

We're not told if the Eye of Harmony in the Doctor's TARDIS is the same Eye of Harmony that was on Gallifrey in The Deadly a.s.sa.s.sin. Perhaps every TARDIS was somehow linked to the same, single Eye of Harmony, which is still on Gallifrey. However, Gallifrey has been lost since the last day of the Time War (which we'll discuss more in Chapter 9) the Doctor thought his home planet had been destroyed before discovering in The Day of the Doctor (2013) that he himself had moved it into a pocket universe. If there was a continuing link between his TARDIS and Gallifrey, surely he would never have believed the planet had been destroyed, and he might even have been able to use that link to make contact again with his people. There would also be no reason for him to refuel the TARDIS with rift energy, as he does in Boom Town (2005).

So it might be that every TARDIS contains its own unique black hole, each one called an Eye of Harmony. But other Time Lords in The Deadly a.s.sa.s.sin and The Horns of Nimon don't seem to know much about black holes. It would be like people driving cars but not knowing about them having engines or needing petrol.

That suggests another possibility: some time between The Deadly a.s.sa.s.sin and the TV movie, did the Doctor take the one and only Eye of Harmony from where it was kept on Gallifrey and use it as a new power source for his TARDIS? The Doctor tells Rose in The Satan Pit that his people 'practically invented black holes ... Well, in fact, they did.' But it seems that he's one of the few Time Lords to know that.

At least, that's one theory, deduced from the available evidence. We might learn in a future episode that it's not right at all that all TARDISes have black holes for engines, and there's nothing special about the one in the Doctor's TARDIS. But that's the point: we need more and better evidence before we can be sure.

Science is a series of statements that are revised and sometimes completely overturned when new evidence comes along. As we saw in Chapter 1, the progress of science has often been the result of people puzzling over the bits of evidence that don't fit the accepted rule: that the position of Mercury doesn't fit what Newton predicted, for example. Our knowledge is provisional rather than certain.

We're still not absolutely certain that Cygnus X-1 is a black hole it's just that the evidence available to us so far seems overwhelmingly to fit that idea. But it's possible that it might turn out to be something else entirely, or that we have to completely revise our ideas about black holes. Time, and better evidence, will tell.

In fact, a vast amount of the universe remains a complete mystery to us. Just four per cent of the universe is made up of baryonic matter the atoms that make up galaxies, stars, planets and people, and that we can detect directly. We can deduce that something else is out there: something that does not absorb or emit light (meaning we can't see it) but that generates enough gravity to affect things around it just like the footprints of the invisible Visians. This mysterious stuff affects the speed that stars...o...b..t in galaxies and the way galaxies cl.u.s.ter together. We can even deduce that this stuff exists in the halo of our own Milky Way galaxy, and that it makes up twenty-three per cent of everything in the universe. But we currently don't know exactly what it's made of scientists simply refer to it as 'dark matter'.

It gets stranger still. Our observations of distant galaxies tell us that the universe is expanding getting bigger and bigger. This is what we would expect for a universe which began in the tremendous explosion of the Big Bang. But over time we would expect the gravity of the galaxies to act as a brake, pulling against the expansion and slowing it down. Instead astronomers have discovered the opposite rather than gradually slowing down, the expansion of the universe is actually getting faster and faster as time goes by. So there must be something else in the universe, some kind of invisible force or energy which acts against gravity. Scientists have named this mysterious quant.i.ty 'dark energy' and we can deduce that it makes up the remaining seventy-three per cent of the universe almost three quarters of everything out there. Together, dark matter and dark energy make up a whopping ninety-six per cent of everything in the universe almost all of it! These invisible components help to explain the large-scale structure of the universe we can see, but they also remind us just how little of what's out there we currently understand.

In spite of these mysteries or because of them we continue to explore the universe with our telescopes and s.p.a.cecraft, looking for answers, slowly deducing the strange and incredible nature of reality.

In fact, that's what really drives the TARDIS not the black hole it has as an engine, but the att.i.tude of its pilot. Other Time Lords have TARDISes, but the Doctor uses his to explore time and s.p.a.ce in search of the weird and wonderful, the stuff he doesn't already know. It's this unknown wonder that he offers each of his companions and so each of us watching his adventures on TV, as well. And that's why Doctor Who remains so successful even after fifty years: it is powered by curiosity.

Antimatter In The Three Doctors, the black hole is the source of an energy beam reaching to the Earth, a bit like the beams of electromagnetic radiation generated by a pulsar. (In fact, scientists think that black holes can also generate jets of matter and energy which are blasted into s.p.a.ce.) Journeying along the beam and through the black hole, the Doctors discover a universe composed entirely of antimatter.

Although scientists don't know exactly what happens to material once it reaches the singularity inside a black hole, it's unlikely to be a gateway to a universe of antimatter. But antimatter does exist and is made of particles that have the same ma.s.s as ordinary matter but with the opposite electrical charge. For example, normal matter contains electrons which have a negative charge, but in antimatter the role of electrons is taken by similar particles with a positive charge called 'positrons'. When matter and antimatter come into contact they destroy each other completely in a tremendous blast of energy a process known as 'annihilation'. This process is important to the ending of both The Three Doctors and Arc of Infinity (1983).

As far as we can tell, our universe is made almost entirely of normal matter. It's still a bit of a mystery why the universe wasn't created with equal amounts of matter and antimatter, but perhaps it's just as well, because it wouldn't have lasted very long if it had been. However, it's possible to imagine universes like the ones in The Three Doctors or Planet of Evil (1975), where antimatter dominates over matter.

Despite its extreme rarity, antimatter is found in our universe, and tiny amounts can even be made artificially in particle accelerators like the Large Hadron Collider at CERN in Geneva. Antimatter particles are also created by natural processes such as lightning strikes and radioactive decay, although with so much normal matter around they don't usually last long. The extreme physical environments around neutron stars and black holes are also good places to look for antimatter particles and astronomers have detected the characteristic bursts of energy as they annihilate with particles of normal matter. Scientists have speculated that combining matter and antimatter could be a very efficient way of producing large amounts of energy, but the main problem would be collecting enough antimatter to be useful in the first place. So we're still a long way from understanding antimatter well enough to use it to power s.p.a.cecraft, as they do in Earthshock (1982).

'"Kiss-me-quick-squeeze-me-slowly"?'

'Yes! Hilarious. See?'

'"Kiss. Me. Quick... Squeeze. Me. Slowly." Nope. Still nothing.'

'It's just a joke.'

'Is speed of central importance in these actions?'

Clara fixed the Doctor with a look, which he ignored as he placed the pink shiny metallic hat back down on the TARDIS console without trying it on.