Black Holes 101

Artist's impression of a black hole.

With new blockbuster movie Interstellar now in cinemas, there's a flurry of interest in black holes and wormholes. Theoretical physicist Kip Thorne was a scientific consultant for the production and insisted that the depiction should stay within legitimate boundaries. Apart from the odd bit of artistic license, of course!

Black holes are scary, right? They suck in everything in their path. They devour whole planets, stars even, ripping them apart like mere wisps of smoke. They condemn anything that confronts them to an unknowable oblivion. It’s the stuff of nightmare, or at least a bad disaster movie.

But I think black holes get a bad press. They are misunderstood, misrepresented. The truth is they are fascinating creatures, if confusing, and not a little bit weird. So, relax for a moment while I give you my quick and dirty guide to black holes. The Black Hole 101, if you like.

Let’s start with a simple definition of a black hole. A black hole is an object of sufficient gravitational force that nothing, not even light, can escape. 

Matter attracts matter due to ‘gravity’, right? The more matter doing the attracting, the stronger the gravitational ‘pull’. With enough matter, the pull of gravity is so great that you cannot get free of it, even if you travel at the speed of light. Since nothing can travel faster than light, nothing can escape the object’s gravitational pull. And if light can’t escape, then the object must be invisible. That’s the basic idea.

Unfortunately, the name ‘black hole’ is somewhat confusing; these aren’t ‘holes’ at all (and it turns out they’re not ‘black’ either!). They aren’t funnel-shaped things sucking stuff in; they are spherical objects, just like stars, and they have a definite surface which we call the ‘event horizon’. Above the event horizon matter and light can still escape the gravitational pull. Below the event horizon, it can’t and is lost forever!

We need to be careful in our definition of a black hole here. It actually isn’t the amount of matter in a black hole that's important; it’s how much space that matter is crammed into.

The strength of gravity depends on two things; the mass of an object and its distance from you. So, if we travel towards a star, for example, its gravity would increase until we reach its surface. If we could keep traveling into the star’s interior, we’d find gravity decreases again. This is because the gravity of the material above us counteracts that lying below. So, for a spherical object like a star, the maximum gravitational force is at the surface. Now imagine we compact the entire star’s mass into a black hole only a few miles wide. This time, as we travel towards it, we pass the point where the original surface of the star lay, but this time gravity continues to increase, not decrease. By the time we reach the black hole’s event horizon, the gravity is far greater than we ever felt from the original star, even though it has the same mass.

So, black holes have strong gravity because they are small for their mass and because we can get close to them! The Earth’s gravity isn’t dangerous to us (unless we fall off something!), but it would be very dangerous if we shrank the Earth down to the size of a marble then tried to stand on it. Same mass: different size. One a planet, the other a black hole.

The obvious conclusion of this is that if the Sun were suddenly to turn into a black hole, it would have no effect on us whatsoever (other than us dying of cold!). The Earth would continue to orbit the resulting black hole in exactly the same way as it does the Sun, since the masses of the two objects haven’t changed. We’d be in trouble though if we tried to get close to our new neighbor!

So, a black hole is lots of ‘stuff’ crammed into a small space. But, wait, I hear you say! How can gravity, which effects matter, prevent light, which consists of massless particles (photons) from escaping a black hole? Huh?

Well, it’s common to think of gravity as ‘pulling’ on matter, just as we have done in the discussion above (oops!). But Einstein showed us that gravity can be envisaged in an entirely different way. Mass, said Einstein, actually distorts ‘space-time’ (the four-dimensional fabric of the Universe) in such a way that the normal rules of geometry no longer apply. Strangely, in this new geometry, the shortest distance between two points is no longer a straight line, but a curve. All matter and energy (including photons) will obey this new geometry and ‘fall’ towards the mass, whether or not they have ‘mass’ themselves. It’s not because the particle is feeling a force, but because the particle is traveling along what is effectively a straight line on a curved surface. And it’s not just black holes that ‘attract’ photons in this way – all matter does.

Due to their extreme gravitational force, black holes are very strange beasts. They seriously mess around with space and time. They actually distort the fabric of space itself. If they are spinning (and most are, very rapidly), they drag space around with them, tying it in an ever tighter knot. Weird, or what?

So, what would happen if you fell headlong into a black hole? As you approach the black hole’s event horizon, the difference in gravitational attraction between your head and feet will be enough to tear you apart. Eww! Astronomers call this process ‘spaghettification’. Oddly though, the more massive the black hole the less this stretching force, so you might just about survive beyond the event horizon if you fell into a really massive black hole.

Surrounding the black hole's event horizon is the ‘ergosphere’ (sounds scary, doesn’t it?). Here, space is being dragged around by the spinning black hole at greater than the speed of light. Yes, faster than light! Matter can’t move faster than light, but it turns out space itself can! And you will be dragged around with it. And due to the warping of space-time around the black hole, you will experience time slowing down relative to the ‘outside world’. Someone watching you fall would see you slowing down until you became frozen in time at the event horizon.

It is a common belief that all this weirdness exists only in the imaginations of astronomers (and science fiction authors). But black holes are a reality.

We know of several processes that can form black holes. The most common way is when a massive star (more than about 3 times the mass of the Sun) reaches the end of its life, runs out of nuclear fuel and begins to collapse. This results in a humongous shockwave that blows the outer parts of the star apart (a ‘supernova’), leaving the star’s core to collapse further in on itself. As the core shrinks its gravity increases until it becomes a black hole.

This process is assumed to be responsible for the formation of stellar mass black holes. But in the very early Universe conditions were such that so-called ‘supermassive’ black holes were formed, i.e. ones with many millions of times the mass of the Sun.

By their very nature black holes are not directly observable. So, how can we be sure they exist? Well, we can detect a black hole’s presence due to its effect on its environment or on other nearby objects. Material in the vicinity of such a compact object is subjected to extremely strong gravitational forces. The resulting turbulence and friction heats the material to many millions of degrees, thus creating very strong X-ray emission (a form of light), which we certainly can detect with our telescopes.

We also know that matter flowing towards an object naturally forms into a flat, rotating disk called an ‘accretion disk’. With enough friction in the accretion disk, and sufficient magnetic fields, a compact object can form beams of extremely hot material which are ejected along the rotation axis of the object. Furthermore, the speed of these jets is found to be comparable to the escape velocity of the central object. Black holes (whose escape velocity exceeds the speed of light) are thus thought to create the fastest and most energetic jets of all. Although the Sun turning into a black hole would not alter the Earth’s orbit, this enormous energy generation would be quite problematic for us!

Taken together, the existence of accretion disks, extremely high velocity jets, and strong X-ray emission is convincing evidence for the existence of black holes. No other mechanism known to science is capable of producing the huge amounts of energy released by these objects. But, what astronomers are observing is not the black hole itself.

A simulation of stars and dusk circling the center of our Galaxy.

These tell-tale phenomena are seen in many different kinds of objects. For example, astronomers are convinced that the center of our own Milky Way Galaxy (which lies about 26,000 light years away in the constellation of Sagittarius) harbors a supermassive black hole. By observing stars in orbit around this region, astronomers estimate that it is about 4.3 million times the mass of the Sun and about as large as the orbit of Uranus.

In fact, although not certain, it seems very likely that most (if not all) spiral galaxies contain a supermassive black hole (sometimes more than one). For many galaxies black holes (or rather their effects on their environments) are clearly observed in the turbulent core regions and the dynamics of their stars often indicate the presence of extremely massive objects. There is also very strong evidence suggesting that black holes may be crucially important, perhaps even required, in the formation of galaxies in the early Universe. This would imply that indeed all types of galaxies (including spirals) contain a gravitational beast at their heart.

Formation of extragalactic jets from a black hole accretion disk.

These ‘active galactic nuclei’ present no danger to the galaxies as a whole since they are actually very small compared to the size of their hosts. This means that, although their gravity is very strong close by, their gravitational pull is comparatively weak far from the galactic core. So although such black holes dominate the inner regions of galaxies, and often power extreme energy production, they don’t have the ability to suck in entire galaxies.

The centers of active galaxies are not the only places that these gravitational monsters reside. They can often be found in association with other objects. For example, a class of objects called ‘X-ray binaries’ often contain a black hole in orbit around another normal star. Astronomers have also discovered isolated, stellar-mass black holes adrift among the stars in our Milky Way Galaxy. These have been found indirectly by measuring how their extreme gravity bends the light of a more distant star behind them.

Artist's impression of an X-ray binary.

Black holes don’t live forever. They die, but in a very slow and mundane fashion. They die because they aren’t entirely ‘black’ – they actually glow very faintly. This glow is known as Hawking radiation after the famous physicist who first postulated its existence. According to quantum physics, ‘empty’ space is actually teeming with virtual particles that flash in and out of existence, often as particle/anti-particle pairs. Normally these particle pairs quickly annihilate each other. But near a black hole’s event horizon it is possible for one particle to disappear inside the black hole and be lost forever, while the other one escapes as Hawking radiation. This process gradually reduces the mass and the energy of the black hole. So black holes that aren’t actively sucking in new material will slowly shrink and ultimately vanish. However, for most black holes this slow death would take many billions of times the age of the Universe!

There is a long history to the idea that black holes may connect with other parts of the universe or other universes entirely, but they remain largely speculation. Some physicists have attempted to combine quantum mechanics with Einstein’s theory of relativity and concluded that black holes may not contain a ‘singularity’, the point within a black hole where the density of matter becomes infinite. This opens up the possibility that they are shortcuts to other universes. Actual tunnels through space-time, called ‘wormholes’, may be a better bet for traversing between universes. Although they are predicted by Einstein’s theories, none have been discovered. In fact, there are doubts that they could occur naturally at all, that they would remain stable for more than a fraction of a second or that they would be anything bigger than vanishingly small. So, although interesting theoretical exercises, we do not yet know if such things are possible. That doesn't stop us from making a blockbuster movie about them though!

That's a quick summary of the fascinating subject of black holes. Far from a scientific myth, these enigmatic gravity machines are alive and well in the Universe. In fact, they are relatively common. But don't worry, we're in no immediate danger of being swallowed up by one...