New York
鈥淎LL hope abandon ye who enter here.鈥� As Stephen Hawking once pointed out, Dante鈥檚 famous words about the entrance to hell might serve just as well as a warning to a traveller approaching a black hole. A black hole鈥檚 gravitational pull is so huge that nothing can escape from the distorted space-time at its heart. This is a place from which travellers can never return to tell their tales.
But black holes are also a source of infinite mystery and even beauty. In fact, if I had to choose the place and manner of my death, I would definitely elect to dive into one. Sometimes I dream of vanishing from our Universe at a speed close to that of light as I fall into a black hole鈥檚 central singularity, where the laws of nature shatter, gravity turns time and space to subatomic putty, and God divides by zero鈥�
Last trip
But to make the most of my ill-fated trip and enjoy the scenery on the way, I鈥檒l need to think a little about black-hole anatomy. A black hole can form when a very massive star runs out of fuel near the end of its life, and starts to shrink under the pull of its own gravity. So strong is this force that nothing can possibly stop the star collapsing to a tiny region of space, or singularity, with inconceivably great density. Within the region closest to the hole, the gravity is so strong that no light, matter, or any kind of signal can escape its tenacious grip. The boundary of this region-the point of no return-is known as the event horizon.
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The circumference of the event horizon is directly proportional to the mass of the hole. For a mass equal to our Sun鈥檚, for instance, the circumference is a few kilometres. For a mass the same as the Earth鈥檚, the horizon would enclose a region the size of a walnut. But because the space around a black hole is so distorted, the radius is larger than the circumference divided by 2
. Conceptually, it鈥檚 a bit like what happens when you drop a lead weight onto the centre of a circle drawn on a rubber sheet; the radius stretches while the circumference remains the same. A circle the size of a wedding ring might have a radius of millions of kilometres if a black hole lurked at its centre.
The first problem to figure out is how I can survive for long enough to explore such a warped region of space. I will feel much stronger gravity at my feet than at my head (assuming that I plunge in feet first), and this will create tidal forces that have a powerful stretching effect. My body can only withstand a difference in acceleration along it of about 12 times the acceleration due to gravity.
When the forces get much greater than that, I鈥檒l be torn apart as easily as a piece of Turkish Delight. For a low-mass black hole, say around 10 times the mass of the Sun, with a circumference of about 185 kilometres, I鈥檇 have a lethal dose of acceleration when my orbit circumference was still several thousand kilometres. I鈥檇 be dead long before I reached the event horizon, where the stretching effect would be the same as if I were hanging from a girder of the Eiffel tower with the entire population of Paris suspended from my knees.
But thankfully this problem can be solved by visiting a much more massive black hole. it turns out that for a given distance from the singularity, the tidal forces are inversely proportional to the square of its mass. In other words, the bigger the hole, the smaller the tidal forces. I could reach the horizon of a 1000 solar mass black hole, for instance, and could even explore just inside a 10 000 solar mass hole before the tidal forces started to rip me apart.
However, this tour would not last for long. The time taken to fall from the event horizon to the central singularity in seconds turns out to be simply 0.0000154 multiplied by the mass of the hole in units of the Sun鈥檚 mass. So for a 10 000 solar mass hole, I would slam into the centre in just 0.154 seconds. To give me time to explore beneath the event horizon, I would prefer to visit the kind of black hole that inhabits quasars, the brightest objects in the Universe. These are thought to be fuelled by black holes as big as 10 billion solar masses. I could then survive inside the event horizon for hours.
So鈥� I am now descending towards a large black hole. As I look up and away from the black hole, the starry sky gets smaller and smaller, as if I have tunnel vision. All the stars are being squeezed into a disc the size of a Frisbee. I feel as if I am trapped in a dark pipe. Because light rays circle the black hole several times, the image of the sky is repeated like a scene in a kaleidoscope. The hole acts like a gravitational lens, bending the light so that all the stars seem concentrated in a bright, circular spot.
Not only do I see the stars congregate into a small region of space, but all the colours are wrong. As I descend, yellow stars are turning green, then blue. This is the gravity of the black hole at work in an effect called a gravitational blueshift. it causes the incoming light to shift to higher energies and shorter wavelengths. As I get very close to the hole鈥檚 event horizon, the visible light shifts to ultraviolet wavelengths, then X-rays.
But when I shine my yellow flashlight away from the hole, the opposite happens. The beam struggles against the hole鈥檚 gravitational pull, and the light is redshifted to longer wavelengths. My friends in the spaceship (safely away from the black hole) see my light turn red.
If I were to return to the ship, my watch would not read the same time as my friends鈥� watches on the ship, because gravity causes time to slow down. Far away from the black hole, where space-time is almost flat, clocks tick at their normal rates. While I am close to the black hole, in regions of increasing space-time curvature, my friends see my movement slowing down. However, I don鈥檛 notice this. My heartbeat and thinking processes slow down by exactly the same factor as my wristwatch.
No escape
Now I am passing the event horizon. Like an ancient ant trapped in amber, I appear to others as if I were permanently frozen at the horizon as my image gradually fades. But in fact, my body pierces the event horizon with steadily increasing speed.
Even though I can鈥檛 escape or send messages to my friends from within the horizon, I can still receive messages from them. They can even send me food. The laws of physics only prevent things from coming out of the event horizon.
But now my fate is sealed. No matter what I do, eventually my body will merge with the singularity at the hole鈥檚 centre, where all the black hole鈥檚 mass is squeezed into a region of space billions of times smaller than an atomic nucleus.
A shiver runs up my spine. I鈥檓 about to have the ultimate mystical experience, to see and explore where no human has been before. Now my body is thoroughly distorted by the huge tidal gravity within the black hole. I am about to contact the singularity. The tidal gravity is so large that it completely deforms all objects in about 10-43 seconds, and quantum gravity takes over.
I merge with quantum foam, the material of the singularity. Quantum gravity rips space and time from one another, then destroys time as a concept, and destroys the definition of space. At this submicroscopic scale, space becomes a 鈥渟eething鈥�, probabilistic froth-like a cosmological milkshake where space has no definite structure.
Quantum gravity gives various probabilities for space having any particular shape and curvature. it might have a 50 per cent chance of being in one shape, a 10 per cent chance of being in another, and a 40 per cent chance of being in a third form.
Quantum foam is everywhere, in singularities, in space, even in your body. But you鈥檇 need a powerful microscope to examine it. We鈥檙e talking of lengths around 10-33 centimetres.
Even though I am now dead, it鈥檚 possible that pieces of me will end up in a different universe. Some astronomers have suggested that black holes act as wormholes-tunnels connecting different regions of the Universe, or even one universe with another.
Can humans ever really expect to truly fathom all the mysteries of black holes? Perhaps not. Humans are just a moment in astronomic time, a transient guest of the Earth. Maybe our minds have not sufficiently evolved to comprehend the mysteries of space and time that surround black holes. Perhaps our brains, which evolved to make us run from lions on the African savanna, are not constructed to penetrate the mathematical mysteries of collapsed stars. Only time will tell.
