COULD you fall into a black hole and avoid being crushed by its intense gravity? Because the laws of physics break down under these conditions, it has widely been considered impossible even to imagine what would happen deep inside these fearsome objects. But Igor Novikov thought it was worth trying anyway. And by probing exactly how the laws of physics break down, he has challenged the conventional view that a trip to a black hole spells certain death.
Of course, Novikov, who heads the Theoretical Astrophysics Center in Copenhagen, Denmark, is not planning to visit a black hole. But physicists still consider the problem worth thinking about; these calculations can give us insight into what happens where the fabric of space-time becomes so tremendously warped that it shatters into droplets or quanta. “Here the general theory of relativity, Einstein’s theory of gravity, is no longer valid,” says Eric Poisson of the University of Guelph in Ontario, Canada. “We are into the mysterious domain of quantum gravity where no theory yet exists to tell us what to expect.” So looking at the internal structure of black holes might tell us how to formulate a more consistent theory of the universe.
Black holes – regions of space-time from which nothing can escape – are rooted in Einstein’s general theory of relativity. Soon after Einstein published his theory describing gravity as warps in space-time, the German astronomer Karl Schwarzschild used the theory to show that if a large star were squeezed into a small enough point, it would create such a strong gravitational field that nothing, not even light, could escape once it had moved beyond a certain point: the “event horizon”.
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Inside the event horizon of a black hole, space-time is radically transformed. Einstein showed that matter or energy stretch space and time, rather like someone standing on a trampoline. Yet no matter how much the fabric of space-time warps, space and time always act together to preserve the speed of light (one of the effects of this is that clocks run slower in the gravitational field close to Earth than they do far out in space). The tremendous gravity inside a black hole distorts space-time so much, that the only way to safeguard the speed of light travelling in this region is for space and time to swap roles. This has an extraordinary consequence: instead of being a place, the centre of a black hole exists in the future and you can no more avoid it than you can avoid tomorrow. At the end of this journey is the singularity, a point of infinite density that is widely believed to destroy anyone or anything that hits it.
The devastation is caused by “tidal” forces: because gravity varies so quickly near a black hole, the pull on your head would be much greater than the pull on your feet. Theorists calculate that these tidal forces increase indefinitely very close to the singularity: matter gets torn apart, space-time would shred and you would be utterly destroyed.
But it turns out that the existence of this murderous singularity does not necessarily spell doom. Because stars and galaxies rotate, there is a good chance that any black hole we encounter would too. In the early 1960s, New Zealand mathematician Roy Kerr worked out that this rotation drags the space-time surrounding the black hole like a tornado, profoundly altering its internal structure. His work showed that a second horizon forms inside a rotating black hole. Novikov has now shown that this second horizon changes everything ().
Shortly after a rotating black hole forms, this inner horizon is little more than a slight wrinkle in space-time whose exact position depends on the speed of rotation. For the fastest spinning black holes, it sits halfway between the event horizon and the singularity. But it does not stay that way for ever. The extreme gravitational field inside the black hole whips all the in-falling light and matter up to extremely high energy. The colossal energy of this junk changes the character of the inner event horizon, warping it so much that the slight wrinkle in space-time becomes a deep fold. To preserve the speed of light in this region, time speeds up – it passes so quickly that the inner horizon concentrates junk from all times, even from events that happen far into the universe’s future.
The result of this is that the inner event horizon concentrates so much light and matter that it rapidly turns into another singularity, one that concentrates an infinite amount of energy into a finite volume. Astrophysicists call it a “mass-inflation” singularity.
And it is this mass-inflation singularity that gets you out of a black hole alive. As well as junk falling into the black hole, Novikov believes that the inner event horizon also dredges up stuff scattered from deep within the black hole. This interacts with the incoming flow of light and matter, and “gravitational feedback”, where the ever-increasing amount of stuff present causes an ever-increasing gravitational pull, causes the radius of the inner horizon to shrink. Eventually the mass-inflation singularity swallows the more dangerous singularity, leaving a tamer, less dangerous kind of black hole.
Calculations show that the tidal forces around a mass-inflation singularity may not last long enough to deform an object. Because space-time is so warped around there, passing near the singularity is the fastest part of the journey. That gives anyone falling into a black hole a fighting chance. “Although you would feel an infinite tidal force as you approached, you would feel it for only a very short time,” Novikov says. “Consequently, you could pass through without being crushed.”
Of course, you will have to choose your black hole carefully: it has certainly got to be rotating, for a start. It has also got to be old, so that there has been enough time for the second singularity to form. The final condition is that your black hole is big – very big.
That’s because of the tidal forces. Near the small black holes formed by collapsing dead stars, they are enough to rip a person limb from limb. But these forces depend on the mass of the black hole, shrinking by a factor of 4 each time the mass doubles. So to increase your chances of survival, it is far better to explore a rotating supermassive black hole weighing hundreds of millions of times as much as the sun. “The tidal effect near such a supermassive black hole is far weaker than near a stellar-mass black hole,” Poisson says. “So you would hardly notice as you slip across the event horizon and into the interior.”
Fortunately, such behemoths do exist. No one is quite sure how they formed but astrophysicists now suspect that supermassive black holes lurk at the heart of every galaxy, including the Milky Way. Our nearest black hole, Sagittarius A*, is rotating, supermassive and old: a perfect candidate for this thrill ride of a lifetime. So, what would the journey be like?
Although the space-time around you is grossly warped – another way of saying that gravity is immensely strong – you notice nothing untoward happening, just as you don’t normally notice the effects of Earth’s gravity. To your friends watching you at a safe distance, however, things seem very different. Everything you do is in slow motion as time appears to stretch out interminably. Not only that, but you gradually fade from view as the visible light reflecting off you decreases in energy and frequency. In effect, gravity stretches the light waves, stretching or “red shifting” their wavelengths towards the infrared part of the spectrum.
Fantastic voyage
Immediately ahead of you lies the event horizon, the point of no return for in-falling light and matter. Here time appears to slow to a standstill, so your friends see your gradually fading image frozen in space forever. The truth (for you, anyway) is that you have long gone over the event horizon and are falling towards the singularity, the point with infinite density.
Once you are inside, your passage to the mass-inflation singularity should take only a few hours. Of course, you could prolong your journey by taking a zigzag route in a rocket through space, but the singularity is unavoidable. “By steering, you could extend the journey perhaps by a factor of 10, but no more than that,” Poisson says.
And what would you find when you got there? So far, no one knows for sure. Many physicists believe that when a star’s core collapses to form a black hole, it does not shrink to nothing, but instead spawns a new region of space-time. Lee Smolin of the Perimeter Institute in Ontario, Canada, for instance, speculates that black holes can give birth to baby universes where the fundamental constants of physics are slightly different. Novikov thinks mass-inflation singularities may act as portals into these regions; if you fall into a black hole you might emerge in a different universe, he says.
Amos Ori of the Israel-Technion Institute of Technology in Haifa, Israel, is exploring an extraordinary scenario that emerges when he applies general relativity to a simplified black hole model. He considered a two-dimensional black hole containing a mass-inflation singularity. “After crossing the mass-inflation singularity, an observer falling into the black hole will return to the external universe,” he predicts. The bad news is, they will return stretched by an enormous factor and billions of years after they fell in. “Probably no observer will survive this,” he says.
Unsurprisingly, many physicists are sceptical that anyone might travel through a black hole unscathed. “We don’t know for sure if it is possible to survive crushing,” says Kip Thorne, a theorist at the California Institute of Technology in Pasadena. “We don’t have enough understanding of the mass-inflation singularity.”
And even if you do not get crushed, another danger awaits you. Today’s universe is filled with completely harmless microwave radiation, the tepid afterglow of the big bang. When these microwaves are sucked into a black hole, they are accelerated to much higher energies and frequencies: these innocuous microwaves get transformed into penetrating gamma radiation. “It is not obvious whether it will be possible to shield someone against these gamma rays,” Ori says, “though there may be a chance.”
If journeying into a black hole sounds too perilous, Novikov has an ingenious way of looking inside one without venturing too close to the event horizon. His idea is to use tunnels in space-time called wormholes, with one end anchored outside the black hole and the other dangling inside. “Light from the interior could then come out, allowing us to peer inside,” he speculates. The big difficulty would be distorting space-time enough to make a wormhole in the first place. And then there is the problem of keeping it open: wormholes tend to snap shut as soon as they are formed. There has been recent progress this problem, however (see “Peeping through a wormhole”). But since it is impossible to change the topology of space-time, we would have to find a wormhole – possibly one left over from the big bang – then inflate it. “That’s a tall order,” Poisson says.
So maybe the best way to learn about the interior of a black hole is to disregard danger and hurl yourself into one. Of course, whatever you learned, you would never be able to tell your friends about it – even if you survived, you would emerge in another universe or at another time. “But it would be a fascinating trip,” Poisson says. Any volunteers?
Peeping through a wormhole
Earlier this year, New Zealand and Indian physicists made a dramatic discovery: wormholes may be easier to maintain than we thought.
Wormholes are perfectly legitimate solutions of Einstein’s equations, and could help us look inside a black hole without fear of falling in. But there is a problem: wormholes snap shut in an instant unless held open by a supply of exotic matter. Unlike the familiar stuff found on Earth, which always feels the pull of gravity, this exotic matter can repel gravity, halting the wormhole’s collapse. Although recent measurements of the big bang afterglow suggest that the universe is made up of a substance with repulsive gravity, no one knows whether it has the right properties, or even if it exists in the quantities needed to prise open a wormhole big enough to look through.
But according to Matt Visser of Victoria University in Wellington, New Zealand, things are not as bad as they initially seemed (Physical Review Letters, vol 90, p 201102). The empty space of the quantum vacuum, where fluctuations in energy allow short-lived particles to pop in and out of existence, shares some of the properties of exotic matter. So it may be possible to keep open the mouth of wormhole with much smaller amounts of exotic matter than previously thought. Our chances of a glance inside black holes just went up – very, very slightly.