How to Destroy the Universe (18 page)

As if 10 dimensions weren't enough, in the mid-1990s US physicist Edward Witten went one better and came
up with new theory that uses 11 dimensions. String theory comes in many different versions. What Witten found is that each type of string theory is just a special case of a broader overarching model, which has become known as M-theory.

Rather than modeling subatomic entities as one-dimensional strings, M-theory treats them as two-dimensional membranes. The strings are still there, but they're just 1D slices through these 2D membranes. And the particular orientation of the slicing is what discriminates between each of the string theory variants. But then, of course, space must have one extra dimension over string theory in order to accommodate the extra spatial extent of the membranes—hence 11 dimensions. And just in case you were wondering, no one really knows what the “M” in M-theory actually stands for. Although suggestions include “membrane,” “master”—and even that it's an upside-down “W,” for “Witten.”

US physicist and author Michio Kaku has suggested that the higher dimensions of string theory could conceivably save humanity from the end of the Universe. Calculations of how the particle physics of the early Universe unfolded in various different string theories show that the expansion of our four-dimensional
space–time was in some way coupled to the compactification of the other six dimensions. Kaku thinks that if our Universe ends in a Big Crunch scenario—where, billions of years in the future, the expansion of space eventually turns back on itself and the Universe recollapses into a hot Big Bang-like state (see
How to destroy the Universe
)—then the compactification of the other six dimensions will also reverse, so that they begin to expand. Kaku believes there will come a point, just before our space shrinks too small, where the size of the other dimensions will have become large enough for human travelers to hop across into them. Perhaps luckily for us, this isn't something we're likely to have to worry about any time soon.

• Curved space

• Degeneracy pressure

• How to find a black hole

• Event horizon

• Ripped apart

• Life preserver

There is no escape from a black hole. Once you have crossed its outer boundary, the gravity is so strong that nothing, not even light, can shake its inexorable pull. Falling into a black hole, your body would be stretched out into spaghetti by the immense forces. But now physicists have found a way for an intrepid space traveler to survive the plunge into a black hole's abyss.

The modern description of black hole physics had to wait until 1915 and the publication of Albert Einstein's general theory of relativity. This theory replaced Newton's law for strong gravitational fields by
describing gravity as curvature of space and time. In Einstein's theory, the gravitational field of an ordinary star forms a bowl-shaped depression in space. Planets orbiting the star can be imagined as rather like marbles rolling around the inside of the bowl. Roll a marble fast enough and it will fly over the edge of the bowl and escape. But as the star gets smaller and denser, so the bowl becomes deeper until—in the case of a black hole—it resembles a long funnel-like tube. Any marbles getting too close are destined to roll in and spiral down the tube, no matter how fast they are moving. General relativity predicted that at the center of a black hole lies a so-called “singularity,” a point of zero size and infinite density, where the curvature of space and time and the gravitational forces become unboundedly large, crushing anything that encounters the singularity out of existence in a heartbeat.

Black holes are fascinating theoretical objects but can they really exist in nature? It seems so. Perhaps the most common way they are thought to form is when a massive star reaches the end of its life. The result is a huge outpouring of energy known as a supernova explosion. The explosion crushes the core of the star to high density, increasing its gravity, which then pulls the star in on itself. Ordinary gas pressure is unable to support a ball of material trying to squash itself down in this way. But that in itself isn't enough to make a black hole, as a young Indian astrophysicist proved in the early 1930s.

Subrahmanyan Chandrasekhar worked out that another much stronger force steps in once gas pressure has been overcome. The force resulted from a principle in the emerging field of quantum theory—the laws of physics describing the behavior of atoms and
molecules. One aspect of quantum theory is called the exclusion principle. It was put forward in the 1920s by Austrian physicist Wolfgang Pauli, and in its most elementary form it says that quantum particles don't like to get too close together. Quantum forces literally push the particles apart. The phenomenon is called degeneracy pressure and Chandrasekhar applied it to electrons to show that it's able to support dying stars with masses up to about 1.4 times the mass of our own Sun. A star supported by electron degeneracy pressure is known as a white dwarf—an incredibly dense object packing the mass of an ordinary star into a sphere the size of Earth.

In the late 1930s, US physicists Robert Oppenheimer, George Volkoff and Richard Tolman repeated Chandrasekhar's calculation for larger neutron particles. They found that degeneracy pressure between neutrons can support stellar corpses with masses up to about three times that of the Sun. These objects are known as neutron stars, and are even denser than white dwarfs—squashing the mass of a star into a sphere about the same diameter as a city. If the star going supernova is heavier than about 3 solar masses, then there is no known force in the laws of physics that can support its gravitational collapse—and it must form a black hole.

If a black hole is black, and space is black, then how do you know if there's one there? This was the problem facing astronomers trying to investigate what the theoretical astrophysicists were telling them about these weird, otherworldly objects. But in fact astronomers have managed to gather convincing evidence that black holes really do exist. There are several ways they've managed to do this. Sometimes a black hole will exist in a binary system with another, normal star. The two stars orbit around their common center of gravity so that the presence of the black hole is revealed by its gravitational influence on the motion of its luminous companion star.

Sometimes when a black hole is orbiting in a close binary system, the hole's gravity will tear material from its companion. The material is then sucked into a belt that orbits around the black hole's equator—known as an accretion disc. Material in the disc gradually loses energy and spirals inwards to ultimately be devoured, but as it does so it gets compressed and heats up, emitting X-rays, which can be detected by telescopes back on Earth. Black holes have also been seen lurking at the centers of some galaxies. These so-called super-massive black holes weigh millions of solar masses. Astronomers have inferred their presence by studying stars orbiting close to the centers of these galaxies. The stars are found to be moving so fast and in such tight
orbits that the central mass cannot be anything but a black hole.

Our own Milky Way is believed to harbor a black hole in its core weighing 2.6 million times the mass of the Sun. The largest known black hole lies at the center of the galaxy QJ 287 and weighs 18 billion solar masses. In 1975, Stephen Hawking famously bet the US astrophysicist Kip Thorne that Cygnus X-1, an X-ray source in the constellation of Cygnus, was not a black hole. If Hawking won the bet he would receive a subscription to
Private Eye
magazine; if Thorne won he would receive a subscription to
Penthouse
. In 1990, Hawking conceded, and most physicists now accept that black holes really do exist. So what might it be like to fall into one?

The outer surface of a black hole is known as its event horizon. It's not a solid surface, but a sphere traced out by the distance from the singularity at the hole's core at which the strength of gravity is too strong for light to escape from it. In 1916, German physicist Karl Schwarzchild calculated the radius of the event horizon around a black hole. For a body of mass
m
, the Schwarzschild radius is 2
Gm/c
²
, where
G
is Newton's gravitational constant (1/15,000,000,000) and
c
is the speed of light (300,000,000 m/s). Any object can
become a black hole if it is squashed small enough. For example, the Schwarzchild radius for the Sun is about 3 km (1.8 miles). If Earth were turned into a black hole it would have an event horizon with a radius of 9 mm (0.35 in).

An astronaut falling toward a black hole event horizon would notice the gravitational field gradually start to increase as she drew close. Another effect would soon kick in too. A bizarre consequence of general relativity makes time in a gravitational field appear to run more slowly, as measured by a distant observer watching the whole process through a telescope. It's a similar effect to the time dilation experienced by observers traveling at close to light speed. This is caused by light having to spend energy climbing out of the hole's gravitational field. It's called the gravitational redshift effect and has been verified experimentally. On the event horizon itself the magnitude of this effect becomes infinite and time there appears to freeze. Gravitational redshift also means that the light from the astronaut is gradually stretched out to lower wavelengths the further she falls toward the black hole until it's eventually shifted outside the visible spectrum and she fades serenely from view. From the astronaut's own point of view it's a far less gentle ride.

As the astronaut starts to approach the hole, she starts to see stars behind it distorted by the intense gravitational field. Starlight that would normally go nowhere near the astronaut's eyes is hooked around the hole by its gravity so that to the astronaut it appears as if they are viewing space through a fisheye lens. As she gets closer, the effect intensifies. At 1.5 times the Schwarzschild radius from the black hole's center, the astronaut encounters the “photon sphere.” At this distance, the gravity is strong enough that light can orbit in a circle around the hole. Firing her jet pack for a moment to hover on the photon sphere, the astronaut looks left and right and finds that she can see all the way around the hole so her gaze falls on the back of her own head.

Falling feet-first toward the event horizon, she notices the difference in the force tugging her feet and her head get larger. The gravity field is so intense that even though her feet are only a meter and a half closer to the black hole than her head, the extra gravitational force they feel is colossal. This force begins to stretch her body out. At the same time, the force squashes her body laterally across the shoulders. The nearer she gets to the singularity, the more pronounced the effect becomes until ultimately her head and her feet are pulled far apart and her body is stretched out into a long, thin strand.

Ordinarily gravitational forces inside a black hole would tear a traveler apart in about 0.1 of a second. But two US physicists—Richard Gott and Deborah Freedman—have come up with a way that the traveler could buy themselves a little extra time. They've calculated that a massive ring of material around the traveler's waist could cancel out some of the force they experience. When they say “massive” they really do mean massive—approximately the weight of a large asteroid. The ring's gravity would act to pull the traveler's feet and head back together and to counteract the squashing forces pressing in on them. Far from the black hole the ring would be about the size of one of the rings of Saturn, but as the traveler got closer it would shrink down, strengthening its gravitational effect to counteract the hole's increasing gravity.

On its own, the ring will only give you an extra tenth of a second, doubling the time you could survive. But this might just be enough to save your life. The secret lies in the physics of rotation. Karl Schwarzschild's early black hole studies just looked at holes created by a stationary mass of material. But in 1963, a New Zealand-born mathematician called Roy Kerr solved Einstein's equations for the black hole created by a mass that's spinning. The so-called Kerr solution made a fascinating prediction. Whereas the singularity at the center of a stationary black hole is a point, which you
can't avoid running into once you've crossed the event horizon, the singularity inside a rotating black hole takes the form of a ring. If the black hole is big enough—and with the help of a Gott—Freedman life preserver—it could just be possible for a human traveler to pass straight through this ring. Calculations suggest that the traveler would emerge on the other side of the ring into a new region of space—though quite where this region would be physicists can't yet say for sure. Some speculate it may be a distant region of our own Universe; others suggest it could be a new universe entirely.