Astronomy, Astrophysics and Space Science
Physicist and cosmologist: Wrote the first logically precise theory for the simplicity of black holes (1967)
"If you really enjoy your work you never need a holiday."
Cosmology is the study of stars and other heavenly bodies. Cosmologists are physicists who ask, How was the universe created? When will it end? How big is the universe? Israel is particularly famous for some of his theories about black holes.
A black hole is not really a hole. It’s a region of space that has so much mass concentrated in it that nothing can escape its gravitational pull. Black holes are thought to be formed when very old stars collapse in upon themselves. Scientists believe that black holes have such strong gravity that they act like gigantic vacuum cleaners, sucking in any matter that comes too close. Whether it is a comet, a planet or a cloud of gas, that matter is crushed to infinite density and disappears forever. The gravity is so intense that it slows down time and stretches out space. Not even light can escape from a black hole, so it’s impossible to see one. That’s why the American physicist John Wheeler named them “black holes” in 1968, even though the first person to think of the idea was British amateur astronomer John Mitchell in 1783.
Despite all this, Werner Israel believed that a black hole was actually a very simple thing. Until he published this theory in 1967, it was thought that the only truly simple things in nature were elementary particles such as electrons or neutrons. An electron only has three properties: mass, spin and charge. Virtually everything in nature is much more complicated and cannot be described so easily. Rocks have jagged cracks. Planets have rugged mountains. Stars have complex magnetic fields. Israel used mathematical techniques to show that black holes are the simplest big objects in the universe. Like an electron, they can be described completely by their mass, spin and charge alone. “The surface of a black hole is as smooth as a soap bubble,” says Israel. But you’d never be able to see this, since light is not reflected by a black hole.
Israel is currently working on several projects involving the internal geometric structure of black holes. He wants to know what goes on inside a black hole. To answer this question he hopes eventually to use superstring theory — the idea that instead of using tiny, point-like particles to describe matter as we have done up to now, perhaps things can be broken down into tiny, line-like “superstrings.” Israel says, “Einstein’s theory is okay for everything until you get very deep inside a black hole.” You need another approach, because relativity theory does not handle infinity very well, and a lot of things become infinite inside a black hole.
For example, Israel is trying to figure out what happens when a black hole evaporates. What becomes of all the information stored inside it? Is it totally randomized and lost when the black hole turns into a huge amount of radiant heat? Or is this radiation only apparently random, and are there subtle correlations stored within it that contain the “lost” information? This is what most string theorists believe today. But Israel can’t understand how this information can rise from the depths of the black hole to its evaporating surface without violating causality, that is, travelling faster than light from the future to the past.
1. If you could watch from the outside as an object falls into a black hole, it would seem to you that it never gets there. The closer the object approaches the hole’s event horizon, the slower it seems to travel. You can think of the event horizon as the surface of the black hole, but it’s not solid. To you, the object would appear to stop, seemingly forever suspended at the event horizon. It would begin to turn orange, then red, then fade fairly rapidly from view. Though the object is gone, you never saw where or how it disappeared.
If you yourself fell into a black hole you would not even notice the event horizon. From there on, everything goes only one way: in. You could not send out messages for help. However, you could still receive messages from outside so, to you, everything would seem okay. You would never know when you had crossed the event horizon — except that the increasing gravity would draw your body longer and longer, squeezing you in from the sides. You wouldn’t last long, which is too bad, because the properties of time and space are so altered in a black hole that some scientists think time travel might be possible. Or you might be able to travel to a parallel universe through a wormhole — holes in the fabric of space and time. The only problem is: how do you survive the tremendous gravity?
2. As you fall deeper into a black hole you would come to the inner horizon. This is the point beyond which you cannot even see out. As you reach the inner horizon, all events in the universe that had ever happened throughout all of time would seem to accelerate and appear to you in a fraction of a second.
3. Nobody knows what happens in the inner regions of a black hole. Theorists like Israel cannot predict what goes on beyond the inner horizon. Ultimately, the black hole becomes a singularity — an infinitely massive point in space.
4. The fuzzy blob to the left of the “4” in the centre is a ground-based photo of the giant elliptical galaxy called NGC 4261, as it appears through a telescope in ordinary light. (Since there are billions of galaxies, most have numbers rather than names. NGC stands for the New General Catalog of galaxies, which was started by astronomers in the 1860s.) NGC 4261 is one of the 12 brightest galaxies in the Virgo cluster, located 45 million light-years away from Earth. It contains hundreds of billions of stars. A superimposed radio image shows a pair of giant opposed jets shooting out of the galaxy, spanning a distance of 88,000 light-years.
The image on the right is an enlarged detail of the same galaxy, taken by the Hubble Space Telescope. The improved quality of the image shows that the centre of the fuzzy blob has a giant disc with a 300-light-year ring of cold gas and dust around a bright central core that is probably feeding matter into a black hole, where gravity compresses and heats the material. Hot gas rushes from the vicinity of the black hole, creating the radio jets. These jets provide strong evidence for a black hole in the centre of NGC 4261. The Hubble Space Telescope has now provided fairly convincing evidence that black holes probably exist at the centre of many galaxies, including ours.
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- Black holes are bodies that have so much mass that they collapse under their own gravity. Find out how to calculate their enormous mass.
A good theory explaining gravity remains one of the biggest mysteries in physics today. Because we don’t really understand how gravity works, nobody knows what happens inside the biggest sources of gravity in the universe, black holes. Israel feels that a quantum theory of gravity is needed to explain what happens to things that fall into black holes. Though people have been working towards this goal since Einstein’s time (for almost 100 years), a quantum theory of gravity has remained elusive because of problems dealing with infinities of mass, energy, and other physical quantities. Israel believes superstring theory may hold the answer.
Perhaps the biggest puzzle facing cosmologists is the problem of the cosmological constant, or “dark energy” as it is known today. In 1917, just after Albert Einstein had finished his theory of gravity, which he called general relativity, the distant stars and galaxies were still believed to be at rest (on average). Einstein was disturbed by the thought that a universe starting out at rest would get pulled together by gravity and should soon collapse, which certainly wasn’t happening. To fix the theory, he needed an extra term in his equations — the cosmological constant — to provide a repulsive force that would counterbalance gravity and hold the universe at rest.
Then, in 1929, the American astronomer Edwin Hubble discovered that the universe was not at rest but actually expanding. There was no longer any possibility of a quick collapse. Gravity could perhaps slow this expansion, but there was certainly no need for a repulsive force. Einstein then dropped the cosmological constant, calling it the biggest blunder of his career.
“It wasn’t,” says Israel, “There were one or two others which vie for that distinction.”
But in 1998 cosmologists got an even bigger surprise. Astronomers found that the expansion was not slowing down. It was speeding up. Something must be there, some kind of dark energy producing a repulsive force stronger than gravity but completely invisible and undetectable to human observers. What’s more, it works only over very long intergalactic distances, not ordinary astronomical distances such as between the Earth and the sun. So it appears that Einstein’s “greatest blunder” was not such a dumb idea after all.
According to Israel, there are really two great mysteries. First, what exactly is dark energy? And secondly, why is its density (that is, the cosmological constant) just about the same as the average present density of matter? According to Einstein, this value should have stayed constant since the beginning of the universe. But the density of matter has decreased by 120 orders of magnitude over the same period — that’s 10 times 10, 120 times! Is it just a fantastic coincidence that the densities of dark energy and present-day matter are the same, or is there something else going on? Cosmologists call this the cosmic coincidence problem. “I think it will take a young Einstein to solve these mysteries,” says Israel, “and, of course, one always hopes that one’s current grad student will be that person!”
Heather Couper & Nigel Hernbest, Black Holes, Dorling Kindersley Publishing, 1996.
Werner Israel, “Imploding Stars, Shifting Continents, and the Inconstancy of Matter,” Foundations of Physics, vol. 26, no. 5, May 1996.
Edwin F. Taylor and John Archibald Wheeler, Exploring Black Holes: Introduction to General Relativity, Addison Wesley, 2000.
Introduction to black holes on Cambridge University website.
Michigan Technological University website with movies of imaginary trips to black holes.