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Facts of the Matter Richard Brill |
Existence of enigmatic
black holes can’t be provedBlack hole describes exactly what scientists mean by the term. It's an object so dense that it creates a region of space from which nothing can escape, not even light. Until a few years ago many astronomers were skeptical that the holes existed, even though their existence is mandated by currently understood laws of physics.
The physics underlying these shadowy entities begins in the late 17th century with Sir Isaac Newton. He conceived the first description of gravity in a flash of inspiration when he made the connection between the force that pulls a falling apple to the ground and the force that holds planets and moons in their orbits.
In his magnum opus, "Mathematical Principles of Natural Philosophy," Newton presented his law of universal gravitation in a "universe" of consistent physical laws. He described various effects of gravity such as the tides and the orbit of comets. In one section he wondered whether light, like planets, might be attracted to a massive object such as the sun. He also defined our modern concepts of space and time as invariant, that is to say, exactly the same everywhere in the universe and unchangeable.
Newton's way of describing gravity stood for 250 years, until 1916, when Albert Einstein published his theory of general relativity. Einstein offered a way of visualizing gravity that revolutionized physics by altering Newton's concept of invariant space and time. Einstein's space and time are not only changeable; they are determined by the presence of matter.
In Einstein's universe, all the observed effects of gravity are the result of distorted space time. Matter distorts space and distorted space determines how matter moves.
Mass "compresses" the space around it, the amount of distortion depending on the amount of matter. A star like our sun causes a large distortion, while a smaller object like Earth or the moon causes a similar, but much smaller one. Anything, including light passing through the distorted space, will follow the shortest path, which will always turn out to be curved, like a marble rolling in a funnel.
According to general relativity, a massive object such as the sun distorts the space around it such that light from distant stars passing near the sun would be bent by the sun's gravity. As seen from Earth they would appear to shift position in the sky as the sun passes near them. Since we normally can't see stars close to the sun in the sky because of its brightness, we can't normally see the shift. Einstein pointed out that during a solar eclipse it should be detectable. He did some calculations and predicted the amount of the distortion to be expected.
In 1919, during a solar eclipse, Sir Arthur Eddington, an eminent English physicist, observed the effect precisely as Einstein had predicted. The event launched Einstein into international fame and set the stage for the concept of the black hole.
As new technology emerged and it became possible to collect more detailed data on more and more stars, astronomers began to see patterns in the birth, development and eventual death of stars of various sizes.
Astrophysicists see a star as a system that is in equilibrium between gravity and pressure. It is a nuclear furnace where the tremendous amount of energy that is released would cause the star to explode, but it cannot because it is held together by gravity. On the other hand gravity would crush the star except for the pressure exerted by the continual nuclear explosion.
Eventually the star will use up all of its nuclear fuel. This may take as little as a few hundred million years, or as long as a few billion years, depending on the mass of the star. In the end, gravity will win and the star will be crushed.
When astronomers began calculating what might happen to stars that spend their fuel, there were some surprising results. One such result was the prediction that stars six to eight times as massive as the sun would be crushed down to a density where the nuclear particles cannot be compressed any further.
Under this tremendous pressure, atoms cannot exist because protons and electrons are squeezed together to become neutrons. A star several times the mass of the sun would be crushed into a neutron star a little more than 10 miles in diameter.
But above a certain critical mass the calculations suggested the unthinkable. Stars more than 10 times the mass of the sun would be crushed with such force that even the neutrons could not withstand the pressure, and the star would be squeezed to singularity of infinite density. The equations indicated that within a certain distance from the center, called the event horizon, the density would be so great that it would generate a gravitational environment so strong that nothing could escape, not even light.
Inside the event horizon, light would be bent by the extreme gravity like the trajectory of a rock thrown upward here on Earth, ultimately to fall back into the hole. The hole would be black because no light, or anything else, could escape it. The only way we could "see" it would be by observing effects that are thought to be associated with such objects.
Although there is no proof that black holes exist, astronomers believe they have observed many objects for which there is no more suitable category. Some of these are compact black holes of solar masses (a solar mass is the mass of our own sun). Others, thought to exist at the center of most galaxies, including our own Milky Way, are supermassive black holes that have masses of anywhere from a million to a billion solar masses.
All of this from Einstein's description of gravity as little more than a space-time warp. Newton's genius notwithstanding, this was a remarkable piece of insight into the workings of this amazing universe.
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu