Understanding Black Holes

As I explained in my last blog, Black holes are regions in space having a gravitational field so strong that even light can’t escape from them and therefore they are invisible. Here are some characteristics:

Features of a Black Hole

No matter or information can flow from the inside of a black hole to the outside universe. The event horizon is the “surface” of a black hole. It is the perimeter at which the escape velocity is exactly the speed of light. The event horizon is marked by the light rays that are just unable to escape out of the black hole’s vicinity. At the centre of a black hole there is a singularity where gravity and escape velocity become infinite. The singularity has infinite density and space-time curvature. Astronomers suspect there are two classes of black holes: stellar mass and supermassive.

Stellar mass black holes have masses between 4 and 15 times the mass of the sun. Supermassive black holes have masses many billions of times greater than the Sun. Supermassive black holes are thought to form at the centres of active galaxies. We have one at the centre of our very own galaxy, the Milky Way.

Roger Penrose proposed the   Cosmic Censorship Hypothesis  which states that “God abhors a naked singularity.” This means that the singularities that occur in the universe are always “hidden” where we cannot see them, such as inside a black hole, hidden by its event horizon. It protects observers who are outside the black hole from the consequences of breakdown of predictability that occurs at the singularity. There are some solutions of the equations of General Relativity where it is possible for one to see a naked singularity and thus enable us to fall through the black hole and into a “wormhole”.

A wormhole is nothing but a black hole with “two ends”. It is a shortcut through space. What is unpredictable is where a wormhole might lead. In fact, there is no experimental evidence for the existence of wormholes. They have been proved to exist only theoretically. Wormholes have been the object of fancy of many sci-fi writers for the simple reason that they are the universe’s natural “time machines”. (Wormholes will be discussed in greater detail in a later blog!)

Detecting Black Holes

Black holes are invisible, but it is possible to detect them by studying their effects. Astronomers observing a star called Cygnus X-I saw that it was giving off enormous amounts of energy. They discovered that this huge, hot, blue star was being dragged around in a circle by an unseen object with a huge gravitational pull. That unseen object, astronomers believe, is a black hole, which is tearing gas from the star. The gas forms a whirling disc before plummeting into the black hole. As it falls, it travels faster and faster until it moves almost at the speed of light itself. Close to the black hole, the gas becomes so hot that it emits massive amounts of energy. Thus, the signatures of a black hole are detected.

Evidence

Only indirect evidence for black holes has been observed. Evidence for the existence of black holes comes from the observation of phenomena close to where a black hole might be.

Accretion disc

Matter falling toward a black hole, but still outside the event horizon, is thought to form a rapidly spinning accretion disc. Friction between different zones of the

accretion disc is possibly the cause of the emission of large amounts of X-rays. Narrow jets of particles moving at a velocity close to the speed of light are supposedly emitted along the polar axis of an accretion disc.

Other evidence

Stars that appear to be orbiting a region of space where no matter is visible may

be orbiting black holes. Gravitational lensing may distort light

from objects behind and far beyond a black hole.

Conclusion:

In his book, A Brief History of Time, Dr. Stephen Hawking goes on to explain why black holes aren’t exactly “sinister” objects found lurking in space. Black holes have several observable effects and of course, they form the centre of several galaxies including our own galaxy, the Milky Way. The most important possible “application” of a black hole could be long distance space travel by making use of wormholes. But, of course, this application is shrouded with complications and paradoxes. Until a time arrives, when the world of Star Trek truly comes alive, wormholes will only remain in our textbooks!