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Question: What is a Black Hole?

What is a black hole? When do black holes form? Can scientists see a black hole? What is the "event horizon" of a black hole?

Answer: A black hole is a theoretical entity predicted by the equations of general relativity. A black hole is formed when a star of sufficient mass undergoes gravitational collapse, with most or all of its mass compressed into a sufficiently small area of space, causing infinite spacetime curvature at that point (a "singularity"). Such a massive spacetime curvature allows nothing, not even light, to escape from the "event horizon," or border.

Black holes have never been directly observed, though predictions of their effects have ...view middle of the document...

.. with unexpected results.

The term expressing the radius had a disturbing feature. It seemed that for a certain radius, the denominator of the term would become zero, which would cause the term to "blow up" mathematically. This radius, known as the Schwartzchild radius, rs, is defined as:

rs = 2GM/c2

G is the gravitational constant, M is the mass, and c is the speed of light.

Since Schwartzchild's work proved crucial to understanding black holes, it is an odd coincidence that the name Schwartzchild translates to "black shield."

Black Hole Properties

An object whose entire mass M lies within rs is considered to be a black hole. Event horizon is the name given to rs, because from that radius the escape velocity from the black hole's gravity is the speed of light. Black holes draw mass in through gravitational forces, but none of that mass can ever escape.

A black hole is often explained in terms of an object or mass "falling into" it.

Y Watches X Fall Into a Black Hole

• Y observes idealized clocks on X slowing down, freezing in time when X hits rs

• Y observes light from X redshift, reaching infinity at rs (thus X becomes invisible - yet somehow we can still see their clocks. Isn't theoretical physics grand?)

• X perceives noticeable change, in theory, though once it crosses rs it is impossible for it to ever escape from the gravity of the black hole. (Even light cannot escape the event horizon.)

Development of Black Hole Theory

In the 1920s, physicists Subrahmanyan Chandrasekhar deduced that any star more massive than 1.44 solar masses (the Chadrasekhar limit) must collapse under general relativity. Physicist Arthur...

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