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 matched observations. There exist a handful of alternate theories, such as Magnetospheric Eternally Collapsing Objects (MECOs), to explain these observations, most of which avoid the spacetime singularity at the center of the black hole, but the vast majority of physicists believe that the black hole explanation is the most likely physical representation of what is taking place.
Black Holes Before Relativity
In the 1700s, there were some who proposed that a supermassive object might draw light into it. Newtonian optics was a corpuscular theory of light, treating light as particles.
John Michell published a paper in 1784 predicting that an object with a radius 500 times that of the sun (but the same density) would have an escape velocity of the speed of light at its surface, and thus be invisible. Interest in the theory died in the 1900s, however, as the wave theory of light took prominence.
When rarely referenced in modern physics, these theoretical entities are referred to as "dark stars" to distinguish them from true black holes.
Black Holes from Relativity
Within months of Einstein's publication of general relativity in 1916, the physicist Karl Schwartzchild produced a solution to Einstein's equation for a spherical mass (called the Schwartzchild metric)… 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 = 2 GM/ c 2
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 Eddington believed some property would prevent the collapse. Both were right, in their own way.
Robert Oppenheimer predicted in 1939 that a supermassive star could collapse, thus forming a "frozen star" in nature, rather than just in mathematics. The collapse would seem to slow down, actually freezing in time at the point it crosses rs. The light from the star would experience a heavy redshift at rs.
Unfortunately, many physicists considered this to only be a feature of the highly symmetrical nature of the Schwartzchild metric, believing that in nature such a collapse would not actually take place due to asymmetries.
It wasn't until 1967 - nearly 50 years after the discovery of rs - that physicists Stephen Hawking and Roger Penrose showed that not only were black holes a direct result of general relativity, but also that there was no way of halting such a collapse. The discovery of pulsars supported this theory and, shortly thereafter, physicist John Wheeler coined the term "black hole" for the phenomenon in a December 29, 1967 lecture.
Subsequent work has included the discovery of Hawking radiation, in which black holes can emit radiation.
Black Hole Speculation
Black holes are a field that draws theorists and experimenters who want a challenge. Today there is almost universal agreement that black holes exist, though their exact nature is still in question. Some believe that the material that falls into black holes may reappear somewhere else in the universe, as in the case of a wormhole.
One significant addition to the theory of black holes is that of Hawking radiation, developed by British physicist Stephen Hawking in 1974.