Tracking black-hole clues
Q: What is the proof of the existence of black holes? -Amtrak
A: A black hole is an object so collapsed and, consequently, with such a large gravity field that its escape
velocity exceeds the speed of light. Earth, for example, is not a black hole because our escape velocity--a
measly 11 kilometers per second-is far less than light speed (300,000 km/s). Even spacecraft escape
Earth. The more compact an object is, the higher its escape velocity. A black hole is so compact
that-within a certain distance of it--not even light can escape. Therefore, we can't see it directly. Within
that "certain distance" (called the event horizon) no information escapes.
[NASA] A cosmic searchlight: a black-hole powered jet of electrons
You ask for "proof" of black-hole existence. That's not exactly how it works. Einstein's General Theory
of Relativity predicts the existence of black holes-as bizarre as they seem. We've verified Einstein's
theory in many ways, which gives us confidence in its predictions. Consequently, we think such objects exist in the real world. His
theory also gives us indirect ways--how gravity affects matter--to detect black-hole candidates.
We verify that an object is a black hole by looking at its effects on the environment. Then we compare what we observe against effects of
other known phenomena. The entity that can best explain these observations, in the simplest way, is the winner.
Supermassive black holes: A disk that spins too rapidly for the number of stars it contains is a likely candidate. Without a dark and
massive object in its middle, the disk would fly apart. That's one clue. Another indication is its size. We can estimate the size of the core
from how the energy varies in time. Suppose the object's mass is a billion or better times the mass of our Sun but suppose it's size is only
about that of our solar system. Then it is probably compact enough to be a black hole.
More good clues pointing to a black hole: Does the star and gas speeds increase rapidly in the core? Does the matter form a disk?
These observations, taken together, pinpoint an invisible object of colossal mass inside a compact space with an extraordinary
gravitational field. Moreover, the object exhibits all the relativistic effects we know how to measure-nothing else fits: it's a black hole.
Stellar-mass black holes: A dark star in a binary system (two stars orbiting each other) lends itself to solid analysis. We determine the
mass of both stars from the orbital velocity of the visible star. The visible star's spectrum tells us its luminosity and radius and from that
we can calculate its mass.
Knowing the total mass of both stars and the mass of the visible one gives the mass of the dark one. The unseen companion can only be
one of three things: a white dwarf (less than 1.4 solar masses), a neutron star (between 1.4 to 3 solar masses) or a black hole. Thus, if the
dark star's mass is much greater than 3 solar masses (say, around 5 to 10), then it must be a black hole.
We have great confidence that we have found stellar-mass black holes, even more so than for super massive black holes.
Unconfirmed yet, but: on Jan. 11, 2001 Joseph Dolan of NASA's Goddard Space Flight Center announced possible direct evidence of a
black hole. He examined photographs taken by the Hubble Space Telescope of the black-hole candidate called Cygnus XR-1. Dolan
found two instances where a hot gas blob appeared to be slipping past the event horizon (point of no return) for the black hole.
That'sdirect evidence of a black hole.
(Answered by April Holladay, science correspondent, May 22, 2002)
U of Texas at Austin: Monsters in galactic nuclei
USATODAY.com: Black hole found at center of Milky Way
BBC- Horizon: Supermassive black holes
NASA: Black holes
Space Telescope Science Institute: Black holes
Space.com: First direct evidence of black hole
NASA: Hubble images of black holes