When matter is dragged into a black hole, where does it go or what happens to it?
Asked by: Daniel Febrer, age 12
Answer
Right back in the time of Isaac Newton (1687) it was determined that the force of gravity
was due to the presence of matter, specifically its mass. In fact the existance of black
holes was postulated not long after Newton's publication of his 'Philosophiae Naturalis
Principia Mathematica' (Mathematical Principles of Natural Philosophy).
It wasn't until 1915 when Albert Einstein published his theory of General Relativity that
the theory of black holes really took off. The first real study into the phenomenon was
undertaken by Karl Schwarzchild in 1916 who derived an equation for the Schwarzchild Radius
of a black hole (Rs = GM / c^2, where Rs is the Schwarzchild radius, G is Newton's
gravitational constant, M is the mass of the blackhole and c the speed of light).
To form a black hole matter collapses under its own gravitational field, such as in the
death of a large star. If the matter in question is massive enough then its gravitational
attraction will be so great that it will overcome all of the other forces trying to resist
the collapse and the matter will continue to shrink until it becomes no more than a point,
known as a singularity. This point will have and infinite density and will be infinitely
small. The effect on space time will be such that it is distorted to the point where light
can no longer escape from the black hole, hence the name black. At singularities the known
laws of physics break down which is why so much time and effort is spent examining these
strange features of our universe.
The Schwarzchild radius describes a property of black holes known as the event horizon.
This is the point between space where light can escape from the black hole's gravitational
field and the space where it cannot. Although the singularity inside the black hole is
infinitely small the black hole would appear to be the size of its event horizon, and to
all effects is.
When matter falls into the event horizon it becomes isolated from the rest of space and
time and has, effectively, disappeared from the universe that we exist in. Once inside the
black hole the matter will be torn apart into its smallest subatomic components which will
be stretched and squeezed until they to become part of the singularity and increase the
radius of the black hole accordingly.
Interestingly enough it has now been shown, by one Stephen Hawking, that the matter inside
a black hole is not completely isolated from the rest of the universe and that given a
sufficient length of time black holes will gradually dissolve by radiating away the energy
of the matter that they contain.
Answered by: Edward Rayne, Physics Undergraduate Student, Cambridge UK
'We are all in the gutter, but some of us are looking at the stars.'