How can you tell a black hole made out of antimatter from a black hole made out of matter?
Published: May 16, 2014
According to our current understanding, there is no way to distinguish an antimatter black hole from a regular-matter black hole. In fact, there is no difference between an antimatter black hole and a regular-matter black hole if they have the same mass, charge, and angular-momentum.
First of all, antimatter is just like regular matter except that its charge and some other properties are flipped. Antimatter has positive mass just like regular matter and experiences gravity the same way. Antimatter is exotic in the sense of being very rare in our universe, but it is not exotic in how it obeys the laws of physics. An antimatter cookie would look just like a regular-matter cookie. Therefore, adding the concept of antimatter to the discussion does not really lead to anything new or exotic. We could just as easily ask, "what is the difference between a black hole made of hydrogen and a black hole made of helium?" The answer is that there is no difference (as long as the total mass, charge, and angular-momentum are the same).
According to the No-Hair Theorem, a black hole has the interesting property that all information and structure that falls into a black hole becomes trapped from the rest of the universe, and perhaps even destroyed, except for its effect on the total mass, charge, and angular momentum of the black hole. The overall mass of a black hole is what determines the strength of its gravity. When scientists talk about large or small black holes, they are actually talking about the mass of the black hole. Large black holes have more mass, more gravity, and therefore more effect on their surroundings. When matter falls into a black hole, it increases the overall mass of the black hole.
The overall electric charge of a black hole determines the strength of the electric field that it creates. When matter with electric charge of the same polarity as the black hole falls in, it increases the charge of the black hole.
The overall angular momentum of a black hole describes how fast it is spinning. When matter falls into a black hole with a swirling motion (as opposed to falling straight in), it can increase the black hole's total angular momentum if the matter swirls in the same direction, or decrease the black hole's total angular momentum if it swirls in the opposite direction.
In the book The Nature of Space and Time by Stephen Hawking and Roger Penrose, Hawking states:
The no-hair theorem, proved by the combined work of Israel, Carter, Robinson, and myself, shows that the only stationary black holes in the absence of matter fields are the Kerr solutions. These are characterized by two parameters, the mass M and the angular momentum J. The no-hair theorem was extended by Robinson to the case where there was an electromagnetic field. This added a third parameter Q, the electric charge... What the no-hair theorems show is that a large amount of information is lost when a body collapses to form a black hole. The collapsing body is described by a very large number of parameters. These are the types of matter and the multipole moments of the mass distribution. Yet the black hole that forms is completely independent of the type of matter and rapidly loses all the multipole moments except the first two: the monopole moment, which is the mass, and the dipole moment, which is the angular momentum.
We don't know exactly what goes on in a black hole. The matter inside a black hole could be condensed down to an indistinguishable blob. Or the matter could retain some structure but remain trapped in the black hole by the black hole's intense gravity. The problem is that a black hole's center is so small that the theory of General Relativity, which describes gravitational effects, becomes inaccurate. We need quantum theory to accurately describe physics on the very small scale. But we have not yet developed a correct theory of quantum gravity. Therefore, we won't have a good idea of what goes on inside a black hole until we have an accurate theory of quantum gravity. The fact that the inside of black holes is shielded from all experimental observations makes the task even more difficult.