Once Before Time - Martin Bojowald [91]
A future mission into a black hole might proceed something like this. But as one may suspect, this is quite unlikely, for who would organize and finance such a mission? The capsule would certainly be destroyed in the end—a fate not essentially different from that of many other satellite missions in the solar system. But here, on a mission into the interior of a black hole, not even signals carrying information about measurements can be transmitted to us as observers remaining at a safe distance. Even if the capsule finds enormous treasures, it will reveal nothing.
The only chance to have this mission funded is the action of quantum gravity! Resulting repulsive forces could prevent the total collapse of a black hole into a singularity and possibly cause conditions allowing matter such as the capsule, or at least its signals, to escape. At present, no physicist would seriously propose such a project, but a theoretical trip into black holes is free of such limitations and provides highly interesting insights. Why, in the first place, is it not possible to escape, not even for light? And how would the capsule perceive the central region of the black hole? Is the center a point in space, like a tiny but extremely massive sun one can see ahead, threateningly bright, while inescapably being pulled in?
As we will see, the central singularity of a black hole in general relativity is of an entirely different kind. One cannot see it even if one has already fallen in; one can feel it only by its ever-growing tidal forces. It is invisible because it simply does not yet exist! The singularity forms only in the instant when one is just about to hit it. To visualize this at least to some degree, we have to take a wider view and first look at the formation of black holes and the impossibility of escape from them.
ON THE WAY TO A BLACK HOLE:
PLAYING WITH FIRE
Gravitational collapse leads to denser and denser compressions of concentrated matter resulting from the gravitational attraction between all constituents. For stable stars, it can be slowed down by counteracting forces only in limited ways. Starting with an average star that has spent its initial fuel—the hydrogen concentrated in it after big bang nucleosynthesis, as discussed in the preceding chapter—there are several steps of stabilization, depending on the initial mass.
THE FIRST STARS: BURNING THROUGH THE FUEL
Most visible stars rely on fusion to draw away the energy they radiate in the form of visible light and other waves. As in big bang nucleosynthesis, the two lightest elements—hydrogen and helium—initially take center stage. Hydrogen contains a single proton in its nucleus, constituting the entire nucleus in the majority of hydrogen atoms; but in heavy hydrogen (deuterium), it is accompanied by a neutron. The two particles stay so close to each other that the resulting nucleus is much smaller than the radius of the atom, whose main extension comes from the space occupied by its electron’s wave function. There is also the hydrogen species tritium, with two neutrons, but it is unstable, with a short decay time of 4,500 days. The neutron does not have electric charge, and thus the nucleus is equally charged in all three cases; it is completed to a neutral atom by a single electron in orbit around the nucleus. All these different