Once Before Time - Martin Bojowald [96]
Neutron stars do not emit much energy after the supernova dwindles down, but one can often find them by their gravitational attraction alone. It happens quite frequently that a neutron star comes sufficiently close to an ordinary one to remain in mutual orbital motion. As a result of the joint motion of both partners around their center of mass, the visible star shows a stagger, allowing a mass estimate of the neutron star to be inferred. Neutron stars also play a large role in pulsars and in some experimental tests of general relativity, as previously described. These objects are clearly confirmed in the cosmos, and they are stable for long intervals.
Does the hard matter of pure neutrons confine gravity within its bounds? No, for here, too, there is a limit to the quantum mechanical force, as again indicated by the presence of a maximal mass of neutron stars. If the neutron star is heavy enough—about twice as massive as the sun—Pauli’s repulsion is overpowered. The precise value of the upper limit is theoretically much less determined than for white dwarfs; neutron matter is, after all, known much less well than the dense matter of a white dwarf. But luckily, one can perform calculations in general relativity that clearly and inescapably show the breakdown of all conceivable forces in matter of sufficiently high pressure, or for sufficiently heavy stars. The generality of this statement, independent of the precise form of matter, once again underlines the elegance of relativity.
When this last rope ruptures, there is no holding back. Matter now collapses unhindered as a result of the unchecked force of gravity. No known physical laws of matter provide further counterforces to play a role here. One could expect hitherto unknown physical laws in such highly compressed and energetic matter, bringing about new stable stellar objects. But nothing of this kind has been found, and nothing that matter could conceive would stand a chance against general relativity’s untethered gravity. What one sees instead, in the actual universe as well as in mathematical solutions of general relativity, are black holes: the final stages of the total collapse of matter.
THE CENTRAL SINGULARITY:
A DREADFUL POINT IN TIME
No one at all sees Death,
No one at all sees the face of Death,
No one at all hears the voice of Death,
Death so savage, who hacks men down.
—GILGAMESH EPOS
The concept of black holes, as it initially arose from solutions of general relativity, can convincingly describe many properties known astrophysically from very dense regions. There is no avoiding them; extremely compact objects far heavier than the maximally allowed masses of neutron stars do exist in the universe. In the center of our own galaxy, for instance, there is a compact mass, called Sagittarius A*, as heavy as about three million suns; and yet its size cannot be measured from Earth. Such a beast can only be a gigantic black hole.
Taking the complete space-time literally, as it results from the classical equations of general relativity, all matter in a black hole collapses to a single point. As at the big bang, we encounter a singularity of infinite density, making the theory’s equations lose their validity. Thus, a wider theory must be developed so as to remain valid and precisely show us the final stage of gravitational collapse. A candidate for such a theory is, of course, again quantum gravity. But even before the total collapse, black holes of general relativity show properties different from those of neutron stars in curious