Once Before Time - Martin Bojowald [90]
Once Hawking radiation reaches an intensity above the background level, energy loss from the black hole, slow but inescapable, results. The covering horizon moves ever closer to the center, further enhancing the radiation. Slowly piling up the loss of energy, the black hole must either evaporate completely after a finite time, or hope that quantum gravity changes the balance once the cover comes to lie close to the central singularity. This could open up new possibilities for observation, provided that such objects have existed in the recent past of the cosmos and did indeed evaporate. What exactly happens with such a black hole remains unclear; possibilities range from complete evaporation to the disappearance of the cover, making the bare interior visible from the outside. The latter case would manifest itself by an explosion revealing a part of the collapsed matter. (This can only be a fraction, since some energy was already radiated away in the Hawking process.)
Can we really employ this effect for observational tests of quantum gravity? Probably not, as fascinating as it would be. The cosmic background radiation is currently too intense, even though it has already cooled down to –270 degrees Celsius. As weak as this radiation is, the Hawking radiation of most black holes is weaker still. Very small black holes are necessary for evaporation to take place at all; but in this case it would happen so quickly that such black holes must have evaporated long ago. New and powerful telescopes might one day glimpse such events.
As one can see, we do not really live at a good cosmic moment in time, at least if one’s aim is to observe Hawking radiation. Going back to the thoughts at the beginning of this chapter, especially in the case of cosmological observations we are obliged to live with what the universe presents us. Later in the history of the universe we might get another chance: The microwave background will be diluted so much as a result of cosmic expansion that even heavy black holes could evaporate, giving rise to entirely new phenomena.
6. BLACK HOLES
COLLAPSE, AND HAPPY ENDINGS?
On the face of the sun its countenance gazes,
Then all of a sudden nothing is there!
—GILGAMESH EPOS
Kruskal”1 the capsule is just about to break into pieces. Ever deeper it penetrates the hot dense mash of matter, only barely recognized as a collapsing star. It is unmanned, for no strong human and not even a single cell would be able to sustain these forces. Using modern femtotechnology, it has specifically been made so tiny that the forces yanking at its different parts are minimized to the greatest possible extent. Its nuclei swing around wildly, whipped by the mightiest anger of a foaming space-time. The miniature instruments withstand those conditions only with luck. No chances of retreat or of repair exist now, for long ago the capsule had passed the black hole horizon. Being pulled ever closer to the center, it would be unable to escape even if it had the strongest engine.
The further the capsule proceeds, the stronger the tidal forces acting on it: Here space-time is curved so much that the gravitational force on one side differs greatly from that on the opposite side, even considering the small size of the tiny capsule. Helplessly the capsule is deformed by the black hole’s forces of nature, as happens on a smaller scale with the tides of the oceans and even the continental land masses in the gravitational tug of war between earth and moon. The material forces can withstand this only for a limited time, and soon the capsule will be torn apart. Even its constituents will splinter ever more, before at last dissolving into the smallest particles.
But until then the capsule’s heart, the Chiao transformer, will do its valuable work. It is the brainchild of the physicist Raymond Chiao, who once postulated the possibility of transforming electromagnetic waves into gravitational ones. Following this principle,