Your Kindle Notes For: White Holes Carlo Rovelli Last accessed on Friday February 28, 2025 13 Highlight(s) | 0 Note(s) Yellow highlight | Location: 454 To understand what happens in a black hole, in other words, we should not think of a stationary cone with the singularity at its center. We need to think of it as a long tube with the star that generated it at the bottom: the tube gets longer and thinner and in the future is squeezed into a line. The singularity is not at the center; it is after. Yellow highlight | Location: 576 We christened the bouncing star as a “Planck star,”[10] because the bounce is expected at the Planck scale.[11] Could the full black hole actually bounce around the star? What would we see if we were to imagine filming the full life of the black hole and then running the film backward? We would see a white hole. Yellow highlight | Location: 616 Applied to space, this basic idea implies that there should be elementary grains of space, of finite size. Quanta of space. That is, there are no arbitrarily small things; there is a lower limit to divisibility. Space is a physical entity, and like any other entity, it is granular. Einstein’s theory and the mathematics of quantum theory combine to imply this fact.[12] Yellow highlight | Location: 645 If space is granular, the inside of a black hole cannot be squashed any further than the size of the individual grains. There is a limit to compression and distortion. The contraction that squeezes the tube of the inside of a black hole must therefore stop before the singularity. What happens next? Yellow highlight | Location: 762 The black and the white horizons are themselves different, but from the outside they look precisely the same. The horizons and the insides distinguish white from black, future from past; the outside does not. Yellow highlight | Location: 855 Heat is the mark of irreversibility. Heat distinguishes past from future.[*] Yellow highlight | Location: 859 But, careful: the shrinking of the horizon does not mean that the interior of the black hole becomes smaller. The interior largely remains what it is, and the interior volume keeps growing. It is only the horizon that shrinks. This is a subtle point that confuses many. Hawking radiation is a phenomenon that regards mainly the horizon, not the deep interior of the hole. Therefore, a very old black hole turns out to have a peculiar geometry: an enormous interior (that continues to grow) and a minuscule (because it has evaporated) horizon that encloses it. Yellow highlight | Location: 1,064 We are the irreversible foam of free energy that was trapped in the disequilibrium between hydrogen and helium, freed by the Sun. Yellow highlight | Location: 1,084 The reason we remember the past and not the future is entirely due to the fact that the universe was further from equilibrium at one point in the past than it is now. Yellow highlight | Location: 1,085 If a system then reaches complete equilibrium, there are no more traces, no more memory, nothing to distinguish the past from the future. Sooner or later, every memory vanishes, canceled through the wear and tear of time. Sooner or later, of our proud civilizations, of everything that we have understood, of the words in books such as this one, of our controversies and of our desperate passions and loves . . . no trace will remain. Yellow highlight | Location: 1,167 But there would be a far more radical consequence for us in such a universe: our very thoughts could not exist. We would not be able to make observations, or to reason, because to think is to dissipate energy. We would not have senses, because our senses record things—in other words, they produce memories. They would not work in a situation of equilibrium. We would not be able to hear music, since music exists in our heads only because we can remember preceding notes. In a universe in equilibrium, we would simply not exist as thinking and feeling beings. Yellow highlight | Location: 1,174 We are children of time. Yellow highlight | Location: 1,250 This, in the end, is a Planck star: a shortcut to the future. A way to hide safely for a moment, while outside eons of time flow slowly by.