And as Lucretius wrote: “our appetite for life is voracious, our thirst for life insatiable” (De rerum natura, bk. III, line 1084).
I believe that our species will not last long. It does not seem to be made of the stuff that has allowed the turtle, for example, to continue to exist more or less unchanged for hundreds of millions of years, for hundreds of times longer, that is, than we have even been in existence. We belong to a short-lived genus of species. All of our cousins are already extinct. What’s more, we do damage. The brutal climate and environmental changes that we have triggered are unlikely to spare us. For Earth they may turn out to be a small irrelevant blip, but I do not think that we will outlast them unscathed—especially since public and political opinion prefers to ignore the dangers that we are running, hiding our heads in the sand.
We have a hundred billion neurons in our brains, as many as there are stars in a galaxy, with an even more astronomical number of links and potential combinations through which they can interact. We are not conscious of all of this. “We” are the process formed by this entire intricacy, not just by the little of it of which we are conscious.
In the awareness that we can always be wrong, and therefore ready at any moment to change direction if a new track appears; but knowing also that if we are good enough we will get it right and will find what we are seeking. This is the nature of science.
As our knowledge has grown, we have learned that our being is only a part of the universe, and a small part at that.
This has been increasingly apparent for centuries, but especially so during the last century. We believed that we were on a planet at the center of the universe, and we are not. We thought that we existed as unique beings, a race apart from the family of animals and plants, and discovered that we are descendants of the same parents as every living thing around us. We have great-grandparents in common with butterflies and larches. We are like an only child who in growing up realizes that the world does not revolve only around himself, as he thought when little. He must learn to be one among others. Mirrored by others, and by other things, we learn who we are.
Using quantum mechanics, Hawking successfully demonstrated that black holes are always “hot.” They emit heat like a stove. It’s the first concrete indication on the nature of “hot space.” No one has ever observed this heat because it is faint in the actual black holes that have been observed so far—but Hawking’s calculation is convincing, it has been repeated in different ways, and the reality of the heat of black holes is generally accepted.
To trust immediate intuitions rather than collective examination that is rational, careful, and intelligent is not wisdom: it is the presumption of an old man who refuses to believe that the great world outside his village is any different from the one that he has always known.
When his great Italian friend Michele Besso died, Einstein wrote a moving letter to Michele’s sister: “Michele has left this strange world a little before me. This means nothing. People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.”
Physicists and philosophers have come to the conclusion that the idea of a present that is common to the whole universe is an illusion and that the universal “flow” of time is a generalization that doesn’t work.
The problem was already present in classical physics and was highlighted in the nineteenth and twentieth centuries by philosophers—but it becomes a great deal more acute in modern physics. Physics describes the world by means of formulas that tell how things vary as a function of “time.” But we can write formulas that tell us how things vary in relation to their “position,” or how the taste of a risotto varies as a function of the “variable quantity of butter.” Time seems to “flow,” whereas the quantity of butter or location in space does not “flow.” Where does the difference come from?
We know what happens to a heated electromagnetic field: in an oven, for instance, there is hot electromagnetic radiation, which cooks a pie, and we know how to describe this. The electromagnetic waves vibrate, randomly sharing energy, and we can imagine the whole as being like a gas of photons that move fast and randomly like the molecules in a hot balloon.
It is not impossible for a hot body to become hotter through contact with a colder one: it is just extremely improbable.
In this way the temperature of objects in contact with each other tends to equalize.
So, again, why, as time goes by, does heat pass from hot things to cold and not the other way around?
The reason was discovered by Boltzmann and is surprisingly simple: it is sheer chance.
Boltzmann’s idea is subtle and brings into play the idea of probability. Heat does not move from hot things to cold things due to an absolute law: it does so only with a large degree of probability. The reason for this is that it is statistically more probable that a quickly moving atom of the hot substance collides with a cold one and leaves it a little of its energy, rather than vice versa.
While there is no friction, for instance, a pendulum can swing forever. If we filmed it and ran the film in reverse, we would see movement that is completely possible. But if there is friction, then the pendulum heats its supports slightly, loses energy, and slows down. Friction produces heat. And immediately we are able to distinguish the future (toward which the pendulum slows) from the past.
In every case in which heat exchange does not occur, or when the heat exchanged is negligible, we see that the future behaves exactly like the past.
What they came to understand is that a hot substance is not one that contains caloric fluid. A hot substance is a substance in which atoms move more quickly. Atoms and molecules, small clusters of atoms bound together, are always moving. They run, vibrate, bounce, and so on. Cold air is air in which atoms, or rather molecules, move more slowly. Hot air is air in which molecules move more rapidly. Beautifully simple. But it doesn’t end there.
If we try to put together what we have learned in the twentieth century about the physical world, the clues point toward something profoundly different from our instinctive understanding of matter, space, and time. Loop quantum gravity is an attempt to decipher these clues and to look a little farther into the distance.
The moment of this bounce, when the universe was contracted into a nutshell, is the true realm of quantum gravity: time and space have disappeared altogether, and the world has dissolved into a swarming cloud of probability that the equations can, however, still describe.
Our world may have actually been born from a preceding universe that contracted under its own weight until it was squeezed into a tiny space before “bouncing” out and beginning to re-expand, thus becoming the expanding universe that we observe around us.
Another of the consequences of the theory, and one of the most spectacular, concerns the origins of the universe. We know how to reconstruct the history of our world back to an initial period when it was tiny in size. But what about before that? Well, the equations of loop theory allow us to go even further back in the reconstruction of that history.
What we find is that when the universe is extremely compressed, quantum theory generates a repulsive force, with the result that the great explosion, or “big bang,” may have actually been a “big bounce.”
A Planck star is not stable: once compressed to the maximum, it rebounds and begins to expand again. This leads to an explosion of the black hole. This process, as seen by a hypothetical observer sitting in the black hole on the Planck star, would be a rebound occurring at great speed. But time does not pass at the same speed for him as for those outside the black hole, for the same reason that in the mountains time passes faster than at sea level. Except that for him, because of the extreme conditions, the difference in the passage of time is enormous, and what for the observer on the star would seem an extremely rapid bounce would appear, seen from outside it, to take place over a very long time. This is why we observe black holes remaining the same for long periods of time: a black hole is a rebounding star seen in extreme slow motion.
Crushed by its own weight, the matter of these stars has collapsed upon itself and disappeared from our view. But where has it gone? If the theory of loop quantum gravity is correct, matter cannot really have collapsed to an infinitesimal point. Because infinitesimal points do not exist—only finite chunks of space. Collapsing under its own weight, matter must have become increasingly dense, up to the point where quantum mechanics must have exerted a contrary, counterbalancing pressure.
The world described by the theory is thus further distanced from the one with which we are familiar. There is no longer space that “contains” the world, and there is no longer time “in which” events occur. There are only elementary processes wherein quanta of space and matter continually interact with one another. The illusion of space and time that continues around us is a blurred vision of this swarming of elementary processes, just as a calm, clear Alpine lake consists in reality of a rapid dance of myriads of minuscule water molecules.
Where are these quanta of space? Nowhere. They are not in space because they are themselves the space. Space is created by the linking of these individual quanta of gravity.
The paradox is that both theories work remarkably well. Nature is behaving with us like that elderly rabbi to whom two men went in order to settle a dispute. Having listened to the first, the rabbi says: “You are in the right.” The second insists on being heard. The rabbi listens to him and says: “You’re also right.” Having overheard from the next room, the rabbi’s wife then calls out, “But they can’t both be in the right!” The rabbi reflects and nods before concluding: “And you’re right too.”
A university student attending lectures on general relativity in the morning and others on quantum mechanics in the afternoon might be forgiven for concluding that his professors are fools or have neglected to communicate with one another for at least a century. In the morning the world is curved space where everything is continuous; in the afternoon it is a flat space where quanta of energy leap.
Without quantum mechanics there would be no transistors. But they remain mysterious. For they do not describe what happens to a physical system but only how a physical system affects another physical system.
Why are precisely these elements listed there, and why does the periodic table have this particular structure, with these periods, and with the elements having these specific properties? The answer is that each element corresponds to one solution of the main equation of quantum mechanics. The whole of chemistry emerges from a single equation.
Einstein predicted that time passes more quickly high up than below, nearer to Earth. This was measured and turned out to be the case. If a person who has lived at sea level meets up with his twin who has lived in the mountains, he will find that his sibling is slightly older than he. And this is just the beginning.
Undistracted by schooling, one studies best during vacations.
You don’t get anywhere by not “wasting” time—something, unfortunately, that the parents of teenagers tend frequently to forget.