Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [72]
Despite this embarrassment, the dark energy has at least one desirable consequence. Recall that inflation requires the Universe to have the critical mass but that all the matter in the Universe adds up to only about a third of the critical mass. Well all forms of energy, as Einstein discovered, have an effective mass. And that includes the dark energy. In fact, it turns out to account for about two-thirds of the critical mass, so that the Universe has exactly the critical mass—just what is predicted by inflation.
Although nobody knows what the dark energy is, one possibility is that it is associated with the repulsive force of empty space proposed by Einstein. In science, it seems, all things begin and end with Einstein. His biggest mistake may yet turn out to be his biggest success.
It is worth stressing, however, that the Big Bang, for all its successes, is still basically a description of how our Universe has evolved from a superdense, superhot state to its present state, with galaxies, stars, and planets. How it all began is still shrouded in mystery.
TO THE SINGULARITY AND BEYOND
Imagine the expansion of the Universe running backwards again like a movie in reverse. As the Universe shrinks down to a speck, its matter content becomes ever more compressed and ever hotter. In fact, there is no limit to this process. At the instant the Universe’s expansion began—the moment of its birth—it was infinitely dense and infinitely hot. Physicists call the point when something skyrockets to infinity a singularity. According to the standard Big Bang picture, the Universe was therefore born in a singularity.
The other place where Einstein’s theory of gravity predicts a singularity is at the heart of a black hole. In this case the matter of a catastrophically shrinking star eventually becomes compressed into zero volume and therefore becomes infinitely dense and infinitely hot. “Black holes,” as someone once said, “are where God divided by zero.”
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A singularity is a nonsense. When such a monstrous entity pops up in a theory of physics, it is telling us that the theory—in this case, Einstein’s theory of gravity—is faulty. We are stretching it beyond the domain where it has anything sensible to say about the world. This is not surprising. General relativity is a theory of the very large. In its earliest stages, however, the Universe was smaller than an atom. And the theory of the atomic realm is quantum theory.
Normally, there is no overlap between these two towering monuments of 20th-century physics. However, they come into conflict at the heart of black holes and at the birth of the Universe. If we are ever going to understand how the Universe came into being, we are going to have to find a better description of reality than Einstein’s theory of gravity. We need a quantum theory of gravity.
The task of finding such a theory is formidable because of the fundamental incompatibility between general relativity and quantum theory. General relativity, like every theory of physics before it, is a recipe for predicting the future. If a planet is here now, in a day’s time it will have moved over there, by following this path. All these things are predictable with 100 per cent certainty. Quantum theory, however, is a recipe for predicting probabilities. If an atom is flying through space, all we can predict is its probable final position, its probable path. Quantum theory therefore undermines the very foundation stones of general relativity.
Currently, physicists are trying to discover the elusive quantum theory of gravity by a number of routes. Undoubtedly, the one getting the most publicity is superstring theory, which views the fundamental building blocks of matter not as pointlike particles but as ultra-tiny pieces of “string.” The string—superconcentrated mass-energy—can vibrate just like a violin string, and each distinct vibration “mode” corresponds to a fundamental particle such as an electron or a photon.
What excites string theorists is that some form