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String theory is the greatest scientific quest of all time. Its goal is nothing other than a complete description of physical reality—at least at the level of fundamental particles, interactions, and perhaps space-time itself. In principle, once the fundamental theory is fully known, one could derive relativity and quantum theory as low energy limits to strings. The theory sets out to do what no other has been able to since the early twentieth century—combine general relativity and quantum theory into a single unified framework. This is an ambitious program that has occupied the best minds in mathematics and physics for decades. Einstein himself failed, but he lacked key ingredients that are necessary to pull it off.
String theory comes attached with a bit of controversy. As anyone who is reading this book likely knows, experimentally testing it is not an immediately accessible option due to the high energies required. It is, after all, a theory of creation itself so the energies associated with string theory are of course very large. Nonetheless, it now appears that some indirect tests are possible and the timing of this book may coincide with some of this program. The first clue will be the continued search for supersymmetry, the theory that proposes fermions and bosons have superpartners, that is, a fermion like an electron has a sister Superpartner particle that is a boson. Superparticles have not been discovered, so if it exists supersymmetry must be broken somehow so that the super partners have high mass. This could explain why we haven’t seen them so far. But the Large Hadron Collider being constructed in Europe as we speak may be able to discover evidence of supersymmetry. This does not prove string theory, because you can have Supersymmetry work just fine with point particles. However, supersymmetry is absolutely essential for string theory to work. If supersymmetry does not exist, string theory cannot be true. If supersymmetry is found, while it does not prove string theory, it is a good indication that string theory might be right.
General relativity and quantum mechanics stand out as the pillars of twentieth century science, able to escribe almost all known phenomena from the scale of subatomic particles all the way up to the rotations of galaxies and even the history of the universe itself. Despite this grand success, which includes stunning agreement with experiment, these two theories represent physics at a crossroads—one that is plagued with crisis and controversy.
The problem is that at first sight, these two theories are at complete odds with each other. The general theory of relativity (GR), Einstein’s crowning achievement, describes gravitational interactions, that is, interactions that occur on the largest scales that we know. But it not only stands out as Einstein’s greatest contribution to science but it also might be called the last classical theory of physics. That is, despite its revolutionary nature, GR does not take quantum mechanics into account at all. Since experiment indicates that quantum mechanics is the correct description for the behavior of matter, this is a serious flaw in the theory of general relativity.
We don’t think about this under normal circumstances because quantum effects only become important in gravitational interactions that are extremely strong or taking place over very small scales. In the situations where we might apply general relativity, say to the motion of the planet mercury around the sun or the motion of the galaxies, quantum effects are not important at all. Two places where they will be important are in black hole physics and in the birth of the universe. We might also see quantum effects on gravity in very high energy particle interactions.
On the other hand, quantum mechanics basically ignores the insights of relativity. It basically pretends gravity doesn’t exist at all, and pretends that space and time are not on the same footing. The notion of space-time does not enter in quantum mechanics, and although special relativity plays a central role in quantum field theory, gravitational interactions are nowhere to be found there either.
General relativity and quantum mechanics stand out as the pillars of twentieth century science, able to escribe almost all known phenomena from the scale of subatomic particles all the way up to the rotations of galaxies and even the history of the universe itself. Despite this grand success, which includes stunning agreement with experiment, these two theories represent physics at a crossroads—one that is plagued with crisis and controversy.
The problem is that at first sight, these two theories are at complete odds with each other. The general theory of relativity (GR), Einstein’s crowning achievement, describes gravitational interactions, that is, interactions that occur on the largest scales that we know. But it not only stands out as Einstein’s greatest contribution to science but it also might be called the last classical theory of physics. That is, despite its revolutionary nature, GR does not take quantum mechanics into account at all. Since experiment indicates that quantum mechanics is the correct description for the behavior of matter, this is a serious flaw in the theory of general relativity.
We don’t think about this under normal circumstances because quantum effects only become important in gravitational interactions that are extremely strong or taking place over very small scales. In the situations where we might apply general relativity, say to the motion of the planet mercury around the sun or the motion of the galaxies, quantum effects are not important at all. Two places where they will be important are in black hole physics and in the birth of the universe. We might also see quantum effects on gravity in very high energy particle interactions.
On the other hand, quantum mechanics basically ignores the insights of relativity. It basically pretends gravity doesn’t exist at all, and pretends that space and time are not on the same footing. The notion of space-time does not enter in quantum mechanics, and although special relativity plays a central role in quantum field theory, gravitational interactions are nowhere to be found there either.
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