Warped Passages - Lisa Randall [137]
Ike was very disappointed when he realized that only the newer Model 7.0 included the latest results. But Ike then caught up with the most recent string theory developments, souped up his Alicxvr, and never got space-sick again.
Einstein’s theory of general relativity was monumental. With it, physicists understood the gravitational field more deeply and could calculate gravity’s influence with unprecedented precision. Relativity gave physicists the tools to predict the evolution of all gravitational systems—even that of the entire universe. However, despite all of its successful predictions, general relativity cannot be the final word on gravity. General relativity fails when it is applied to extremely short distances. At very tiny length scales, only a new gravitational paradigm can succeed. Many physicists believe that that paradigm must be string theory.
If string theory is correct, it embraces the successful predictions of general relativity, quantum mechanics, and particle physics. But it also extends physics to distance and energy domains that these other theories are not equipped to handle. String theory is not yet sufficiently developed for us to evaluate its high-energy predictions and validate its efficacy in these elusive distance and energy regimes. But string theory does have several remarkable features that lend credence to this promising picture.
We’ll now take a look at string theory and how this dramatic new theory evolved, culminating in the “superstring revolution” of 1984, when physicists demonstrated that pieces of string theory fit together miraculously well. The superstring revolution was only the beginning of an intense research program that actively engages many physicists today. In this and the following chapters, we’ll review the history of string theory and some of the recent exciting string theory developments. We’ll see that string theory has made remarkable advances and has numerous promising aspects. But we’ll also see that string theory faces many crucial challenges that physicists will have to resolve before using it to make predictions about our world.
Incipient Unrest
Quantum mechanics and general relativity peacefully coexist over a wide range of distances, including all those that are accessible to experiments. Although both theories should apply on all length scales, the two theories have a mutual understanding about which of them dominates at measurably long and short distances. Quantum mechanics and general relativity can peacefully share territory because each respects the other’s authority in its designated domain. General relativity is important for massive extended objects, such as stars or the galaxy. But gravity’s influence on an atom is negligible, so you can safely study an atom ignoring general relativity. Quantum mechanics, on the other hand, is critical at atomic distances because its predictions for an atom are substantial and differ significantly from those of classical physics.
However, quantum mechanics and relativity do not have an entirely harmonious relationship. These two very different theories never adequately negotiated the extremely tiny distance known as the Planck scale length, 10-33 cm. From Newton’s gravitational force law, we know that the strength of gravity is proportional to masses and inversely proportional to distance squared. Even though on atomic scales, gravity is weak, the gravitational force law tells us that on even tinier scales, the force of gravity is enormous. Gravity is important not only for very massive extended objects, but also for objects that are in extremely close proximity, separated by the minuscule Planck scale length. If we try to make predictions about this unmeasurably small distance, both quantum mechanics and general relativity would contribute significantly, and the two theories’ contributions would be incompatible. Neither quantum mechanics nor gravity can be neglected in this contested territory, where quantum mechanical and general relativity calculations fail