Authors: Lisa Randall

led him to use more theoretical methods later in his career. Looking to Einstein won’t resolve the issue, however. Despite his successful application of mathematics to general relativity, his later mathematical search for a unified theory never reached fruition.

As Einstein’s research demonstrated, there are different types of scientific truth and different ways of finding them. One is based in observations; this is how we learned about quasars and pulsars, for example. The other is based on abstract principles and logic: for example, Karl Schwarzschild first derived black holes as a mathematical consequence of general relativity. Ultimately, we would like these to converge—black holes have now been deduced from both the mathematical description of observations and from pure theory—but in the first phases of investigation, the advances we make based on the two types of truth are rarely the same. And in the case of string theory, the principles and equations are not nearly so well laid out asare those of general relativity, making deriving its consequences that much harder.

When string theory first rose to prominence, it sharply divided the particle physics world. I was a graduate student in the mid-1980s when the “string revolution” first split the world of particle physics asunder. At that time, one community of physicists decided to devote themselves wholeheartedly to the ethereal, mathematical realm of string theory.

String theory’s basic premise is that strings—not particles—are the most fundamental objects of nature. The particles we observe in the world around us are mere consequences of strings: they arise from the different vibrational modes of an oscillating string, much as different musical notes arise from a vibrating violin string. String theory gained favor because physicists were looking for a theory that consistently includes quantum mechanics and general relativity and that can make predictions down to the tiniest conceivable distance scales. To many people, string theory looked like the most promising candidate.

However, another group of physicists decided to stay in touch with the relatively low-energy world that experiments could explore. I was at Harvard, and the particle physicists there—which included the excellent model builders Howard Georgi and Sheldon Glashow, along with many talented postdoctoral fellows and students, were among the stalwarts who continued with the model building approach.

Early on, the battles between the merits of the two opposing view-points—string theory and model building—were fierce, with each side claiming better footing on the road to truth. Model builders thought that string theorists were in mathematical dreamland, whereas string theorists thought that model builders were wasting their time and ignoring the truth.

Because of the many brilliant model builders at Harvard, and because I relished the challenges of model building, when I first entered the world of particle physics I stayed within that camp. String theory is a magnificent theory which has already led to profound mathematical and physical insights, and it might well contain the correct ingredients to ultimately describe nature. But finding the connection between string theory and the real world is a daunting task. Theproblem is that string theory is defined at an energy scale that is about ten million billion times larger than those we can experimentally explore with our current instruments. We still don’t even know what will happen when the energy of particle colliders increases by a factor of ten!

An enormous theoretical gulf separates string theory, as it is currently understood, from predictions that describe our world. String theory’s equations describe objects that are so incredibly tiny and possess such extraordinarily high energy that any detectors we could imagine making with conceivable technologies would be unlikely ever to see them. Not only is it mathematically

As Einstein’s research demonstrated, there are different types of scientific truth and different ways of finding them. One is based in observations; this is how we learned about quasars and pulsars, for example. The other is based on abstract principles and logic: for example, Karl Schwarzschild first derived black holes as a mathematical consequence of general relativity. Ultimately, we would like these to converge—black holes have now been deduced from both the mathematical description of observations and from pure theory—but in the first phases of investigation, the advances we make based on the two types of truth are rarely the same. And in the case of string theory, the principles and equations are not nearly so well laid out asare those of general relativity, making deriving its consequences that much harder.

When string theory first rose to prominence, it sharply divided the particle physics world. I was a graduate student in the mid-1980s when the “string revolution” first split the world of particle physics asunder. At that time, one community of physicists decided to devote themselves wholeheartedly to the ethereal, mathematical realm of string theory.

String theory’s basic premise is that strings—not particles—are the most fundamental objects of nature. The particles we observe in the world around us are mere consequences of strings: they arise from the different vibrational modes of an oscillating string, much as different musical notes arise from a vibrating violin string. String theory gained favor because physicists were looking for a theory that consistently includes quantum mechanics and general relativity and that can make predictions down to the tiniest conceivable distance scales. To many people, string theory looked like the most promising candidate.

However, another group of physicists decided to stay in touch with the relatively low-energy world that experiments could explore. I was at Harvard, and the particle physicists there—which included the excellent model builders Howard Georgi and Sheldon Glashow, along with many talented postdoctoral fellows and students, were among the stalwarts who continued with the model building approach.

Early on, the battles between the merits of the two opposing view-points—string theory and model building—were fierce, with each side claiming better footing on the road to truth. Model builders thought that string theorists were in mathematical dreamland, whereas string theorists thought that model builders were wasting their time and ignoring the truth.

Because of the many brilliant model builders at Harvard, and because I relished the challenges of model building, when I first entered the world of particle physics I stayed within that camp. String theory is a magnificent theory which has already led to profound mathematical and physical insights, and it might well contain the correct ingredients to ultimately describe nature. But finding the connection between string theory and the real world is a daunting task. Theproblem is that string theory is defined at an energy scale that is about ten million billion times larger than those we can experimentally explore with our current instruments. We still don’t even know what will happen when the energy of particle colliders increases by a factor of ten!

An enormous theoretical gulf separates string theory, as it is currently understood, from predictions that describe our world. String theory’s equations describe objects that are so incredibly tiny and possess such extraordinarily high energy that any detectors we could imagine making with conceivable technologies would be unlikely ever to see them. Not only is it mathematically

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