tremendously challenging to derive string theory’s consequences and predictions, it is not even always clear how to organize string theory’s ingredients and determine which mathematical problem to solve. It is too easy to get lost in a thicket of detail.
String theory can lead to a plethora of possible predictions at distances we actually see—the particles that are predicted depend on the as yet undetermined configuration of fundamental ingredients in the theory. Without some speculative assumptions, string theory looks like it contains more particles, more forces, and more dimensions than we see in our world. We need to know what separates the extra particles, forces, and dimensions from the visible ones. We don’t yet know what physical features, if any, favor one configuration over another, or even how to find a single manifestation of string theory that conforms to our world. We would have to be very lucky to extract all the correct physical principles that will make the predictions of string theory match what we see.
For example, string theory’s invisible extra dimensions have to be different from the three that we see. The gravity of string theory is more complex than the gravity we see around us—the force that caused Newton’s apple to fall on his head. Instead, string theory’s gravity operates in six or seven additional dimensions of space. Fascinating and remarkable as string theory is, puzzling features such as its extra dimensions obscure its connection to the visible universe. What distinguishes those extra dimensions from the visible ones? Why aren’t they all the same? Discovering how and why nature hides string theory’s extra dimensions would be a stunning achievement, makingit worthwhile to investigate all possible ways in which this might happen.
So far, however, all attempts to make string theory realistic have had something of the flavor of cosmetic surgery. In order to make its predictions conform to our world, theorists have to find ways to cut away the pieces that shouldn’t be there, removing particles and tucking dimensions demurely away. Although the resulting sets of particles come tantalizingly close to the correct set, you can nonetheless tell that they aren’t quite right. Elegance might well be the hallmark of a correct theory, but we can only really judge a theory’s beauty once we’ve fully understood all its implications. String theory is captivating at first, but ultimately string theorists have to address these fundamental problems.
When exploring mountainous territory without a map, you can rarely tell what the most direct route to your destination will turn out to be. In the world of ideas, as in complex terrain, the best path to follow is not always clear at the outset. Even if string theory does ultimately unify all the known forces and particles, we don’t yet know whether it contains a single peak representing a particular set of particles, forces, and interactions, or a more complicated landscape with many possible implications. If the paths were smooth, well-signposted grids, route-finding would be simple. But that is rarely the case.
So, the approach to advancing beyond the Standard Model that I will emphasize is model building. The term “model” might evoke a small-scale battleship or castle you built in your childhood. Or you might think of numerical simulations on a computer that are meant to reproduce known dynamics—how a population grows, for example, or how water moves in the ocean. Modeling in particle physics is not the same as either of these definitions. However, it’s not entirely different from the use of the word in magazines or fashion shows: models, both on runways * and in physics, demonstrate imaginative creations and come in a variety of shapes and forms. And the beautiful ones get all the attention.
Needless to say, the similarities end there. Particle physics modelsare guesses at alternative physical theories that might underlie
String theory can lead to a plethora of possible predictions at distances we actually see—the particles that are predicted depend on the as yet undetermined configuration of fundamental ingredients in the theory. Without some speculative assumptions, string theory looks like it contains more particles, more forces, and more dimensions than we see in our world. We need to know what separates the extra particles, forces, and dimensions from the visible ones. We don’t yet know what physical features, if any, favor one configuration over another, or even how to find a single manifestation of string theory that conforms to our world. We would have to be very lucky to extract all the correct physical principles that will make the predictions of string theory match what we see.
For example, string theory’s invisible extra dimensions have to be different from the three that we see. The gravity of string theory is more complex than the gravity we see around us—the force that caused Newton’s apple to fall on his head. Instead, string theory’s gravity operates in six or seven additional dimensions of space. Fascinating and remarkable as string theory is, puzzling features such as its extra dimensions obscure its connection to the visible universe. What distinguishes those extra dimensions from the visible ones? Why aren’t they all the same? Discovering how and why nature hides string theory’s extra dimensions would be a stunning achievement, makingit worthwhile to investigate all possible ways in which this might happen.
So far, however, all attempts to make string theory realistic have had something of the flavor of cosmetic surgery. In order to make its predictions conform to our world, theorists have to find ways to cut away the pieces that shouldn’t be there, removing particles and tucking dimensions demurely away. Although the resulting sets of particles come tantalizingly close to the correct set, you can nonetheless tell that they aren’t quite right. Elegance might well be the hallmark of a correct theory, but we can only really judge a theory’s beauty once we’ve fully understood all its implications. String theory is captivating at first, but ultimately string theorists have to address these fundamental problems.
When exploring mountainous territory without a map, you can rarely tell what the most direct route to your destination will turn out to be. In the world of ideas, as in complex terrain, the best path to follow is not always clear at the outset. Even if string theory does ultimately unify all the known forces and particles, we don’t yet know whether it contains a single peak representing a particular set of particles, forces, and interactions, or a more complicated landscape with many possible implications. If the paths were smooth, well-signposted grids, route-finding would be simple. But that is rarely the case.
So, the approach to advancing beyond the Standard Model that I will emphasize is model building. The term “model” might evoke a small-scale battleship or castle you built in your childhood. Or you might think of numerical simulations on a computer that are meant to reproduce known dynamics—how a population grows, for example, or how water moves in the ocean. Modeling in particle physics is not the same as either of these definitions. However, it’s not entirely different from the use of the word in magazines or fashion shows: models, both on runways * and in physics, demonstrate imaginative creations and come in a variety of shapes and forms. And the beautiful ones get all the attention.
Needless to say, the similarities end there. Particle physics modelsare guesses at alternative physical theories that might underlie
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