Warped Passages - Lisa Randall [34]
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. The problem 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 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, making it 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,