String Theory: Space-Time Should Be Fluid
One of the hallmarks of modern physics is general relativity. The clash between general relativity and quantum physics is part of the motivation for looking for a string theory, but some critics believe that string theory is designed in such a way that it doesn’t faithfully maintain the principles of general relativity.
Which principles of general relativity aren’t maintained in string theory? Specifically, the idea that space-time is a dynamic entity that responds to the presence of matter around it. In other words, space-time is flexible.
In physics terminology, general relativity is a background-independent theory, because the background (space-time) is incorporated into the theory. A background-dependent theory is one where objects in the theory are sort of plugged in to a space-time framework.
Right now, string theory is a background-dependent framework. Space-time is rigid, instead of flexible. If you are given a certain configuration of space-time, you can discuss how a given version of string theory would behave in that system.
The question is whether string theory, which right now can only be formulated in fixed space-time environments, can really accommodate a fundamentally dynamic space-time framework. How can you turn the rigid space-time of string theory into the flexible space-time of general relativity? The pessimist replies You can’t and works on loop quantum gravity.
The optimist, however, believes that string theory still has hope. Even with a rigid background of space-time, it’s possible to get general relativity as a limiting case of string theory. This isn’t quite as good as getting a flexible space-time, but it means that string theory certainly doesn’t exclude general relativity.
Instead of getting the full high-definition version of space-time, though, you’re left with something more like a flipbook, which treats each image as static but, overall, provides the impression of smooth motion.
String theory is a work-in-progress, and it’s still hoped that physical and mathematical principles might be developed that will allow for the expression of a fully dynamic background in string theory. String theorists are forced to talk about the theory in a rigid space-time (background dependent) only because they haven’t yet found a mathematical language that will let them talk about it in a flexible space-time (background-independent).
Some believe that Maldacena’s AdS/CFT correspondence may provide a means of incorporating this background-independent language. It’s also possible that the principles that allow this new language will come from an unexpected direction.
Or, of course, such principles may not exist at all, and the skeptic’s inclination to criticize string theory may therefore be justified.