String theory isn’t complete. There are a vast number of different string theory solutions — literally billions of billions of billions of billions of different possible variants of string theory, depending on the parameters introduced into the theory. So, in order to test string theory, scientists have to figure out which predictions the theory actually makes.
Before testing on string theory can take place, physicists need to filter through the massive possible number of solutions to find a manageable amount that may describe our universe.
Most of the current tests related to string theory are measurements that are helping to define the current parameters of the theory. Then, after the remaining theoretical solutions are somehow assessed in a reasonable way, scientists can begin testing the unique predictions they make.
There are two features common to (almost) all versions of string theory, and scientists who are looking for evidence of string theory are testing these ideas even now:
Supersymmetry
Extra dimensions
These are string theory’s two cornerstone ideas (aside from the existence of strings themselves, of course), which have been around since the theory was reformulated into superstring theory in the 1970s. No theory that has tried to eliminate them has lasted very long.
Testing an incomplete theory with indistinct predictions
Right now, there is a great deal of confusion over what physical properties (other than supersymmetry and extra dimensions) lie at the heart of string theory. The holographic principle, anthropic principle, brane world scenarios, and other such approaches are becoming more popular, but scientists don’t know for certain how they apply in the case of our universe.
The energy constraints on string theory experiments are obviously a big obstacle, but for most skeptical theorists, lack of specific, distinct experiments is the more disturbing issue. The variants of string theory make few distinct predictions, so it’s hard to even think about testing it.
Scientists can continue to test aspects of the Standard Model, to make sure that string theory predictions remain consistent, and they can look for properties such as supersymmetry or extra dimensions, but these are very general predictions, many of which are made not just by string theory. The first step in testing string theory is to figure out what the theory is telling us that is distinct from other theories.
Test versus proof
There’s really no way to prove something like string theory, as a whole. You can prove that a specific prediction (such as supersymmetry) is true, but that doesn’t prove that the theory as a whole is true. In a very real sense, string theory can never be proved; it can just meet the test of time, the same way that other theories have done.
For scientists, this slight distinction is known and accepted, but there’s some confusion about it among nonscientists. Most people believe that science proves things about the laws of nature beyond a shadow of a doubt, but the truth is that science dictates there is always a shadow of a doubt in any theory.
A theory can be tested in two ways. The first is to apply the theory to explain existing data (called a postdiction). The second is to apply the theory to determine new data, which experiments can then look for. String theory has been very successful at coming up with postdictions, but it hasn’t been as successful at making clear predictions.
String theory has some valid criticisms that need to be addressed. Even if they are addressed, string theory will never be proved, but the longer it makes predictions that match experiments, the more support it will gain.
For this to happen, of course, string theory has to start making predictions that can be tested.
