One of the biggest criticisms of string theory is that it has never been confirmed by experiment, despite its long history. The difficulty lies with the size of the strings, which can be as small as 10-33 cm in length. This is a hundred million billion times smaller than the protons that are smashed together at the Large Hadron Collider (LHC) in Geneva.
The LHC is the largest particle accelerator in the world, with a circumference of 27 km. In order to see strings themselves we’d need to build an accelerator the size of the galaxy! This means it’s going to be pretty impossible to see strings directly. We must look for indirect evidence.
One of the biggest hopes for discovery is supersymmetry. This theory predicts that every particle has a heavier twin. These so-called superparticles could appear at the LHC, perhaps solving the hierarchy problem of the Standard Model. All realistic string theories require supersymmetry so its detection is of paramount importance. But supersymmetry alone wouldn’t be enough to prove that string theory is right.
It might also be possible to detect the extra dimensions predicted by string theory. Some conjecture that gravity is weak compared to the other forces because it works in many more dimensions. The messenger particle of gravity, the graviton, should be able to move between these dimensions. If a graviton were produced in the LHC and subsequently moved to another dimension, it would seem like a chunk of energy had gone missing.
If extra dimensions exist they might cause microscopic black holes to form. The properties of these black holes would depend on the number and size of the extra dimensions. Observing these would lend great credibility to the string perspective.
Detection of extra dimensions would constitute strong support for string theory. However, we’ve seen no evidence for them at the LHC, and theoretical models predicting their detection in current colliders are generally quite contrived. It remains a spectacular – but unlikely – scenario.
One experimental application of string theory comes from an unlikely source – condensed matter. In 2005, the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory noticed unusual behaviour in an exotic state of matter called a quark-gluon plasma. String theorists have used the AdS/CFT correspondence to provide a reasonably successful explanation of the phenomena.
Finally, there are a few outlandish attempts to locate the smoking gun. Remember that the quantum description of strings predicted an infinite number of particles of increasing mass. The lightest of the non-standard particles would be 1014 times heavier than those that the LHC can detect, but there might be indirect ways to spot them.
Maybe hypothesised cosmic strings will turn up. These strings would have formed during a time of very fast expansion at the beginning of the universe. As a result they would have been stretched to a very large size and could be identified by telescopes.
Ultimately, the cleanest and most certain experimental predictions of string theory lie at energies inaccessible to current technology. Definite confirmation in the near future would require either a brilliant idea or wonderful good luck.
In the meantime, string theory continues to offer many insights into different areas of theoretical physics. Moreover it does produce definite – if unconfirmed – claims about the universe at its smallest scales. Few other theories have this power.