Updated: May 28, 2020
At STARMUS 2016 Brian Greene will tell us about String Theory and the Nature of Reality. He will do this in just 20 minutes. So I’ll try to explain it to you in just a few paragraphs but from the perspective of a non-physicist.
String theory rose from the quest to explain inconsistencies in existing theories in physics. Gravity seems to be the bad boy that doesn’t fall in line with quantum physics and general relativity. If scientists could reconcile that, they might even have the holy grail—one theory that explains everything in the physical universe.
The idea of unifying explanations of the universe is laudable, but the “stringicists” have given rise to even more theories—strings, superstrings, supersymmetry, branes, m-branes, and more. They generated so many types of string theories—Type 1, Type IIA, Type IIB, Type HO, Type HE, and so on—that they themselves had to devise a unified theory of string theory call M-Theory (the mother of all string theories).
So what’s it all about? Strings are tiny, one-dimensional vibrating entities. The vibrational state of the string manifests itself as different physical particles. We don’t see a string, but we can observe ordinary particles and measure the particle’s mass and charge. But under the hood, that particle is just vibrations. The apple in the image is, ultimately, good vibrations! (Image courtesy of Wikimedia Commons.)
Scientific theories are supposed to have predictive power—you should be able to predict a consequence of the theory and then measure those consequences in the real world. So far nothing has panned out for string theory, which is why some people refer to it as a “theoretical framework.” There are some elegant mathematics behind strings, which is the primary reason why this area of study has survived from 1960’s until now. That and the fact it has cool jargon.
For more information, see Brian Greene's TED Talk on string theory.