One of the toughest problems in physics is unifying Einstein’s theory of general relativity, which describes gravity on a large scale, with quantum mechanics, which describes the other three fundamental forces acting on the atomic scale. Finding this “theory of everything” has stirred great debate in the scientific community. Devising experiments to test it has been unsuccessful so far, but that may soon change.
After Einstein finished his work on general relativity, he spent the next three decades of his life trying to expand his theory of gravitation to include electromagnetism (the strong and weak nuclear forces had not yet been discovered). This search left Einstein isolated from the main body of physics which was consumed by the emerging field of quantum mechanics, and which Einstein never fully embraced. In the early 1940s, Einstein wrote “I have become a lonely old chap who is mainly known because he doesn’t wear socks and is exhibited as a curiosity on special occasions.” In 1950, he described his unified field theory and although his work was ahead of its time, his efforts were ultimately unsuccessful. Einstein’s dream of unifying the fundamental forces of nature would have to wait for physics and mathematics to catch up.
The worldsheet on the left shows a string splitting apart and
the worldsheet on the right shows two strings joining together.
The arrows indicate the direction of travel through spacetime.
Since then, theoretical physicists have taken up the challenge. The most successful theory thus far is string theory. At its essence, string theory is an attempt at describing gravity at the quantum level. In string theory, all fundamental particles are not point like, but instead made out of tiny, one-dimensional, vibrating loops of string—like incredibly small rubber bands. These strings use gravitons to exchange the force of gravity back and forth. A graviton is a hypothetical particle that is itself another little loop of string. You start with a loop of string and that string splits in half, creating a second string. If you have many strings doing this at the same time, every once in a while the split strings will get mixed up with one another and exchange back and forth, and that is the origin of gravity in string theory. This splitting and joining can be viewed diagrammatically as a worldsheet. A worldsheet is the name coined by Leonard Susskind for a two-dimensional surface which describes the embedding of a string in spacetime.
Leonard Susskind is widely regarded as the father of string theory. In 1969, he and other physicists began to explore the possibility that particles were made up of strings. What they discovered, to their amazement, was that these particles behaved as if they had gravitational forces between them. Split off little pieces, exchange them, and they create forces very similar to gravity.
Initially, string theory was intended to describe protons and neutrons. But the strings that physicists describe now are a billion billion times smaller than a proton. Much smaller than we can see with any kind of microscope that exists and smaller than anything that can be detected by the Large Hadron Collider. Because of this, graviton detection is impossible. Even if we could build a detector the size of Jupiter, we would only be able to observe one graviton every 10 years at best and it would be impossible to distinguish it from a neutrino. And a neutrino shield would be so massive that it would collapse into a black hole!
Next week we will finish up our discussion of string theory by talking about why it requires extra spacial dimensions and what experiments are underway to test string theory.