The two great pillars of modern physics are quantum mechanics and general relativity. These theories describe small-scale and large-scale phenomena, respectively. While quantum mechanics predicts the shape of a hydrogen atom, general relativity explains the properties of the visible universe on the largest scales.
A long-standing goal of physics is to construct a new theory that embraces large and small scales in a consistent way, producing what is sometimes called the “theory of everything”. If successful, this enterprise promises to provide a new basis for understanding the laws of physics and the history of the universe.
Historically, physicists viewed electrons as point particles but in string theory they are understood as vibrating strings with zero thickness and minute length. In the early development of the theory, the fundamental entities studied – matter and fields – were described in terms of microscopic strings. To examine the strings in a direct fashion, we would need accelerators many orders of magnitude more powerful than those available today.
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Some years ago the Large Hadron Collider (LHC) at CERN successfully detected the Higgs boson, the particle that gives mass to other particles. But even the LHC cannot directly “see” the strings that make up particles. So, practitioners in the field are searching for indications of “stringy physics”, predictions of the unified theory that go beyond quantum mechanics and general relativity and that would validate the general approach. String theory also has many vociferous critics who decry its lack of verifiable predictions.
String theory has produced some beautiful results in a symbiosis of mathematics and physics. New ideas are continually emerging. The discovery of branes – objects unrecognised as part and parcel of the theory prior to 1995 – considerably enriches its contents. A point corresponds to a 0-brane, a string to a 1-brane, and a membrane to a 2-brane, and there are branes of higher orders too. A D-brane is like a capstan to which a string can be tethered. Strings may extend between D-branes and the physical properties of the branes are quite different from the strings with which the story first began. It is not yet clear how best to utilise the full contents that are present to construct the elusive theory of everything.
Validation from the skies
String theory continues to amaze those who study it. We live in a world with three spatial dimensions and one time dimension. Four-dimensional spacetime is the arena in which general relativity is played out. But string theory requires extra dimensions if it is to make physical sense: supersymmetric string theory requires 10 dimensions. If the universe has 10 dimensions and we only see three, where are the missing ones? One possibility is that the “extra” dimensions are so small that they are effectively hidden from view.
We generally consider strings representing subatomic particles to be extremely small. However, the history of the universe muddles the division between small and large scales. Our universe likely suffered a period of rapid expansion at the earliest times, a process called inflation, stretching microscopic to macroscopic scales. I spoke to Prof David Chernoff of Cornell University, who is visiting UCD. He explained how astronomers are working to devise observational techniques to study the consequences of inflation.
Roughly speaking, the stretching forms a network of long strings that extend across the visible universe and smaller loops that oscillate wildly. Both these relics from the epoch of inflation may be detectable: they may deflect light or generate a background radiation field. Direct detection would provide evidence supporting string theory and lack of detection (within limits) would constrain the theory itself.
We may have to wait some time for a new theory that can unify our current models. Ed Witten, one of the leading lights of string theory, has described it as “a 20th-century theory for the 21st century”.
Peter Lynch is emeritus professor at the School of Mathematics & Statistics in University College Dublin. He blogs at thatsmaths.com