Introduction to M-theory
From Wikipedia, the free encyclopedia
- This article is intended as a general, non-technical introduction. For the main encyclopedia article, please see M-theory.
M-theory is a cutting-edge theory of physics that deals with the extension of superstring theory. It is somewhat contentious in the physics community, as it lacks empirical evidence. If ever experimentally verified, M-theory and string theory would represent remarkable advances in science.
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[edit] String theory
To understand M-theory it is necessary to first get some understanding of string theory. For hundreds of years physics has operated on the paradigm that the fundamental particles, like the familiar electron, are point-like or (in mathematical jargon) zero-dimensional. If string theory were to be summed up in a single idea, it is that this assumption is incorrect. Instead, string theory posits that the Universe is fundamentally composed of 1-dimensional objects—things that are similar to a string. These strings would be so small that on even the tiny scale of particles they would seem like points. In string theory, each fundamental particle is created in some sense by different patterns of vibration of the strings. One might ask why physicists have constrained themselves to 0-dimensional points for all this time; the answer is that 1-dimensional objects are much harder to work with and often cause technical problems with causality and violations of special relativity's mandate that nothing can travel faster than the speed of light.
String theory's development has come primarily because of an extremely important problem that has plagued physics for almost 100 years. The problem is that general relativity, the theory developed by Albert Einstein that explains things on very large or cosmological scales, is irreconcilable with quantum mechanics and the Standard Model, which describe the Universe on the small subatomic scale. Additionally, there are problems with the Standard Model: it has around 20 free parameters that must be plugged in by hand, and has a large number of particles it declares fundamental (there are three copies of every particle organized into generations, whose only difference from one another is mass). Also, because it can't be reconciled with general relativity, it lacks a description of gravity, the most familiar of the four fundamental forces.
It turns out that using 1-dimensional objects instead of point particles solves many of these problems. The number of free parameters in the theory drops from 20 to one (a parameter that corresponds to the size of the strings), and there is hope that details of the theory will explain why the three families of particles exist. Most importantly, string theorists were delighted to find that string theory necessarily contains gravitons, the particle that causes gravity. Edward Witten, the founder of M-theory, jokes that string theory does have the remarkable experimental evidence that gravity exists all around us. Thus, string theory successfully unites General Relativity with Quantum Mechanics.
However, there are some problems with string theory. First of all, it requires 10 dimensions for the strings to vibrate in as opposed to the 4 (3 space and 1 time) dimensions that we commonly observe. This may seem ludicrous; it is, however, possible if the extra 6 dimensions are extremely tiny and curled up. For example, if you look at a tube from a great distance it seems to be just a line (1-dimensional). Closer inspection of the tube reveals 2-dimensions, though: one that is along the tube (the one we saw from far away) and one that is going around the tube. Thus, the tube is really a 2-dimensional surface despite the fact that it appears to be a line from far away.
Another problem with string theory is that there are 5 different formulations of it. This is a key to understanding where M-theory comes in. The 5 theories differ in what kind of strings they allow and in how they implement supersymmetry, a technical part of string theory that leads to the often-used name superstring theory. These 5 different theories (esoterically called Type I, Type IIA, Type IIB, SO(32), and E8×E8 (pronounced E8 "times" E8, not E8 "ex" E8)) are something of an embarrassment of riches for string theorists. Since string theory claims to be a theory of everything, there should really only be one consistent formulation of the theory, but instead there are 5. Here is where M-theory comes to the rescue.
[edit] M-theory
In 1995, Edward Witten initiated what has been called the Second Superstring Revolution by introducing M-theory to the world. This theory combines the 5 different string theories (along with a previously abandoned attempt to unify general relativity and quantum mechanics called eleven-dimensional supergravity) into one theory. This is accomplished by knitting together a web of relationships between each of the theories called dualities (specifically, S-duality, T-duality, and U-duality). Each of these dualities provides a way of converting one of the string theories into another.
T-duality is probably the most easily explained of the dualities. It has to do with the size, denoted by R, of the curled up dimensions of the string theories. It was discovered that if you take a Type IIA string theory that has a size R and change the radius to 1/R then you will end up getting what is equivalent to a Type IIB theory of size R. This duality, along with the others, creates connections between all 5 (or 6, if you count supergravity) theories.
Factually, these dualities' existence had been known before Witten came up with the idea of M-theory. What Witten did was to predict that the fact that all of these different theories were connected was as a result of there being some underlying theory to which they were all approximations. Additionally, it was found that the equations that required string theory to exist in 10 dimensions were also approximations. The proposed (and somewhat nebulous) M-theory would instead be a theory that took place in 11 dimensions, although the details have not been pinned down.
[edit] Skepticism
M-theory has been the target of increasing skepticism as some (notably Peter Woit and Lee Smolin) argue that string theorists have overstated many of the theory's strengths while underplaying its weaknesses. One argument against string theory questions the predictive power of the theory, pointing out that string theory has not produced any "sharp" (in the sense of numerical) predictions that have been verified by experiment. Critics go on to say that because of the string theory landscape, the theory may be unable to make these predictions even in principle and would hence not be falsifiable. Another version of this argument is to say that string theory is not yet well-defined because it is largely based on perturbative equations. Proponents of string theory respond that these arguments are just a sign that string theory is not yet fully developed. Another argument against string theory is that it does not keep some of the properties of general relativity such as background independence and that the theory displays a particle physics bias. Proponents of string theory respond that particle physics has been more accurately tested than general relativity. There is a final sociological argument that string theory has dominated funding and tenure positions for high energy physics making the exploration of alternatives difficult.
[edit] What "M" stands for
According to its creator, Ed Witten, as quoted in the PBS documentary based on Brian Greene's book "The Elegant Universe", the "M" in M-theory "stands for magic, mystery, or matrix, according to taste." He also added, "Some cynics have occasionally suggested that M also stands for 'murky,' because our level of understanding of the theory is in fact so primitive." Then, jokingly, he said, "Maybe I shouldn't have told you that!"
Skeptics of M-theory have joked that the "M" means "Moronic", "Moron!", or just plain "Mud", which represents dirt. Some also suggest that the M is an upside down W, for "Witten".
[edit] Books
- Brian Greene has written books explaining string theory and M-theory for the layperson in 1999, The Elegant Universe, ISBN 0-375-70811-1 and in 2004, The Fabric of the Cosmos, ISBN 0-375-41288-3.
[edit] External links
- The Elegant Universe - A Three-Hour Miniseries with Brian Greene by NOVA (original PBS Broadcast Dates: October 28th, 8-10 p.m. and November 4, 8-9 p.m., 2003). Various images, texts, videos and animations explaining string theory and M-theory.
- Superstringtheory.com - The "Official String Theory Web Site", created by Patricia Schwarz. Excellent references on string theory and M-theory for the layperson and expert.
- Basics of M-Theory by A. Miemiec and I. Schnakenburg is a lecture note on M-Theory published in Fortsch.Phys.54:5-72,2006.
