According to the models we currently operate with there are four basic forces at work in the universe: The strong and weak nuclear forces, electromagnetism and gravity. Of these, gravity is by far the weakest and is therefore more or less ignored when dealing with particles at the subatomic level, yet it is the most important force when dealing with the universe at large; both the strong and the weak nuclear forces act over extremely short ranges. As authors Neil F. Comins and William J. Kaufmann III state in Discovering the Universe, Eighth Edition:
“Their influences extend only over atomic nuclei, distances less than about 10?15 m. The strong nuclear force holds protons and neutrons together. Without this force, nuclei would disintegrate because of the electromagnetic repulsion of their positively charged protons. Thus, the strong nuclear force overpowers the electromagnetic force inside nuclei. The weak nuclear force is at work in certain kinds of radioactive decay, such as the transformation of a neutron into a proton. Protons and neutrons are composed of more basic particles, called quarks. A proton is composed of two ‘up’ quarks and one ‘down’ quark, whereas a neutron is made of two ‘down’ quarks and one ‘up’ quark. The weak nuclear force is at play whenever a quark changes from one variety to another….at extremely high temperatures the electromagnetic force, which works over all distances under ‘normal’ circumstances, and the weak force, which only works over very short distances under the same ‘normal’ circumstances, become identical. They are no longer separate forces, but become a single force called the electroweak force. The experiments verifying this were done at the CERN particle accelerator in Europe in the 1980s.”
The physicist Abdus Salam (1926-1996) from present-day Pakistan together with Sheldon Lee Glashow (born 1932) and Steven Weinberg (born 1933) from the United States shared the 1979 Nobel Prize in Physics for their work in this field. Following this, the Dutch accelerator physicist Simon van der Meer (born 1925) was awarded the Nobel Prize in Physics in 1984 together with the Italian particle physicist Carlo Rubbia (born 1934) for work at CERN and for contributions to the discovery of the W and Z particles, short-lived subatomic particles.
Physicists have found that at sufficiently high temperatures, these various forces begin to behave in the same way. If you believe the current theoretical models, from the Big Bang until 10–43 seconds afterward, a moment which has been called Planck time, all the four forces are believed to have been united as one. Before the Planck time, the universe was so hot and dense that the known laws of physics do not describe the behavior of spacetime, matter and energy. At this point, matter as we think of it did not yet exist, but the temperature of the tiny and rapidly expanding universe may have been in the order of 1032 K. After this, gravity separated from the three other forces, which then separated from each other a little later.
The twentieth century brought two great new theories of physics: general relativity and quantum mechanics. Albert Einstein introduced the general theory of relativity and was also, along with the great German physicist Max Planck, a co-founder of quantum physics, although he famously had serious reservations about this field in later years. It is often said that Einstein “disproved” or “overturned” Newton’s theories of gravitation, but this is misleading. Newton’s theories work reasonably well for objects that are not extremely massive or move with velocities that approach the speed of light. It would be more accurate to say that Newton’s work on gravity should be considered a special case of general relativity.
The general theory of relativity is one of the best-tested theories of modern physics. It is not “wrong.” There may well be phenomena awaiting discovery that it does not explain, or fail to adequately explain. If we do discover such phenomena, any new theory must contain all that is good about general relativity within itself, just like Einstein’s theory carried Newton’s theory within itself. The problem is that while the theory of relativity works quite well for describing large objects in the universe, it is next to useless when dealing with what happens on the subatomic level. This is where quantum mechanics takes over. It, too, has so far been quite successful at predicting observed behavior. Physicists currently have to use two sets of rules, one for the very large and one for the very small. The challenge is to bridge these two.
To combine gravity and the other three fundamental forces into one comprehensive “Theory of Everything,” some scientists try to imagine that the universe consists of more than the traditional four dimensions we are familiar with when we think of spacetime (three of space plus time). Theories that attempt to mathematically describe this new formulation of the universe are called superstring theories. There are several versions which assume that spacetime has 10 dimensions, or 11 according to M-theory. In addition to the four traditional ones are six that are rolled up into such tiny volumes that we cannot detect them directly.
The difficulty in reconciling quantum mechanics (describing the weak, strong and electromagnetic forces) and general relativity (describing gravitation) is that the three former forces are quantized whereas general relativity is not, as far as we know today. The weak, strong and electromagnetic forces are transmitted by particles; photons are the quanta of electromagnetism; gluons are the exchange particles between quarks involved in the strong nuclear force and the W and Z bosons are the particles involved in the weak nuclear force.
A hypothetical particle, the graviton, has been suggested as the force carrier for gravity analogous to the photon, but it has not yet been detected, and no theory of quantum gravity has succeeded. Gravity as described in the general theory of relativity is based on a continuous rather than quantized force; the distortion of spacetime by matter and energy creates the gravitational force, which is to say that gravity is a property of space itself.
Superstring theories assert that what we perceive as a particles are actually tiny vibrating strings, with different particles vibrating at different rates. The interactions between these strings create all of the properties of matter and energy that we can observe. Calabi-Yau manifolds are six-dimensional spaces that, according to string theory, lurk in the tiniest regions of spacetime, down at the Planck length where the quantized nature of gravity should become evident, at 1.6 x 10-35 m, which is extremely tiny even compared to an atomic nucleus. The Planck time, 10-43 seconds, is the time it would take a photon travelling at the speed of light to cross a distance equal to the Planck length.
The German mathematician Erich Kähler (1906-2000) defined a family of manifolds with certain interesting properties. The Italian American Eugenio Calabi (born 1923) in the following generation identified a subclass of Kähler manifolds and conjectured that their curvature should have a special kind of simplicity. Shing-Tung Yau (born 1949), a mathematician from China based in the USA, proved the Calabi conjecture in 1977. These types of spaces are called Calabi-Yau manifolds. For believers in string theory they form a critical element of the explanation of what appear to us as a variety of natural forces and subatomic particles. They are six-dimensional, but the extra dimensions are “folded up” out of sight from our vantage point in the macroscopic world. At least, that is how the theory goes.
The Italian physicist Gabriele Veneziano (born 1942), working at CERN in Western Europe in the 1960s, made early contributions to the field, but interest in string theory took off in a major way in the 1980s and 1990s. The Englishman Michael Green (born 1946), currently professor of theoretical physics at Cambridge University, together with his American colleague John Schwarz (born 1941) in 1984 extended string theory, which treats elementary particles as vibrations of minute strings, into “superstring” theory. It incorporated a novel relationship called supersymmetry that placed particles and force carriers on an equal footing.
Many leading scholars have since joined this debate. They include Leonard Susskind (born 1940), a Jewish professor of theoretical physics at Stanford University in the United States, Juan Maldacena (born 1968) from Buenos Aires, Argentina who is now a professor at the Institute for Advanced Study at Princeton in the USA, the Iranian-American Cumrun Vafa (born 1960) at Harvard University in the USA as well as David Olive (born 1937) and Peter Goddard (born 1945), both mathematical physicists from Britain. The American physicist Joseph Polchinski (born 1954) in the 1990s introduced a novel concept called D-branes.
By the mid-1990s there were as many as five competing string theories, which nevertheless had many things in common. The great American Jewish mathematical physicist Edward Witten (born 1951), a professor at the Institute for Advanced Study at Princeton, during a conference in 1995 provided a completely new perspective which was named “M-theory.” According to Witten himself, M stands for magic, mystery or matrix. Before M-theory, strings seemed to operate in a world with 10 dimensions, but M-theory would demand yet another spatial dimension, bringing the total to 11. The extra dimension Witten added allows a string to stretch into something like a membrane or “brane” that could grow to an enormous size.
The American physicist Brian Greene (born 1963), a professor at Columbia University in the USA, has done much to popularize these new string theories. According to Greene, “Just as the strings on a cello can vibrate at different frequencies, making all the individual musical notes, in the same way, the tiny strings of string theory vibrate and dance in different patterns, creating all the fundamental particles of nature. If this view is right, then put them all together and we get the grand and beautiful symphony that is our universe. What’s really exciting about this is that it offers an amazing possibility. If we could only master the rhythms of strings, then we’d stand a good chance of explaining all the matter and all the forces of nature, from the tiniest subatomic particles to the galaxies of outer space.”
Superstring theories are consistent with what we know, but critics, of which there are still quite a few, claim that they are too mathematically abstract to predict anything which can be experimentally tested and verified, as should be possible with a proper scientific theory. Its supporters claim that the theories suggest that there should be a class of particles called supersymmetric particles, where every particle should have a partner particle. CERN, the European Organization for Nuclear Research, recently opened their Large Hadron Collider (LHC), the world’s largest and highest-energy particle accelerator, near Geneva on the border between France and Switzerland. There are those who hope that the LHC will be able to detect signs of supersymmetric particles. If so, this finding will not by itself prove superstring theory, but it would constitute a strong piece of circumstantial evidence in its favor.
Personally, I would still count myself among the skeptical half regarding string theory. The critics who complain that it is unnecessarily complex with precious little experimental evidence in its favor have a point. It does seem rather drastic to go from four to eleven dimensions, thereby nearly tripling the amount of dimensions in the universe. As it is now, the theory contains too many epicycles for my taste. However, just because a theory is complex and seemingly counter-intuitive does not necessarily mean that it is wrong, as quantum mechanics and to some extent the theory of relativity showed us in the twentieth century.
One humorous illustration of how hard it is to imagine extra dimensions was provided by the English writer Edwin A. Abbott (1838-1926). His satirical novel Flatland: A Romance of Many Dimensions from 1884 is narrated by a being who calls himself “Square” and lives in Flatland, a world populated by two-dimensional creatures with a system of social ranks, where creatures with more sides rank higher and circles highest of all. Women are merely line segments and are subject to various social disabilities. In a dream, Square visits the one-dimensional Lineland, and is later visited by a three-dimensional Sphere from Spaceland. The Sphere tries to convince Square of the existence of a third dimension and mentions Pointland, a world of zero dimensions, populated by a single creature who is completely full of himself.
Perhaps we are all a bit like Square, who finds it very hard to imagine extra dimensions. And most of us have encountered individuals who live in Pointland, occupied only by themselves.