It's uncertain whether string theory was originally Italian or Japanese. Some sources say that Yoichiro Nambu came up with it in the late 1950's to explain the odd results that came from mathematically describing hadron-hadron collisions. The products and results of every particle collision is related using an S-Matrix, an invention of John Wheeler. Richard Feynman, his student, broke these down into an infinite series of simpler expressions that could be added together to produce a finite answer. With lepton-lepton or hadron-lepton collisions, each successive term got smaller, so the end-result was finite. With hadron-hadron collisions, however, each successive term did not get smaller, and the result was infinite. Nambu found that by thinking of particles as strings, this problem could be resolved.
On the other hand, in 1968, Gabriele Veneziano used strings to solve an equally pesky problem. He was trying to figure out why no one had ever seen an independent quark. He tried the idea of thinking of the nucleons as strings, with the ends representing strings, and it all worked out. One cannot have independent quark in the same way that one cannot have independent ends of strings. They both had to do with solving problems arising from calculations involving the string interaction.
Regardless of which one came up with the "real" original string theory (or maybe they were both the same), they both fell big time in the 70's. Nambu's got torn to shreds by contradictions and the tachyons he predicted (particles going faster than the speed of light), and Veneziano's was marginalized by the emerging quark theory that explained the strong interaction in terms of exchanging gluons that got stronger as distance increased. It was displaced by a theory called quantum chromodynamics.
In 1979, Michael Greene and John Schwarz met at CERN (European Center for Nuclear Research), and over a cup of coffee, they decided to give the strings thing another go. Perhaps string theory could be used as a theory of quantum gravity instead to solve the incompatibility that supergravity could not. They met every summer for five years, and in 1984, they put the finishing pen strokes on a theory they called Superstrings.
In 1995, Ed Witten started the Second Superstrings Revolution by announcing his vision of M-Theory, an amalgamation of all the different string theories that have survived the test of hordes of nitpicking physicists (only five theories remained afterwards) in addition to 11-dimensional supergravity, which is M-Theory's zero-limit.
Considering the previous concept of a particle was a dimensionless point, it's not too far a stretch too see how they arrived at strings. A one-dimensional object is a natural extension of a point-particle. These strings have to be incredibly small, 10-33 cm, in order to incorporate gravity. This is because the scale of quantum gravity is at Planck's length, 10-33 cm (like Planck's constant, another extremely small number that has been very useful for describing our universe). In order to be this small and still produce the larger masses seen in nature, strings must have a gigantic tension. It comes out to approximately 1039 tons. These strings act like really smooth, elastic playdoh. They can bend, vibrate, spin, and even join their ends together. The "super" in "superstrings" comes from the fact that supersymmetry has been incorporated into the theory. Insights from Grand Unification are used as well. String theories are grouped by whether they include fermions in their particle spectrum and whether they have open or closed loops.
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By making a string, we mean coming up with the various mathematically possible strings that can describe nature. First, we must make sure the string does not go out of its way to do unnecessary actions. According to the principle of least action, nature chooses, out of all possible paths of motion, the path that minimizes the action; therefore, they minimize the mathematical expression and get the infinity of legitimate possible vibrations and movements of a string.
Next, a string must be made compatible with relativity. According to special relativity, the same observers can record different and equally valid observations for the same phenomena if traveling in different speeds. The laws of physics, however, must hold fast for all observers. Since we are traveling at relatively low speeds, we all share the same maps and coordinate systems. But at close to the speed of light, we each have our own coordinate maps. When making changes to our maps, or making coordinate transformations, we still have to make sure we are not changing the underlying laws of physics. This also applies to strings. This actually severely restricts the equations of superstrings, but this is what Schwarz and Green wanted because the more restricted it is, the less arbitrary choices physicists have to make. A perfect theory should be forced upon us by nature.
Switching to a relativistic string requires a change in description. Usually, we'd map out the strings coordinates and see how it evolves over time, but with a relativistic string, the time coordinate is brought in right from the beginning. Now the string is no longer one-dimensional. In four dimensions, the string becomes a world sheet; it is now two-dimensional. Again, the area of this world surface must be minimalized. Some implications of these restrictions are that the string must be massless and its ends must move at the speed of light.
"Ghosts", anomalies, and pesky infinities usually begin to surface around the stage at which quantum mechanics is incorporated. Ghosts are negative probabilities. Just remember that it's impossible to have a -30% chance of winning the lottery. In quantum mechanics, particles are not defined by coordinates, but by wave functions. Certain terms of the equations for the strings have to be replaced by quantum mechanical operators. An operator corresponds to a measurement of the whole quantum system. Since most particles are represented by probabilities, a measurement is made on the whole system to produce solutions that correspond to different quantum states or wave functions, each with their own energies, quantum numbers, and symmetries. (If this paragraph makes no sense, just think of it as an elaborate way (but the only way) of avoiding saying anything precise about a particle or string. Since, of course, that's what quantum mechanics is all about.)
Chirality describes whether an object can be made to look like its mirror image through rotations or translations. Chiral figures come in pairs (i.e. left and right hand). Left/right-handed particles refer to its spin. It used to be thought that nature does not favor any one hand over the other, but a group of scientists discovered that the decaying nucleus of cobalt were spinning in a preferred direction. Any Theory of Everything must exhibit the correct choice of handedness. This is a considerable obstacle because handedness often flies out the window when theorists try to compact ten or so dimensions to our familiar three spatial, one time.
In 1980, Green and Schwarz came up with a superstrings theory with open strings called Type I. The open loops explained the gauge forces of nature when it conformed to a symmetry called SO(32). A symmetry (such as SO(32)), is like a series of transformations that can make a series of seemingly unrelated things look like each other. A year later, they created Type II. Type IIA came first, but when it was discovered that it was non-chiral, they came up with Type IIB, which is chiral. In closed loops, fermions and bosons traveled as waves around the loop. Fermions went one way, the bosons went the other. The closed loops not only had chirality, they also did not blow up into infinities. These two theories were promising because they were chiral, supersymmetric, and free from ghosts, infinities, and particles that travel faster than the speed of light (a problem encountered with the original string theory).
Quantum numbers are the things at the ends of strings that act as labels and allow them to interact with each other. David Gross of Princeton University challenged the idea that quantum numbers had to be on the ends of strings. He suggested instead that on closed loops the quantum numbers actually run around the loops. When the mathematics were worked out, this still allowed for fermion numbers and bosonic numbers to run in opposite directions to each other without ever getting mixed. The name "heterotic" comes from the fact that different dimensions are associated with it. It's a theory that suggests that a two-dimensional world sheet or tube is moving in a ten-dimensional universe, which compactifies down to the four dimensions we experience. Gross suggested that a sixteen-dimensional bosonic field runs through the string. The sixteen dimensions came from the original string theory. It (originally) needed twenty-six dimensions to avoid ghosts and infinities. Gross came up with heterotic strings when he tried to combine the two theories (26 - 10 = 16). There are also two fully functioning heterotic string theories. One is Heterotic HO, named because it used SO(32) (note the O), and one called Heterotic HE, which used a symmetry called E8 x E8 (note the E).
It looks odd when several theories describing the ultimate structure of the universe could all be right. This is why, years later, Ed Witten of Princeton University's Institute for Advanced Study (he's still there, by the way) came up with the concept of M-Theory.
What are elementary particles? What is mass?
Superstrings don't just explain hadrons, like the original string theory, they describe all elementary particles, including force particles such as the graviton. Strings are massless but they can still represent massive particles. As the string vibrates and rotates, it will have a series of discrete energy levels. Since E=mc2, these energy levels will also produce masses.
What are forces?
Strings can split and join together. An open string can join itself into a closed one. Like the Feynman diagrams, string interactions can also be reduced to a series of simpler expressions and added together to calculate the overall size of the interaction. Each string has a series of quantum numbers that identify it, acting as a label. As strings join and split, it appears as if quantum numbers are exchanging. From a distance, it appears as if the point particles (really strings) are exchanging vector bosons (quantum numbers). It appears to be the work of a quantum force field.
What is gravity?
An open string in the 4th dimension traces out a world sheet. A closed string traces out a world tube. Quantum fluctuations would cause this world tube to fluctuate. These distortions could be interpreted as interactions between the closed loops. If we take "time slices" of this fluctuating tube, it looks as if closed loops come out of nowhere from background space just to die back into same. These interactions looks just like the exchange of vector bosons with spin 2, or the graviton.
How many dimensions are there?
A dimensions are the number of coordinates needed to specify a location. For instance, if a girl asked you to come to her apartment located on an intersection, you'd have to know which street (dimension #1) by which street (dimension #2), what floor she lives on (dimension #3), and what time to meet her at (dimension #4). Special and general relativity have no problem adjusting to additional dimensions. However many special dimensions are found, you just add an additional one for time and all the physics still works. Adding extra dimensions makes it possible to create unified theories of physics. Without the extra dimensions things like negative probabilities, or "ghosts", would crop up. Two main proposals have come up to relate these extra dimensions to our four-dimensional world.
Kaluza-Klein compactification: In the 1920's a five-dimensional theory of relativity was developed where the 5th dimension was rolled up into a tiny circle, existing everywhere but not able to be measured by our current instruments. The vibrations of the gravitational field in the extra dimension looked like the vibrations of the electromagnetic field in the other three spatial dimensions. With superstrings, this has to be done with six dimensions at once. One proposal is to take the heterotic strings and have the six dimensions compact together to form a Calabi-Yau space.
Braneworlds: The idea is that we live in a universe of higher dimensions, but we are only able to move in a few of those dimensions (i.e. being a 2D cartoon character in 3D world). We only live on a brane, which is a physical representation of a solution to an equation dealing with dimensions. The total area is called the bulk.
By this time, you may have come up with a few guesses as to what these physicists did with strings to make it possible for quantum mechanics and relativity to coexist. The real answer may come as a complete surprise to you.
According to string theorists, the problem never existed in the first place. You see, the quantum fluctuations that got them tearing their hair out only happens at the sub-Planck level. This means they involve scales smaller than 10-33 cm. Sound familiar? This is also the length of a string. Since particles are the vibrations of strings, and strings can't be smaller than Planck's length, then anything smaller than Planck's length is non-existant. It may sound like dodging the issue at first, but it's similar to Stephen Hawking's refusal to speculate about what happened before the singularity since anything before is mathematically undefined.
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