STRING THEORY - 2
The details of the various string theories are complex to grasp for those whi do not possess sophisticated knowledge of mathematics, but some properties are intuitive.
Strings would be subject to tension just like the strings of an instrument. The tension is directly connected to the size of the string: for example, the tauter a ring string will be, the more it will contract and tighten the ring. However, it cannot be reduced to a point due to Heisenberg's Uncertainty Principle and therefore the characteristic dimension of the string will be an equilibrium between the tension, that tends to tighten it, and the uncertainty effect, that tends to widen it.
Before the 90s there were five types of superstrings, with as many corresponding theories, and it was thought that only one of these was the correct Theory of Everything. With the M-Theory, however, it was understood that they are part of a greater system. They are connected by transformations dubbed duality: if two theories are in a duality relationship, one of the two can transform and become the second. Practically, they are different mathematical descriptions of the same phenomenon.
These dualities, therefore, connect quantities that were thought to be separate, and strings can eliminate the differences between large and small, strong and weak, just as the five seemingly different theories are in reality related to each other.
For example, to fully understand the quantum behaviour of electromagnetism, it was divided into more manageable pieces, and each one was solved with a different power of the constant underlying the electromagnetism itself. At ordinary levels, the constant is small, so the calculations produce well-approximated results, but if it grows the results become null.
You can see in detail the various types of dualities in the Theory M page.
This is perhaps the most interesting and famous property of the various string theories. If for Maxwell's Theory of Electromagnetism and Einstein's Theory of Relativity it was required that physicists enter the number of dimensions of the Universe by hand, String Theory instead calculates it by itself. This is because the Lorentz Principle of Invariance, according to which if you draw a line between two points and then rotate the observer by a certain angle and measure the line again, it maintains the same length, this can only be satisfied in a particular number of dimensions.
The surprising thing that makes the system famous and intriguing is that the calculations tell us that there are not only four dimensions, time plus the three space dimensions, but, depending on the theory, 10, 11 or even 26.
However, this contradicts the experimental observations, and therefore there is a tendency to compact the extra dimensions since it is believed they produce physical effects on such a modest scale that they cannot be revealed by an experiment. A famous example is the rubber tube: if viewed from distance, we note only its length, and this would correspond to the four dimensions we are used to; when we get nearer, we discover the circumference, visible only from very close as the extra dimensions.
Representation of a vibrating closed string
PIC CREDITS
Physics for 21st century 4 string theory – Annenberg Learner