|
|
|
Stringing theories together
Physicists have been trying for centuries to understand the universe and all the physical phenomena that take place anywhere in it, from Big Bang to black holes. For this, a complete understanding of the laws and forces that govern the universe is necessary. Also needed is a theory that explains the universe’s darkest and deepest secrets.
Let’s start with the times of the pioneer of classical physics, Sir Isaac Newton, who discovered gravity and gave the theory of gravitation. He showed that the force that made the apple fall on the ground is the same that causes moons to orbit the Sun.
In this way, he described the planetary motion and the story of defining interactions in terms of forces began. Newton also presented the equations to find the magnitude and effects of this force.
Later, as electricity and magnetism were discovered, Maxwell made the first attempt in combining the forces by presenting his four equations of “Electro-Magnetism” along with his Electro-Magnetic Theory which combines the two forces as one.
In the 20th century the pioneer of modern physics, Albert Einstein, gave a new picture of gravity in his General Theory of Relativity, which holds that the space-time is warped by the matter and energy in it. This theory revolutionised the field of physics.
Einstein presented his gravitational field equations to show how matter influences space to curve and how space influences matter to move. Initially, it was hard to understand this theory due to the extraordinary mathematics involved in it. But today, this stands as a pillar of modern physics.
At the time, electro-magnetism and gravity were the only forces. Einstein made a second attempt in this regard and spent his last thirty years in trying to combine these forces and to present a complete theory describing the universe fully. However, even the genius failed.
During his last days, two new forces of nuclear physics were discovered which were based on Heisenberg’s Uncertainty principle. Einstein never accepted them.
Quantum physics is the second pillar of theoretical physics. General relativity and quantum mechanics are the two, yet-to-be-combined aspects of physics. The interesting part is, the uncertainty equation was derived by De-Broglie’s matter wave’s equation which shows the dual nature of light. But it involved the Einstein’s equation (E=mc2) which was a result of his special relativity theory, and general relativity is due to special relativity. But even general relativity and quantum mechanics do not combine. In fact, they are regarded as two different principles of physics.
In the 1970s, the standard model of particle physics was presented by Abdus Salam, Sheldon Glashow and Steven Weinberg. The model includes strong force along with the combined electromagnetic and weak force, known as “electro-weak force”, which won the trio the Nobel prize.
The standard model consists of elementary particles grouped into two classes: bosons (particles that transmit forces) and fermions (particles that make up matter). The bosons have particle spin that is 0, 1 or 2. The fermions have spin of 1/2. Particles that transmit forces are gravit on which have a spin 2, photon, gluon, W+, W-and Zo bosons which have a spin 1 and Higgs that have a spin 0. The internal symmetry group of the standard model is represented by:
They proposed a theory named “String’s Theory” for the first time but it was rejected as “nothing but a mathematical curiosity”. Physicists considered it as a mathematical game play, but later on the work was unveiled again and scientists tried to pursue the so-called “Einstein’s dream”. The errors pointed out were removed and many versions of this theory were presented.
This theory basically tells us that there exists such tiny strings of Planck’s length that vibrate in hyperspace (multiple dimensions). These are the sources of all forces generated in universe. In other words, the entire universe is made up of these tiny “open and closed” strings.
Another name of the theory is “M-theory” (Membrane or magic theory). Now let us see what the theory describes:
Think of a guitar string that has been tuned by stretching the string under tension across the guitar. Depending on how the string is plucked and how much tension is in the string, different musical notes will be created by the string. These musical notes could be said to be excitation modes of that guitar string under tension.
In a similar manner, in string theory, the elementary particles we observe in particle accelerators could be thought of as “musical notes” or excitation modes of elementary strings.
In the String theory, as in playing guitar, the string must be stretched under tension in order to become excited. However, the strings in string theory are floating in space-time; they are not tied down. Nonetheless, they have tension.
The string tension is denoted by the quantity 1/(2 p a’), where a’ is pronounced “alpha prime” and is equal to the square of the string length scale. If string theory is to be a theory of quantum gravity, then the average size of a string should be somewhere near the length scale of quantum gravity, called the Planck length, which is about 10 by the power of -33 centimetres, or about a millionth of a billionth of a billionth of a billionth of a centimeter.
Unfortunately, this means that strings are way too small to see by current or expected particle physics technology. So string theorists must devise more clever methods to test the theory than just looking for little strings in particle experiments.
String theories are classified according to whether or not the strings are required to be closed loops, and whether or not the particle spectrum includes fermions. In order to include fermions in string theory, there must be a special kind of symmetry called super symmetry, which means for every boson (particles that transmit a force) there is corresponding fermions (particles that make up matter). Super symmetry connects the particles that transmit forces to the particles that make up the matter.
Super-symmetric partners to currently known particles have not been observed in particle experiments, but theorists believe this is because super-symmetric particles are too massive to be detected at current accelerators. But particle accelerators could be on the verge of finding evidence for high energy super symmetry in the next decade. Evidence for super symmetry at high energy would be compelling proof that string theory was a good mathematical model for Nature at the smallest distance scales.
There are several ways theorists can build string theories. Start with the elementary ingredient: a wiggling tiny string. Next decide: should it be an open string or a closed string? Then ask: will I settle for only bosons or will I ask for fermions as well? If the answer to the last question is bosons only, then one gets bosonic string theory.
However, if the answer is in the negative, advocating the existence of matter, then we need super symmetry, which means an equal matching between bosons and fermions. A super symmetric string theory is called a superstring theory.
We still do not know what the fundamental theory behind string theory is, but judging from all of these relationships, it must be very interesting, one where distance scales, coupling strengths and even the number of dimensions in space-time are not fixed concepts but fluid entities that shift with our point of view.
Today, this theory survives. It holds mathematically, but not experimentally, due to the lack of technology present today. But, the supporters of this theory believe that someday, it would be proved. Shall we keep our fingers crossed?
The writer rahim_gowani@yahoo.com is a freelance contributor
In this sense, string theory is still in a “larval stage”: it possesses many features of mathematical interest, and it may yet become supremely important in our understanding of the universe, but it requires further developments before it is accepted or falsified.
Since string theory may not be tested in the foreseeable future, some scientists have asked if it even deserves to be called a scientific theory: it is not yet falsifiable in the sense of Popper.
It is by no means the only theory currently being developed which suffers from this difficulty; any new development can pass through a stage of uncertainty before it becomes conclusively accepted or rejected. As Richard Feynman noted in The Character of Physical Law that the key test of a scientific theory was whether its consequences agreed with the measurements taken in experiments.
It does not matter who invented the theory, “what his name is”, or even how aesthetically appealing the theory may be — “if it disagrees with experiment, it’s wrong.” (Of course, there are subsidiary issues: something may have gone wrong with the experiment, or perhaps the person computing the consequences of the theory made a mistake. All these possibilities must be checked, which may take a considerable time.) These developments may be in the theory itself, such as new methods of performing calculations and deriving predictions, or they may be advances in experimental science, which make formerly ungraspable quantities measurable.
On a more mathematical level, a problem is that, like quantum field theory, much of string theory is still only formulated perturbatively (i.e., as a series of approximations rather than as an exact solution). Although nonperturbative techniques have progressed considerably — including conjectured complete definitions in space-times satisfying certain asymptotics — a full nonperturbative definition of the theory is still lacking.
— http://en.wikipedia.org/wiki/ String_theory
|
|
|
|
