Particle Physics – Anyone For a Piece of String? In a Nutshell, All You Ever Wanted to Know About String Theory
Physicists have been trying for decades to bring together the interlinked worlds of matter and force. They have been successful to some extent and explained many natural phenomena. However, there still remain great gaping holes in our knowledge and understanding. The main aim, the Philosopher’s Stone of Physics, has always been to have one theory, that would in effect explain everything.
And there lies the problem. At present there are two distinct areas in physics, the physics of the very small and the physics of the very large. Strange as it may seem, each is at odds with the other. The very small world of elementary particles(e.g. protons, neutrons) is beautifully described by relativistic quantum field theory. However in order for the theory to work, we must discount gravity.
The behaviour of the whole universe and its constituent planets, stars and galaxies is clearly predicted by Einstein’s General Theory, but only if we discount any reference to quantum mechanics.
String theory allows us to combine both gravity and quantum mechanics. Essentially, the theory proposes that elementary particles and the particles responsible for transmitting force are made of energy strings. These are incredibly small and either ‘open’ or ‘closed’ in a loop.
The strings vibrate, like strings on a stringed musical instrument. So each particle is identified by the length of its string and the frequency of its vibration.
In reality the theory is incredibly complex and becoming more so. At the last count there were six main string theories, requiring from 10 to 26 space-time dimensions.
When reading about String Theory you will come across the concept of ‘Super Symmetry’. Essentially this
is a prediction that for every fundamental matter particle there exists a particle associated with it carrying force. Unfortunately at the moment there is no experimental evidence for this. However, these are still early days and the search is still on, with Fermilab and C.E.R.N. leading the field.
One of the problems in verifying the existence of strings is their minute size. The dimensions of strings is believed to be around the Planck Length 10^-35 m. Events at this size are not observable with present day technology. To give some perspective to these sizes, compare the atom which has a diameter of approximately 10^-11 m and its nucleus 10,000 times smaller at 10^-15 m. This means that you could stretch 10^-24 strings (i.e. 1,000,000,000,000,000,000,000,000) across the diameter of a nucleus!
Clearly experimental physics has a long way to go before reaching into this ‘brave new world’ of the extremely small. At the present rate of progress it is not outside the bounds of possibility that one day these dimensions will be reached, but for now that realm exists only in the heads of physicists and science fiction writers.
