Research that employs quantum physics remains at the centre of contemporary physics. One aspect of this research involves the search for approximate methods that can be applied with the basic principles of quantum physics in studies of situations that are so complex that they cannot be dealt with exactly. Much of the research in condensed-matter physics is of this nature. An important discovery in this area is that in some situations the discreteness of physical quantities that usually occurs on the subatomic level can also occur on the macroscopic level. The quantised Hall effect, a property of electrical resistance of certain substances under the influence of electrical and magnetic forces, is a recently discovered example of this.
Today, quantum physics plays an increasingly important role in our life. We have already named some of the applications in their context in the previous chapters. Lasers, for instance, are a direct application of Bohr's theory. If you are reading an electronic version of this page on a computer, then the microprocessor on which it runs is ultimately based on Bohr's theory. Much of the microelectronic devices, indeed the electronic revolution itself, is indebted to quantum physics. Scanning tunnelling microscope (STM) is another very useful device based on a concept known as barrier tunnelling, which is derived from quantum theoretical principles.
It has been learnt that there are three fundamental forces (also known as interactions) that govern the world of the very small: electromagnetic force, strong nuclear force, and weak nuclear force. The zoo of subatomic particles, along with the understanding of these three fundamental forces, combined with the laws of the quantum world today make up what is commonly called the Standard Model of Particle Physics. For a very specific reason it is clear that the Standard Model is not a complete theory.
In order for the Standard Model to be a complete theory, it would have to be able to account for all objects, events, and forces which come under its purview. There is one force which is not incorporated into the Standard Model: Gravitation. The gravitational force is so weak at subatomic levels that it can safely be ignored in most calculations. Nevertheless, there are circumstances under which the gravitational force is strong enough on very small scales that it cannot be ignored. Two of these cases that have been frequently noted are the singularity at the centre of a black hole, and on the very small scale but high density conditions of the Big Bang at the beginning of the Universe.
Gravitation is very well described by Albert Einstein's General Theory of Relativity; it works so well, in fact, that even though it has been tested under very many conditions in very many experiments, virtually no exceptions have been found to the theory. On the other hand, relativity is a 'classical' theory, in the sense that it deals with smooth continuities and so has not been 'quantised.' Every attempt so far to bring quantum theory and relativity into agreement has failed for various mathematical reasons.
One possible exception can be made to this rule. There is a branch of particle physics called Superstring Theory, which describes subatomic particles not as dimensionless points but as one-dimensional strings (of various lengths with various properties, some of the open like a line, others closed in a loop). Superstring theory is not only very elegant in a mathematical sense; it also includes a mathematical equivalent of Einstein's General Theory of Relativity. The only problem is that Superstring Theory cannot be tested with any existing experimental equipment -- in fact, nobody can even imagine what sort of equipment can be used to test it.
For the present, the Standard Model in general and quantum physics must remain incomplete. Still, there is much work to be done within the framework that physicists already have, and indeed one day someone working on the edge of physics (as Planck, Einstein and others were doing a century ago) may make a breakthrough discovery that will lead to yet another new age of our understanding of the physical world. Meanwhile, there is no doubt that quantum physics is the most successful theory of physical phenomena yet invented by the human mind.