Quantum mechanics does a spectacularly good job at describing the phenomena we observe in nature. So far, we have no compelling reason to replace it with an extended version. All but one of the known fundamental forces show up in distinct, measurable ways on the quantum level.
Three of the four "fundamental" forces have well-established quantum descriptions. The first to be described was electricity and magnetism, with the work of Dirac, Schwinger, Feynman, and Tomonaga, among others, in the 1930ís and 1940ís. They described how electrical and magnetic interactions happen because of the exchange of photons, which are the "carriers" of the electromagnetic force. Photons come one by one -- they are the "quanta" of the electromagnetic field, and what gave quantum mechanics its name. The quantum description of electricity and magnetism, called QED (Quantum Electrodynamics) makes very precise predictions which have been carefully tested experimentally and found to be true. When describing large objects with interactions that happen slowly, it correctly reproduces the classical description.
The next force to be described was the weak nuclear force. Glashow, Weinberg, and Salam, in the 1960ís produced a description of the weak force which was similar to that of the electromagnetic force. The carriers of the weak force are the charged W particle and the neutral Z particle. The W weighs about 80 times that of a proton, and the Z about 90 times that of a proton. The W, because it is charged, changes the charge of stuff it interacts with, enabling it to change one kind of particle into another. Glashow, Weinberg, and Salam did something even more wonderful -- they found that electricity, magnetism, and the weak nuclear force were really all just different manifestations of the same force, called now the "electroweak force".
The next one to have a currently accepted quantum description was the strong nuclear force, which is carried by massless "gluons", the quanta of the strong interaction. These gluons interact with each other, and the strength of the interaction increases with decreasing energy. If you hit something which interacts with gluons very hard, it seems to recoil like a free particle. If you pull on it slowly, it seems to be stuck in a very strong field. There isnít any convincing way to unify the strong force with the electroweak one in the same way the weak and electromagnetic forces were. But given that the quantum descriptions rely on the same kinds of techniques, it seems plausible to think that someday we will understand how the strong force is related to the electroweak one (if it is at all).
The oddball now is gravity. There is a proposed quantum carrier of the gravitational force, called the graviton. Thereís no experimental evidence for quantum gravity, like there is for the other forces, and many existing quantum gravity theories have internal consistency problems when asked to make certain predictions. Theorists are working hard on this problem (string theory is currently in vogue for including gravity with the other forces), but it is not clear how to test these models experimentally to see if they are right or wrong.
(published on 10/22/2007)