Quantum Theory

For a good web index see http://www.dmoz.org/Science/Physics/Quantum_Mechanics/ (see OpenDirectoryProject)

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There's a limit to how finely you can subdivide things. Inquiries beneath this limit are empirically meaningless. This is a consequence of Schrodinger's wave equation, the mathematical foundation of classical QuantumTheory, which has by now been verified experimentally in many ways. Many physical consequences flow from QT, including those necessary to invent much of the electronic hardware of the last fifty years, from the first transistor to the latest integrated circuit or microprocessor. QuantumTheory isn't the last theory that we'll have about physics, as it is known to be inconsistent with GeneralRelativity. It's the most widely tested physical theory we have today although interestingly GeneralRelativity has recently beaten it in the all time accuracy stakes.

Opinion is still sharply divided on the meaning of QT, with popular books focusing on all kinds of wild speculation about squillions of universes being generated per second, and cats being both alive and dead at the same time, and correlations travelling instantaneously across the universe and back, and so on and so forth. The one thing we can say is the expectations created by our sensory experience do not apply to a wide range of phenomena involving high and low extremes of distance, time, and energy. If anyone tells you that their interpretation of the meaning of QuantumTheory, and no other, is the correct one, you may safely assume they are a crank or perhaps a future Nobel prize winner.

Opinion is still sharply divided on the meaning of QT, -- Not anymore. Nowadays, most Physicists accept the Bell's Theorem as true. The speculation that you mention is curretnly considered at the fringe. -- JimmyCerra

It's interesting to note that although the devices now used to build radios, TVs and computers are largely quantum mechanical in nature, each of these three technologies were invented using classical techniques.

This depends where you draw the line for QT. Certainly the theory the inventors needed to draw on included Maxwell's equations, which were later shown to be derivable from QT.

Hmmm. That "were later shown to be" lets a lot of air escape from the balloon, doesn't it?

Did Helmholtz/Marconi/Tesla or Zworykin/Baird or <your favourite inventor of the computer here> make use of QM principles in their inventions? It doesn't seem like it. It hardly seems likely that they could have, given the level of development that QM had reached at the time of their inventions. The line I'd want to draw is: could these inventions have been made without an understanding of QM? I'd say that they could. Wasn't the BipolarTransistor? (invented in 1948) the first practical application of QM? It certainly couldn't have been invented without an understanding of QM.

The atomic bomb came before then, though whether nuclear weapons are practical is a philosophical question. And I suspect that QM had other practical applications (material science? spectroscopy?) in the two decades after it was formulated.