Why the World Is Not Quantum Mechanical
It is common to hear quantum computing experts stating that quantum mechanics is a fundamental theory of nature.
If you are familiar with popularizations of quantum computing, you may have certainly heard that Feynman said, "Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical."
This phrase is often used as an ontological justification for the development of quantum computers.
In fact, if the world is quantum, we would rather build a computing machine based on the principles of quantum mechanics.
Well, you might be surprised to know that quantum mechanics (regardless of who says it!) is not a fundamental theory of nature.
Feynman knew this very well, and any high-energy physics student knows it as well.
The quantum mechanics at the foundations of quantum computation and quantum communication is the result of almost three decades of constant struggle by the physicists of the time, including notable figures like Planck, Einstein, and Bohr, to explain nuclear phenomena such as the radiation of black bodies, the photoelectric effect, and the emission spectra of atoms.
The theory was finally formulated by Erwin Schrödinger and Werner Heisenberg in 1925-1926.
However, this theory is not relativistic.
The special theory of relativity was discovered more than two decades before, and physicists knew that the non-relativistic quantum mechanics of Heisenberg and Schrödinger could not be a fundamental theory of nature.
A few years after the formulation of Heisenberg and Schrödinger’s non-relativistic quantum mechanics, Dirac wrote a relativistic equation that generalized Schrödinger's equation for the electron.
The theory predicted the existence of particles with identical mass to the electron but with a positive charge.
This particle, the positron, was soon discovered in 1931 by Carl Anderson.
Despite this experimental success, Dirac's theory of the relativistic electron presented some mathematical inconsistencies that were only resolved many years later by Schwinger, Tomonaga, and Feynman himself.
This theory is called quantum electrodynamics.
Two other theories explain the nuclear forces at a fundamental level, the so-called weak and strong nuclear forces.
The general framework for these theories is known as quantum field theory.
According to Steven Weinberg, one of the greatest theoretical physicists of the second half of the 20th century, "quantum field theory is the way it is because this is the only way to reconcile quantum mechanics with special relativity."
In conclusion, non-relativistic quantum mechanics, the one used in quantum computing, is not a fundamental theory of nature.
It is simply a low-energy limit (velocities much smaller than the speed of light) of more fundamental theories.
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