Author: haroonkhan
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The Aspect Experiment
Bell derived his inequality in 1965, believing that an actual experimental test was still a long way off. But in 1972, John Clauser at the University of California, Berkeley figured out a nifty way to test it (though using paired photons instead of electrons). The result was that Bell’s inequality was violated, indicating that nature…
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Bell’s Inequality
Preoccupied by the lead-up and aftermath of the Second World War, physicists temporarily set aside further attempts to figure out the more esoteric implications of quantum physics. The next big step had to wait for the Irish physicist John Bell to take a sabbatical year in 1964. Bell had long been interested in the strangeness…
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Hidden Variables
The only way to preserve the ultimate speed limit of special relativity in the EPR scenario was to say that the spin state of each individual electron must have actually existed before the measurement, even if there was no way to know what it was. In other words, there must be some “hidden variable” that…
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Spooky Action at a Distance
But wait, something even stranger is going on here. Given the geometry that we have assumed (which is perfectly acceptable by the laws of physics) the “influence” has to travel from A to B much faster than the speed of light. Since the theory of special relativity tells us that no matter or energy could possibly travel faster than…
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The EPR Paradox
In 1935, after a lull in the long-running debate between Bohr and Einstein, the latter genius stunned the physics community with another important Gedankenexperiment. Even though Bohr had successfully refuted all of his arguments to date, Einstein was not quite ready to accept the Copenhagen interpretation. He still wasn’t happy with the inherent uncertainty in…
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Introduction
We will continue to dig deeper into the basic question of what quantum physics actually means. We have become familiar with many ways in which this theory challenges traditional understandings of science, reality, and even common sense. Now we will look at some of the ways quantum physics responds to these challenges. Along the way,…
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The Bohr-Einstein Debates
Schroedinger’s cat has become a sort of diagnostic tool for testing all kinds of interpretations of quantum physics. It wasn’t the last such Gedankenexperiment, nor was it the first. It was preceded by a set of lively debates between the chief architect of the Copenhagen interpretation, Niels Bohr, and Albert Einstein, its fiercest critic. These…
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Schroedinger’s Cat
The Copenhagen interpretation says that before a wave function is pinned down by a measurement, things that you might potentially measure, like position or energy state, can take many possible values. Some values may be more probable than others, as determined by the details of the wave function, but each remains possible. As we have…
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Measurement and Objective Reality
A bigger issue may be the introduction of a conscious, self-aware observer into the processes of nature at a very fundamental level. The whole idea of measurement seems to require someone to do the measuring. Given the concept of wave function collapse, the act of measurement has a profound effect on nature itself. If no…
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Superposition and Collapse
We explain that there is typically more than one solution to the Schroedinger equation for a given quantum system. Each solution is an Eigenfunction that we can denote as Ψi, and the various Eigenfunctions correspond to valid quantum states of the system. We also explained that any linear combination of Eigenfunctions is itself a valid…