New Scientist | Physics & Math | 22 July 2010
Quantum Mechanics Not In JeopardyIt is one of the all-time greatest physics experiments: such a classic that it's taken a century to go one better.
In the double-slit experiment of 1908, a photon fired at a pair of slits passed through both simultaneously and interfered with itself. This surprising effect provided one of the first clues to the weird world of quantum mechanics.
Now precise measurements have been made on a version with three slits – and they again confirm the predictions of quantum mechanics.
Why are we still testing such predictions? It is not just tilting at windmills: physicists have long struggled to unite quantum mechanics with general relativity, which describes gravity, and some believe quantum mechanics will need tweaking to make this work.
Those tweaks, some physicists have argued, might include altering a quantum dictum called Born's rule. It predicts that interference patterns from three or more slits is equivalent to combining the effects of several double-slit experiments.
But although it is easy to add a third slit to the double-slit experiment, it has been more challenging to do it in a way that allows the precise measurements needed to check the validity of quantum mechanics.
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The result was a wavy interference pattern that matched the predictions of Born's rule to within the experiment's error margin of 1 per cent.
PhysOrg | Quantum Physics | 22 July 2010
Quantum mechanics survives triple-slit photon testPhysicists Confirm Decades-Old Key Principle Experimentally
When waves -- regardless of whether light or sound -- collide, they overlap creating interferences. Austrian and Canadian quantum physicists have now been able to rule out the existence of higher-order interferences experimentally and thereby confirmed an axiom in quantum physics: Born's rule. They have published their findings in the scientific journal Science.Copyright: IQC
In quantum mechanics many propositions are made in probabilities. In 1926 German physicist Max Born postulated that the probability to find a quantum object at a certain place at a certain time equals the square of its wave function. A direct consequence of this rule is the interference pattern as shown in the double slit diffraction experiment.
Born's rule is one of the key laws in quantum mechanics and it proposes that interference occurs in pairs of possibilities. Interferences of higher order are ruled out. There was no experimental verification of this proposition until now, when the research group led by Prof. Gregor Weihs from the University of Innsbruck and the University of Waterloo has confirmed the accuracy of Born’s law in a triple-slit experiment.
ars technica | Nobel Intent | 22 July 2010
Quantum mechanics flummoxes physicists againOne of the best-known examples of the counterintuitive behavior seen in the quantum world is the double-slit experiment. Take a piece of material that blocks light and cut two small slits in it; hit the slits with a flood of photons, and they'll interfere with each other as they exit the far side, creating a pattern of peaks and valleys corresponding to where the wavelengths of the photons interfered constructively and destructively. Do the same thing, but send the photons at the slits one at a time, and you still see the same pattern. In effect, the photon flows through both slits as a probability wave, and these probabilities interfere with each other.
Stranger still, the same behavior can be observed with particles. In many circumstances, an electron will behave as a simple, easy-to-quantify particle. But it will also act as a probability wave when confronted with a double slit.
It's a great example, but it also raises a question that's obvious only after you've heard it asked: what happens if there's more than two slits? Is there a limit to how many probabilities a single particle can adopt as it flows through the quantum realm? A paper that will be released by Science today provides a pretty clear answer: two, just as quantum mechanics have suspected. That deceptively simple answer, however, has major implications for our attempts to combine quantum mechanics and gravity into a unified theory of everything.
Nature News | 22 July 2010
Ruling Out Multi-Order Interference in Quantum MechanicsA fresh take on a classic experiment makes no progress in unifying quantum mechanics and relativity.
If you ever want to get your head around the riddle that is quantum mechanics, look no further than the double-slit experiment. This shows, with perfect simplicity, how just watching a wave or a particle can change its behaviour. The idea is so unpalatable to physicists that they have spent decades trying to find new ways to test it. The latest such attempt, by physicists in Europe and Canada, used a three-slit version — but quantum mechanics won out again.
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The new three-slit version of the experiment, performed by Gregor Weihs at the University of Innsbruck in Austria and his colleagues, sought to uncover gaps in our understanding of quantum mechanics through which modern physics might make some headway. Perhaps the greatest problem in modern physics is how to reconcile quantum mechanics, which allows for seemingly instantaneous communication, with Einstein's theories of special and general relativity, which imply that nothing should travel faster than light.
- Science 329 (23 July 2010) DOI: 10.1126/science.1190545
