Tag: Conservation

Articles in New Scientist, Scientific American & Spektrum.de

Our paper, “Angular Momentum Flows without anything carrying it”, was recently featured in New Scientist, in New spin on quantum theory forces rethink of a fundamental physics law. Then it was featured in Spectrum.de, in New quantum paradox triggers controversy (in German). This was translated into English, appearing in Scientific American as A new Quantum Cheshire Cat thought experiment is out of the box.

Conservation Laws For Every Quantum Measurement Outcome

In Quantum Mechanics, a cylinder can simultaneously rotate clockwise and anti-clockwise at the same time. This phenomenon, called superposition, is crucial for paradoxes like Schrödinger’s cat, and applications such as Quantum Computing. If we measure which way the cylinder rotates, we will find it to be clockwise or anti-clockwise. This gives us a paradox, as there is a law in physics which states that angular momentum (rotational mass * rotational speed) should be conserved. e.g. if the cylinder rotates clockwise, it shouldn’t flip to rotating anti-clockwise unless something physically stops it and sends it spinning the other way.

This paradox, involving the randomness at the heart of quantum mechanics, has troubled physicists since the dawn of quantum mechanics. Several solutions have been presented. However we have written a new paper which, we believe, solves the paradox. The change in angular momentum of the cylinder when it is measured is perfectly offset by a change in the device which prepared the rotating superposition in the first place.

This solution, inspired by a paper by Aharonov, Popescu and Rohrlich, says that this preparation device is also the frame of reference for the angle of rotation of the cylinder. Without such a frame, we would not be able to make the superposition of clockwise and anti-clockwise rotation in the first place.

Not only that, but we also show that the preparation can be arranged so that the preparation device is a finite object. In principle we can perform a laboratory experiment to test this, i.e. to show that the total angular momentum of the system and preparation device is conserved.

We can, we believe, rewrite the conservation laws in quantum mechanics, which so far only talked about conservation of the distribution over many repeated experiments of e.g. angular momentum. Now we say conservation applies for each individual outcome. We do have some remaining work to prove this holds in all cases: so far we only dealt with angular momentum on a circle. Pursuing this solution to the end should be fun, and lead to a better understanding of randomness, frames of reference, and conservation laws in physics.

Angular Momentum Flows without anything carrying it

Yakir Aharonov, Sandu Popescu and I have written a new paper where we show how Angular Momentum, a conserved quantity, can travel from one place to another across a region of space which is empty except for a vanishingly small probability of containing a particle. Remarkably, this probability, which can be made as small as we like, allows the angular momentum to flow, apparently without any carrier. This is an advance in the interaction free research field, which began with Elitzur and Vaidman’s interaction free measurement.