By Eugenie Samuel Reich of Nature magazine
At the heart of the weirdness for which the field of quantum mechanics is famous is the wavefunction, a powerful but mysterious entity that is used to determine the probabilities that quantum particles will have certain properties. Now, a preprint posted online on November 14 reopens the question of what the wavefunction represents--with an answer that could rock quantum theory to its core. Whereas many physicists have generally interpreted the wavefunction as a statistical tool that reflects our ignorance of the particles being measured, the authors of the latest paper argue that, instead, it is physically real.
"I don't like to sound hyperbolic, but I think the word 'seismic' is likely to apply to this paper," says Antony Valentini, a theoretical physicist specializing in quantum foundations at Clemson University in South Carolina.
Valentini believes that this result may be the most important general theorem relating to the foundations of quantum mechanics since Bell's theorem, the 1964 result in which Northern Irish physicist John Stewart Bell proved that if quantum mechanics describes real entities, it has to include mysterious "action at a distance".
Action at a distance occurs when pairs of quantum particles interact in such a way that they become entangled. But the new paper, by a trio of physicists led by Matthew Pusey at Imperial College London, presents a theorem showing that if a quantum wavefunction were purely a statistical tool, then even quantum states that are unconnected across space and time would be able to communicate with each other. As that seems very unlikely to be true, the researchers conclude that the wavefunction must be physically real after all.
David Wallace, a philosopher of physics at the University of Oxford, UK, says that the theorem is the most important result in the foundations of quantum mechanics that he has seen in his 15-year professional career. "This strips away obscurity and shows you can't have an interpretation of a quantum state as probabilistic," he says.
Historical debate
The debate over how to understand the wavefunction goes back to the 1920s. In the `Copenhagen interpretation' pioneered by Danish physicist Niels Bohr, the wavefunction was considered a computational tool: it gave correct results when used to calculate the probability of particles having various properties, but physicists were encouraged not to look for a deeper explanation of what the wavefunction is.
Albert Einstein also favoured a statistical interpretation of the wavefunction, although he thought that there had to be some other as-yet-unknown underlying reality. But others, such as Austrian physicist Erwin Schrödinger, considered the wavefunction, at least initially, to be a real physical object.
The Copenhagen interpretation later fell out of popularity, but the idea that the wavefunction reflects what we can know about the world, rather than physical reality, has come back into vogue in the past 15 years with the rise of quantum information theory, Valentini says.
Rudolph and his colleagues may put a stop to that trend. Their theorem effectively says that individual quantum systems must "know" exactly what state they have been prepared in, or the results of measurements on them would lead to results at odds with quantum mechanics. They declined to comment while their preprint is undergoing the journal-submission process, but say in their paper that their finding is similar to the notion that an individual coin being flipped in a biased way--for example, so that it comes up 'heads' six out of ten times--has the intrinsic, physical property of being biased, in contrast to the idea that the bias is simply a statistical property of many coin-flip outcomes.
Quantum information
Robert Spekkens, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, who has favoured a statistical interpretation of the wavefunction, says that Pusey's theorem is correct and a "fantastic" result, but that he disagrees about what conclusion should be drawn from it. He favours an interpretation in which all quantum states, including non-entangled ones, are related after all.
Spekkens adds that he does expect the theorem to have broader consequences for physics, as have Bell's and other fundamental theorems. No one foresaw in 1964 that Bell's theorem would sow the seeds for quantum information theory and quantum cryptography--both of which rely on phenomena that aren't possible in classical physics. Spekkens thinks this theorem may ultimately have a similar impact. "It's very important and beautiful in its simplicity," he says.
This article is reproduced with permission from the magazine Nature. The article was first published on November 17, 2011.
See what we're tweeting about
6 Comments
Add CommentSince no corpuscle at rest does exist in the universe, everywhere there is matter there are also waves. To say that the energy of a particle is proportional to its frequency (for Einstein) is the same as saying that such energy is inversely proportional to its wavelength (for de Broglie). We would say that energy is a quantity measurable only as a product of itself by space-time, because what is quantized is the exchange of energy in space-time. Then energy behaves in time and space respectively as a corpuscle of a particular frequency or as a wavelength of a particular length. Consequently, we could say that time and space are not continuous magnitudes as it is generally assumed.
Reply | Report Abuse | Link to thisWe have to revise our understanding of the universe we live and then a concept of multiverse in line with the balloon inside balloon theory of matter and antimatter universe on opposite entropy path producing dark energy by annihilation of matter and antimatter at common boundary and injected into our universe as expanding cosmic fabric not isotropic but non uniform fild strength causing laws and constants different at large distances and speed of light varying as well new NEWTONS LAW F=P.G.M.m/R.R where P is factor of permeability . Again gravitation is not pull of matter but monomagnetic reaction of matter at molecular level by dark energy causing AVOGADROS PRINCIPLE AS WELL AS GALILEOS THEOREM OF ACCELERATION ETC ETC. wE CAN EVEN DERIVE mendeleefs law BY THE PRINCIPLES OF MONOMAGNETIC COUPLING. READ THEORIES PUBLISHED BY DURGADAS DATTA IN ASTRONOMY.NET IN YEAR 2002 AND AVAILABLE IN DURGADAS DATTA SEARCH IN GOOGLE AND DURGADAS DATTA FACEBOOK.--durgadas.ddatta@gmail.com
Reply | Report Abuse | Link to thisI think Spekkens is on the right track, because Einstein showed us that for light there is no time, so light from the most distant galaxy taking billions of years to fall on your retina on a dark night experiences no time at all, it exists continuously along its path in its own frame of reference that we find very difficult to comprehend from our time-bound frame of reference. That means all the light in the universe experiences no time either and it has to all be co-existing simultaneously in its own frame of reference within the volume of universe it occupies. Light can and does interfere with other light, thus so will the light from today crossing a space filled with yesterday's light, all the way back to the origin of the universe, whatever that may be. Actually for light, space and distance may not exist at all but we need those concepts to understand light so we'll keep them for now. The point is that light's frame of reference provides a mechanism for every photon to interact with every other photon at some point in the history of the universe (including its indefinite future which to us hasn't even happened yet, and remembering that photons also really exist as waves spread out over spacetime), thus being 'entangled' in a giant web the size of the universe in a way that PROBABILISTICALLY interacts with our time-bound frame of reference to produce effects that we are just beginning to understand and be able to work with. Presently we can deliberately create entangled photons that are emitted simultaneously and detect that entanglement because the probabilities of their entanglement approach 100% in our frame of reference when they are emitted simultaneously from the same source. Spekkens seems to be saying that ALL photons have some interrelationship with one another even if we can't measure and perceive them to be entangled, but I would extend his view to a probabilistic interpretation extending outward from our time-based frame of reference such that near (in time) photons have a higher probability of being entangled than do photons farther away in time from one another. And in the same way, photons have a higher probability of being entangled when closer together in space than do photons farther away in space from one another, since space and time are interconvertible like matter and energy are. Finally to make things complicated and completely baffling, multiverses may vary from one another by only one single entanglement pair across the age of the universe being out of register by one Planck length.
Reply | Report Abuse | Link to thisWhere does light go when we're done with it?
Reply | Report Abuse | Link to thisDoes it fall to the floor like dust? Pile up in the bottom of our eyes? Is excess light waste the reason some older folk go blind? And this is critical; Televisions left on all day, do they die of dead light pile-up?
See?
Entanglement is the very demon.
Reply | Report Abuse | Link to thisOf course the wave is physical.
I have suggested this before,that we should watch for wavefunction drop in starlight.by which a means of sending messages back and forth faster than the speed of light may be doable.You need only have some 2 slit experiments set up on both ends with both watchers at each end looking at a same distance star from each other.If wave function is there then that equals a o and if not a 1.By doings this you could carry on a binary communication in real time.Who knows maybe someone out there is already trying to send us a message.
Reply | Report Abuse | Link to this