new theory emerging that doesn’t need an observer to collapse wave functions. instead, the wave function collapses when the particle interacts with the measuring device. to me that just means that the measuring device is the observer. but here
QUANTUM mechanics – simultaneously physics’s most successful and most baffling theory – may be in for an upgrade.It faces a challenge from a modified version that would solve a largely ignored puzzle at the heart of the theory: why do subatomic particles never let us catch them in the act of being in many places at once but instead “collapse” into a single position as soon as we observe them?
Unlike most attempts at modification, this latest upgrade is generating special excitement as it meshes with another pillar of physics, Einstein’s special relativity.
It also comes at the same time as a proposed test for such modifications – and a recent plea from a Nobel laureate to stop ignoring the collapse problem.
“Like many physicists, I have used quantum mechanics throughout my working life, cheerfully ignoring the deep questions about its meaning, but with a nagging feeling that this is something I ought to understand,” says Steven Weinberg of the University of Texas at Austin, who recently posted a blueprint for devising such modifications online.
If the recent refinement holds true, much of the “spookiness” that still surrounds quantum theory would melt away.
When subatomic particles aren’t being measured, they behave very strangely indeed, occupying many positions all at once. These superpositions are represented by a wave function, which can extend out into space. When a measurement is made, however, the wave function collapses and we only ever see the particle in a definite spot, never the blurry wave itself.
How can a particle possibly “know” when it is and isn’t being watched? And why should observation change its behaviour, anyway? The majority of physicists aren’t at all bothered by these unknowns, content in the knowledge that the theory has passed every single experimental test thrown at it. But a small band of rebels, including Weinberg, are very bothered indeed.
Since the 1980s they have wanted to modify quantum mechanics so that wave function collapse doesn’t require an observer. Instead, they argue that collapse happens at random and is simply more likely when a measurement is made. The sticking point was that all attempts to mesh these theories with special relativity had failed.
Recently though, Daniel Bedingham, who splits his time between work in the financial industry and physics research at Imperial College London, has come up with a way to do just that. His theory has its roots in an approach called GRW, named after its 1986 inventors GianCarlo Ghirardi, Alberto Rimini and Tullio Weber.
GRW says that collapses are extremely rare for an individual particle, but that making an actual measurement on a particle forces it to interact with the measuring equipment. The particle becomes intimately linked, or entangled, with the many atoms that make up the measuring equipment.
Because these atoms are numerous, one of their wave functions is bound to collapse during the measuring process. Thanks to entanglement, that triggers the collapse of the rest – including that of the particle being measured. So the particle’s wave function collapses on measurement, without needing any spooky reason for a change dependent on the observer.
In 1989, Philip Pearle of Hamilton College in Clinton, New York, finessed GRW with another theory called continuous spontaneous localisation. CSL attributes the random collapses of GRW to fluctuations in an entity that fills the universe, rather as a force field does, and varies across time and space. When physicists rewrote their equations to make CSL fit with the predictions of special relativity, they hit a speed bump. Unworkable “sharp jerks” emerged in the wave functions that would inject an infinite amount of energy into the universe, something we know wave functions don’t actually do.
Bedingham’s contribution is to come up with a way to make CSL relativistic, which avoids the infinities. Rather than allowing the fluctuating field to act directly on the wave functions, he introduces an intermediary field that smooths out its effects and prevents the sharp jerks.
Unlike a previous relativistic version of CSL, Bedingham’s idea describes not only individual particles but also the forces between them – a must for any theory seeking to replace quantum mechanics.
Bedingham first posted it online in October 2010, following up with a clearer version in March of this year. The idea is creating excitement: Ghirardi for one is now joining forces with Bedingham to probe relativistic collapse models further.
However, quantum mechanics is so elegant that most physicists are still sceptical of modifications. That could change if deviations were to emerge in experiment, says Kurt Jacobs of the University of Massachusetts in Boston.
An experiment that could turn up such deviations is now being proposed by Stefan Nimmrichter of the University of Vienna, Austria, and colleagues.
When two wave functions meet, they can interfere with one another – cancelling out where a peak meets a trough and reinforcing each other where peaks align, like waves in water. Nimmrichter and colleagues propose examining the interference patterns produced when the waves associated with little clouds of atoms are made to interfere with each other (Physical Review A, DOI: 10.1103/PhysRevA.83.043621).
In contrast to existing quantum theory, both the original CSL and Bedingham’s relativistic version of it predict that this interference pattern should be less pronounced or even absent for sufficiently dense clusters of atoms. This is because these entangled clusters are highly likely to undergo spontaneous collapse of their wave functions.
Carrying out this test will require clouds of atoms that are extremely cold and dense – pushing technology to its limits.
“We are working on the implementation of an experiment,” says Markus Arndt, who leads the group at the University of Vienna. “But it is still too premature to define a definite time frame.” Several other groups in Europe are also trying to perform the experiment.
Seeing this effect would constitute the first modification to quantum mechanics since its conception in the 1920s, Pearle says. “It will be an extraordinarily exciting occurrence.”
Embrace many worlds, or shut up
A single subatomic particle is in many places at once, but whenever we look at it, the particle seems to decide on just one position. We never get to see the superposition. One way to explain such a bizarre, observation-dependent “collapse” is to modify quantum mechanics itself (see main story). But a range of interpretations can explain collapse within standard quantum mechanics.
The “many worlds” interpretation is probably the most famous of these and certainly the most mind-bending. It says that every possible position of an object is actually realised, but each in a separate universe: the act of observing causes the universe to split into two or more versions, so an observer only ever sees one position.
For those who find that a bit perplexing, there’s always quantum Darwinism, where measurement causes a kind of natural selection to take place among the many possible states of an object. Certain states are better at imprinting information onto their surroundings, including measuring devices. Thus we tend to see one of these states when making a measurement.
By far the most popular approach, however, is the more prosaic “Shut up and calculate”, which boils down to “It gets the right answers, so why worry about how or why”.