Transformation of magnetic monopoles

A team of physicists led by David Hall from Amherst College, USA, and Mikko Möttönen from Aalto University, Finland, has experimentally demonstrated the relationship between two different analogues of magnetic monopoles.  The results, published in Physical Review X, provide the first demonstration of quantum monopole dynamics.

The new research builds on a decade of earlier work, by Hall and Möttönen as well as by other teams, which focused on trying to synthesize monopole analogues in the first place. Real magnetic monopoles – the magnetic counterparts of electrons and protons, the fundamental negative and positive electric charges that make up the atoms in our universe – have yet to be observed. Magnets always have two poles, north and south, and so far no amount of metaphorical slicing and dicing has been able to isolate separate north and south poles: rather, cutting a magnet in two simply produces two magnets, each with a north and a south pole.

This asymmetry between electricity and magnetism has long puzzled physicists. It also spoils the beauty of James Clerk Maxwell’s celebrated 1864 equations of electromagnetism. But there is no theoretical reason not to put the symmetry back into Maxwell’s equations, by adding in magnetic “charges” (monopoles) analogous to the electric charges, and in 1931, pioneering British quantum physicist Paul Dirac showed how to reinterpret the relevant quantum mechanical equations in this light. He found that the force between two opposite magnetic monopoles would be nearly 5000 times as strong as the force between an electron and a proton. No wonder, he mused, that no-one has yet been able to separate magnetic poles. Which is why physicists have recently turned to simulating monopoles.

“My feeling is that some of the details associated with the Dirac monopole are not fully appreciated by the wider physics community,” says Hall. Experiments can help physicists to better understand this elegant theory, and ultimately, perhaps, point to ways of discovering whether or not real monopoles exist. But there are also potential practical benefits.

Back in 2009, Jonathan Morris was part of a team from Berlin’s Helmholtz Centre that found magnetic monopole analogues in strange structures known as “spin-ice”, and he believes we could be in for a slew of new technologies using simulated monopoles. But first, he cautions, “we must get to the bottom of how monopoles behave”.

And that means spending many hours in the lab – hours that often involve “a lot of unglamorous day-to-day problem-solving,” as Hall puts it. Working out how to remove “noise” from everyday magnetic fields created by overhead power lines, computers, and the Earth itself was a real headache in the early research, Hall laments; in these latest delicate experiments, even something as simple as a pair of steel scissors had to be banned from the lab.

To isolate and study their monopole analogues, Hall, Möttönen and their colleagues used a cloud of extremely cold rubidium atoms.

(This is known as a Bose–Einstein condensate, or BEC for short. Theoretically predicted in 1924, the first BEC was not actually made until 1995; its creators received the 2001 Nobel Prize for physics. Following in their footsteps, Hall and his undergraduate students at Amherst made their own atomic refrigerator in 2002, and it is still going strong.)

A BEC acts as a sort of magnifying glass, because the cloud of atoms, cooled to almost absolute zero, behaves in just the same way as if it were a single quantum particle. This “magnification” makes it possible to observe and photograph the way a BEC “electron” behaves in a simulated magnetic monopolar field, or the way a “monopole” forms. It’s about making a model of something that is not really electromagnetic, but which behaves just the way quantum mechanics says that an electron or a magnetic monopole should behave.

By contrast, a number of international teams have found that “spin-ice” does seem to contain a lattice of monopoles that are really magnetic, although they, too, are analogues of the free-moving real monopoles that would parallel electrons and protons. Each experimental analogue adds to physicists’ knowledge, and in the latest research, Hall, Möttönen and their colleagues have taken their model to a new level by demonstrating the relationship between analogues of Dirac monopoles and “isolated” or “topological” monopoles.

Predicted by t’Hooft and Polyakov in 1974, an “isolated” monopole is mathematically different from Dirac’s version, but theory says that at a suitable distance it effectively becomes a Dirac monopole.

Hall’s team began by allowing a simulated “isolated” monopole to evolve in time.

“This is where noise can really wreak havoc,” says Hall. “The problem is compounded because to study the process over time, we don’t simply take a movie of a sample, one frame after the other, but we have to take each frame with a different sample, waiting a little longer after the creation [of the isolated monopole analogue] to take each successive frame. It’s as if you create the movie set, take a picture, and then the set is destroyed. Then you recreate the set, wait a little longer, take the picture, and it is destroyed again. It’s annoying enough to have to recreate the set every time you need another frame of the movie. Now imagine that every time the director calls ‘Action!’ the scene props are being blown randomly all over the place because it is violently windy.” The winds are the “noise” that needs to be filtered out before the data can be interpreted.

But these laborious experiments have hit paydirt: for the first time, physicists have observed the spontaneous creation of a Dirac monopole analogue from the decay of a simulated t’Hooft–Polyakov monopole.

“I was jumping in the air the first time I saw it,” says Möttönen. As for Hall, “I knew to expect this from the theory, but to see it in the data – that was pretty wild. It felt like watching a sculpture take form from a block of marble.”