# On the Trail of the Elusive Magnetic Monopole

Break a magnet into two pieces, and what do you obtain?  What you get, unsurprisingly perhaps, are two new magnets – each one with two sides of opposite polarity.  You don’t get a north half and a south half.  Back to square one, it seems…

Ever since British physicist Paul Dirac developed a comprehensive theory of magnetic monopoles in 1931, physicists have been eager to isolate these hypothetical particles featuring a north or south pole only.  Dirac demonstrated that if even a single monopole exists, then all electrical charge must come in discrete packets – which had indeed previously been demonstrated by the Millikan experiment.

# The Lowdown on Electricity and Magnetism

People have known about electricity and magnetism for many centuries.  The ancient Greeks noted that pieces of amber would attract light objects upon being rubbed.  That observation is responsible for the name electricity itself, since in Greek amber is elektron (ηλεκτρσνισ).

Magnetism was also known centuries ago.  The ancient Greeks knew that certain minerals attracted iron and other pieces of the same mineral.  About a thousand years ago, the Chinese noticed that a magnetised needle always points in the same direction and thus, can be used for navigation.

In time, people recognised that there are in fact two types of electric charges: positive and negative (after Benjamin Franklin (1706-1790)), and that opposite charges attract each other, while like charges repel each other.  In the 20th Century, Robert Millikan showed that the electric charge is quantized: that is to say, all electric charges are multiples of  an elementary electric charge found on the electron.

However, unlike electric charges which can be isolated, magnetic materials always have two poles (called North and South, after the directions they point towards on Earth).  If one breaks a compass needle into two pieces, each will again have both north and south poles.  It was apparently impossible to isolate a single magnetic pole.  Only the combination of north and south poles (called a dipole) seems to exist.

The absence of a single magnetic charge (called a monopole) implies the laws of electricity and magnetism are different, and this lack of symmetry has bothered physicists for years.

# Along came Paul Dirac…

In 1931, one of the founders of quantum mechanics, Paul Dirac (1902-1984) showed that if a  magnetic monopole existed, it could help to explain the puzzling fact that electric charge is quantized.  Dirac found that the product of the electric charge e and a magnetic monopole charge g is necessarily an integer multiple of the fundamental constant in quantum mechanics, $2 \hslash c$ (where $\hslash$ is Planck’s constant which relates the energy and the frequency of a photon, and c is the speed of light).  Given the values of $\hslash$, c, and e, the minimum monopole charge g must be at least a few thousand times larger than e.

This implies that the monopoles, if they indeed exist, could produce very strong scattering of light (photons) compared with ordinary electrically charged particles.  The monopole could exist with intrinsic angular momentum (spin) of 0, 1/2, or 1.  For comparison, the spin of the electron is 1/2.

Phew… right?!

Although magnetic monopole analogues have been found in spin ices and other exotic systems, no Dirac monopoles have been observed directly within a medium described by a quantum field.

But now, David Hall and colleagues report the experimental observation of Dirac monopoles in the synthetic magnetic field produced by a spinor Bose–Einstein condensate.  They describe their breakthrough in Nature.  The authors obtain real-space images of monopoles at the termini of vortex lines within the condensate, providing evidence of the existence of Dirac monopoles.

# Dirac Monopoles in a Quantum Field

For the first time, scientists have engineered a synthetic monopole in a quantum system, allowing for its mysterious properties to be explored.  It provides an unprecedented glimpse on these quantum mechanical entities, and the opportunity to manipulate monopoles in a controlled environment.

To observe and test monopoles in the lab, scientists engineered a quantum system – the magnetic field of a cloud of rubidium atoms in an unusual state of matter, known as a Bose-Einstein condensate.  Using direct imaging, the team detected the distinct signature of a Dirac monopole, known as a “Dirac string“.

The researchers highlight that – while other teams have previously made analogues of monopoles, their demonstration is the first in a quantum system which can be tested by experiment.

“This creation of a Dirac monopole is a beautiful demonstration of quantum simulation,” commented Lindsay LeBlanc of the University of Alberta, a physicist not involved in the study.  “Although these results offer only an analogy to a magnetic monopole, their compatibility with theory reinforces the expectation that this particle will be detected experimentally.”

Detecting a natural magnetic monopole would be a revolutionary event comparable to the discovery of the electron,” wrote the team from Aalto University, Finland, and Amherst College, United States, in their paper.

Hall et al.’s work provides conclusive and long-awaited experimental evidence of the existence of Dirac monopoles.

Researchers have hunted for them since Paul Dirac first theorised their quantum-mechanical characteristics in 1931.  The discovery of magnetic monopoles has been long-awaited because they can help to explain a variety of physical phenomena.  As Dirac said: ‘Under these circumstances one would be surprised if Nature had made no use of it’.”

#### The creation and manipulation of Dirac monopoles in a controlled environment opens up a wide range of experimental and theoretical investigations.

Shut up, Horatio!

They have found them.  Kinda…