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Can corkscrewing lasers solve an enduring particle physics mystery?


A physicist may have finally figured out how to create as-yet evasive magnetic monopoles. Actually doing it, though, is still a little way off. Cathal O’Connell reports.


Physicists, such as those at the now-defunct Monopole, Astrophysics and Cosmic Ray Observatory at the Gran Sasso National Laboratory in Italy (pictured), have tried to find magnetic monopoles, but efforts have so far been in vain.
TOMMASO GUICCIARDINI / INFN / Getty Images

What happens when you fire two intensely powerful lasers at one another? You might just solve a century-long physics mystery by creating a weird fundamental particle called a magnetic monopole, a physicist from the US suggests.

Magnetic monopoles are a theorised fundamental particle carrying a net "magnetic charge" – a north without a south, or vice versa. Physicists have been trying to detect them in particle accelerators for decades. But now, reporting in Physical Review Letters, Tanmay Vachaspati from Arizona State University predicts that monopoles could be created by colliding two waves of corkscrewing light.

If true, this would be a huge deal. Magnetic monopoles were first theorised more than 80 years ago, making their existence one of the longest running questions in particle physics.

If one were ever detected, it would resolve a big question that’s been hanging over physics since the 19th century – why are electricity and magnetism so different? Namely, why is there such thing as an electric “charge”, but no magnetic equivalent?

Plus, it would bring us a step closer to a grand unified theory encompassing all forces of nature.

This story turns on physics so basic we learn it in primary school: the fact that magnets always have two poles, a north and a south.

Unlike the electric force, where charges can be portioned individually – giving you the negative electron, the positive proton and so on – you never see an individual magnetic "charge" of north or south alone.

This difference between electricity and magnetism bothered Scottish physicist James Clerk Maxwell in the 19th century as it upset the symmetry of his equations describing electromagnetism.

In 1894, Pierre Curie (Marie’s husband) delved a little deeper into the theory and pointed out that there was actually no hardline reason why an individual magnetic pole could not exist. Perhaps it was just that none had yet been detected?

Around four decades later, British physicist Paul Dirac realised that if there was such thing as a magnet with just one pole – a monopole – he could explain why electric charge came in discrete units, thus making a vital link between electromagnetism and quantum mechanics.

Years later, theorists found that monopoles tended to pop up in grand unified theories, such as string theory, which try to connect electromagnetism with the forces that hold an atom’s nucleus together.

In a 2003 presentation, American string theorist Joseph Polchinski described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen".

Cue the grand quest to discover a magnetic monopole.

Unfortunately, you can’t just make one by chopping a magnet into two pieces – that just gives you two smaller magnets, each with a north and south end.

No matter how many times you split a magnet, even down to single particles – such as a single electron – you still have two poles (in this case directed according to the electron’s "spin").

Some theories say magnetic monopoles were created during the Big Bang and may still pervade the universe. So physicists have set up special instruments, such as the Monopole, Astrophysics and Cosmic Ray Observatory (MACRO) at the Gran Sasso National Laboratory in Italy, that might detect monopoles floating through space. Alas, none have been reliably found.

THINGS MIGHT GET ESPECIALLY INTERESTING WHEN THE COLLIDING WAVES WERE CIRCULARLY POLARISED – THAT IS, CORKSCREWING THROUGH SPACE

Others have tried to create them, watching for them to pop out of particle smashing experiments such as CERN’s Large Hadron Collider – but, as yet, to no avail.

Now, Vachaspati suggests there may be a better way: collide photons, not particles.

He ran numerical simulations describing what happens when two waves of force carrying particles, such as photons which carry the electromagnetic force or the W bosons that carry the weak force, collide.

In particular, Vachaspati reasoned that things might get especially interesting when the colliding waves were circularly polarised – that is, corkscrewing through space.

He wondered whether the twisting action might pull apart the positive and negative magnetic poles, so creating two separated monopoles.

In the simulations, that’s exactly what he saw. Emerging from the fallout, Vachaspati observed particles with a magnetic field of an isolated north or south pole. This is exactly what a magnetic monopole should look like, though it’s not yet clear if these are real or just a quirk of the simulation.

These monopoles only appeared when the two light waves were corkscrewing in the same direction, supporting the idea that the twistiness of the fields are crucial.

The next step is to try it out for real. Normally, photons pass straight through each other, but in rare cases photon-photon collisions are possible. Plans for photon colliders have been worked out in detail and one may be included as part of the new International Linear Collider machine, slated for Japan.

In the meantime, Vachaspati suggests blasting two high-intensity lasers at one another.

Now that sounds like a fun experiment.

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Cathal O'Connell is a science writer based in Melbourne.
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