# Bob’s birthday presents: a surprise party and magnetic monopoles

I was in Bristol on Thursday for a surprise 70th birthday party, of a friend a colleague, Bob Evans. About 40 of us waited at a restaurant to surprise him; he thought he was going there for a quiet dinner with his wife and few friends. This was a lot of fun. The next day there was a little symposium in his honour.

One of the talks was on making magnetic monopoles from particles dispersed in liquids. Particles physicists looked for particles with a magnetic monopole for decades but couldn’t find them, it looks like particles with magnetic monopoles don’t exist in nature. A magnetic monopole is the magnetic analog of a charge. So such a particle would look a bit like an electron except that it would a have magnetic charge, i.e., monopole, not an electric charge as an electron does. Around a charge the electric field points in or out (depending on if it is a negative or positive charge) and decays as one over the distance squared. If a particle with a magnetic monopole existed the magnetic field would behave in the same way.

What Daan Frenkel talked about (paper here) is what happens when you heat a particle floating in liquid surrounded by smaller particles which have magnetic dipole moments (which do exist in nature). With a hot particle immersed in the colder liquid, you have heat diffusing away from hot particle, this creates a heat gradient surrounding the particle.

Gradients tend to align things as they define a direction (away from the hot particle in this case) and so the temperature gradient will align the magnetic dipoles. These aligned magnetic dipoles in turn create a magnetic field. The clever bit is that the temperature gradient is given mathematically by a solution to Laplace’s equation, and this is the same equation that gives the field around a magnetic monopole. So the magnetic field around the hot particle looks just like that around the elusive magnetic monopole particle. Cute. The magnetic monopole is probably not going to be very strong so this is not going to revolutionise nanotechnology but it is ingenious and shows that just heating up a particle in liquid can have consequences that are lot more exotic than you might have thought.