# Do we have magnetic monopole? Is charge, according to yesterdays paper on Nature by Ray-Roukokoski-Kandel-Möttönen-Hall (30.01.14) quantized?

As everybody here knows, Maxwell's equation would look more beautiful if a magnetig charge were present. Beyond the aesthetics question, if a single magnetic Dirac monopole would be found, the consequence is that the electric charge would be quantized.

While, I guess, everybody should be happy with the news, my experimentalist background doesn't allow me to understand the experimental setting. I therefore ask for the following advice -- a rather theoretically oriented question.

A recent paper,

Observation of Dirac monopoles in a synthetic magnetic field. M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen and D. S. Hall. Nature 505, pp. 657–660 (30 January 2014).

claims to have observed such monopoles.

• Can somebody shed light on the theoretical implicacions of this work? Concretely, should we now take care before saying that it's been proven that the electric charge is quantized?
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I've only seen the abstract, but based on that, to my reading, it's not clear that they have discovered true magnetic monopoles. – David Z Jan 30 '14 at 7:47
Is a quasiparticles in spin ice. – Vlad Jan 30 '14 at 7:48
Related: arxiv.org/abs/0903.4732 and arxiv.org/abs/1110.3955. These are not monopoles in the sense that an electron is a charge monopole. Interesting physics though. – John Rennie Jan 30 '14 at 8:12
@Vlad The recent work is in BECs and not in spin ice, but it is still work on quasiparticles. – Emilio Pisanty Jan 30 '14 at 16:09
Well as matter is made up of elementary particles and charges come in +/-1 or +/-2/3 or +/-1/3 I would say that experimentally charge is quantized. – anna v Jan 30 '14 at 16:13

There are no consequences concerning the quantization of the charge or the existence of real magnetic monopoles.

The connection with the monopoles is only formal. What the experimentalists study is the (superfluid) velocity field $v$ and the corresponding vorticity $\Omega=\nabla \times v$ in the gas and the spin orientation of the atoms (the system is described by spin one bosons). By using a real magnetic field, they can force the velocity to have a special form, such that the vorticity looks like the magnetic field of a monopole. The velocity corresponds then to the vector potential. The fun thing is that this imposes that the velocity field is not well defined along a line connecting the monopole to the outside of the gas, it corresponds to a vortex in a superfluid language, or to a Dirac string if it were a real monopole.

But the connection is only formal, there's nothing to do with real EM fields, and no cosmological/HEP/... implications (it just says that Dirac did the math correctly). But of course, that's not how they present that in the abstract, if you want to be published to Nature...

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## protected by Qmechanic♦May 22 '14 at 12:31

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