Observation of Dirac monopoles in a synthetic magnetic field | Hacker News Reader

Observation of Dirac monopoles in a synthetic magnetic field

nature.com

85 pointsrmserror16 comments

The BBC coverage quotes a 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."

http://www.bbc.co.uk/news/science-environment-25946734

The overall phrasing seems to say that it's still just an analogue of a monopole, but that it's a far more useful analogue in a system that you could do meaningful experiments upon.

Physicists tend to be metaphysical sceptics on the whole, meaning that they arent really convinced that the implied objects (/particles/fields/etc.) of physical theories actually exist^. Merely that "something exists which makes the mathematics work". Seeing so many different kinds of math work for the same physical system tends to engender a kind of scepticism towards the "metaphysical power" of mathematics to imply anything ontologically particular about the world.

Now, when it comes to results like this, they are interesting because it doesnt really matter if there are magnetic monopoles if you "just wanted to use a magnetic monopole" - you could create some in supfl. He.

The point is that whatever system enables the math to work is irrelevant to what ever uses theories will have for this math.

Of course it is still practically relevant, because a simple monopole experiment might be extremely expensive/difficult in Supfl. He.

NB^: Pop physicists who wax philosophical on TV shows tend to be sceptics but appear "hardcore realists" for TV.

> The point is that whatever system enables the math to work is irrelevant to what ever uses theories will have for this math.

Additional explanation from someone who (also?) used to do research, with papers and all: we actually used objects we knew did not exist to do actual science. By "do not exist" I mean "they are make-believe theoretical objects that do not exist per se, but they are good models for things that do exist".

Science and engineering is full of these things. Holes in transistors: they don't exist, they're actually a certain type of electron shifting their positions in a particular way, but the math would get a lot hairier (at no gain!) if you treated them as electrons. Electrical charges in classic electromagnetism aren't electrons at all -- they're small dots with charge on them, but the math would get a lot hairier (at no gain -- again!) if you didn't treat them like this.

Being able to model a phenomenon is very important, and being able to model it simply is, for lack of a better word, God-given. Phenomenological models (i.e. models that model only the behaviour, not the inner working of a certain phenomenon) are as good as any other model, as long as you bear in mind that they are phenomenological: you can use them to reason about certain effects, but not necessarily about what causes them.

Before anyone jumps in to point out this is bullshit and that there's really more to life, I just want to remind everyone that we've been building buildings and bridges and shit based on a phenomenological model. Classical mechanics knows absolutely shit about how gravity works -- it just tells you useful stuff about what it does -- and I'll personally grab all the beer in the world for someone who finds an actual material point.

Perhaps it's phenomenological models all the way down. If that is, indeed, the case, is a hole significantly less "real" than a mobile electron?

I guess an electron is "an actual point".

But it's not the only charge carrier. To put things in better context, classical electromagnetism actually predates knowledge about the structure of atoms. The charges in its model are very much hypothetical particles, and that's why it only really works at macroscopic scales. Turns out that's actually enough for a lot of things :).

As others have pointed out, these are indeed just analogues of (fundamental) monopoles.

IMHO, this is an example of a deliberately-slightly-misleading abstract for a paper. The authors write: "Although analogues of magnetic monopoles have been found [in others' work] there has been no direct experimental observation of Dirac monopoles within a medium described by a quantum field [...]. Here we demonstrate the controlled creation of Dirac monopoles in [...]. Monopoles are identified, in [...]"

If I were a referee for the paper, I would have registered the following objection: When referring to others' work, the authors use the phrase "analogues of monopoles" whereas they reserve the phrases "monopole" and "Dirac monopole" for references to their own. Their terminology seems especially egregious on account of their earlier statement: "The existence of even a single Dirac magnetic monopole would have far-reaching physical consequences, most famously explaining the quantization of electric charge". In fact they too have only discovered analogues of monopoles and the abstract is confusing for non-specialists, as evidenced by this thread.

Nature is a journal that has been known to give in to the temptation to sensationalise so I am not all that surprised.

Can anyone with a background in physics and/or interpreting Nature publications comment on this? It appears to be a peer-reviewed publication, rather than something merely published.

My understanding of Dirac's monopole hypothesis implies that these results would be very significant for particle theory . . .

This isn't a newly discovered fundamental particle. This is a new form of behavior observed in ensembles of particles, in this case superfluid helium. This is an exciting new result in condensed matter physics, not particle physics. It's kind of surprising how much the mechanisms of particle physics (gauge fields and all that) carry over to describing the collective behavior of particles that's studied in condensed matter physics.

As to the sibling's comment from Lindsay LeBlanc, I don't believe that at all. While true magnetic monopoles are allowed by our theories, we just haven't seen any real evidence that they exist, and that's a bit strange. Personally, I don't believe they'll ever be observed.

(I used to be a particle physicist, for what that's worth. Take my opinions at face value!)

I just came here to say pretty much exactly what you've already said (both paragraphs, in fact). So I'll just chime in (as a physics professor and string theorist) to endorse your take on the article.

This is a cool result, and it's neat to see a realization of the Dirac monopole in a thoroughly quantum field. But it's not a detection of a Dirac monopole for the usual electromagnetic field (which would be an amazingly big deal, and which I think most people don't ever expect to see).

The existence of a magnetic monopole would make maxwell's equations so much prettier. Plus validate dirac's proof of why electric charge is quantized. Sigh...

Thank you (and also to fennecfoxen)! My background in physics isn't sufficient to be sure of my interpretation of the details here.

I usually ignore implications of immediate world-changing revolution in popular science reporting (they're invariably nonsense in those fields where I'm qualified to come to a definite conclusion), but Nature (and certain other publications) are more circumspect, at least often enough to check first.

Of course, I'm still happy that the results are interesting!

One point to mention -- the results appear to apply to a quasiparticle rather than a naked monopole.

There's an additional article in Nature that describes the research:

http://www.nature.com/news/quantum-cloud-simulates-magnetic-...

No it's not a monopole, Div B is still zero everywhere. Yes it is interesting. :)

Full article for anyone curious: http://ge.tt/2O8mr1H1/v/0?c

(link seems not to work any longer, mirror: http://bayfiles.net/file/15Fqf/KfQTOj/nature12954.pdf)

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