Why is striking woo so easy on the internet?

My knee has been bothering me, and the knee brace I have used in the past (which AFAICT stabilizes against some lateral movement) wasn’t helping me. The problem appears to be consistent with patellar tendinitis, aka jumper’s knee (and yes, mister fancy-pants word-check software, thatisthe correct spelling for tendinitis). It’s not bad enough to send me to the doctor’s office — that event has a fairly high activation potential — so I bought a patellar knee strap, and it seems to be helping thus far.

But when you go out on the web and search on things like “knee pain brace,” you end up hitting a lot of sites selling magnetic therapy bracelets. And, of course, these are bunk — total crap. Now it’s not insane to wonder if magnetic fields will affect the body, but the explanations of the way the bracelets are supposed to work are just clearly bogus. The fields of these bracelets/wraps/jockstraps are weak, and hemoglobin isn’t ferromagnetic. Depending its the oxygenation state, you can get diamagnetic or paramagnetic behavior in the hemoglobin, which allows you to do functional MRI, but this uses fields several orders of magnitudes stronger to see effects. People have done tests that show that these devices don’t work, but it’s also true that these suffer from some problems, namely, that you can’t have a magnetic “placebo” device, since it isn’t difficult to tell if a bracelet is magnetic.

But there’s actually a deeper level to some of the shenanigans, and this is where the basic physics breaks down – badly. Some of the purveyors tout more specific benefits: the south pole of the magnet, they say, heals by speeding processes up, and the north pole inhibits activity, so it’s used for treatment of inflammation or even cancer. (I think I’ve gotten that right. If not, multiply the preceding by -1) Some sell “unipolar” magnets which they imply have only one pole! Maxwell says otherwise. The divergence of a magnetic field is zero, which means that a field line can’t end at a point (unlike electric fields, which start and end on charges). They must loop around on themselves so that in any arbitrary volume (and as almost any physics book will confirm, “arbitrary” seems to be latin for “kidney-shaped”) so a field line which enters the volume must also leave it.

What does that mean for our magnetic therapy? Nothing good. We draw our field lines with a direction from north to south and a field line from a north pole, leaving a volume, looks exactly like a south pole entering. And any field line entering the body also has to leave to satisfy the zero divergence Maxwell’s equations. If we have a row of magnets with their north poles all facing down and look at the field, we’ll get the same answer if we flip it over — the same number of field lines entering as leaving. There’s no difference.