Experimental physics is easy on paper

By Marcus Wilson 18/10/2012

Currently, down in the depths of C-building, there’s a master’s student trying to carry out the Stern-Gerlach experiment. (and also here). This is one of the classic experiments in quantum mechanics – specifically demonstrating the quantisation of angular momentum.

If you look at the text books, it’s simple enough. Pass a beam of atoms through an inhomogeneous magnetic field (i.e. one that is stronger in one region of space than another) and, hey-presto, the beam gets split into two (or more) beams, depending on the magnetic moment of the atoms. The non-uniformity of the field is essential. If the atom has a magnetic moment that lines up with the field, then it will have a negative potential energy due to the field and will move towards the region of strong field, where its energy is most negative. Conversely, if it has a magnetic moment that is against the field, it will have a positive potential energy and will move towards the region of weak field, where its energy is least positive. So the beam splits. The key result, though, is that the beam doesn’t split into a continuum, which would mean any magnetic moment were possible, it splits into discrete beams, showing that only certain values of magnetic moment are allowed. This is what quantization means – things are split into discrete amounts. What the experiment is doing, is measuring the magnetic moment in a particular direction.

Stern and Gerlach did this experiment in 1922. Having seen our poor student struggle with the apparatus, they must have put in some considerable effort, that’s now been glossed over in most books. There are all kinds of issues that need attending to. Preparing a beam of atoms (in our case sodium – we’d like to use potassium but that’s a little bit too exciting from a safety point of view) is tricky. The sample needs to be heated so that atoms are evaporated. We need a high vacuum, meaning that atoms do not collide with air molecules on their way down the apparatus. We need to make sure that we are detecting our sodium atoms not contaminant atoms that are coming from elsewhere.  And, most frustratingly this afternoon for student, we need to find where the beam is going an align it so that it falls on the detector.

The stereotypical drawing of the apparatus we see in the quantum textbooks overlooks most of what actually has to go on to get this to work. It’s slow going, tedious, and frustrating, but hopefully the student will nail it in the end. This is all too reminiscent of the reasons why I became a theoretical physicist rather than an experimental one, and the old adage…"Biology experiments wriggle, Chemistry experiments smell, and Physics experiments don’t work"

0 Responses to “Experimental physics is easy on paper”

• Well I will comment on this. I believe this to be one of the most important discoveries made this century so far. In fact I believe the higgs boson pales compared to this. After all the universe is comprised of magnets and magnets help generate electricity. At last a magnetic field is shown not to be static and the nature the vortex plays such an important role (as does in the center of the universe). The implications of this could have profound implications for reducing our oil addiction.

• Marcus Wilson says:

Unlike you I’m not so excited by this. There’s a lot of science words used, but not used with their correct meanings, suggesting some quackery going on here. The comment about ‘conclusive proof’ is worrying, as is, ‘it can be used for healing’, without any evidence shown of this application. So I’m suspicious right from the outset.

What’s going on?
Things that come to my mind:
1. Only works when a voltage is applied (I assume it doesn’t work without one – but the video didn’t show this.) I’d like to know how large the voltage is and whether it is a.c. or d.c. Also, where is the other electrode – is it on the plate at the bottom of the water?)
2, Doesn’t work with an iron slug. Again, though, it wasn’t clear whether this lump of iron was magnetized or not. I assume not.
3. Direction of spiral can be reversed (was this by flipping the magnet over? Not clear)

The fact that you get helical movement of something in a static magnetic field is entirely well known and accounted for with our present theories of electromagnetism. Charged particles will move in helices around field lines – they do it at the earth’s poles (these give the Northern and Southern lights). We also do it with our students in the lab – by accelerating a beam of electrons which is then curved by a STATIC magnetic field. The magnetic force on a moving charged particle is PERPENDICULAR to the direction of motion and the magnetic field – therefore resulting in a circular motion around the field line, but drifting along it (a helix).

So, my best guess – and there is some guesswork based on the evidence shown – is that the voltage sets up a strong electric field between the magnet and the plate at the bottom (the two are insulated from each other). This is sufficient to cause gas bubbles to form in the water – some of which may be charged. The movement of these bubbles then spirals around the field lines. Of course, one must get equal amounts of positive and negative charge, and the two would spiral different ways. Why does one dominate then? Maybe, say, the bubbles carry a positive charge while the remaining electrons are still attached to water molecules. I don’t know. That’s what experimentation could help track down.

But, to summarize, I’m not convinced there is anything new here. Helical patterns are readily caused when a charged particle moves in a static magnetic field.