March 2011 Archives

Yes, I know. I've not blogged for ages. Anyway, I'm blogging again, and then will probably fall silent for months again.

Two things have inspired me to write today, both physics demonstrations or analogies.

The first is a very common description of gravity as described by Einstein's Theory of General Relativity (GR). GR essentially describes gravity as due to the curvature of spacetime, and that curvature as being due to matter. Almost as soon as someone starts to try to explain it they use the rubber sheet analogy. Now I link to that page to describe the analogy, but that page does not do a bad job of using it. Essentially the analogy is that you have a rubber sheet, and a ball on that sheet curves it, and the curvature of the sheet affects the motion of other objects upon it. Now, I've no criticism of that page. There are two major problems with this analogy.

• The ball deforms the sheet - why? It's because the weight of the ball is pushing down upon that sheet. What gives the ball weight? Gravity, which is the thing this analogy is trying to describe. Now that's not too serious, you can simply point that out and say that the mechanism by which mass curves spacetime is not described - it simply does so.
• If you roll a small ball slowly past the larger ball it curves in and rolls toward the big ball. This is because the small ball rolls downhill. Why does it do this? Because of gravity. This is a serious problem. There's a path deflection due to the geometry of the sheet, but there's also a path deflection because of a preexisting downward force upon the ball. This is almost never pointed out, and most worryingly the effect exactly looks like what people generally think of as a gravitational attraction - a movement towards rather than a deflection of a path from what would naively be considered straight.

Professor Brian Cox, using this description in this week's Wonders of the Universe, I'm looking at you.

When you first studied magnetism at school, what demonstrations of it do you recall? I bet that a very early one was putting a sheet of paper over a magnet and sprinkling iron filings over it. You get something like this image - iron filings lining up upon field lines. You're then also shown a diagram like this showing discrete field lines.

What's wrong with this? Well, field lines don't come in discrete chunks. They're continuous. Every point in space has a magnetic field line passing through it, and the field lines do not vary in strength in some onion-skin like way. There's nothing special about where those iron filings are lining up. The field lines are no more existing in a particular number than the field lines of Earth's gravitational pull exist in particular places, rather than smoothly over the entire surface. I bet you that every kid comes out of that lesson thinking magnetic fields look something like an onion. I did, and it took me a disturbingly long time to figure out that they weren't, and why, because noone ever corrected that misconception.

What is actually happening is that every iron filing is itself becoming magnetised and is drawing adjacent filings towards itself. It's like they're concentrating the field where they are. The filings are an active part of the field - they're not what a physicist might call a 'test particle' that doesn't affect the things around it and only traces out some physical phenomenon.

This is really problematic. People come away from seeing this thinking that magnetic fields are hairy.

Now both these demonstrations are actually useful if explained properly, and I honestly have no idea if there's a better demonstration of these physics concepts. Do you have one? Or do you have a pet hate amongst common physics demonstrations yourself?