Measure The Speed Of Light With Your Microwave

maddmike writes "There is a very interesting article on About.com that shows how to measure the speed of light using your microwave to melt chocolate. "

Comments

Half the experiment is missing

[panurge]

In a proper physics experiment all the quantities that affect the result have to be measured. In this one the frequency of the microwaves is taken for granted, so it is not a proper experiment. Just reading the value off the label doesn;t count. All the experiment actually allows you to work out is the wavelength of the microwaves, which is not hugely interesting. So how do you measure the frequency?

You cannot do it by measuring the dimensions of the magnetron cavities, because the calculation of the frequency based on dimensions assumes the thing you are trying to work out - the speed of light. Frequency counters that go up to 2.5GHz are a bit difficult to come by in most homes. One possibility might be to extract some energy from the cavity using a suitable antenna and mix it with the clock signal from a 2.4 or 2.53GHz motherboard, then try and pick up the resulting beat signal using a short wave or VHF radio. However, I'm not at all sure how to get the signal out of the P4.

Has anybody got a better and reasonably practical method of measuring the frequency?

Re:Half the experiment is missing

[panurge]

I really don't see why this is modded insightful. It's actually possible to reference the clock of a computer, or any other clock, to the cesium standard because it is possible to make use of radio broadcasts referred to clocks based on that standard. So in theory you could measure the frequency of the microwave oven - the frequency, notice - in terms of the international standard.

The measurement you do in the microwave oven is a real-world measurement, i.e. of a displacement. If you have a real frequency and a real displacement, you can measure the speed of light in terms of your measured displacement and a time interval related tot he international standard.

Your comment that "refusing to accept c would be problematic" is invalid because, in fact, the experiment doesn't assume a value for the meter, or for c. You could compare your measured interval to anything convenient - using compasses, just like a Greek philosopher - and measure the speed of light in terms of a reference in your house. It would be a valid outcome of the experiment to measure the speed of light in teaspoons per cesium transition. It isn't a question of making up units: it is a question of deciding on the real world analog of the reference unit, whether it be a standard meter, standard kilogram, or standard sack of sugar. The international definition of the meter is handy if you have a suitable interferometer, but you can still only use it to produce a substandard by making marks on something, or counting interference fringes.

Anyway, why am I bothering? As I remind myself, this is /.

This won't work for most microwaves

[ChrisCampbell47]

I'm surprised that nobody's posted this yet (or that that comment hasn't made it up above +2).

This only works if you can stop the mechanism by which the microwaves are scattered around to make for even heating. If you have a turntable in the bottom of your microwave, then removed it might do the trick, but most microwave ovens have a rotating metal "fan" that is enclosed in the upper surface over the cooking cavity, and that metal fan spins to scatter the waves around -- think of it like a flashlight and a mirrored pinwheel. Hence no turntable is required.

I'm not aware of any way of disabling that "fan", although I suppose you could drill a tiny hole in the shroud and poke in something to stop the spin, a la stopping a grinding PC fan. But I personally am not terribly interested in poking a drill into a microwave oven ...