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A Lot of Knowledge Is a Dangerous Thing

作者:Chad Orz…    文章来源:scienceblogs    点击数:    更新时间:2010-1-14 【我来说两句

I was looking at some polling about science over the weekend, and discovered that they helpfully provide an online quiz consisting of the factual questions asked of the general public as part of the survey. Amusingly, one of them is actually more difficult to answer correctly if you know a lot about the field than if you only know a little. I'll reproduce it here first, if you would like to take a crack at it, and then I'll explain why it's tricky below the fold.

Choose only one answer-- this is being recorded for SCIENCE!

So, what's the problem? The correct answer is obviously "Satellites," right?

The problem is that two of the answers are technically correct. GPS is based on a constellation of satellites sending out radio signals, but those satellites contain atomic clocks. And the atomic clocks in the satellites rely on magnets to function.

The key idea behind the atomic clock is that atoms have discrete energy states, determined by quantum physics, and move between those states by absorbing or emitting photons of light. Light, in turn, has a frequency associated with it that is determined by the energy of a single photon.

An atomic clock uses the discrete energy levels of an atom as a frequency reference. Unlike mechanical clocks which depend on the construction of the pendulum, or quartz clocks that depend on the properties of an individual quartz crystal, every cesium atom in the universe is guaranteed to be identical to every other cesium atom in the universe, with the same basic separation between energy levels. If you have a frequency source, and tune it so that it is at exactly the frequency that cesium atoms like to absorb, you can use that frequency to measure the progress of time-- to be specific, every 9,192,631,770 oscillations of the light absorbed or emitted by a cesium atom moving from one hyperfine ground state to the other is one second.

GPS relies on precise timing-- your GPS receiver determines its position on the surface of the Earth by measuring the time required for signals from at least three different satellites to reach its position. The orbital positions of the satellites are well known, and light travels at a fixed speed, so the travel time determines your distance from three of those satellites. That, in turn, fixes your position at a specific point on the surface of the Earth.

So, where do the magnets come in? Well, in order for the clock to work, you need to take a bunch of cesium atoms, expose them to the light that you're hoping to use as a reference, and see if they move from one state to the other. In order to know this, though, you need to know which state they started in, and which state they ended up in. Magnets can help you do this state selection, as shown in this schematic of an atomic clock taken from

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