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 the PDF file at the bottom of this NIST page:
The clock starts with an oven containing a chunk of cesium. Atoms from the oven stream out in an atomic beam, but the atoms are initially distributed over many states, when clock operation demands that they be in a single well-specified state. In older atomic clocks, and the clocks on the GPS satellites, the specific state of interest is selected out by using magnets to deflect the other states out of the beam, so they don’t enter the area where they are exposed to the light.
The physics behind this deflection is the same as the Stern-Gerlach experiment, done by Otto Stern and Walter Gerlach in 1921. Stern and Gerlach sent a beam of silver atoms between the poles of a tapered magnet, so that the magnetic field was stronger on one side than the other. The beam of atoms split into two beams, which they detected by letting them fall on a photographic plate.
As is typical of pioneering experiments, Stern and Gerlach actually measured something other than what they thought they were measuring– they thought they were just seeing an effect cause by an electron orbiting the nucleus of a silver atom, with atoms whose electrons orbited clockwise experiencing a different force than atoms whose electrons orbited counter-clockwise. but the planetary type model behind that picture doesn’t work. Instead, what they discovered was the intrinsic spin of the electron, which has its own angular momentum as if it were a spinning ball of charge. That angular momentum can take on one of two possible values, and in the case of silver, leads to a force on an atom between Stern and Gerlach’s magnets that is either up or down, depending on the particular electron.
Stern and Gerlach were fortunate that they were using silver, which only has two states. Other atoms have more complicated level structures, and split into more than two beams, but the essential physics is the same. This splitting is the basis for the state selection in an atomic clock– the cesium atomic beam passes through a Stern-Gerlach magnet, and then a pinhole blocks everything but a single state that happens to be bent onto exactly the right trajectory.
After the cesium atoms are exposed to the light source, they enter a second set of magnets, which again splits the atomic beam, with the atoms that have changed states being sent one way, and atoms that have not changed states sent another way. Measuring the fraction that changed tells you how close the frequency of the light is to the frequency the cesium wants to absorb, and allows you to correct the frequency to keep it right on target. That, in turn, provides an absolute reference for the GPS satellite to use in determining the time, which it encodes into the signal that it broadcasts to your GPS receiver.
So, while satellites are the really important part of GPS, and the correct answer to the question, the satellites wouldn’t work without magnets. And now you, too, know enough to be confused by a simple and straightforward poll question.
(Anticipating a possible comment: the schematic shown is for a Ramsey-type clock, with the atoms interacting with the light field twice. A comment in one of the linked references suggests that the actual GPS clocks may be Rabi-type clocks, with a single large interaction region (though I could be misreading that). I couldn’t find a picture of the clocks used in GPS, though, so you’ll have to live with this one.)