Friday, November 04, 2011

It's About Time

A while back, a group of physicists at CERN reported a remarkable result: they had observed a beam of neutrinos traveling faster than light. Which, near as we can tell, ought to be impossible. They are currently attempting to repeat their experiment to verify their results, and are also looking into possible measurement errors.

One recent rebuttal was interesting. They claim that, if they really were moving faster than light, they ought to have shed energy in the form of specific particles, which the CERN experiment did not detect.

Another theory holds that something that's moving faster than light is also moving backwards in time. Leading to the physicists' joke: "We don't serve faster-than-light neutrinos here," said the barman. A neutrino walks into the bar. I'm not sure I buy the theory, but then again, I pretty much hate time travel in all its forms.

Another competing theory caught my attention: the neutrinos only seemed to be traveling faster than light, because the laboratory used GPS satellites as a time reference. Since the satellites were also moving with respect to the experiment, that motion also has to be accounted for, leading to a margin of error that almost exactly covers the gap.

"Now, wait one second," I can hear you saying. "GPS satellites are used to find where you are. What do clocks have to do with it?" Everything, my friend. Everything. The clock is by far the most important thing a GPS satellite carries. It's the key to the whole process.

You see, one of the things we think we know about the Universe is that light always moves at the same speed. We're fairly sure about that. At least, we haven't been able to design an experiment to disprove that fact. So, if you have two clocks, and Clock A broadcasts a time signal to Clock B, then the difference between Clock A's time at arrival and the received time from Clock B tells you the instantaneous distance between the two clocks. With me so far?

Now, each of the GPS satellites is continually broadcasting two pieces of information: an identifier, and a time signal. The identifier tells you which satellite it is, and that information plus your current time should tell you where that satellite ought to be. So, once your GPS receiver gets a signal from Satellite A, it compares the broadcast time with the current system time, and computes a distance. Using that distance, and the satellite's position, it figuratively draws a circle on the globe. You're somewhere on that circle.

That's not enough. So, it looks for a second signal. If it can find Satellite B, it compares Satellite B's broadcast time with the current system time, converts that into a distance, and draws another circle on the globe. Now, those two circles intersect in at most two places. So, you're at one, or the other. You still don't know which.

Well, that doesn't really work, either. So, you need a third signal. Now, it looks for Satellite C, and does the same thing. It compares times, finds a distance, draws a third circle ... and now, if the Earth were a perfect sphere, all three circles would meet in harmony in one and only one place. But the Earth isn't a perfect sphere. It's not a perfect anything. It's awfully damned lumpy. So...

So it looks for a fourth signal. Once it finds Satellite D, it goes through the same process. And basically, it finds the point where all four circles more or less meet up. And hey presto, you know where you're at. And, if you have a relatively new unit, it'll also tell you that you need to take the next exit to get to your Aunt Sally's house. Oh, and you need to pick up a loaf of bread and some eggs while you're at it.

But, mad scientists being the innovative chaps that they are, they found a new and interesting "off-label" use for GPS technology. Why worry about synchronizing your clocks if the U.S. Department of Defense has already gone to the trouble of synchronizing one for you? Well, now we think we know a good reason: because it's whizzing around the planet at 17,500 miles per hour, that's why. And if you don't take that into account, weird things might happen.

Although the jury's still out. Weird things might still be happening. We won't know until the re-test is over. Measurements talk, and we just don't have enough of 'em yet.

Still, we should know something in six months or so. Keep your fingers crossed.

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