Good News! Relativity Is Saved! (Probably)

Speedy neutrino mystery likely solved, relativity safe after all

By Evan Ackerman

6:59PM on Oct 14, 2011

http://dvice.com/archives/2011/10/speedy-neutrino.php#more

Those weird faster-than-light neutrinos that CERN thought they saw last month may have just gotten slowed down to a speed that'll keep them from completely destroying physics as we know it. In an ironic twist, the very theory that these neutrinos would have disproved may explain exactly what happened.

Back in September, physicists ran an experiment where they sent bunches of neutrinos from Switzerland to Italy and measured how long the particles took to make the trip. Over 15,000 experiments, the neutrinos consistently arrived about 60 nanoseconds early, which means 60 nanoseconds faster than the speed of light. Einstein's special theory of relativity says this should be impossible: nothing can travel faster than light.

The fact that the experiment gave the same result so many times suggested that one of two things was true: either the neutrinos really were speeding past light itself and heralding a new era of physics, or there was some fundamental flaw with the experiment, which was much more likely. It's now looking as though the faster-than-light result was a fundamental flaw, and appropriately enough, it's a flaw that actually helps to reinforce relativity rather than question it.

The Experiment

Here's the deal: neutrinos move very very fast (at or close to light speed, at least), and the distance that they traveled in this experiment was (to a neutrino) not that far, only 450 miles. This means that in order to figure out exactly how long it takes a given neutrino to make the trip, you need to know two things very, very precisely: the distance between the two points, and the time the neutrino leaves the first point (the source) and arrives at the second point (the detector).

In the original experiment, the CERN researchers used GPS to make both the distance measurement and the time measurement. They figured out the distance down to about 20 centimeters, which is certainly possible with GPS, and since GPS satellites all broadcast an extremely accurate time signal by radio, they were also used as a way to sync the clocks that measured the neutrino's travel time. The CERN team had to account for a lot of different variables to do this, like the time that it takes for the clock signal to make it from the satellite in orbit to the ground, but they may have forgotten one critical thing: relativity.

It's All Relative

Relativity is really, really weird. It says that things like distance and time can change depending on how you look at them, especially if you're moving very fast relative to something else. In the case of the neutrino experiment, we've got two things to think about: the detectors on the ground that measure where and when the neutrinos depart and arrive, and the GPS satellites up in space that we're using as a basis for these measurements. Since the satellites are orbiting the Earth and moving way faster than the detectors, we say that they're in a different "reference frame," which just means that the motion of the satellites is significantly different than the motion of the Earth.

Part of the deal with relativity is that neither of these reference frames are the "correct" one. From our perspective here on Earth, the satellites are whizzing around in orbit at about 9,000 miles per hour. But the perspective of the satellites, the Earth is whizzing around just as fast, and the difference in velocities between these two reference frames is large enough that some strange things start to happen.

A Satellite's Perspective

To understand how relativity altered the neutrino experiment, it helps to pretend that we're hanging out on one of those GPS satellites, watching the Earth go by underneath you. Remember, from the reference frame of someone on the satellite, we're not moving, but the Earth is. As the neutrino experiment goes by, we start timing one of the neutrinos as it exits the source in Switzerland. Meanwhile, the detector in Italy is moving just as fast as the rest of the Earth, and from our perspective it's moving towards the source. This means that the neutrino will have a slightly shorter distance to travel than it would if the experiment were stationary. We stop timing the neutrino when it arrives in Italy, and calculate that it moves at a speed that's comfortably below the speed of light.

"That makes sense," we say, and send the start time and the stop time down to our colleagues on Earth, who take one look at our numbers and freak out. "That doesn't make sense," they say. "There's no way that a neutrino could have covered the distance we're measuring down here in the time you measured up there without going faster than light!"

And they're totally, 100% correct, because the distance that the neutrinos had to travel in their reference frame is longer than the distance that the neutrinos had to travel in our reference frame, because in our reference frame, the detector was moving towards the source. In other words, the GPS clock is bang on the nose, but since the clock is in a different reference frame, you have to compensate for relativity if you're going to use it to make highly accurate measurements.

Not So Fast

Researchers at the University of Groningen in the Netherlands went and crunched the numbers on how much relativity should have effected the experiment, and found that the correct compensation should be about 32 additional nanoseconds on each end, which neatly takes care of the 60 nanosecond speed boost that the neutrinos originally seemed to have. This all has to be peer-reviewed and confirmed, of course, but at least for now, it seems like the theory of relativity is not only safe, but confirmed once again.