The Sciences

Followup: FTL neutrinos explained? Not so fast, folks.

Bad AstronomyBy Phil PlaitOct 15, 2011 7:20 PM


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If you haven't heard about the experiment that apparently showed that subatomic particles called neutrinos might move faster than light (what we in the know call FTL, to make us look cooler), then I assume this is your first time on the internet. If that's the case, then you can read my writeup on what happened. Basically, neutrinos move very very fast, almost at the speed of light. Some scientists created neutrinos at CERN in Geneva, and then measured how long it took them to reach a detector called OPERA, located in Italy. When they did the math, it looked like the neutrinos actually got there by traveling a hair faster than the speed of light! 60 nanoseconds faster, to be accurate. Was relativity doomed? Nope. In fact, relativity may very well be what saves the day here.

First, most scientists were skeptical. Even the people running the experiment were skeptical, and were basically asking everyone else for help. They figured they might have made a mistake as well, and couldn't figure out what had happened. Relativity is an extremely well-tested theory, and doesn't (easily) allow for FTL. Despite some headlines screaming that Einstein might be wrong, most everyone figured the problem lay elsewhere. Most everyone zeroed in on the timing of the experiment, which has to be extremely accurate. The entire flight time of a neutrino from Switzerland to Italy is only about 2.4 milliseconds, and the measurement accuracy needs to be to only a few nanoseconds -- mind you, a nanosecond is a billionth of a second! The scientists used a very sophisticated GPS setup to determine the timing, so that has been the focus of a lot of scrutiny as well. And a new paper just posted on the Physics Preprint Archive may have the answer... and it uses relativity. Basically, what Einstein found is that the speed of light is the same for all observers. If I'm moving at 0.9 times the speed of light toward you and turn on my flashlight, I see those photons moving away from me at the speed of light. The thing is, you see those photons moving toward you at the speed of light! This goes against common sense, which tells us that velocities add together; if I throw a baseball out car window, the velocity of the ball add to that of the car. But light doesn't behave that way. And this changes a lot of things, including how two objects moving relative to each other measure distance, and even how they measure time. I might measure a meter stick in my hand as being (duh) one meter long, but an observer moving past me at a significant fraction of the speed of light would see it being shorter. It's just a consequence of the Universe making sure we all see the same speed of light. And that's where neutrinos come in. In this new paper, author Ronald A.J. van Elburg lays out his case. The timing was measured using a GPS satellite orbiting the Earth, and moving relative to CERN and OPERA. That means the distance traveled by the neutrinos would be less as measured by the GPS sat as it would be from the ground, and therefore wouldn't take as long to cover it. Doing the detailed math, van Elburg calculates how much faster the neutrinos would be expected to arrive accounting for the satellite's motion, and he gets... 64 nanoseconds. That's almost exactly the discrepancy measured by the original experimenters. Case closed! Well, maybe. As I recall from the foofooraw that unfolded after the initial announcement, the original experimenters said they accounted for all relativistic effects. The paper they published, however, didn't include the details of how they did this, so it's not clear what they included and what they might have left out. It's possible van Elburg might be right, but I expect we haven't seen the end of this. After all, not long after the announcement, a physicist asked if they had accounted for gravitational time dilation -- like relative velocity, gravity can also affect the flow of time, throwing off the measurement -- and the experimenters said they had. I had thought of something like this as well. CERN and OPERA are at different latitudes, and since the Earth rotates, they are moving around the Earth's axis at different speeds. Could that be it? I did the math, and the answer is no. Too bad; it would've been fun to be the person to have figured this out! The bottom line here is that this experiment is still very interesting. I don't think we know exactly what's going on here yet -- my bet is still on the statistics, since they didn't measure the speeds of individual neutrinos, but clouds of them, making the exact timing much harder -- but it's hard to say. Like most other scientists, I think somewhere down the line here a mistake was made, and the neutrinos, like everything else we know of made of matter, travel slower than light. But if we're wrong, then we get new physics, which is great! And if we're right and figure out how, it means that future experiments will benefit from this. Win/win. Either way, my bet is that we're not done here. This new result is interesting and may very well be right, and be the dampening field that bursts the neutrino FTL warp bubble. But I'll wait for the reaction from the original experimenters to see what they say. If we've learned one thing from all this, it's that it's best not to jump to conclusions.

Related posts: - Faster-than-light travel discovered? Slow down, folks - A (very) smart kid and a solid theory - Wall Street Journal: neutrinos show climate change isn’t real - Followup on the WSJ climate denial OpEd

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