Faster than light? Has relativity been violated?
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The OPERA experiment (OPERA stands for “Oscillation Project with Emulsion-tRacking Apparatus.” It’s a stretch, but who can blame particle physicists for liking fun acronyms?), presented results on 23-September-2011 indicating that a beam of neutrinos had exceeded the speed of light. Ever cautious, the collaboration concluded the paper with this:
"Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results."
OPERA is a group of about 200 physicists and, in the tradition of such particle physics experiments, tends to be very careful, even conservative, when it comes to presenting evidence for previously unobserved phenomena.
Matter, Antimatter and the Forces of Nature
What the OPERA experiment does
Neutrinos are subatomic particles with no electric charge. They are not to be confused with neutrons! Neutrons are components of atomic nuclei which, themselves, are formed of constituent particles called quarks and are quite heavy on the scale of these things. Neutrinos, on the other hand, are extremely light, perhaps even massless particles. They are believed to be point-like, fundamental particles that are all but invisible to even quite sensitive detectors.
Neutrinos do not interact through the electromagnetic force or through the strong nuclear force. If they interacted through either of these forces, they’d be easy to detect. Since they only interact through the aptly named “weak nuclear force” and gravity, which is far weaker than the “weak” force, they are very difficult to detect.
Zillions of neutrinos blaze through your body every day but don’t worry about it, they don’t do anything. If they did, they’d be easy to detect. See how it works?
At CERN, neutrinos are created from the decay of other subatomic particles. Since the neutrinos themselves are so hard to detect, their speed is calculated by measuring the byproducts of the decay from which they were created.
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The neutrino beam then travels from CERN in Geneva Swtizerland, through the earth’s curst, about 454 miles, to the OPERA experiment in a tunnel at the base of a mountain in central Italy called Gran Sasso. Again, since neutrinos barely interact with matter, all that dirt and rock is essentially invisible to them. The OPERA detector is built of 1300 tons of lead interspersed with photographic film and special plastic that scintillates with light when electrically charged particles pass through it. Since lead is so dense, the probability that a few of the neutrinos will interact goes up. While most of the neutrinos zip right through, a few leave tracks behind. Literal tracks in the film and light in the plastic.
The OPERA experiment can measure the time that the neutrinos leave Switzerland and the time that they arrive in Italy. Since they know the distance traveled, it’s easy to calculate the speed. The speed they measured was a tiny fraction faster than that of light,
neutrino speed = speed of light + 0.0000248 x speed of light
OPERA estimates that the uncertainty of this measurement is about 17%.
If the OPERA physicists have an excellent understanding of their experiment and a good understanding of what they don’t understand (estimating systematic uncertainty is tricky business) then the probability that they’re truly seeing faster-than-light phenomena is roughly 99.9999998% certain.
In the tradition of human activity since the dawn of civilization, every other team of physicists capable of confirming or contradicting OPERA’s measurement is scrambling into action. At Fermilab in the United States, the MINOS collaboration (MINOS stands for Main Injector Neutrino Oscillation Search) and, in Japan, the T2K collaboration (T2K stands for Tokai to Kamioka), are both all over this, so we’ll see pretty soon whether Relativity needs to be rewritten again.
(Though the author, Ransom Stephens, seriously doubts that the result will hold up, he hopes like hell that it will. Physics desperately needs a little shake up now and then and we're overdo. PS: You must click this link: http://xkcd.com/955/ - hilarious)
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Hoping you get that shake up! Will we need to re-write the science books?
(By the way, there was a typo in the original post, I had 0.0000348 instead of the correct 0.0000248 (you totally caught that one, huh?) and I also filled out a little on the distinction between "neutrons" and "neutrinos")
If it holds up, the science books will need to be revised, but probably not rewritten. The existing science is "right" insofar as it makes useful predictions. What usually happens, as happened when relativity and quantum mechanics were discovered a hundred years ago, is that new layer of understanding is revealed. More like adding a course to the degree program than starting from scratch.
For example, mechanical engineers use Newtonian physics, not Einstein-ian physics to build stuff. The extra layer of complexity just doesn't matter for building bridges.
Danette! If this holds up, your son's career will be a whole lot more exciting. CERN is a great place to work. It has the best cafeteria in the world - French cooks.
This is certainly exciting!! So if another team finds the same results, that is enough for the theory to hold up? Oh I hope that happens! Got any takers on your bet??
If another team confirms these results, a dedicated experiment will be designed and a whole mess of previous measurements will be re-examined. Only really decisive results - and I suspect that neutrinos are too hard to detect to provide that level of decisiveness - will overturn relativity. It's been so successful.
What makes relativity and quantum mechanics and, before those, Newtonian physics and classical electrodynamics so successful is that they do much more than explain previously known results. They predict great vats of new things.
This is a fun story, but I also seriously doubt that the results will hold up. There is some subtle thing somewhere that no one has thought of yet, which is throwing off their measurements.
A pretty serious argument against faster-than-light neutrinos is that we detected the neutrinos from supernova SN1987a about the same time as the light from it, but if the neutrinos had traveled 0.00248% faster, they would have arrived four or five years before we saw it optically.
But what if it's confirmed, and the SN1987a neutrinos were just not going quite as fast as the OPERA neutrinos? Einsteinian relativity says that there's a speed limit to the universe, and that light travels at that speed because it's massless. But what if light actually goes very slightly slower than the speed limit? I understand that the photon has been confirmed to have a rest mass lower than some extremely low value, but what if it's just way below even that? Could that be something that would need just a small adjustment to current theories?
Although, looking back at why we thought that the speed of light could be an issue at all, more than a century ago, Maxwell's equations say that light travels at a certain speed but don't indicate what reference frame that's relative to. Slightly-slower-than the limit light would get us back to that old problem again.
This is above my brain level, but awesomely interesting. How cool for you to have been there! Crazy cool stuff, right?
From slashdot today "It would appear that the hotly debated faster-than-light neutrino observation at CERN is the result of a fault in the connection between a GPS unit and a computer. This connection was used to correct for time delays in the neutrino flight, and after fixing the correction the researchers have found that the time discrepancy appears to have vanished."
It was fun while it lasted, though!
This is how science is done, by people. OPERA has thousands of cables, one was loose. In scientific terms, it means that they underestimated their "systematic uncertainty" - what they really got wrong was the "99.9999998% certain" statement.
Should they have sat on the result for four months to see if they found the problem? Or reported it, as they did, and let other experiments check their results while they went back through their fundamentals?
Consider this, though, how about the adrenal rush of the person (probably a grad student) who discovered the loose cable? "Uh oh."
Danette Watt Level 7 Commenter 7 months ago
My son is a college student going for a degree in physics/math. He texted me last week about this: CERN finds neutrinos (sub atomic particles) may be able to travel faster than light. theoretically impossible since they have mass.
Can't attest one way or the other on the matter. But it's still interesting