Faster than a Speeding Neutrino...

David Kaiser

One of the T-shirts you’ll see quite often around MIT says: ‘Speed limit: 186,000 miles per second. It’s not just a good idea. It’s the law.’ The speed in question is the speed of light, and the law comes from Albert Einstein’s theory of relativity. Relativity is predicated on the notion that the speed of light is unsurpassable, and most of modern physics is predicated on relativity. So this morning’s announcement that a team of physicists at CERN may have measured tiny particles, known as neutrinos, travelling faster than light has the potential to eclipse all other news that ever has or may yet come out of CERN – Higgs particles, supersymmetry and all else combined. The key word, though, is ‘potential’. By the physicists’ own reckoning, their results require a lot more scrutiny before anyone concludes that physics has one fewer leg to stand on.

Neutrinos were first postulated in 1930 as an accounting gimmick: only by imagining that some tiny, unseen particles were whisking away energy and momentum in certain nuclear reactions could physicists balance the books on those processes. The first direct detection of neutrinos came about 25 years later. The original plan for the 1956 experiment was to place a massive instrument near an above-ground test explosion of a nuclear bomb. (Nukes, like stars, are prodigious producers of neutrinos.) It ended up, less spectacularly, using neutrinos spewed out of a nuclear reactor.

Physicists had to go to such extreme lengths to detect neutrinos because they hardly react with ordinary matter at all. Unlike electrons or protons, neutrinos carry no electric charge. This means they are impervious to the electromagnetic forces by which most particles (and larger collections of particles, like atoms and molecules) nudge or jostle each other. We are bathed by trillions of neutrinos every second without even knowing it; most of the neutrinos that come our way from the sun pass right through the earth.

In the late 1990s, physicists discovered that neutrinos have a very tiny mass, around one million times smaller than the mass of an electron, or two billion times smaller than the mass of a hydrogen atom. (They were previously thought to have no mass at all.) More precisely, they have three different identities, with three slightly different masses, which they can switch between astonishingly quickly.

The experiment at CERN that’s causing all the fuss was designed to scrutinise the oscillations in neutrinos’ identities. Protons are revved up to enormous speeds in a huge particle accelerator, then slammed into a graphite target to bring about nuclear reactions. Out of that mess come, among other things, streams of neutrinos. For three years, the CERN team has sent those neutrinos from Geneva to a lab in Italy about 450 miles away. Given their tiny but nonzero mass, they should have made the trip ever so slightly slower than the speed of light, getting there in just over 0.002 seconds: 100 times faster than the blink of an eye.

But they seem to have shown up at the Italian laboratory about 60 nanoseconds – 60 billionths of a second, roughly 100 million times quicker than the blink of an eye – earlier than expected. That corresponds to a travelling speed roughly 0.003 per cent faster than the speed of light. According to Einstein’s relativity, faster-than-light means backwards-in-time, which should be impossible. (For some reason physicists tend to illustrate the paradox with gruesome tales of people travelling backwards in time to murder their grandparents, thereby ensuring that they could never be born. Less self-destructively, you could send your forebears – or yourself – a message to get out of the stock market before a major crash.)

We shouldn’t write Einstein off just yet, however. His relativity has passed every single major test in a century. Measurements of speed rely on precise knowledge of both the distance travelled and the time taken for the journey. To attribute the discrepancy in the neutrinos’ arrival time to an anomalous speed requires knowing the distance they have travelled to unstinting accuracy. In this case, the physicists need to know that they can measure a 30-foot distance to an accuracy better than the width of a human hair. Such measurements can be made, but they are not routine. (And it’s possible that even if the results are completely accurate, they may not show that neutrinos can travel faster than light but point to other exotic effects, such as the existence of extra dimensions of space that may provide a short cut for tiny particles whizzing from point A to point B.)

There is another good reason to wait and see. Several hundred thousand years ago, a star in a nearby galaxy exploded. Light from the supernova first became visible on earth in 1987. Physicists at several laboratories detected a surge of neutrinos from the supernova at more or less the same time as they detected light from the explosion. If the neutrinos had been travelling at the rate reported by the CERN team, they would have reached us four years before the light got here. Four years ago the world looked like a different place. Lehman Brothers was still in business; few people had heard of mortgage-backed securities or collateralised debt obligations. If these neutrinos really are travelling faster than light, then I have a few messages to send.


  • 25 September 2011 at 8:54pm
    fenster says:
    two questions

    "For three years, the CERN team has sent those neutrinos from Geneva to a lab in Italy about 450 miles away."

    how are they doing this?

    "We are bathed by trillions of neutrinos every second without even knowing it; "

    how does the Italian Lab know which neutrinos are the CERN ones they are supposed to be gathering?

    • 26 September 2011 at 3:24pm
      cigar says: @ fenster
      Here's the CERN's neutrino project page:

    • 26 September 2011 at 3:35pm
      Norman Gray says: @ fenster
      The neutrinos are generated in a specially-dug tunnel at CERN, lined up with Gran Sasso, under the Alps. Protons are diverted from the Super Proton Synchrotron (once, the king of the road; now a feeder for the LHC) and led into a block of graphite. The collision with the graphite produces other subatomic particles, which in turn promptly decay into neutrinos, which keep going past the end of the tunnel, heading for Italy.

      The Gran Sasso detector does indeed detect neutrinos from the cosmos, too, but since they know with good accuracy what time the ones from CERN are due to arrive, they know which other ones to ignore.

      The separation between the start of the neutrino beam and the detector is 73053461 cm (give or take 20cm). That number would have to be wrong by 18m to explain away the result via a mis-calibration of the length.

      All the gory details are in

  • 25 September 2011 at 9:23pm
    HC says:
    the barman says 'we don't serve neutrinos in here'. a neutrino walks into a bar.

    • 26 September 2011 at 11:39am
      Harry Stopes says: @ HC
      I didn't see that joke coming.

    • 26 September 2011 at 10:41pm
      Bob Beck says: @ Harry Stopes
      Then of course there's the famous Ray Bradbury story. A stray neutrino somehow kills a butterfly. A hundred million years later, Rick Perry is elected President of the United States.

    • 28 September 2011 at 8:45am
      Geoff Roberts says: @ Bob Beck
      Has this all happened tomorrow?

    • 28 September 2011 at 1:39pm
      Bob Beck says: @ Geoff Roberts
      And will happen again yesterday, I expect.

  • 28 September 2011 at 8:48am
    Geoff Roberts says:
    It was really good to read that these neutrinos are whistling down tunnels into Italy, but what do they do when they get there? Was this a plan to help Italy win the World Cup? If so, when do the neutrinos hit New Zealand?

    • 28 September 2011 at 7:05pm
      Bob Beck says: @ Geoff Roberts
      Me, I can't answer that -- alas, I know less about rugby than I do about, say, 1970s prog-rock. And so I can't reject the ghastly possibility that at some obscure TV or radio station, somewhere, one of the other 97 people who remembers the Canadian band Klaatu has dusted off their first LP and, to accompany a jokey version of this story, played the song "Little Neutrino."

    • 29 September 2011 at 7:29am
      Geoff Roberts says: @ Bob Beck
      That's OK, I knew the answer, but I've forgotten it.

  • 17 October 2011 at 7:51pm
    Jon Day says:
    So it was all the fault of GPS:

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