The ICARUS experiment is built inside a large underground tunnel at Italy's Gran Sasso National Laboratory.
updated 3/16/2012 12:37:27 PM ET 2012-03-16T16:37:27

European researchers said Friday they have measured the speed of neutrinos and found that the subatomic particles don't travel faster than light after all, refuting another team's measurements that prompted widespread disbelief among scientists last year.

Scientists with the rival OPERA experiment said in September that their tests appeared to show neutrinos speeding faster than light, a feat that goes against Albert Einstein's special theory of relativity which underlies much of modern physics.

Nobel Prize winning physicist Carlo Rubbia said his team, called ICARUS, used a similar experiment to trap neutrinos fired from the European Organization for Nuclear Research, or CERN, in Switzerland to a detector hundreds of miles (kilometers) away in Italy.

"It's a perfectly straightforward experiment, very clean," Rubbia told The Associated Press in a telephone interview. "The results are very convincing, and they tell us essentially that there was something not quite right with the results of OPERA."

Doubts about original experiment
Einstein's famous theory of relativity — a pillar of modern physics — says the speed of light in a vacuum, approximately 186,282 miles per second (299,792 kilometers per second), is the ultimate speed limit and that nothing in the universe can travel faster.

That speed factors into everything from estimates about the size and age of the universe to the radius of black holes to the power generated by nuclear reactors.

Doubts about the OPERA results were heightened last month when it was announced that researchers had found a flaw in the technical setup that could have distorted the experiment's figures.

Antonio Ereditato, a member of the OPERA team and the head of the Albert Einstein Center for Fundamental Physics in Bern, Switzerland, said he welcomed the latest results.

"These results are in line with our recent findings about the possible misfunctioning of some of the components of our experimental setup," he told the AP in an email.

Asked whether he was disappointed that the prospect of breaking light speed likely remains in the realm of science fiction, he said: "This is the way science goes. What matters is the global progress of scientific knowledge."

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Results from trial run
The ICARUS team's results came from a trial run for a longer experiment planned to take place in April or May. OPERA, too, will repeat their experiment, this time with the technical glitches ironed out.

"The evidence is beginning to point toward the OPERA result being an artifact of the measurement," CERN Research Director Sergio Bertolucci said in a statement.

"Whatever the result, the OPERA experiment has behaved with perfect scientific integrity in opening their measurement to broad scrutiny, and inviting independent measurements."

The ICARUS team confirmed that, as Einstein predicted, neutrinos travel at the speed of light.

"I'm not displeased that Einstein was right again," said Rubbia.

Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Explainer: What’s a hadron? Tour the particle zoo

  • Image: Elementary particles

    The Standard Model of particle physics is one of science's most successful theories, enabling the development of devices ranging from light bulbs, to microwave ovens and television, to quantum computing devices. The Standard Model is also one of the oddest theories, because it lays out a dizzying menagerie of hundreds of subatomic particles. At its heart are 16 types of elementary particles ... plus at least one more mysterious particle that scientists are spending billions of dollars to detect.

    Click on "Next" to get the full rundown.

  • Quarks

    Image: Quarks and gluons
    Berkeley Lab

    Six "flavors" of quarks have been detected: up and down, charm and strange, top and bottom. Quarks are almost always found in different combinations, bound together by gluons (more on those later). Particles built up from quarks and gluons are called hadrons. The Large Hadron Collider is so named because it's a large collider that smashes hadrons together.

    Three-quark combinations fit in the category of baryons. The best-known baryons are the proton (with two up quarks and one down quark) and the neutron (with two down quarks and one up quark).

    Particles that have one quark and one antiquark fit in the category of mesons. For example, the pion, or pi meson, contains an up quark and an anti-down quark.

  • Leptons

    Image: Single electrons in helium
    Brown University

    Six "flavors" of leptons have been detected: The negatively charged electron is the best-known lepton — along with its antimatter counterpart, the positron. This photo shows the path of single electrons passing through liquid helium, in an experiment devised by Brown University researchers.

    The muon is also negatively charged, but it's about 207 times as massive as the electron. ("Who ordered that?" physicist Isidor Rabi reportedly asked.) The negatively charged tau particle is even bigger — 3,477 times as massive as the electron — but it decays into other particles in less than a trillionth of a second.

    Each of those leptons has a neutrino associated with it: the electron neutrino, the muon neutrino and the tau neutrino. Neutrinos interact only weakly with other particles, and they zip through our planet virtually without a trace. Physicists only recently determined that they have mass, but there's still a great deal of mystery surrounding the ghostly particles.

  • Force carriers

    Image: Graviton

    The Standard Model sets aside a category for particles that are associated with force fields. The effect of a field can be viewed as involving an exchange of such force-carrying particles.

    Four elementary force-carrying particles have been detected. The best-known force carrier is the photon — which plays a part in the electromagnetic spectrum, including visible light. The gluon binds quarks together through the strong nuclear force. The weak nuclear force involves the exchange of W and Z bosons. The W boson can carry a positive or a negative charge, while the Z boson is neutral.

    If gravity could be incorporated into the Standard Model, the force-carrying particle would be called the graviton (shown here in an artist's depiction). However, gravitons have not yet been detected, and at least for now, such particles are not accounted for in the Standard Model.

  • Bosons vs. fermions

    Image: Bosons vs. fermions
    Rice Univ. via AIP

    All force-carrying particles are bosons, but not all bosons are force carriers. The difference has to do with a property known as particle spin. Particles with a fractional spin value (for example, electrons, protons and neutrons) are fermions. Two identical fermions cannot occupy the same quantum state. This is a property that keeps electrons from collapsing into a jumble, and thus makes chemical reactions possible.

    All particles with a whole-integer spin value are classified as bosons, and such particles can occupy the same quantum state even if they're identical. The photon is the best-known type of boson.

    Even atoms can be classified as fermions and bosons. This photo shows how atom clouds of lithium-7 (bosons) and lithium-6 (fermions) behave at low temperatures. The bosons collapse into a compact cloud, while the fermions can't squeeze that closely together.

  • The mysterious Higgs

    Image: Higgs as seen by CMS
    Ianna Osborne / CERN / CMS Collection

    The Higgs boson is the only particle predicted by the Standard Model that has not yet been detected. The Higgs is the main quarry for physicists at the Large Hadron Collider. This image is a simulation of the Higgs' signature as it might appear in one of the LHC's detectors.

    The Higgs boson, named after Scottish theorist Peter Higgs, is thought to be associated with a field that endows some particles (such as the weak nuclear force's W and Z bosons) with mass, while leaving the electromagnetic force's photons without mass.

    This Higgs field may have played a role at the very beginnings of the universe: Physicists believe that at the highest energies, the electromagnetic and weak nuclear forces were unified, but something led to "electroweak symmetry breaking" as the infant cosmos cooled. That would be why the electromagnetic force and the weak nuclear force are distinct in the current universe. The Large Hadron Collider could shed new light on this mysterious Higgs mechanism.

  • Why so complicated?

    Image: Particle zoo
    Tim Jones / McDonald Observatory / HETDEX

    Hadrons and leptons? Baryons and mesons? Fermions and bosons? Sometimes it seems as if particle physicists set up these classifications just to keep outsiders totally confused. But for researchers, these occasionally overlapping categories are useful for figuring out how different types of particles interact with each other.

    In a sense, it's as if we've been talking about the game of chess but have gotten only to the point of naming the different pieces on the board: black pieces and white ones, pawns and knights, bishops and rooks, kings and queens. The real meaning of the game comes out when you start studying how the pieces perform and interact.

    To delve into the deeper meaning of the Standard Model, you can visit The Particle Adventure at Lawrence Berkeley National Laboratory, "A Subatomic Venture" at CERN, or Particle Physics UK.


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