HK physicists prove single photons do not exceed the speed of light

Discussion in 'Members Corner' started by Vyom, Jul 24, 2011.

  1. Vyom

    Vyom Seeker Elite Member

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    Hong Kong physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light -- demonstrating that outside science fiction, time travel is impossible.

    A group of physicists at the Hong Kong University of Science and Technology (HKUST) led by Prof Shengwang Du reported the direct observation of optical precursor of a single photon and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon.

    Prof Shengwang Du, Assistant Professor in HKUST's Department of Physics, and his research team have published their study in Physical Review Letters recently. Co-authors include three postgraduate students Shanchao Zhang, Jiefei Chen and Chang Liu, Chair Professors Michael M T Loy and George K L Wong. The paper was selected as editors' suggestion for reading. This research was also highlighted as a Physics Synopsis by American Physical Society with a title "Single photons obey the speed limits."

    Prof Du's study demonstrates that a single photon, the fundamental quanta of light, also obeys the traffic law of the universe just like classical EM waves. Einstein claimed that the speed of light was the traffic law of the universe or in simple language, nothing can travel faster than light. HKUST's team is the first to experimentally show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon wave packet even in a so called 'superluminal' medium.

    "The results add to our understanding of how a single photon moves. They also confirm the upper bound on how fast information travels with light," said Prof Du. "By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon. Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."
    HKUST President Tony F Chan said, "We are most delighted that Prof Shengwang Du and his research group have confirmed a key feature of a fundamental law of physics, which also has important implications for communication technology. It epitomizes the mission of our university - to both produce fundamental knowledge and technological impact."

    Discovery of superluminal propagation of optical pulses in some specific medium 10 years ago has evoked the world's dream of time travel, but later scientists realized that it is only a visual effect where the superluminal 'group' velocity of many photons could not be used for transmitting any real information. Then people set their hope on single photons because in the strange quantum world nothing seems impossible -- a single photon may be possible to travel faster than the speed limit in the classical world. Because of lack of experimental evidence of single photon velocity, this is also an open debate among physicists. To tackle the problem, Prof Du's team measured the ultimate speed of a single photon with controllable waveforms. The study, which showed that single photons also obey the speed limit c, confirms Einstein's causality; that is, an effect cannot occur before its cause.

    HKUST's team used a demonstration which required not only producing single photons, but separating the optical precursor, which is the wave-like propagation at the front of an optical pulse, from the rest of the photon wave packet. To do so, Prof Du's team generated a pair of photons, and then passed one of them through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency (EIT). For the first time, they successfully observed optical precursors of a single photon.

    The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum. The main wave packet of the single photon travels no faster than the speed of light in vacuum in any dispersive medium, and can be delayed up to 500 nanoseconds in a slow light medium. Even in a superluminal medium where the group velocity (of an optical pulse peak) is faster than the speed of light in vacuum, the main part of the single photon has no possibility to travel faster than its precursor.

    HK physicists prove single photons do not exceed the speed of light
     
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  3. Vyom

    Vyom Seeker Elite Member

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    People would find it intense that while light sets the maximum speed for everything, two photons of equal but opposite angular momentum (produced out of a single burst of a subatomic particle) "communicate" with each instantly no matter how far they are in the universe, even if on opposite corners of the universe, thus baffling the physicists with the claim that information cannot travel faster than light.
     
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  4. Vyom

    Vyom Seeker Elite Member

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    Here is another related article. While the first tries to enforce the Standard Model of particle physics, this one dooms it.

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    Asymmetric quarks defy standard model of physics

    Newly released observations of the top quark — the heaviest of all known fundamental particles — could topple the standard model of particle physics. Data from collisions at the Tevatron particle accelerator at Fermilab in Batavia, Illinois, hint that some of the top quark's interactions are governed by an as-yet unknown force, communicated by a hypothetical particle called the top gluon. The standard model does not allow for such a force or particle.

    The results, presented1 today at the Europhysics Conference on High-Energy Physics in Grenoble, France, could help researchers to understand the origins of mass. According to one theoretical interpretation, a top quark bound by to its anti-matter partner, the antitop, would act as a version of the elusive Higgs boson, conferring mass on other particles.

    Regina Demina, a physicist at the University of Rochester in New York, and her colleagues sifted through eight years' worth of particle-collision data recorded by one of the Tevatron's two detectors, known as DZero. Top quarks produced during collisions can fly off in the direction of the accelerator's proton beam or its antiproton beam; Demina and her team discovered that more travel towards the proton beam than is predicted in the standard model of physics. A different model would seem to be needed to explain the discrepancy.

    Paired particles

    One possible model has been suggested by Christopher Hill, a theorist at Fermilab who 20 years ago proposed how a top quark and its antiparticle could impart mass to the W and Z bosons, particles that carry the weak nuclear force responsible for radioactive decay. The work, updated in 20032, draws heavily on an analogy with some types of low-temperature superconductors, materials that have no electrical resistance at temperatures just a few degrees above absolute zero.

    In some superconductors, electrons pair up, bound by particle-like vibrations in the material. The bound electrons limit the range over which the electromagnetic force can act within the material, an effect that in turn imparts an effective mass to nearby photons — particles of light, which carry the long-range electromagnetic force and are normally weightless.

    In a similar way, Hill suggests, top quarks and anti-top quarks might pair up throughout the cosmos, bound by a force carried by an as-yet undiscovered particle dubbed the top gluon. "It's as if the entire universe was a special kind of superconductor," says physicist Matthew Schwartz of Harvard University in Cambridge, Massachusetts. The theory explains the origin of mass throughout the universe as a team effort, First, the top gluon would act to make both the top quark and the antitop heavy, just like the force binding electrons in a superconductor makes nearby photons heavy. Then, the top–anti-top pair would itself explain the origin of mass throughout the rest of the universe, conferring mass, for instance, on the W and Z bosons, the carriers of the weak nuclear force. The relatively heavy mass acquired by the W and Z particles limits the range of the weak force, breaking the symmetry between this force and the long-range electromagnetic force that theorists believe exists at very high energies.

    In a study posted online3 on 16 June, Schwartz and his Harvard colleagues show that Hill's model could also account for the top-quark asymmetry observed at the Tevatron. The details have to do with the way the up quark, a component of the proton, couples with the top quark in the new theory.

    Independent confirmation

    The asymmetry observed at DZero is not certain enough to constitute proof of the existence of the top gluon, but it does independently match findings reported4 earlier this year by researchers at the Tevatron's other detector, the Collider Detector at Fermilab (CDF).

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    Dmitri Denisov, a spokesman for the DZero experiment, agrees that the results are similar to the directional preference of the top quark seen with CDF. He cautions, however, that the standard model of particle physics is so complicated that it is difficult to accurately describe with equations. The observed top-quark asymmetry is being compared to an imperfect surrogate for the true standard model, so the supposed discrepancy might fall within the uncertainty of the model.

    Schwartz's theory is easily testable. The top gluon has a predicted energy within the current range of the world's most powerful particle collider — the Large Hadron Collider (LHC) near Geneva, Switzerland — so it could be found within a year, says Schwartz.

    A research team working with the LHC's Compact Muon Solenoid detector reported5 on 21 July that they see no evidence of the top-quark asymmetry. But Schwartz notes that the asymmetry is much harder to see at the LHC than at the Tevatron, because the LHC starts with an intrinsically symmetrical setup — smashing a proton beam into another proton beam — so it's more difficult to discern if the top quark has a directional preference at the LHC than at the Tevatron. "I suspect that you can't rule out anything with this data," he says, "and it doesn't negate any models."

    Dan Hooper, a theoretical physicist at Fermilab, notes that the top-quark asymmetry is just one of many cracks in the standard model of particle physics. And although Schwartz agrees that it is unlikely that any one theory will explain all the defects, he says that accounting for the odd behaviour of the top quark would be a promising start.

    Asymmetric quarks defy standard model of physics
     
  5. Godless-Kafir

    Godless-Kafir DFI Buddha Senior Member

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    Time travel means to reorganize the entire sphere of matter the whole planet contains to an "date" of our choosing! We want to reorganize the whole structure of the planet to say when your 15years old, atom by atom and there are countless trillions of atoms, then that is impossible.

    Even the existence of other dimensions separated from each other is questionable, because the idea of separation only exists as long as your awake and thinking. The only thing that says your separate is your thought. Other wise everything is one, the void and matter are not two things but the expression of one.

    Moreover If there was time travel wont people from the future already be here! lol..
     
  6. Vyom

    Vyom Seeker Elite Member

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    That is an interesting point. If we go through countless cycles of birth and rebirth, what preserves our identity?
     

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