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  1. #1
    Mojibake sprinkles's Avatar
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    Default Speed of light not so constant after all

    Speed of light not so constant after all

    Pulse structure can slow photons, even in a vacuum


    BY
    ANDREW GRANT

    5:00PM, JANUARY 17, 2015




    Light doesn’t always travel at the speed of light. A new experiment reveals that focusing or manipulating the structure of light pulses reduces their speed, even in vacuum conditions.

    A paper reporting the research, posted online at arXiv.org and accepted for publication, describes hard experimental evidence that the speed of light, one of the most important constants in physics, should be thought of as a limit rather than an invariable rate for light zipping through a vacuum.

    “It’s very impressive work,” says Robert Boyd, an optical physicist at the University of Rochester in New York. “It’s the sort of thing that’s so obvious, you wonder why you didn’t think of it first.”

    Researchers led by optical physicist Miles Padgett at the University of Glasgow demonstrated the effect by racing photons that were identical except for their structure. The structured light consistently arrived a tad late. Though the effect is not recognizable in everyday life and in most technological applications, the new research highlights a fundamental and previously unappreciated subtlety in the behavior of light.

    The speed of light in a vacuum, usually denoted c, is a fundamental constant central to much of physics, particularly Einstein’s theory of relativity. While measuring c was once considered an important experimental problem, it is now simply specified to be 299,792,458 meters per second, as the meter itself is defined in terms of light’s vacuum speed. Generally if light is not traveling at c it is because it is moving through a material. For example, light slows down when passing through glass or water.

    Padgett and his team wondered if there were fundamental factors that could change the speed of light in a vacuum. Previous studies had hinted that the structure of light could play a role. Physics textbooks idealize light as plane waves, in which the fronts of each wave move in parallel, much like ocean waves approaching a straight shoreline. But while light can usually be approximated as plane waves, its structure is actually more complicated. For instance, light can converge upon a point after passing through a lens. Lasers can shape light into concentrated or even bull’s-eye–shaped beams.

    The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber. The other photon went through a pair of devices that manipulated the structure of the light and then switched it back. Had structure not mattered, the two photons would have arrived at the same time. But that didn’t happen. Measurements revealed that the structured light consistently arrived several micrometers late per meter of distance traveled.

    “I’m not surprised the effect exists,” Boyd says. “But it’s surprising that the effect is so large and robust.”

    Greg Gbur, an optical physicist at the University of North Carolina at Charlotte, says the findings won’t change the way physicists look at the aura emanating from a lamp or flashlight. But he says the speed corrections could be important for physicists studying extremely short light pulses.
    https://www.sciencenews.org/article/...tant-after-all
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  2. #2
    Senior Member Passacaglia's Avatar
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    Hm, I wonder what they mean by 'structure,' and how they changed it...

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    Quote Originally Posted by Passacaglia View Post
    Hm, I wonder what they mean by 'structure,' and how they changed it...
    The paper is on arxiv. Which is free and open to everyone in the form that is available to scientists Like PLOS also.

    "One sentence summary: The group velocity of light in free space is reduced by
    controlling the transverse spatial structure of the light beam."

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    Senior Member Mal12345's Avatar
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    Quote Originally Posted by ygolo View Post
    The paper is on arxiv. Which is free and open to everyone in the form that is available to scientists Like PLOS also.

    "One sentence summary: The group velocity of light in free space is reduced by
    controlling the transverse spatial structure of the light beam."
    Hmm. I would ask my scientist friend on facebook about this, but I unfriended him for being a communist.
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    Senior Member INTP's Avatar
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    I asked my friend about this because it sounds like a really big news, his explanation after a quick glance about it was this:



    Or not exactly that, thats the basic double slit experiment, but he said that its basically the same thing. You see the time it takes for the photon to travel 1m in open space is shorter than it would take for it to go past the two slits. And what the scientists have done is that they were able to make the photons structure(the same structure than it would have going through the double slit vs going in straight line) last even after it came out from what ever they used to change its structure. Thus the photon was actually moving a "longer distance" due to its structure, even tho the measured distance was the same.

    = they didnt make photon move slower, they just made it move longer distance in same distance, similar to moving 1m long measuring tape in straight line vs 1m long measurement tape in a curved line. Like this:

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    The Typing Tabby grey_beard's Avatar
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    Hmm, are there implications for the apparent age of galaxies subject to gravitational lensing? /out-of-my-field speculation>
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    Mojibake sprinkles's Avatar
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    Quote Originally Posted by grey_beard View Post
    Hmm, are there implications for the apparent age of galaxies subject to gravitational lensing? /out-of-my-field speculation>
    I don't know tons about it either but my intuition says 'probably' - but not enough to make huge differences.

    It probably would make a difference if one day we decide to navigate to a body in another galaxy though. Those small mistakes would add up to huge course corrections.

  8. #8
    Mojibake sprinkles's Avatar
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    Quote Originally Posted by grey_beard View Post
    Hmm, are there implications for the apparent age of galaxies subject to gravitational lensing? /out-of-my-field speculation>
    Also here's a bit of a thing

    Gravitational lensing creates 'Einstein's cross' of distant supernova - physicsworld.com

    Gravitational lensing creates 'Einstein's cross' of distant supernova

    Mar 5, 2015 8 comments
    Multiple images of a supernova created by gravitational lensing have been captured for the first time by an international team of astronomers using the Hubble Space Telescope (HST). The "Einstein cross" pattern comprises four images of a distant supernova created by the gravitational lensing of its light as it passed a distant galaxy within a cluster of galaxies on its way to Earth. In addition to giving us a closer look at the dynamics of distant supernovae, the team says that its discovery will help to improve our understanding of the distribution of dark matter in the lensing galaxy and galaxy cluster, as well as to test Einstein's general theory of relativity and measure the rate of cosmic expansion in the universe.
    A gravitational lens is a large galaxy or group of galaxies that bends or "lenses" light from a distant source as it travels towards an observer. The effect was predicted by Einstein's general theory of relativity and the first such lens was discovered in 1979. Sometimes, the distant light source, lensing galaxy and the observer line up precisely, and we can see an "Einstein ring" – a perfect loop of light from the source encircling the lensing mass. But if there is any misalignment along the way, we observe partial arcs or spots. Depending on the relative positions of the bodies, four such spots can be seen, forming an Einstein cross. The lensing effect serves as a "natural telescope" for astronomers, who can determine the mass of the lensing galaxy and its dark-matter content based on the amount of distortion observed.
    Long search

    "It's a wonderful discovery," says Alex Filippenko of the University of California, Berkeley, who is part of the team that found the latest quadruple-lensed supernova image, explaining that researchers have been "searching for a strongly lensed supernova for 50 years, and now we've found one". Thanks to the many conditions that need to be fulfilled for a gravitational lens to be seen from Earth, and the relatively short lifetime of a supernova, such a lensed supernova with four images has never been seen before.
    Even more interesting, thanks to an understanding of the peculiarities of gravitational lensing, the team already knows that a fifth image will appear in the next decade. This will give astronomers a "replay" of the supernova, because light can take various paths around and through a gravitational lens and therefore arrive at Earth at different times. This is particularly rare and useful, because astronomy is not normally a predictive science. "The longer the pathlength, or the stronger the gravitational field through which the light moves, the greater the time delay," says Filippenko.
    The team used a computer model to predict the pathways that the light from the supernova can take around the lensing cluster, which also suggests that we already missed out on seeing earlier images of the exploding star 10 and 50 years ago. The team has dubbed the distant supernova SN Refsdal (after the late pioneering astrophysicist Sjur Refsdal), and it is located about 9.3 billion light-years away (redshift 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light-years (redshift 0.5) from Earth.
    Multiple replays

    "Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses," says team member Patrick Kelly, also at Berkeley, who discovered the supernova while looking through infrared images taken by the HST last November.
    The galaxy that splits the supernova's light is part of a large cluster – MACS J1149.6+2223 – that was discovered more than 10 years ago. In 2009 astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The more distant galaxy appears in multiple images around the foreground lensing cluster and it hosts the supernova in one of the galaxy's spiral arms. "We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system," explains Kelly.
    Kelly hopes that measuring the time delays between the phases of the supernova in the four images will let them put better constraints on the mass distribution of the foreground galaxies, as well as the expansion and geometry of the universe. If the researchers identify it as a Type Ia supernova (these have relatively standard brightness) by studying its spectrum, they could place even stronger limits on both the matter distribution and cosmological parameters.
    The work is published in Science.

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