bron:->Scientists slow the speed of light
By Kenneth Macdonald
BBC Scotland Science Correspondent
A team of Scottish scientists has made light travel slower than the speed of light.
They sent photons - individual particles of light - through a special mask. It changed the photons' shape - and slowed them to less than light speed.
The photons remained travelling at the lower speed even when they returned to free space.
The experiment is likely to alter how science looks at light.
The collaborators - from Glasgow and Heriot-Watt universities - are members of the Scottish Universities Physics Alliance. They have published their results in the journal Science Express.
The speed of light is regarded as an absolute. It is 186,282 miles per second in free space.
Light propagates more slowly when passing through materials like water or glass but goes back to its higher velocity as soon as it returns to free space again.
Or at least it did until now.
Two and a half years ago, the experimenters set out to see if they could slow down light just a little - and keep it moving more slowly.
In a laboratory at Glasgow university, Dr Jacquiline Romero, Dr Daniel Giovannini and colleagues built what amounts to a racetrack for photons, the individual particles of light.
Then they raced them in pairs. One photon they left in its normal state. The other photon was sent through a special mask.
The mask forced the photon to change its shape and travel slower than the speed of light.
Dr Romero explains: "After the mask, the photon is launched into a sort of racetrack about a metre in length.
"Then we take the time in which the unshaped photon finishes the racetrack, and the shaped photon's time as well, and then compare the two times."
If they had both been travelling at the speed of light it would have been a dead heat. But the re-shaped photon came in second.
Not by much - a few millionths of a metre - but it showed that it had not just been slowed by the mask, but had continued to travel at less than light speed even after it had returned to free space.
Light travelling at less than the speed of light. Whose bright idea was that?
It grew from a conversation between Prof Daniele Faccio at Heriot-Watt University and Prof Miles Padgett at Glasgow.
Prof Padgett says the crucial component is the mask - a software controlled liquid crystal device: "That mask looks a little bit like a bull's-eye target.
"And that mask patterns the light beam, and we show that it's the patterning of the light beam that slows it down.
"But once that pattern has been imposed - even now the light is no longer in the mask, it's just propagating in free space - the speed is still slow."
But hang on a minute. If a photon is a particle, how is it possible to impose a pattern on it?
It's because photons exist in the exotic and rather wonderful quantum realm, where the rules of the reassuringly solid world in which we live tend to lose their grip.
They exhibit what physicists call "wave-particle duality": they behave like both a wave and a particle. So you can send them round a racetrack two by two like particles, yet change the shape of one of them as if it was a wave.
Complicated? Oh yes. Which is why the researchers say it might help to think of a bicycle race.
The peloton - the main bunch of riders - may be moving at a constant speed. But within the bunch an individual rider may be moving more slowly, dropping back for a rest or a drink.
Meanwhile other riders in the bunch are moving faster to get to the front.
The bunch is a beam of light, travelling at - yes - the speed of light. The riders are photons, travelling at their individual speeds.
For Dr Giovannini it's been a satisfying intellectual and experimental challenge: "It mostly comes from a question we asked ourselves two and a half years ago. We just kept working on it.
"It's really, really interesting. It's just one of those big, fundamental questions you may want to ask yourself at some point in the pub one night.
"And if you follow through and you actually measure it it's quite amazing, isn't it?"
Prof Padgett says: "What makes our experiment different, and what has brought clarity to this, is that rather than looking at a light pulse which contains many, many, many photons we've reduced the experiment down to a single photon.
"So we measure the speed of a single photon as it propagates.
"And we find it's actually being slowed below the speed of light."
There are some practical implications. Light is used to make extremely precise measurements such as how far the Moon is from Earth.
The good news is that we are not in for any nasty surprises on that scale. But researchers using large aperture lenses to accurately measure very short distances may be forced to take a second look for tardy photons.
Beyond that, Dr Giovannini says practical, everyday uses for the discovery are possible. Although he concedes the physics is more fundamental than applied right now.
"But," he says, "who knows?"
Hier tref je allerlei QFF materiaal aan uit de afgelopen jaren QFF.
Tja het is u overal breeQend...
via:-> http://www.sciencedaily.com/releases/20 ... 144158.htmScientists slow down the speed of light travelling in free space
Scentists have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass.
However, it has generally been thought impossible for particles of light, known as photons, to be slowed as they travel through free space, unimpeded by interactions with any materials.
In a new paper published in Science Express today (Friday 23 January), researchers from the University of Glasgow and Heriot-Watt University describe how they have managed to slow photons in free space for the first time. They have demonstrated that applying a mask to an optical beam to give photons a spatial structure can reduce their speed.
The team compare a beam of light, containing many photons, to a team of cyclists who share the work by taking it in turns to cycle at the front. Although the group travels along the road as a unit, the speed of individual cyclists can vary as they swap position.
The group formation can make it difficult to define a single velocity for all cyclists, and the same applies to light. A single pulse of light contains many photons, and scientists know that light pulses are characterised by a number of different velocities.
The team's experiment was configured like a time trial race, with two photons released simultaneously across identical distances towards a defined finish line. The researchers found that one photon reached the finish line as predicted, but the structured photon which had been reshaped by the mask arrived later, meaning it was travelling more slowly in free space. Over a distance of one metre, the team measured a slowing of up to 20 wavelengths, many times greater than the measurement precision.
The work demonstrates that, after passing the light beam through a mask, photons move more slowly through space. Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material -- it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.
The work was carried out by a team from the University of Glasgow's Optics Group, led by Professor Miles Padgett, working with theoretical physicists led by Stephen Barnett, and in partnership with Daniele Faccio from Heriot-Watt University.
Daniel Giovannini, one of the lead authors of the paper, said: "The delay we've introduced to the structured beam is small, measured at several micrometres over a propagation distance of one metre, but it is significant. We've measured similar effects in two different types of beams known as Bessel beams and Gaussian beams."
Co-lead author Jacquiline Romero said: "We've achieved this slowing effect with some subtle but widely-known optical principles. This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum.
"Although we measure the effect for a single photon, it applies to bright light beams too. The effect is biggest when the lenses used to create the beam are large and when the distance over which the light is focused is small, meaning the effect only applies at short range."
Professor Padgett added: "It might seem surprising that light can be made to travel more slowly like this, but the effect has a solid theoretical foundation and we're confident that our observations are correct.
"The results give us a new way to think about the properties of light and we're keen to continue exploring the potential of this discovery in future applications. We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example."
The team's paper, titled 'Spatially Structured Photons that Travel in Free Space Slower than the Speed of Light', is published in Science Express, which provides electronic publication of selected papers in advance of print in the journal Science.
via:->> http://www.popsci.com/researchers-who-f ... t-it-wrongEVIDENCE OF EXPANDING UNIVERSE MAY HAVE BEEN JUST DIRT
SUPPORT FOR COSMIC INFLATION JUST DEFLATED A BIT
By Loren Grush Posted January 30, 2015
The BICEP2 telescope at twilight
It was a huge win for Big Bang supporters when, in March 2014, a team of astronomers claimed they had found direct evidence to support the concept of cosmic inflation—the super-rapid expansion of the Universe that occurred just fractions of a second after it exploded into existence. The discovery was monumental, completely altering our perception of the early moments of space and time. “This is huge, as big as it gets,” Marc Kamionkowski, a theoretical physicist at Johns Hopkins University who was not part of the team, told the New York Times.
But alas, sometimes these things are just too good to be true. In a new paper (that leaked a week early), the same researchers who made the momentous discovery say that they were probably mistaken about their findings. It turns out that cosmic dust within our own galaxy muddled the signals detected by their instrument BICEP2, a huge telescope near the South Pole. So the primordial signatures they thought they had found were actually just a bunch of space trash.
For years, the researchers, from the Harvard-Smithsonian Center for Astrophysics, had been looking for unique ripples in space-time. A physicist named Alan Guth theorized in 1979 that if the Big Bang and cosmic inflation did indeed occur, then the event would have created phenomena called gravitational waves—traces of the Universe aggressively bursting into being. Up until last year, the ripples were only a hypothetical concept, as no evidence had been found to support their existence. Then, the Harvard-Smithsonian team claimed they had found signatures of these waves in the sky, creating some major metaphorical waves among the early-Universe community.
The swirl patterns captured by BICEP2
Yet it seems the team didn’t account for similar-looking patterns. BICEP2 aims to measure light coming from the Cosmic Microwave Background (CMB)—extremely old radiation that was created shortly after the Big Bang. Specifically, it looks for swirls in the light’s polarization, and BICEP2 stumbled upon certain swirl patterns that the researchers thought were direct imprints of gravitational waves.
The only problem? Spinning grains of dust in the foreground of the galaxy can produce the exact same swirl patterns, something the team failed to properly account for. After the researchers published their findings, the Planck space telescope made a much more detailed map of the intergalactic dust circulating in the area of the sky BICEP2 observed, revealing that the results may have been just dust in the wind.