Scottish scientist have slowed down light

[dragon]

The speed of light travelling through air has been slowed down for the first time, breaking what was thought to be a constant physical measurement.

A team of physicists at the University of Glasgow sent photons through a mask to change their shape and then raced an altered photon against an unaltered one. Over a distance of one metre the team observed that the altered photon was slowed by up to 20 wavelengths, demonstrating for the first time that light can be slowed in free space.

"The results give us a new way to think about the properties of light," said professor Miles Padgett from the University of Glasgow's optics group. The research was carried out in conjunction with Heriot-Watt University with the findings being published in the journal Science Express.

The speed of light in free space is 186,282 miles per second, which until now was thought to be a constant. While light slows down when passing through water or glass it returns to the speed of light when it comes out the other side. In this experiment the speed of a photon remains slightly slower.

As light behaves both like a wave and a particle it is possible to change the shape of an individual photon as if it were a wave and then race the two particles of light.

To explain how the experiment worked the researchers compared the behaviour of a beam of light to that of a team of cyclists. While the team of cyclists travels at a constant speed, the individual cyclists will all be travelling at their own speeds as they swap positions.

The same applies to a beam of light, which has a number of different velocities. In this analogy the group of cyclists is the beam of light travelling at the speed of light, but each cyclist is an individual photon with its own speed.

"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," said Daniel Giovannini, one of the lead authors of the paper.
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Importantly the experiment is only applicable over short distances. The researchers explained that the effect was biggest when the lenses used to create the beam of light were large and the distance travelled was small. This means our fundamental understanding of how the universe works remains unaltered, but our understanding of light has changed.

link

[Purple]

Say what?!

[Eternal_Freedom]

Yeah, this is so utterly what-the-fuck I have no idea what's going on.

I mean, awesome and everything, but still? What the fuck?

[Darmalus]

Over a distance of one metre the team observed that the altered photon was slowed by up to 20 wavelengths,

That doesn't make any sense as far as I understand those words.

[Eternal_Freedom]

I think it means that whilst the unslowed photon travelled one metre, the slowed phton travelled one metre minus 20 wavelengths.

[Terralthra]

Assuming it was visible light, 20 wavelengths is 7.8-14 μm.

[SpottedKitty]

Might not seem like much, but (if verified and duplicated) it's as mindboggling and shouldn't-be-happening as all the molecules of the tea in the cup on my desk here all deciding to vibrate upwards at the same time.

<splash>

[Napoleon the Clown]

So... large lens, small distance covered? I'd be interested in a test that makes sure the light's travel-time is measured based on from when it leaves the lens and arrives at its destination, rather than including time spent in the lens. And making sure that all the equipment is in proper working order. I'm sure we all remember the "neutrinos clocked going faster than light!" story that turned out to be incorrect thanks to a loose wire.

Until this gets repeated by somebody else, I'm going to have as much faith in it as I did the FTL neutrino story. Meaning, it's possible the result is accurate, but I wouldn't put any amount of money on it, regardless of how much I could get back.

[General Zod]

How is running it through a mask different from running it through a medium? I'm not really clear on that.

[loomer]

The excitement is that it stayed slowed after exiting the medium.

[Feil]

Popular science journalism contains as much falsehood as truth. Fortunately, this news article contains a link to the actual journal article. Usually I have to look. Unfortunately, this particular journal article is a Science Express pre-publication, which most libraries (including mine) won't carry until it hits the print version of Science in several months.

Fortunately, the abstract is information-dense. Reading an abstract is a learned skill and it doesn't define technical terms, so I'll go through it briefly. Please bear in mind that I am not an expert and my reading may be flawed.

That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities.

Stating background facts. The phase velocity of a wave is how its phase (its shape, the relative positioning of its peaks and troughs) moves. The group velocity of a wave is how the overall wave packet (the collection of peaks and troughs that form one complete link in the repeating pattern of the full wave) moves. Note again, this is background information that anybody in their field would already have known.

We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam’s transverse spatial structure.

Stating the purpose of the experiment. Transverse spatial structure being the dynamic "shape" of the wave - i.e. the relative positioning of peaks and troughs in the wave packet, over time.

Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several micrometers over a propagation distance of the order of 1 m.

Stating the method and results. Bessel beam and Gaussian beam being important presumably because they create patterns on a target that allow for easy indirect measurement of their arrival time, since one can't very well measure femtosecond differences with a clock.

Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.

Explaining the results. Note that the choice of language suggests that they are adding to the body of evidence supporting an already-accepted, but insufficiently confirmed and insufficiently well-known, theoretical prediction, not (as the WIRED article would have you believe) making a revolutionary claim. Also note the mention of plane waves. Across distances that are long relative to their wavelength, all waves approximate plane waves, so what they're describing is a strictly local phenomenon.

Worth pointing out, since several people have expressed disbelief: The authors include well-published experts in related fields (I checked). Science is a very respected journal, and it didn't get that way by publishing articles with unsubstantiated conclusions. The journal article (although not the news site article) should be reliable.

[Kuroneko]

Take a cone and superpose all plane waves that travel in directions perpendicular to the surface of said cone, all with same amplitude and phase. The result is a Bessel beam, which is axially symmetric and propagates along the axis of the original cone. Since it's formed by superposition of plane waves that travel off-axis, it's sensible that its speed along the axis is slower than those of the original plane waves. The peaks and troughs of the wave form a concentric pattern that spreads outward, except for the central one, so

It's interesting, but I don't think their point about invariance is a good one because even the full Bessel beam satisfies the usual Lorentz-invariant wave equation with speed c.

[jwl]

Was that article a really long-winded way of explaining the difference between a phase velocity and a group velocity?

[cadbrowser]

The speed of light travelling through air has been slowed down for the first time, breaking what was thought to be a constant physical measurement.

The speed of light in free space is 186,282 miles per second, which until now was thought to be a constant.

While light slows down when passing through water or glass it returns to the speed of light when it comes out the other side.

In this experiment the speed of a photon remains slightly slower.

The researchers explained that the effect was biggest when the lenses used to create the beam of light were large and the distance travelled was small.

Is it just me or does all of this sound goofy?

How does the researchers observing a photon slowing down in air change the speed of light constant in a vacuum (free space)?

I always understood that photons had different (slower than c) speeds in anything other than a vacuum, was I mistaken?

What is the given distance after a photon passing through water/glass does it require to regain it's full speed potential?

[jwl]

Is it just me or does all of this sound goofy?

How does the researchers observing a photon slowing down in air change the speed of light constant in a vacuum (free space)?

I always understood that photons had different (slower than c) speeds in anything other than a vacuum, was I mistaken?

What is the given distance after a photon passing through water/glass does it require to regain it's full speed potential?

I've just clicked on the article:

An earlier version of this article incorrectly defined free space as air.

Problem solved.





Anyway, here's the article about it on the University of Glasgow website:
http://www.gla.ac.uk/news/headline_388852_en.html

Scientists have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass.

BBC News: Scientists slow the speed of light
New Scientist: Shapely photons break rules to fly slower than light
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.

Their experiment was configured like a race, with two photons released simultaneously across identical distances towards a defined finish line.

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.

Slow light‌The 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 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.

And here is the arxiv preprint of the paper:
http://arxiv.org/ftp/arxiv/papers/1411/1411.3987.pdf

Abstract: That the speed of light in free space is constant is a cornerstone of modern
physics. However, light beams have finite transverse size, which leads to a modification
of their wavevectors resulting in a change to their phase and group velocities. We study
the group velocity of single photons by measuring a change in their arrival time that
results from changing the beam’s transverse spatial structure. Using time-correlated
photon pairs we show a reduction of the group velocity of photons in both a Bessel beam
and photons in a focused Gaussian beam. In both cases, the delay is several microns over
a propagation distance of the order of 1 m. Our work highlights that, even in free space,
the invariance of the speed of light only applies to plane waves. Introducing spatial
structure to an optical beam, even for a single photon, reduces the group velocity of the
light by a readily measurable amount.

[jwl]

One interesting bit about the paper omitted in the newspaper article and to an extent the university of glasgow website is that this is not as new as their tone leads us to believe. From the paper preprint:

Even in the absence of a medium, the modification of the speed of light has previously been known. For example, within a hollow waveguide, the wavevector along the guide is reduced below the free-space value, leading to a phase velocity vphi greater than c. Within the hollow waveguide, the product of the phase and group velocities is given as vphivg,z = c2, thereby resulting in a group velocity vg,z along the waveguide less than c.

Also, here's how they measured the timings:

Measuring the arrival time of single photons with femtosecond precision is challenging. Consequently, we adopt a method relying upon a quantum effect, namely, the Hong-Ou-Mandel (HOM) interference. We use a parametric down-conversion source to produce photon pairs that are very strongly correlated in their wavelengths and their generation time. One photon can then act as a reference, against which the arrival of the other photon can be compared. When the arrival times of the two photons incident on a beam splitter are matched to a precision better than their coherence time, both photons emerge from the same output port. Under this matched condition, the coincidence rate for detection at the two output ports of the beam splitter falls to zero, which results in what is known as a Hong-Ou-Mandel dip.

[SpottedKitty]

What is the given distance after a photon passing through water/glass does it require to regain it's full speed potential?

Doesn't work that way. It's not like the difference between you walking through chest-deep water and walking on solid ground, where your muscles have to adjust the way you walk, allowing you to gradually move faster. The speed of a photon is determined by the material it's passing through at the moment you measure its speed — the answer to your question is "none", it will instantly change speed when it moves from one material to another.

[cadbrowser]

I've just clicked on the article:

An earlier version of this article incorrectly defined free space as air.

Problem solved.

Ah, indeed it is then.

What is the given distance after a photon passing through water/glass does it require to regain it's full speed potential?

Doesn't work that way. It's not like the difference between you walking through chest-deep water and walking on solid ground, where your muscles have to adjust the way you walk, allowing you to gradually move faster. The speed of a photon is determined by the material it's passing through at the moment you measure its speed — the answer to your question is "none", it will instantly change speed when it moves from one material to another.

I honestly didn't think it really did, but it has been so long since I did any research or "freshening up" on photons I just wanted to make sure. Like jwl pointed out, what really messed me up was their error. Thank you so much for the follow up.