UGlasgow: Speed of light slowed down through FREE SPACE

[MargaritaMc]

University of Glasgow:
Scientists slow down the speed of light travelling through air
Issued: Fri, 23 Jan 2015 00:01:00 GMT

Scientists 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 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.

Science Express:
Spatially structured photons that travel in free space slower than the speed of light
Daniel Giovannini, Jacquiline Romero et al
Science DOI: 10.1126/science.aaa3035

http://arxiv.org/abs/1411.3987
Photons that travel in free space slower than the speed of light
Daniel Giovannini, Jacquiline Romero, Vaclav Potocek, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, Miles J. Padgett
(Submitted on 14 Nov 2014)

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.

Margarita

[Chris Peterson]

University of Glasgow:
Scientists slow down the speed of light travelling through air
Issued: Fri, 23 Jan 2015 00:01:00 GMT

Scientists 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.

Terrible headline. Light certainly travels slower than c in air, which is neither a new discovery nor anything at all mysterious. But apparently the actual work is unrelated to the headline.

I always try to emphasize that the speed of light is not a constant. There is an important universal constant, c, which shows up in many places. Under ideal conditions, particles with a rest mass of zero travel at c. It is the limit of the fastest speed we can accelerate anything to, and is the fastest speed we can transmit information. But it really isn't the speed of light.

[MargaritaMc]

University of Glasgow:
Scientists slow down the speed of light travelling through air
Issued: Fri, 23 Jan 2015 00:01:00 GMT

Terrible headline.

I agree, Chris. Press release writers again, perhaps? I've altered the thread subject line to reflect the title of the paper in Science Express, but not really adequately.

M

[Speculator]

University of Glasgow:
Scientists slow down the speed of light travelling through air
Issued: Fri, 23 Jan 2015 00:01:00 GMT

Scientists 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.

Terrible headline. Light certainly travels slower than c in air, which is neither a new discovery nor anything at all mysterious. But apparently the actual work is unrelated to the headline.

I always try to emphasize that the speed of light is not a constant. There is an important universal constant, c, which shows up in many places. Under ideal conditions, particles with a rest mass of zero travel at c. It is the limit of the fastest speed we can accelerate anything to, and is the fastest speed we can transmit information. But it really isn't the speed of light.

Your statement, Chris, " It is the limit of the fastest speed we can accelerate anything to, and is the fastest speed we can transmit information" should be amended to 'at this point in time." At one time 'c' was considered an inviolable law, but that too was 'at that point in time.'

[Chris Peterson]

Your statement, Chris, " It is the limit of the fastest speed we can accelerate anything to, and is the fastest speed we can transmit information" should be amended to 'at this point in time." At one time 'c' was considered an inviolable law, but that too was 'at that point in time.'

We have no reason to believe that anything can be accelerated to or past c. We have no reason to think that c isn't the speed limit for information transfer. And we have very good reasons, based on both theory and experiment, to believe these are true.

I feel no need to say "at this point in time". That would be misleading, and possibly lead people to think that our understanding on this matter is likely to change. It isn't.

Not sure what you mean with your last reference to c. Nothing has changed in a very long time with respect to our understanding of the limit that c represents.

[Speculator]

Your statement, Chris, " It is the limit of the fastest speed we can accelerate anything to, and is the fastest speed we can transmit information" should be amended to 'at this point in time." At one time 'c' was considered an inviolable law, but that too was 'at that point in time.'

We have no reason to believe that anything can be accelerated to or past c. We have no reason to think that c isn't the speed limit for information transfer. And we have very good reasons, based on both theory and experiment, to believe these are true.

I feel no need to say "at this point in time". That would be misleading, and possibly lead people to think that our understanding on this matter is likely to change. It isn't.

Not sure what you mean with your last reference to c. Nothing has changed in a very long time with respect to our understanding of the limit that c represents.

Light has, as you know, been 'accelerated' faster than the speed of light. I notice you are careful in the wording of one statement, "...possibly lead people to think that our understanding on this matter is likely to change, it isn't. ("is likely" being the key, leaving option for change open.) However, that statement contradicts your statement that "at this point in time" would be "misleading" which means the understanding won't change. You obviously don't want to believe your understanding will change, yet you are open to that change, slightly. Your openness is commendable, and you would benefit by enlarging that openness to change.

[Chris Peterson]

Light has, as you know, been 'accelerated' faster than the speed of light.

No, it has not. First of all, light is made of photons, which have zero rest mass. So they are not accelerated at all. In a vacuum, they simply exist moving at c. In any other medium, they travel slower than c. Do not confuse phase velocities and group velocities with the actual motion of photons, or with the speed that information can be transmitted.

I notice you are careful in the wording of one statement, "...possibly lead people to think that our understanding on this matter is likely to change, it isn't. ("is likely" being the key, leaving option for change open.) However, that statement contradicts your statement that "at this point in time" would be "misleading" which means the understanding won't change. You obviously don't want to believe your understanding will change, yet you are open to that change, slightly. Your openness is commendable, and you would benefit by enlarging that openness to change.

It would be silly to preface every statement as you suggest. "At this point in time we believe gravity is an attractive force." "At this point in time, we believe genetic information is carried primarily by DNA." "At this point in time we believe spacetime is expanding."

These things are so well supported that we can safely treat them as facts (along with the limits imposed by c). Does that mean it's impossible that some new evidence will require us to change our understanding? Of course not. But it means that we are so sure of this knowledge, that it is so well supported by evidence, that there is no point in additional research, that we can and should treat these things as facts. That allows science to move down promising and related avenues.

So I will not be using any qualifiers when discussing things that have risen to that level of fact. The speed of light IS as I described it.

[Speculator]

Light has, as you know, been 'accelerated' faster than the speed of light.

No, it has not. First of all, light is made of photons, which have zero rest mass. So they are not accelerated at all. In a vacuum, they simply exist moving at c. In any other medium, they travel slower than c. Do not confuse phase velocities and group velocities with the actual motion of photons, or with the speed that information can be transmitted.

I notice you are careful in the wording of one statement, "...possibly lead people to think that our understanding on this matter is likely to change, it isn't. ("is likely" being the key, leaving option for change open.) However, that statement contradicts your statement that "at this point in time" would be "misleading" which means the understanding won't change. You obviously don't want to believe your understanding will change, yet you are open to that change, slightly. Your openness is commendable, and you would benefit by enlarging that openness to change.

It would be silly to preface every statement as you suggest. "At this point in time we believe gravity is an attractive force." "At this point in time, we believe genetic information is carried primarily by DNA." "At this point in time we believe spacetime is expanding."

These things are so well supported that we can safely treat them as facts (along with the limits imposed by c). Does that mean it's impossible that some new evidence will require us to change our understanding? Of course not. But it means that we are so sure of this knowledge, that it is so well supported by evidence, that there is no point in additional research, that we can and should treat these things as facts. That allows science to move down promising and related avenues.

So I will not be using any qualifiers when discussing things that have risen to that level of fact. The speed of light IS as I described it.

Spoken as a True Believer, Chris, as if you really have FAITH in what you are saying.

[Chris Peterson]

Spoken as a True Believer, Chris, as if you really have FAITH in what you are saying.

Faith is the basis of an epistemological system that doesn't weigh beliefs according to the quality of evidence. Faith is the last thing that makes up my views of the natural world.

[Speculator]

Spoken as a True Believer, Chris, as if you really have FAITH in what you are saying.

Faith is the basis of an epistemological system that doesn't weigh beliefs according to the quality of evidence. Faith is the last thing that makes up my views of the natural world.

http://www.newscientist.com/article/dn6 ... Muq5-lMvPw

[Chris Peterson]

Spoken as a True Believer, Chris, as if you really have FAITH in what you are saying.

Faith is the basis of an epistemological system that doesn't weigh beliefs according to the quality of evidence. Faith is the last thing that makes up my views of the natural world.

http://www.newscientist.com/article/dn6 ... Muq5-lMvPw

What is your point? It has long been accepted that physical constants may not, in fact, be constant over time (although it is certain that any change with time is very, very tiny). This has no bearing on my comments at all, either about how we acquire knowledge, or about the limits created by c.

[MargaritaMc]

This post is related to the opening post, not to the intervening discussion.

BBC News, Scotland
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.

I needed to check the definition of free space

*free space
n
1. (General Physics) a region that has no gravitational and electromagnetic fields: used as an absolute standard. Also called (no longer in technical usage): vacuum

free space. (n.d.) Collins English Dictionary – Complete and Unabridged. (1991, 1994, 1998, 2000, 2003). Retrieved January 31 2015 from http://www.thefreedictionary.com/free+space

Wanting further information about how this absolute standard is approximated within a laboratory, I referred to this site‡:

http://www.newworldencyclopedia.org/entry/Free_space
Realization of free space in a laboratory

By "realization" is meant the reduction to practice, or experimental embodiment, of the term "free space," for example, a partial vacuum. What is the operational definition of free space? Although in principle free space is unattainable, like the absolute zero of temperature, the SI units are referred to free space, and so an estimate of the necessary correction to a real measurement is needed. An example might be a correction for non-zero pressure of a partial vacuum

. Regarding measurements taken in a real environment (for example, partial vacuum) that are to be related to "free space," the CIPM cautions that:[29]

In all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum.

In practice, a partial vacuum can be produced in the laboratory that is a very good realization of free space. Some of the issues involved in obtaining a high vacuum are described in the article on ultra high vacuum. The lowest measurable pressure today is about 10^−11 Pa.[30] (The abbreviation Pa stands for the unit pascal, 1 pascal = 1 N/m^2.)

‡ I haven't used this new world encyclopaedia site before and, since posting that quotation, see that it was founded by Sun Myung Moon, which makes me cautious. I'd be pleased if any Asteriskean who is more knowledgeable than I am would check that what the encyclopaedia has written about the realisation of free space in a laboratory is accurate. It seems to be an exact copy of this Wikipedia user page, which doesn't appear in the main Wikipedia entry. Many thanks!
M

[geckzilla]

Here's a blog post by the author of Skulls in the Stars which attempts to explain the phenomenon as well as how it was measured. (I say "attempts" because I'm not sure I got it, not because of any shortcoming on behalf of the author.)
http://skullsinthestars.com/2015/01/31/ ... ght-light/

[Markus Schwarz]

This post is related to the opening post, not to the intervening discussion.

BBC News, Scotland
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.

I needed to check the definition of free space

‡ I haven't used this new world encyclopaedia site before and, since posting that quotation, see that it was founded by Sun Myung Moon, which makes me cautious. I'd be pleased if any Asteriskean who is more knowledgeable than I am would check that what the encyclopaedia has written about the realisation of free space in a laboratory is accurate. It seems to be an exact copy of this Wikipedia user page, which doesn't appear in the main Wikipedia entry. Many thanks!
M

Thank you Magarita for pointing out the Science Express article to me. As is usually the case, the article itself is more accurate (and less sensational) than various press releases. The experiment and conclusions indeed hinges on the definition of free space.

Let's, for the sake of this discussion, define empty space as a perfect vacuum with infinite spatial extend (not realistic, since we couldn't breath or wouldn't even be there). Under these boundary conditions, Maxwell's equations, which describe the way electric and magnetic fields interact, have the solution of plane waves. For various reasons (being simple to work with is one of them), these are the solutions every physics student gets taught and many scientist work with. However, once you drop the assumption of infinite vacuum, these solutions, and all conclusions derived from them, are no longer valid. For example, as Chris pointed out above, if you consider electro-magnetic waves in a medium, the speed of light is no longer a constant.

This is how I understand the article: the authors used optical equipment ("lenses") to shape a beam of light. This means that they introduced boundary conditions that allow for a particular propagation of light, which they call a Bessel beam and which is not a plane wave. So, what the authors mean by free space is that they have vacuum bounded by material. Yes there is vacuum but it is not of infinite extend. The Bessel beam then travels through vacuum, but behaves different from your plane wave.

A more mundane example are pipe organs. In air, without walls, sound waves are described as plane waves as well. All wavelengths are possible. The longer the wavelength, the deeper the sound we perceive. What the organ pipe does, however, is to introduce boundary conditions. A given length of the pipe allows only for a particular set of wavelengths (aka musical note). The sound wave produced by an organ pipe in a church is the analog of the Bessel beam in the experiment. Both waves travel through their natural medium (vacuum or air), but their properties are effected by the boundaries ("lenses" or pipes).

I hope my explanations were helpful, as the subject of boundary conditions is a subtle and often glossed over one .

[MargaritaMc]

Thank you, Markus. Together with the blog to which geckzilla linked in her post, this has helped me to get the beginnings of a grasp of the implications of this research. Thank you to geck as well

Margarita