Scientists drag light by slowing it to speed of sound

Jul 06, 2011
The green laser is shown as it leaves the ruby crystal.

(PhysOrg.com) -- Scientists at the University of Glasgow have, for the first time, been able to drag light by slowing it down to the speed of sound and sending it through a rotating crystal.

Most people may think the is constant, but this is only the case in a vacuum, such as space, where it travels at 671million mph.

However, when it travels through different substances, such as water or solids, its speed is reduced, with different wavelengths (colours) travelling at different speeds.

In addition, it has also been observed, but is not widely appreciated, that light can be dragged when it travels through a moving substance, such as glass, air or water – a phenomenon first predicted by Augustin-Jean Fresnel in 1818 and observed a hundred years later.

Prof. Miles Padgett in the Optics Group in the School of Physics & Astronomy, said: “The speed of light is a constant only in . When light travels through glass, movement of the glass drags the light with it too.

“Spinning a window as fast as you could is predicted to rotate the image of the world behind it ever so slightly. This rotation would be about a millionth of a degree and imperceptible to the human eye.”

In research detailed in the latest edition of the journal Science, researchers Dr Sonja Franke-Arnold, Dr Graham Gibson and Prof Padgett, in collaboration with their colleague Professor Robert Boyd at the Universities of Ottowa and Rochester, took a different approach and set up an experiment: shining a primitive image made up of the elliptical profile of a green laser through a ruby rod spinning on its axis at up to 3,000 rpm.

Once the light enters the ruby, its speed is slowed down to around the speed of sound (approximately 741mph) and the spinning motion of the rod drags the light with it, resulting in the image being rotated by almost five degrees: large enough to see with the naked eye.

Dr Franke-Arnold, who came up with the idea of using slow in ruby to observe the photon drag, said: “We mainly wanted to demonstrate a fundamental optical principle, but this work has possible applications too.

“Images are information and the ability to store their intensity and phase is an important step to the optical storage and processing of quantum information, potentially achieving what no classical computer can ever match.

“The option to rotate an image by a set arbitrary angle presents a new way to code information, a possibility not accessed by any image coding protocol so far.”

Explore further: 'Dressed' laser aimed at clouds may be key to inducing rain, lightning

More information: 'Rotary photon drag enhanced by a Slow-Light Medium' - www.sciencemag.org/content/333/6038/65.full

Provided by University of Glasgow

4.7 /5 (27 votes)

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User comments : 43

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Nik_2213
3.5 / 5 (2) Jul 06, 2011
Is this akin to gravitational frame-dragging around a massive, spinning object ??
jjoensuu
4 / 5 (1) Jul 06, 2011
So if e=mc2 (and c=speed of light), does this mean that when light goes slower, there is somehow less energy somewhere...hmmm...around it?
Jaeherys
not rated yet Jul 06, 2011
Wouldn't the mass equivalent go up then if the photon slows down to keep E the same?
antialias_physorg
4.4 / 5 (7) Jul 06, 2011
Energy of a photon is calculated via:

E = hv

Where h is the Planck constant and v is the frequency.

Neither of these is affected by the medium so the energy of the photon doesn't change (unless it's absorbed, scattered, ... ).

Is this akin to gravitational frame-dragging

No. Frame dragging effects are much much MUCH smaller.
SlightlyMad
5 / 5 (5) Jul 06, 2011
the c in e=mc2 is a constant, the speed of light through vacuum. This has nothing to do with the speed of the photon through a medium.
Ricochet
2 / 5 (2) Jul 06, 2011
Maybe some of the energy is bled off into the medium that's changing its velocity? I guess the question would be whether or not the light returns to its full speed, brightness, etc. after leaving the medium that altered it.
antialias_physorg
4.4 / 5 (7) Jul 06, 2011
I guess the question would be whether or not the light returns to its full speed

Of course. There is no 'slow' light in vacuum. In a medium it can travel slower (and there that speed can even be dependent on the frequency). But for a defined frequency the photon will always travel at the speed of light for that frequency in that medium.

(There is even the theory that that speed varies ever so slightly for different frequencies in a vacuum - but that has not been confirmed by experiment yet)

brightness

Brightness is a measure of the number of photons that pass a given area in a given amount of time. Individual photons, on the other hand, are characterized by their frequency.
Increasing/decreasing the energy of a single photon does not change the brightness but the frequency. (Other effects can also change the apparent frequency: the expansion of space can cause a redshift, as will the relative motion of sender and observer away fom each other.)

Raygunner
5 / 5 (2) Jul 06, 2011
I think the photon has to absorbed/emitted again and again through the medium it is traveling through, thus causing a delay depending on the density and properties of the material. At least I think that's right.
physpuppy
5 / 5 (1) Jul 06, 2011
I find it amazing that they are able to slow the speed of light by 6 orders of magnitude.

I wonder if there are useful applications for this. Time delays of light pulses are used for measurements - would longer delays be useful?

The light is interacting with the medium in some way and this interaction keeps the directional information. I wonder if some loss of resolution occurs, or perhaps the rotational speed is too slow and medium interaction too fast for something like that to occur.

I remember a Sci-Fi/Mystery where someone made glass that slowed light so much that it took 10 years to go through - they would store the glass in some scenic place and then sell to customers (and you can guess the rest of the plot)
SteveL
4 / 5 (1) Jul 06, 2011
Since the speed of light can be affected by the medium it is going through, does dark matter count? Because of dark matter, disregarding gasses and other baryonic matter for the moment, when we think of the vacuum of space, it's not really a vacuum is it? Could dark matter actually be slowing down light compared to photonic speed in a "true" vacuum? I wonder if light would even have a frequency without dark matter.
antialias_physorg
4.5 / 5 (11) Jul 06, 2011
I find it amazing that they are able to slow the speed of light by 6 orders of magnitude.
I wonder if there are useful applications for this.

Yes. They are called 'lenses'.

I wonder if light would even have a frequency without dark matter.

Could you rephrase that so it makes sense?
Could dark matter actually be slowing down light compared to photonic speed in a "true" vacuum?

No. If it did we would get another lensing effect. Dark matter does not seem to interact with electromagnetic radiation at all (other than by gravitation - which is how we can infer that it exists)
FrankHerbert
1.9 / 5 (9) Jul 06, 2011
Yes. They are called 'lenses'.


Lol, blunt and funny.

@ SteveL

No. If it did we would get another lensing effect. Dark matter does not seem to interact with electromagnetic radiation at all (other than by gravitation - which is how we can infer that it exists)

--antialias_physorg


This is why it's called 'dark' matter. It doesn't interact with light. Neutrinos are actually dark matter as they don't interact with photons. However, generally the term 'dark matter' is reserved for not yet discovered particles that fit this description. How else would people be able to claim dark matter doesn't exist?
Temple
5 / 5 (4) Jul 06, 2011
(There is even the theory that that speed varies ever so slightly for different frequencies in a vacuum - but that has not been confirmed by experiment yet)


I can't imagine any validity to that theory. The spectra and intensity curves of well measured supernovae 10-12 billion lightyears away seem to convincingly refute any possible differences in the speed of light through vacuum.

Over those distances and times, just about any detectable event would show incredibly obvious evidence if such a theory had any merit.
CSharpner
4 / 5 (6) Jul 06, 2011
Maybe some of the energy is bled off into the medium that's changing its velocity? I guess the question would be whether or not the light returns to its full speed, brightness, etc. after leaving the medium that altered it.


When light moves through a medium, each photon is absorbed by an electron in the median, stored for a (very) short while, then it (or a new photon, with identical quantum properties) is then emitted in the same direction. Between electrons, it's moving through empty space and travels at full speed (c). The repetitive stalls while each photon is absorbed by electrons causes more and more of a delay. The light, itself, never slows down. When an electron absorbs a photon, the photon ceases to be "light" and instead is an electron with a higher energy state. Eventually, the electron loses the higher energy state, creating a new photon. While it's a photon, it's always traveling at c.

Photon @ c -> high energy electron state (delay) -> photon @ c, etc...
Flap
5 / 5 (1) Jul 06, 2011
I assume the begining of this article should say:

(PhysOrg.com) -- Scientists at the University of Glasgow have, for the first time, been able to drag light by slowing it down to the speed of sound BY sending it through a rotating crystal.

Rather than:
(PhysOrg.com) -- Scientists at the University of Glasgow have, for the first time, been able to drag light by slowing it down to the speed of sound and sending it through a rotating crystal.
Osiris1
not rated yet Jul 06, 2011
let us say that we launch a probe using some yet new tech directly at the star Sirius A and it has now reached the velocity of 'c' with respect to our Sun. Unknown to us, a civilization on a planet about Sirius A....we will assume that they exist, disregarding all we think we know about the place....has launched a similar probe toward us, and it has acheived a velocity of 'c' with respect to our star, the Sun. Now we have two objects approaching each other on diametrically opposed courses, one with a velocity of positive 'c' and one with a velocity of negative 'c' with respect to the same object, the Sun. We know that space is not empty, so there will be some friction with the interstellar mediums, slowing the craft just a tiny bit. Will each see their launch location appear to 'red shift out'? What will be the color of each other's probe as they approach..how 'blue shifted' to what color? How many will say that the sum of V(earth probe) V(Sirian probe) though equal, is not 2V?
Auxon
not rated yet Jul 06, 2011
Same story ... different author: http://www.physor...ges.html
Flap
2.3 / 5 (3) Jul 06, 2011
Lets make it more interesting by turning the headlight on in both vehicles.
Osiris1
4 / 5 (2) Jul 06, 2011
I find it interesting also that different COLORS have different velocities in the 'slow' medium. This militates that reality is different than we think in what we call a vacuum that really is not. Space is not empty, and neither is any so called vacuum that we create in any lab here. So any measurement claimed to be a vacuum measurement is false on its face. The best measurement equipment that we have cannot really recognize a true vacuum....its 'measurement impedence' is too low, giving rise to a schroedingerlike statistical imprecision in which all manner of sins called assumptions can live. On top of this, we claim a matter-like component of light called a photon with a claimed mass of 'a bit less than 1(10^(-18))eV', and that a light beam has pressure. This means that light photons are subject to accelerations, positive and negative. Therefor any claimed instaneous |acceleration| [ a = (d^2)v/d(s^) ] {as a second order derivative} = c/sec/sec seems a violation of physics.
CSharpner
not rated yet Jul 06, 2011
let us say that we launch a probe using some yet new tech directly at the star Sirius A and it has now reached the velocity of 'c' with respect to our Sun.

That would require an infinite amount of energy and an infinite amount of time to achieve and violates relativity. But, I'll assume you meant something like 99%c.

Will each see their launch location appear to 'red shift out'?

Yes.

What will be the color of each other's probe as they approach..

They will see each other as blue shifted.

how 'blue shifted' to what color?

I don't know off hand. Would have to do some calculations.

How many will say that the sum of V(earth probe) V(Sirian probe) though equal, is not 2V?

Everything is relative. I presume you mean "relative to our sun"? If so, mathematically yes, 2V. In reality, they might each receive a small boost from their respective solar winds at the start, then drag from interstellar particles & g through most of their journey, never exactly 2V
holoman
not rated yet Jul 06, 2011
Might want to research Colossal Storage Corp. 3D optical
data storage or Holographic data storage.
Deesky
4 / 5 (4) Jul 07, 2011
When light moves through a medium, each photon is absorbed by an electron in the median, stored for a (very) short while, then it (or a new photon, with identical quantum properties) is then emitted in the same direction.

Sorry, but that isn't strictly correct. If it were, then the absorption spectrum should be discrete, as atoms have only discrete energy states. For example, in glass we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. Also, the index of refraction varies continuously rather than abruptly with the frequency of light.

It's about the bulk property of the medium, where the collective behavior of a large number of atoms (a lattice forming "collective vibrational modes") interacting with each other, rather than individual, isolated atoms, by means of phonon interactions. It's a bit technical as some knowledge of solid state physics is required, but that's the gist of it.
extinct
2.3 / 5 (4) Jul 07, 2011
Is this a joke? This was done 12 years ago, if not longer. Do an internet search on physicist Lene Hau at Harvard University, who in 1999 slowed the speed of light down to just 38 miles per hour.
FroShow
not rated yet Jul 07, 2011
@extinct; the point of the article is that they were able to 'drag' the light by slowing it down, not just slowing it down.

...light can be dragged when it travels through a moving substance, such as glass, air or water a phenomenon first predicted by Augustin-Jean Fresnel in 1818 and observed a hundred years later.

Doesn't this contradict the whole premise of this article? That for the first time they were able to drag light?

BTW, thank you everyone for the knowledgeable responses, very helpful (with a few obvious exceptions).
RobertKarlStonjek
not rated yet Jul 07, 2011
So, Mr. Tompkins could reach the speed of light on his bicycle after all?

[See 'Mr.Tompkins in Paperback' by George Gamow]
MIBO
3 / 5 (1) Jul 07, 2011
A real application for this would be in the delay of high speed signals for phase alignment.
One such example is for the SKA telescope where there is a requirement to delay hundreds of gigabits of data from the nearby radio telescopes so that is can be correlated with the signals from the more distant receivers.
All signals from all telescopes need to arrive at the same time for correlation but this is a big problem when there are a few distant receivers and hundreds of gigabits/second of real time data from the nearby receivers.
Currently one solution would be to pass all signals from near receivers through long optical fibers to match the delays but this takes a lot of fiber.
This has some quite interesting scientific uses, particularly if multiple streams can be passed through the same crystal, just amplify the signal and send it multiple times through and you have a very elegant delay line.
antialias_physorg
2 / 5 (2) Jul 07, 2011
Is this a joke? This was done 12 years ago, if not longer. Do an internet search on physicist Lene Hau at Harvard University, who in 1999 slowed the speed of light down to just 38 miles per hour.

The point of the article wasn't the slowing down of the speed of light (which, as you point out has been done before) but the dragging of light by motion of the medium.

So, Mr. Tompkins could reach the speed of light on his bicycle after all?

In a medium: yes

Something like this happens when electrons move at a speed greater than the speed of light in that medium (then we get Cherenkov radiation)
CSharpner
5 / 5 (1) Jul 07, 2011
When light moves through a medium, each photon is absorbed by an electron in the median, stored for a (very) short while, then it (or a new photon, with identical quantum properties) is then emitted in the same direction.

Sorry, but that isn't strictly correct. ...

Thanks for the update. What I know about light propagation through a medium is limited to Richard P. Feynman's book of lectures titled "QED". There was a section that explained it as above, IIRC, but it was probably a simplified explanation. Question: Is the seemingly slow down of light caused by another physical process? Or was my explanation appropriate for the slow down? In other words, is it still true that when light is not interacting with matter, (when it's between atoms in a medium) that it travels at full speed? Or perhaps, there's some more complex interaction going on there?
Deesky
4.3 / 5 (6) Jul 07, 2011
There was a section that explained it as above, IIRC, but it was probably a simplified explanation.

Yes, that is a common and simplified explanation.

Or perhaps, there's some more complex interaction going on there?

This. I'll point you to a physics forum faq which goes into some detail on this and where I brushed up on the details in my previous reply:
http://www.physic...tcount=4
CSharpner
2 / 5 (2) Jul 08, 2011
Excellent link Deesky! Thanks! Now I know more detail of how light is repeatedly delayed in a mesium. It's not delayed by an electron at a time, but by a collection of atoms at a time.
roseman
5 / 5 (1) Jul 09, 2011
Light drag in the universe's imperfect vacuum would explain the "red shift" of distant stars and galaxies. We will have to rethink our theory of universal expansion and our model of the universe.
Isaacsname
not rated yet Jul 09, 2011
A video for those of us who need a layman's explanation.

http://www.youtub...0JNyDBI0
Vibishan
3 / 5 (2) Jul 10, 2011
forgive me if i'm wrong but doesn't Faraday say that the speed of light is 'c' in any frame of reference??
whatsthat
5 / 5 (1) Jul 11, 2011
Not a scientist, but it appears to me that what they have done is to prove (again) that light has mass and can be affected by outside forces... like gravity.
Deesky
3.7 / 5 (3) Jul 11, 2011
Not a scientist, but it appears to me that what they have done is to prove (again) that light has mass

Light does NOT have mass!
antialias_physorg
not rated yet Jul 12, 2011
...and the shift was not due to gravity. We're dealing here with an effect that is due to the periodic nature of atoms in a ruby rod which tends to shift the electrica and magnetic components of electromagnetic waves towards preferred planes of vibration.

When they spun up the rod that preferred plane spun along with it 'dragging' the alignemnt of the photons with it. The cool thing is that with such 'low' number of revolutions and over such a short distance the effect is still visible (mainly due to the slow speed and consequently the high travel time of the photons within the rod.

It shows that matter can impart a significant angular momentum to the electromagnetic components of light.
rawa1
1 / 5 (1) Jul 12, 2011
.. where I brushed up on the details in my previous reply..
You're saying, in transparent solid the collective phonon mode of vibrations isn't available and the photon is re-emitted after while. How it differs from explanation, which you're trying to criticize, after then? We have apparent paradox here: the photon must be absorbed with many atoms for to not exhibit the spectral lines, nevertheless the glass still slows down the light without spectral lines and collective phonon mode of vibrations.
antialias_physorg
not rated yet Jul 12, 2011
If you notice: It's a green laser (which usually operate at a 532nm frequency). What kind of spectral lines were you expecting from such a monochromatic source?
nickelsworth
not rated yet Jul 12, 2011
Perhaps, the Universe has an infinate amount of frequencies? Can anyone explain what the wave-length is for gravity? Magnetism? Every source of energy is generated by one means or another. For every transmitter there is a receiver.. but if disparate antennae are used there can be no communication. If 'shadow zones' exist here on earth, it's quite possible that they exist on a universal scale. We know how to escape a shadow zone on a local scale. Should we not be able to do the same in a universal sense?
bluehigh
1 / 5 (2) Jul 12, 2011
Light does NOT have mass!


Until you can devise an experiment that can establish that photon rest mass is exactly zero then the best you can do is suggest lower limits of photon mass. In the meanwhile moving photons/light at any velocity have mass equivalence in relation to energy transfer. Aside from differing definitions of Mass designed to obfuscate with semantics your lack of understanding regarding the nature of light, the fact remains that photons/light with velocity, exhibits properties of mass.
Deesky
5 / 5 (1) Jul 12, 2011
Aside from differing definitions of Mass designed to obfuscate with semantics your lack of understanding regarding the nature of light

Bullshit. Light or photons, if you will, have no mass. By simply stating 'mass' it is assumed one is talking about rest mass. Photons, by definition, cannot be at rest. There is no obfuscation here, merely convention. It is you who is trying to muddy the waters. Relativistic mass however, is quite a different matter and is why semantics ARE important.
antialias_physorg
5 / 5 (1) Jul 12, 2011
Perhaps, the Universe has an infinate amount of frequencies?

No. Because a frequency is directly related to an energy.
(By E = hv where h is the Planck Constant and v is the frequency). From this you can see that even if we pumped the entire energy of the universe (which is large but finite) into one photon the frequency would also be finite.

Can anyone explain what the wave-length is for gravity? Magnetism?

Go to wikipedia. Electromagnetic radiation.
As for gravity: The 'wave' concept is a bit different there.

I have no clue what you are trying to say in the rest of your post.

Until you can devise an experiment that can establish that photon rest mass is exactly zero then the best you can do is suggest lower limits of photon mass.

If Einstein's equations are true then the mass of a photon would be infinite (if it had any) when travelling at c (which a photon does). At the very least we should see an enormous increase in impulse (which we don't)
bluehigh
1 / 5 (2) Jul 13, 2011
Even though photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of special relativity by the trick of considering them to have zero rest mass. No proof, just an assumption. That way, the relationship between energy, mass and momentum can be expressed with well defined properties.

The notion that photons have zero invariant mass is a convenience designed wholly to ensure that charge conservation is absolutely guaranteed.

The established upper limit for the invariant mass of a photon is 7 × 10-17 eV

NOT and NEVER zero in reality.

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