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### Author Topic: an experiment of slowing down the speed of light  (Read 6205 times)

#### engrByDayPianstByNight

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##### an experiment of slowing down the speed of light
« on: 18/07/2007 06:44:26 »
Hi,

About three or four years ago I read from a website that a researcher in Denmark was able to slow down the speed of light to about 30 miles an hour. I don't remember the details, but the way she did it was by shooting the beam of light through a highly condensed matter (I don't know what kind), thereby blocking its passage to simulate a slow-down in speed.

I'm curious if anybody has any idea about that experiment and can comment on that. Also, what may be the possible application of it (if indeed the experiment was carried out correctly)?

#### Soul Surfer

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##### an experiment of slowing down the speed of light
« Reply #1 on: 18/07/2007 12:14:38 »
Googling  "slow light"  produces lots of references including this one

http://en.wikipedia.org/wiki/Slow_light

that should get you started

#### lightarrow

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##### an experiment of slowing down the speed of light
« Reply #2 on: 18/07/2007 15:33:21 »
Hi,
About three or four years ago I read from a website that a researcher in Denmark was able to slow down the speed of light to about 30 miles an hour. I don't remember the details, but the way she did it was by shooting the beam of light through a highly condensed matter (I don't know what kind), thereby blocking its passage to simulate a slow-down in speed.
I'm curious if anybody has any idea about that experiment and can comment on that. Also, what may be the possible application of it (if indeed the experiment was carried out correctly)?
Light propagating inside a medium travels with a slower speed than in the void: in glass with refractive index n = 1.5, for example, light's speed is 299,792,458/1.5 m/s. If the refractive index of the medium is much higher, the slowing is very high. They uses Bose-Einstein Condensates in such experiments, as medium.

A possible, fascinating application would be data storage of a new kind: storing pulses of light "freezed" inside their slowing matrix (it seems incredible, isnt'it?).
« Last Edit: 18/07/2007 15:40:50 by lightarrow »

#### JP

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##### an experiment of slowing down the speed of light
« Reply #3 on: 18/07/2007 22:36:08 »
I work in optics, and work with a lot of people who do research on slow light and fast light.  There is the technique mentioned here, that the speed of light can be slowed down by the refractive index of a medium.  v=c/n.  Where n is larger than 1, and c is the speed of light in a vacuum.

A more powerful technique (and what is usually referred to when one says 'slow light') is the "group velocity."  This measures the speed at which a light pulse travels.  A pulse can actually be thought of as a whole bunch of different frequency light waves added up, and if you change how fast they all move with respect to each other, you can dramatically change how fast the pulse appears to move.  You can even make the pulse move faster than the speed of light in a vacuum, slower, or even go so slow it appears to go backwards.  This technique depends not on the refractive index itself, but on how it differs for different frequencies of light.
(http://www.phy.duke.edu/research/photon/qelectron/proj/infv/fast_tut.ptml this site has a neat demonstration, as well as an experiment you can do to see how this works).

What people are mostly using these for is to delay information being sent along optical lines.  In theory it could be more energy efficient than the current technology, which uses electronics to read and buffer the information.

#### engrByDayPianstByNight

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##### an experiment of slowing down the speed of light
« Reply #4 on: 19/07/2007 01:52:33 »
jpetruccelli,

If I understand your explanation and the Duke website material correctly, the speed-of-light, c, is really the group speed of the multitude of single-frequency waves, right?

Since I don't have time right now to read the other reference material on that website, I'm going to just ask the question here: At present, at which frequency have researchers measured the fastest light wave speed? Presumably it is faster than c. And if so, what's its value? Thanks.

#### lightarrow

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##### an experiment of slowing down the speed of light
« Reply #5 on: 19/07/2007 12:23:55 »
jpetruccelli,
If I understand your explanation and the Duke website material correctly, the speed-of-light, c, is really the group speed of the multitude of single-frequency waves, right?
Since I don't have time right now to read the other reference material on that website, I'm going to just ask the question here: At present, at which frequency have researchers measured the fastest light wave speed? Presumably it is faster than c. And if so, what's its value? Thanks.

Waiting for jpetruccelli's answer, I can anticipate you some informations:

1.A generic wave packet is made of many frequencies added together, and it's usually described by 3 kinds of velocities:
phase velocity, group velocity and signal velocity. Said in simple terms: Phase velocity is the speed at which moves a single frequency; group velocity is the speed at which moves the maximum amplitude of the packet; signal velocity is the speed at which moves the information.
2.With the symbol "c" we mean "speed of light in the void". In this case phase velocity, group velocity and signal velocity are the same (= c).
3.Inside a medium, phase, group and signal velocities are not, in general, the same.
4.Inside a medium where group velocity is greater than c, the "actual speed of light", that is, the speed at which information is carried along with the light pulse, is given by signal velocity and this is still < c.
« Last Edit: 19/07/2007 12:33:45 by lightarrow »

#### JP

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##### an experiment of slowing down the speed of light
« Reply #6 on: 20/07/2007 17:18:53 »
Lightarrow has it.  To put my own spin on the explanation:

The phase velocity, which is usually thought of as the "speed of light" measures how fast a single frequency of light moves.  This is always less than c.  In a medium, it turns out that different wavelengths move at different speeds.  (This is why prisms work to separate different frequencies of light).

The group velocity is a measure of how fast a pulse of light moves in a material, and may be greater than 'the speed of light', c.  The group velocity turns out to depend on how the phase velocity varies between frequencies of light.  The more variation there is between the speeds of different single-frequency waves, the bigger the group velocity.  This is because a pulse of light consists of a lot of different single-frequency waves all added up.  If you change the speed at which each of these single-frequency waves moves, they can interfere with each other in such a way that they make the pulse move faster.

The signal velocity or information velocity always appears to be limited by the 'speed of light', c.  In order to send information, you need to turn some pulse on, which means that in detecting the signal, the first bit of information you get is the point at which you go from having zero signal to having some non-zero signal.  It turns out that this point moves (it was proven  by the physicist Leon Brillouin) at the speed of light, c, always.  (Now, it's usually so small that you can't measure it because of background noise, so in actuality, information travels slower than c).

So to answer the question "at what frequencies have researchers measured the fastest speed of light"?
(1) For phase velocity, the fastest any frequency can go is c.
(2) For group velocity, the fastest is some number many times bigger than c, but I don't know it off the top of my head.
(3) For information velocity, the duke team showed that the fastest propagation of information seems to be at c. (Even in cases where the group velocity, and pulse, appeared to move faster than c).

For a nice picture of it, look at the java applet here:
http://gregegan.customer.netspace.net.au/APPLETS/20/20.html
The top plots show each individual frequency that makes up the light.  The total light signal is shown at the bottom, and is a sum of all the frequencies shown up top.

It shows the phase velocities up top, which are how fast each color(frequency) is moving (blue faster than red).
It shows signal velocity (the leading edge of the bottom plot) which moves along with the fastest component of the frequency (c).  It shows the group velocity, which moves along faster than the speed of light, and is evident in how fast the "pulses" move along in the bottom plot.

#### lightarrow

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##### an experiment of slowing down the speed of light
« Reply #7 on: 20/07/2007 22:49:28 »
Very nice applet!

#### engrByDayPianstByNight

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##### an experiment of slowing down the speed of light
« Reply #8 on: 22/07/2007 00:53:31 »
Yes, very good applet and explanation indeed. Thanks.

#### Soul Surfer

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##### an experiment of slowing down the speed of light
« Reply #9 on: 22/07/2007 09:31:21 »
Jpetrucelli  You have got the group and phase velocities mixed up.  The phase velocity (basically the speed that the peaks or zero crossings in the electromagnetic waveform) is the one that can be faster than the velocity of light.   The group velocity is the one that carries the information and cannot be faster than the speed of light.

This is how it applies in waveguides at microwave frequencies  (inside a waveguide the phase velocity is generally faster than the speed of light and approaches infinity as the waveguide approaches cut off but the group velocity gradually gets slower and is always below the speed of light) as you say there are some peculier media in which the phase velocity behaves in such a way as a "pulse" appears to travel faster than light but this is not a true information pulse and not a true group velocity,
« Last Edit: 22/07/2007 09:35:45 by Soul Surfer »

#### lightarrow

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##### an experiment of slowing down the speed of light
« Reply #10 on: 22/07/2007 14:40:23 »
Jpetrucelli  You have got the group and phase velocities mixed up.  The phase velocity (basically the speed that the peaks or zero crossings in the electromagnetic waveform) is the one that can be faster than the velocity of light.   The group velocity is the one that carries the information and cannot be faster than the speed of light.
Sorry but this is wrong. Both phase and group velocities could be greater than c, under specific conditions: the first case for example in waveguides, as you said, the second in "anomalous refraction", that is when refraction index decreases with frequency, instead of increasing (as usual). In the first case, group velocity equals signal velocity, and so you can say that "The group velocity is the one that carries the information", but this is not true in general, because what really carries information is "signal velocity", which is something else; in the second case not: group velocity is greater than signal velocity.

Compare:
http://www.mathpages.com/home/kmath210/kmath210.htm
Quote
To see why the group velocity need not correspond to the speed of information in a wave, notice that in general, by superimposing simple waves with different frequencies and wavelengths, we can easily produce a waveform with a group velocity that is arbitrarily great, even though the propagation speeds of the constituent waves are all low. A snapshot of such a case is shown below. In this figure the sinusoidal wave denoted as "A" has a wave number of kA = 2 rad/meter and an angular frequency of wA = 2 rad/sec, so it's individual phase velocity is vA = 1 meter/sec. The sinusoidal wave denoted as "B" has a wave number of kB = 2.2 rad/meter and an angular frequency of wB = 8 rad/sec, so it's individual phase velocity is vB = 3.63 meters/sec.
The sum of these two signals is denoted as "A+B" and, according to the formulas given above, it follows that this sum can be expressed in the form 2cos(kx-wt)cos(Dkx-Dwt) where k = 5, w = 2.1, Dk = 0.1, and Dw = 3. Consequently, the "envelope wave" represented by the second factor has a phase velocity of 30 meters/sec.
Notice that the "envelope wave phase velocity " corresponds to the "Group velocity".
« Last Edit: 22/07/2007 14:53:02 by lightarrow »

#### Soul Surfer

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##### an experiment of slowing down the speed of light
« Reply #11 on: 22/07/2007 23:54:18 »
I have just learned something  thanks

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##### an experiment of slowing down the speed of light
« Reply #11 on: 22/07/2007 23:54:18 »