Relativity and stopping light

[jakesteele]

http://www.news.harvard.edu/gazette/...stoplight.html

I was blown away when I heard about this. I was wondering if the theory of relativity took into account something like this? What would have been the scientific reaction if we could go back in the Wayback machine and give that era of scientists a demo?
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[Reality Check]

http://www.news.harvard.edu/gazette/...stoplight.html

I was blown away when I heard about this. I was wondering if the theory of relativity took into account something like this? What would have been the scientific reaction if we could go back in the Wayback machine and give that era of scientists a demo?
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The link you include is broken.
If you mean the experiments that stop light then no one is surprised and it is not to do with relativity. In relativity the speed of light in a vacuum is constant. Many experiments have confirmed this.

[Skeptic Ginger]

http://www.news.harvard.edu/gazette/...stoplight.html

I was blown away when I heard about this. I was wondering if the theory of relativity took into account something like this? What would have been the scientific reaction if we could go back in the Wayback machine and give that era of scientists a demo?
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Try this link. (http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html)

I think this does have implications regarding relativity, RC.Hau, 41, a professor of physics at Harvard, admits that the famous genius [Einstein] would "probably be stunned" at the results of her experiments.

I love this stuff:In the mid-1990s, she and her colleagues became excited about experiments aimed at crowding atoms so close together that unusual things happen. The key is to cool them to within a billionth of a degree of minus 459.7 degrees F. Called "absolute zero," this is the temperature at which atoms have the least possible energy, and they all but cease to move around.

Hau was one of several researchers who succeeded in creating this novel state of matter. She corresponded with a colleague, Stepen Harris at Stanford University, and they came up with the idea that it might be possible to use a small ball of cold atoms to slow down light.

Hau and her group then figured out a way to make it work.


And this is just as interesting as messing with the speed of light:Lene Hau and her colleagues created a new form of matter

[Reality Check]

Try this link. (http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html)

I think this does have implications regarding relativity, RC.

I love this stuff:

And this is just as interesting as messing with the speed of light:
That is what I thought it was - the fact that the speed of light in a vacuum is different from the speed of light in matter. Thus you can have matter that slows or even stops light.

Relativity uses the postulate that the speed of light is a constant in a vacuum. The experiment does not use a vacuum.

[Soapy Sam]

Ye gods.
Slow Glass.

[Ziggurat]

And this is just as interesting as messing with the speed of light:
Lene Hau and her colleagues created a new form of matter

This is the sort of stuff in science journalism that bugs me. It's not a new form of matter. It's just a Bose-Einstein condensate, which was predicted over 80 years ago, first observed in superfluid helium over 70 years ago, and produced with laser-trapped gasses (as in this case) over a decade ago. They're really cool, but the production of this state of matter is not new at all.

[Skeptic Ginger]

That is what I thought it was - the fact that the speed of light in a vacuum is different from the speed of light in matter. Thus you can have matter that slows or even stops light.

Relativity uses the postulate that the speed of light is a constant in a vacuum. The experiment does not use a vacuum.But when you stop light by an opaque object, you can't then take that object away and recover the light (assuming the source is shut off)

This experiment changes the actual speed of light. That does at least require relativity be modified.

[Reality Check]

But when you stop light by an opaque object, you can't then take that object away and recover the light (assuming the source is shut off)
That is right.
It is also not what the experiments that stops light does. The object remains.


This experiment changes the actual speed of light. That does at least require relativity be modified.

The experiment does not change the speed of light in a vacuum. It thus does not require relativity to be changed because the postulates of relativity are (Special Relativity (http://en.wikipedia.org/wiki/Special_relativity#Postulates)):

The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.
The Principle of Invariant Light Speed – Light in vacuum propagates with the speed c (a fixed constant) in terms of any system of inertial coordinates, regardless of the state of motion of the light source.
The fact that light travels slower in materials than vacuum has been known for centuries. The big surprise from the Hau group (and others) is that the speed of light in matter can be made very small and even zero. The press release is a bit simplistic. A better description is in the Slow Light (http://en.wikipedia.org/wiki/Slow_light) Wikipedia article.

[Ziggurat]

This experiment changes the actual speed of light. That does at least require relativity be modified.

No, it doesn't. It's a cool trick, but it's got nothing to do with relativity at all. As the article itself states, relativity only puts an upper limit on the speed of light under any scenario equal to its (constant) speed in a vacuum, but it says nothing about how much lower it can get inside matter or whether it can change. And changing the speed of light in matter in general is old hat anyways. What do you think causes mirages? Variations in the speed of light through air.

[ben m]

This experiment changes the actual speed of light. That does at least require relativity be modified.

If you take this cloud and launch an electron, or a muon, or a spaceship, or a photon of the wrong frequency, (etc.) through it, you will find as always that "c", the fundamental constant of relativity, is 3x10^8 meters per second as usual. All this experiment is doing is making some photons move slower than the vacuum speed of light.

Remember, an optical photon in water is always going 30% slower than the speed of light. A photon in diamond is going 50% slower. These experiments are just bringing them (within a very, very narrow frequency band) down to 99.9999% slower.

[Skeptic Ginger]

No, it doesn't. ... relativity only puts an upper limit on the speed of light under any scenario equal to its (constant) speed in a vacuum, but it says nothing about how much lower it can get inside matter or whether it can change. I'll accept that on face value.

And changing the speed of light in matter in general is old hat anyways. What do you think causes mirages? Variations in the speed of light through air.I was under the impression light was bending, not changing speed. If you have to go around something it takes longer, but you don't necessarily go more slowly. And the Doppler effect changes the wave frequency, not the speed, doesn't it?

[Skeptic Ginger]

If you take this cloud and launch an electron, or a muon, or a spaceship, or a photon of the wrong frequency, (etc.) through it, you will find as always that "c", the fundamental constant of relativity, is 3x10^8 meters per second as usual. All this experiment is doing is making some photons move slower than the vacuum speed of light.

Remember, an optical photon in water is always going 30% slower than the speed of light. A photon in diamond is going 50% slower. These experiments are just bringing them (within a very, very narrow frequency band) down to 99.9999% slower.I knew I shouldn't have gotten into this discussion. ;)

[Ziggurat]

I was under the impression light was bending, not changing speed.

It bends because (http://en.wikipedia.org/wiki/Mirage#Cause) it changes speed. See index of refraction. (http://en.wikipedia.org/wiki/Index_of_refraction)

[Soapy Sam]

Can some smart person explain in words of 2 or fewer syllables, WHY light goes slower in a cloud of cold atoms ?

[sphenisc]

I was under the impression light was bending, not changing speed. If you have to go around something it takes longer, but you don't necessarily go more slowly. And the Doppler effect changes the wave frequency, not the speed, doesn't it?


If you turn round a corner, you'll find that one shoulder has go slower than the other.

[Soapy Sam]

Do photons have shoulders?

[Andrew Wiggin]

Can some smart person explain in words of 2 or fewer syllables, WHY light goes slower in a cloud of cold atoms ?

If the light is going through nothing, it can go really fast. If it's going through stuff, the denser the stuff the slower it goes. If stuff isn't clear, light can't go through at all, and lots of dense stuff isn't clear. They found a way of making some really dense stuff that was still clear enough that the light could go through, but only real slow.

Hope that helps.

A

[Andrew Wiggin]

Do photons have shoulders?

They must. I think it was the late great John Denver who was singing about how sunlight on his shoulder makes him happy.

A

[sphenisc]

Do photons have shoulders?

How do you think they wave?

[Dave Rogers]

Ye gods.
Slow Glass.

Would be nice to see, for sure. I suspect that it wouldn't work in practice, though. The refractive index would be so high that even a microscopic amount of dispersion would scramble the images to the point of unrecognisability.

Dave

[Soapy Sam]

I'm trying to think how this works either in terms of photons or waves.
I can see how a spherical wave front expanding through stuff gets dissipated, because the stuff is in the way, so the wave is diffracted, refracted, interferes with itself and generally loses energy... but why would that make it slower rather than just messing up its frequency?

In terms of photons, ok, some bounce off the stuff or are absorbed and re-emitted by the bits of stuff, presumably warming the stuff up a bit in the process, so transferring energy from photon to stuff , but that makes them less energetic photons - ie red shifted. Not slower.

[ponderingturtle]

But when you stop light by an opaque object, you can't then take that object away and recover the light (assuming the source is shut off)

This experiment changes the actual speed of light. That does at least require relativity be modified.

So what? Did you know it can take a million years for a photon from the core of the sun to reach the surface of the sun?

It is an interesting effect, but the writting about it is garbage, it has nothing to do with relativity. It would be like wondering what the relativistic effects of a mirror are.

[ponderingturtle]

Would be nice to see, for sure. I suspect that it wouldn't work in practice, though. The refractive index would be so high that even a microscopic amount of dispersion would scramble the images to the point of unrecognisability.

Dave

And that it needs to be at near absolute 0 would raise all kinds of problems building a window out of it, just think of the AC bills...

[ponderingturtle]

I'm trying to think how this works either in terms of photons or waves.
I can see how a spherical wave front expanding through stuff gets dissipated, because the stuff is in the way, so the wave is diffracted, refracted, interferes with itself and generally loses energy... but why would that make it slower rather than just messing up its frequency?

Well look at sound, it does not have a uniform frequency in all materials, but is very fast in something like steel compared to its speed in air. Thinking of it as an eletromagnetic wave, why shouldn't it have wave like properties of being effected by the material it travels through? Its frequency shouldn't change, you are not going to suddenly get extra crests and troughs per minute going past a point just because the speed of the travel changed.

Think of a road with no onramps or offramps. It has a section that is 60 mph and section that is 30 mph, now should the 30 mph see more cars per minute pass him? If so where did the cars come from?

[Andrew Wiggin]

Would be nice to see, for sure. I suspect that it wouldn't work in practice, though. The refractive index would be so high that even a microscopic amount of dispersion would scramble the images to the point of unrecognisability.

Dave

More than that, I seem to recall that if there's too big a difference between indexes of refraction the light won't go in at all and you get a reflector. I read a sci-fi story once that used the slow glass idea, and I seem to remember them explaining it as forcing light into a spiral, not by index of refraction or density.

I think I'll pass on slow glass windows. In the story, people were using them for landscapes, but they worked both ways, so people outside could see what you'd been doing however many years ago. The creepy element was a guy still mourning his dead wife and watching her through the window.

On the other hand, they missed out on the potential use as a flashlight/vampire weapon. Keep a thick cylinder of glass pointed at the sun, and in a couple of years sun will shine out of it. If it's in the right phase, sun will shine out of it all night. Focus that incoming light with a great big fresnel lens or solar death ray mirror array, and the light coming out will be weapon grade.

A.

[3point14]

Okay, this could be me being thick, but I thought light always moved at the speed of light in a vaccuum, and it slowing down when in any other medium was just a product of it bouncing around a little in the medium it's travelling through.

Have I got this wrong? That happens a lot.

[sol invictus]

This is the sort of stuff in science journalism that bugs me.

Agreed.


This experiment changes the actual speed of light. That does at least require relativity be modified.

Case in point (not your fault skeptigirl, we all get misled by bad science articles in fields we don't know well).

Can some smart person explain in words of 2 or fewer syllables, WHY light goes slower in a cloud of cold atoms ?

I'll try, in words of two syllab...b... aw, forget it.

[ponderingturtle]

More than that, I seem to recall that if there's too big a difference between indexes of refraction the light won't go in at all and you get a reflector. I read a sci-fi story once that used the slow glass idea, and I seem to remember them explaining it as forcing light into a spiral, not by index of refraction or density.

This has to do with the angle change. If the light would be bent such that it would either end up running along the interface surface or even more it has a total internal reflection. Now waves to always reflect a bit at these interfaces as well.

[Brian-M]

Slow Glass.


Science fiction is weirder than fact. :)