# Stopping light?
**URL:** https://boards.straightdope.com/t/stopping-light/398075
**Category:** Factual Questions
**Created:** [March 30, 2007, 3:24am UTC](https://boards.straightdope.com/t/stopping-light/398075 "2007-03-30T03:24:51Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Cronux](https://boards.straightdope.com/u/Cronux)
#### Post date: [March 30, 2007, 3:24am UTC](https://boards.straightdope.com/t/stopping-light/398075/1 "2007-03-30T03:24:51Z")
An interesting question was raised to me from a friend of mine (Mike, his comment is mentioned below). His friend told him that they used temperature to actually /stop light/. I thought it was hard to believe, but he assured me claiming he saw a video where the light was shining, then suddenly, it stopped. I know a
decent amount of knowledge as far as theoretical physics as far is
concerned, so I ask you, the world’s most intelligent human being this:
is it really possible to stop light with a change of the temperature?
note: Mike - How Sam described my “friend”, he is a credible source.
Dirk Williams is known in many circles for a variety of different
things. One of those happen to be engineering, and he happens to work
with chemist’s. While I’m not 100% sure of the way the went about
stopping light, I think Mr. Williams mentioned the uses of extreme
temperatures.
P.S. - I am trying to impose as little bias as possible while describing
my “friend”. Thanks for your time. 😮
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### Author:  [@Cronux](https://boards.straightdope.com/u/Cronux)
#### Post date: [March 30, 2007, 3:26am UTC](https://boards.straightdope.com/t/stopping-light/398075/2 "2007-03-30T03:26:26Z")
This was aimed at Cecil, but it was turned down. So I go to the next best place. His message board. And yes, I was too lazy to edit it so it’s directed elsewhere. 😛
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### Author:  [@Jragon](https://boards.straightdope.com/u/Jragon)
#### Post date: [March 30, 2007, 3:36am UTC](https://boards.straightdope.com/t/stopping-light/398075/3 "2007-03-30T03:36:16Z")
Well, I doubt the **stopped** light, but they may have slowed it down. The colder it is, the less energy there is. If you recall your old chemistry and physics classes once you reach absolute zero (0 degrees kelvin) matter basically stops.
Now i’m not entirely certain if photons count as matter (but I’m pretty sure the same property applies even if it’s not). So it would be entirely possible to reach a cool enough temperature to freeze light. But liek I said, we haven’t reached absolute zero yet, so they probably just drastically slowed it down.
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [March 30, 2007, 3:40am UTC](https://boards.straightdope.com/t/stopping-light/398075/4 "2007-03-30T03:40:33Z")
[Harvard scientists stop light](http://www.spokesmanreview.com/blogs/commons/archive.asp?postID=4807)
> [@](#):
>
> In 1999, (Lene Vestergaard) Hau headed a team of scientists that slowed light, which travels a brisk 186,282 miles a second when unimpeded, to a leisurely 38 miles an hour by shining it into an exotic, ultracooled cloud of sodium atoms. At temperatures a fraction of a degree above absolute zero, the atoms coalesce into a single quantum mechanical entity known as a Bose-Einstein condensate. Shining a laser on the cloud tunes its optical properties so that it becomes molasses when a second light pulse enters it.
> Then, in 2001, Dr. Hau and a second team of physicists, this one from the Harvard-Smithsonian Center for Astrophysics, brought light to a complete halt by slowly turning off the laser. The Bose-Einstein cloud turned opaque, trapping the light pulse inside. When the laser was turned back on, the trapped light pulse flew out.
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### Author:  [@Jragon](https://boards.straightdope.com/u/Jragon)
#### Post date: [March 30, 2007, 3:55am UTC](https://boards.straightdope.com/t/stopping-light/398075/5 "2007-03-30T03:55:00Z")
Hmm… apparently theres some odd property of liight then that allows it to do that. Not entirely certain how it works, but apparently it’s some odd anomaly (although it could have easily also been going to some painfully slow rate completely inconcievable by human reasoning, but that’s “stopped” enough for me).
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### Author:  [@A.R.Cane](https://boards.straightdope.com/u/A.R.Cane)
#### Post date: [March 30, 2007, 4:34am UTC](https://boards.straightdope.com/t/stopping-light/398075/6 "2007-03-30T04:34:49Z")
There’s been more work done on stopping light, I posted this about six weeks back: [Storing light. - Miscellaneous and Personal Stuff I Must Share - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=407885)
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### Author:  [@BobPi](https://boards.straightdope.com/u/BobPi)
#### Post date: [March 30, 2007, 4:36am UTC](https://boards.straightdope.com/t/stopping-light/398075/7 "2007-03-30T04:36:16Z")
[QUOTE=Jragon]
Well, I doubt the **stopped** light, but they may have slowed it down. The colder it is, the less energy there is. If you recall your old chemistry and physics classes once you reach absolute zero (0 degrees kelvin) matter basically stops.
Now i’m not entirely certain if photons count as matter (but I’m pretty sure the same property applies even if it’s not). So it would be entirely possible to reach a cool enough temperature to freeze light. But liek I said, we haven’t reached absolute zero yet, so they probably just drastically slowed it down.
[/QUOTE]
The light that existed before it was stopped isn’t the same light as the light that’s released when the stopping is over. They use an exotic state of matter called a Bose-Einstein Condensate to capture the quantum properties of a photon when it’s absorbed, and then when a photon is emitted it has the same quantum properties. But is it the same photon? There aren’t a whole lot of properties of photons, just frequency and polarization direction, so one photon of that frequency and orientation looks a lot like another.
Other report you might see about light being slowed down always refer to a large collection of photons, and here it’s important to distinguish between group velocity and phase velocity. Think of the ripples on a pond, the waves move from one shore to another at one speed, but the molecules of water that make up the wave move at a different speed.
A photon always moves at c, the speed of light in a vacuum. That is a given fact of physics, and it’s absolute. If anyone talks about light moving at something other than c, they’re really talking about something else.
And just as it’s impossible for light to move at anything other than c, it is also impossible for anything to be at absolute zero. You can get close to absolute zero, but you can’t actually get all the way there.