PDA

View Full Version : How can the speed of light be 38 MPH??!!!???

LonesomePolecat
12-14-1999, 05:27 PM
I just read something in the paper that's freaking me out a bit. It seems that a team of scientists at Harvard managed to get within five billionths of a degree of absolute zero (-273.15 degrees Celsius) and thus managed to reduce the speed of light to 38 miles per hour. Would someone PLEASE explain in a manner intelligible to a layman how this is possible to I can get some sleep tonight???!!!??? Why would extreme cold reduce the speed of light to practically nothing???!!!??? Oh, yeah, and how exactly did they achieve that degree of cold?

DrFidelius
12-14-1999, 05:43 PM
The sodium they cooled down to that point became something called a Bose-Einstein Condensate. Basically, so far as I understand it,the wave functions of the individual atoms collapsed into a single wave. This rendered the material transparent to light with a wavelength that resonated with this wave function. They shined a laser tuned to that frequency into the sodium, and because of the way the atoms were behaving the light had to take the long way around every atom. Or something like that.

Pretty neat, figuring that sodium is opaque at temperatures above that where a Bose-Einstein Condensate forms.

Anyway, light passing through ANY medium slows down. The "speed of light" so often referenced is the speed of light in a vaccuum.

dougie_monty
12-14-1999, 05:54 PM
So how come it takes light only 8 minutes (assuming a normal speed of light of 186,000 miles per second) to get to the earth from the sun, through outer space which is mostly a vacuum? Huh??????

Boris B
12-14-1999, 06:05 PM
The 186,000 miles per second thing is the speed of light in a vacuum .. the 38 mph speed is the speed in a super-duper dense medium. (I'm not trying to varify they 38 mph speed, I'm just trying to sum up what Dr. Fi. was saying.)

------------------
I don't want to make people think like me, I want them to think like me of their own free will.

12-14-1999, 07:03 PM
I have found an article (http://www.news.harvard.edu/gazette/1999/02.18/light.html) about the experiment with the speed of light within a Bose-Einstein condensate, in case anyone is interested in reading it. The speed of light, which is a fundamental constant in a vacuum, is different when it is propagated through any medium. Glass lenses are able to bend light because of the inherent fact that the change in velocity at the interface between glass and air causes the light to bend if the surface is not perpendicular to the direction of the light. Bose-Einstein condensates are simply the most striking example of difference in speed. The speed in a vacuum is the greatest possible speed that in any other medium will be less.

<P ALIGN="CENTER">Tris</P>
------------------
Have a place for everything and keep the thing somewhere else. This is not advice, it is custom.
-- Mark Twain

Stephen
12-14-1999, 07:15 PM
Um, Dougie... What are you asking exactly? The Sun is about 93,000,000 miles from Earth. The speed of light in a vacuum is 186,000 mps and there are sixty seconds in a minute, so 93,000,000 / 186,000 / 60 = 8.333 minutes. What's the problem?

------------------
Stephen
Stephen's Website (http://stephen.fathom.org)
Satellite Hunting 1.1.0 visible satellite pass prediction
Satellite Hunting (http://stephen.fathom.org/sathunt.html)

handy
12-14-1999, 07:49 PM
If you put a brick in a light's path, the speed drops to 0. wow

LonesomePolecat
12-14-1999, 08:27 PM
I just read that article that Trisk found, and all I can say is -- damn, I'm amazed.

Llardball
12-14-1999, 08:46 PM
HANDY FOR PRESIDENT.

jab1
12-14-1999, 08:48 PM
So that's how a laser beam can be used to freeze something. And next they're going to try to slow light down to 120 feet per hour?!

What does a slowed-down light beam look like? It wasn't clear from the article.

------------------
&gt;&lt; DARWIN &gt;
____L___L__

AWB
12-14-1999, 09:08 PM
dougie_monty: So how come it takes light only 8 minutes (assuming a normal speed of light of 186,000 miles per second) to get to the earth from the sun, through outer space which is mostly a vacuum? Huh??????

186,000 mi/sec * 60 sec/min * 8 min = 89,280,000 (mi/sec)*(sec/min)*min
= 89,280,000 miles

That's about how far we are from the Sun.

AWB
12-14-1999, 09:09 PM
oops, sorry Stephen. I guess I should scroll down more before I answer. :)

LongHrn99
12-14-1999, 10:01 PM
Handy: What if it's got holes in it?

Kilgore Trout
12-14-1999, 10:10 PM
"What does a slowed-down light beam look like? It wasn't clear from the article."

look like?

Stephen
12-15-1999, 09:49 AM
I don't think anyone has seen a 'slowed down light beam'. The BEC is glowing brightly to begin with and when the light exits, it resumes normal (for air/glass/etc.) speed. My own WAG is that in the BEC it would look like a little light saber, slowly growing in length, until it reached the other side.

------------------
Stephen
Stephen's Website (http://stephen.fathom.org)
Satellite Hunting 1.1.0 visible satellite pass prediction
Satellite Hunting (http://stephen.fathom.org/sathunt.html)

RealityChuck
12-15-1999, 10:09 AM
You see "slowed down light beams" all the time. It's called "refraction."

In other words, light bends in a lens because it slows down as it passes through glass. The difference in speed isn't much, but it's enough to produce noticeable effects.

I'd guess that if you slowed down light to 38 MPH, the refraction will be even more noticeable.

------------------
"East is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does." -- Marx

Read "Sundials" in the new issue of Aboriginal Science Fiction. www.sff.net/people/rothman (http://www.sff.net/people/rothman)

handy
12-15-1999, 10:35 AM
Who has seen light anyway? Shucks, few can even define it, but hey, at least we can see it reflecting off things.

knappy
12-15-1999, 02:41 PM
Something has been bugging me about this report. The angle of refraction is related to the ratio of the speeds of light in the 2 media involved, for example, air & the Bose-Einstein Condensate. The bigger the slowdown the sharper the refraction angle. With this much slowdown, it seems like you would need an angle if incidence of almost exactly 90 degrees, or the light would get reflected & wouldn't pass thru.

Of course, I'm tacitly assuming that the B-E Condensate has a suface, which it doesn't really. More of a fuzz. The location of the individual atoms is severely smeared due to the fact that their momentum is so precisely determined to be near zero, and the Uncertainty Principle.

Can someone shed some light?

mr john
12-15-1999, 02:53 PM
I think dougie got distracted by the glare off the vacuum of space/temperatureof space/absolute zero deal,which is a whole nother thread or subeject that cecil addressed. All I know is ,as my grandmammy Light Horse Harper Lee used to say, Light Beer sux.

------------------

"Pardon me while I have a strange interlude."-Marx

12-15-1999, 03:20 PM
The ninety-degree angle referred to in the article is the angle of a laser beam introduced into the B-EC across the path of the beam to be measured. For reasons I barely comprehend, and cannot hope to explain the presence of the first laser is a part of the conditions being examined, which produce the slow speed for the second light.

<P ALIGN="CENTER">Tris</P>
------------------
Politicians are the same all over. They promise to build a bridge even where there is no river.
--Nikita Khrushchev

Stephen
12-15-1999, 03:28 PM
You see "slowed down light beams" all the time. It's called "refraction."

Yeah, but how often do you see a refractive index of 17,621,052? For that matter, how often do you encounter a photon travelling slow enough that you could hope to see it actually moving? I think KT was looking for an observation a bit out of the ordinary.

The angle of refraction is related to the ratio of the speeds of light in the 2 media involved

Don't forget, it is also related to the incidence angle. If you go straight in, you're gonna come straight out. Also, remember that the angle of refraction is related to wavelength and they are working in the infra-red.

------------------
Stephen
Stephen's Website (http://stephen.fathom.org)
Satellite Hunting 1.1.0 visible satellite pass prediction
Satellite Hunting (http://stephen.fathom.org/sathunt.html)

RickG
12-15-1999, 04:46 PM
The angle of the refracted beam is related to the angle of the incident beam (measured from a line perpendicular to the surface), by the equation

n_i * sin(theta_i) = n_r * sin(theta_r)

where n_i and n_r are the indices of refraction in the incident and refractive media, respectively, and theta_i and theta_r are the angles of incidence and refraction.
The BEC has an extremely large index of refraction, and the vacuum has index of refraction = 1. What this means is that the angle of refraction is extremely *small* compared to the angle of incidence. Therefore, no matter what the angle of incidence, the beam will enter the BEC pretty much perpendicularly.

Pretty neat.

Rick

Boris B
12-15-1999, 04:59 PM
All right now people, I don't know if you're really thinking what I think you're thinking, but let's all remember that we see photons only when they hit us in the eyeball. When light leaves the dense media we've been talking about, its speed increases. And since that dense media doesn't exist in our eyeballs, we're never going to see a photon at the super-low speeds. We always see light at the same speed; I don't know what the speed of light through vitreous humour is, but that's the speed.

Y'all seem to be thinking about a purely (and necessarily) hypothetical case whereby said 38 mph photon could carry a little flashlight and point it at us. The flashlight would send out photons at right angles to the vector of the flashlight-armed photon in the dense medium. As soon as the baby photons crawled to the edge of the medium, they could speed up and hit us in the eyeballs. Then we could see the (results of) the photon moving that slowly.

It's necessarily a fictitious case cause photons don't emit photons.

------------------
I don't want to make people think like me, I want them to think like me of their own free will.

Stephen
12-15-1999, 05:04 PM
Maybe, but various atoms and molecules excited by photons, emit photons. Rather than each photon carrying a flashlight, to indicate its position as it moved through the BEC, imagine instead, that it is moving through a forest, and igniting the leaves as it goes.

------------------
Stephen
Stephen's Website (http://stephen.fathom.org)
Satellite Hunting 1.1.0 visible satellite pass prediction
Satellite Hunting (http://stephen.fathom.org/sathunt.html)

Stephen
12-15-1999, 05:08 PM
Before I got cut off, I meant to say that we're still dealing in hypotheticals, but if you could excite certain atoms, without upsetting the BEC as a whole and you could differenciate between the illumination of the laser trap and the slow moving beam, who knows what you might be able to observe?

------------------
Stephen
Stephen's Website (http://stephen.fathom.org)
Satellite Hunting 1.1.0 visible satellite pass prediction
Satellite Hunting (http://stephen.fathom.org/sathunt.html)

DrFidelius
12-15-1999, 07:06 PM
If anyone is interested in a site that has far too many equations and charts for my blood:

http://www.spie.org/web/oer/may/may99/cover2.html

Boris B
12-16-1999, 11:14 AM
That makes sense as a hypothetical, Stephen. We could watch a pattern of trees ignite as the incredibly slow photon touched them off, and we could find out more about the whole wave/particle thing that I never understood.

MrSCOTT
12-16-1999, 04:16 PM
Interestingly enough, the speed of light in water is slow enough that scientists can accelerate particles to speeds faster than light in that medium.

When that happens, an interesting phenomena called "cherenkov effect" occurs. Even more interestingly enough, when the Starship Enterprise accelerates to warp speed, they show this Cherenkov Effect :)

Star Trek Rules.

-MrSCOTT

DrFidelius
12-16-1999, 04:21 PM

I thought the "wake" was caused by pseudogravitational lensing initiated by the warp field...

dougie_monty
12-16-1999, 06:41 PM
I said that because I understand that--well, sound waves cannot travel through a vacuum--how can light waves? Unless there really is a hypothetical "ether" in outer space...

12-16-1999, 08:30 PM

The Smithsonian does a very good job of explaining how this works: Putting the Brakes on Light (http://www.smithsonianmag.si.edu/smithsonian/issues99/jun99/phenom_jun99.html)
Essentially sodium is being cooled down to such low temperatures that normal physics does (do?) not apply. Read the article, it's fascinating.

It's also the source of my new sig...

------------------
...in a state so nonintuitive it can only be called weird...

Stephen
12-17-1999, 09:29 AM
Well Dougie, to put it simply, a photon behaves like a wave and a particle. The particle/wave duality allows light to interact with constructive/destructive interference (like irridescence) while also imparting energy to solar cells. So, vacuum is no problem.

------------------
Stephen
Stephen's Website (http://stephen.fathom.org)
Satellite Hunting 1.1.0 visible satellite pass prediction
Satellite Hunting (http://stephen.fathom.org/sathunt.html)

John W. Kennedy
12-17-1999, 11:28 AM
I said that because I understand that--well, sound waves cannot travel through a vacuum--how can light waves? Unless there really is a hypothetical "ether" in outer space...

Nope. Relativity proves that there is no "ether". (Making up "ether" to explain something does no good when, even if there were an "ether", it wouldn't explain the way things actually are.)

Electromagnetic "waves" -- just are.

------------------
John W. Kennedy
"Compact is becoming contract; man only earns and pays."
-- Charles Williams

DrFidelius
12-17-1999, 11:42 AM
So basically a photon is its own medium of propagation.

mumble mumble Maxwell's Equations mumble mumble wavicle mumble mumble compression wave in a medium versus the vibration of a packet of energy mumble mumble magnetic component balances against electrical component mumble mumble at right angles to its direction of motion mumble mumble

Hope that helps.

drewbert
12-17-1999, 11:56 AM
Let me add: mumble mumble something or other mumble Michelson-Morley experiment mumble mumble gotta go mumble later bye

dasmoocher
12-17-1999, 01:46 PM
This topic caught my eye because just the other day I attended a lecture by William Phillips, whose work relates to this. Two web sites http://www.nobel.se/announcement-97/phyback97.html http://www.almaz.com/nobel/physics/1997c.html

The lecture was for a general audience and mostly about atomic clocks. He showed a short video of what I understood to be a BEC trapped by lasers and a magnetic field. Computer-colorized, of course, so maybe they really don't glow like light sabers. My physic is weak, so maybe it wasn't a BEC, but it definitely was an image of super-cooled cesium atoms.

If anybody out there is interested and in the Wash. D.C. area, he'll be giving a series of lectures about this next month at the Univ. of Maryland. He's an entertaining speaker.

Surgoshan
12-17-1999, 06:54 PM
I recall someone asking how a laser could be used to cool something down. THe exact same question came up in my chem or physics class one or two years ago. I think the way it works is they wait until two molecules or atoms collide and one of them has almost no speed. Then they use the laser to getthe other one going really fast out of the system. THus, the system is cooler.

Oh, it was jab1 what asked.