Bose–Einstein condensate

[Icemelt]

What is the explanation for the slowing down of light to a snail's pace when it enters a Bose–Einstein condensate, only to regain its normal speed as it leaves ?

Experiments have shown that a laser pulse several miles long is reduced to just a few microns in length when it strikes a Bose–Einstein condensate.

Then, slowly passing through the medium, as it reaches its exit point, the laser pulse regains its length and speed.

Must we now assume that, when testing the limits of our physical environment, there are in reality no constants ?

[insane_alien]

the light gets absorbed, stored, then re-emitted. the bose einstein condensate has the remarkable property of releasing a photon exactly how it entered.

[iNow]

in other words, it doesn't "slow down," it just traverses a greater distance.

[Icemelt]

Well yes indeed !

You have just repeated exactly what I've already stated

But what is the explanation ?

It's no good just saying "it does" !

What we need to theorisze on is why, and what are the implications ?

[Kyrisch]

Bose-Einstein condensates are incredibly dense. The situation is similar to a ping pong ball falling from an airplane through a dense forest. While it may take longer for the thing to reach the ground because of all the impacts and reflections off of the obstacles, it doesn't invalidate the constant of acceleration due to gravity.

[Icemelt]

in other words, it doesn't "slow down," it just traverses a greater distance.

 

OK but we can measure this distance since it the condensate is trapped in a tiny magnetic bottle in a lab, and it takes the light several seconds to traverse it !

[Kyrisch]

Yes... And if you take that distance and do [math] \frac{\Delta d}{\Delta t}[/math] you will get the known constant c. The constant is NOT invalidated.

 

~

 

Oh and as a side note w00t just used LaTeX editor for the first time and it is f*in awesome xP

[Icemelt]

in other words, it doesn't "slow down," it just traverses a greater distance.

 

Bose-Einstein condensates are incredibly dense. The situation is similar to a ping pong ball falling from an airplane through a dense forest. While it may take longer for the thing to reach the ground because of all the impacts and reflections off of the obstacles, it doesn't invalidate the constant of acceleration due to gravity.

 

No !

I can't agree with that

If the light was bouncing off the particles due to the high density of the condensate, then it would be scattered as it leaves the medium.

But it isn't !

It leaves as the same nice concentrated tight laser beam exactly as it entered.

[iNow]

I'd like to see some empirical evidence of those claims before I take them as anything more than your (potentially seriously mistaken) personal opinion.

[Kyrisch]

It was an analogy because it seemed to me as though you were not understanding the concept. In truth, the molecules absorb the photons, waits a tick, and then re-emits them in exactly the same way, producing that "nice concentrated tight laser beam exactly as it entered". Regardless, [math]

 

\frac{\Delta d}{\Delta t}

[/math] remains constant.

 

Edit: iNow, to whom was that post addressed?

[swansont]

Forgive me quoting myself, but I recently addressed this

http://www.scienceforums.net/forum/showpost.php?p=410859&postcount=29

 

Slow light happens because you prepare a sample with a really sharp resonance, and use light near that resonance.

 

low light arises from the really large changes in the absorption profile, leading to a rapidly changing index of refraction for group velocity, which gives rise to a classical explanation of slow light. I think quantum-mechanically it'll still be absorption and re-emission, but you've got lots of absorption (you're near resonance)

 

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

Scroll down to the 6th equation and ask yourself what happens if you create a sample where [math]\frac{dn}{d\lambda}[/math] is negative and large, and what must then be true of the group velocity in the preceding equation.

 

Lena Hau's paper explains it a little differently, but the conclusion is that the index they create is about a million times larger than the typical index for a normal sample of cold atoms

http://www.seas.harvard.edu/haulab/publications/pdf/Slow_Light_1999.pdf

[iNow]

Edit: iNow, to whom was that post addressed?

 

Sorry. I got lazy and didn't use the Quote function. It was to the post immediately prior to my own.

[Kyrisch]

Oh, alright. I was a bit confused because he didn't really make any claims, just seemed to refuse the ones proffered and we sort of had a conflict the other day in the post about where space ends and all I just wanted to make sure things were smoothed over and stuff.

[Icemelt]

Lena Hau's paper explains it a little differently, but the conclusion is that the index they create is about a million times larger than the typical index for a normal sample of cold atoms

http://www.seas.harvard.edu/haulab/p...Light_1999.pdf

 

Good grief !

That was nearly ten years ago ?

So what's happened since ?

 

"With improved frequency stability of our set-up and lower coupling intensities, even lower light speeds would be possible, perhaps of the order of centimetres per second"

Edited June 8, 2008 by swansont
fix quote tag

[swansont]

Other stuff. Slow light is not an end in and of itself.

 

edit/update: http://www.news.harvard.edu/gazette/2007/02.08/99-hau.html But you have to get past the horrendous mangling of the physics in the first three paragraphs — slow light did not shake any physicist's beliefs or contradict relativity, "stopped light" is absorption (albeit in special conditions), and light was not converted into matter.

 

I'll be blogging about this act of mayhem.

Edited June 9, 2008 by swansont
multiple post merged

[elas]

Light passes through a given number of particles in a given unit of time. An observer in the same frame observes the light traveling at c, an observer in a different frame sees it traveling at a different speed. In the Michelson-Morley experiment the observers measured the speed of light in their own frame; Hau times a movement in a different frame.

The difference between frames is a difference in the density of particles.

[swansont]

A BEC is a dilute gas with about 10^14 atoms/cm^3. An ideal gas at STP will have a density a million times greater. Slow light is a trick of specially-prepared atomic states.

[amiya]

A very apt question by icemelt. Global warming really affects the forum now. I, personally, believe, however unscientific it sounds, that nothing in the world is absolute; except the statement itself. Take light's speed (versus Tachyons, particles supposedly having higher velocity than light, defying the theory of relativity) or absolute temperature (maybe in some day scientists will go below 0 degree Kelvin). This is only expected, as absolute or relative, is relative to the basis of the present knowledge.

 

Regarding the 'trapping' of light in this instance, it happens to me that this property could be used in 'compacting' of information; much like 'zipping' as in Win-Zip. The strange behavior of light will take time to be explained, by using 'quantum theory', perhaps.

[swansont]

Tachyons don't defy relativity. They are hypothesized particles that would conform to relativity, but that requires imaginary energy or imaginary mass (and perhaps negative as well), and we don't know what that means, physically.

 

Negative temperatures are another area that is interesting, because you can do it in non-equilibrium conditions — the population inversion of a laser, for example, gives a negative temperature. However, for any kind of equilibrium condition, no, that breaks physics. You can't have any less center-of-mass motion than "none at all."

[nstansbury]

Tachyons, particles supposedly having higher velocity than light, defying the theory of relativity

 

Relativity doesn't preclude superluminal activity, it says bodies can't be accelerated FTL. Constant superluminal activity is not precluded.

[Cosmodot]

Take light's speed (versus Tachyons, particles supposedly having higher velocity than light, defying the theory of relativity) or absolute temperature (maybe in some day scientists will go below 0 degree Kelvin).

 

Light speed does not actually describe how fast light travels. It only describes how fast it could travel in a vacuum. In our Universe, there is no such thing as a perfect vacuum (across physically quantifiable units of space), but in many places it gets close enough to accurately estimate. It gives us a "pressure spectrum" through which to measure the speed of light. This is a theory based on facts that have been tested by people who, at first, doubted it... but they also tested it and did so rigorously. This is why Relativity is so widely accepted in the scientific community. It has yet to be debunked.

 

I don't think light actually moves at all. I don't think anything really "moves," because to me, everything already exists within the same place. I don't think light actually moves, because the matter in between two other particles share an electromagnetic relationship. Without particles of matter, you can't have light. Light exists AS the relationship between particles of matter. The further any two particles are apart from each other, the faster the relationship is realized. This is called conjecture. It's an idea that has no basis in experimentation (that I know of.) but it's something I've come to believe (however poorly refined such thoughts/beliefs may be.)

 

I'm an amateur and an enthusiast, but it doesn't take much more than an open state of mind to understand what the pursuit of science is all about. It's not about finding answers to your questions. Science is about finding better questions and being satisfied in the pursuit of better answers while never being satisfied with the answers. It's about being satisfied with never being fully satisfied. There are no complete answers. There is no end in sight nor a hope for one. That is science. It's about admitting that you might not know what you're talking about.

 

Personally, I believe that in a perfect vacuum, not even light can travel. There just is no such thing as a "perfect vacuum." I believe that if there was, it would destroy itself, possibly creating what we see now and it would occur at every moment. The destruction of the vacuum is what composes everything. It's hard to describe physically (or philosophically), but I believe that light would somehow travel through this reverberation as well as electrons and the nuclear components. This is also conjecture.

 

Tachyons are part of a hypothesis. They're not yet theory, because facts which prove the hypothesis of tachyons have not yet been discovered. If the prediction is someday proved by experiment, we will acquire a theory on tachyons. They're more than conjecture because of the mathematical and hypothetical background that they have been derived from. My level of mathematics is practically inept, but I understand the idea of tachyons intuitively. I have a feeling that the unseen parts of our Universe are much more complex than particles that move faster than light. I think that they don't really move at all, but there's hardly a point to get into anything more. I've already spouted more than I can back up with any type of evidence, but at least I know the difference between a scientific theory and a personal opinion.

 

For my first post, I'm almost glad I exposed this much of my... self. Call it what you will, but I'm glad I stumbled across this forum and I hope to learn more than there is to know in the moments past. After reading a few other threads, my expectations sunk but my hopes hang high. I'll end my rant by saying that I think some of you seem genuinely enlightened and I pray such knowledge is contagious.

Edited July 12, 2008 by Cosmodot

[Deja Vu]

Light speed does not actually describe how fast light travels.

 

Really? I always thought that this is precisely what is meant by light speed.

 

I don't think light actually moves at all. I don't think anything really "moves," because to me, everything already exists within the same place (Mostly conjecture which I fail to explain).

 

Nothing can move?! If nothing ever moves, then how am I typing on a computer, or how does my room stay at 20 C ?

 

 

I don't think light actually moves, because the matter in between two other particles share an electromagnetic relationship (duh). Without particles of matter, you can't have light. Light exists AS the relationship between particles of matter. The further any two particles are apart from each other, the faster the relationship is realized.

 

How so? Light is made out of photons, which are massless particles, so I'm not seeing how you come to this conclusion.

 

This is called conjecture. It's an idea that has no basis in experimentation (that I know of.) but it's something I've come to believe (however poorly refined such thoughts/beliefs may be.)

 

I think this is more correctly termed a faith or an opinion. The behavior of the rest of the universe does not seem to confirm your speculations.

 

 

Anyways, tachyons are part of a hypothesis. They're not yet theory, because facts which support the hypothesis of tachyons have not yet been discovered. If the prediction is someday proved by experiment, we will acquire a theory on tachyons.

 

Exactly. The problem with detecting tachyons is that they move faster than light, and will move even faster when they lose energy, so you have a very small window of time before their velocity becomes infinite.

 

I have a feeling that the unseen parts of our Universe are much more complex than particles that move faster than light. I think that they don't really move at all.

 

Do you mind elaborating more on this? Or, make a new thread on it as I believe this is very much off topic.

 

For my first post, I'm almost glad I exposed this much of my... self. Call it what you will, but I'm glad I stumbled across this forum and I hope to learn more than there is to know in the moments past. After reading a few other threads, my expectations sunk but my hopes hang high. I'll end my rant by saying that I think some of you seem genuinely enlightened and I pray such knowledge is contagious.

 

This is my first time on this site too . Welcome aboard! From what I am finding, there is lots to learn on this site, and plenty of people to talk to who are very knowledgeable on various subjects. I hope you enjoy this site.

[Cosmodot]

Really? I always thought that this is precisely what is meant by light speed.

 

1. Really? I always thought that this is precisely what is meant by light speed.

 

2. Nothing can move?! If nothing ever moves, then how am I typing on a computer, or how does my room stay at 20 C ?

 

3. How so? Light is made out of photons, which are massless particles, so I'm not seeing how you come to this conclusion.

 

4. I think this is more correctly termed a faith or an opinion. The behavior of the rest of the universe does not seem to confirm your speculations.

 

5. Do you mind elaborating more on this? Or, make a new thread on it as I believe this is very much off topic.

 

I apologize because I haven't yet figured out how to more conveniently quote and address specific comments and questions within another post so I numbered them.

 

1. Light speed is the theoretical speed of light or C. Light travels slower than C through matter. Matter is everywhere in the Universe.

 

2. That comment was more philosophical than scientific (I tend to mix them more often than I should), related to classical perceptions of motion. You're typing on a computer because the forces composing the matter of both you and the keyboard you're typing on are closely interacting, in a manner sufficient to cause a reaction perceivable by you. Your room stays at 20 C for a number of reasons. One has to do with the relationship of the matter and energy between your house and the sun. Another involves that of the matter and energy 'moving' through your air conditioner. The particles are moving relative to each other, but what are they moving through? I think it really comes down to what composes space. Like I said, it was more philosophical than scientific.

 

3. Photons have not yet been conclusively proved to be massless. We have a classical definition and maybe soon we'll have a quantum clarification, but there's hardly a promise of completion to any theory in existence, much less the mystery of gravity. Light reacts to gravity through space, but it doesn't actually exhibit mass of it's own? I understand (vaguely) the notion of space-time, but it's just not complete. I think there needs to be more and perhaps there will always need to be more. I don't think that light is the same thing as matter, but rather an additional property of the way that matter interacts with itself. That much is proven, but I don't see light nor space as a separate entity. It's intuition, speculation or imagination. I just feel as if everything is, in some sense, attached or all part of the same tangible thing. Light and space are the attachment and matter is the 'thing.'

 

4. I won't argue with that. You're right. It's opinion or speculation. I don't really like the word faith, because I refuse to conform to even my own opinions.

 

5. Maybe I'll make a thread on this later, but first I think it would serve us all better if I could better grasp, verbally, the concepts I'm trying to convey.

 

I was actually somewhat shocked that someone responded so promptly. I hope this clarifies a bit more of my perspective, however 'off-course' it may be. I'll be the first to admit that I'm not a contemporary thinker. It helps to relate on an intuitive level, but I'm always willing to readjust to a course that is perceptively more reasonable than my own. Any further responses to this will warrant the creation of a new thread. I agree that this has little to do, specifically, with Bose-Einstein condensates.

Edited July 12, 2008 by Cosmodot

[Kyrisch]

Take light's speed (versus Tachyons, particles supposedly having higher velocity than light, defying the theory of relativity)

 

It doesn't quite 'defy' the Theory of Relativity. All the equations in the theory say that an object with nonzero mass can never travel at the speed of light because as its velocity approaches c, its relativistic mass approaches infinity. However, the equation is as follows:

 

[math]m_{relativistic} = \frac {m_{rest}}{\sqrt{1-\frac{v^2}{c^2}}}[/math]

 

At the point where [math]v=c\,,m_{relativistic}[/math] is undefined. However, if [math] v>c\,, m_{rest}[/math] simply needs to be negative or imaginary, depending on which equation you use, and a coherent answer still comes out. What these terms translate to in the physical world is anyone's guess, but it essentially still follows the equations of the theory.

[Cosmodot]

It doesn't quite 'defy' the Theory of Relativity. All the equations in the theory say that an object with nonzero mass can never travel at the speed of light because as its velocity approaches c, its relativistic mass approaches infinity. However, the equation is as follows:

 

[math]m_{relativistic} = \frac {m_{rest}}{\sqrt{1-\frac{v^2}{c^2}}}[/math]

 

At the point where [math]v=c\,,m_{relativistic}[/math] is undefined. However, if [math] v>c\,, m_{rest}[/math] simply needs to be negative or imaginary, depending on which equation you use, and a coherent answer still comes out. What these terms translate to in the physical world is anyone's guess, but it essentially still follows the equations of the theory.

 

Agreed. Initially that comment was made by someone else up the list. I was actually responding to it, not stating it and I seem to have gone completely off track. Sorry for the confusion and the rambling.

 

Like I said, I'm new here and since I first posted, I've spent a little time in the stickies trying to get a better perspective on how this forum operates. I think I'm getting the basic idea, although it would be helpful to figure out how to most conveniently quote and respond. I'll get the hang of it sooner or later, but likely before I decide to jump back into the ring, so to speak.

 

There seems to be enough dreaming and delusion around here even without my 2 cents. I'm just glad I found a place with enough skeptics to keep the nonsense and pseudoscience to a minimum.