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Light gets ticketed, slows down.

By aphrael in News
Sat Feb 10, 2001 at 11:55:39 AM EST
Tags: Science (all tags)
Science

The January 25 issue of Nature reports that a group of researchers at Harvard have, under certain complicated conditions, reduced the speed of light to zero.


The experiment builds on previous work by the same group which has been replicated by numerous groups of researchers elsewhere. The basic idea is that you cool a particular type of gas to within one millionth of a degree above absolute zero and then pass two lasers through it. One of the lasers, referred to as a coupling pulse, will adjust tweak the states of the atoms in the gas so as to make it more difficult for them to absorb the other pulse.

In their original experiment, the group demonstrated that they could slow the movement of light through the gas to such an extent that the pulse did not exit the gas until after it had been completely absorbed by it. In this experiment, by turning the coupling laser off, the group was able to prevent the continued propogation of the pulse until after the coupling laser was turned back on.

They suggest that this might have applications to quantum computing. That's the obvious use for it (although it will be a while; cooling something to just above absolute zero is expensive, and while their original results have been replicated in warmer gases, the length of the delay seems to decrease the warmer the gas gets). But I wonder if there are other applications, including perhaps:

  • Can something derived from this technology be used to shield a building from emitting electromagnetic radiation, achieving something similar to the Ixian no-room described in God Emperor of Dune? Failing that, would it be possible to tune the gas to also change the direction of the radiation, causing everything emitted from a building to radiate back into it?
  • Can this be applied somehow to defense from space-based lasers used as weapons? (The day when those weapons will exist is unfortunately coming).
  • (Less likely) Is it possible to adapt this technology to stop something other than electromagnetic radiation?

Another curiosity lies in this question: right now the maximum length of time that light can be stopped in the gas is one millisecond (for a 6-microsecond pulse). If it were to become possible to stop light for longer, would the result be that the gas would heat up? Eg., could you use the gas to convert the energy in the pulse into heat?

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Light gets ticketed, slows down. | 43 comments (33 topical, 10 editorial, 0 hidden)
I find this news incredibally worrying. (2.43 / 16) (#2)
by Kiss the Blade on Fri Feb 09, 2001 at 08:19:40 PM EST

Okay, so they have reduced the speed of light in this gas to near 0. Now, what happens if you spin the gas about, to create a vortex like in a sink? You then have a medium moving faster than the speed of light!

Effectively, you will have a black hole; a region of the universe with no known physical laws. But instead of being safely millions of miles away in space, it will be right here on Earth.

Has anyone thought of the implications? It could well be dangerous, we just don't know. The point about a black hole is that nobody knows what the hell it is likely to do next. It could very well be extremely dangerous, unless we are very careful.

I just wish the scientists would be more careful, and perhaps think before creating dangerous situations. I know the risks are small, but when the consquences would be so great, it is best to consider them in depth.

KTB:Lover, Poet, Artiste, Aesthete, Programmer.
There is no contradiction.

speed of light... (4.66 / 3) (#5)
by ism on Fri Feb 09, 2001 at 08:40:45 PM EST

Okay, to our knowledge, things can't go faster than the speed of light. That's slightly misphrased. Things can't go faster than the fastest light can go. Light is routinely slowed down. When light refracts through a glass of water, it actually travels slower than c, which is why it "bends".

Secondly, about black holes; they are objects with extreme density such that the escape velocity is equal or greater than the speed of light. Theoretically, since nothing can go faster than light, nothing can escape the black hole's gravitational field. A black hole (which is currently theoretical, by the way) is not a region where physics laws do not apply. There are several theories as to what happens at the center of a black hole, and they are explained with physics (quantum mechanics, superstring theory).

[ Parent ]

Misunderstandings (5.00 / 2) (#6)
by jasonab on Fri Feb 09, 2001 at 08:43:07 PM EST

I think your physics is a little messed up. Light slows down in lots of media (glass, water, etc). The trick is, nothing can go faster than the speed of light in a vacuum. Clearly, this slowed light is not in a vacuum.

A black hole has nothing to do with light directly, except that the force of gravity is such that the escape velocity is higher than the speed of light. We don't have anything like that kind of energy here on Earth.


--
America is a great country. One of the freest in the world. -- greenrd
[ Parent ]
Ummm. . . (3.00 / 1) (#7)
by slick willie on Fri Feb 09, 2001 at 08:58:49 PM EST

My understanding of black holes is that they are extremely large, extremely dense masses. (Not unlike some of my co-workers...)

I doubt if there's enough mass in a lab to create the forces required for a singularity.

"...there is no limit to what a man can do or where he can go if he doesn't mind who gets the credit."
--Ronald Reagan, First Inaugural Address

[ Parent ]

An Earth black hole probably wouldn't be dangerous (5.00 / 3) (#8)
by jesterzog on Fri Feb 09, 2001 at 09:03:54 PM EST

Others have already discussed the speed of light issues, so I won't go there. On the other hand, even if we did end up with a Black Hole on Earth, it wouldn't necessarily be dangerous. Please note that all of this is current generally accepted theory. Even then I'm a bit rusty so anyone who wants to can jump in and correct me.

It is true that in current theory, light can't escape from a black hole, and what goes on in there can be pretty weird. But all of this happens inside the event horizon. The key is how far out the event horizon goes.

Black holes can be of any mass. But contrary to popular belief, if our Sun ever turned into a black hole (which it won't), we wouldn't die from being sucked into it. Instead, we would die from lack of energy coming from the Sun to keep us warm, and give us light. The Earth would still continue to orbit the Sun just as it always has, as long as the Sun was the same mass, since the forces (where the Earth is) would remain the same. ie. Outside the event horizon, gravitation forces stay the same as they would be if it wasn't a black hole.

Now with regard to that bit up there where I said that black holes can have any mass. Because you usualy need a really massive star to go supernova before a black hole forms, a lot of people get the impression that lots of mass is needed. This mass is really just a trigger level for dying stars, but in theory the final mass of a black hole can be any amount.

It would in fact be theoreically possible to make a black hole from the Earth. To do this would require compressing all the matter of the Earth into a very small space. I think with the Earth's mass, this works out to be about 1cm. (Less than half an inch.) This is because 1cm is the Earth's event horizon. If all the matter is inside it pulling in, nothing can escape. But it isn't.

If anyone managed to create a black hole somewhere on Earth, they only have a minutely tiny amount of matter to work with. Likely it would have an almost unmeasurable event horizon. We might need to contain it in a special jar, but it's detructive potential wouldn't be massive. It would be hard enough for it's ordinary gravitational range to have an effect on anything, let alone getting something within it's event horizon.

This whole theory could be completely changed within a few years following future discoveries. Things like that always happen in science, which is why it's so interesting.


jesterzog Fight the light


[ Parent ]
Sun as black hole (4.00 / 1) (#40)
by sigwinch on Mon Feb 12, 2001 at 05:46:27 PM EST

But contrary to popular belief, if our Sun ever turned into a black hole (which it won't), we wouldn't die from being sucked into it. Instead, we would die from lack of energy coming from the Sun to keep us warm, and give us light.

Instead, we would die from the tremendous flash of energy released as Sol was crushed into a black hole. Gravitational collapse is very efficient at converting mass to energy -- makes a uranium bomb look frigid by comparison. The neutrinos alone would be lethal.

--
I don't want the world, I just want your half.
[ Parent ]

Maybe =P (none / 0) (#43)
by jesterzog on Tue Feb 13, 2001 at 04:37:47 PM EST

lol. This is getting subjective, because it comes back to the problem that the Sun doesn't have enough mass to produce the flash of energy in the first place.

I should have said "if the Sun were a black hole" without even considering how it would get there. Thanks for pointing that out.


jesterzog Fight the light


[ Parent ]
Do you know what you're talking about? (3.50 / 2) (#9)
by Anonymous 6522 on Fri Feb 09, 2001 at 09:16:58 PM EST

First off, I know from my high school physics class that light, when it travels through anything, goes slower than light when it travels through a vacuum. The speed of light is not a fixed speed, the speed of light in a vacuum is. The light in this experiment is not traveling through a vacuum.

The definition of a black hole is, "an object in space so dense that no light or radiation can escape from it." (from here) These researchers were just experimenting with a very cold cloud of gas that happens to slow light down much more than normal. I don't see how this could be related to black holes at all.

I know exactally what a black hole would do if one suddenly appeared in my bedroom, it would suck in everything around it, including the planet. It's not going to do anything strange and unpredictable, it will only suck. That's it, it would only suck. It would do nothing but suck. There is absolutely no question about it in anyone's mind but yours.

I am not a physicist, and I doubt you are, but that's exactally what the people who do these experiments are. They know what they are doing, they think things through. They don't go doing experiments willy nilly, they design them to test ceritan hypotheses, and it takes years to set up experiments like this. Even if they could create a black hole (which I'm pretty damn sure they can't) they wouldn't do it because I'm sure they value their lives very much.

If you're a troll, you're not a very good one. I'd be suprised if anyone takes your post seriously.

[ Parent ]

Besides which (4.66 / 3) (#12)
by aphrael on Fri Feb 09, 2001 at 09:24:38 PM EST

there's no reason why a black hole of mass [x] would do any more sucking than a computer of the same mass --- the gravitational attraction would be the same either way. It just wouldn't let go of anything it got a hold of.

If we could get low-energy, low-mass black holes, they'd make great trash cans. :)

[ Parent ]

It still would suck... (none / 0) (#20)
by Anonymous 6522 on Fri Feb 09, 2001 at 09:59:31 PM EST

If I had a black hole the mass of my computer on my desk, it would:

* suck in all the air around it
* fall tward the center of the Earth, sucking in even more mass
* once it's at the center, it would continue to suck in matter until everything that was the planet Earth is inside of it.

The only way to stop it would be to contain it somehow. You could put it in a vacuum, but (I think) the only force that affect black holes is gravity, and we can't control it like we do, say, magnatism.

If we ever make black hole trash can, I'm getting the hell off the planet. I just know some moron would kick his the wrong way and end up getting everyone killed.

[ Parent ]
Air (5.00 / 1) (#22)
by aphrael on Fri Feb 09, 2001 at 10:07:03 PM EST

ok, I can see that: random motion of the air would cause particles to fall into it and never be released. And certainly, if it were not contained in some fashion, it would swallow up anything it *touched*; I have a hard time following that to destroy-the-earth proportions, tho.

[ Parent ]
Reply... (none / 0) (#23)
by Anonymous 6522 on Fri Feb 09, 2001 at 10:43:58 PM EST

The black hole would be pulled down, so it will touch the ground. It would suck everything it touches into it, so it would get progressivly deeper into the Earth. It would be very unlikely that it would settle in the center of the Earth with a vacuum around it. Stuff would be continually pressed into contact with it because of the pressure. Eventually its gravity will be so strong that it would be able to overcome any structure that resists being pulled in. This may take a long time, but it would eventually pull all of the Earth into itself.

[ Parent ]
Besides which (5.00 / 1) (#33)
by spiralx on Sat Feb 10, 2001 at 07:02:03 AM EST

It would oscillate around the centre for quite a while before settling down anyway, it wouldn't stop straight away. And as for pulling the entire Earth in... depends on how strong the pull was after pulling in all of the liquid matter available. If the other stuff was strong and dense enough it might be able to resist. Hmmm, interesting :)

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

Actuall that small a black hole..... (5.00 / 1) (#36)
by Mycroft_VII on Sun Feb 11, 2001 at 02:23:18 AM EST

All black holes radiate (Hawking radtiation) a Black hole THAT mass ( <10kg ) would: a: have a event horrizon smaller than could fit an electron. b: detonate through hawking radiation and probably take out a few blocks of real estate min.

[ Parent ]
Hawking radiation (5.00 / 1) (#38)
by a humble lich on Mon Feb 12, 2001 at 12:50:07 AM EST

Yes but Hawking's calculation was done semiclassically, assuming a large black hole. Although I have not looked at the details of the calcualtion, it was my impression that it had questionable validity for black holes of that size. To really see what would happen for small black holes you would have to use a complete theory of quantum gravity (which does not yet exist).



[ Parent ]
Re: Hawking radiation (5.00 / 1) (#41)
by sigwinch on Mon Feb 12, 2001 at 05:55:30 PM EST

The Hawking radiation equations might not apply exactly as you approach the Planck scale, but they are probably qualitatively correct. Experimentally, there are many processes that could create small black holes (supernovae, Big Bang, neutron star collisions), yet no one has observed stable supermassive particles, which is consistent with spontaneous evaporation of black holes.

--
I don't want the world, I just want your half.
[ Parent ]

Sci-Fi Novel (5.00 / 1) (#39)
by guinsu on Mon Feb 12, 2001 at 01:35:00 PM EST

I just read a Sci-Fi novel (it was from the 70's I think, but just republished) about this, mini black holes that were accidentally produced. They oscillated up and down from the center of the earth to the surface. I believe it was by Gregory Benford, but I could be mistaken. It was a tad dated, the scientists in the novel were using PDP-2000's or something and it was set in the 90's :)

[ Parent ]
Optical black holes (none / 0) (#28)
by guffin on Fri Feb 09, 2001 at 11:45:16 PM EST

You may be confused about Optical Black Holes. Very VERY different phenomenon. See here for a popular media view. Nice paper here(pdf) and some nice links from the aip.

[ Parent ]
Joining the chorus (5.00 / 1) (#34)
by mdxi on Sat Feb 10, 2001 at 08:11:40 AM EST

I just also want to go on record as pointing out that the universal speed limit is the constant defined by the speed of light in a vacuum.

Still, even within a certain medium, it is possible for particles to exceed the LOCAL speed of light. For example, during the incident at teh nuke fuel processing plant in Japan last year, some workers reported seeing a bluish glow in the tank where the boo-boo happened.

This phenomena is known as Cherenkov Radiation, and it is released when excited particles exceed the local speed of light (in this case, the speed of light in the water-filled fuel holding tank).

On a side note, seeing Cherenkov radiation is a Very Bad Thing because it means you have just been exposed to lethal doses of radiation.

--
SYN SYN NAK
[ Parent ]

I don't think you're right (5.00 / 1) (#35)
by guffin on Sat Feb 10, 2001 at 01:01:33 PM EST

Because in the old days (1950's i think), the canonical method to see if your particle accelerator was turned on was to stick your eye in the beam. If you saw Cherenkov radiation, you knew it was on. Now, although those physicsts are walking around today with big coke-bottle glasses, they are still walking around.

[ Parent ]
More information: NY Times (3.20 / 5) (#3)
by jrh on Fri Feb 09, 2001 at 08:21:20 PM EST

A NY Times article on this is here.

No-room... (4.50 / 2) (#11)
by Anonymous 6522 on Fri Feb 09, 2001 at 09:24:21 PM EST

I don't see how this could be used it a no-room type device, but would you really want to be cooled down to one millionth of a degree above absolute zero, just so the God Emperor couldn't see you?

You wouldn't have to be. (5.00 / 2) (#13)
by aphrael on Fri Feb 09, 2001 at 09:27:16 PM EST

I'm imagining a room surrounded by this substance. Since radiation can't pass through it, it's an effective barricade against just about all forms of spying ... and the fact that it's surrounding the room doesn't mean the room itself is near absolute zero.

[ Parent ]
As I understand it... (none / 0) (#16)
by Anonymous 6522 on Fri Feb 09, 2001 at 09:40:48 PM EST

this substance allows radiation to pass through, it just slows light down alot in some curcumstances. I also believe that the light has to be at a specific frequency to be slowed down so extreamly.

[ Parent ]
Your understanding (4.00 / 2) (#18)
by aphrael on Fri Feb 09, 2001 at 09:48:09 PM EST

is correct for the technology as it currently exists. I'm speculating about future developments.

[ Parent ]
Very Unlikely (5.00 / 3) (#27)
by guffin on Fri Feb 09, 2001 at 11:37:57 PM EST

I'm pretty sure such a thing will never happen. The reason the light is absorbed is because the atoms in the gas have an atomic transition whose energy cooresponds to the frequency of the light absorbed times plank's constant. These transitions, or energy levels, are quantized. Thus, you can't absorb all available wavelengths (i'm assuming no Doppler broadening of the absorption lines, since the gas is at such a low temperature)

[ Parent ]
Well... (5.00 / 5) (#14)
by guffin on Fri Feb 09, 2001 at 09:30:06 PM EST

Cooling things to just above absolute zero isn't that expensive. I'm building a device to do it right now in my lab course. The technology is actually rather old: it was first demonstrated in the mid-eighties I believe. There is a very nice article in the American Journal of Physics (Inexpensive laser cooling and trapping experiment for undergraduate laboratories, Am.J. Phys 63, April 1995, p.317) that has pretty much step-by-step instructions on how to do it. From the abstract of said article:
We present a design which has reduced the cost to less than $3000 and does not require any machining or glassblowing skills in construction... These features make the [atom] trap simple and reliable to operate...


Let's try this (3.60 / 5) (#15)
by fluffy grue on Fri Feb 09, 2001 at 09:33:58 PM EST

Okay, first of all, there's easier ways to shield something from electromagnetic radiation. It's called a "mirror." If you want to get really fancy, you can absorb the radiation and route around something; this is called a "Faraday cage," and almost every piece of consumer electronics has one, as required by FCC regulations. Spook-type agencies and other people who need to stay secure (DoD, DoE, etc.) typically have very good electromagnetic shielding for this purpose - having been an intern at Sandia National Labs, I'm well aware of the huge degrees they go to in order to avoid leaking out any EM.

Space-based lasers are still lasers. They might put out HUGE amounts of EM, but it's still EM - a perfectly-efficient mirror will still reflect most of it. A dome made of, say, pure gold will do a pretty good job of reflecting most of the energy away.

Stopping something other than electromagnetic radiation? Well, there's more elegant ways of doing this, especially considering that photons (with zero rest mass) have literally infinitely less momentum than practically everything else (barring neutrinos); the energy has to go somewhere. Concrete comes to mind.

In all of these cases, stopping light only does that - stops it. The energy still remains, and would (theoretically) have to be released eventually. Conservation of energy, etc. Which also answers your last question - the gas wouldn't heat up, assuming the pulse of light were to remain intact.
--
"Is not a quine" is not a quine.
I have a master's degree in science!

[ Hug Your Trikuare ]

Err.. (none / 0) (#19)
by guffin on Fri Feb 09, 2001 at 09:58:51 PM EST

Actually you're incorrect on a few points.
  • It's not that lasers put out a huge amount of photons that makes them so intense, it's that the light is coherent.
  • Although photons are massless (some theories say they aren't though), they still have momentum.
  • From your statements on neutrinos, i'm assuming you think they are massless. There is actually quite a debate on this subject.
  • The light is not actually stopped. See my other comment.


[ Parent ]
Space-based lasers (none / 0) (#42)
by davidduncanscott on Tue Feb 13, 2001 at 09:03:09 AM EST

Jeez, let's be pedantic. The poster was talking about space-based laser weapons, and in order to reach the ground such devices would indeed put out huge amounts of EM, so much so that nuclear explosions have been proposed to power them. Yes, the light is coherent, but quantity counts too, which is why I can't shoot down airplanes with my laser pointer.

[ Parent ]
Grrr... second try (4.00 / 1) (#31)
by spiralx on Sat Feb 10, 2001 at 04:11:24 AM EST

Okay k5 ate my first comment, so here goes again :)

Space-based lasers are still lasers. They might put out HUGE amounts of EM, but it's still EM - a perfectly-efficient mirror will still reflect most of it.

A laser puts out no more EM than any other light source. The difference is that the light is coherent - all of the photons have the same frequency and their phases are in step. This is why a laser can travel so far without losing focus - there are little or no (in theory) interference effects.

Stopping something other than electromagnetic radiation? Well, there's more elegant ways of doing this, especially considering that photons (with zero rest mass) have literally infinitely less momentum than practically everything else (barring neutrinos); the energy has to go somewhere.

How do photons have infinitely less momentum than other particles? Considering that p=E/c for a massless particle it is possible to have a vast range of momenta for any given photon, and the gamma rays that impact on our atmosphere can have huge amounts of energy, far more than anything we can yet do in the lab.

Neutrinos are thought to have zero momentum and as such have a similar range of momenta, however they don't hardly interact with anything at all. However it is possible for neutrinos to have a small mass (less than 10 electron volts) which would also present a solution to the solar neutrino problem (we only see a third of the amoount of solar neutrinos that we expect, but if neutrinos have mass then they can cycle between the three types of neutrinos and we would only detect electron neutrinos not the other kinds).

And gravitation is the only other long-range force, but this kind of technique is probably not effective due to the extremely weak nature of the force.

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

Zero rest mass? (4.00 / 1) (#37)
by inpHilltr8r on Sun Feb 11, 2001 at 04:39:22 AM EST

What is the Mass of a Photon?

[ Parent ]
They don't actually stop the light (5.00 / 5) (#17)
by guffin on Fri Feb 09, 2001 at 09:41:57 PM EST

What happens is the atoms basically absorb and 'remember' the state of the incoming photons. Then, the pump beam causes the atoms to emit a photon in the same state as the one it absorbed. It's a very neat quantum mechanical trick, but it's not as though the photons are actually being lasso'ed and stopped.

  • Can it be used as a defense against space based weapons?
    Probably not, but who knows.

  • Sheild a building from EM waves?
    Try a faraday cage

  • Is it possible to adapt this technology to stop something other than electromagnetic radiation?
    No. This is an entirely electromagnetic phenomenon. Besides, what other kinds of things would you want to 'stop'?

  • could you use the gas to convert the energy in the pulse into heat?
    What is it you think happens when an atom absorbs a photon? Absorbed photon => higher energy => higher temperature.



Light gets ticketed, slows down. | 43 comments (33 topical, 10 editorial, 0 hidden)
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