Light slowed down to 38 mph | Hacker News Reader

Light slowed down to 38 mph

news.harvard.edu

124 pointsaliasaria39 comments

The 38mph result was from 11 years ago, the title of this (2007) article is "Light and matter united" and it's far more startling...

> She and her team made a light pulse disappear from one cold cloud then retrieved it from another cloud nearby. In the process, light was converted into matter then back into light.

"She and her team made a light pulse disappear from one cold cloud then retrieved it from another cloud nearby. In the process, light was converted into matter then back into light."

When I first read that, the only response I had was "Beam me up Scotty"

I think it was more of a stunned response than anything. Mind = Blown.

Photon number isn't conserved.

More accurate, to my layman's interpretation, would be to say that light was converted to information, encoded in existing matter, then converted back to light. Saying it was converted to matter seems to imply that new matter was created in the process which doesn't appear to be the case.

Unless light and matter consist of the same fundamental building blocks. During nuclear tests they've found that a tiny amount of matter actually disappears (which may SOUND like it violates the Law of Conservation of Matter), but it's actually believed that that tiny amount of matter was converted into an enormous amount of electromagnetic energy (hence e=mc^2) (i.e. light, for all intents and purposes).

If that's true, then this experiment is probably just reversing the process. The light is converted into a tiny amount of matter that's barely detectable (that 'fingerprint' - possibly just a tiny amount of matter that's slightly different from the matter around it), and can be converted back into energy easily.

Personally, my theory is that once we get down to more and more subatomic particles, we're going to start seeing that it has more and more properties in common with electromagnetic radiation. Electromagnetic radiation itself consists of a magnetic 'part' and an electro 'part', and if you coiled enough of it together millions and millions of times in a certain state, maybe you'd have an atomic particle that contained a phenomenal amount of energy, and a tiny amount of mass.

My physics is a little rusty, so please forgive me if I'm wrong. I think the law you're referring to is the Law of Conservation of Mass. As energy has mass and subatomic particles have mass, you can convert back and forth between them all day (good luck with that), and never violate it. In particular, Einstein's E=mc^2 is applicable here.

It was a surprise to me to find that the term 'matter' is actually poorly defined. At least according to Wikipedia.

Ah yes - I did mean conservation of mass, thank you.

I think most people use 'matter' to refer to the regular objects we interact with that are composed of atoms, and that's how I intended to use it in the above post. However, if I had my way, electromagnetic radiation would also be consider 'matter', because it's the same stuff, just a different form.

>> As energy has mass

Is that generally accepted, btw? Because I've always heard people talk about light as having no mass, and it never made sense, because that what I understood e=mc^2 to represent. I think it does have mass, it's just so minute (smaller than anything we know by such an order of magnitude that the speed of light squared is used to express it) that it doesn't seem like it.

I'm no expert, but my understanding is a little different. Its not that energy "has" mass but that energy and mass are two different forms of the same thing, i.e. they are equivalent. If some process is able to convert mass to energy then you could find out much mass you'd get but multiplying the energy by c^2 (hence E = mc^2).

Massless particles always move at the speed of light.

Mass is simply a form of energy, converted as E=mc^2. Energy is what's conserved. The model breaks down dramatically once you're doing dramatic forms of mass-energy conversion, like with nuclear reactions.

EDIT: I appreciate the correction on my misinformation about other types of reactions, and will not continue to spread that untruth here. Thank you, Locke1689.

"Even simple exothermic chemical reactions convert mass to energy by the same ratio as nuclear reactions."

That's not exactly true. Most exothermic reactions are simply the restructuring of chemical bonds (molecular bonds) to lower energy states. Consider that every molecular bond stores a certain amount of chemical potential energy. By breaking that bond, the potential energy is released in the form of kinetic energy, or heat.

There is no fission or fusion in normal chemical exothermic reactions and no matter is created or destroyed. Instead, the process is more similar to the powering of a motor via electricity -- electric potential energy is converted into kinetic energy through the actions of electromagnetism.

Maybe I'm dense, but I don't see how this experiment shows that the original light beam was affected in any way. Seems to me that the original light beam etched information onto the "cold atomic cloud", and then later an entirely new beam of light (with identical properties) was emitted (or reflected) from the laser blast's impact on this "etching".

How can they reach the conclusion that the original light beam itself was altered from this experiment?

They mislabeled the event when they said that the light was "converted" into matter. This is sad because I think many people will miss the greatness of what was really done because of it. A large (long) beam of light (everything about it) was stored in a very small amount of matter, then recreated exactly. It wasn't light like what was there before, diffracted from new light (like a hologram), it was the same light.

Once this becomes practical, we have practically infinite data storage, not to mention the underpinnings of computing with pure light, so perhaps practically infinite computational speed as well.

Saying "the same" about light (and other subatomic particles) is meaningless. If the have the same state, then are the same, in the sense that they are indistinguishable.

So it was "just like" what was there, but not "the same" photon in the sense the photon did not exist for a while, and was recreated. And at the same time it was also "the same" photon, in the sense that the before and after photons are not distinguishable.

It should be noted that the "no cloning" theorem adds a twist: It says you can not copy a photon. This means that the matter that stored the photon can not be induced to create another one (like a hologram does).

So in that sense it was the "same" photon - because you can only ever create the original, and no additionals.

True, true. Saying a lot of things about subatomic particles is meaningless. Their behavior doesn't map to english very well. The point is that that information could be stored in the beam basically at the photon level, trapped in matter, and then pulled out later. That's as cool as ultracold atoms.

So if you could figure out just how much storage, you could use Moore's law to figure out just when this will become practical? If "nearly infinite", then this will likely never happen (unless you go with the "accelerating returns").

"then later an entirely new beam of light (with identical properties) "

I think, if you understand quantum physics, it's meaningless to say that it's a "new beam of light with identical properties." If it "has the same properties", then it is the "same beam of light." It doesn't just look the same. It is the same in a physical (and meta-physical?) sense.

Then again, I don't really understand quantum physics, so I might be misunderstanding this. I'm telling you what I picked up from Eliezer Yudkowsky's series on quantum electrodynamics.

Perhaps a physicist can elaborate?

it's meaningless to say that it's a "new beam of light with identical properties."

You can think of it as information (bits). The same information that was encoded was decoded later.

It is not as if photons are items on the macro scale, that were somehow labeled (for example, photon #134), then disappeared and reappeared and the scientists compared the label and proclaimed that its the same photon (#134).

Every time light reflects of some surface this happens. The light excites the (electrons in the) material which then gives off 'new' light.

We still call it reflected light as though the photons somehow bounce off. They don't. They get adsorbed and are then re-emitted. But they might as well be the same ones because we can not tell the difference.

There are variations on this, specular reflection (think of a mirror) and diffuse reflection (think of a wall), the difference lies in the direction of the outgoing photons.

> It is not as if photons are items on the macro scale, that were somehow labeled (for example, photon #134), then disappeared and reappeared and the scientists compared the label and proclaimed that its the same photon (#134).

Quantum theory doesnt quite work like that.

What's amazing about this case is that all of the quantum states were maintained as well. The light maintained entanglement with other photons. This is not something that could happen by simply storing information and retrieving it later.

Currently, quantum encryption setups require a fiber optic cable to reach all the way from the sender to the recipient, because repeaters would destroy the entanglement. But with this device, you could store the data in a Bose-Einstein condensate, ship it to the destination, then read the data back out.

<quote>Einstein and just about every other physicist insisted that light travels 186,000 miles a second in free space, and that it can't be speeded-up or slowed down. But in 1998, Hau, for the first time in history, slowed light to 38 miles an hour, about the speed of rush-hour traffic. <quote>

I wouldn't have been surprised if a newspaper journalist used dramatization that indicated as if Einstein is proven wrong. I didn't expect that kind of dramatization when reporting scientific matters on news.harvard.

Of course, Einstein is still right. All these experiments slow down the speed of light when it's traveling in special matter at special temperatures. Constant speed of light in free space is a necessary condition for Theory of Relativity be correct. If that condition is found to be incorrect, our current understanding of the universe completely goes for a toss;

Indeed. To be even more specific, the reason why it is slowed down is because of absorption and re-emittance of particles of light (photons). If you go down the the extremely small scale, what you will actually see is a photon moving at the speed of light, colliding with an atom, being absorbed by an electron, then re-emitted on the other side. Repeat this process ad nauseam and the constant absorption/re-emittance will slow down the speed of light to a varying degree. Most importantly, however, is that the c is constant in free space.

This reminds me of something that I think I first read about in Michio Kaku's book Physics of the Impossible where a Bose-Einstein condensate was used as a medium for "teleportation". I believe in his book he referred to the experiment referenced here. http://www.physorg.com/news102681027.html

They are careful to note that it is a transmission of information rather than photons. It sounds like this experiment is similar, but I am certainly not qualified to comment authoritatively on that.

It would make sense that teleportation (or perceived teleportation) would be possible from it, since all matter is is stored information. Now the question is if matter is the only way to store information aside from light. If this is so, then if you reduced an object or human to its light representation, you would be able to travel at the speed of light, but no faster, which would be perceived teleportation - I'm assuming teleportation is instantaneous whereas going the speed of light is not. Hopefully no one would slow you down in the meant time.

I think that what you've described is essentially the basis for the Australian "teleportation" idea. They distinguish it from quantum entanglement approaches which could theoretically (or not, depends on who you ask) transmit the information faster than light.

Ah, good to know. I'm just now getting to the link.

As described in the article linked the speed of the teleportation is limited by the speed of the signal beam which is necessarily capped at c.

The article seems very loose with its terminology which makes it really confusing. For example:

> Albert Einstein and just about every other physicist insisted that light travels 186,000 miles a second in free space, and that it can't be speeded-up or slowed down. But in 1998, Hau, for the first time in history, slowed light to 38 miles an hour, about the speed of rush-hour traffic.

Hau did not slow down light in free space so this has nothing to do with Einstein's statement.

What I would like to know is, is underlying fundamental mechanism here similar to or the same as what we know as refraction? From all the descriptions it sounds awfully similar, other than the extreme(!) nature of the slowdown.

Yes. It's the same, just on a more extreme level.

Light can NOT be slowed down - at all! What happens with refraction is the light is absorbed by matter, then re-emitted, over and over, which effectively slows it down.

In this case the material that absorbed the light takes a very long time to re-emit it.

This is way beyond my head, but I can fell the awesomeness!

> For the first time in history, this gives science a way to control light with matter and vice versa.

Typical bad science writing.

Mirrors have existed before this year...

Though mirrors are not too good at the vice versa part.

Ever heard of vanity? ;)

Check out this great docu http://www.pbs.org/wgbh/nova/zero/ witch shows more of the Bose-Einstein condensate.

Not just 60km/h, it was completely stopped. It was on (Danish) TV years ago.

the key phrase here is "Despite all the intriguing possibilities, "there are no immediate practical uses," Hau admits."

The experiment and its results are intriguing nevertheless.

Thanks for posting this. count one more in the mind=blown camp!

This is sure to revolutionize all communications done at 460 degrees below zero Fahrenheit!

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