Laser pulse travels 300 times faster than light (2002) | Hacker News Reader

Laser pulse travels 300 times faster than light (2002)

optics.org

53 pointsnixass39 comments

>The 3-microsecond long pulse of light would normally take only 0.2 nanoseconds to pass through the chamber in a vacuum. But when passed through the specially prepared chamber, light emerged 62 nanoseconds earlier than it would have had it passed through the chamber in a vacuum.

Am I stupid or does this not make sense to anyone else

This explains anomalous dispersion a bit better: http://www.astro.ucla.edu/~wright/anomalous-dispersion.html

Nice explanation. But if "it is still correct to say that information cannot be transmitted faster than the vacuum speed of light" what are some possible use cases of this affect?

Maybe this helps. From the Nature article abstract:

"in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. "

You have to admit though, it would be very impressive if it made it through the tube in -61.8 nanoseconds.

That’s actually what it did though!

Something was definitely lost in translation.

My guess is that for some definition of "emerged", the three microsecond-long (3000 ns-long) pulse emerged 62 ns early.

Most of the results like this involve some sort of anomalous dispersion and careful definition of the term "emerge". Relevant keywords you're looking for if you delve into the work are "group velocity" and "phase velocity".

TL;DR: Details. They'll matter in this case.

Like, is this just pulse compression due to nonlinear wave effects? If so, it's quite yawn (but it did win a Nobel prize recently so...)

Group velocity [1]:

Since the 1980s, various experiments have verified that it is possible for the group velocity (as defined above) of laser light pulses sent through lossy materials, or gainful materials, to significantly exceed the speed of light in vacuum c. The peaks of wavepackets were also seen to move faster than c.

In all these cases, however, there is no possibility that signals could be carried faster than the speed of light in vacuum, since the high value of vg does not help to speed up the true motion of the sharp wavefront that would occur at the start of any real signal. Essentially the seemingly superluminal transmission is an artifact of the narrow band approximation used above to define group velocity and happens because of resonance phenomena in the intervening medium. In a wide band analysis it is seen that the apparently paradoxical speed of propagation of the signal envelope is actually the result of local interference of a wider band of frequencies over many cycles, all of which propagate perfectly causally and at phase velocity. The result is akin to the fact that shadows can travel faster than light, even if the light causing them always propagates at light speed; since the phenomenon being measured is only loosely connected with causality, it does not necessarily respect the rules of causal propagation, even if it under normal circumstances does so and leads to a common intuition.

[1] https://en.wikipedia.org/wiki/Group_velocity#Superluminal_gr...

The simplest example of something that "can travel faster than light" without transmitting information faster than light is rapidly moving the little dot a laser pointer makes. The dot itself can apparently travel across a surface faster than lightspeed (300,000 km/s), but the dot can't cause faster than light information transfer between the targets it flicks between. You could have the dot rapidly blinking a Morse code message, for example, then shine the dot at Alice and then flick the dot over to Bob, and this would let you send a message to both Alice and Bob, but not send a message from Alice to Bob faster than light.

Thanks! That was the simple example I was looking for. I was thinking about Moire patterns and other wave patterns where the pattern can move faster than the constituent.

The dot is not moving though. By the time you have it in another surface FTL it is a totally different dot.

The hypotenuse of a triangle is shorter than the sum of the sides, so Bob could always send a message directly to Alice faster than Bob could send a message to you and have you propagate that message to Alice.

The Nature paper that this press release talks about is [0], and even better, the same issue of Nature also published an accompanying "News and Views" explanation of the paper for non-experts [1] which I thought was clarifying:

> A report by Wang et al.5 ( page 277 of this issue) now demonstrates a very large superluminal effect for pulses of visible light, in which a pulse propagates in a specially prepared medium [...] not only faster than a pulse travelling in a vacuum, but so fast that the peak of the pulse exits the medium before it enters it! [...]

> their light pulses do not violate causality. [...] They use smooth, well-defined light pulses, so that the peak of the pulse at the output results from the forward rising edge of the input pulse, which occurs far earlier in time, making it consistent with causality. An abrupt feature in the light pulse would not be able to travel faster than c.

[0] https://www.nature.com/articles/35018520?proof=t [1] https://www.nature.com/articles/35018657?draft=journal&proof...

IMHO, just another blatant article claiming revolutionary discovery without details, papers, peer reviews aka clickbait. Moreover, this is an article you expect to find in generalist press, not in a supposed scientific journal. It's an article without signature also contradictory in some points, it starts with: "Despite exceeding the vacuum speed of light, the experiment is not at odds with Einstein's theory of relativity and is explainable by existing physical theory" and it ends with " however, because it is still correct to say that information cannot be transmitted faster than the vacuum speed of light". Is this the kind of journalism we like ?

They should send 0.2 ns long random bits through it, see if they get the data faster than light out the other end. Me thinks that won’t work.

The abstract concludes with, "...we will continue to study the nature of light and hopefully it will provide us with a better insight about the natural world and further stimulate new thinking towards peaceful applications that will benefit all humanity."

If the rate of information transmission is still limited by the speed of light in a vacuum, I fail to see how this finding leads toward "peaceful applications that will benefit humanity." Can someone enlighten me?

Maybe it just means "hey Nobel prize committee, look over here."

I don't understand it much, but it did occur to me when I was reading it that if equipment can detect the light emerging the tunnel earlier, then it has a physical effect on the world. And perhaps that physical effect could be used to make a novel ion thruster or solar sail that is 300x more efficient. E=mc2 thus c=sqrt(E/m) and m for photon is constant -- proportionally higher c is balanced by a more efficient E?

Might be useful for a lot of things if the research holds up. Maybe we could get a realtime video feed of Mars, instead of being 15 minutes or so behind :) That would make exploration and space development and such a lot easier.

Incidentally, if we can't assume c as a constant, we might not be able to assume m as a constant for photons either.

Paper is 2000 Jul has an Erratum - Published: 21 June 2001

Correction: Gain-assisted superluminal light propagation https://www.nature.com/articles/35082117

and a follow up where authors claim a more detailed explanation of their experiment:

Published 12 April 2001: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.63.05...

“The 3-microsecond long pulse of light would normally take only 0.2 nanoseconds to pass through the chamber in a vacuum. But when passed through the specially prepared chamber, light emerged 62 nanoseconds earlier than it would have had it passed through the chamber in a vacuum.”

So, the pulse emerged 61.8 ns before it entered? It traveled backwards in time? Seems like there’s a typo somewhere.

No, I think what it's saying is a pulse of duration 3000ns arrived at its destination over the course of 2938.2ns.

Oh ok. Maybe there’s some confusion over the meaning of “emerged” then. I interpret “emerged” as “when the leading edge first becomes detectable coming out”.

Yes from the point of view of observers. From the Nature article abstract: "in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. "

https://pubmed.ncbi.nlm.nih.gov/10917523/ The nature paper is probably a better link thank optics.org, this is a few years before the demonstration, however it has the same claim of 300x speed of super-luminal (faster than light) group velocity.

This is fascinating. And after reading every comment - it makes me wonder when people are saying something like “right, it arrived before it left”

1) I am understanding the affirming responses? Aka is the above summary a correct characterization?

2) it feels like this (and related older research) would be the closest thing to a future telling machine - but with limited application “did I press the button?” The observer would perhaps know before the button presser albeit by tiny margin.

This can’t possibly be right, someone help me out here if you would be so kind.

This release is written in an extremely misleading way. Check this comment

https://news.ycombinator.com/item?id=27417390

should clear it right up

Bizarre editorial choice to contradict one of the paper authors. From the article,

> The research work, which may result in significantly faster information transfer speeds across networks and in computers

But also from the article, further down:

> "Einstein's Theory of Relativity still stands, however, because it is still correct to say that information cannot be transmitted faster than the vacuum speed of light," said Dr. Lijun Wang

It's not a contradiction. Maximum speeds for information transfer in communications and computers is in the .67c zone. Research that could speed that up closer to 1c is possible (and welcome), which doesn't mean information will ever, at least within the current paradigm of physics, travel > 1c.

Isn't the .67c usually around the limitations of currently deployed optical fiber, but hollow core gets to .997c although it has distance limitations.

Also electrical signals propagating along metal wires, traces on a circuit board, and etc. (https://physics.stackexchange.com/a/643614)

Ah, fair enough, I misread the editorial claim, and stand corrected. Thanks for pointing that out!

Another "superluminal group velocity" or?

That's exactly, what this is. In particular, they manage to create a set up demonstrating strong anomalous dispersion, so that in the medium, longer wavelengths become slower (you'd usually expect longer wavelengths to have higher phase velocity), whereas shorter wavelengths become faster. The reshapes your pulse in such a way that the group velocity ends up appearing negative.

Alright, so a move towards more in-depth understanding of interaction of group velocity with medium in these situations / observations.. not as media spun it towards "faster than light" BS. Media's gonna med, I guess. Still valuable research, but phenomenon is decades old (in research).

> Einstein's Theory of Relativity still stands, however, because it is still correct to say that information cannot be transmitted faster than the vacuum speed of light," said Dr. Lijun Wang

I feel as though I'm not understanding something. What would stop one from transmitting information using the laser pulse?

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