Refraction at the interface between two materials is fundamental to the interaction of light with photonic
devices and to the propagation of light through the atmosphere at large. Underpinning the traditional rules for the refraction of an optical field is the tacit presumption of the separability of its spatial and temporal degrees-of-freedom. We show here that endowing a pulsed beam with precise spatio-temporal spectral correlations unveils remarkable refractory phenomena, such as group-velocity invariance with respect to the refractive index, group-delay cancellation, anomalous group-velocity increase in higherindex materials, and tunable group velocity by varying the angle of incidence. A law of refraction for ‘space-time’ wave packets encompassing these effects is verified experimentally in a variety of optical
materials. Space-time refraction defies our expectations derived from Fermat’s principle and offers new opportunities for molding the flow of light and other wave phenomena.
Thanks for sharing. The paper is interesting, and not clickbaitty like the article. The link should probably be changed to the arXiv web page: "Anomalous refraction of optical space-time wave packets" (https://arxiv.org/abs/1912.13341).
Essentially, in this paper it's a light wave were the spatial and temporal dimensions are not easily separable. What the authors seem to do here is essentially do a time dependent spatial correlation of a light pulse, which then exhibits
"unintuitive" properties. The paper reads a little bit, like several similar papers in the field, where something relatively trivial is packed into a nice sounding story. That said I have not read the details yet and there might be something more profound.
Or an alternative take – A reader comes across something that seems interesting and submits it to HN to see what the more knowledgable (in the relevant topic) think.
As for the number of junk/low grade scientific papers that are being published at the moment, it is way higher than it should be, however a reader of the submitted article link would take it at face value as having some solid science behind it. Paywalled research papers are a bane and can hide a multitude of sins (so thanks for the Arxiv link jari_mustonen). I tried to read the paper but was soon way out of my depth so the comments here that more or less say ‘this is junk for the following reasons’ are extremely helpful.
Yes! This is what I'm thankful about on /r/science and HN. A few times I find an article and as a layman, do not really fully understand. I generally look to see if it has been submitted on /r/science or HN to see what people more knowledgeable than me say about the article.
It's almost certainly not junk when it makes it to Nature Photonics. This is the top journal in the field along with Science. The usability might not be there, but the science is almost guaranteed to be interesting. The news release spins are what they always are, and unrelated to the actual merits of the paper.
This may be harder to do, because dang is doing most of the lifting, but it may be advantageous to shift these comments to a link that has the arxiv or nature link. In that way we wouldn't be supporting these clickbait type of articles. But depends if HN wants to take a stance in that fight, clearly this is difficult for users to do (because we are in essence priming the well).
I also do appreciate the domain knowledge that HN has over many other forums and there's the old adage "they best way to get an answer is to post the wrong one." I was able to come to the HN comments and get: the arxiv link, the abstract, and both a high level and mid level explanation of the content. Seems pretty effective to me.
It's of course a click-bait title, while some of the classical aspects of light refraction can't be applied here it is for sure within the normal treatment of light physics (as is even stated in the article). Still, cool engineering and light hacking!
'Spacetime wave packets' sounded to me like gravitational wave packets. Highly misleading term in this context, but probably also intended to attract attention.
Agreed. Still, it mislead me into thinking it was something else and had I known what this was really about I probably wouldn’t have clicked. While it is hard to tell that this technical term was deliberately coined to be click bait, there are certainly other ways to name it which wouldn’t be as misleading.
I think that really depends on what you are used to reading and is in neither intentionally or unintentionally misleading. There is quite a bit of literature about spatio-temporal correllations, spacetime correlations... outside the gravitational fields. It simply means that space and time are coupled.
I would understand spatio-temporal correlations different from spacetime correlations: the first are the spacio-temporal correlations of something which exists in space time while the latter would imply some kind of correlations of space time.
>"But the new laser beams don’t follow this basic law of light. And it’s not just Snell’s Law either – the team says they also ignore Fermat’s Principle, which says that light always takes the shortest possible path.
“This new class of laser beams has unique properties that are not shared by common laser beams,” says Ayman Abouraddy, principal investigator of the study. “Spacetime wave packets can be arranged to behave in the usual manner, to not change speed at all, or even to anomalously speed up in denser materials. As such, these pulses of light can arrive at different points in space at the same time.”
PDS: Or, quite possibly the same point in space at different times (think out-of-order as a possibility, since different points in space at the same time would imply this to be true as well...)
[...]
>"That’s because they’re not messing with the oscillations of the light waves themselves – instead, they’re controlling the speeds at which the peaks of the light pulses travel. This is done using a device called a spatial light modulator, which reorganizes the energy of each pulse of light to intertwine its properties in space and time."
"“Space-time refraction defies our expectations derived from Fermat’s principle and offers new opportunities for molding the flow of light and other wave phenomena,” says Basanta Bhaduri, co-author of the study."
I wonder if some combination of different materials and different modulations could allow these pulses to arrive at the same point at the same time? If so, how much could you scale that and would it be more useful than just building a higher powered laser?
That's more or less how an ultrashort pulse laser works already.
Different wavelengths (colors) take different paths so the high energy of the pulse is spread out for processing over different parts of the optical components.
The spreading is obvious in space. Less obvious is that this also spreads it in time, because each region of space gets a narrower frequency range than if the different wavelengths weren't taking different paths, and a narrower frequency range corresponds directly to a longer timescale for the pulse (see Fourier transforms).
Then the different wavelengths are focused back together to a small spot. This obviously focuses them in space.
Their path lengths are arranged carefully so they also arrive in just the right relative phase, and this causes the longer pulses at each narrower frequency to combine into a short pulse at a broad range of frequencies.
The careful attention to relative phase is literally focusing the pulse in time as well as space, to make an ultrashort pulse.
The current record for "most focused in time" pulse is 43 attoseconds, which is 43 nano-nanoseconds, 43 micro-micro-microseconds, and 43 milli-milli-milli-milli-milli-milliseconds. It uses this technique.
Very neat I wonder how many applications this will create. I was just yesterday wondering if there was such a thing as an underwater telescope which I obviously assumed this was not possible due to distortion but wondered if laser technology could be used to give a computer image of what is in the distance. Perhaps with this laser it would be possible to do distance imagining like a fish finder but with much higher details.
I wonder if this can be used to build a 3D display?
Since they can control propagation speed, you can control the intersection point of two laser pulses.
Let's say that each pulse is not enough to excite some medium to emit a photon, but if you time both pulses to intersect at a specific point in space, the combined energy of both pulses could trigger the excitation so the medium emits a photon.
You could even use the same laser to select the position, send the slow pulse first, then the fast one exactly right so they intersect at the spatial point where you want a "dot" to appear.
You can obviously do something to the same effect without this kind of modulation, and it has been done.
It might not be impossible that something like this could make it easier, but iirc correctly, the primary problems were more related the noise created and possibly the chemical results of air turned plasma.
Clickbait aside, does this effectively mean we can produce latency-free games in the sense that all players could have the exact same amount of latency?
So long as the speed of light is an ultimate speed limit, the only way to ensure identical latency is to make everyone experience the same latency as the person with the worst latency since every 1000 miles of straight-line distance adds 5.4ms one-way latency under ideal conditions. So a signal following undersea cables will incur worse latency as it's not a perfectly straight line point-to-point. Better communication technologies can get us closer to that ideal, but everything we know points at the speed of light being an upper bound for any signal propagation.
The best latencies observing "causal fairness" you can achieve is to simulate/emulate the information propagation properties of light, rescaled so that "light speed" is related to whatever time it takes to traverse the slowest link, and that all faster links are path dependently delayed as to create a similated equidistance between players.
To do this anywhere close to perfectly you need absolute local time measurements with reasonable precision to determine the parameters.
Anything with less latency than that will have some kind of paradoxes, as that'd essentially be the equivalent of someone transferring data (or traveling) faster than light. It's kind of odd that it turns out this way, but it seems that any general case communicating network that's paradox free must be reducible to an equivalent physical light circuit.
It is still possible to make something very very counterintuitive that has a better perceived latency, is still fair for most purposes, and is at least eventually consistent. That is however more based on the relative predictability of humans over very short terms and the same of any physical simulation, and not at all causally consistent in any strict sense.
That's already effectively possible and doesn't solve any of the problems that come with propagation delay other than keeping the effects of latency "fair".
It doesn’t “defy laws of light physics”. Our “laws” were just incomplete. Why does science media always have to act like they correctly understand nearly everything and we are just 1 step away from figuring it all out? Is it just because of click bait or is there some hubris as well?
Abstract for those who don't bother:
Refraction at the interface between two materials is fundamental to the interaction of light with photonic devices and to the propagation of light through the atmosphere at large. Underpinning the traditional rules for the refraction of an optical field is the tacit presumption of the separability of its spatial and temporal degrees-of-freedom. We show here that endowing a pulsed beam with precise spatio-temporal spectral correlations unveils remarkable refractory phenomena, such as group-velocity invariance with respect to the refractive index, group-delay cancellation, anomalous group-velocity increase in higherindex materials, and tunable group velocity by varying the angle of incidence. A law of refraction for ‘space-time’ wave packets encompassing these effects is verified experimentally in a variety of optical materials. Space-time refraction defies our expectations derived from Fermat’s principle and offers new opportunities for molding the flow of light and other wave phenomena.