Spacetime wave packets: New class of laser defies laws of light physics
newatlas.com
newatlas.com
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.
> text: may sound like it does, but doesn't
Yes, of course, the clickbait. Or the "How he got more clicks with this one simple trick" aka "The readers hate him for that".
Sad to see stuff like this pop-up almost daily on HN lately.
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.
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.
“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."
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.
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.
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.
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.