
Spacetime wave packets: New class of laser defies laws of light physics - jack_pp
https://newatlas.com/physics/spacetime-wave-packets-laser-light-refraction/
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jari_mustonen
Here's arxiv.org version (Nature Photonics version is of course pay walled):
[https://arxiv.org/pdf/1912.13341.pdf](https://arxiv.org/pdf/1912.13341.pdf)

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.

~~~
khana
what are 'space-time' wave packets then?

~~~
cycomanic
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.

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numlock86
> title: defies laws of light physics

> 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.

~~~
Pick-A-Hill2019
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.

~~~
Accacin
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.

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l33tman
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!

~~~
edna314
'Spacetime wave packets' sounded to me like gravitational wave packets. Highly
misleading term in this context, but probably also intended to attract
attention.

~~~
willis936
I think that’s just the technical term. It fits the dichotomy.

~~~
edna314
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.

~~~
cycomanic
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.

~~~
edna314
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.

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peter_d_sherman
>"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."

~~~
pharke
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?

~~~
jlokier
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.

------
14
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.

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juancn
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.

~~~
lostmyoldone
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.

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m3kw9
Let’s see if they can miniaturize this to compete with finer optics
transceivers

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bawana
could someone give me a high school explanation of this?

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hanselot
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?

~~~
Bjartr
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.

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jonplackett
Waiting for ISPs to adopt and start charging for the faster route

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LargoLasskhyfv
Äh? _Raumzeitwellenbündel?_ I can haz _Zeitkristalle_ with that?

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oxymoran
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?

