Physicists Create Time-Reversed Waves of Optical Light | Hacker News Reader

Physicists Create Time-Reversed Waves of Optical Light

sciencealert.com

119 pointsbookofjoe45 comments

I found the ~6 min video embedded in the article worth watching.

If I'm understanding it correctly, this technology could ultimately provide a means to generate very realistic holograms (after many iterations and lots of capital investment).

> I found the ~6 min video embedded in the article worth watching.

This is very good advice. They also have an hour long presentation as well. I haven't watched it yet but I plan to. That 6 min clip was fantastic.

> If I'm understanding it correctly, this technology could ultimately provide a means to generate very realistic holograms (after many iterations and lots of capital investment).

Well, they first used this to disrupt kidney stones but there will be numerous applications to this. Being able to model and control light in ways that we never have before opens up many fields.

It was a very clear and intuitive explanation as well. I was most intrigued by their optical system used to encode and decode light structures in space and time. Though, I didn't quite understand their discussion of 4D holograms at the end, they seem to imply that we can now achieve actual 3D holograms of images using time-reversed light waves and also have these holograms move over time?

Edit: the author has a longer video with timestamps[0] as well, perhaps the details and implications are discussed there.

[0] https://www.youtube.com/watch?v=9hVEJvfWjRQ

Agreed. That was really well presented in my opinion.

So cool to see this work on HN!

This is a former postdoc of mine who is leading this effort. I highly recommend everyone who is more interested in this work to go to Joel's YouTube channel at https://m.youtube.com/channel/UC6sIgT149AlEx0PAEzx015g

His videos are outstanding and highly educational.

I just watched Tenet a few times, so I feel ready to understand this article

Thank you for your contribution to this thread.

> We have found a way to precisely measure where all that scattered light arrives and at what times, then create a 'backwards' version of that light, and send it back through the fog

I'm assuming this is just a sort of analogy, because unless the fog was somehow frozen in time, there's no way the light would (de)scatter in the same way going backwards, it just wouldn't be hitting the same spots within the fog. Now, if they are able to predict the exact flow of the fog, that would be incredibly impressive.

I think the scattering medium is indeed assumed to be frozen in time. This would be the case for any solid of course, but also for a sufficiently homogeneous fog. I assume you are only inverting the statistical average of the scattering medium properties, so as long as those are frozen in time it's sufficient.

Maybe the fog droplets move slowly compared to light? Don't know if that's true.

The article mentions that this can be applied to sound waves. Could that be used to spy on conversations that happened recently in a room, kind of like the Observers did towards the end of Fringe?

Not really, this is about reversing waves that have already happened. The technique requires you to fully capture the outgoing propagation in order to reverse it. It isn't a way to "extract" previous vibrations without recording equipment.

Full length video: https://youtu.be/9hVEJvfWjRQ

Could this be used for making something perfectly black or something like that? There was an A.C.Clarke story about a silence machine that reemits the sound with a delay to cancel it.

Re-emits the opposite sound?

Like noise - canceling headphones, presumably.

Exactly like noise cancelling headphones. Also like any number of industrial noise cancelling systems.

The story is “Silence Please” (1).

(1) https://en.wikipedia.org/wiki/Silence_Please

Not a physicist, but this might not work quite the same since light is both a particle and a wave, whereas sound is just a pressure wave on a given medium. Would happily be proven wrong though!

Seems like the cancelation happening in your scenario would likely be from a delay of half the period of the sound waves, effectively making them roughly "complementary" or "inverse". Can't say I'm familiar with the research, though.

On a sillier note, have you heard of the Speech Jammer? [0]

It's a device to disrupt a person's speech by playing it back to them with a delay of a few milliseconds. Hilarious results. Also easily reproducible at home if you're bored.

[0] https://phys.org/news/2012-09-speech-jammer-ig-nobel-winners...

This may be extremely dumb question, but given a snapshot of a wave, what properties of that snapshot tell you which direction that wave is moving in?

(I'm neither a physicist nor a philosopher and happy to be corrected.)

I don't believe you can. Given a snapshot of an arrow suspended in air, how do you know its direction and speed (or if it has been shot at all rather than just been let go in mid-air)? That's basically Zeno's paradox of the arrow[0]. A snapshot description without speed doesn't tell you anything about speed.

In the case of light-waves, their propagation is governed by Maxwell's equations, which have have a dynamic component (induction).

[0]: https://en.wikipedia.org/wiki/Zeno%27s_paradox#Arrow_paradox

(Edit: typo.)

Interesting to think about the experiment of copying some chunk of the universe with a wave in it and letting the time tick in a simulation: which direction should the wave propagate? Surely there must be some information encoded somewhere about that fact.

If by snapshot you mean a full (static) physical description of the scene, then the "air" part of your "arrow suspended in air" will have wake turbulence behind the arrow and not in front of it, and I'd have thought the magnitude of that turbulence should in principle allow you to deduce the airspeed.

There might also be lengthwise compression of the arrow shaft as a result of acceleration.

And if the concept described in the article is valid, how do you differentiate between plain and “time-reversed” version of those waves?

Length compression and other relativistic effects, as I understand relate to those properties measured in time. How does that manifest when we’re talking about a static snapshot?

I don't pretend to have much of a grasp of what the article is describing, but it sounds very "frictionless spherical cow in a vacuum". I'd be amazed if this technique could be applied to something as messy as turbulence.

For lengthwise compression I didn't mean anything as abstruse as relativistic effects, just normal physical compression. If you accelerate something by pushing on the back of it, it's going to squish to some extent.

Turbulence and length compression are good evidence, but in theory not sufficient to infer the speed of the arrow: The arrow could have been squeezed and air turbulence induced some other way. These alone then would not make the arrow fly in the suspected direction.

Hmm. If the arrow was actually travelling in a different direction, doesn't that imply that you'd need to be able to cancel the turbulence from that motion? Otherwise it'd appear in the snapshot as contradictory evidence.

Sure! I'm assuming if we have the means to fake the one turbulence, we can also cancel the other. Or the arrow is just not moving, thus no "natural" turbulance to cancel.

I'm not saying that air turbulence wasn't a very strong indicator for motion! And I am not sure the technology to fake them completely does exist. But I don't see why it shouldn't be possible, maybe with a very elaborate set of fans, air ducts and loudspeakers.

> I'm assuming if we have the means to fake the one turbulence, we can also cancel the other

That seems like a leap. There's a huge difference between generating some plausible-looking turbulence and generating one absolutely precise pattern of turbulence at one exact instant.

For classical electromagnetic waves it's the relative position of the electric and magnetic fields. [1]

There is a difference to Zeno's paradox, as a sibling comment suggests the impossibility. For a flying arrow we can assume that it's motion is driven by Newton's law, which is a second order differential equation for the position. Therefore you can't use the position only for initial condition to solve the equation, ie. you can't decide where the arrow travels from a snapshot of time.

However the Maxwell equations that describe classical electromagnetism are a system of first order partial differential equations in terms of electric and magnetic fields. So if you know both the electric and magnetic fields everywhere at a given time point then in theory you can predict it at every future time points.

[1] https://en.wikipedia.org/wiki/Electromagnetic_radiation#/med...

Would those positions not reverse for described “time-reversed” waves? Or would the simulated time-reversion look like normal wave because, after all, it’s generated normally?

Well, the time-reversed waves can't have the same electric and magnetic fields as the original forward waves, because they travel in the opposite direction. In principle the reversal can be achieved by flipping the sign either the electric or magnetic fields. This reversed wave still would "look like" the reversal of the original wave, as the light intensities match.

In practice I expect that the mechanism to be much more involved than this. Traditional holography works by capturing a fine grained picture of light intensities on a surface or even in a volume, but it doesn't capture all possible information, certainly not both the electric and magnetic fields at a given time point. It looks like the researchers use a novel holographic technique to capture more information than normally possible.

> In traditional holography, a 2D diffractive element encodes the complex amplitude of a 2D wavefront, which can be recreated by illuminating the element with a spatial reference beam. This new device can be thought of as an extension of this to an extra dimension; a three-dimensional (3D) diffractive element, which when illuminated with a reference pulse in a reference spatial mode, will reconstruct a fully volumetric optical field (2 transverse space and 1 time/longitudinal space). It is a type of reprogrammable space-time hologram.

https://www.nature.com/articles/s41467-%20020-19601-3

Thanks for the explanation and correction!

At the end of the video they explain this is like a hologram of 4d space-time instead of just 3d space. Pretty cool!

I have hopes for holographic projection directly into the eye one day allowing us to live in a Matrix like world.

The technology would be cool, but I have already grown cynical of the faucets of information we currently orchestrate. When I imagine stuff like that I also subconciously change the setting to a world without advertising, user accounts, terms and conditions, all the legal and commercial bullshit that truly makes up the information revolution experience.

Imagine having all the interruptions the current web has but in your eyeballs. Oh want to bring up some price matching on the items you are looking at? Whoops, gotta reauth the app with my Google account. Yes, I accept the cookies. Yes. Terms are fine. No I am not a robot. Oh my plan ran out, better find my credit chip...

Sexy singles in my area? The advert is showing my waypoints on the ground! Hey, this just took me to McDonalds... bloody SEO spambots.

I do the same, but am just now realizing it after reading your comment. My hypothetical future also seems to be missing a certain massive surveillance machine, now that you mention it. Is there a phrase akin to "rose-colored glasses" but for the future?

It's called the Technium

Something like this would be my total nightmare!

https://vimeo.com/166807261

the closer they get the to the brain, the more important it is to have user controlled software... aka open source system, no javascriptable "content"

Movies like the Matrix and Ready Player One seem more like warnings than aspirations to me.

Sounds like a phase-conjugate mirror.

I saw that coming.

light saber?

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