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Agreed on the first point, the headline and lead-in are grossly sensationalized, but later they do pretty clearly explain what’s going on.

The article’s author says the “time mirror” means you send a signal into the experimental setup of wave guides e.g. “a-b-c” and it comes back “c-b-a”.

Furthermore, when the researchers introduce interference during the reversal they don’t see superposition as you might expect (wave-like behavior), but kinetic interactions (signals are acting like a collection of particles).

It may not be true (we know a large percentage of papers can’t be replicated I.e. they’re bullshit), and I have no context to know how important that finding would be, but it is explained.

Maybe I’m missing the importance of this happening in this experiment specifically, but isn’t this already a known phenomena seen in the double-slit experiment?

According to TFA, it’s different.

> Just a few months after developing the device, Alù and his colleagues observed more surprising behavior when they tried creating a time reflection in that waveguide while shooting two beams of light at each other inside it. Normally colliding beams of light behave as waves, producing interference patterns where their overlapping peaks and troughs add up or cancel out like ripples on water (in “constructive” or “destructive” interference, respectively). But light can, in fact, act as a pointlike projectile, a photon, as well as a wavelike oscillating field—that is, it has “wave-particle duality.” Generally a particular scenario will distinctly elicit just one behavior or the other, however. For instance, colliding beams of light don’t bounce off each other like billiard balls! But according to Alù and his team’s experiments, when a time reflection occurs, it seems that they do.

> The article’s author says the “time mirror” means you send a signal into the experimental setup of wave guides e.g. “a-b-c” and it comes back “c-b-a”.

It would be great if the author said that. She doesn’t say that, though. She says, “For instance, what is at the beginning of the original signal will be at the end of the reflected signal — a situation akin to looking at yourself in a mirror and seeing the back of your head.”

Which is a surprising thing for a person with a PhD in nanophotonics to say.

Key line from the article:

> what is at the beginning of the original signal will be at the end of the reflected signal

Leading with that would have made the whole thing a lot less interesting to non-physicists (possibly everyone, dunno).

Completely agree about the headline and lead, but if you slog through it, the article did an ok job explaining the concept:

> The trick is to create a certain kind of reflection. First, imagine a regular spatial reflection, like one you see in a silver-backed glass mirror. Here reflection occurs because for a ray of light, silver is a very different transmission medium than air; the sudden change in optical properties causes the light to bounce back, like a Ping-Pong ball hitting a wall. Now imagine that instead of changing at particular points in space, the optical properties all along the ray’s path change sharply at a specific moment in time. Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it. This is a “time reflection.”

> To do so, physicist Andrea Alù and his colleagues devised a “metamaterial” with adjustable optical properties that they could tweak within fractions of a nanosecond to halve or double how quickly light passes through.

So you have some volume of material and wave of light is passing through it. You alter the material everywhere in that volume so that the light does a 180 at every point along the wave at the same instant, reversing the stream of light. Any signal encoded in the light comes back in the opposite order you put it in, like you played a recording backward, or popped a stack until it was empty.

Can this be buffered or can it only reflect the light in the machine’s volume?

> halve or double how quickly light passes through

Isn’t that just like any material that light passes thru, which explains refraction?

Just sounds like they read Feynman’s QED book

Disclaimer: not a quantumelectrodynamacist

Yes, but typically the different optical properties are strictly localized in space. Air here and water there. In this paper it's basically like you have a pool full of water with a light beam traversing halfway to the ground and then you instantly turn the air in the pool into water and the light goes, "oh hell no" and backs out of there

Hardware stack running at the speed of causality sounds useful for quantum computing.

You’re a more generous reader than me if you think this is illuminating:

> Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it.

Because to my mind, a ping pong ball returning to the player who last hit it seems like something I would describe as ‘recoiling in space’, not ‘recoiling in time’.

Returning stuff back the way it came is actually a thing people associate with ordinary mirrors - so if you’re trying to tell me how a time mirror differs from a normal mirror this ping pong ball metaphor is not exactly helpful.

This comes up from time to time with photons. It's usually the result of confusion over the difference between phase velocity and group velocity.

3blue1brown has recently released good YouTube videos on phase velocity.

Question: how do I check group velocity in media, to know how fast information traverses through that media? For example in water - how long does it take for my waterproof flashlight to get across a small body of water? As in where to find that information. I always thought I just have to look up the index of refraction, but that's just the phase velocity.

The index of refraction (including its dependence on frequency) fully describes the physics, there's nothing else to look up. Group velocity is a derived property taking into account the mathematics of waves, and in the typical textbook example of a very-nearly-monochromatic wave can be computed from the derivative of the index of refraction with respect to frequency.

Waait. OK. I get it. It's photons which make me look fat?

Yep, sorry. You look the size of a football pitch, but your mass is only the size of a pea!

Its a common problem. I remember getting really frustrated over articles announcing time crystals. What is a time crystal? They never actually said.

FWIW, and IANAP, a time crystal is a structure that causes things to apparently reverse their structure back and forth without additional energy. This is similar to a spatial crystal in that the physical configuration repeats in space, except the configuration repeats in time without additional energy.

A time mirror causes a configuration to reverse in a way that’s a reflection of the original. Someone in a parallel thread gives examples of this.

They’re not the same thing.

IMO time crystals are more amazing.

Also NAP, but how is a time crystal different than the metamaterial used for a time mirror, wouldn’t a reversed light beam be reversed again, when it is reflected?

The time mirror applies energy to create the transformation. A time crystal requires no energy but you still see perpetual motion (not violating thermodynamics fwiw).

Quantum physics does lots of fuzzy things that look like they'd have to violate thermodynamics, but "really don't" due to uncertainty mainly.

Indeed all time crystals created to date leverage quantum effects to work.

Time crystals are neat! A normal crystal is one that has a definite repeating pattern in space. A time crystal is similar, but instead of repeating in space, it repeats in time.

https://en.wikipedia.org/wiki/Time_crystal

Here's a gratutious example thanks to ChatGPT:

"Imagine a line of atoms that flip their spins periodically, creating a pattern that repeats not only in space (like the atoms are arranged in a regular grid) but also in time (like the atoms flip their spins at regular intervals). This repeating pattern in time, which occurs without continuously adding energy, is what makes time crystals so intriguing and different from other states of matter."

Someone should make a time-image about time-crystals.

Lisa, I'd like to buy your florb warper.

It may be a MacGuffin.

https://en.wikipedia.org/wiki/MacGuffin

It looks like this is talking about https://en.wikipedia.org/wiki/Time_reversal_signal_processin... but if so it's unclear what the innovation is, since that's been done for decades in the optical domain.

It must be done with Time Crystals, which were the article of the day a few months ago. Never did find out what they really were.

the headlines of articles have now completely morphed from being a summary of the contents in a short sentence (which somehow includes the same information but less precisely) into full blown clickbait

I blame google just because they are famous

Very good, funny.

Most science articles read like this to me. I wish this was the exception.

>What is the purpose of this kind of science journalism?

Clickbait.

Thanks, I'm not reading it after this comment. I was hoping to be explained something, but as you put it the article is merely explaining the title is clickbait.