Light can be reflected not only in space but also in time

https://www.scientificamerican.com/article/light-can-travel-backward-in-time-sort-of/

by rbanffy • 3 years ago

119 74 3 years ago

scientificamerican.com

tibanne• 3 years ago

Ok, but how can I use this practically in my day to day?

TechPlasma• 3 years ago

Time mirrors are useful to know if youll cut yourself shaving tomorrow.

(This is not an endorsement of Future Mirrors Inc.)

doublerabbit• 3 years ago

Highly recommend the Ƒπ Model -- doesn't require the UltraGalaxy Subscription and supports the Gigaflib connector.

Aachen2• 3 years ago

Can't tell if you're trolling. Surely you've heard of the concept where people look into stuff before realising it's something we can practically use today? Like, before electricity was discovered, nobody could possibly have known the force existed, let alone what they could use it for and this whether/how useful it was going to be

Among the first reasons for getting electricity was light. It took many centuries to go from "huh, this metal behaves odd when I put it in a jar of acid" (around the year 200) to light bulbs being invented to them making it into your day-to-day life around the year 1880 (if you were a rich person in the right country)

bigbillheck• 3 years ago

If you have to ask....

Aachen2• 3 years ago

... then?

Based on https://www.usingenglish.com/forum/threads/if-you-have-to-as... maybe you meant "you'll never know / I'm not going to tell you" (doesn't sound very nice) or "you've answered it" (umm, how?)

bigmattystyles• 3 years ago

Only time will tell

robertlagrant• 3 years ago

That's worth reflecting on.

n6h6• 3 years ago

You shouldn't wear your ignorance with such pride.

Nthringas4• 3 years ago

you should learn how to do humilty

coldtea2• 3 years ago

One could use it to make their day to day life less petty and boring by expanding their knowledge and their intellectual horizons, and thinking about beyond the limited necessities or niceties of their "day to day life".

kloch2• 3 years ago

General Relativity has identical solutions for forward and reverse time.

The Schwarzschild metric for example is expressed in terms of time squared so time itself can take positive or negative values and give the same result. It also allows negative mass but those solutions are not symmetric with positive mass - the rs term that contains mass is not quadratic.

One consequence of this combination is if you consider "negative mass" to be just positive mass operating in reverse time (to avoid having to accept the ugly idea of negative mass in positive time), you end up with asymmetric effects.

Nthringas4• 3 years ago

IMO, the fact that GR is time-symmetric is actually a quality of the math underlying the physics

i.e. the theory, not reality, is where time symmetry comes from. and this is also exactly why those theories have predictive capabilities

but reality is still asymetric in the temporal dimension

chowells• 3 years ago

reality is still asymetric in the temporal dimension

That's an interesting claim. Do you have any evidence of it?

Oh, wait. Do you actually believe time passes? You're going to need to start by proving we're not in a block universe, then.

As far as I know, there is no such thing as an experiment that can disprove either assertion. As such, they're more claims of faith than claims about reality.

fnordpiglet4• 3 years ago

A wonderful aspect of using math to describe physics is the things admitted as possibly by the math might describe things possible in the physical reality but are very hard to observe given our frame of reference. The fact we’ve really found absolutely nothing observable that contradicts going backwards in time hints that it might in fact be possible under the right conditions. Our sense from our frame of reference that it isn’t doesn’t prove anything, just that we have only observed an asymmetry so far.

Sabine made a great episode on this recently, fwiw, and it’s worth watching if the subject interests you.

https://youtu.be/4ICIObFtWjM?si=XJ2t0P_I6XCWTl1Q

aptitude_moo• 3 years ago

Not entirely sure what time reflection is, but I'm pretty sure it doesn't mean anything related to anything going backward in time. Here I think it's explained better https://spectrum.ieee.org/time-reversal-interface

Edit: Now that I've read a bit more, it seems to me this is just reflecting light back but "all at once". Like if a light signal were a train going right and the first wagon is A (>>>[D][C][B][A]>>>), a normal mirror would make it bounce and the train would go back going left and wagon A arrives first (<<<[A][B][C][D]<<<). A """time reflection""" would make the train return backwards and wagon D arrives first like this (<<<[D][C][B][A]<<<). I understand it is like reflecting each wagon at the same time so the train comes back reversed. This looks cool but I think the article deliberatively makes it confusing so it sound like the movie TENET when it starts talking about entropy

plusplusungood• 3 years ago

Really going above and beyond on that, "write a function that reverses a string in place" interview question, aren't they? I'm afraid I'll have to dock points in simplicity though.

jasondavies• 3 years ago

Interesting to compare with a time-reversal mirror for water waves that uses a high-acceleration "jolt" to disrupt the wave propagation at all points in space simultaneously: https://pubs.aip.org/physicstoday/Online/11398/Reversing-tim...

wokwokwok• 3 years ago

-_- it’s extremely disappointing to see the number of articles claiming a time reflection mirror would,let you see the back of your head, for example (1):

“ In the acoustic version of this observation, you would hear sound similar to what is emitted during the rewinding of a tape.”

Yes.

“As a result, if you were to look into a time mirror, your reflection would be flipped, and you would see your back instead of your face.”

No.

That is not how it works.

Practically speaking what you would see is a frequency shift (color change).

Where does this bs come from?

Reversing a light signal (light bouncing off your face) does not result in a signal that doesn’t exist (light bouncing off the back of your head). This is just absolute nonsense. If no light bounces off the back of your head, there is no possible way to make light bouncing off the front of your face transform into the former… unless the front of your face and the back of your head are indistinguishable.

There’s science reporting trying to explain complex ideas in simple terms, and there’s just making stuff up. :|

Read the paper (2).

“At such a time interface, a portion of the input signal is time reversed, and its frequency spectrum is homogeneously translated as its momentum is conserved, forming the temporal counterpart of a spatial interface”

[1] https://asrc.gc.cuny.edu/headlines/2023/03/scientists-demons...

[2] https://www.nature.com/articles/s41567-023-01975-y

AssertErNullNPE• 3 years ago

Perhaps someone with a better understanding of physics could provide an ELI5 but for a software engineer (ELISE)? Have they literally created a device that causes photons to travel into the past? It seems like this is something different.

phkahler• 3 years ago

> Have they literally created a device that causes photons to travel into the past?

No. If you ever see a headline that gives you that impression, its just someone sensationalizing something much more mundane. I've seen several of these over the years and find it pretty annoying.

BitwiseFool• 3 years ago

"provide an ELI5 but for a software engineer (ELISE)? Have they literally created a device that causes photons to travel into the past?"

I spoke with the client and I still don't understand what they're getting at. Sales said that our requirements are to create a device that can reflect light through time.

ineptech• 3 years ago

ELI5 attempt:

Suppose you're using Newtonian mechanics to model the behavior of a moving baseball that bounces off a brick wall. You write down some equations, do the experiment, the results match, yay science.

Now imagine your lazy coworker was supposed to do the same experiment, but he couldn't find any bricks, so instead of a wall he just built a tiny rocket engine inside the tennis ball that can exert exactly enough force to reverse its velocity. The ball's velocity changes in exactly the same way, except that it changes at a certain time (when your lazy coworker triggers it) rather than at a certain place (where the wall is). Would his results match yours?

For a single ball, yes. The behavior would be identical between these two scenarios. If you looked at a graph of the ball's location over time, you wouldn't be able to tell whether it had bounced off a wall or been "bounced" by a little rocket engine. Right?

Now consider a stream of baseballs. In your lab, your baseball cannon fires a stream of balls which all reverse velocity at a certain place in space, but your coworker is firing a stream of balls that all reverse velocity at a certain time. Would they also behave identically?

No, this time there are differences. For one example, the wall-balls would collide with each other (the ball that just hit the wall would run into the ball that's about to hit the wall) and the rocket-balls wouldn't (each ball reverses direction at the same time, so the spacing between the balls never changes). For another difference, they'd bounce back in a different order (the first wall-ball to be shot would be the first to return, whereas the last rocket-ball to leave the cannon would be the first to return).

That is more or less what this experiment is doing except they're using light rather than baseballs, and a substance with variable reflectiveness rather than tiny rockets. This might have repercussions for time symmetry, that's above my pay grade, but definitely doesn't involve time travel.

jebarker• 3 years ago

Is physics finally going to explain the plot of Tenet for us?

jameshart3• 3 years ago

What is the purpose of this kind of science journalism?

The basic structure of the piece is to put a claim in the headline that sounds like a sci-fi breakthrough, then spend the article explaining that this isn’t actually a sci-fi breakthrough.

Like, the entire article here is basically saying ‘there is such a thing as a time mirror. No, it doesn’t make light travel back in time.’

What it doesn’t do is remotely explain what a time mirror is and what it actually does do. No, ‘scientists made a time mirror out of a metamaterial that uses capacitive switches to add and remove matter’ does not answer what a time mirror is.

I can tell you I made a florb warper out of a piece of zinc suspended in a cup of tea and you’re still none the wiser what a florb warper is or what it means for florbs to get warped. But please don’t get the wrong idea - it’s nothing like the warp drive from Star Trek! After all, it uses tea and zinc, not dilithium! Still, we can headline the article about it ‘Scientists test warp device’.

kloch2• 3 years ago

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.

H8crilA• 3 years ago

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.

b112• 3 years ago

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

cies• 3 years ago

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.

thereddaikon• 3 years ago

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

fnordpiglet4• 3 years ago

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.

embwbam• 3 years ago

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."

oneshtein• 3 years ago

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

karaterobot• 3 years ago

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

aranchelk2• 3 years ago

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.

willy_k• 3 years ago

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?

jameshart3• 3 years ago

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.

verisimi2• 3 years ago

Very good, funny.

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

icambron• 3 years ago

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.

0cf8612b2e1e• 3 years ago

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

louthy• 3 years ago

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

teaearlgraycold• 3 years ago

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

jameshart3• 3 years ago

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.

zozbot234• 3 years ago

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.

ysofunny• 3 years ago

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

leptons• 3 years ago

What is the purpose of this kind of science journalism?

Clickbait.

m463• 3 years ago

It may be a MacGuffin.

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

Tagbert• 3 years ago

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.

nimbius• 3 years ago

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” officer and (obviously) proves i wasnt speeding.

bilsbie• 3 years ago

Maybe it’s time being reflected off of light.

peter_d_sherman• 3 years ago

"...a rich and growing subfield of wave physics shows that such “time reversal” is possible."

Let's call this burgeoning branch of physics "Wave Reversal Physics".

That sounds like a good name for now, Wave Reversal Physics...

Plus, that would be a better sell to the "Time can't be reversed" crowd.

Because anyone who's everyone who's anyone who's everyone in Physics -- who knows anything about waves -- know that they can be reversed.

Perhaps not in gigantic superclusters (as might be required for actual Time Travel), perhaps not yet, but as a single discrete wave in various mediums -- wave reversal definitely is possible.

Also, I should point out that that's for any wave which is not a pure sine wave.

That's because pure sine waves have a period -- but they do not have a reversed, (AKA phase-conjugate) form.

They are basically their own reversal at half-period.

Well, I take that back -- it could be argued that a 180 degree out-of-phase secondary sine wave -- is the reversed form of an initial one...

So phase could be one component of reversal in some wave types...

Anyway, point is, let's call this new subfield of Physics:

Wave Reversal Physics

...At least until/unless a better moniker is to be had! :-) <g> :-)

1a_user• 3 years ago

I broke a time mirror once. Had 7 kilometres of bad luck.

m3kw9• 3 years ago

They saying they created an experiment that looks like time has reversed but for light only, in which they made light not lose information due to them preventing entropy from happening?

diracs_stache• 3 years ago

Yeah this article should be more forthright in describing meta-materials/left handed media. This media is highly dispersive and described by changes in constituent properties (permittivity, permeability) usually coinciding with frequency/wave propagation constant changes. From there we can describe the energy propagation via Poyntings theorem and this is no violation in a physical sense. This media is usually very complex/anisotropic and incredibly challenging to fabricate outside of a lab, wake me up when we can manufacture these /s

dustingetz• 3 years ago

Offtopic - check out A Student's Guide to Waves, a math book I just worked through as an adult which is awesome

https://www.danfleisch.com/sgw/ https://www.amazon.com/Students-Guide-Waves-Guides/dp/110764...

JKCalhoun• 3 years ago

Would light passing around a gravitational lens arrive to an observer on Earth at different times?

I would imagine that due to the bending of space, some of the light would have taken a longer path and seem therefore delayed.

UncleSlacky• 3 years ago

I thought this was going to be about phase-conjugate mirrors: https://en.wikipedia.org/wiki/Nonlinear_optics#Optical_phase...

Demo here: https://www.youtube.com/watch?v=gAy39ErqV34

Taniwha• 3 years ago

You'll know they have a working time machine when all the university physics departments suddenly start getting their research grants funded by lottery winners

Light can be reflected not only in space but also in time

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