A mirror that reverses how light travels in time | Hacker News Reader

A mirror that reverses how light travels in time

spectrum.ieee.org

131 pointsmfiguiere35 comments

The title is very click batey.

It doesn't reverse how light travels in time, it just reverses light in terms of the order of photons, as in "Last in First out" instead of "First in First out". The concept is quite natural to grasp. The article makes it harder to understand than necessary by making it sound like science fiction.

The "reflector" is actually a medium that encompass the entire signal being reversed... in the same way that you would need to buffer the entire segment of a signal in memory before reversing it (you can't send what you haven't yet received), you would also need the entire segment of light to be travelling through the medium at the same time in order to reverse it. This is why their experiment involves such long sections of material.

It's an interesting and useful phenomenon, effectively being able to instantly change a material's properties to reverse the direction of photons currently passing through it, which probably has good application in signal processing performance (it would be comparable to hardware signal processing using electrical components). But "time" is not reversed; the direction of photon's currently traversing a medium is dynamically reversed, but unlike a reflective surface, a reflective medium could change the direction of multiple photons simultaneously instead of one at a time, which affects the order of that segment..

So photons would effectively be reflected back in a LIFO manner rather than FIFO?

If so, thank you for the translation.

Yes, exactly, I came to the same realisation that was an appropriate label and made an edit to describe it as such. In fact comparing it to FIFO as you have makes even more sense so I've edited it again.

Surface reflections are FIFO, because there is no medium, no "storage"; This novel medium based reflection is LIFO, based upon whatever is traversing the medium at the time of the change to the medium.

It's not magic, but it's an interesting way to achieve this in pure photonics, without first converting to something more manageable like electronics... it also doesn't require discrete memory like electronics, it's kind of analogue processing but with a buffer? but can only support analogue processing.

I think you misunderstand. What's being described is a Phase Conjugate Mirror (PCM) -- Think of it as a mirror where the angle of incidence equals the angle of reflection, but without being translated into the opposite quadrant.

It does not "buffer" signals incident upon it, but reflects it along a path that it took to get to the PCM. So it's the same path through space that you would get if you reversed time from the moment the signal hit the mirror.

I'm sure that was clear as mud, here's a wikipedia link: https://en.wikipedia.org/wiki/Nonlinear_optics#Optical_phase...

So the photons re-trace their steps instead of continuing forward. I think the picture of the tiger seen through the distorion of a glass bottle on the linked page explains it pretty well. But I'm a layperson.

No I don’t think that’s right:

When a light wave enters the new time interface and the device changes its optical properties, the signal keeps moving forward in space. However, the signal gets reversed—if it were a spoken word, it would sound as if it were getting played backwards. In contrast, with a conventional reflection, a light or sound wave would travel back at its source but mostly look or sound the same as it did before the reflection.

— They continue forward, but they behave in the reverse with respect to time alone.

I did interpret it as effectively reflecting along the same path, I was using "buffer" in a more abstract sense to compare how it would be processed digitally. But I don't doubt I've misunderstood something, my description of photons traversing the medium is probably what sounds physically inaccurate, I'm no expert in photonics and don't really know what a PCM is.

As someone who does, are you able to corroborate whether the effect differs abstractly from what I described in terms of reversing the signal, as in a LIFO? Because this was my gripe with the article, the way the concept was explained makes it sounds like something more than it is.

Also referred to as a "corner cube": https://en.wikipedia.org/wiki/Corner_reflector

Corner reflectors don't reverse the order of the incoming beam.

No. This is not a corner reflector. Read the wikipedia reference I posted.

So instead of:

light: --123-> :Wall

light: <-321-- :Wall

it is:

light: --123-> :Wall

light: <-123-- :Wall

No:

—

So it’s Light -123-> wall -321->

Where wall isn’t a wall but a time mirror.

Thank you!

I think it’s right. The mirror takes f(x,y,z,t) and maps it to f(x,y,z-t), thus reversing it in the time dimension but not space. It doesn’t make it go backwards through time, it just reverses lights behavior with respect to time.

> The concept is quite natural to grasp.

I'd use that term to explain perhaps understanding seeing an Apple falling from a tree.

> very click batey

click bait would entice you to click, like baiting a fish.

click bate would stop you from clicking, like abating your breathing and therefore holding your bated breath.

>When a light wave enters the new time interface and the device changes its optical properties, the signal keeps moving forward in space. However, the signal gets reversed—if it was a spoken word, it would sound as if it was getting played backwards. In contrast, with a conventional reflection, a light or sound wave would travel back at its source but mostly look or sound the same as it did before

So it's reflecting in a novel way. It's not sending information into the past. The headline gave me the wrong impression. Relativity lives. Time travel doesn't exist.

In fairness this technique is called time reversal because it is equivalent to playing back the waves but with the time axis reversed. (I assume that's it anyway - that's what you do in acoustics.)

I agree the headline is misleading though. It doesn't affect how light travels in time.

This. I really thought light was literally going back in time, not getting rewound like a cassette tape.

It's hard to read between the aggrandized lines, but it appears that this is a discrete phenomenon based on precisely switched gates/reflectors. It's super interesting but not exactly science fiction.

Imagine your signal looks like _A_B_C_ where each letter represents a discrete subpacket. The reflectors look like _1_2_3_. As long as those gates are timed to turn on precisely between the subpackets, the reflected signal would be the reverse order of packets.

We could already make light "slow", and then we can move our eyeballs faster than the light, hitting photons in the reverse of the order they were emitted in. It certainly isn't "time reversed light", it's a time reversed signal. I didn't RTFA though.

Sounds really interesting, but I'm struggling to imagine the implications of this. Can anyone expound on it?

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

Disclaimer: I don't know if anything said in the article is true -- or not...

I do know that the article is interesting however...

Here's the thing -- I notice that a lot of posters are using the words "forward" and "backward" with respect to time, with respect to waves...

Here is my problem with those words, used in this context:

If you are one of the posters who used the words "forward" or "backward" in your posts -- I ask you to first consider a simple Sine Wave (https://en.wikipedia.org/wiki/Sine_wave).

OK, now if you used the words "forward" and/or "backward" in your post -- please explain to me the following:

Does a simple, periodic Sine Wave -- go "forwards" through time -- or "backwards"?

Forwards or backwards?

Note that I'm NOT asking about a complex information-carrying radio signal as might be emitted by a radio station or other source -- I'm asking about the simplest of simple Sine Waves -- as might be output by a frequency generator, pendulum, or other simple pure sine wave generating device...

So, is that wave moving forwards or backwards?

Or more specifically forwards or backwards -- in Time?

Forwards or backwards?

I thought we already had phase-conjugate mirrors for EM? Is this not the case? "Real Genius" (1985) mentions a tracking system using one.

Or rather how is this different than a phase-conjugate mirror?

This is how the effects of Magnetorheological fluids work with sound and vibrational damping. Glad to see they have figured out a corrolary with EM waves.

The analogy of "if it was a spoken word, it would sound as if it was getting played backwards" was helpful, but I'm still struggling to understand what's happening here. Thinking of the audio analogy, I could imagine implementing that as a buffer you record into, and after some amount of time, you play back the buffer in reverse. Is that what's going on here?

https://news.ycombinator.com/item?id=35263734 helped explain it for me.

The phenomenon in the article still seems like a kind of reflection, but I wonder if a time crystal with quantum size facets would mess with light's actual space/time dynamics.

Do you want to release the Shrike? Because this is how you release the Shrike.

Oh hell yes. About time someone references Hyperion.

I discovered this effect in mirrors in bright hotel bathrooms. They somehow distort time by inacurately making me look older. They need to fix that.

Usage suggestion: can we please call this material a 'turnstile'?

"All I have for you is a gesture, in combination with a word. Tenet. Use it carefully. It'll open the right doors, but some of the wrong ones, too."

That movie lost me when I realized someone had to pour the concrete with the bullet and bullet hole and cracks already in it, but the same doesn't happen when the bullet comes out of human flesh.

I'm imagining the horrifying disease a child must have been born with, that legitimately cured by putting a time-reversed gun to its body and pulling the trigger.

I quite enjoyed the film, but you're absolutely correct: it falls apart when you think about it too clearly.

All the same, it got folks thinking enough about physics (and time!) to notice that it falls apart! That's powerful.

Sort of like how ChatGPT's confident hallucinations make everyone read everything more carefully. It's not the intended outcome, but I value it.

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