[HN Gopher] A mirror that reverses how light travels in time
___________________________________________________________________
A mirror that reverses how light travels in time
Author : mfiguiere
Score : 90 points
Date : 2023-03-22 14:17 UTC (8 hours ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| [deleted]
| wheaties wrote:
| 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.
| timwis wrote:
| Sounds really interesting, but I'm struggling to imagine the
| implications of this. Can anyone expound on it?
| kiviuq wrote:
| https://www.youtube.com/watch?v=gAy39ErqV34
| 1attice wrote:
| 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."
| cwillu wrote:
| 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.
| 1attice wrote:
| 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.
| karmakaze wrote:
| 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?
| greenhearth wrote:
| 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.
| tomxor wrote:
| 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..
| retrocryptid wrote:
| 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...
| tomxor wrote:
| 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.
| nielsbot wrote:
| 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.
| fnordpiglet wrote:
| 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.
| anonymousiam wrote:
| Also referred to as a "corner cube":
| https://en.wikipedia.org/wiki/Corner_reflector
| iamerroragent wrote:
| So instead of:
|
| light: --123-> :Wall
|
| light: <-321-- :Wall
|
| it is:
|
| light: --123-> :Wall
|
| light: <-123-- :Wall
|
| ?
| fnordpiglet wrote:
| No:
|
| 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.
|
| --
|
| So it's Light -123-> wall -321->
|
| Where wall isn't a wall but a time mirror.
| iamerroragent wrote:
| Thank you!
| nobodyandproud wrote:
| So photons would effectively be reflected back in a LIFO manner
| rather than FIFO?
|
| If so, thank you for the translation.
| tomxor wrote:
| 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.
| [deleted]
| fnordpiglet wrote:
| 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.
| [deleted]
| nonrandomstring wrote:
| 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.
| chrisshroba wrote:
| 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?
| Shared404 wrote:
| https://news.ycombinator.com/item?id=35263734 helped explain it
| for me.
| achr2 wrote:
| 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.
| ouid wrote:
| 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.
| mcdonje wrote:
| >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.
| IshKebab wrote:
| 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.
| greenhearth wrote:
| This. I really thought light was literally going back in time,
| not getting rewound like a cassette tape.
___________________________________________________________________
(page generated 2023-03-22 23:01 UTC)