[HN Gopher] What is a zero refractive index material?
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What is a zero refractive index material?
Author : _Microft
Score : 217 points
Date : 2023-12-10 08:28 UTC (1 days ago)
(HTM) web link (skullsinthestars.com)
(TXT) w3m dump (skullsinthestars.com)
| knob wrote:
| Fascinating. Thank you for putting it together.
| carterschonwald wrote:
| This is cool. This is the sort of stuff I like to see on the
| internet.
| jacobolus wrote:
| For anyone who hasn't studied electrodynamics or advanced optics,
| Grant Sanderson (3Blue1Brown) recently made a detailed video
| explaining what the refractive index really means, probably the
| clearest reasonably accessible explanation of this topic anywhere
| to date:
|
| https://www.youtube.com/watch?v=KTzGBJPuJwM
|
| https://www.youtube.com/watch?v=Cz4Q4QOuoo8
|
| The first video is really a tour de force. Too bad it saw such
| limited traction here:
| https://news.ycombinator.com/item?id=38482549
| hadlock wrote:
| For those without a lot of patience, skip ahead to 6:45 or so
| (in the fist linked video) which is a really cool, "a-ha!"
| moment of why light "slows down" when it hits light. He also
| points out that he's really just animating feinman lectures, so
| giving credit where it's due. Gonna have to go dig out those
| books. Big thanks to jacobolus for linking these videos
| Kranar wrote:
| To be clear there are numerous explanations for why light
| slows down in a medium and they are almost all incorrect in
| one way or another.
|
| This video uses the explanation that light interacts with
| electrons in a medium which causes the electrons to produce
| an electromagnetic wave whose interference pattern changes
| the phase of the light wave, kicking it back.
|
| The problem is that an interference pattern can never change
| the speed of a wave, it can change the phase of the wave and
| adjust it, but that won't explain how light passing through a
| medium takes longer to traverse that medium than light
| travelling in a vacuum.
|
| I don't know of a visualization or explanation that captures
| why light slows down in a medium, and the video linked is
| still a good way to get a sense of what's happening, but it's
| worth knowing that there is still a great deal being left out
| of that explanation.
|
| It is nevertheless better than explanations involving the
| absorption an reemission of photons or that light bounces
| around within the internal structure of a medium. Both of
| those explanations manage to explain how light takes longer
| to pass through a medium but fails to explain how light keeps
| a consistent direction as opposed to scattering randomly.
|
| The explanation in the video manages to explain how light
| maintains a consistent direction but fails to explain how
| light will actually take longer to pass through a medium
| compared to a vacuum.
| mikewarot wrote:
| I thought that Grant's video did a great job of explaining
| it was all phase kicks, and how it all depended on resonant
| frequencies of the medium.
|
| [Edit]In the second video above, he notes that phase kicks
| don't change the speed of the wave, even though the
| wavelengths can stretch out, it's still lower than the
| speed of light in vacuum.
| Kranar wrote:
| Phase kicks don't change the speed of a wave. If you
| setup a similar experiment with sound or water where you
| constructed an interference pattern that kicked back the
| phase of the wave, the wave wouldn't slow down.
|
| If you want an actual non-layman explanation for why
| light slows down ib a medium you need to express the
| light and medium interaction in terms of polaritons. Of
| course this is incredibly difficult to do so for even
| simple cases, so alas a whole suite of simplified
| explanations exist that seek to explain some aspect of
| the situation while failing to explain others.
|
| https://en.m.wikipedia.org/wiki/Polariton
| deely3 wrote:
| Could you elaborate please?
|
| So, there no slowdown by phase shift at all? But.. are
| you sure, because it looks like the phase shift is the
| real effect and logically speaking result of the phase
| shift will be the effect that look like slowdown for
| outside observer.
|
| Also, how can you simulate similar effect in water and
| sound if there no sound polarization?
| mikewarot wrote:
| The phase shift is dependent on both the resonance
| frequency of the medium and the incident light. In the
| case of X-rays, the refractive index can go below 1,
| resulting in total external reflection for a few tenths
| of a degree. Even in this case, the speed of the X-ray
| itself stays below that of C, even though the
| "wavelength" appears to be longer than in vacuum.
| michaelt wrote:
| _> logically speaking result of the phase shift will be
| the effect that look like slowdown for outside observer._
|
| If there was a phase shift but no slowdown, wouldn't we
| expect individual photons to propagate through a 10km
| loop of fibre optic cable without any slowdown, which
| would be measurable?
| Kranar wrote:
| You would certainly see an apparent slowdown due to phase
| shift if you consider a continuous wave that spans the
| entire medium, yes for sure and so this explanation and
| model does have merit, I don't want to dismiss it
| entirely.
|
| But this model won't be useful for understanding pulses
| of light through a medium. For example if I emit a very
| short pulse of light through a medium and measure how
| long it takes the head of the pulse to traverse the
| medium, using this model won't work. A phase shift won't
| move the head of a pulse of light backwards, it will just
| adjust the amplitude of the head as it propagates through
| the medium.
|
| You still need a way to explain how it is that the head
| of the pulse actually takes longer to travel through the
| medium and that's not something a phase shift can
| explain.
| rocqua wrote:
| The understanding of a refractive index only cares about
| 'phase speed' in steady state. At least, that is what the
| video claims to be about. For that case, phase kicks are
| a sufficient explanation no?
|
| I can imagine things get really weird with pulses, but
| that's out of scope for the video.
| planede wrote:
| > Phase kicks don't change the speed of a wave.
|
| "Phase kicks" explain the phase velocity. The speed of a
| wave is the group velocity. Phase velocity is omega(k)/k,
| group velocity is omega'(k). So if "phase kicks" explain
| phase velocity sufficiently so it's valid for a range of
| frequencies, then it also explains group velocity by
| taking the derivative of the dispersion relation.
| Kranar wrote:
| That's true for a steady-state wave, and hence this model
| is accurate for a steady-state light wave. But if you
| were to consider short pulses of light then there is no
| longer a relationship between the group velocity and
| phase velocity.
|
| The group velocity of these pulses through a medium will
| still be lower than the speed of light in a vacuum and
| phase kicks won't influence the group velocity.
|
| Explaining how the group velocity of light can be slower
| in a medium than in a vacuum requires analyzing the
| coupling of photons with electrons to form polariton
| quasiparticles. You can then calculate the mass of these
| polaritons which in turn gives you the speed and get the
| full picture. Doing this, however, is incredibly complex
| and so it's much easier to consider simplified scenarios
| like either the steady-state case where you can simply
| reason about the scenario in terms of interference
| patterns between the light wave and the electromagnetic
| waves produced by oscillating electrons, or you can
| consider some non-steady state scenarios involving
| photons themselves being absorbed and reemitted by
| electrons but neither of these explanations fully capture
| the phenomenon.
| gpsx wrote:
| I'm not sure if this will be satisfying because it is a
| mathematical explanation rather than an intuitive one. If
| you combine two of maxwell's equations, you get a wave
| equation for electric/magnetic fields. However, there is an
| extra term in there, which is the effect of a time changing
| current on the magnetic field. In a material, the time
| changing electric field induces such a current, which in
| turn has the mathematical effect of reducing the coupling
| between the electric and magnetic field, creating the
| equation for a wave that is slower.
|
| Something that might be comforting about light going slower
| than the speed of light is that the speed of light is not a
| special property of light, but it is a property of space
| time. It just so happens that light is the only way we have
| experience on this magic speed. Light isn't physically
| bound to go "the speed of light".
| Tazerenix wrote:
| The actual answer is the assumptions which define a self-
| propagating wave do not apply once the wave leaves a
| vacuum. When it becomes incident onto some medium, due to
| the coupling of electrons within the medium to the
| electromagnetic field, the pure electromagnetic wave gets
| transformed into a phonon, which is a combination of
| electromagnetic and mechanical oscillation within the
| medium (and therefore has speed <c, depending on the
| particular properties of the medium). When the phonon
| subsequently leaves the system, those traveling
| oscillations induce a new self-propagating wave on the
| other side, sending the light on its way as usual.
| MichaelZuo wrote:
| What does it mean for something to oscillate
| electromagnetically and mechanically simultaneously?
|
| Doesn't mechanical oscillation already occupy all three
| spatial dimensions plus a time dimension?
| zmgsabst wrote:
| The phonon oscillates both the electrons and the EM
| field.
|
| A photon is EM only.
|
| So you're correct that it occupies all four dimensions,
| but we're discussing an excitation in one field (photon)
| changing to an oscillation in multiple (phonon) and then
| back to only one field (photon).
| MichaelZuo wrote:
| Can you explain this in a step by step manner?
|
| I really can't quite grasp how the photon/phonon switches
| back and forth like this.
| zmgsabst wrote:
| 1. The photon moves through free space, where only the EM
| field is disturbed.
|
| 2. The photon enters a material, where the disturbance in
| the EM field couples to the electron waves -- creating a
| phonon.
|
| 3. The phonon travels through the material, as an
| oscillation in both electrons and EM.
|
| 4. The phonon reaches the edge of the material, where
| there are no more electrons and reverts to a wave of just
| EM -- a photon.
|
| 5. The photon continues in free space.
|
| The reason this happens is the photon changes the
| electron behavior, which in turn changes the photon
| behavior. This is because EM interacts with charged
| particles like electrons.
|
| For the time it is in a material, a EM wave can't be
| separated from the behavior of the electrons present:
| both and their mutual interaction are required to explain
| what happens.
|
| That disturbance of both is "coupled" -- and called a
| phonon.
| cbolton wrote:
| Is this more than a change of terminology? Is it wrong to
| say that the electromagnetic oscillation is still there
| in the medium but coupled to a mechanical oscillation,
| and the coupled system is called a phonon? (I mean
| there's still an electromagnetic field in the material,
| it still gets excited, it's not replaced with a new field
| that fuses the EM and matter fields). Then the question
| is by which process exactly is the electromagnetic
| oscillation slowed down, and is the answer much different
| from the explanations above? I.e. the moving charges due
| to the mechanical oscillation add perturbations to the EM
| field such that the net effect is a slower propagating
| wave.
| dustingetz wrote:
| visualized well in "Picture Book of Quantum Mechanics" ch 1
| (a fairly hardcore physics text that does indeed have many
| diagrams)
| martincmartin wrote:
| No need to dig out the books, they're available online, well
| formatted and a pleasure to read:
|
| https://www.feynmanlectures.caltech.edu/
|
| https://www.feynmanlectures.caltech.edu/II_32.html
| dcreater wrote:
| Grant has singlehandedly taught me more than all of my elite
| education combined. Man is an absolute treasure
| platz wrote:
| One remaining question I had after watching Grant's video was
| that, although the collective behavior of a bunch of light
| waves made sense via interference, _how would a single photon
| 's direction would change upon entering the medium_?
|
| The best answer I could find on physics stack exchange was that
| the single photon's wavefunction is delocalized, so that the
| photon's wavefunction, in fact, interacts with the entire
| medium, instead of at a single point.
|
| Is this the correct way to state this phenomenon or is there a
| better understanding of how the light emitted from a single
| photon interacting with a medium would bend?
| SAI_Peregrinus wrote:
| Huygens Optics (pretty much the only dedicated optics channel
| on Youtube) also recently made a video on this subject[1].He
| made some diffractive lenses via photolithography, and shows
| how the interference caused by variably spaced diffraction
| patterns causes refraction.
|
| [1] https://youtu.be/SS2AbZVdk2A?si=5KTvkX-KVnxRUN7G
| datadrivenangel wrote:
| This kind of thing is super important for fiber optics.
| luispauloml wrote:
| AhnLab Safe Transaction's anti-phishing protection erroneously
| blocks this domain. The log says: Date:
| <redacted> Security Threat: Phishing site Target:
| skullinthestars.com/ Status: Block Scan Method:
| Anti-Phishing
|
| _Sigh._ I wonder how much worse can these South Korean "security
| applications" get. For instance, AhnLab's website1 doesn't even
| list Ahnlab Safe Transaction in their products.
|
| For the curious ones, a related thread: "South Korea's online
| security dead end" https://news.ycombinator.com/item?id=34231364
|
| 1 https://www.ahnlab.com/
| rendall wrote:
| > _" Privacy & Cookies: We and our 772 advertising partners store
| and/or access information on your device and also process
| personal data, like unique identifiers, browsing activity, and
| other standard information sent by your device including your IP
| address..."_
|
| There is no opt-out, which violates GDPR. If my data was
| collected on landing, I would like my data removed, please.
| uxp8u61q wrote:
| Have you contacted the owners of the website to get your data
| removed?
| saagarjha wrote:
| And give them more personal information?
| rendall wrote:
| I assumed OP was the website owner. But while my request is
| serious and legally binding, it was somewhat rhetorical as I
| do not expect the owners of a site with such a data policy
| would really be conscientious about contacting me to sort out
| which data is mine and removing it.
| eru wrote:
| > But while my request is serious and legally binding,
| [...]
|
| Posting a comment on HN is somehow legally binding for a
| third party website whose operator likely never even heard
| of HN? Or what are you trying to say?
| rendall wrote:
| As I understand it, the request to have data removed is
| what's legally binding, not the format in which the user
| makes the request. If the site owners see it or otherwise
| become aware of it, they are obligated to remove my data
| even though it was a post on HN. Whether all of that
| actually happens is a different story. They could
| probably claim truthfully they don't know which data is
| mine. If I were determined, I'd contact them directly.
| uxp8u61q wrote:
| Don't be absurd. I'm a big proponent of GDPR but you seem
| to be under the impression that it's a magic spell. You
| seriously believe that, legally, if you post a comment on
| HN, you're putting an obligation on someone? I have a
| piece of the moon to sell you, then!
| rendall wrote:
| That is a misapprehension of what I wrote.
| rendall wrote:
| Also, I threw that in there because, if the site
| operators were to see "I don't expect them to actually
| remove my data", I do not want them to interpret that as
| releasing them from the obligation. It does not. It's
| just my prediction of how things will go.
| groby_b wrote:
| Yes, there is an opt out, see the fourth sentence: * Otherwise,
| you can get more information, customize your consent
| preferences, or decline consent by selecting "Learn More".*
|
| It's a dark pattern to only call it "Learn More", but I
| wouldn't bet it's a GDPR violation.
| rendall wrote:
| Thanks for the tip.
|
| The opt out is supposed to be as easy as the opt in, so I'd
| bet it crosses the line.
|
| Edit: furthermore, even after opting out, you nevertheless
| "agree" that (that is to say, you cannot opt out of):
| Certain information (like an IP address or device
| capabilities) is used to ensure the technical compatibility
| of the content or advertising, and to facilitate the
| transmission of the content or ad to your device.
| Match and combine data from other data sources
| Link different devices Identify devices based on
| information transmitted automatically
| sokoloff wrote:
| The first of those is surely a valid technical requirement,
| right? (I'm not a GDPR expert, by any means.)
|
| In order to send you bits via IP, the server needs your IP
| address. In order to know the type of bits to send, it
| needs to understand the content-type, encoding, etc that
| your device supports.
| rendall wrote:
| From what I understand, GDPR applies only to retention of
| data, and not such technical requirements. It does not
| require operators to inform users unless the server will
| be retaining the IP address.
| sokoloff wrote:
| In any distributed system, what "retention" means is
| unclear* and, in the face of that lack of clarity, a
| company deciding to play it safe and require consent for
| things that GDPR says needs consent seems reasonable.
|
| * If I put your content-type and IP address onto a queue
| for processing by another system, that's arguably
| retaining the data, even if that length of retention is
| typically measured in seconds. Given that, the safest
| thing for me to do is to require your consent for the
| retention of IP address and related data.
| rendall wrote:
| I suppose? The GDPR is very clear: if you serve European
| residents, you cannot collect their identifiable data
| without their informed consent, nor can you make consent
| the price of viewing your content. If you are found to do
| so, you are liable to huge fines.
|
| The language of the law itself is not very far from that
| clarity. Check it out for yourself:
| https://commission.europa.eu/law/law-topic/data-
| protection/d...
|
| I find it not credible that a company would read that
| second part, think "My goodness that sounds awful! Out of
| an abundance of caution, we better include language
| around arcane technical edge cases!" but then go on to
| violate the very, very clearly stated core intent of the
| law regarding consent and actual data retention and
| brokering.
|
| So... it is confusing. Probably intentionally so. Perhaps
| their legal strategy would be to somehow conflate these
| edge cases? Try to baffle judges?
| 6510 wrote:
| > The EDPB adopted Guidelines on dark patterns in social
| media platform interfaces[2]. The guidelines offer practical
| recommendations to designers and users of social media
| platforms on how to assess and avoid so-called "dark
| patterns" in social media interfaces that infringe on GDPR
| requirements.[1]
|
| There calling it "deceptive design patterns". In short,
| tricking people is not consent.
|
| [1] - https://edpb.europa.eu/news/news/2022/edpb-adopts-
| guidelines...
|
| [2] - https://edpb.europa.eu/system/files/2023-02/edpb_03-202
| 2_gui...
| lm28469 wrote:
| Just click on "disagree to all", no ?
| rendall wrote:
| At the time i wrote that, I did not understand that was
| behind the "learn more" button. However, if their data
| retention policy is accurate, even clicking "disagree to all"
| means they will retain data that individually identifies
| their users and they exchange this data with other brokers.
| This is strictly against the letter and spirit of the GDPR,
| opening them up to the possibility of massive fines.
| kazinator wrote:
| I would love to have a mug that is nearly invisible (when clean)
| until you pour beer into it.
| johanvts wrote:
| If you want to play around with different refraction values and
| see what happens I made a MetaPost script for that once:
| http://www.hugoblog.onrender.com/post/prism/
| zdimension wrote:
| I'm getting an SSL error when trying to load that URL
| ipdashc wrote:
| https://hugoblog.onrender.com/post/prism/
| zppln wrote:
| > _We and our 772 advertising partners store and /or access
| information on your device [...]_
|
| > _772_
|
| Seriously...
| lm28469 wrote:
| A lot sites are like that (most big ones), that's the whole
| reason they started GDPR. All you have to do is add a few
| widgets/plugin/third party ads/traffic monitors/tracking and
| their dependencies and you'll get hundreds of "partners"
|
| arstechnica, cnet, &c. all have 200+ "partners"
| xnorswap wrote:
| And unfortunately they've all settled on the apparently
| untested, "legitimate interest" loophole where these hundreds
| of partners have a "legitimate interest" in serving you
| personalised adverts.
| ycombinatrix wrote:
| thanks for the heads up. i wasn't that interested in this topic
| anyway.
| jeffparsons wrote:
| I have no physics education beyond high school, but I briefly
| became obsessed with the idea of efficient collimation of light.
| I eventually concluded that it must be impossible, because it
| would imply a global decrease in entropy, much like Maxwell's
| demon.
|
| But this article seems to suggest that it might still be
| possible. Have I misread it? Or maybe misinterpreted -- e.g.
| maybe "efficient" would cover a system that does require energy
| input, and just doesn't require discarding a lot of the input
| light?
|
| Can anyone with more of a clue comment on this?
|
| I find the idea exciting because I can imagine endless practical
| uses for an efficient collimator.
| mnw21cam wrote:
| It'd be balanced by the difficulty in getting the light _into_
| the material in the first place. You 're right, you can't
| change the laws of physics. Light will only enter an object
| with lower refractive index if its angle of incidence is close
| enough to straight on that the beam coming through into the
| material has somewhere to go that satisfies Snell's law. When
| the object's refractive index is near zero, the object will
| only accept incoming light across an extremely narrow angle.
| This solves your concerns about having a global decrease in
| entropy.
|
| You can fudge this by making the incoming surface of the object
| match the shape of the incoming waves. Then all the light will
| be allowed in. However, that surface will only match the shape
| of the incoming waves for light originating from a tiny point
| in space. If your light source is larger than a tiny point in
| space, then the light coming from the larger area will bounce
| off the material and not enter it. In effect, the magic
| material will magically collimate a light source, but only if
| the light source is already fairly magic, and in that case a
| simple lens would suffice.
| dekhn wrote:
| it sounds like etendue
| denton-scratch wrote:
| You can model refraction using waves in a water-tank; the waves
| slow down in shallow water, and speed up in deeper water. You can
| do things like make lenses.
|
| So I suppose a near-zero refractive index would be modeled by a
| region of "infinite depth". Is there a relationship between
| wavelength and depth, such that for a given wavelength, there is
| some finite depth that behaves for practical purposes as if it
| were infinite?
| Jeff_Brown wrote:
| I don't understand why this does not imply the possibility of
| faster than light communication. If you had a long tube with a
| zero refractive index, couldn't you type Morse code into one end,
| by simply turning the signal on and off, and have the receiver
| receive the message immediately?
| edgineer wrote:
| 3blue1brown's video covers this. The effect of "changing speed"
| is a change to the light's wavelength, not the speed of an
| impulse through the material--imagine taking a standing wave
| and pushing the peaks and valleys up and down so the wavelength
| looks really long. And since the frequency is unchanged,
| there's your appearance of high speed. This happens because the
| material changes the phase of the wave continuously.
| gcanyon wrote:
| For anyone who's interested, Fermilab has done several videos on
| this topic as well that I found highly informative:
|
| https://www.youtube.com/watch?v=CUjt36SD3h8
|
| https://www.youtube.com/watch?v=NLmpNM0sgYk
| aurizon wrote:
| There is an old science fiction concept of 'slow glass', where
| light travels very very slowly, with 1/2 day glass used for
| windows that allow light to dally for 1/2 day before it exits -
| also good for street lights. Extending it to week/glass, year
| glass and so on, with the glass becoming a defacto streaming
| video display to watch burglars/traffic etc. Interesting concept,
| smashing a pane of year glass - would it release all that energy
| as an e=mc2 blast?? https://sf-encyclopedia.com/entry/slow_glass
| JoshGG wrote:
| This is a great article. Very interesting. Thanks for posting!
|
| I recently developed a new interest in optics and it's cool to
| see this posted on HN.
| cbondurant wrote:
| The first application that comes to mind for me, and of course I
| am not a quantum physicist so its entirely possible that my
| conceptions here are misguided, would be with creating even more
| perfectly coherent beams of laser light for use in the high
| precision environments of quantum laser experiments.
|
| Possibly either at more affordable rates or to allow for an
| extremely temporally coherent wavefront in other shapes besides
| just the beam that is most commonly used? I could easily believe
| that there are potential experiments that are just waiting for
| the right technology to be viable.
| FireInsight wrote:
| I'm on my phone, and the privacy banner takes so much space I
| just see the title, a divider, and then the banner. I started
| reading the banner thinking it was the article...
| swayvil wrote:
| Note that the sun produces collimated rays too. And the stars
| even moreso.
|
| Which is to say, the rays are always parallel if the source is
| far enough away. Which feels like a good metaphor for something.
| the-mitr wrote:
| of possible interest: _Discussions on Refraction of Light_
|
| https://archive.org/details/TarasovTarasovaDiscussionsOnRefr...
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