I didn't believe that light slows down in water (part 1)

[@LookingGlassUniverse]

Part 2 of this story: kzbin.info/www/bejne/q6CWlaZme7ujjtUsi=TaUPxDsOUi2k4yW5 3Blue1Brown's explanation of Feynman's proof is so great: kzbin.info/www/bejne/gYXdeHWAhdp9ra8 Here's a doc with more of the mathematics behind all of this: colab.research.google.com/drive/1L9X_tq-Kjt-foEhcnSXpvNujbbJEedBz?usp=sharing The 3D simulation lives on my github. I would love to update it though, so if you have ideas or time to work on it, I'd love to hear about it : github.com/mithuna-y/speed_of_light_in_a_medium/tree/main/multiple_layers

[@bogdy72000]

kzbin.info/www/bejne/mqDIeYRmo711nas

[@MarcusAndersonsBlog]

IMHO, speed of light has nothing to do with light but rather the speed of entropy (aka causality), which is directly related to matter because entropy can only be observed in matter. Light just tags along for the ride. 🙂

[@SpotterVideo]

Conservation of Spatial Curvature: Both Matter and Energy described as "Quanta" of Spatial Curvature. (A string is revealed to be a twisted cord when viewed up close.) Is there an alternative interpretation of "Asymptotic Freedom"? What if Quarks are actually made up of twisted tubes which become physically entangled with two other twisted tubes to produce a proton? Instead of the Strong Force being mediated by the constant exchange of gluons, it would be mediated by the physical entanglement of these twisted tubes. When only two twisted tubules are entangled, a meson is produced which is unstable and rapidly unwinds (decays) into something else. A proton would be analogous to three twisted rubber bands becoming entangled and the "Quarks" would be the places where the tubes are tangled together. The behavior would be the same as rubber balls (representing the Quarks) connected with twisted rubber bands being separated from each other or placed closer together producing the exact same phenomenon as "Asymptotic Freedom" in protons and neutrons. The force would become greater as the balls are separated, but the force would become less if the balls were placed closer together. Therefore, the gluon is a synthetic particle (zero mass, zero charge) invented to explain the Strong Force. An artificial Christmas tree can hold the ornaments in place, but it is not a real tree. String Theory was not a waste of time, because Geometry is the key to Math and Physics. However, can we describe Standard Model interactions using only one extra spatial dimension? What did some of the old clockmakers use to store the energy to power the clock? Was it a string or was it a spring? What if we describe subatomic particles as spatial curvature, instead of trying to describe General Relativity as being mediated by particles? Fixing the Standard Model with more particles is like trying to mend a torn fishing net with small rubber balls, instead of a piece of twisted twine. Quantum Entangled Twisted Tubules: “We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct.” Neils Bohr (lecture on a theory of elementary particles given by Wolfgang Pauli in New York, c. 1957-8, in Scientific American vol. 199, no. 3, 1958) The following is meant to be a generalized framework for an extension of Kaluza-Klein Theory. Does it agree with some aspects of the “Twistor Theory” of Roger Penrose, and the work of Eric Weinstein on “Geometric Unity”, and the work of Dr. Lisa Randall on the possibility of one extra spatial dimension? During the early history of mankind, the twisting of fibers was used to produce thread, and this thread was used to produce fabrics. The twist of the thread is locked up within these fabrics. Is matter made up of twisted 3D-4D structures which store spatial curvature that we describe as “particles"? Are the twist cycles the "quanta" of Quantum Mechanics? When we draw a sine wave on a blackboard, we are representing spatial curvature. Does a photon transfer spatial curvature from one location to another? Wrap a piece of wire around a pencil and it can produce a 3D coil of wire, much like a spring. When viewed from the side it can look like a two-dimensional sine wave. You could coil the wire with either a right-hand twist, or with a left-hand twist. Could Planck's Constant be proportional to the twist cycles. A photon with a higher frequency has more energy. ( E=hf, More spatial curvature as the frequency increases = more Energy ). What if Quark/Gluons are actually made up of these twisted tubes which become entangled with other tubes to produce quarks where the tubes are entangled? (In the same way twisted electrical extension cords can become entangled.) Therefore, the gluons are a part of the quarks. Quarks cannot exist without gluons, and vice-versa. Mesons are made up of two entangled tubes (Quarks/Gluons), while protons and neutrons would be made up of three entangled tubes. (Quarks/Gluons) The "Color Charge" would be related to the XYZ coordinates (orientation) of entanglement. "Asymptotic Freedom", and "flux tubes" are logically based on this concept. The Dirac “belt trick” also reveals the concept of twist in the ½ spin of subatomic particles. If each twist cycle is proportional to h, we have identified the source of Quantum Mechanics as a consequence twist cycle geometry. Modern physicists say the Strong Force is mediated by a constant exchange of Gluons. The diagrams produced by some modern physicists actually represent the Strong Force like a spring connecting the two quarks. Asymptotic Freedom acts like real springs. Their drawing is actually more correct than their theory and matches perfectly to what I am saying in this model. You cannot separate the Gluons from the Quarks because they are a part of the same thing. The Quarks are the places where the Gluons are entangled with each other. Neutrinos would be made up of a twisted torus (like a twisted donut) within this model. The twist in the torus can either be Right-Hand or Left-Hand. Some twisted donuts can be larger than others, which can produce three different types of neutrinos. If a twisted tube winds up on one end and unwinds on the other end as it moves through space, this would help explain the “spin” of normal particles, and perhaps also the “Higgs Field”. However, if the end of the twisted tube joins to the other end of the twisted tube forming a twisted torus (neutrino), would this help explain “Parity Symmetry” violation in Beta Decay? Could the conversion of twist cycles to writhe cycles through the process of supercoiling help explain “neutrino oscillations”? Spatial curvature (mass) would be conserved, but the structure could change. ===================== Gravity is a result of a very small curvature imbalance within atoms. (This is why the force of gravity is so small.) Instead of attempting to explain matter as "particles", this concept attempts to explain matter more in the manner of our current understanding of the space-time curvature of gravity. If an electron has qualities of both a particle and a wave, it cannot be either one. It must be something else. Therefore, a "particle" is actually a structure which stores spatial curvature. Can an electron-positron pair (which are made up of opposite directions of twist) annihilate each other by unwinding into each other producing Gamma Ray photons? Does an electron travel through space like a threaded nut traveling down a threaded rod, with each twist cycle proportional to Planck’s Constant? Does it wind up on one end, while unwinding on the other end? Is this related to the Higgs field? Does this help explain the strange ½ spin of many subatomic particles? Does the 720 degree rotation of a 1/2 spin particle require at least one extra dimension? Alpha decay occurs when the two protons and two neutrons (which are bound together by entangled tubes), become un-entangled from the rest of the nucleons . Beta decay occurs when the tube of a down quark/gluon in a neutron becomes overtwisted and breaks producing a twisted torus (neutrino) and an up quark, and the ejected electron. The production of the torus may help explain the “Symmetry Violation” in Beta Decay, because one end of the broken tube section is connected to the other end of the tube produced, like a snake eating its tail. The phenomenon of Supercoiling involving twist and writhe cycles may reveal how overtwisted quarks can produce these new particles. The conversion of twists into writhes, and vice-versa, is an interesting process, which is also found in DNA molecules. Could the production of multiple writhe cycles help explain the three generations of quarks and neutrinos? If the twist cycles increase, the writhe cycles would also have a tendency to increase. Gamma photons are produced when a tube unwinds producing electromagnetic waves. ( Mass=1/Length ) The “Electric Charge” of electrons or positrons would be the result of one twist cycle being displayed at the 3D-4D surface interface of the particle. The physical entanglement of twisted tubes in quarks within protons and neutrons and mesons displays an overall external surface charge of an integer number. Because the neutrinos do not have open tube ends, (They are a twisted torus.) they have no overall electric charge. Within this model a black hole could represent a quantum of gravity, because it is one cycle of spatial gravitational curvature. Therefore, instead of a graviton being a subatomic particle it could be considered to be a black hole. The overall gravitational attraction would be caused by a very tiny curvature imbalance within atoms. In this model Alpha equals the compactification ratio within the twistor cone, which is approximately 1/137. 1= Hypertubule diameter at 4D interface 137= Cone’s larger end diameter at 3D interface where the photons are absorbed or emitted. The 4D twisted Hypertubule gets longer or shorter as twisting or untwisting occurs. (720 degrees per twist cycle.) How many neutrinos are left over from the Big Bang? They have a small mass, but they could be very large in number. Could this help explain Dark Matter? Why did Paul Dirac use the twist in a belt to help explain particle spin? Is Dirac’s belt trick related to this model? Is the “Quantum” unit based on twist cycles? I started out imagining a subatomic Einstein-Rosen Bridge whose internal surface is twisted with either a Right-Hand twist, or a Left-Hand twist producing a twisted 3D/4D membrane. This topological Soliton model grew out of that simple idea. I was also trying to imagine a way to stuff the curvature of a 3 D sine wave into subatomic particles. ------------------

[@bogdy72000]

what a crok of bs try the rope hypothesis @@SpotterVideo

[@tusharpotdar5762]

Wow very nice experiment I have another concept about time, I think time is not a thing that really exist, time is imaginary, so time travel is just a myth tossed by pseudo science fellow. only distance and angle, and directions are real thing.

[@ModelThree64]

I rarely comment on videos, but for this one, I feel I must. As someone who teaches physics and continues to study it in my free time, I feel like your video is such an honest and amazing look into the scientific method. You found something you were curious about, did research, came up with a very well argued hypothesis, tested your hypothesis experimentally, and found the result that you didn't expect, which you finally accepted. You should be really proud - I'm definitely going to be showing this video to my students every year.

[@LookingGlassUniverse]

Aww thank you!! It sounds like your students have a great teacher

[@mcamp9445]

@@LookingGlassUniversethe cool part is that once it leaves the water it instantly is back to lightpseed

[@durragas4671]

​@@mcamp9445conservation of energy be damned. 😮

[@MarcusAndersonsBlog]

@@LookingGlassUniverse @3:20 "That's not we see..." is absolutely not correct. Actually THAT is exactly what we see. The light you see in the water is being emitted from the water molecules. You cannot see light unless you are looking at the emitter, in this case by either looking directly into the laser (extremely dangerous) or at the path it takes through the water. The light beam path between the laser source and the water is not visible because it is not encountering enough air molecules to be scattered, and when it encounters water the beam is visible only BECAUSE the light is being scattered by the water molecules. You can prove it is being scattered because the beam is visible in the water regardless of where it is viewed from.

[@franciscomagalhaes7457]

I wish teachers would have the ability to communicate this sort of stuff like this in the 90s... I feel like 75% of my education on a variety of subjects was, worse than useless, just plain wrong.

[@rosuav]

"Light's whole purpose is to harass charged particles." That means that playing with a laser pointer and a cat is fundamentally correct: a cat is just a really really BIG charged particle.

[@hughobyrne2588]

Indeed. A... cat-ion.

[@IndirectlyResponsible]

@@hughobyrne2588A cat-ion 🤣

[@IB4U2Cme]

I remember measuring the speed of light in my college lab 52 years ago. I am so blown away with how a person today can challenge physical concepts with commonly available devices.

[@Baaqel]

Even more interesting- you measured the speed the light that it makes in round trip, but no one has measured it in one direction.

[@IB4U2Cme]

@@Baaqel And it is interesting that no one can measure the passing of time except at a single pint in space.

[@Heracles_FE]

Light doesn't slow down through water because light doesn't actually travel . Light is always there , what travels is the wave of excitation . Einstein was wrong and ruined physics. Tesla was correct , the aether is real . Light is aether excited .

[@IB4U2Cme]

@myspeechles I am 72, yes I am amazed at today’s technology. But rocket fuel is still pretty much the same technology as when we put twelve men on the moon. But I remember how difficult it was for me to repeat the Michaelson Morley test. And she was doing the equivalent with modern common technology.

[@SytRReD]

I've been following you for about the last ten years, and although you don't post often, your videos are of utmost quality, and I truuuuuuly love them! So glad to have this tonight, plus it's a question that bugged me for years so I look forward to your discoveries and explanations!

[@KitagumaIgen]

As an researcher with a experiment-observation job it pleases me to see a theoretician struggle with experimental problems - this is not just a little embarrassing but a lot embarrassing to admit. But the most important thing to remember: It is when we find out that we're wrong about something we have the opportunity to learn something new! Great job!

[@INT41O]

Some high frequency traders made a lot of money by building a radio connection between stock exchanges, since it was faster than the existing fiber optics connection (speed of light reduced to 2/3 compared to air).

[@LookingGlassUniverse]

That is an amazing fun fact (that would have saved me a lot of pain)

[@chrisking7603]

Is that related to an exchange that has a huge coil of fibre to deliberately slow the transactions in/out the building?

[@pineapplegodguy]

just wow. the journey was crazy, and we've got a mouthful of emotions - somewhat of a recurrent theme of scientific research (and of your channel) is the expectations, the joy in anticipation of the results, the moment reality brings you back to earth with the experimental results. and we've got all of that filmed. incredible.

[@RealNovgorod]

The "original" wave does indeed travel at c through the medium. However, what comes out at the other side is NOT the original wave but a superposition of it with the (non-resonantly) stimulated dipole emitters in the medium. Every infinitesimal "layer" of the medium adds a phase delay to the emitted wave (because of the molecular harmonic potential, but that's not important at this point), and this delayed wave interferes with the original wave and shifts its phase by a little bit - but the resulting wave still travels at c. The thing to realize is that this tiny phase delay happens continuously at every single layer of the medium, so you're continuously adding a bit of delay to the original wave at some constant _rate_ which is the same as changing the (spatial) frequency. That explains the change in wavelength inside a medium in the 3b1b video. In your case, you're dealing with short pulses of light and the TOF LIDAR measures when the peak of the pulse arrives back at the detector. This peak is slowed down inside the medium due to the same superposition mechanism with continuously phase-delayed versions of itself so it takes longer to pass through the medium. That's the definition of group velocity. What stays constant is the _phase velocity_ which is the oscillation speed of the E-field _underneath_ the pulse envelope. In a simulation you can see that the wave peaks underneath the envelope travel faster (i.e., at c) than the envelope itself (at c/n), but because of the interference you never detect these c-speed wave peaks at the output after the medium because they are destructively suppressed by the pulse envelope effectively "riding the wave backwards" (from the reference frame of the input wave). There's also broadening of the pulse due to group velocity dispersion in most media, but that's a story for another day...

[@Shadow_B4nned]

Good explanation. I reached much the same conclusion. Light doesn't slow down. It's only absorbed and emitted during the phase kick when it travels through a medium.

[@ArbitraryConstant]

The speed of light being slower in fiber optics is pretty important to understanding network latency, I assume similar principles apply there.

[@andrewharrison8436]

Good shout.

[@ikocheratcr]

Not only on fiber optic, you can also measure the increase in travel time in coaxial cable, parallel wires, etc. Also if the pair is twisted the delay is less than not twisted.

[@LookingGlassUniverse]

Wow, I had no idea that it slowed down enough to account for in a fibre optic network- it makes sense but that's mind blowing

[@ArbitraryConstant]

@@LookingGlassUniverseit's Typically about 2/3c so not tremendously slower, but enough you need to account for it over long distances. High frequency trading firms use very expensive microwave networks because microwaves through air gives them a small latency advantage.

[@der.Schtefan]

It's faster to beam data to starlink satellites at 500 km height in vacuum, them transmitting the data, between each other, and then back down, than to send the data from London to Ny in fiber optics.

[@333dsteele1]

I did a PhD in physics many years ago and thats a really excellent video you made. You demonstrate the scientific method, repeatedly testing theory against experiment (reality testing), revising your theories, whilst recognising the complexity of experiments that are not as simple as they might seem. The scientific method is so important because its the only way to bootstrap new knowledge (avoiding theorising in the absence of experiment, which is very tempting as it avoids the hard work of experiment, which is philosophy).

[@elinope4745]

3Blue1Brown recommended your channel to me, subscribed. That's a damned good recommendation, and you lived up to it.

[@zyxzevn]

Light is always an electromagnetic wave. It is actually very simple. You may have missed that the electron keeps accelerating for the full positive electrical field of the wave. So their generated dielectric wave is always 90degrees (1/2 PI) in phase off (is that + or - 90 degrees?). The dielectric wave slowly replaces the original wave, per atom-layer. So after a small layer the original light is no more. That is why the light "slows down". There are also fun relativity experiments with light through moving water. The more difficult part is how the electrons stay in the electronshell. The electron stays in the shell, even when it is harassed by alternating electric fields (light). Except when the frequency is matching the electron-shell resonance. That is where "quantum magic" happens.

[@RealNovgorod]

Don't forget nonlinear optics, buddy. Glass can become a conductor even with an infrared laser if the E-field is strong enough.

[@zyxzevn]

@@RealNovgorod Yes. Light gets even more fun with Zeeman effect, the Evanescent field. And the phase conjugate mirror: kzbin.info/www/bejne/nXLcZGx7p9aJaZY

[@kavehguilanpour6875]

Your videos are amazing. Please don’t stop making them. I started out as a physics undergrad years ago, but switched because I did not think my maths was good enough. Your videos have inspired me to start studying physics again for fun. From this video the things that blows my mind the most is the notion of a maximum speed of causality, and your example that the Earth would not ‘know’ for 8 mins if the sun disappeared. It completely upended my notion of the nature of the ‘force’ connecting the sun and the Earth.

[@ghahrai]

I am so glad I found you. I will follow your enthusiasm from now on. Thank you

[@theodavies8754]

Never give up. I wish I had been shown diatoms in dark field microscopy at school. Light is not caused by particles is something I would be reasonably confident to say. Citizen science is a valuable resource.

[@peetiegonzalez1845]

I can't get get over how cute those anthropomorphized electrons are. Excellent pair of videos, here. As usual. I haven't seen you in my recommended feed for quite a while so I'm very glad Grant pointed me in this direction!

[@Curt_Sampson]

Except that they're all frowny! Hey, electrons, why are you all so negative? Don't you realise your potential?

[@serversurfer6169]

The pixies do be angry tho. 🤔

[@Niohimself]

Please make more science videos like this one, this is amazing!

[@muffins4tots]

I just wanted to take a minute to say I absolutely love how this was presented. A personal journey of discovery that I felt like I was making with you. I'm glad you didn't shy away from showing your failures as it really helped me to understand the topic much better. Well done!

[@ThatGuyFromDetroit]

Great video! The reason your phone doesn't detect touch-screen input while underwater, is because the screen works by detecting a tiny electric current from your finger ('capacitive touch screen'), and since water is conductive, your phone is basically detecting that "the entire screen is being touched everywhere that the wet plastic bag is touching it, all at once". It's the same reason why my phone's Fingerprint Sensor doesn't work when my hands are wet, and it actually knows well enough to say 'dry your finger and try again'. You could potentially use some object, maybe two pieces of foam or rolled up paper towel, to keep the rest of the bag from touching the screen until your finger presses down on it? You only need enough 'functionality' to double-tap But you've already found an answer, so carry on! (...I think it's funny how I didn't understand a single thing until you explained why that one formula was 'potentially missing a set of brackets', and the rest made sense to me from there. I am not normally mathematically inclined.)

[@LookingGlassUniverse]

ah, that makes a lot of sense! Great point

[@ClaytonMacleod]

Or you could just use a laser ranging device like construction workers use. The hardware store will have some. Edit: haha, that’s what you do a couple minutes later.

[@Juni_Dingo]

I know I'm a bit too late, but I want to first say that I love this video, it's rare to see this genuine curiosity, experiments and most importantly, admitting of being wrong. One comment for the lidar though, the device you got at a hardware store isn't a lidar, these measure distance using parallax between the "aiming" laser and a short IR laser pulse that it emits during measurement. There's a camera and an IR laser inside the fancy optics and it basically just measures the angular distance between the two laser spots. Meaning that the measurement depends on refraction of the two beams, not their propagation velocity.

[@duggydo]

I like the way you made this video. Taking us through the whole process rather than editing it to show just the outcome is very genuine and I can appreciate that.

[@nataliem4434]

this is some seriously inspiring science, it takes a lot of courage to post something like this! Looking forward to the next video!

[@astrokevin92]

I really loved this whole approach of thinking critically about the popular explanations, consulting multiple good-quality sources, putting your own understanding to the test, and even sharing when your experiment shows your understanding to be wrong. An excellent and honest demonstration of how science is supposed to work.

[@MrMeltdown]

My understanding was light is a photon and subject to QED . Looked at one at a time you only know the photon has left and might be absorbed by your detector. If it is absorbed by your detector..... then you don't really know which route it has taken. the more particles in between the routes available being absorbed and re-emitted. however this is now a quantum calculation to work out the probability that you detected the light. any slight variation in the distance taken will mean the wave equation will likely cancel out the nearby routes when they reach 180degree phase shift, buy amplify those at 0 (or multiple of 360 degree ). Due to the density of water being vastly more than air (or a vacuum) there is a very high probability that the light interacts with particles and very few that make it through without interacting until it hits your detector (if it ever does). You have to do the combination of these to work out the total probability of hitting your detector at all. When the experiment is carried out you are performing this many many times so the average tends to towards the highest probability which happens to be not the straightest route but one that bounces a bit so the average distance is "more". Therefore taking just a little bit longer, on average..... I'm probably wrong and the maths is still beyond me but it was my takeaway from watching feynman lectures etc......

[@mananthakkar1451]

Great explanation, came here after "The Action Lab" mentioned about this video.

[@rajat0610]

this was so interesting! i have a doubt though - 28:36, rather than water and air, was it supposed to be air and vacuum?

[@guuslohlefink378]

This is really great. An honest experiment, like it should be. Finding experiments when the answer is known is so much easier the discovering the truth by experimenting. I love this.

[@rdtyphon6684]

26:30 but the electron above to upper electron here would also produce waves that would cancel the sin component of the wave of the lower electron no?

[@rdtyphon6684]

Mb i shouldve seen the whole thing

[@LarrySiden]

Totally cool. I always accepted that light slows down in water and different wavelengths slow more than others, but you made the effort to actually examine it with tools millions have access to.

[@A_few_words]

Proper science, reporting failed predictions. The only way to learn. Brilliant. Engaging, asf. I am hooked.

[@Baaqel]

The universe still has so much to be understood. I’m currently stuck on the fact that we can’t prove that c is the same in different directions because there’s no way to measure the speed the light in one direction.

[@kxs7267]

Really happy to have found this channel. Appreciate hearing the thought process, the hand-crafted animations, the honest reporting - and the fact you're adressing a topic that's niggled at the back of my mind. Looks like the argument still continues in the comments section here...!

[@alonamaloh]

I am not a physicist, but I have a bunch of friends who are, and this is the best explanation I've gotten from them about index of refraction: A photon is a good way to model how light propagates in a vacuum, and it's a massless particle; but when you look at light propagating in a medium with fixed charges that can oscillate, a better model for light propagation is a quasi-particle called a "polariton" (en.wikipedia.org/wiki/Polariton), which does have mass and therefore goes slower than c.

[@JonBrase]

Vertices in QED Feynmann diagrams always have two fermion lines and one photon line, so interactions between light and matter *have* to involve absorption and reemission. Another way to look at it is that, by conservation of energy, any movement of charged particles in the water/glass *has* to involve light being absorbed (and the creation of an interfering component *has* to involve reemission). But to understand what's happening, we need to understand exactly *how* the absorbtion and reemission is happening.

[@samanthagarg14]

I've been asking this ques for almost forever...if light is a wave then what is waving like Isn't it a particle oscillating finally understood what it is! Thank you so much!!

[@HuygensOptics]

You got the video out, congratulations! About the measurement device containing the red laser: aren't those measurements based on triangulation instead of actual time of flight? I don't think these contain a LiDAR.

[@WanJae42]

I don't know the answer, but you make excellent videos, too! Thanks, Huygens!

[@LookingGlassUniverse]

Thank you!! I'm not sure, but I'd be very surprised if they use triangulation? That would require multiple sources of light at known locations, wouldn't it? This one only had the continuous looking red laser. The sources I saw said they're usually lidar or phase based. I don't understand how phase based measurements work for longer distances though- wouldn't it be constrained to measure distances within one wavelength? I don't know. If anyone has more info about how these work I'd love to know.

[@HuygensOptics]

​@@LookingGlassUniversesorry,I guess I was wrong. I was confusing these with laser line scanners that can measure the topology of objects. These generally have a much higher accuracy of measurement and use triangulation.

[@LookingGlassUniverse]

Ah yeah, I know the ones you mean!

[@PaulSinnett]

The exact info on the sensor in an iPhone is somewhat secret other than it's Sony. But it seems that it sends out a continuous wave and then changes the phase of the signal. It detects the transit "time" by detecting the change in the phase in the returning light. But if the phase gets changed as it goes through the water as well then that might skew your result@@LookingGlassUniverse​

[@1495978707]

What textbook said that??? Anyone who works with optics knows that dispersion is an issue, and if you work with pulsed lasers, it absolutely affects timing and can spread out your pulse in time. Edit: it’s apparently “Matter and Interactions” 16:15. Yeah that book is just wrong. You could even try a light speed measurement in the water, though you will be limited by scattering and absorption how far you can do the measurement over. It absolutely does slow down. Heck, your LIDAR sensor was a test of this. Great job not taking a textbook as gospel

[@Petch85]

I had forgotten how much I missed videos from this channel.❤

[@infra-cyan]

Historically the idea that the speed of light is slower in a denser medium first appeared in Fermat's principle which sought to answer the question of why a transparent medium refracts light. The mechanical philosopher's of the time were not satisfied with this answer because it did not include a mechanism for why light would slow down.

[@infra-cyan]

The wikipedia page on Fermat's principle has an interesting historical account of its roots.

[@RachelsSweetie]

Love the low-tech animations! So much more visceral and believable than some python-scripted mumbo jumbo.

[@billant2]

Right?! No needs for fancy graphics, what really counts is clear and simple presentation. :)

[@domy6827]

love the moment when it all comes together

[@SlyPearTree]

I really wish had found something like this video back when I was a kid, it would not have had to be in video form, a magazine article would have been great. And it's not only about what you explain but how you explain it. I would have had a much better understanding of science as I went through school.

[@Peanutcat]

Actually top tier explanation. The light as vectors explanation at 08:35 made it instantly click.

[@martifingers]

It was lovely to watch this and have that moment when, as we say, the penny dropped about the connection between causality an the speed of light. I am very grateful for having this had someone with the intellectual honesty and curiosity to work their way through Feynman's lectures and provide such clarity.

[@Tomas.Malina]

I can't help but wonder why you dismissed the phase vs group velocity "explanation" at the very beginning as "absolutely not". You might find Cherenkov radiation interesting 🙂 Also, I'm surprised that the LiDARs can handle a reflective water surface and ignore the reflection from it - I would have guessed they would give you the distance to the water level.

[@somefellow2]

I really wish this kind of genuine, spirit of discovery stuff was what KZbin and honestly learning spheres in general was full of. Great video.

[@JessWLStuart]

I love how you look into this, and keep at it until you have your results! Science isn't about being "right" or "wrong", it's about seeing if nature works the way we think.

[@davidhand9721]

Points for integrity. We all forget too often that learning is always preceded by being wrong.

[@awboqm]

13:30 on Desmos, instead of manually moving the slider back, you can set up the animation to keep looping. You do this by clicking the arrows under the play button. It defaults to bi-directional motion, but there is also an option to send it in one direction over and over again, and I think there is a third option that I don't remember.

[@LookingGlassUniverse]

Oh my god, thank you

[@awboqm]

@@LookingGlassUniverse you can also change the limits of the animation by clicking the 10 and -10 below the animation slider (if you use both features, you could make it animate over a full period, so even when the animation resets, it looks like the wave continually moves forward)

[@cowvintube]

This is such a great video. I love how you walk us through your whole process of figuring out how this all really works. Your simple visuals illustrating things are fantastic.

[@bubblecast]

Incredibly good video. Makes me realise how much I've just taken for granted!

[@Saitama62181]

So good to see someone who can admit when they're wrong. You shouldn't feel bad about being wrong, it's a way to learn.

[@johng.1703]

a very simply way to test if the light does go in a straight line is to use laser light, in air you can't see the laser light from the side, but if the light is being diverted you would be able to see it from the side.

[@ChristianBoyer]

I really like your channel. You’re very intelligent and you focus on the deeper issues which is what I love

[@KipIngram]

This is a complicated business, that you really can't fully understand without using quantum field theory. It is true that if you launch optical energy from a source and detect it at some other location distance D away, it will take D/c time to get there if the region the light moves through is a vacuum. But if you replace some of the path with a transparent medium, it will take D/c to make the trip. So it's hard to argue against the idea that the energy moves forward more slowly in the medium. But - it's actually not valid to talk about the details of how the light makes the trip (that is, how it's behaving at specific locations along the path) unless you actually measure something there, and if you do that you've totally changed the experiment - if the light gets absorbed somewhere along the way, then that light never shows up at the final detector. There is no way to "keep tabs" on the light that eventually reaches the detector. Saying something like "photons are moving faster here and slower there" is just not a valid thing to say. If you pull out quantum field theory and work the whole thing out completely, you will get the right prediction as to when photons arrive at the detector. The theory complies with the observed behaviors. But that doesn't tell you what's "happening" in the middle either - the theory is constructed in a way that just doesn't give you that information. The theory tells you about the probabilities of obtaining particular measurement results. But, as I noted in the first paragraph, the global situation certainly the light slows down while it's in the medium, so that's how we generally think about it. We just don't think about the instantaneous behavior of photons while they're en route.

[@sasabnis]

I'm not sure about the missing brackets around the 15:00 mark. Without those parentheses, we get the wrong dimensions. wt is in units of "cycles per second", but x/c is in units of "seconds". That cannot be the right correction.

[@LookingGlassUniverse]

Yup, it’s the correct units (and correct eq) with the brackets, not without

[@CharlesFout]

If the original light is passing through unimpeded, a sufficiently fast recording medium should record the original wavefront and then a rapid change to the summed wavefront as it stabilizes.

[@sabeehb9514]

Love your videos as you are not afraid to ask fundamental questions and then you actually work really hard to experimentally prove / disprove it one way or another. This is physics at its best, well done !!

[@Ckay2552]

I normally watch everything on double speed to save time. I can't here and i love it! Your explanations are strong and i am sure you went over your script many times. It's almost like a textbook. Not sure you would be happy about it, since it takes you lots of time to prepare but i imagine this kind of explanation would making you a great professor.

[@peterp-a-n4743]

The wording of the quote at 15:46 is so eerily accurate. You couldn't summarize it any better, but you only appreciate it when you have understood it yourself.

[@Skellborn]

Omg, so looking forward to watching this! I just couldnt wrap my head around the slowing down exactly because of this argument and our professor just brushed over it...

[@MrBeklager]

Very brave of you to upload this video❤️

[@pauls5745]

Yeah, my thoughts, too. Using a sketchy phone and covering in plastic bag, as well as a non-uniform container just gave ammo to difficulty understanding and testing, and the nay sayers.

[@dorrisziffel]

love the honest curiosity and the simple pleasure that comes in understanding something well

[@danielwallace1968]

My absolute favorite physics channel. Spectacular video.

[@nickcaci7238]

Photons, dark matter and gravity are my favorite topics to ponder. The way that scientists come up with how to depict explanations through simplified animations vary and yours is simple and smart. But what I find in common with all good displays is it’s always depicted and displayed in 2 dimensions description which doesn’t allow for the massive infusion and scattering effects of all energy and matter, water, atmosphere and so on in a 3 dimensional concept.

[@Atm_0s]

Feynman's explanation about the refractive index is about where I ran out of steam in the Feynnman lectures. I hope I can go back to it better informed and primed to understand after these, thank you.

[@russellsharpe288]

29:20 "You can't make this stuff up" ChatGPT: Hold my beer.

[@LookingGlassUniverse]

The huygen's thing really cracks me up in hindsight. The actual experiment wasn't done till 200 years later and definitely didn't involve a telescope

[@benjamin_markus]

I did a very superficial search about it and many places quite convincingly say that the prompt re-emission of light is not, in fact, random, but influenced quite a bit by the stimulation of the surrounding electromagnetic field, so even it's directionality can be preserved.

[@theworldneedsweird8625]

You should definitely look into the "superfluid theory of everything". Imagine if light travels slower in water because the medium in which it travels gets less dense, similar to sound waves traveling slower in less dense air. I know it's a little out there but there are some good papers on it and it solves so many mysteries!

[@WestOfEarth]

A question regarding your statement at 23:00. You say that as soon as the electron feels the force of the elecromagnetic field of the light it emits its own field. But it can't be instantaneous, can it? Due to the Uncertainty Principle, there must be an imprecision in time, shouldn't there?

[@DeadCatX2]

One way to tell time of flight from phased-based range finding is that using phase tends to be more accurate and operate over shorter distances. That would be why the store rangefinder was able to operate over shorter distances while your phone would fail. Phase based is also usually about an order of magnitude more accurate (mm or less) As a circuits guy it makes intuitive sense to me that it slows, after all a voltage in a vacuum propagates at c but run it through copper and it's about 2/3 c. But intuition does not a proof make. Also as a circuits guy, I would do this as follows. Take two LEDs, and two photocells. Place one LED right over one photocell, place the other LED outside the body of water aiming in and photo sensor on the other side. Run the same source voltage to both LEDs and use a high bandwidth oscilloscope to measure the response of both photocells. This is a step input and the difference between the rising edges of the photocells is your propagation time, accuracy limited only by your scope. Remember that it's still moving 30cm/ns so if you use a 10 GHz scope your limit is increments of 3cm (this is why time of flight has less accuracy). This would work best with a more water and faster scopes

[@MrHichammohsen1]

6:30 this is where you have to dig deeper and not take information for granted. If the 'speed' of gravity is the same as C or causality, the earth would be orbiting the point where the sun was 8 minutes ago so the orbit would turn elliptical and eventually gets thrown out of the system. Gravity doesn't have a speed because it is not a 'force' or nature. It is the warping in spacetime caused by the presence of mass. You standing on earth now is not gravity pushing you down, but the earth pushing you up. Sabine the KZbinr has a great video on that explaining where the confusion came from :) Thank you so much for everything you do, the work is amazing!

[@coolstar7819]

I love that you linked down references for us to further study

[@amitmishra8061]

Finally your video came out at first i also use to think that the electrons absorb certain wavelength of light and then spit it out in random direction which causes the delay same as in prism but i learned a lot of new things in this video like adding up light, electron absorbing electron remitted light where the tough and crests are different and learned how noise cancellation works nice video going to watch 2nd part GG

[@companyjoe]

This is by far the most fascinating science experiment video I've ever seen!

[@Admimistrator]

So amazing to be on this journey with you to discover what is going on with light travelling through a medium. Love this format.

[@aniksamiurrahman6365]

I can see several small mistakes in the exeriment. 1. Which surface the light is bouncing off? The glass? Or the desk bellow it? An opaque paper/plastic should've been put at the bottom of the container b4 pouring the water. 2. Instead of hanging the phone from air, you coupd've easily buy a 30+ cm long beaker or measuring cyllinder (or borrow them from Uni lab). And made sure the lidar is touching the waster surface. 3. Why buy container with slanted surface? Another good thing of measuring cyllinder is that, the walls are perpendicular.

[@RagnarTube]

I wish I had that curiosity. The world needs more people like you.

[@georgetirebiter6437]

As a former land surveying tech from the electronics revolution of the 80s, calibrating the “gun” requires knowing the density of the atmosphere (barometric pressure and humidity). Conversely, if you know the distance between two points, you can work backwards to find the density of the atmosphere. The manual for, say, a Topcon, Wild, or Zeiss total station will explain this far more eloquently than I can here.

[@lundsweden]

I cant believe you didn't reach the right answer! Haven't you noticed how much harder it us to swim in water, while people just in regular air run much faster alongside you at the pool?!

[@galaflux9897]

Did you not watch her part discussing the common explanations 💀

[@lundsweden]

​@@galaflux9897 It was pretty obviously a joke!

[@fluoroantimonic9950]

8:31 shouldnt the electron be pushed in the direction opposite to the electric field? As electrons are negatively charged?

[@mckenziekeith7434]

The way I learned it in electrical engineering is that the propagation speed of electromagnetic radiation is a function of physical properties of the medium in which it travels: the relative permittivity and relative permeability. In electrical engineering we encounter slowed down EM radiation all the time, so of course we know it is a real phenomenon.

[@chrisking7603]

It's important to remember particles/waves/yada are all just models. Their only requirement to be useful is that they can predict. They're allowed to defy our intuition, and we're allowed to make up consistent models to affirm our belief in other models.

[@tcaDNAp]

Yesss perfect timing for an LGU video with finals coming to an end!

[@rickwhite4137]

It's worth to mention that c stand for speed of light in vacuum, which is the maximum speed of light! It's also worth to mention that when the light comes out at the other side of the water, it has the same speed as it had before! If the speed of light in water slows down, does it accelerate when it comes out at the other side?

[@bloodink9508]

What an absolutely wonderful result. Well done.

[@johanrheeder1640]

Would You be able to measure the amount of light entering the water and the amount of light coming out of the water with a lightmeter perhaps. does the amount of light increase or decrease?

[@tnb178]

3:26 that is exactly what we see. The beam is visible in the water from the side but not in air. You probably put something in water to show the effect but if you want to be rigorous you gotta take what you see seriously. Not just pretend to confirm what you believe.

[@Gredunza]

A day with a Looking Glass Universe video is a good day.

[@gnanay8555]

13:55 : shouldn't the pulsations of the original light and of the electron's light be the same here ?

[@gnanay8555]

(regardless of the parenthesis error)

[@viralsheddingzombie5324]

IF you are saying that electrons are physically moving in response to the light energy wave, it seems to me the light emitted by the electrons would be phase delayed or possibly slowed in velocity because electrons have mass, while the original exciting light/photons do not have mass. So the secondary light would at least be out of phase with the original light. Then the question is whether the phase difference can affect C as the light waves progress through the denser water medium. In radio communications engineering we are taught that EM/radio waves in the Earth's atmosphere travel at roughly .95c compared to vacuum space. But I don't recall how that was calculated.

[@mrigeshparashar6719]

Wow, this is incredible! You're truly thinking through first principles, bravely discarding any fluff, just as Mr. Feynman would have approached it.

[@stapler942]

The low-tech paper visuals give a fun little flavour to this discussion. It reminds me of the UI prototyping people sometimes do using slips of paper and foldouts. 🙂 Also makes me think of those relaxing puppet shows on TV when I was a kid.

[@milogonzalez1334]

thought i'd post this here too. there is an inherent relationship between capacitance, voltage, and refraction. adding capacitance by adding the dielectric of the glass lowers the voltage of the incoming wave. the rate of propagation of light induced in a medium should only remain constant as long as the properties of the medium remain constant. this is why the dielectric constant varies depending on the material and hence so does the rate of induction of light through that material. if the light can be made to appear to slow, it can also be made to appear fast. i would argue the ferranti effect is an example of this. i think i have come up with an experiment you might be interested in trying to perform. if you put a magnet on a crookes radiometer and then put that radiometer near a tesla coil, the radiometer will ionize and the vanes will spin from the heat and glow. what would you expect to see if you put this in an infinity mirror and pulsed a green laser at it? what if you pulse the laser at the same frequency that the tesla coil ionizes the bulb? even if the ionized air in the crookes radiometer doesnt scatter the laser light and even if the infinity mirror doesnt have enough reflections to accurately simulate distance, it would still be an amazing visual display that would make for good content, no? also to play gpt advocate, there is a lot of water between the surface of earth and the moon. lots of moisture in the atmosphere

[@creeperroese]

27:00 but waves emanate/radiate? Like they’re repeated circles whose area increases cubically? They wouldn’t travel ‘linearly’ like that? No?

[@leftyrighter8662]

You are like a bridge builder. This idea was visibleb to me with the level of understanding I had, but it wasn't accessible. With this video, I could connect what I earlier understood & this new idea. Thanks. I am massively understating how much I was moved by this video. But I'll wait till I watch part to and hopefully try to put my feeling into words. 🇮🇳

[@confused_lefty]

I've never understood(and I still don't) how reflection, refraction, dispersion, scattering of light really works in atomic level. This video kinda helped me understand some of it. Thank you!

[@janasandeep]

3:24 How camera captured that laser? Because water absorbed and re-emitted, right

[@ChandrasegaranNarasimhan]

In your first experiment, you placed the camera close to the glass. There might be reflection from first surface. So perform the experiment with distance away from glass surface. And with phase detector as well.