Why is light slower in glass? - Sixty Symbols

[@tomdrowry]

Professor Moriaty , what a great name.

[@SiddharthSharma15]

i was thinking the exact same thing.

[@fullyawakened]

Thomas Drowry They should name a douche after him. Garbage human being

[@desertfox2020]

Why is that?

[@anthropomorphousdodecahedr6504]

Would you care for an elaboration?

[@hsterts]

Hilmar Zonneveld I always love myself some Sherlock Holmes. Reading the Hound of Baskerville right now!

[@benmacdonald4702]

I love the videos where Prof. Merrifield just gets let loose on a problem, gives a bunch of different points of view and disproves them while finally leading to our best idea of what's going on yet still leaving us something to think about.

[@saltybits9954]

And gets everything wrong?

[@Xasperato]

All I wanted to know is why light traveled a bit slower through a medium such as glass, but now I find myself with all these questions about quantum physics and the nature of fundamental reality, and it's a bloody mess.

[@charliedobbie8916]

A certain Ghork Yes, that sounds about right!

[@cleebe823]

+A certain Ghork you cant just know one thing, you need prior knowledge, its the same with everything, i just want to take the car to the shop, now i need to learn to drive.

[@mrembeh1848]

+A certain Ghork That is how physics works. that is what makes it fascinating :D

[@scottwheeler1641]

A certain Ghork But light travels a lot slower through glass! I don't believe Atoms exist, just electro magnetic waves. I don't think anybody knows why light is slower through glass or what a magnet is!

[@hanspeterfake3130]

+A certain Ghork You have no idea how deep your comment is… I'm actually thinking about adding this as a citation to my physics phd thesis :)

[@ReedCBowman]

"Barry the beam of light"?? Surely his name should be Ray!

[@Haitchpeasauce]

Barray.

[@brokenwave6125]

Barry Allen...The Flash

[@VeronicaGorositoMusic]

That's why nobody loves Raymond (ba dummmm)

[@godfreyallen4094]

Reed Bowman Lenses

[@NickRoman]

But Barry's a beam.

[@wonderpookie]

This is one of my favourite videos on YT, of all time. A huge thank you to all involved in its making.

[@nonexistence5135]

This topic is much more interesting than I thought it would be. This really shows that the more questions you have answered, the more questions you are left with

[@TrabberShir]

best part of this video in my opinion is at 16:04 as you try to imagine Brady's face before vocalizing his question.

[@rikschaaf]

Weirdly enough, that last explanation makes the most sense

[@DANGJOS]

@Billy Willy I think he was referring to the 'Polariton' explanation.

[@thefinder8087]

@@DANGJOS I don't know, to me both quantum mechanical explanations make more sense than the Newtonian one. It didn't seem like he ever said why having all the other fields around slows light, just that it does.

[@DANGJOS]

@@thefinder8087 Pretty sure the reemitted EM waves are phase shifted, and this combines to form a slower wave. Also, the path integral explanation doesn't really have an intuitive feel for why it travels slower.

[@geronimomiles312]

A polariton exists when the , matter absorbs the light , elevates an electron to the conduction band , leaving an electron hole in the valence band. Thus it is the ' treacle' model ' of light passing through matter , which was dismissed as implausible due to the stochastic nature of re-emission. ( Essentially this suggests a scattering ,rather than uniform transmission of light... And also it would exhibit differential absorption of photons ) Right? The hybrid situation should be called a valoton , as an ephemeral valence state which manifests the propagation of the photon which is arbitrary in wavelength. Emerging from the circumstance of substrate , the c speed photon resumes it's trajectory.

[@A3Kr0n]

I'm so glad we get to see Brady more often. It really improves the videos.

[@frabuleuse]

What a lovely conversation! I especially like the fact that Prof. Merrifield explains how physicists work with models trying to explain reality. Did you already made a video about what a model is? I would love to hear all the professors explanations on how we go from reality to a model and than use the model to make assumptions that we can verify or not...

[@saltybits9954]

They cant answer real questions. Its not written down for them.

[@davecrupel2817]

i love how he got quiet at "traveling faster than the speed of light" xD

[@yigitsezer6696]

thats kind of a taboo

[@nmarbletoe8210]

@@yigitsezer6696 yet it is in Nature and has been replicated. They just don't want to have to explain how it doesn't break causality. I don't blame them lol

[@nin10dorox]

He literally explained how it doesn't break causality in this video.

[@mw0099]

​@@nin10doroxI don't get this though: he says the reason that you can't send information at the group velocity of light is because you can't do pulses. But couldn't you still send just a single bit of information, ie a Boolean? If it's agreed beforehand what the Boolean means, you could send information that way and have preknowledge of an event

[@midi5581]

​@@mw0099 Because the group velocity is just an effect of interference of many frequencies that travel at normal speeds so if you want to send a signal (0 or 1) you have to change the emitting frequencies and this change won't propagate faster than c

[@bezveze8047]

There is a little math error at 1:38. Light travels 40% *faster in vacuum than it does in glass* The reverse with same percentage is not true though. In glass, the speed is 1 - 1/1.4 = 29% slower than in vacuum Percentages man :P

[@robertelessar]

The description of the quantum interactions and summations of the light beam traveling through a medium and slowing down reminds me just a little of the interactions of particles with the Higgs field generating mass.

[@shabe99]

Coming back to this now 8 years later, i've changed degree paths a couple times and i'm about to finish my physics degree. And i can say with certainty that this is one of those very instructive concepts that shows why physicists bother with all this damn math(s). The balance of classical vs quantum and different types of velocities for waves, phases, blah, blah, etc are so obtuse and hard to understand independent from the math(s). Which is why we take the time to walk through the math(s) all the way from algebra and trigonometry to optics, E&M, Classical and Quantum Mechanics and beyond: it makes so much more sense in that framework. And I also have a new appreciation for Prof Merrifield's ability to explain this stuff. It's like he just casually sat down and tried to explain his music to a bunch of deaf people without any musical notation to help, and he pulls it off splendidly.

[@shok24199]

I think Professor Merrifield has created his own physical constant. 3.0x10^8 words per second, in a monologue.

[@FatManLaughing]

This is one of the best explanations I've ever heard on the subject.. And definitely the clearest of them.

[@ostheer]

Thank you for this very informational video.

[@sixtysymbols]

Mathijs Verhaegh you are welcome

[@jonathanolson772]

+Mathijs Verhaegh Yes! This is probably the best description of what actually happens on the quantum level that I have found so far. This is a great video.

[@G4mm4G0bl1n]

The shown material is completly wrong and missunderstood from the original postulation from Albert Einstein. The Light becomes not slower. The radiant from the longitude movement will be longer. So the Light becomes not slower, the way for the Light becomes longer! Lightspeed is constance and fix! Thats the first rule of E=mc². Baddest fail I ever seen and what is he, a Professor? Where is the Vending Machine for 25¢ to get the title?

[@G4mm4G0bl1n]

Joel White The Explanations of him are useless complicated. I can show you a picture which explains all what he said over the complete video and more.

[@god_damn9661]

lol...i bet u are more confused now and didnt understand a sh!t!!!

[@MarcCastellsBallesta]

I already am a physicist but I couldn't have answered the question without having to revisit many class notes and books. I wish one day I can explain stuff like he did. What's the English expression... *hats off to you!*

[@flurng]

I do enjoy all of your videos, but I think I prefer videos of this type; un-edited, with just one person presenting a concept, start to finish, rather than jumping back and forth between two people. I find it much easier to follow & thus understand in this format. Well done & keep up the good work!

[@saiprasadrm97]

Did he make a mistake? I think he meant 40% faster in vacuum, not 40% slower in glass (ya, they aren't the same). 150 is 50% more than 100 but 100 is only 33.3% lesser than 150. Tell me if I am wrong.

[@TLJGames]

You are right - he was just roughly estimating I guess.

[@Lamnom]

well, only if you consider 150 as 100%. If you keep value of 100 as 100%. Then 100 is still 50% lesser than 150...

[@michaelmjh23]

Lamnom :D

[@Crazy_Diamond_75]

When you say something is 40% smaller, you are saying that you divide by 1.4 to achieve your result. For example, 10 is 40% lower than 14 -> 14 / 1.4 is 10. 14 is 40% larger than 10 -> 10 * 1.4 = 14.

[@Lamnom]

so 71,43 is 40% less than 100??? how did you come to this conclusion. by what logic?something is 40% less, it means it's -40% of the original value. If 100 is original value, then 40% less is 60. How did you come to "divide by 1,4"? makes no sense? The only way I can see how would you come to this, is that you thought that if to add 40% you need to multiply by 1,4, then to take 40% you need to divide by 1,4 (???) No.

[@rynieryarom4277]

I watch and rewatch these videos over years and still can figure it out

[@quintonwilson8565]

In all three of his explanations, I don't understand how the photon wave comes out of the glass going the original speed before entering the glass. So the original photon wave "energy/speed" was never even affected by the lattice/electrons waves of the glass it passed through? In the classical explanation, he even says the photon wave loses energy because some of it goes to microvibrations of atoms, doesn't the photon somehow have to regain this lost energy?

[@SimulatingPhysics]

Photons have a fixed energy, equal to E = h*f where h is the planck constant and f the frequency. The frequency of the photon never changes, neither does its energy. What happens is when the light (bunch of photons) enters in the material, it can absorb an amount of them. So the energy change of the intensity of light is due the loss of photons, not about the change of the energy of the individual photons.

[@quintonwilson8565]

@@SimulatingPhysics I'm talking speed, not flux.

[@SimulatingPhysics]

@@quintonwilson8565 Photon speed is always the same too. The apparent slowdown is because the emitted photons by the atoms of the material destructively interfere with incident ones in the ends of their wavefronts so the photons appear to travel at lower speed, but they are not. When the photons get out of the material they don't interfere anymore and the slow down effect dissapear.

[@peterisawesomeplease]

Yea I am confused too. Like if you sent a single photo through glass you would think that jiggling the elections in the glass would cause a loss of energy. So you would expect that the photo would be remitted at a lower frequency. But of course that does not happen. I think it must have something to do with the how the double slit experiment works though. Like it looks like there is interference without actually giving up any heat or something to the glass.

[@olevik2005]

Prof. Merrifield is definitely my favourite with Prof. Moriarty a close second I hope they live forever!

[@teefkay2]

Something doesn’t make sense (to me, anyway). The explanation for a material being transparent is that the energy gap for the electrons is larger than the photon’s energy, & therefore the photon cannot interact with the glass molecules’ electrons & therefore passes right thru the material (whereas for opaque substances, the energy gap is less than the photon’s energy, the photon gets absorbed by “promoting” the electron to a higher energy state, and does not pass thru the material. But in this video, the prof says that in a transparent substance (eg., glass) the photon DOES interact with the electron, which produces other em waves, which exhibit superposition with the original light wave, thereby slowing down the original light wave. Which is it? In a transparent substance like glass, can a photon interact with the glass molecules’ electrons? Or not? Or is there some “lesser” type of interaction which requires less energy (& complicates the whole concept of quantized energy levels)?

[@PlasmaFuzer]

Not sure if you have done one, but a very interesting video you could do, which is related to this one and is briefly touched on (not by name), would be on Cherenkov radiation. Granted it is much more complicated to explain properly, however I have always found it to be quite extraordinary that it is possible to exceed the speed of light (phase velocity; in a medium) without breaking the laws of physics. Despite there being other videos on the subject, I think the public could only benefit by input from your channel.

[@jackgude3969]

8 years later, hopefully a productive comment for future readers, I ended up here while going down the rabbit-hole of Cherenkov radiation. I agree with the sentiment that the physics is truly awesome. I think it makes a lot of sense when you consider the momentum of an electron relative to a photon. It's really only because these charged particles have mass that they're able to temporarily continue at some speed faster than light in a medium. The velocity graph of the charged particle is continuous but decreasing through the material. Analogous to a ball through a vacuum suddenly encountering air and being slowed by friction, generating heat. The electrons move at 90-something% of the speed of light through a vacuum until they hit a medium and are slowed by the interaction, generating light. (which is, as you said, much more complicated than simple friction, or the classical photon interactions described in this video) Like the professor said, photons are weird. They're massless but still have momentum and don't seem to experience time. I think classically, you can think of it similarly, with a photon velocity graph being continuous but decreasing much more sharply in the case of photons than electrons as soon as you hit the medium, because most photons simply don't have anywhere near the momentum of a massive particle moving at close to the speed of light. Velocity drops so sharply that, depending on the scale, the photon velocity graph would appear discontinuous where it enters and exits the medium. This classical model doesn't really explain why the photon would speed back up upon exiting the material though.

[@alaaakkoush1135]

we hope you can make a video about Polaritons.

[@TtttTt-ub5xb]

ALice Akkush هاي

[@itsalongday]

I hope so, too

[@kashmirha]

... and Magneto, and Cryptonite! :D

[@crackedemerald4930]

What, whaaaat aareeee thooseeee??

[@TimbavatiLion]

I found the last model to be the easiest to understand. Photons becoming Polaritons, no longer behaving like photons, is not as mind-bending as a photon being everywhere at once :)

[@ahmedshinwari]

I read it over the internet that "For every complex problem, there is an answer that is clear, simple, and wrong."

[@cretaceoussteve3527]

Sure, but remember that this Polariton thing is just a model, a mathematical symbol... which is also true of a photon. Quantum theory shows that in any measurable sense, particles that are small enough to be subatomic move according to probability waves. So in fact the term "particle" is misleading, but it's very difficult for us to wrap our minds around the idea of the universe and all its contents including our own bodies and brains as consisting of probabilistic fields of "energy", whatever that is...

[@michalkacko4408]

You think that is mind-bending? Cause you are also everywhere at once.. but just a little bit :D

[@randomnobody660]

@@cretaceoussteve3527 here's a simple solution (to having to wrap our minds around quantum physics): don't. How physicists model reality has no bearing on how reality actually is, as explained by both the prof and you just earlier in your comment. I do hope we get a simpler way to interpret these models though.

[@Robin.Tussin]

@@michalkacko4408 How can you be? Do you feel, "everywhere at once"?.. Isn't it self-evidently the case that you're, not, everywhere at once? Aren't your components obviously collapsed into a hard and immutable probability matrix that is the being of you, at the sub-atomic level? If that's so then surely, you are, where you are - and, by that token, where you can possibly be, as that matrix, has always been strictly limited to being wherever it was that your own matrix determined you should most probably be, in the next infinitesimal moment of time - based on, where your matrix actually was, in the previous infinitesimal moment of time, ago?

[@neshploda17]

15:33 if the polariton has mass, when the light slows down. And you can make the photon move faster through a solid faster than c0. Does that mean the polariton on that faster than c case, has negative mass? I assume it happens through some other mechanism that I just don't understand.

[@mikeatyouttube]

It was only briefly mentioned at the end of the video, but not much is said about the fact that light exiting the glass immediately speeds up again. Over what distance does this acceleration take place? And, for that matter, when the light enters the glass, how quickly and over what distance does it slow down? Presumably you could do an experiment to measure the slow down and speed up transition by passing light through extremely thin pieces of glass. At some very thin thickness of glass the light wont have slowed to the equilibrium speed (the speed of light in a thick piece of glass) before it has to speed up again as it exits this thin piece of glass.

[@ecyor0]

Time to start using 'Polariton' in sci-fi stories :3

[@wbeaty]

Excellent video! Especially excellent because it "debunks" the common (and wrong) explanations which are offered on many other websites. Oddly enough, Reddit gets it right too. Their science moderators pro-actively delete the highly-upvoted wrong explanations which Merrifield also debunks.

[@lezbriddon]

i'm a bit thick but... if they go slower through glass, then they lose momentum, but when they exit, how do they speed back up......

[@bentoth9555]

From my understanding of it, that's right. The equation for how much energy it takes to accelerate something is e=0.5M(V^2). Having no mass a photon would zero out the entire equation, meaning they don't have to have any energy added to accelerate to C.

[@kenseto9266]

Because light is being transmitted by a structured aether. Inside the glass the aether is more curved and thus light goes through the glass at a slower speed. When light exists the glass it is being transmitted by a less curved aether and thus it speeds up again.

[@Toni999985]

The wavelength and frequency change when it slows down and vice versa. That's where the change happens

[@IEVISCERATEU]

You can think of the photon as imparting some of it's momentum on the particles in the glass and then recollecting that momentum (being pushed) as it leaves the glass. That's just an analogue though we need quantum mechanics to properly describe what's happening.

[@fullyawakened]

They don't require energy to speed back up. Photons always travel at C as C is the default speed of anything that does not interact with the Higgs field. It's a bit counterintuitive that we should start at the fastest speed possible and require some kind of interference to slow down or stop but that is the way it actually is. If matter did not interact with the Higgs field then everything would travel at the default speed of existence, C.

[@fractalnomics]

A fun 'fact' I came to in my research this week: liquid water and clear glass have something in common with the GHGs; they are all transparent to the visible and opaque to the IR. Water and glass are said to be perfect absorbers of IR; so, glass is a greenhouse solid (a GHS), and water a greenhouse liquid (a GHL).

[@kashmirha]

Prof. Merrifield looks such a nice guy, humble, smart, diligent, somehow sensitive, reflective, king. He could be a great teacher.

[@Hack3r91]

I may be wrong but, isn't that kind of (classical) scattering a dipole radiation? Light would be re emitted in some distribution which is not keeping track of the direction that the original light had.

[@adamunruh2931]

Very interesting. First time I've grasped quantum vs classical models

[@jasdeepyou]

So if I find a medium in which the speed of light is really slow and it is transparent like glass. I take a huge block of that material and do something on one side of it and then quickly run to the other side faster than the speed of light in that medium, so on reaching the other side will I be seeing the past?

[@katiebennie9245]

+Jasdeep Singh No. If you are not talking about c then time isn't compressed the same way. You would just be seeing the light coming towards you slowly. You would never be about to see into the past because you haven't gone beyond the speed of light c.

[@anitatromp6295]

Technically you will. Just like the light you are seeing from Andromeda is technically its light from the past. Just like in a photograph all the objects in the distance is their photons from a different time "the past" than the objects in the foreground.

[@jcxmej]

You cant go faster than light but yea reach the other side faster than the light travelling in the thick medium. You see the light that you had sent some while ago.

[@tanc94]

pff. would be much cooler if you find a material to see the future! :P

[@jxshannon2]

Actually, you're always seeing the past. And looking in a mirror, you see yourself in the past. Albeit a very recent past.

[@Veptis]

The refractive index of Germanium is about 4. Which is one of the highest I believe. While visible light doesn't travel through Germanium, infrared does. And it does so really well. So Lenses for thermal cameras are made out of Germanium. But it never occured to me that a quarter of the speed of light is really really fast, because the speed of light in the first place is already vers fast. But matter going a quarter the speed of light isn't impossible.

[@jrpg0]

11:57 ok, so if light acts in this situation the same way as in the double slit experiment, if we observe each individual foton that travels through the glass it will act as a particle. So because it doesn't produce an interference patern anymore, therefore no superpositions, would the final speed change in glass?

[@sixtysymbols]

soon I hope - been a bit busy here!

Ah, ok then...

[@avantesma1]

@ Some reeeal busy 8 years, those must've been.

[@TheZetr0]

I really am enjoying this subscription! I just want to say a huge thanks to all those involved with their time and efforts, its amazing to have three models that for the most part describe the photon affect when effected by a medium. To be fair this is fundamental and inspirational work which I am thoroughly enjoying to watch and listen. Thanks for sharing.

[@EdM66410]

8:40 funny hearing words used how they're supposed to be used.

[@padenzimmermann1892]

The 3 second pause before cracked me up

[@jeffreysokal7264]

I love the Professors but Brady is the best! He has the innate ability to generate the right questions at the right time.

[@padsoneil]

I was surprised to discover that the "polariton" description was simplest to follow and understand. It also gave me a greater insight into the other descriptions which, beforehand seemed messy & unsatisfying. That was cool - many thanks! :-)

[@vinayseth1114]

1:42 - No that's not a reduction by 40 percent but 28.57 percent lol !

[@RTD553]

Yes, I was thinking this. One has to be careful with percentages. 1.4:1 means It travels 40% faster in a vacuum compared to glass, but 1:1.4 means about 29 percent slower in glass compared to a vacuum.

[@DANGJOS]

+Nick Allen Didn't think of it that way thanks

[@yusuf1597]

I don't get it isn't 1:1.4 the same as 1.4:1?

[@DANGJOS]

Pedro Numerically no, but depending on how you look at it, they are the same

[@NyanSten]

+exitbag123 No, one is the inverse of the other. For example, when you increase something by 100% (1 → 2) and then decrease it by 50% (2 → 1), you end up at the same value. Instead of percentages, you can also write these changes as ratios 2:1 and 1:2 respectively.

[@DarkNemesis25]

so what is so fundamentally different about light turning into a polariton in glass vs air... at what point does the light fail to gain mass through a medium? is it not a binary question or does it gain infinitesimally small amounts of mass through every medium

[@jasonslade6259]

+DarkNemesis25 I think that you could describe a photon in air as a Polariton but the mass of the Polariton that is created depends on the medium that it is passing though. The density of air is so much less than the density of glass that the resulting Air-Polariton would be nearly indistinguishable from a normal vacuum photon. The Glass-Polariton would be on the order of 2500x heavier.

[@rogerwilco2]

I clicked a like on this video before even watching it. With this subject I just knew it was going to be good.

[@JackFou]

So if the interaction of a photon with the lattice of a solid material creates a new kind of particle which has mass and subsequently travels at less than the speed of light, what does this model say in those curious cases briefly mentioned where you can cause the speed of light inside the material to be higher than the speed of light in vacuum?

[@HectorSalamnca]

So I looked it up and it turns out Sommerfeld and Brillouin showed that GROUP velocity can ALSO exceed c, but apparently this doesn't violate causality either. I'm very confused.

[@waltermeerschaert]

I have a question. Does the density of space make an appreciable difference in the speed of light? is space considered a medium, as opposed to vacuum? there are theoretically particles coming into existence all the time, and then disappearing. wouldn't their mass change the speed of light? it might be small but we are talking up to 15 billion light years.

[@amineaboutalib]

Wally Meerschaert but I guess they take the ratio in calculations?

[@dhvsheabdh]

This process happens everywhere though uncontrollably, and it linearly affects every instance, so how could you tell?

[@omikronweapon]

the question lacks a fundamental variable. What constitutes "an appreciable difference"? If the difference is small, it will always be proportionally small, wouldn't it? Even if it's, say, (just making up a random number) one-thousand years, it would be over HUGE distances. Is that appreciable? Are we even capable of determining that accurately over such distances? A tiny miscalculation or unforeseen phenomenon would through it out of wack. With no way of confirming it.

[@lupus7194]

I vaguely remember a result which I think comes from the Maxwell theory - the speed of light is the square root of (permeability/permittivity) of space. Could the slow down in light speed also be explained as being due to changes in these electrical and magnetic properties within a solid.

[@G4mm4G0bl1n]

pssst.... Square and negate square this digit! ;) 3,1622776601683793319988935444327^ 2 = 3,1622776601683793319988935444327^-2 = Its so bad. Im really the only Once which understands this complete? I feeling like Nikola Tesla. A man far away from his centurie.

[@davidwuhrer6704]

G4mm4G0bl1n​ Ich kann dich beruhigen: Du bist nicht der einzige, der das versteht. Allerdings verstehe ich nicht, was daran so schwierig zu verstehen ist. √(10)² = 10. √(10)⁻¹=0,1. Oder anders gesagt: 10^(1/2)^(-2)=10^(-2/2)=10^(-1). Ich persönlich finde das eher banal, und auch gar nicht relevant für Maxwells Gleichungen. +lupus I think so, but that wouldn't tell us what makes these values different in glass or water, just what these values are.

[@davidwuhrer6704]

G4mm4G0bl1n Du bist wahrscheinlich auch der einzige Mensch der Welt, der weiß, was ein „Planck Paket“ sein soll.

[@colemarc]

Sure, but the Maxwell theory says nothing about what is the source of permeability and permittivity, it just postulates that each medium (even vacuum) somehow can be characterized by these two parameters. If I am not mistaken, Prof. Merrifield tries to explain that, from a classical point of view, the wave front travels through the glass at velocity c (speed of light in vacuum) but the material emits another wave in the same direction that somehow is just so that when superposed it results in a delayed version of the original.

[@amineaboutalib]

G4mm4G0bl1n /r/Iamverysmart

[@TheZooman22]

OK, so the speed of light c is a constant 299,792,458 ms. The velocity doesn't really change, does it? Just the time it takes to navigate, though stuff.

[@titaniumdiveknife]

I love the serious tone of these special two vidoes. All of Brady's sixty symbols should be like this. Serious.

[@AgnostosGnostos]

There are different kinds of glasses. For example the common glass from silicon that is used in cheap glass bottles is less dense than the lead glass that is used in expensive crystal bottles. Different glasses have different refractive index and that is very useful in optics. In microscopes, telescopes or the common camera lens of your smartphones or mirrorless camera.

[@rays5163]

8:40 it prefers to be called a differently abled light wave :(

[@the_real_vdegenne]

I wear glasses from my birth, are you telling me i've been sort of living in the past all that time ? i am 28 y.o now

[@shuriken188]

We're all living somewhat in the past, the light has to travel through air, the lens of your eye, and the fluid inside your eye. Then the signals have to travel along your nerves much slower than light before reaching your brain to be processed. By the time this has all happened, extremely little time has passed, even if you have a thin layer of glass in front of your eye.

[@the_real_vdegenne]

sure. I was just joking, that is why when we look the bright stars in a clear night sky we somehow visualize what the past looked like. But who cares, there is just Present anyways

[@The_Real_Indiana_Joe]

Bet that was painful for mom.

[@carultch]

Even the speed of light in glass, as slow as it is (40% slower than light in empty space), is fast enough that the time it takes to pass that third of a centimeter through glasses, is insignificant for most human time scales. Human reaction time is about a tenth to a third of a second. If you are looking at something that is 10 meters away through glasses that are 3 mm thick (1.4 refractive index assumed), the light spends 33.333 nanoseconds in the air and 14 picoseconds in the glasses. That's a total of 33.3357 nanoseconds to get from the object to your eye. (ignoring significant digits in this calculation)

[@EdMcF1]

Perhaps you have aged 40% slower...

[@Gryffster]

Atlas Of Creation? WTF????

Gryffster i hope that book is there just for those moments of fun...

[@VeronicaGorositoMusic]

It seems that fear of death reaches not only average people, but also physics?

[@CntRational]

Late response, but he talked about it in another video. He got sent the book by some creationist group. It's a thing they do, apparently.

[@UNRELATIVE]

Yeah, I had to look this up...

[@sebastiangeorge9252]

Some researchers have managed to do something sort of like this. In a supercooled gas of rubidium atoms, light slows down to a few meters per second (around 0.000001% of its speed in vacuum). When you contain the gas in a chamber coated with an extremely reflective material, you can essentially trap light in the chamber for a significant amount of time. Trapping the whole universe is something else...

[@markusantonious8192]

Merrifield is always the most concise, precise and eloquent of these 'sixty symbols' presenters.

[@PrivacyKingdoms]

10:07 he doesnt answer the question at all and now i dont know WHY the net effect of all the light created by the jiggling atoms makes the light slower. just gives some bull shit roundabout answer.

[@roblaquiere8220]

Another way of thinking of the problem is to imagine light as a wave front moving through the medium and not as a group of photons. Remember, photons are dual wave-like/particle-like phenomena, and therefore talking about the wave aspects of the light wave front is just as legit as the particle aspects. Like the professor explained in the video when you view the light like a wave front then the natural explanation is that the wave will interfere with the medium (and itself, view double slit experiment) and this sum of interferences results in a group velocity less then that of light. It's important to understand that photons ARE WAVES TOO. Waves can construct, destruct, and interfere in many ways; such that it appears to create a net effect of slowing the light wave fronts. No individual photon is slower than C, but the intensity peaks of the light wave fronts are moving slower than C by the refractive index of the medium. Thinking in only particle ways in QM will only confuse you.

[@onetwoBias]

mazdaplz Actually that's a misunderstanding, as explained by the professor in the video. It's not really that it takes a longer route, it's just that the original light wave gets into a superposition (read is basically spread out across a volume in the form of more waves) And then the sum of these waves, because they interfere with each other, the sum of their velocities is less than that of light in a vacuum. So the resulting speed is really less than that of light, even though each individual wave is moving at the speed of light in a vacuum. That's what I gathered from the explanation given in the video - maybe I got it wrong, at least it still really confuses me quite a bit, so don't be discouraged if my explanation didn't clarify much.

[@raykent3211]

Tobias Knudsen mazdaplz I just came back here from minutephysics where Henri gives what is called here the pinball explanation. A poster says: but surely that would result in scattering (the material looks milky) because he's given no reason why the light would go all over the place within the material, but then choose to come out in exactly the same direction it went in (assuming perpendicular incidence). Quite. Prof Merrifield says it's wrong here and I'll go with Merrifield rather than Henry.

[@GodmyX]

Ray Kent Yeah, Henry's damn smart, but even he as a non-professional physicist not working in the field (unlike the professors) is sometimes guilty into buying into the simplier scenarios which are more mainstream, but as seen, after a careful inspection, quite wrong.

[@wcsxwcsx]

Does anyone remember a science fiction short story of many decades ago that dealt with this? Panes of glass were created called "slow glass" where light took years to pass through. People would buy the panes so they could view the past as the light finally came out.

[@juliantreidiii]

Do you know its name.

[@bernardfinucane2061]

Light of Other Days

[@ulilulable]

"How'd you expect me to edit this?" :D

[@SanyaLOLZ]

So, seen some questions about reflection. If i remember correctly, metals reflect light when their electrons are so densely packed, and most often free, so when the light hits the material the interaction is so big that it's reflected.. Mirrors also depend on the crystalline structure, if materials aren't crystalline they're usually transparent or diffusive reflectors. I got a question tho. What happens to the photon when it exit a material that slowed it down. I mean, I guess not every photon have the a bit lower energy, then we would see redshifts, right? So are there just less of them?

[@googleyoutubechannel8554]

Do we still not know, I can't find anything in the last 10 years that isn't just speculation without a testable hypothesis?

[@danvez5656]

so many experts in the comments, makes you wonder why they even watch this if you already know everything XD

[@Serdar54321]

I can't stop laughting when I watch 8:35

[@miles11we]

Why?...

[@Serdar54321]

Miles Eaton yea but he said it in a way that made it sound like he mean the other way and in a funny way with all the things he did with his hands... So yea, it just made me laugh. So what

[@miles11we]

Weirdos

[@bobbobson2061]

Teorik Redstone'cu What is it like to be twelve?

[@nightangel7239]

Bob Bobson What's it like to be hyper-sensitive about words?

[@jameswilson8270]

0.6*1.4 = 0.84

[@lokeasplund8412]

I have two questions: Nr. 1: When the light have passed through the glass, does it resume in the "regular" speed of light or is it still slower? And if it continues at the "regular" speed of light, does the change in speed happen instantly or is it some kind of acceleration involved. Nr. 2: Can anything travel faster than light inside glass (neutrinos for example?) or can nothing travel faster than the speed of light even when the speed of the light is slowed down by the glass?

[@simonruszczak5563]

No, time is slower (dilated) in glass, the speed of light is unchanged (1c).

[@alext9067]

11:50 question of having such a weak light that only photons go thru the slits. How do we know that they are discrete photons and not just waves? Are there detectors arranged around the area to rule out this possibility?

[@kshgarg147]

Is it possible? That the light travelling through glass doesn't slow down (maintaining that the light always travels at the speed of c. It is actually TIME that slows down inside the glass. The mass of the piece of glass would warp the spacetime (although a little bit) following the general theory of relativity. This in effect is gravitational lensing at a smaller level. So, the light appears to take longer to get through glass because of the warp in spacetime and since the material is transparent, you can actually see gravitational lensing at play. I may be wrong though but I am surprised why general theory of relativity is never thought of as a plausible reason for refraction.

[@rcgamer7780]

Kshitij Garg I don't think that is the case as only massive mass like star can bend light, the mass of a glass is too small to actually bend light.

[@kshgarg147]

Yes, you are right. Probably an experiment should be conducted to see if the deviation in light agrees with eistein's equations. Just to be sure :P

[@rcgamer7780]

Kshitij Garg I think there were already experiment conducted which backed eistein's equations long time ago in 1919. When general relativity had been publish, scientists are trying to confirm the theory by observing deflection of light by the Sun, However, normally you cannot observe the stars near the Sun because of sunlight. So in 1919, solar eclipse happened and scientists were able to observed deflection of light by the Sun by observing the change in position of stars when the sun is there or not. In the end the experiment were successful as the change in position of stars and the mass of the sun is matching eistein's general relativity equations. en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun

[@rcgamer7780]

Also before the solar ellipse experiment conducted in 1919, not many people know about Einstein and his theory. However after the experiment conducted,The result was considered spectacular news and made the front page of most major newspapers. It made Einstein and his theory of general relativity world-famous.

[@scottwheeler1641]

RCgamer 77 But the light isn't passing around the glass, It has to pass through it. I think Kshitij Garg may be right that the speed of light stays the same.

[@arundhatisharma5743]

Professor Moriarty? hmmm.... Sherlock, anyone?

[@amineaboutalib]

Do U miss me?

[@TheRumpusView]

This explanation is very unsatisfactory. At one point while refuting one of the proposed mechanisms the professor states that photons can only interact with atoms at specific frequencies, presumably because of the electron energy levels, yet later, in his explanation, he says that the photons interact with the atoms and "jiggles" them about such that they radiate light as well which superposes with the original light to generate the light at a group velocity which is less than the speed of light. At no point does the professor state what sort of interaction this is, and whether it is in any way related to the interactions described above which were dismissed as a possible mechanism. So we seem to have either a contradiction or a new magical mechanism which is utterly crucial to understanding what is going. Either way the explanation is completely unsatisfactory, and alas is so often the result of these sixty symbols videos.

[@brodaclop]

Try paying more attention to the exact wording and context because it's important. He says that "atoms ABSORB light at very specific frequencies". And indeed, absorption is out of the question, it isn't mentioned any more. The jiggling thing is entirely different from absorption. He doesn't say that photons jiggle atoms because at that point he's talking about the wave model. (Context!) In this model there are no photons, just an oscillating electromagnetic field, which, as we know, exerts a force on charged particles (electrons for example), which in turn emit their own oscillating electromagnetic field and so on. The other, quantum model doesn't require any of this jiggling.

[@LetalisLatrodectus]

You say "At one point while refuting one of the proposed mechanisms the professor states that photons can only interact with atoms at specific frequencies" The professor never stated this. He said photons can only be ABSORBED at specific frequencies. They can interact with photons as much as they want at any frequency though but not absorb them. So his explanation holds.

[@squidb8]

in case you didn't understand. When light interacts with an atom it cause an electron to jump to a higher energy state, go from an orbit that is further from the core, eventually the atom will have to come down to it rest state, and it will emit a photon. Therefore a photon is a packet of energy. I think the problem is that these are university professors they never taught high school, and hardly remember when they were first introduced to physics. They fail to understand the difficulty of some people have at grasping quantum physics.

[@LetalisLatrodectus]

squidb8 You are talking about absorption and emission which is one way a photon interacts with atoms but not the only way. A photon can also interact with atoms without being absorbed, without making an electron jump to a higher orbit.

[@SuperJonny7]

the explanation is that the photons excite electrons in the conduction band to recombine with holes in the valence band, forming an electron-hole recombination pair, which then emits a photon

[@Nehmo]

Professor Merrifield laughed a lot and rocked from side to side. The body language translation from that is that he was unsure and felt he was being put on the spot. Considering the answer was as definite as an un-collapsed wave function, this makes sense.

[@seanki98]

12:26 if this is true, how come a narrow beam of light seems to go in a straight line over all? like Professor merrifield said in 4:35?

[@TheDetonadoBR]

+Sean Thrasher Just watch the video

[@johnnyllooddte3415]

light is so complex no one understands it

[@johnnyllooddte3415]

wow an honest physicist..im impressed

[@Graeme_Lastname]

I feel like we know only the roughest approximation of what is really going on. In my life, I have gone from vacuum tubes to tiny boxes with billions of components. What's next? ;)

[@Graeme_Lastname]

I feel like we know only the roughest approximation of what is really going on. In my life, I have gone from vacuum tubes to tiny boxes with billions of components. What's next? ;)

[@omikronweapon]

@@johnnyllooddte3415 that's a bit of a cop-out though, isn't it? It's too easy to say "I don't understand it, so it's probably beyond all humans to understand". At the same time it sort of implies "why bother with trying to grasp things". There's also a matter of degrees of understanding. Of all the things out there, I suspect light isn't very complex by comparison, in any case. Fundamental particles and forces are relatively 'simple'. It's when they start to make up a large system when things start to get "complex". The biggest problem for most people, seems to me to be, imagining something at that size, and subjected to forces a layman really doesn't grasp either.

[@CreatorOfJoy1]

So in other words scientist have no idea why light slows down

[@masansr]

He just explained 3 ideas, didn't you watch the video?

[@CreatorOfJoy1]

masansr So. which of the 3 ideas is correct?

[@masansr]

All three of them. Just from different viewpoints.

[@alucardwhitehair]

I love it when people mistake their own ignorance for the ignorance of others. Its quite entertaining.

[@myothersoul1953]

FrankCoffman - Science is a buffet of choices, it's that and a very careful way of choosing. Three explanations that work are three explanations, choose the one that works best for the particular case. Reality isn't a buffet of choices, there's only one and there's no choice.

[@IterativeTheoryRocks]

I like the last explanation best. It makes sense that if the polariton has mass, it would travel slower than c. Also, for the first classical explanation, I always interpreted it as the ‘interference’ from the juggling atoms continuously ‘steals’ from the front of the photon wave and adds it back at the tail end, thus slowing it down overall.

[@hbloops]

In the start he says that a refractive index of 1.4 means that the light travels 40% slower inn glass than in vacuum. since n=c/v 1.4=3*10^8/v v=3*10^8/1.4=2.14*10^8 which is not 40% slower. Am i misunderstanding this or was he wrong?

[@nogood237]

Question: If I shine a light on a piece of glass that is 0.5 a light year thick and then on the other side of it I have another 0.5 a light year of vacuum before I have a detector, does the light reach the detector in 1 light year still or dose it take 1.4*0.5 + 0.5 light years now?

[@89sfry]

For the classical view, thinking of this in terms of the time it takes a pulse to transit a piece of glass. Does this mean that for the original wave of light that the first wave cycles that pass through the medium are nullified though destructive interference caused by the movement of atoms within the medium? Then in turn constructive interference between the waves generated within the glass continue to generate the tail end of the pulse? This is the only way I can see the system working where the speed of light is fixed. What am I missing?

[@XylyXylyX]

There actually is a big concept he didn’t mention called the Sommerfeld precursor. Plot twist: the arriving wave of light DOES travel through the medium at the speed of light, but that part attenuates quickly. I can’t believe he didn’t mention it, it is critical to understanding basic physics: you cant slow down what you cant catch.

[@24kGoldenRocket]

Okay. let's see if I have this right. From a vacuum we have light enter a dense medium and it slows down as a result creating these "polarotrons".. When these "polarotrons" leave the dense medium and enter the vacuum again, after losing momentum, they somehow instantaneously accelerate to Light Speed once again? Do tell me...WHAT FORCE ACTED UPON THE "POLAROTRONS" to accelerate them to Light Speed? Was the mass they attained, when slowed, converted back to Energy? Okay. That is where the force comes from. Zero sum works for me. At what speed does Light become material and stay material. How much speed must I lose, how much do I need to slow it down, before it transforms into mass permanently? How can I permanently manifest matter from energy? (We can manifest mass temporarily through refraction as has been demonstrated in this lecture.) Energy was transformed into matter at the Singularity event. Energy cannot be created or destroyed. It can only be transformed and matter is just one of the transformation states. Mass is Energy after all. E = m₀c². That is what the Mass Energy Equivalence Statement means. We can transform matter into energy in a nuclear furnace. That is how te Bomb works. That is how Nuclear Reactors work. That is how matter/antimatter annihilation works. But how can we take Energy and transform it into matter when we need it, in what form we need it, and at our beck and call?

[@ozzzer]

One this i don't quite get, as the light enters the glass, it imposes a wave which 'counter balances' it. What does this mean exactly? Does this mean the two waves are out of phase? Transmitting energy in opposite directions? And even in this case, i always thought when waves superpose (which i'm guessing they do, in the resultant wave), doesn't this mean you sum the displacements, not the velocities, meaning the velocity is the same but the amplitude or frequency of the wave has changed? But after saying this i realize that, if the frequency has reduced then according to c = wavelength * frequency, then c would decrease as a result of frequency decreasing. Please elaborate on his, thanks!

[@skrame01]

I like the more educational/serious attempts at explanation and teaching / understanding on your channel. Too much entertainment is fun but distracting if you want to actually learn things, I'd rather have useful content. I'd love to see this style of interview/explanation applied to discussions dedicated to the explanation and understanding of advanced physics equations and maths! I'm sure there's a market for something like this, all technically educated people and enthusiasts who would appreciate and are interested in deeper understanding of the actual maths and their implications for a variety of reasons.

[@DaylightDigital]

Why does the superposition of the re-radiated EM waves change the speed of the effective wave, as opposed to the amplitude due to interference patterns?

[@SimulatingPhysics]

It does the two things. About the speed, you can image that the re-radiated EM waves interfere destructively at the end of the effective wave.

[@josephfinkelstein1843]

Am I the only one that thinks its awesome that there is actually a professor called Professor Moriarty? This seriously just made my day. haha

[@questionare]

I love listening to these things, learning something new. The problem is people in the comments trying to say they understand something that is more than likely WAY beyond their comprehension and argue against it.

[@dejremi8190]

I know it's not the actual picture. But however, I stumbled onto something in his line of reasoning to explain why the "bumping into atoms and springing back in another direction" suggestion that made me react a bit. He says it's a problem because of the fact light would emerge out of the medium in all sorts of directions. But couldn't we see atoms and photons like a physical Galton board, with the ball being the photon and the pins being the atoms. Then there would be a higher probability for the photon not to draw away to much from it's original straight pass, so that statistically the light beam would stay mostly one ?

[@manipunation]

At about the 13 minutes mark, I imagined one of those canes with hook on the end coming out after there was a buzzer sound, and him begin pulled off camera, indicating that his little skit of pretending he understood why light is slower in glass had finally been voted down by enough of his audience, like they used to do on stage at talent shows I think...

[@dissidify]

The Polariton? When there was a discussion of Super position, more than one photon will be present I know photons have mass because they are affected by large gravitational fields. therefore is the Polariton not just the combined masses of the photons that are interfering with each other in there special quantum way? Also the wavelength of light passing through the medium will be of different phase relation to the presumably constant photons inside the medium, does that explain why there is a spectrum effect? I think the interference must effect the superposition and therefore the angle, assuming all the photons inside the medium are a not a net zero and travel in all directions, resulting in a different resultant velocity for each wavelegnth

[@Ashes42g]

What happens if we do the double slit experiment and one slit has glass in it? Do we still get an interference pattern, or is the time differential mean it cannot interfere with itself?

[@jimmyriba]

Something doesn't add up to me. If the correct explanation is that the wave still travels at the speed of light, but the superposition of all the generated waves along the way makes it appear to propogate more slowly: 1) Why does this not change the colour of the original light wave? Why is the result only a phase shift? And 2) If it's just a phase shift, does that mean that the light actually does propagate at the speed of light but only appears to move at c/n? Does that mean that if I have a kilometer thick piece of glass with a sensor on the other end, and I shine a laser through it, the first photons would arrive at the other end just as fast as if I shone it through a kilometer of vacuum?

[@RGF19651]

Essentially, from a classical point of view one should solve the Maxwell equations, in the glass and apply the boundary conditions at the interface. These equations, especially the ones dealing with the Electric vector take into account the permittivity of the medium (glass), which is a measure of the strength of the formation of electric dipoles in the material, which will oscillate in resonance to the frequency of the incident light. It is the counter fields from these dipoles that interfere with the original light wave that causes the interference that causes the slow down of the light in the medium as mentioned in the video.

[@matthewmiller5486]

Question, a problem with this is throughput. How does something slow down then speed up again and not lose throughput or density or frequency of the photons?

[@yahiaAly]

for the first explanation of the quantum view of this we are saying that every photon is following every possible path and is absorbed by every atom and re-emitted but the atoms emit the photons at random times so wouldn't that mean that every time we shed the light it would travel at a different speed (since the sum of all times of every atom can't be constant every time)? for the second explanation of the quantum view of this problem wouldn't this means that the mass of the glass would change as light goes through it which can then be detected by maybe a very sensitive device ? and what about the way it will travel through the glass since if it has mass then it will take different paths every time it is produced inside the glass causing it to travel at random speeds ..or is the mass of the particle random in the first place ...or would it then travel the same way the first explanation sees it ?

[@hippzhipos2385]

i m just a year 10 student and might sound silly but this question always bothers me. how can a light slows down if it experiences no time. also general relativity says that light speed is same for all observers .so how does that woork??

[@myth-termoth1621]

so is the speed of light still one over the square root of the magnetic permeability times the dielectric permitivity? in other words can you calculate refractive index from the permeability and the dielectric constant of a material?

[@danielgc857]

So I have a question. When the proffessor mentioned the energy approach (meaning when he talked about the wave losing energy inside the medium), it made me ask myself: then how does it gain the energy back after exiting the medium? Maybe this way of loking at the problem is wrong, and if it's not, what's the reason?

[@bharath_b]

But how do you explain the bending towards and away from the normal while traveling between media of varying refractive index?

[@dLimboStick]

Does the frequency of the wave change (speed up or slow down) in the glass, and does it return to its original frequency when it exits the glass medium?

[@dhvsheabdh]

The frequency remains. In a sense, it has to. If you think about it in the wave picture, the frequency is essentially associated with the wavefronts themselves, pinging over. The number of wavefronts must remain the same, and since the speed changes, the wavelength must change!