Solving The Slow Light Paradox
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- čas přidán 19. 04. 2024
- In this video I explain why the light slows down in different materials and how the speed of light never actually changes
See 3Blue1Brown's video here:
• But why would light "s...
Looking Glass Universe Videos:
• I didn't believe that ...
• I don't know why light...
Harvard video: • Prof. Lene Hau: Light ...
Online Simulators I used in this video:
javalab.org/en/light_refracti...
javalab.org/en/wave_propagati...
www.compadre.org/osp/EJSS/403...
javalab.org/en/electromagneti... - Věda a technologie
There's a classic sci-fi story about glass-like material that is so dense it takes ten years for light to pass through. The idea is that you store a nice view, say from an alien planet, and sell the "glass" after ten years to someone who wants a "living window" to another world.
so the view lasts for ten years but also takes ten years to store. would it just be black when the glass is brand new?
@@elio7610 should be, but i'm wondering about the corners which light could possibly go through in a shorter time frame, causing weird distortions
not to mention light bouncing off of materials and all of that, making the sight a blur
@@elio7610 Probably 🤔.
mindblowing!
A literal window to the past 🪟.
Even without the need for a vacuum, he finds a way to use the vacuum chamber for experiements. What a legend.
Should have measured the time of flight though a chamber full of air then vacuum.
Exactly, this man tries to make the most of his vacuum chamber in all his videos. That's one of his prized possessions or smth and why wouldn't it be
If I lived next door, I would want to be his helper free of charge.
When you buy a shiny new hammer, everything starts looking like a nail...
@@FractalShoggoth only in a vacuum
0:29 "the most famous constant in the universe, the speed of light, named after the speed of light" made me giggle ( cannot say cracked me so hard coz some guys do not catch the vibes)
Yes 😅. Also; the constant, called: ”the speed of light”, is actually just the speed of light, in a vacuum.
@@PC_Simo It's actually not the speed of light that is the constant. The actual constant is the speed of causality=C. This is the speed of how fast a change can occur. Light itself could be infinite in speed, but is limited by the speed of causality.
Cannot tell if joke, or AI generated script... >.>
@@thingsiplay That’s fascinating 😮! Might that be connected to Planck time? 🤔
@@thingsiplayfrom our prospective. From lights prospective it traveled to its destination instantaneously
Appreciate all the free education you provide.
I second that motion! The Action Lab is excellent and awesome!
I second that motion! The Action Lab is excellent and awesome!
Absolutely!!
But his channel is dead 😢
@@alien3200 no u
This is a hell of a rabbit hole, the mathematics behind it is crystal clear, models can show aspects of whats going on, but a complete intuitive explanation of whats actually physically happening is incredibly difficult to grasp. Like a lot of things in physics the more you learn the more you realise the naive explanations just dont cover the full range of weirdness of it all.
Exactly my feeling. The problem is, you can visualize the behaviour only for a very tiny section of the whol thing (1 wave cross section + 1 electron). The real process in 3 dimensions and for multiple elctrons and waves is overwhelmingly more complex. I can't imagine anything of this tbh.
I picture this as like the light gets pushed backwards a bit when it comes across a medium, and then it continues to move at c. But because it got pushed back when it touched the medium, it'll reach the other side a bit late even though it's still moving at c. So from our perspective, light seems to be slowing when in fact it just got offset backwards a bit.
🚩This is why many people have started hating scientists 🤢🤢🤮They lie & manipulate with ZERO shame🤢🤢🤮 Light NEVER slows😡It just scatters in denser material, which makes it takes longer time to pass through🤏
@@Dipj01 interesting thanks
" the mathematics behind it is crystal clear" You've earned a gold star for that line!
There are lots of "authoritative" misstatements on CZcams on this topic. I'm glad you reference 3Blue1Brown and his illustration of Feynman's explanation ... the best I've seen. Nice demo, as usual!
3Blue1Brown indeed has some good animations on it (KTzGBJPuJwM & aXRTczANuIs). I also like FloatHeadPhysics (JZOn1B8K8mA) although the "pull back" animation isn't that good - it has some really interesting comments though. I don't like the FermiLab version as it does not really explain it properly (well at least as you can say "properly" as of course it's far more complex in real life than any of those videos imply).
Someone explain to me how this works as a photon of light. Whenever I see videos or explanations done about this, everyone starts showing waves. While I know light behaves both as a wave and as a particle, if we don't know how an individual photon 'knows' it transitioned into a different medium, there's a possibility this wave theory is also incorrect. Also, the way it's explained here and everywhere else is that a single atom's electrons changes the wave and then it gets passed on to the next atom's electrons.... but isn't this additive? If that were the case, enough length of the material would make it completely opaque, and well before that the refractive index would be observed as being dynamic. It makes 0 sense that this is considered the proper explanation.
Totally off-topic comment, but please consider to have your cheek checked for skin cancer. I have seen such a slight discolorisation before in someone, and it turned out to be cancerous and was indeed surgically removed.
I just had a long discussion with my wife about how come light slows down through different mediums when it's massless and so can never slow down - then I open my laptop and this pops up. Now, that really is strange...
That stuff happens a lot to me too.
you being spied on by your phone and any mic on laptop
Google heard you then forced action lab to make the video
I mean, as long as it has speed it can slow down.
That is just a normal thing, no need to freak out. Tons of information fly through our head everyday but we never give them a damn. When you focus on one topic, you'll pay attention to information of it, if it fly through your day again. And then you just think "Wait, does universe listen to me?"
I think addressing "What makes a material transparent?" would have been a good addition to this video.
Great work.
Wait- holy hell this just gave me actual virtigo. This explanation means that in the truest sense of the word, no mateiral is transparant, it's simply a material that has an electromagnetic-wave-propagation effect. 'Air' would be a good case study becuase I'm uncertain if it has enough density to compare to something like Water or Acrylic... but except in a true vacuum, you will never receive the electron that the originating atom created... And in low density fluids... how deeply could an EM wave penetrate before it MUST hit another atom to propagate and potentially delay? Your eyes are constantly re-propagating EM waves so for that matter... *head asplode*
Methinks this question has the wrong direction. It e.g. does not explain why vacuum is perfectly transparent. And it gets difficult to explain why water is transparent and snow is not. The question is why some material is not transparent.
@@bosslca9630 A vacuum, and gases in general, will be transparent because the photons can actually pass through without interacting with anything. Gases are mostly empty space, part of the reason their densities at normal pressure are so low compared t liquids and solids where the atoms do contact each other. There is some atomic interaction between light and gases - that interaction is why the sky is blue for example.
@@peglor Compared to vacuum, air is incredibly dense. It is as dense as it can be without increasing the temperature in its space. It is so dense it will knock you over if it travels as 20m/s. "Speed of light" in materials is indeed a conundrum that very likely proves we don't understand what light is. It is neither hardness, element or state deciding if a material is transparent. Coal is the perfect light stopper, unless you compress is insanely hard. Crude oil is also blocking almost all light, until you increase the mass of the molecules and turn it into plastic. No metal is ever transparent, unless you "fix" it in a ceramic grid.
The reason light "slows" down in different materials is probably because it never travels through it. It's only the waves going through. Sort of like sound through a wall.
Thenproblem is that youbare thinking in particles, everything is transparent and opaque, it depende on the wavelenght, the window of your microwave oven is opaque to microwaves but transparent to visible waves.
We need more optics experiments 🔥
I'm glad so many science CZcamsrs are tackling this. Most quantum things require many different analogies and explanations to illustrate an idea without using math. This was probably the most concise yet complete explanation I've seen yet.
🚩This is why many people have started hating scientists 🤢🤢🤮They lie & manipulate with ZERO shame🤢🤢🤮 Light NEVER slows😡It just scatters in denser material, which makes it takes longer time to pass through🤏
@@duran9664did you even watch the full video
21/4/2024 Sunday 4:21PM
You made a very deep question very accessible. The true speed of light divided by the speed of light in the material in question. Great job James. The speed of light through the material in question IS the true speed of light... through that material
I'm a new sub...delighted to find you answering many questions I've had for years!
Great video! Props for doing the shoutouts at the end, I thought of those videos instantly when I saw this!
You made a very deep question very accessible. Kudos!
🚩This is why many people have started hating scientists 🤢🤢🤮They lie & manipulate with ZERO shame🤢🤢🤮 Light NEVER slows😡It just scatters in denser material, which makes it takes longer time to pass through🤏
@@duran9664did you even watch the full video
21/4/2024 Sunday 4:21PM
these preprint papers claimed reversing entropy.
What do you think about this?
Experimental Demonstration of Energy Harvesting by Maxwell's Demon Device
And
An Autonomous Mechanical Maxwell's Demon
A beautiful explanation. I had heard the claim that the photons were constantly being absorbed and re-emitted, and that that's what caused the delay, and I'd heard the statement that no, that wasn't accurate. But I hadn't heard what the real explanation was. Thank you for providing it.
you can do this w one photon, but the absorbed an remitted is very misleading. The photon remains coherent , which wouldn't work if it were absorbed and remitted classically..but then it gets into more semantics of coherent vs incoherent scattering. The juice is not worth the squeeze,
🚩This is why many people have started hating scientists 🤢🤢🤮They lie & manipulate with ZERO shame🤢🤢🤮 Light NEVER slows😡It just scatters in denser material, which makes it takes longer time to pass through🤏
A very small portion of the photons actually are being absorbed and re-emitted, it's just that the majority are making it through and just being interfered with over and over again.
In other mediums, the ratio is different, and so transparent-ish things can take on colors.
No please forget about this idea. It's completely wrong and bogus. Just think about one thing: Why would the re-emitted photon fly in the direction it needs to fly and not in any other random direction?
Very interesting! Upvoted for mentioning 3B1B video on this topic, which is outstanding!
I was always puzzled by this question. Thanks for your great explanation
Great job James you have done a great job on the action lab. You are so good.
*Light slows down in Diamonds by 25% which is why they have so much "fire".*
I watched the 3Blue1Brown video recently and sort of understood it, but your explanation is much more understandable. Thanks.
This excellent vid reminded me of one of my favorite science fiction short stories, "Light of Other Days", by Bob Shaw. (1966)
About "...a window made of Bose-Einstein Condensate that slows light to a snail's pace.", ....thereby allowing it to be set up in, say, a beautiful wilderness scene, uncovered for 10 years to 'record' that, then sold to people who wanted that scene to be 'played' for another 10 years in their home, as that original light finally makes it through.
(Not to be confused with Arthur C. Clarke and Stephen Baxter's "_The_ Light of Other Days".)
That could make a good murder mystery element. Where the owner of the window is sitting their watching the view and he sees someone being murdered ten years ago.
@@robertgreen7593Bob Shaw wrote another "slow glass" story called Burden of Proof that is vaguely similar to your suggestion.
I love it when good science channels shout out videos from other good science channels.
Your content might be targeting kids but that's honestly a good thing with how good your quality is :D
Its hands on broken into simple small components explained using simple terms and showcases that are using real life models.
Keep it up! you def deserve the 5mio subs
This is one of the best takes on the topic. Much is semantics. Yes, light slows down in acrylic, and when we say "the speed of light", we mean c/n, and don't appreciated "achtually's". (we: ppl who've built Cherenkov detectors)--it doesn't mean we think gamma rays slow down too.
It's perfectly fine to use Maxwell's eqs. in media (the ones with D and H in addition to E and B), where the permittivity is > that of free space. Ofc that is a bulk model and ignores atoms...which you can do since the wavelength is much bigger than an atom.
Nevertheless, the most unambiguous approach is to use Jefimenko's eqs (equivalent to Maxwell's, but manifestly casual). This is gives you the *correct* physics view that changing E fields do not cause B field (and likewise for dB/dt -> E, an a-causal idea). Rather, the fields at a point/time are *only* due to charges and currents on the past light cone, so effect from cause propagates at "c", but because the medium, the presence of max E-field appears to move at c/n. Note that nothing is moving, waves don't propagate...only effect propagates from cause on the past light cone. In media and in vacuum, the wave description is just a model of an emergent phenomenon--I hate to use this cliché, but it's correct here: like a Mexican wave at soccer game. There is no wave, just people standing up and sitting down in what looks like a wave, so with that: the wave does move slower than c.
Holy sh-t bro, you writing books on YT.
@@EffectualPoet well, I worry about this one, and I got some work churning on the other computer that needs attending every 20m. If I put on movie or sports I'll forget about the work...this is actually max efficiency.
Great demonstration and explanation. Worth noting that the "true" speed of light is measured in a vacuum, so the phone's software already compensates for the air molecules affecting the beam. If you used it to measure distance in a vacuum it would underestimate.
Oh no, he might need to use his vacuum chamber, and he never uses it😊
As a long time engineer and physics enthusiast it is often so easy to forget things like these are not common sense to everyone. Great video on the topic 🙂
Fantastic video! I really love these strange observations of light. Especially in relation to the very cold, the very dense, and now the very electric.
Sometimes I wonder if the apparent expansion of the universe is a function of the lensing of light through varying degrees of cold pockets, transparent matter, and gravity wells. If a galaxy we observed 100 years ago seems farther away, it might not be, but the light red-shifting towards us from it might be running up more hills to get here than it used to, because of the vast movements of new anomalies in its path to get here.
Edit: But that would mean we should observe some cosmic neighbours seemingly getting 'closer' to us, if their light was once struggling through more to get here than it is now...
Really goes to show how much depth there is to the things we think we already know. I had no idea about this
The constant "C" is defined as "The speed of light in a vacuum". This speed is the constant. The context matters.
Light, like every other wave, travels at different speeds in different mediums. This doesn't change the constant, just the context.
The explanation was known to me as it's the standard one, but I always wondered about three things that so far nobody answered:
1) The secondary waves created by the "wiggled" electrons move along with the incoming initial wave. But what happens to those waves once they all exit the glass on the other side? They all still move with c, but why does the "kickback" now reverse and the wavefront no longer (apparently) slows down?
2) You talk about "second order propagation" - ok but why are there no third, fourth,... until light slows down to 0?
3) Wiggling an electron needs energy. Thus a photon should lose energy and thus change its frequency (hf). Why does it not do that?
A wave decrease in amplitude when it loses energy, not frequency. On a microscopic scale, the release of energy takes place in the form of quanta, which are released in their entirety. At least that's what quantum physic says.
Yes, there might be a wave going through glass without slowing down, but only at the beginning, until the transient regime passes.
Nice demonstrations with the ToF Lidar
Your content never ceases to amaze me and serve as great education tools!
these preprint papers claimed reversing entropy.
What do you think about this?
Experimental Demonstration of Energy Harvesting by Maxwell's Demon Device
And
An Autonomous Mechanical Maxwell's Demon
That star you have in your name is kinda rare
@@mdkamruzzaman8605I am a paying member of his channel.
Always great stuff! And really appreciate your recommending other channels! CZcams is always throwing new ones at me and they are not always worth much. Nice to know in advance!
Nice explanation. I like that you show it experimentally with cell phone lidar
3B1B's and LGU's were more intuitive for me, but this video was good. And I loved seeing the acrylic blocks; your experiments always make the concepts a lot more concrete.
The simplicity of demonstrating the speed of light with your phone app is stunning! We can NEVER directly measure the speed of light, only indirectly by reflection.
You should attempt at a zero point energy experiment in a vacuum. Using the Casimir Effect.
wow you always can show something in such creative manner
fantastic. so simple yet so deep.
This channel (and others mentioned in description) are the real gems of CZcams ❤
🚩This is why many people have started hating scientists 🤢🤢🤮They lie & manipulate with ZERO shame🤢🤢🤮 Light NEVER slows😡It just scatters in denser material, which makes it takes longer time to pass through🤏
@@duran9664did you even watch the full video
21/4/2024 Sunday 4:21PM
if you would use 2 walls of that vacuum chamber, that would be enough to see the relativistic effect of light. We could see a little delay on the moving image below the walls. You should try something more that could make the relativistic effect visible by just eyes.
Good job - my compliments. I have an issue with the idea that the incident photon pumps an electron in the acrylic material then continue on its merry way unchanged to later couple with the photon emitted by the pumped electron as it drops back a level and create a third wave front that is slower. I understand the concept of the standing wave - well. think I do at least)
The only way for the photon to pump the electron is to dump energy into the electron. The energy level of a photon/electron is in discrete steps. The photon either has to dump all its energy and disappear or dump a significant part of its energy, then drop to a longer wave length (lower energy level) to continue on.
This raises the question of the recombining of photons of different wavelengths recreating the same color of light as the primary product (ignoring scattering) . As a ham radio guy it suggests to me that recombination should result in an I/Q modulation with three different wave lengths appearing at the output (as a mix of colors).
This all proven by Major Armstrong at much longer wavelengths..I will let the standing wave slide for now.
73
Fascinating video.
Quadrature shifting. Used to modulate and demodulate radio waves. Very interesting stuff. I work with RF for hobby and experiment purposes and have come across this concept.
Fantastic video as always! I still don’t quite understand, but I’ll take your word for it 😊
That is a fantastic video. Thank you!
7:53
Looking Glass Universe videos on this topic are great, I can recommend them.
I was researching this for a while long ago but only read that atoms absorb and re-emit light. Thanks.
Sweet - the acrylic acts like a delay line or tardis for light waves!
Wow that’s amazing! 👍
Great explanation and animation, I wish you would have really hammered down the point that its a result of continuous phase change "backwards" with some sort animation of the continuous phenomena
You're my favorite science teacher
I've always thought of light moving "slower" through some medium is just the light "bouncing" around in said medium.
While the full explanation may be more accurate, it's simply easier to wrap your head around it.
If you jump into a pool of water, you now fall slower than you would in pure air.
And you can still see all the way to the bottom as if it was air.
But the denser medium simply makes you move slower because you are bumping into more stuff on the way.
And that's what i think of when light "slows down" when hitting something.
You can think that. It isn't right though. The problem with analogy is we are the quantum scale and our intuition is all wrong. None of these things are objects as we understand them and the concept of a medium is meaningless.
@@101Mant Yep and i did say that.
But to wrap your head around the phenomenon in a practically usable way, it does help to look at it in simpler terms.
Us:Hey does light ever slow down? His answer: Yesn't
THIS WAS SO AWESOME!
I ALMOST UNDERSTOOD IT!!! 😃
Cherenkov Radiation is a beautiful example of light moving slower through a material than other particles.
Folk should check it out. Imagine an impossible haunting blue colour, that's Cherenkov Radiation.
I could listen to a 10 minute evil villain monologue about the speed of light
An interesting thing to consider is how lights "speeds up" again when it exits the denser medium, since there's no way it could gain energy when it exits the medium must mean that it never slowed down in the first place. I hope this makes sense scientifically as well.
how do you know that it speeds up again? is there another experiement for that?
@@elio7610It’s essentially the same example. It’s the push back at each layer of electrons that causes the final waveform to propagate slower. So without it, it’s that combined waveform that still travels at c. Think of it like you are taking two steps forward but each time you cross an electron layer, they push you back one step. You yourself are always still taking two steps each time, but your overall progress is one step. Once the electrons aren’t there, you continue at your two step pace but it’s not like you gained any energy, there just isn’t the thing holding you back anymore
@@PieMan061 that's not what i am asking. the experiment doesn't show when the light speeds up or slowed down, it only shows that the combined travel time was slower, so how do we know that the light returned to its previous state after leaving the medium?
now that i think about it, there is a very simple way to test it, you just need the start and finish to be in different places. by having the light travel through both the air and the glass (or whatever medium) we can see if the light returns to its normal speed by comparing if the speed of the light changes when the medium is placed at the starting point vs the end point.
@@elio7610 isn't the fact that it bends back to it's incident angle with denser medium enough to know that it returns to its original state? Though it could be tested and I'm sure a quick Google search will yield the answer.
@@MuhammadTaimurBaber i am not sure what you mean.
This explanation is very interesting. Good Job. This knocked a couple of cells together and sparked a thought: Passing it through materials would be a wave resistor... I wonder could there would be a way to make light transistors using prisms or ....
Or lenses?
Optical Computing is a field of ongoing research, and there is a lot of fascinating work being done in building logic mechanisms using light’s properties. Typically these techniques leverage quantum effects such as wave interference and polarization - check out Huygens Optics channel
If only it were that easy.
Photonic logic is the use of photons (light) in logic gates (NOT, AND, OR, NAND, NOR, XOR, XNOR). Switching is obtained using nonlinear optical effects when two or more signals are combined.
Resonators are especially useful in photonic logic, since they allow a build-up of energy from constructive interference, thus enhancing optical nonlinear effects.
Other approaches that have been investigated include photonic logic at a molecular level, using photoluminescent chemicals. In a demonstration, Witlicki et al. performed logical operations using molecules and SERS.
The basic idea is to delay light (or any other signal) in order to perform useful computations. Of interest would be to solve NP-complete problems as those are difficult problems for the conventional computers.
There are two basic properties of light that are actually used in this approach:
The light can be delayed by passing it through an optical fiber of a certain length.
The light can be split into multiple (sub)rays. This property is also essential because we can evaluate multiple solutions in the same time.
When solving a problem with time-delays the following steps must be followed:
The first step is to create a graph-like structure made from optical cables and splitters. Each graph has a start node and a destination node.
The light enters through the start node and traverses the graph until it reaches the destination. It is delayed when passing through arcs and divided inside nodes.
The light is marked when passing through an arc or through a node so that we can easily identify that fact at the destination node.
At the destination node we will wait for a signal (fluctuation in the intensity of the signal) which arrives at a particular moment(s) in time. If there is no signal arriving at that moment, it means that we have no solution for our problem. Otherwise the problem has a solution. Fluctuations can be read with a photodetector and an oscilloscope.
The first problem attacked in this way was the Hamiltonian path problem.
The simplest one is the subset sum problem. An optical device solving an instance with four numbers {a1, a2, a3, a4}.
The light will enter in Start node. It will be divided into two (sub)rays of smaller intensity. These two rays will arrive into the second node at moments a1 and 0. Each of them will be divided into two subrays which will arrive in the third node at moments 0, a1, a2 and a1 + a2. These represents the all subsets of the set {a1, a2}. We expect fluctuations in the intensity of the signal at no more than four different moments. In the destination node we expect fluctuations at no more than 16 different moments (which are all the subsets of the given). If we have a fluctuation in the target moment B, it means that we have a solution of the problem, otherwise there is no subset whose sum of elements equals B. For the practical implementation we cannot have zero-length cables, thus all cables are increased with a small (fixed for all) value k'. In this case the solution is expected at moment B+n×k.
Awesome explanation.
U never fail to entertain me with science
Basically the interaction of the incident light and the matter wave takes some time to produce the resulting wave and the denser the material or the more fine-tuned the "shift" the slower the resulting wave propagates through the material. That is why it's possible to almost stop the light - it's like when a photon is traveling from withing the Sun's core to Earth. The core is so dense that there's so much interaction that the photon seems like it's slowed down until it reached the corona where it zips right through reaching Earth in 8m20sec.
What makes the material clear or opaque is if the resulting wave still has the same direction as the incident light but clear materials can occupy the photon for as much time as the opaque materials do.
The radiative pressure of the sun's photons forcing through the dense core (1.622 x 10 to the 5 kg/m3) and the surrounding currently created elements as well as those present in the initial molecular cloud does slow the straight pathway of a lot of the photons.
This is what holds the rest of the gases in place as the density decreases. Eventually these photons WILL leave the sun and reach us, about 200,000 years after their creation.
The Action Lab,
The ocean of matter (aether) we are all in "jiggles" but the acrylic electrons do not move, for acrylic is nonconductive.
You are therefore mistaken to blame the electrons for what the aether is doing.
Jiggling electrons (in my mind) doesn't explain why the speed of light decreases, only that a narrow pulse would get smeared out a bit. The leading edge of the pulse would still be the speed of light in a vacuum.
A question I had for a long time and u made a video on it. 😃
Thank you. What is the specific app you used for measuring distance to sticker on table?
I was always told the the speed of like is slower in different materials bc it has to bounce off all the molecules to get out the other side.. kinda like hitting a golf ball into dense woods
That's how Vantablack works to trap the light in
If this were any other youtube channel I would refuse to believe phones have sensors accurate enough to measure the speed of light. that is just wild
The light did not slow down it just followed the jiggle. The jiggle made the light distance traveled 1/3 time as long as what it was traveling through in a strait line. Neat.
I've actually watched all of those videos talked about here from both channels trying to understand this, and i think i actually do now. For the most part, still some ways until it's intuitive. But it's right there on the tip of my intuition. haha.
The constant we think about is not the speed of light. It is the speed of light in a vacuum. Any mass that interacts with the wave-form or blocks the particle of light will change the speed.
I agree with you. But I think the iPhone only judges the distance between the object and the iPhone based on the distance between the points of the infrared dot matrix (the time algorithm is useful, but only as an aid). The acrylic board/water magnifies the object behind it, which makes the distance of the dots look far away.
To test the hypothesis, we need to compare the measurement differentiate by magnifies is 1/2 the of the time differentiate.
Hey, whoever you are, if you're reading this, I have a thought experiment for you. It's an interesting one, because it makes sense. Consider two simple assumptions: the Universe expands, and expansion is not uniform. Expansion only happens in the absence of energy/matter/interactions. That's the strong assumption. The other assumption is that massless particles do not move, but stand still. They only appear to move because the Universe expands relatively. If you're somewhat accustomed to general relativity, you can consider the expansion rate of the Universe to be a parameter of a frame of reference. "How fast does the Universe expand between here and there?". Now, in your head, construct a Universe based on those assumptions in the image of our own, and bonus points if you can find gravity as an emergent effect (negative relative expansion rates). It's fun. I'm not saying it's true, but it's fun, because it might be true. It might even explain the randomness found in QM. That's why it's a fun thought experiment, it hints at having answers to tons of questions still unanswered, but there's no way to prove any of it. So it's all pointless, besides the fun for the mind.
From an e.m. point of view, the dielectric constant of the clear plastic is higher than that of the air/vacuum. The light is being interfered with by matter rather than just virtual particle pairs.
I always thought it was because the light has to travel "around" the molecules in whatever substance it goes through. Like water, glass, acrylic etc all have a denser mass and the light can't pas through molecules, as it's repelled. Kind of like they take a million or more years to move from the core of the sun to the surface, but hits earth 8+ minutes after leaving the sun.
3b1b is one of my most favourite channels
Now i'm curious to see how the difference between constructive and destructive phase change in a dual slit experiment under various dense materials(air, water, salty water etcetera).
I have a different explanation - the light has to travel extra distance when in denser material. light particle enters the material and starts to "wiggle" while affected by field forces around, the light is not slowed down actually, it's forced to do extra route. if you run straight line, of course you run to end in shorter time than if you do zig zag run even your run speed is the same.
I love this video. Thanks for rhess
Yeah, 3Brown1Bkue did a great job 😎
I had always assumed light just took a slightly longer path through materials, and by going the same speed, but covering more distance, it takes more time
*The speed of light indeed never changes.* It's just the distance it has to cover increases from all the interactions with particles in denser media. So the lidar readings aren't really wrong.
You might want to do an experiment with moving medium, to see what the effect would be... like light beam passing through flowing water with flow direction the same as/opposite to the beam
idk, a relativistic water fall sounds dangerous.
@@DrDeuteron bbbut it would be COOL though
I'm most interested when light accelerates after leaving an encumbering medium.
From the photon's POV nothing changes, but from a stationary observer they observe energy being created per the formula E=HV.
😮Wow! Awesome!
Sixty Symbols, Brady Haran's physics channel, also attempts to explain refractive index with two physics professors with their own but similar explanations (Phil Moriarty and Mike Merrifield). Another 60S video with Phil explains why some materials are transparent while others are not.
I remember not too long ago that there were experiments done on light to slow it down as much as possible and have it interact with other photons, and they were able to create a new type of matter that was based on light, complete with "chemical" reactions, which resulted in photons conjoining to create "molecules" It was really bizarre. Maybe this could be exploited for photonic computing so that photons can be stored as bits in a kind of RAM.
cool! didn't even know this was a thing!
you have to try an experiment where the IR laser goes though transparent materials of different densities, for example a transparent cylinder filled with air at different pressures (take precautions), and the same optical path. If you can use solid lidar IR-transparent materials with different densities, even better.
Once you found materials of different densities that allow you to observe diferent distances at the same optical paths you can change the temperature of the material, that will drop the measured "apparent" speed, and explain Hau's team observations near the 0K boundary. Not by slowing light down but by slowing down atom's internal mechanics. Yeah, something moves inside atoms and that movement is driven by ...
temperature, not by fiction science.
Sound refracts the same way. My ultrasound machine needs to have the correct material velocity to measure the time of flight properly. My machine can find a defect the size of a grain of sand through 50 inches of steel. Did ultrasonic non destructive testing at a steel casting foundry in Berkeley California. We use lucite shoes coupled to the transducer to do shear wave inspection that looks for flaws at an angle where straight longitudinal waves can’t reach. Sound does not propagate through air as well as it does through water or other viscous coupling agent.
For years I had the same question, thanks
The problem is that although this explains things like refraction, it doesn't explain why the LIDAR shows a slower time. If the wavefront is still travelling at C, regardless of the summed waves' "speed" then there should be no extra delay.
that edge of your acrylic vacuum chamber is very nicely polished *or broken, not cut ;)
Dispersion can be seen using a prisma. Does that mean, that red light and blue light (as the extremes) also have different velocities going through glas?
Got me curious, considering that atmosphere can affect the speed of light, does the lidar system on the phone notice a distance difference from sea level or higher up?
Perhaps a vacuum chamber could help.
This is cool and grand for slowing down light. But that doesn't explain getting light to STOP completely. Move it to another location. And then release it agai. To start moving
Buen video. Instructivo.
light itself is NOT a constant as going from A to B the speed of light will increase and decrease, but all light will reach its termination at the measured constant. What this means is if you measured light only from point A to point B you will measure a constant, but if you measure light between point A and point B the speed will vary constantly. also light does carry mass, but not in the photon itself as a photon is only the radiation being emitted from the field, its the field that carries the mass. see it as if you are the light and you are crossing a stream using stepping stones, its the stepping stones that carry the mass.
Think of all the error involved in calculating the distance of deep space objects. Is the space all vacuum or the light rays pass through various mediums that alter its speed along billions of kilometers of distance that it travels. In the end, if it affects our measurements, how are we supposed to know if there was a delay or not? Also, is it observable with space telescopes and earth telescope because of the atmosphere? Very interesting topic.