Solving The Slow Light Paradox

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:

Looking Glass Universe Videos:

Online Simulators I used in this video:

[CaptainTedStryker]

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.

[GuagoFruit]

Even without the need for a vacuum, he finds a way to use the vacuum chamber for experiements. What a legend.

[micahrunyon]

Appreciate all the free education you provide.

[elemental]

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)

[xtieburn]

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.

[dugldoo]

There are lots of "authoritative" misstatements on YouTube 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!

[BaggyCatEntertainment]

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...

[OC]

I think addressing "What makes a material transparent?" would have been a good addition to this video.

Great work.

[WanJae]

I'm glad so many science YouTubers 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.

[SammieHarrison-hdkw]

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

[stevestarcke]

You made a very deep question very accessible. Kudos!

[hunux]

You should attempt at a zero point energy experiment in a vacuum. Using the Casimir Effect.

[KalebPeters]

Great video! Props for doing the shoutouts at the end, I thought of those videos instantly when I saw this!

[johnslugger]

*Light slows down in Diamonds by 25% which is why they have so much "fire".*

[bxdanny]

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.

[DrDeuteron]

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.

[physics]

I was always puzzled by this question. Thanks for your great explanation

[whuzzzup]

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?

[GhostSenshi]

This brings a great perspective to understand what’s going on. Essentially light isn’t slowing down, it’s just taking extra steps as it moves through the medium, thus taking more time to exit through it, creating the observation of it moving “slower”
Thank you very much for sharing with us and making something like this comprehensible

[junatah]

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.