Measuring the Speed of Light Throughout History

Not only am I a lover of science, but I’m also very interested in the history of science and so I wanted to do an occasional series on some of our most important scientific breakthroughs and discoveries. For the first in this series I want to look at the speed of light, how we first measured it, and how we measure it today. Let's find out more.
Before we start let us look at what the speed of light actually is. It is 299 million, 792 thousand 458 metres per second. That is 186 thousand 282 point 4 miles per second. That value has now been set and won’t change, it was actually set in 1983.
The speed of light isn’t just the speed of light though, it’s the speed of causality. It is the universes ultimate speed limit. Objects and particles with mass are slowed down by that mass and so must travel slower than the speed of light. Massless things such as electromagnetic waves of which visible light is just one type are not slowed down by any mass and so are able to travel at this ultimate speed.
Chapters
0:00 Introduction
2:00 Ancient Times
3:07 Beeckman and Galileo
4:45 Ole Roemer
6:52 James Bradley
8:15 Hippolyte Fizeau
9:50 Leon Foucault
10:50 Weber & Kohlrausch
11:50 James Clerk Maxwell
12:50 Albert Michelson
14:50 Cavity Resonance
16:15 Home Experiment
17:00 Interferometry
Ole Roemer Image
Jacob Coning, Public domain, via Wikimedia Commons
Hasan Ibn al Haytham image
artwork drawn by Adolph Boÿ, engraved by Jeremias Falck, Public domain, via Wikimedia Commons
Christiaan Huygens Image
Caspar Netscher, Public domain, via Wikimedia Commons
Albert Michelson Image
The original uploader was Bunzil at English Wikipedia., Public domain, via Wikimedia Commons

[kayliibensen]

This is easily one of my favorite recently found channels, you are putting out outstanding quality! Thank you!

[t_ylr]

Fizeau's math may not have been perfect, but he designed a very simple yet incredibly clever experiment. Also, It really must have been a task getting everything lined up perfectly.

[anelectofgod]

Thank you for the video. I do have a few questions.

1. How can we accurately say the speed of light is ~186, 000 mi/s in space if all of the experiments were conducted on earth (including the wavelength measurements)?

2. How do we confidently calculate the speed if refraction is known to slow down light?
(The experiments involving mirrors are affecting the speed when it passes through the glass)

3. How can we be sure the "actual value" of light traveling the diameter of earth's orbit is 16.40 light minutes when the earth's orbit is not perfect, but rather elliptical? (We should be wary of calculations based on assumptions)

4. How can any measurements involving stars be accurately determined if we don't know the rate speed at which the entire milky way galaxy is moving through space as well as the speed of expansion (stars are believed to still be moving outward from center)?

[JafoTHEgreat]

What absolutely crazy ideas these guys used to try and figure this out. We kind of owe our tech to these guys. Human foot steps but each step is another generation. They had information that was too early for them to use. Kind of like "born too early" like we say today. You know the "born too early to explore the universe, born too late to explore earth"

Edit: American cheese isnt all bad if you can measure the speed of light with it LOL

[nikapanda]

But why is the accepted number 299 792 458 m/s and not 2m/s less like the calculations you showed at the end?

[joz]

In bed ill, this has dropped at just the right time, thanks in advance for another great video.

[mjproebstle]

I was fortunate enough to attend the U. S Naval Academy. At the time in the basement corridors of the science hall named for Michelson, were displayed the actual apparatus used in the famous experiments. I marveled at their simple yet robust construction for their time, and was privileged and inspired to see them first hand as I walked by the displays daily on my way to classes.

[stephanieparker]

I love science history!! Thanks for another fantastic video, well done! ❤🎉✨🙌

[Hydroverse]

I believe I have a solution for the problem. You don't actually use a clock, but light itself. The key is interferometry. Instead of your normal double-slit experiment in which the laser beam enters through both slits simultaneously, you instead shine the beam parallel to the plane of the slits. This way one wave enters through one slit before the other, and you should be able to make predictions about the interference pattern that is made if light travels at a constant rate. This is using light to tell us if it is moving faster in one direction over the other by interfering with itself with a modification to the classic interferometer that treats light waves like water waves in which the two waves that exit from each slit will modify the interference pattern made depending on the one-way speed of said waves. I've tried this myself, and I don't see any difference between shining my laser parallel to the plane of the slits when doing in both directions. Should I be correct, then all it means is that the simplest convention is true, which is that light does travel the same in both directions.

[mhklein]

Wait! How was the accepted value of 299, 792, 458 m/s derived?

[jimothybikael]

6:33 How did Edmond Halley calculate that it should take 17 mins for light to complete one Earth orbit?

[dogastus]

Just found your channel and subscribed right after watching this excellent video. Superbly narrated and really good graphics, I can only imagine the amount of time it must have taken you to create them all.

[DougVandegrift]

Excellent presentation. I'm curious if each color in the visible spectrum has a slight difference in speed?

[PraveenSrJ]

It takes a whopping 2.5 million years for light to travel from the Andromeda Galaxy 🌌 which is just mind blowing!!!

[sickboy]

Two things are happening today. I am going to measure the speed of light and make a run to the grocery store to buy cheese.

[adamlong]

How did they figure out the currently accepted value?

[Beltalowda]

Excellent presentation. Nice and relevant animations and progression of the tests. Each was well explained. I really liked the Sun to the Earth speed of light graphic at the bottom. Well done!

[robertmccormack]

theres so much information in these videos. this channel and @LearningCurveScience are mad underrated

[randy]

Love your content man. Pleasant infographics and animations. And your video's are not bombarded with equasions, but are there when necessary. Thanks!

[林士堯-dc]

I had goosebumps. Thank you for this work.