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

[@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.

[@anelectofgod114]

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)?

[@joz6683]

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

[@LearningCurveScience]

I hope you're feeling better soon

[@joz6683]

@@LearningCurveScience 👍

[@dancingwiththedogsdj]

Hope you are doing better! And yes, another enjoyable video. 👍🌎❤️

[@taotaostrong]

Hope you feel better.

[@PraveenSriram]

Hope you are feeling much better now than you did 11 days ago

[@kayliibensen387]

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

[@nikapanda160]

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

[@rodrigoappendino]

Because we can calculate the speed of light using electromagnetism. The 4 Maxwell's equation lead to a wave equation where the wave propagates with the speed of 1 over the square root of the product of two constants from the electric and from the magnetic fields.

[@jacquesvermeulen748]

Only reflected light ever measured, one way speed of light not possible to measure

[@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

[@mhklein57]

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

[@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.

[@Beltalowda55]

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!

[@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.

[@stephanieparker1250]

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

[@davidwagner6116]

Thank you! I have been wanting a good answer to this question for a while. Well done!

[@j.anthonybattaglini6650]

Fantastic history lesson!

[@user-cr5en4rx1k]

I had goosebumps. Thank you for this work.

[@randy7894]

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

[@jimothybikael]

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

[@dieago12345]

Great science / history video dude - Bravo.

[@baolujiang9060]

Interesting! Thanks for putting all the stuff together.

[@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.

[@simo9445tsns]

Interesting video well presented again, thanks

[@makqua2001]

Great channel - many thanks : )

[@adamlong9031]

How did they figure out the currently accepted value?

[@nethoncho]

I love the history of science also. Thanks for sharing your videos 😎

[@LearningCurveScience]

Glad you like them!

[@Marisad]

This is my favourite comment on all of youtube. I also love these videos!

[@benjerman4438]

Agreed, the first Ben said it perfectly.

[@williamwalker39]

​@@LearningCurveScience The speed of light was determined by measuring the phase shift of a sinusoidal signal transmitted from one dipole antenna to another dipole antenna as the antennas were moved from the nearfield to the farfield. Then the phase speed and group speed were determined from the slope and curvature of the resultant phase vs distance curves. This method revealed that the speed of the radio waves (light) was instantaneous in the nearfield and reduces to the speed of light in the farfield after about one wavelength. The results were checked theoretically using Maxwells equations and numerically using an electromagentic simulator, and the results matched perfectly. These results have been checked by many independent researchers over the past 20 years. The result fits in with our understanding of electromagnetic fields, where the Coulomb law for the electric field and Biot-Savart law for the magnetic field are independent of time, and known to be valid for slow moving sources, implying that they are instantaneous in the nearfield, and in the farfield, radio waves (light) has been measured to be speed c. These results are not compatible with Relativity, which assumes light propagates only at speed c. So, Relativity needs to be reanalyzed. Derivation of Relativity using instantaneous nearfield light shows that Relativity reduces to Galilean Relativity. This is because as c=infinity, the Relativistic gamma function becomes equal to one, causing the Lorentz transforms to reduce to the Galilean transforms. But using farfield speed c fields yields the Lorentz transforms. But since time and space are real and can not depend on whether nearfield or farfield fields are used (i.e what frequency of light is used), then the effects of Relativity on time and space must be an optical illusion. Time and space are absolute as indicated by Galilean Relativity. When moving objects are observed using farfield electromagnetic fields then time can appear to dilate and space can appear contract, but the effects are not real and can be verfied by using nearfield electromagenic fields which will show time and space have not changed. Since General Relativity is based on Special Relativity, then the effects of General Relativity on time and space must also be an optical illusion. So what is a better theory of gravity if General Relativity is wrong. It is well known that General Relativity reduces to Gravitoelectromaganetism for weak gravitational fields, which is all that we observe. Consequently Gravitoelectromagnetism predicts all known observed gravitational effects. But the difference is that Gravitoelectromagnetism is a field theory, whereas General Relativity is a geometric theory. Gravitoelectromagnetism assumes gravity is modeled by 4 equivalent Maxwell Equations as for Electromagnetism. The equations only differ in the constants used. Gravitoelectromagnetism is field theory of gravity and assumes there are both an Electric and Magnetic components of gravity. Just like for electromagnetic fields, changing gravito-magnetic fields create gravito-electric fields and visa versa. Gravitoelectromagnetism also assumes time and space are absolute and not flexible as General Relativity predicts. So gravity is a propagating field that can finally be quantized enabling the unification of gravity and quantum mechanics. The current interpretation of quantum mechanics makes no sense, involving particles that are not real until measured, and in a fuzzy superposition of states. On the other hand, the Pilot Wave interpretation of Quantum Mechanics makes makes much more sense, which says particles are always real with real positions and velocities. The particles also interact with an energetic quantum field that permeates all of space, forming a pilot wave that guides the particle. This simpler deterministic explanation explains all known quantum phenomena. The only problem is that the Pilot Wave is known to interact instantaneously with all other particles, and this is completely incompatible with Relativity, but is compatible with Galilean Relativity. But because of the evidence presented here, this is no longer a problem, and elevates the Pilot Interpretation to our best explanation of Quantum Mechanics. *KZbin presentation of above argument: kzbin.info/www/bejne/qZazlX1tq7iErLM *Paper it is based on: William D. Walker and Dag Stranneby, A New Interpretation of Relativity, 2023: vixra.org/abs/2309.0145

[@MrSamPhoenix]

Amazing video! I’d love for this guy to measure warp factors.

[@333crazymonkey]

Excellent visuals. As a teacher I appreciate the clear explanation.

[@paulwoolcock6364]

This is the content I am looking for

[@DougVandegrift]

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

[@megan7258]

All electromagnetic waves propagate at the same speed in a vacuum (the speed of light); the distinct colors of the visible spectrum is how the human eye perceives it. Although EM waves can travel slower than the speed of light in different mediums, the relative color that we view it is irrelevant to how fast it travels. Hope that helps!

[@SpontaneityJD]

This video is gold

[@thamzource8971]

Absolutely fantastic videos, you sir have earned yourself a new fan. Cheers from Sweden!

[@LearningCurveScience]

Thank you very much!

[@pederlindstrom3132]

Umeå här.

[@KeyPeele1]

Thank you for this educational video. I had difficulty visualising Galileo's experiment.

[@PraveenSriram]

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

[@y5mgisi]

I love the history of science. Great video. Love this channel.

[@LearningCurveScience]

Thanks for watching!

[@defeatSpace]

And thank you for sharing!

[@teppC]

Thank you so much

[@sciencetalks909]

That is amazing...hats off sir 🙏

[@LearningCurveScience]

Thank you so much.

[@MyLinguine]

this really made my week! Thank you

[@mtylerw]

Thank you.

[@seanmortazyt]

fantastic

[@i-am-evil-morty6710]

The cool thing about the speed of light is that it is not the speed of light, its the speed of information. Of causality. Fascinating.

[@wolfhram3996]

It always amazed me how some smart people who lived among mostly illiterate population and without electricity, toilet paper or understanding that you should wash your hands before eating or touching open wounds were able to discover beyond-the-endge things simply by ability to calculate stuff and by practical application of their imagination to create WHAT to calculate.

[@bsadewitz]

"Instruments that come in contact with the body during all surgical procedures and many non-surgical procedures must be free of all microbial elements. This is to ensure that the risk of infection is kept to a minimum. Throughout history different methods were adopted using materials that were available at that time. In 3000 BC the Egyptians used pitch and tar as antiseptics. In later years the fumes from burning sulfur were found to cleanse objects of infectious material. In 1680 a French physicist, Denis Papin invented a pressure cooker that would trap boiling water, convert it into steam, and was found to cleanse objects by cooking them. This device was further improved upon during the next two hundred years and it became possible to additionally sterilize linens, dressings, gowns using steam. Two major contributions to the art of sterilization came in the 1860’s when the French chemist and microbiologist Louis Pasteur wrote extensively on how germs cause disease and the English physician, Joseph Lister, developed a technique that used carbolic acid as a spray to disinfect instruments. " www.flushinghospital.org/newsletter/history-of-instrument-sterilization/

[@pederlindstrom3132]

I do love the videos,, relaxed and lighthearted yet science. Of course I will accompany you on the travels through time and space.

[@jannis11]

Nice

[@shoutitallloud]

Would not Roemer had to observe some slight color shift to red and blue in different Earth position due to Doppler effect ?

[@aaronhrk]

I really enjoyed this. Please keep up the great work!

[@sickboy703]

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.

[@crazkurtz]

Well, how did we get the actual measurement?

[@Dudleymiddleton]

1:27 This is true to scale of how far away the Moon is, most of us seem to think it's quite near us - but it isn't! Thank you for sharing.

[@willi80636]

I suddenly have the urge to measure the speed of cheese 🧀👍🏼

[@1pcfred]

The speed of cheese is the square of how hungry you are.

[@gooberclown]

What about the speed of mice when they taste the cheese? 😮

[@1pcfred]

@@gooberclown if you set the trap right they don't even get to taste it. BTW peanut butter works better. Cheese is just for cartoons. Peanut butter is like crack cocaine for rodents. It's way easier to apply too. Although since I've been using peanut butter as bait I can't bring myself to eat it anymore.

[@williamwalker39]

The speed of light was determined by measuring the phase shift of a sinusoidal signal transmitted from one dipole antenna to another dipole antenna as the antennas were moved from the nearfield to the farfield. Then the phase speed and group speed were determined from the slope and curvature of the resultant phase vs distance curves. This method revealed that the speed of the radio waves (light) was instantaneous in the nearfield and reduces to the speed of light in the farfield after about one wavelength. The results were checked theoretically using Maxwells equations and numerically using an electromagentic simulator, and the results matched perfectly. These results have been checked by many independent researchers over the past 20 years. The result fits in with our understanding of electromagnetic fields, where the Coulomb law for the electric field and Biot-Savart law for the magnetic field are independent of time, and known to be valid for slow moving sources, implying that they are instantaneous in the nearfield, and in the farfield, radio waves (light) has been measured to be speed c. These results are not compatible with Relativity, which assumes light propagates only at speed c. So, Relativity needs to be reanalyzed. Derivation of Relativity using instantaneous nearfield light shows that Relativity reduces to Galilean Relativity. This is because as c=infinity, the Relativistic gamma function becomes equal to one, causing the Lorentz transforms to reduce to the Galilean transforms. But using farfield speed c fields yields the Lorentz transforms. But since time and space are real and can not depend on whether nearfield or farfield fields are used (i.e what frequency of light is used), then the effects of Relativity on time and space must be an optical illusion. Time and space are absolute as indicated by Galilean Relativity. When moving objects are observed using farfield electromagnetic fields then time can appear to dilate and space can appear contract, but the effects are not real and can be verfied by using nearfield electromagenic fields which will show time and space have not changed. Since General Relativity is based on Special Relativity, then the effects of General Relativity on time and space must also be an optical illusion. So what is a better theory of gravity if General Relativity is wrong. It is well known that General Relativity reduces to Gravitoelectromaganetism for weak gravitational fields, which is all that we observe. Consequently Gravitoelectromagnetism predicts all known observed gravitational effects. But the difference is that Gravitoelectromagnetism is a field theory, whereas General Relativity is a geometric theory. Gravitoelectromagnetism assumes gravity is modeled by 4 equivalent Maxwell Equations as for Electromagnetism. The equations only differ in the constants used. Gravitoelectromagnetism is field theory of gravity and assumes there are both an Electric and Magnetic components of gravity. Just like for electromagnetic fields, changing gravito-magnetic fields create gravito-electric fields and visa versa. Gravitoelectromagnetism also assumes time and space are absolute and not flexible as General Relativity predicts. So gravity is a propagating field that can finally be quantized enabling the unification of gravity and quantum mechanics. The current interpretation of quantum mechanics makes no sense, involving particles that are not real until measured, and in a fuzzy superposition of states. On the other hand, the Pilot Wave interpretation of Quantum Mechanics makes makes much more sense, which says particles are always real with real positions and velocities. The particles also interact with an energetic quantum field that permeates all of space, forming a pilot wave that guides the particle. This simpler deterministic explanation explains all known quantum phenomena. The only problem is that the Pilot Wave is known to interact instantaneously with all other particles, and this is completely incompatible with Relativity, but is compatible with Galilean Relativity. But because of the evidence presented here, this is no longer a problem, and elevates the Pilot Interpretation to our best explanation of Quantum Mechanics. *KZbin presentation of above argument: kzbin.info/www/bejne/qZazlX1tq7iErLM *Paper it is based on: William D. Walker and Dag Stranneby, A New Interpretation of Relativity, 2023: vixra.org/abs/2309.0145

[@mrcleanisin]

I am suggesting that a stick is faster than the speed of light. I have had this idea since the 70's and I have presented it to many people, but of course I have no way to actually conduct the experiment. We know that light travels at 186,000 miles per second and that the moon is 240,000 miles from earth, and it would take 1.3 seconds for light to reach the moon. If a bell was placed on the moon and could be rung by a photo sensor on the moon activated by someone firing a laser beam at it from earth, it would ring 1.3 seconds from the button being pushed. Now if it were possible to extend a solid object from the earth to the moon and that object was an inch away from the bell and the earth end was pushed, how long would it take to ring the bell? [Note: On 2-2-2023 I am adding this clarification to this puzzle because all the other comments to questions similar to mine focus on the stick traveling at some ridiculous speed that causes it to explode or whatever. My stick is not moving fast at all. It's taking 1 second to travel 1 inch, but it still rings the bell before the laser which takes 1.3 seconds.] In the experiment I used wires to run to my simulated moon because I don't have a photo sensor setup. Note: light and electricity both travel at the same speed.

[@Beltalowda55]

The atoms in the stick interact with each other via the electromagnetic force which is a speed of light interaction. In a perfect setup with no other realistic influences, the force on the stick could only propagate at the speed of light at the maximum. The speed of light is the maximum speed of causality. (quantum entanglement not withstanding 🙂)

[@mrcleanisin]

@@Beltalowda55 My stick is only traveling 1 inch per second which is 0.0568182 miles per hour, so I don't think the atoms are smashing into each other too violently. The reason it rings the bell faster than the speed of light is because one end is already 1 inch away from the bell on the moon. For some reason most people cannot grasp this puzzle. You might want to try the 3 doors puzzle.

[@Tsathogguah]

One thing I don't get is how Edmund Halley calculated that it took light 17 minutes to cross the diameter of Earth's orbit. He's been dead over 450 years and he's still smarter than me.

[@annaclarafenyo8185]

Michaelson's 1890s experiment WAS NOT the start of the path to special relativity, Fizeau's 1830s or 1840s experiment was. This measured the speed of light through a moving medium, flowing water, and produced a value which is the relativistic sum of the velocity of the water and the velocity of light in the medium. This experiment was what inspired Einstein, not Michaelson's, which was a much better experiment, but came decades later.

[@capjus]

Fizeau's experiment is still the most exciting for me, the first step advanced tech within the planet, the ingenious way of thinking, even if it's so simple, but trying to "trick" the nature in order to get the difference and number with a very clear output. The light come out or not - with which rotation speed. So fascinating

[@anelectofgod114]

I don't see how Fizeau's experiment could be reliable if he did not account for the light speed being affected by the atmosphere (air) AND the glass in the mirror.

[@capjus]

@@anelectofgod114 at least it lead to one of the first steps to ask questions .. like wait a minute.. we got something in such rotation frequency

[@ThunderBassistJay]

Very interesting! Thanks! 🙏

[@jonathanberry1111]

If you ask most people, most physicists, and most LLM's (Large Language Models) if the one way speed of light is constant they all will say it is and that it is part of Special Relativity (SR). If you ask most, "how can that be", they will answer the contraction of space and dilation of time, but if you drill down deeper you learn that actually it isn't, it is a postulate of the 1905 paper on Special Relativity and postulate is a fancy word for an assumption that is made but not typically explained within. But if you drill down deeper, you find it isn't even that! The constancy of the speed of light (in each direction, AKA one way speed of light) is neither explained by, nor necessary for, nor a postulate of the 1905 paper! What the 1905 paper DOES say is essentially two key things, both postulates (again, postulates = assumptions typically not covered in the theory being presented, but the foundation of it).... The first is that the speed of light is not affected by the velocity of the emitter.

[@hypercomms2001]

The speed of light unlike the speed of causality, does vary depending upon the permitivity Of the medium that it is passing through.

[@y5mgisi]

It was fun watching the light travel from the sun to the earth in "real time".

[@JungleJargon]

How do expect to measure the speed of light when time and distance are not constant?

[@AD-nx1xd]

Brilliant video shows me how lousy my education was. Q.) Does the speed of light change each time the scientific powers that be change the definition of the metre? (I think ,I read somewhere the old bras rod is now considered inaccurate.) As a none scientific Brit, of course, I still define it as 3 feet there and a bit inches. Thus ( so long as I don't move my light😗), and using a Chinese made Imperial measure rule, I can make some brilliant non rectilinear kitchen cabinets!!😅😅😅.

[@magicmulder]

Michaelson/Morley: The idea that some magic field permeates everything is nonsense. Higgs: Ahem…

[@stephanieparker1250]

I’d like to see an explanation to how the effect gravity is the same speed as light..

[@atheistaetherist2747]

13:40 The MMX of 1887 was not null for the aetherwind -- it indicated approx 6 km/s -- Munera corrected an averaging error & the 6 km/s became 7 km/s -- Prof Reg Cahill derived the correct calibration for the MMX & the 7 km/s became over 300 km/s (needs checking) -- VV Demjanov made an MMX that showed a horizontal component of the aetherwind of 180 km/s to 480 km/s at Obninsk -- aetherists accept that the background aetherwind blows throo the solar system at 500 km/s south to north about 20 deg off Earth's axis (RA 4hr30min). Most of the Earthly measurements of the SOL c are ave 2-way measurements, & hence wont be affected much by the aetherwind. However DeWitte measured the 1-way SOL which included a varying component due to the aetherwind which of course changed due to Earth's orientation during a day. Torr & Kolen also measured the 1-way SOL including an aetherwind effect. And there was at least one other such SOL measurement which i forget. Also, em radiation is not light & light is not em radiation. However the SOemR is close to the SOL in the far field at least.

[@iaindavis4417]

Fascinating and informative

[@leovolin7525]

That is only an approximation. 2 things are missing in no particular order: first it should be stated 'measured at, let's say 250 meters over the sea level', second that the the length of the second(pun intended) is measured using in, far from ideal, some conditions. Both units will measure differently in the vicinity of Sun or outside of the Solar System. A lot of controversy there. But overall the show is needed, useful, and well presented. Just use it with a grain of salt😉

[@ExSheriffFattyBoySkinnyArms]

A microwave? Mind blown. Liked and subbed

[@SBMPLYMA]

History and science must be synonyms

[@skivvy3565]

Been binging the history of physics lately, would love goin along for that ride

[@gooberclown]

Marilyn Vos Savant mentioned the possibility that scientists may eventually discover the speed of light is not constant.

[@DBREW]

How did we arrive at today's the figure from the interferometry experiments?

I came here to ask this. Felt a bit disappointed that the ultimate experiment/method wasn't explained.

[@baolujiang9060]

Micheson-Morley experiment only proved that there is no relative motion between Ether and Earth. Without a medium, you can not explain how wave works.

[@incription]

First, or technically, Last, depending on how fast the speed of light is

[@sipoppy984]

What takes 2nd place for speed after Light?... Electron flow?

[@AlpaOmega-nb5jm]

Well done

[@hurle0409]

Is there an experiment that if light travels at the same speed in any direction?

[@Novastar.SaberCombat]

Reflection is key. 🐲✨🐲✨🐲✨ "Before I start, I must see my end. Destination known, my mind’s journey now begins. Upon my chariot, heart and soul’s fate revealed. In time, all points converge, hope’s strength re-steeled. But to earn final peace at the universe’s endless refrain, We must see all in nothingness... before we start again." 🐲✨🐲✨🐲✨ --Diamond Dragons (series)

[@FrankAnzalone]

How would Fizeau know it was the next tooth to block the light and not the second tooth which would give a different speed calculation

[@LearningCurveScience]

Good question. He did the experiment a bunch of times with different rotations. he found a number of speeds that blocked out the light corresponding to the next tooth, and the next but one tooth and so on. 12 revolutions was the lowest speed that blocked out the light so must have been the next tooth. Hope that answers your question.

[@dnfielding]

The metre was set to be a dividion of the speed of light. The speed of light was not "set".

[@therealmatthewsmith]

Why does it take longer to travel through water?

[@chrisoakey9841]

if lights is slowing by around 0.5mm/s/year then the universe is not expanding at an exponentially increasing amount. hubble was looking from the wrong end. it is not the proximity of galaxies to us, but the travel light has gone through to us. so the further away the light source, the longer it has travelled. so the redshift is not necessarily due to expansion or collapse, but the photon having slowed.

[@Shonade_Malik]

This leads me to a possible problem. If light slows down through a medium, that means the constant we have is slower than the true value. Testing the speed of light on Earth is not an accurate measurement, but you would need to do it in a vacuum, or in space.

[@Beltalowda55]

Several of the experiments described in the video were done in a vacuum. You are correct, but most if not all current speed of light experiments are done in a vacuum and adjust for the gravitational effects of being in a gravity well. (i.e. here on earth)

[@Shonade_Malik]

@@Beltalowda55 Ohhh, alright.

[@TwoFoxGibbon]

Do photons accelerate? Or are they always at maximum speed?

[@stewiesaidthat]

Light has an induction rate. That rate is dependent on the permittivity and permeability of space. Light blueshifts entering earth and redshifts leaving earth. Light slows down in water - Cherenkov radiation. Light has been slowed down to zero in a lab.

[@martinstubs6203]

Iterestingly, and despite of all that's said in this video, the speed of light has never actually been measured. As it turns out, the speed of light can only be measured in a two-way fashion and that doesn't rule out the possibility of light travelling at two different speeds there and back.

[@kyzercube]

" melty bits " 🤣

[@internet_introvert]

Meme time: Albert Michelson kinda looks like We Have Einstein At Home

[@sciencetalks909]

Actually its important to teach science with historical timeline....when I was in school, we just got dry equations in the name of science

[@Willhardwood]

Is good.

[@Georgia-Vic]

These all were reflected beams and not one-way which is impossible to measure.We assume that the speed is constant each way so we subtract the total time and cut it in half which is logical but the time reflected could be different.It's called a "convention" and hasn't been substantiated as of yet because it's a theory that hasn't been proven or disproven!

[@Napoleonic_S]

What, this video is equivalent with telling a story but cut out the ending... So how did we figure out the missing 2 m/s? And then why is the speed of light equate to that number relative to meter and second? Also this is something I have yet to see being discussed on the internet : if you're an alien that have completely different measurement standard for distance and time, wouldn't that mean you would have completely different speed of light measurement?

[@LearningCurveScience]

I couldn't find an adequate explanation for the difference it's probably a bunch of experiments that they averaged. If an alien had a different measurement system the speed of light wouldn't change. They might say the speed of light is 7200 blifblaffs per medung. But the speed of light is still the same they just use different units to measure it.

[@Napoleonic_S]

@@LearningCurveScience You're right, aliens would only have different symbols, oh wait, there's still this nagging curiosity in my mind... Has anyone ever attempted to translate our human physics equation into alien physics equation? Including with alien maths... Hmmm maybe I need to ask that at reddit... Yeah, I'll do just that...

[@hosh1313]

The MM Experiment is the most misread experiment in the history of physics. The assumption was that light would travel in aether the same way sound travels in air. But we know that light has an extra force (magnetic) that is somehow perpendicular to travel, thus it is simply not the same thing as sound and thus it should have been of no great surprise that the result was what it was. No Aether is not a conclusion to this experiment - it is merely a lazy interpretation. Aether is the only thing that explains all of the wave like behaviour of EMR.

[@satyris410]

I think it should go without saying that you should read A Short History of Nearly Everything by Bill Bryson

[@angusmackaskill3035]

We travel to the future and see how long it takes

[@jonathandaflon1769]

The history of the gravity with Socrates is really cool too

[@erbalumkan369]

How do you explain that between 1928 and 1945, in science labs all over the world, the speed of light dropped bij 20 Km/s ?

[@stephanieparker1250]

Ibn al-Haytham wrote a great book… “Book of Optics”. Check it out.

[@primemagi]

very well presented. you need to add that these speed are all two way divided by half. they are approximately near speed. all the ones on earth has been taken horizontally. not vertically. DLR, ESA, NASA REFUSE TO TEST THE SPEED ONE WAY IN SPACE. it will undermine GR and mankind need new physic. light is a stream of photon. photon has mass and complex structure. it is disk shape and gravity has little effect from side, but maximum effect on speed from front and back of travel direction. that is why they took picture of starlight bent by sun's corona (matter space dust and plasma) and lied telling public it was by gravity.

[@bonetiredtoo]

Because the metre is now defined in terms of the speed of light, that's it - the speed of light is now fixed.

[@erbalumkan369]

Circular reasoning

[@bonetiredtoo]

@@erbalumkan369 But that is the way it is. "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second."

[@adamrspears1981]

According to Veritasium, the one-way speed of light has not been measured.

[@Yora21]

That's basically a laser experiment in 1849.

[@aguskhusaeni]

Alhaitham 😳

[@marktorch9079]

Maybe not Ether, but couldn't blackholes be proof that light needs a medium like sound to travel. It's theorized that the curvature is so extreme, so light travels toward the singularaty. This I'm aware of. Some astromers say that all physics literally break down and all matter is crushed into nothingness. That however doesn't make sense to me because with no mass present the fabric of spacetime wouldn''t continue to be bent. How can gravity continue to exist if the source of said gravity is not there anymore.