history of the speed of light and how its speed was determined

What is the history behind determining the speed of light? How did past scientists determine it'salue value
This video covers Rømer, Bradley, Fizeau and Foucault and how they determined the speed of light.
CORRECTION: As some have noted I have made an error in the discussion on Rømer. I acknowledge this , and here is my correction as well as animation: • Ole Rømer correction
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An Intro to Special Relativity which relies on the constancy of the speed of light - • Einstein and the clock...
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[@Squishergeo]

Just a note to say thank you! This was a great video. Have subscribed. Keep up the excellent work!.

[@PhysicsHigh]

Thanks Daniel

[@AbhayKumar-um8vl]

Maybe the scientist of that time didn't know about the earth orbit is in elleptical path and this also could be the reason for the change in interval of time for stars circular rotation and jupiters moon rotation period. Which means why we thought light to be travelling still has no base for it.

[@kennethmacleod5926]

@@PhysicsHigh llllllĺĺllllĺllllll2

[@archimedesmaid3602]

@@AbhayKumar-um8vl It demod that light IS traveling, and that was possible w/o great accuracies. Lol, we dont today depend upon 18th century observation to determine whether or not light is traveling. The discrepancy was that in the 17th century they didnt have a very accurate figure for our distance from the sun. Btw, earths orbit is VERY circular. The minor axis is only about 1/7000ths shorter than the major. That fact makes so the earth varies (in distance from the sun) by only about 3.3%

[@alexandrekassiantchouk1632]

You will be surprised that combined with Newton-Laplace formula for a wave speed in any medium, it solves gravity - no more GR needed. Read 2-page chapter 92 in "Time Matters, 9th edition": gravity (acceleration g) is directly related to time dilation D (and nothing else) g = -(0.5c²/D²)' ≈ c²×D' ~ ∇D - formula from that book back-cover.

[@IngmarSweep]

Thanks for not disturbing your interesting story with background music.

[@primemagi]

they do that so your brain is distracted by irritating music so their garbage enter your memory by passing your critical sensors which discriminate between some thing useful and their crap.Ferydoon Shirazi. MG1

[@jimlassiter749]

Amen...!

[@MrSorbias]

​@@primemagithat's not why we do it tho 😅 The music is added to combine it all together and hide for example background noise.

[@keithtomey5046]

​@@MrSorbiasIt makes videos impossible to endure - such a shame when the content is otherwise good. (Dot)

[@savage22bolt32]

Thanks for the wonderful video, and a huge thanks for not ruining it with crappy background music. I don't know why some people are compelled to add annoying background music throughout their videos. I always let content creators know that I didn't watch their vid because of an added soundtrack. It's constructive criticism, not bichin.

[@PhysicsHigh]

Just letting you know I am currently working on a follow up to this video with more recent experiments on determining the speed of the light. Stay tuned

[@PhysicsHigh]

Both follow up videos have now been released. Check the descriptions or end screens

[@Squishergeo]

Great! I'll do that. Thank you. Best Regards Dan.

[@firefox7801]

Well, whoever built the great pyramids of gezza, knew the exact speed of light. And also the meter.

[@lucvl4557]

Worth mentioning : XVIIth century judge Fermat (of famous math conjecture fame) had the intuition that light takes the shortest route in TIME between A and B. This implies a finite speed. From this principle, Descartes and Snell independantly derived the law of refraction, fundamental to lens design.

[@beenaplumber8379]

My high school physics teacher explained Foucault's method to me in 1983, and it was my first experience of finding out that something that seemed unknowable to mere mortals like me was actually a simple matter to explain and understand. I had that same experience again as an undergrad when I asked my biology prof how we knew about transmembrane cell proteins. (The amount we seemed to know about molecular cell structure baffled me.) In about one minute he explained freeze-fracturing to me. It was so simple! So I went into research. I think people might be surprised how much they can understand if they just have the confidence to ask, and if they ask someone who is good at explaining things. Kids, ask your teachers how we know the things they are teaching! (But don't be a snot about it... that won't help.)

[@ptgr12]

It’s all incorrect knowledge, and should be retracted. Roemer would never witness a delay in anything, because he was using a telescope.

[@beenaplumber8379]

@@ptgr12 People who have dedicated their lives to studying this stuff over the past few centuries, and who know a lot more about it than you do, overwhelmingly disagree that it's all incorrect. Odd are that there's something about this that you don't understand. That's the most likely explanation for your outlier opinion. You could be right, of course, but I think it's exceedingly unlikely.

[@ptgr12]

Jesus Christ. You believe it? Isn’t that the one with the beam travelling 8 kms before electricity was even a thing? Did you hear that? Process it? It’s like you’re all brain dead.

[@ptgr12]

@@beenaplumber8379 I am right. 100%, and all it takes is a little common sense, which is something lacking everywhere. Obviously.

[@beenaplumber8379]

@@ptgr12 Common sense is never common (varies by cultural & personal differences), and it is not useful as evidence. So far the only case you've made is that your brain worked this out in a certain way that makes sense to you, but no one else. But you're 100% right. Science doesn't work that way. Maybe science isn't your thing?

[@xaplomian7294]

Timestamps for each speed of light experiment Ole Romer 1:04 James Bradly 5:12 Hippolyte Fizeau 7:53 Leon Focault 12:15

[@dalitnahipehlehinduhu6569]

Vedas said first

[@mobashwerabegummrs.7857]

Thanks

[@dwtalley]

Wow. At age 65, I am suddenly a high school student studying a subject that teenaged-me dismissed as boring. Thanks for that!

[@AlexThompson171]

Hi Paul, thanks for another fantastic video! I think there is a slight error in your explanation of Romer's data... the apparent period should depend on the relative velocity of Jupiter/Io and Earth, not upon their distances (as suggested at 3:05). This would mean the apparent period is larger when earth is moving from J to K, and smaller from F to G. Consider: if Earth were *stationary* at K, the apparent period would = true period, despite light taking longer to travel a greater distance. It seems analogous to the Doppler effect: when source and observer are moving apart, the apparent f decreases (hence apparent T increases)

[@PhysicsHigh]

Thanks Alex. I stand corrected.

[@avnertishby]

@@PhysicsHigh This part of your explanation bothered me too. I'm not sure if the changing relative velocities between earth and Jupiter were enough to account for a large enough time difference - it's not difficult to do the math and check what that time difference would be, so I could be wrong. But a simpler interpretation of the story doesn't require this: If we assume Roemer established the approximate period over several measurements taken while Earth and Jupiter were closer to each other, all he had to do was mark his calender at equal intervals into the future (with each interval being the period he observed) and then look at Jupiter when those times came. So if for example the measured period was 45 hours, he'd mark his calender at 45 hours in the future, 90 hours in the future, 135 hours etc. for several months ahead. If he came back and checked he'd discover that for the near future his predictions were reasonably accurate, but later in the year they were several minutes too early. Similarly if he started out determining the period when Earth and Jupiter were far apart and then marked his calender at equal intervals he'd find that after a few months those predictions were too late. This seems to me like a reasonable interpretation of what happened, based on the familiar points of the story. Edit: also @14:55 please correct your units. The first number should be in m/s.

[@PhysicsHigh]

...and I am working on a fix.

[@PhysicsHigh]

Here is my correction: kzbin.info/www/bejne/inKsnaatmM1ompI

[@justinclark3199]

THANK YOU SO MUCH I WAS SO CONFUSED AS TO HOW HIS EXPLANATION WOULD HAVE WORKED

[@marksimpson2321]

A fabulously clear explanation of how Roemer calculated the speed of light! Worth it for that alone! Ty

[@robertgoss4842]

Excellent presentation! Easily one of the best I've seen on the subject. I'm just a hack layman on physics, but I do relish videos like this. Thanks a million!

[@PhysicsHigh]

That’s great feedback. Thanks. I do hope you share.

[@TiniDarer]

This is amazing! This video definitely deserves more views! Thank you.

[@morley7584]

This video deserves more attention. You guys really helped me with my uni report on thr measurment of light speed

Good watch after videos about the Maxwell's equations and his realisation that light is but an electromagnetic wave.

[@WitchingNumbers]

It's appreciated that this is a history/basic concept video , yet an introduction to the measurement being relative to Earth normal space with time dilation would be appropriate.

[@dqvissmyph2968]

Thank you for great video. We learned about the early scientists at school, and I was amazed, and still am, at the intellect and the determination of Rømer, Bradley and Fizeau, and many others, using basic astronomical equipment etc. I mean, if I'm correct, Fizeau's mirror was five and a half miles away, and he was using something like an oil lamp or a candle. I know there was not a lot of light pollution in thise days, but still!

[@CrochetIsLife54]

My first thought was about the attempt to measure the “aether” by Michaelson and Morley. I also thought of the slit experiment which shows that light behaves like both a wave and a particle. That always fascinates me.

[@profdc9501]

This video is such a good explanation that you appreciate the genius of the scientists who study nature and become more aware of our universe. Now if we could have an explanation of quantum entanglement and quantum measurement like this, that would be brilliant. But the finest minds are still pondering what quantum mechanics means.

[@JustsomeSteve]

Found a small mistake: At 14:54 It's 299,792,472 m/s not km/s. Otherwise, awesome video! I want to thank you for it!

[@josenriqueha]

The real reason is as fallows. As the Earth and Jupiter move in their orbits there are periods when the distance between them becomes smaller, they are closing to each other, and there are periods where the distance becomes larger and larger. In both situations you see Io starts an eclipse and measures the time when it is visible again. In the first situation the light had to travel a shorter distance that in the second situation, so the eclipse is "shorter" than when the planets are moving away from each other. So the important factor is the relative movement between Earth and Jupiter.

[@adrianpjones]

Exactly right

[@adrianpjones]

Such a fundamental mistake seriously reduces the credibility of the video.

[@mr.e7379]

jUST CONTENT!!!! nO ADS??? I have GOT to support you good sir.!!

[@nugget4life420]

Our teacher loves to use your videos

[@ripsumrall8018]

In an episode of the television series Bonanza ("Look to the Stars", broadcast March 18, 1962), Ben Cartwright (Lorne Greene) helps the 16-year-old Michelson (portrayed by 25-year-old Douglas Lambert (1936-1986)) obtain an appointment to the U.S. Naval Academy, despite the opposition of the bigoted town schoolteacher (played by William Schallert). Bonanza was set in and around Virginia City, Nevada, where Michelson lived with his parents prior to leaving for the Naval Academy. In a voice-over at the end of the episode, Greene mentions Michelson's 1907 Nobel Prize. I remember this one. A cowboy show with physics!

[@daleeasternbrat816]

The speed of light was a theoretical thing in those days. Nothing to do with everyday life. Now, the speed of light is factored into z lot of technology we use every day.

[@ripsumrall8018]

@@daleeasternbrat816 I know that!

[@MegaLokopo]

Why do we pretend the speed of light is a constant when even in a vacuum in the best conditions we have access to the actual number calculated varies drastically?

[@petefluffy7420]

They set it going and after that no one could catch up to it to turn it down. Always been that way, and forever will be.

[@webjammer1]

A simple way to test the speed of light is to try and open the refrigerator door before the light comes on.

[@rudolphguarnacci197]

It's actually faster because it's cooled. And everyone knows light travels faster in a vacuum cleaner and a refrigerator.

[@stefanc4520]

So if I open my fridge I could somehow deduce the exact speed of light? Lol

[@OmniGuy]

Or hit the light switch and be in bed before it gets dark.

[@ColinWatters]

Get through the road junction before you see the red light.

[@jpapan1]

This is fascinating. Have no idea if all is true or accurate...but really curious who is giving this a thumbs down...but more importantly...why? Is the info not good? Do they not like this guy? His presentation? What? I've never thought of this question before...but find this a great place for me to start.

[@PhysicsHigh]

thanks for the support

[@wplg]

Light-Speed is measured by the around trip a+b=c. Take away "a" light speed speed will change.

[@rushilpatel7418]

This was extremely interesting... high school science should also include the brilliant ways our ancestors made their discoveries

[@tanishmalik9807]

There is the chapter in 10 standard ncert book . If you study you will know .😅

[@basudevsamantaray2363]

God.....still boy you don't get enough from Newton's apple????😮😮

[@user-vk3sf2qp4d]

Excellent! A bit closer to understanding the speed of light! Thank you!. Absolutely fantastic! Thank you so much. Keep them coming..

[@robertsynclair9046]

excellent video, thank you for investing your time and effort on this. I am sure that with time this will become a very popular channel.

[@PhysicsHigh]

Thanks. Hope so

[@JoeBlowUK]

Except the speed of light c is really the average speed over a round-trip journey, and we cannot be certain that the speed is the same in both directions.

[@mpolier]

Excellent! A bit closer to understanding the speed of light! Thank you!

[@damienbull8160]

When I watch this program I understand the great minds and brain power that these individuals possess. These are truly remarkable individuals and they were working from the foundation that was laying down thousands and thousands of years ago by their ancestors. And when I say Foundation I mean genetically.

[@josepacheco4093]

Good job. Very nice explanations. You provide the right insight without going into much detail.

[@bill.strain]

It astounds me that back in the horse-and-cart days, before steam engines, before electricity, when timepieces were mechanical, that there were people with imagination so bold that they would set about to determine the speed of light.

[@BobtheScienceGuy]

very nice video, I've been getting questions about this and may adapt it for my channel.

[@lordgarion514]

Considering that the cog was literally handmade, the accuracy was astounding TBH. His friend was an absolute master.

[@hvymettle]

Curiously, if you take the speed of light at 299,792,458 m/s and perform gematria on the number triplets the result equals 1. 2+9+9=20, 2+0=2. 7+9+2=18, 1+8=9. 4+5+8=17, 1+7=8. 2+9+8=19, 1+9=10, 1+0=1. If we perform gematria on the whole number the result is magic number 9. Add 9 for the whole number and 1 for the triplets and we get 10 which adds back to one. Don't know what any of this means but there is music in that number.

[@PasajeroDelToro]

That's meaningless.

[@fa6805]

Suggestion. Please put captions. It was kinda hard to hear the names of the scientist you mentioned.

[@hariprasadreddy108]

Its simple ele vara#$&*"&eee

[@PhysicsHigh]

Captions are available - you just click the caption icon

[@janedoe5229]

click the little "cc" button in the lower right of the video screen.

[@kimmariager3420]

Please take a look at Ole Rømer's wiki, if You're not already familiar with this amazing man. I promise You will find it worth Your time.

[@videovideo4587]

The celestial and planetary experiments don't appear to factor in the movement of our solar system through space (~500k mph), or the motion of the stars either. With those factors taken into consideration, the actual distances involved would be different, greater or perhaps less, depending on the relative direction of travel.

[@mariorqmsilveira3270]

14:54:..... this align so much with the speed of light (events) that Maxwell could state light was a form o electromagnetic wave! To me, in this resides the greatness of this video! Thanks!!

[@howardcallahan6692]

I love this video. I imagine the series is very interesting. Thank you for doing this. I have a question about Romer's experiment. I'm pretty sure that the difference in time was about 35 seconds, but NOT longer at K and F than at L and G. Rather, L and K were about the same and F and G were about the same; however, L and K were about 35 seconds longer than F and G. This would be because on the F and G side the Earth was speeding TOWARD Jupiter while on the L and K side Earth was speeding AWAY from Jupiter. The 45-hour orbit allowed the Earth to cover about 2 degrees of the Earth's orbit, which is a significant distance--requiring about 17 seconds of time for light to travel. So on the F and G side the 45 hours is 17 seconds shorter; on the L and K side the 45 hours is 17 seconds longer.

[@PhysicsHigh]

thanks Howard, you are correct and my error, I am working on a fix

[@PhysicsHigh]

Here is my correction: kzbin.info/www/bejne/inKsnaatmM1ompI

[@user-ky5dy5hl4d]

@@PhysicsHigh I think you have a mistake in the video. At 14:54 min., you have 299,792,472 km/s. I think it should be meters/second not kilometers per second.

[@ramadash2472]

really good information which is not available at text books👌

[@bobhoward6750]

Excellent video, thank you, I now have a better understanding of the history. An interesting consequence of a finite speed of light; as demonstrated by Olaf Romer, the observed orbital period of Io slows down while Earth moves away from Jupiter (i.e. from L to K in the diagram at approximately 3 minutes in). It is interesting because, in an expanding universe, distant galaxies are apparently moving away with a recessional velocity dependent on the distance from the observer. Therefore the further away the galaxy the slower the observed rotational velocity relative to its actual rotational velocity. At the Hubble Sphere (HS), the galaxies should appear stationary. Galaxies beyond the HS should appear to rotate in reverse time order; if the recessional velocity is a real movement. I conclude that the movement is fictitious and that the redshift-to-distance relationship is due to a cosmological dilation of relative clock time over absolute time whose rate of change is inversely proportionate to one another. This produces a scalar model where time and distance change interdependently, keeping the speed of light and other constants of nature constant as the universe evolves. I would love to discuss this with someone better qualified as it may explain the nature of dark energy, and dark matter, and why we are seeing the most distant galaxies with the JWST that appear too mature for our current theories.

[@funlover1977]

Thank you for the video. However, I'd like to share some important fact at 14:45 if I may. James Clerk Maxwell wasn't calculating the speed of light. He was calculating the speed of how the 'elecrto-magnetic' force is propagating throughout space according to his new-born theory. It was only after he came up with the result and compared it to what was already known, and this way, he become the first man on planet Earth to realise and undarstand that light is 'just' electro-magnetic force. It must have been an important day in his life.

[@acmefixer1]

For his experiment, Michelson built a hollow tube about a meter in diameter and about a mile long in what is now Irvine, California. It's part of the history of Southern California.

[@colt4667]

The tube (a pipe) was pumped down to a vacuum. Does it still exist?

[@Whiteyy191]

I have a couple of questions: how did fizeau determine the rotating speed of his wheel and how did he maintain a constant speed without an electric motor?

[@2869may]

It's all Bullsh!t....!

[@2869may]

And what was use as a source of light at that time....?

[@graemetho9805]

This wasn't taught when I went to high school, and I have always wondered how they measured before modern technology

[@andywander]

How does being further away from IO make the period appear longer? Wouldn't the start of the period be delayed just as much as the end of the period?

[@adrianpjones]

Exactly right

[@PhysicsHigh]

Did you see my errata?

[@robertgoss4842]

Excellent! A fine blend of mathematics and ordinary human language. Well done! I quickly subsccribed.

[@jakekp4739]

I kinda understand and get how mechanical waves move forward but how does light or any electromagnetic wave move? Or what does make it move and with that kind of velocity? All from that I’m just blown away with this universe🤯 WOW

[@vtbn53]

It moves because a change in the electric field causes a change in the magnetic field, that change in the magnetic field then causes a change in the electric field and so on. The rate of change of of the electric field is determined by the universal constant the permittivity of free space, and the rate of change of the magnetic field is determined by the universal constant the permeability of free space, together they determine the speed at which light travels. No one knows why free space has these values, it just does.

[@sandmonke21]

Damn. This really is good, physics is one of the most interesting things in life.

[@acmefixer1]

@Creed Follower **Physics is everything**

[@sandmonke21]

@@acmefixer1 pure facts

[@daleeasternbrat816]

I have had to explain the speed of light to kids and older people before. From now on, I will send them this video.

[@2869may]

How did Fizeau set up a 4,315 meter experiment? what was the light source? what produced the constant rotation of the cog?

[@colt4667]

Good questions, Nick. And how were the revs per second of the wheel measured with accuracy?

[@2869may]

@@colt4667 Right, What kind of time instrument could record a speed of 186,000 per second at a distance of 8,630 meters...? That would be in the 1,000ths of a second a if not 10,000ths or 100,000ths.... I don't believe anything...! LOL

[@olegyamleq7796]

WOWWWWWW!!!!!!!! This was brilliant !!!!!!!!!!! Thank you. Excellent explanation !!!!!!!!!!!!!!!

[@PhysicsHigh]

Thanks!!!!

[@archimedesspiral]

Good explanation and genius experimenter

[@KokkiePiet]

Christian Huygens was a brilliant guy, sadly even in the Netherlands he is underestimated

[@PhysicsHigh]

As a fellow Dutchman, I agree

[@tehleelmir4407]

thanks -> to all those people

[@bpolat]

Wonderful content. Thank you very much for great video.

[@backnineblues5843]

There were so many brilliant scientists who toiled in relative obscurity. Of course Maxwell was famous but I had never heard of these other gentlemen.

[@samk6042]

Thanks sir! Will u be releasing more videos for physics hsc syllabus module 7? Really wish u were my physics teacher at school!!

[@PhysicsHigh]

Yes. I have already quite a few videos for module 7. See the playlist, but there is more in development too. Make sure you are subscribed and click the bell. Thanks for the encouragement and share with your peers

[@lambda4931]

The angle of the reflection needs to be divided by 2. If a mirror rotates 5 degrees the mirror’s reflection will move 10 degrees.

[@randallblake1213]

Excellent video. Subscribed!

[@jamesbarringer2737]

This is brilliant.

[@andreysavin1931]

All these men had great minds, wow

[@alexeyl22]

More rigorous explanation of James Bradley experiment would be appreciated. Everything with thorough details of wave vector directions, animated diagrams of stars, earth movement. I know it is hard work to be thorough with this )). Maybe someone has already done it?

[@greggweber9967]

About a year ago, I saw a KZbin video saying that because of the new way of measuring the Speed of Light, we wouldn't know if it changed. Can we detect any change?

[@rickintexas1584]

I wish I had these videos when I was in high school. I’m 58 now.

[@Overitall805]

Has anyone measured the 1 way TOF for light? Also, how does relativity play into it and... in the 2 way TOF, is the return speed the same as the send speed? Essentially , how do we Actually know if the speed remains constant between send and return?

[@stewiesaidthat]

They don't know. All their physics and theories fall apart unless light speed is constant. Which it isn't.

[@videovideo4587]

The celestial and planetary experiments don't appear to factor in the movement of our solar system through space (~500k mph), or the motion of the stars either. With those factors taken into consideration, the distances would be greater.

[@BadPennyDogBoy]

Great video. But what happens if it turns out that light travels at different speeds coming back from the mirror to that going towards the mirror? These measurements assume that the speed is constant in both directions, and measures the average speed!?

[@PhysicsHigh]

🤓 have you been watching Derek Muller’s video?

[@DoggosAndJiuJitsu]

False. We have never determined the speed of light. We have only determined twice the speed of light divided by two. There’s no way (yet) to measure it in one direction or (yet) eliminate the possibility that some unknown universal constant has light moving at different speeds in different directions.

[@MostlyPennyCat]

The "and then he calculated the frequency" bit is disappointing, frequency of what? How? How did they know when the light was passing through the gap but hitting a tooth? How did they know when light just hit the tooth outbound?

[@thalesnemo2841]

Excellent video! Clear and concise.

[@walterkersting9922]

I find it interesting that it comes out even money; 300,000 km a second 671 million mph, not a nano particle more or less.

[@whirledpeas3477]

Wrong, Google the speed of light 😮

[@burntsider8457]

Interesting topic well presented. Refreshing to view a STEM video without puerile inanity.

[@davez4285]

Excellent video, thank you! I have a question : James’ method demonstrates the speed of light is relative to earth’s motion. If every two years or two turns, the angle will be twice. If the reference frame is on earth, the light speed c, will be a function of the speed of earth. Why we think the speed of the light is considered as constant regardless of reference frame?

[@okaro6595]

The speed of light is same relative to all observers. Of course he could not know it.

[@behnamashjari3003]

Great video. More than high school level.

[@ProProboscis]

2:33 Hi, Please if you have a minute: why around H, Jupiter would get in the way? Doesn't its natural satellite end up on one side or the other of the orbit where it is visible again?

[@cmillerg6306]

Didn't Galileo conduct a SOL experiment? It involved two distant observers each with covered lanterns. Galileo correctly realized that the finite human reaction time obscured any short or zero light delays

[@IBITZEE]

Nice video... thanks for your effort... I did not know the period of IO was so short... ~45h Nevertheless the period observed should be the same... if the observer are closer... or distant from Jupiter... I can only see how this values would be of a perceptible magnitude... if there was a significant distance difference between the start and the end of the observation of a IO period (not only ~45min) The ~80º arc or Earth orbit you mention between points in Ole Romer diagram is done in roughly 3 months,,, About: c=2L/t of Fizeau t=2L/c of Foucault Could you please elaborate on why 'c' was chosen for the name?

[@PhysicsHigh]

c stands for celeritas, which means swift in Latin .

[@GeorgeSPAMTindle]

@@PhysicsHigh Which is also where we get 'acceleration' from, it means an increase in swiftness.

[@GeorgeSPAMTindle]

@MichaelKingsfordGray Accelerate comes from the Latin word 'accelerarre' which is derived from the Latin words 'ad' (meaining 'towards') and 'celere' (meaning 'swift'), so the literal meaning is 'to move towards swiftness'.

[@AnujKumar-sx6ws]

and delay in second experiment is also because earth is having elleptical path . so it would take longer time in one part of orbit and small amount of time in other part of orbit which seems as delay of light reaching

[@boonteetan4448]

At one time, physicists looked for ether that was supposed to be the medium for light travel. They could not find it. Now one would suppose universal permeating dark energy could be the medium that enables light to travel so fast.. (?)

[@edwardmartin6052]

Ponder this: let's say you have a lever with a distance of 8 light minutes in length (about the distance from Earth to the Sun). The opposite end of you pivots. If you move (assuming inertia is overtaked) the lever for 10 seconds. The wave is 10×c ≈ length traveling light speed to the pivotal point?

[@schmetterling4477]

Displacements propagate in mechanical systems at roughly the speed of sound. They don't travel at the speed of light.

[@edwardmartin6052]

@@schmetterling4477 l did some research on this. I think your right. The distance from the sun on average is 94,343,000 miles. The speed of sound through iron is 767.269 miles per hour. With a wave length about 32 miles, it will take about 14 years for the movement to complete. Hahaha

[@schmetterling4477]

@@edwardmartin6052 Sounds about right. ;-)

[@astrospeedcuber]

This is an amazing channel

[@PhysicsHigh]

Thanks

[@outlawzgosu]

What software is being used here? It looks like all of those are object you drag with your mouse and not just animations.

[@okaro6595]

2:53 How did that affect the period? It would affect when they occurred in some absolute clock but not the period*. In fact the idea was to create such an absolute clock independent of earth's rotation for navigational purposes. Finding the speed of te light was a side effect. * now of course there has to be a tiny effect on the period but it is way way too small to detect. It is like if a clock is one second too fast a day you cannot detect it during a minute, but wait 30 days and you will detect it as the minute changes at a different time.

[@theo3000]

The explanation in the video misses the mark. To an observer on earth it is the time BETWEEN eclipses that changes. The duration of each eclipse remains constant, as you and Galileo made clear.

[@deinauge7894]

at 3:10 there is a glitch in the argument? because the measured orbit time is independent of the distance - it depends on the speed of earth relative to jupiter. therefore it appears shorter between F and G, longer between L and K

[@PhysicsHigh]

Did you see my errata?

[@deinauge7894]

@@PhysicsHigh no, sorry to bring this up again

[@philo5923]

Absolutely fantastic! Thank you so much. Keep them coming.

[@PhysicsHigh]

Thank you! Will do!

[@meibing4912]

Did the Fizeau experiment in High School Physics. Could drive you crazy setting it up right. A sneeze and it was ruined.

[@PhysicsHigh]

that would be a challenge. Did you get a credible result?

[@meibing4912]

@@PhysicsHigh not really. It was too difficult to get a stable setup. I'd call the results very "biased" towards merciful interpretation of the facts.

[@daved3494]

Excellent overview of the early days of establishing the value of C. Really well done.

[@PhysicsHigh]

Thank you Dave

[@erickrajan7401]

How does different positions of earths orbit produce different periods for io? Although light takes longer to travel, this will just delay when we first see the moon (start of period) and when it finishes its orbit (end of period). But won’t the duration of the period itself remain constant ?

[@josenriqueha]

Erick + You are right. A lot of YT videos give the same reasoning that this one, but there are videos where they give the correct reasoning. As the Earth and Jupiter move in their orbits there are periods when the distance between them becomes smaller, they are closing to each other, and there are periods where the distance becomes larger and larger. In both situations you see Io starts an eclipse and measures the time when it is visible again. In the first situation the light had to travel a shorter distance that in the second situation, so the eclipse is "shorter" than when the planets are moving away each other. So the important factor is the relative movement between Earth and Jupiter.

[@erickrajan7401]

@@josenriqueha right yea this does make sense. Thank you

[@josenriqueha]

@@erickrajan7401 You're welcome. HighSchoolPhysicsExplained gives this explanation in the link that appears at the end of this one.

[@wWvwvV]

So the definition of the length of the meter now depends on the speed of light. And the speed of light depends on the length of the meter per definition. Why wasn't it possible to define the speed of light to 300,000 km/s? The original definition of the meter was a percentage of the distance from an earth pole to the equator. I'm sure the historical measurements of that distance had a big error span that would allow a 300,000 km/s definition to stay consistent with old publications.

[@christobotha5378]

Major issue I have with topic. Does the light travel that fast or is it the speed of illumination that travels that fast. Light must have a certain value of power, strength or intensity. Does this mean the speed of light is variable, but the speed of illumination is the same. Speed of lght using planets is not scientific. Is the universe not expanding? How do they have exact distance to planets?

[@lancearmada]

So the hippolyte gear test worked by testing frequencies of moving the cog until the light passed theough the cog but did not come back. How did he confirm this though?

[@schmetterling4477]

He looked at it. ;-)

[@gorkhe8602]

There's a verse from Sayana when we was explaining a verse from Rigveda in the 1300s that says light travels 2202 yojanas (8-9 miles) in 1/2 nimesha (6/25 secs). Im not buying these numbers because these values might have been adjusted later to match the speed of light but the verse is old and I figured when the British colonized India they decided the value of yojana to be about 5 miles and its argued over the years based on old texts. Kudos to all the physicists who did experiments to find the precise value over the years but it's interesting that an old scripture has something like this even considered.

[@DutchZippy]

Excellent video, a difficult concept explained really well.

[@PhysicsHigh]

Glad it was helpful!

[@BBQDad463]

Thank you for this excellent presentation of the history of the speed of light.

[@PhysicsHigh]

You’re welcome. Please do share

[@syb36]

I missed Morley!

[@GerardHammond]

Wonderful. Subscribed immediately

[@PhysicsHigh]

Awesome, thank you!

[@kimba381]

Nice. Should have started with Galileo, though.