How did we measure the fastest speed there is? | The History of the Speed of Light Part I

The speed of light is 299,792,458 meters per second. That's an incredibly fast speed. But how did we even figure that out in the first place? And how do we even know it that accurately?
Part 2 - Why can’t anything go faster than the speed of light? • Why can’t anything go ...
My phases of the Moon necklace is from Eclectic Eccentricity, who have a great selection of space-themed jewellery: eclecticeccentricity.co.uk/pr...
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More details on measuring the speed of light (or really, the length of a meter!) with your microwave: physicamechanica.wordpress.co...
Here are links to the papers mentioned:
Rømer (1667) - gallica.bnf.fr/ark:/12148/bpt...
Huygens (1677) - gallica.bnf.fr/ark:/12148/bpt...
Newton (1704) - gallica.bnf.fr/ark:/12148/bpt...
Bradley (1729) - royalsocietypublishing.org/do...
Fizeau (1849) - onlinelibrary.wiley.com/doi/a...
Weber & Kohlrausch (1856) - onlinelibrary.wiley.com/doi/e...
Maxwell (1864) - upload.wikimedia.org/wikipedi...
Essen (1950) - www.jstor.org/stable/pdf/9843...
Evenson et al. (1972) - journals.aps.org/prl/pdf/10.1...
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📚 "Space: 10 Things You Should Know": bit.ly/SpaceDrBecky
📚 US & Canada version: "Space at the speed of light" (same book, different title): www.penguinrandomhouse.com/bo...
📚 German translation "Das kleine Buch vom großen Knall" : www.dtv.de/buch/becky-smethur...
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📹 Dr. Becky also presents videos on Sixty Symbols: / sixtysymbolsand Deep Sky Videos: / deepskyvideos
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👩🏽‍💻 Dr Becky Smethurst is an astrophysicist researching galaxies and supermassive black holes at Christ Church at the University of Oxford.
drbecky.uk.com
rebeccasmethurst.co.uk

[@DrBecky]

Sounds like I had a brain typo at the beginning of this video and mixed up the digits when I quoted the speed of light! It should be 299,792,458m/s Also, we didn’t define the speed of light as 300,000,000 m/S because then the dimensions of everything around the world would have changed!

[@mr51406]

Thanks for solving my “What if”! ☮️❤️⭐️

[@macgyveratlarge2133]

I was taught that is was 186,000 mi/sec in a vacuum. I simply went from there, since it still uses a common standard. Conversions from that are fairly easy.

[@Mister_Pedantic]

@@Paygelove Standing on the shoreline of Lake Ontario here in Toronto, I can see the curvature.

[@LeonardLeon]

​@@Paygelove Actually the coriolis effect can be measured. There is actually a formula to calculate the coriolis force strength. Sailplane pilots and meteorologists can show it to you. They can also explain how the atmosphere moves even with the coriolis force. But this is a half hour discussion with somebody smart. You will need at least 5 hours.

[@omanshsharma292]

mam u made my day

[@neuvocastezero1838]

That parallax star field animation is super cool. Also, "The meter is something we humans have invented as just sort of like... a yardstick." Classic.

[@allangibson8494]

And the meter exists because the French couldn’t agree on how long a “yard” was. Every town in France had a different length yard, inch and foot (with different numbers of inches in a foot as well)).

[@pimwillemsma9547]

The fact that Rømer actually drew a face on the sun in his diagram, brings a smile to my face everytime I see it.

[@barthvapour]

Good ole Romer!

[@shadowprince4482]

I'm a total science nerd and somehow I never heard about the Jupiter/Io experiment. At first I thought it was completely stupid to put it lightly then I was massively humbled when I saw how it was done. It was possibly the most brilliant thing I have ever seen!

[@I_dreamed_my_name_was_Brandon]

I made a joke about this while debunking these "Scientists". I did it sarcastically. To know they actually did this just makes it better.

[@antoniobragancamartins3165]

Dr Becky, I'm Brazilian man, I'm not a physic and I'm not an astronomer, I'm a telecommunications specialist, but I'm an enthusiast with space, and i want to say you explain in an way that we understand perfectly. Thanks!

[@Bollibompa]

P H Y S I C

[@pythondrink]

Even if you didn't mention you were Brazilian, I would have still figured out that English wasn't your native language bcoz your English was quite formal. Btw, you clearly meant _physicist_ .

[@DarkMatterBurrito]

I'm probably crazy, but it seems like using a meter to define the speed of light but then using the speed of light to define the meter is circular reasoning in a way.

[@aidanjt]

I love these astrophysics history episodes, they're fantastic.

[@valeriobertoncello1809]

They are

[@rachel_v_k]

I agree.

[@RezaRob3]

I loved watching that. Thank you Dr Becky Smethurst.

[@michael.forkert]

aidanjt One of the meanings of FANTASTIC is: BASED on FANTASY. Another meaning for FANTASTIC is UNREAL.

[@rachel_v_k]

@@michael.forkert So, therefore... What? Were you going to say something?

[@samrodian919]

You have just delivered the most entertaining physics lesson I have ever heard! Thank you Dr. Becky.

[@zachzorn9930]

Looking at a smart and pretty woman makes it much more entertaining

[@Insightful_Truth]

@@zachzorn9930 Who is smart and pretty ?

[@zachzorn9930]

@@Insightful_Truth dr becky.

[@BotanyDegreePilkerton]

she lied right TO YOU FACE lol moron

[@PatricRogers]

My 11yo daughter and I have long shared a love of astronomy, and we enjoy your channel together. She recently started sitting in the front seat of the car with me, and she *asks* me to put on your channel while we drive, instead of playing music on the radio, or music videos. Thank you for being awesome and making such incredible science and the amazing beauty of the complicated real life universe fun and kid-friendly.

[@Jackie-wn5hx]

That's wonderful to hear. Parent and child both interested in astronomy is great. My dad and I both loved the Star Trek franchise, but that doesn't count as science.

[@kayinoue2497]

Your excitement for science and sharing it with others is incredibly endearing and such a joy to see. Thank you for what you do.

[@DrBecky]

Thank you Kay for your kind words!

[@garytyme9384]

Pity that she is incorrect.

[@sethtenrec]

@@garytyme9384 troll any?

[@garytyme9384]

@@sethtenrec Hardly, just facts that this "Dr" will not engage with.

[@AndrewMerts]

At 2:07 there's a note that there should have been 11ms of delay between observations. That figure is off by a factor of ~2000. 11 light-milliseconds is equal to 2049.1 miles. The actual delay should have been only 5.37 microseconds.

[@bottlekruiser]

I think it's a rounded round-trip delay. So it's off by exactly one SI prefix

[@AndrewMerts]

@@bottlekruiser Yeah, they made two mistakes. They used the round trip time, which is wrong as it should only be the one way delay, and they also swapped microseconds for milliseconds.

[@Geosearchef]

I scrolled way to long to find this :) .

[@OrcinusDrake]

Thanks I just calculated myself cause that number looked very wrong lol.

[@AWildBard]

This is the kind of question I've always asked. How do we know what we know? Love this video Can't say I understand it completely, but I guessed there was a process, and this is a great history / science lesson.

[@planetyouranus22]

When I was just a wee lad, still at school. Many many years ago ( I'm 74 now) The speed. of light was a given at 186,000 miles per second. I haven't yet caught up with metres and kilometres and the like. I doubt I ever will! But I do enjoy your videos Dr. Becky. You do have a knack for explaining complex scientific matters in the simplest of layman's terms. Which helps an old geezer like me to absorb some fresh knowledge. So you see!....You really can teach an old dog new tricks!

[@wayneyadams]

I don't know where you live, but the United States is right there with you. They tried to force us to change in the 70s, but failed so we still use inches, feet, yards, miles and so forth.

[@stevegladden1364]

Microwaves (ovens) use a free running oscillator (magnetron) which varies in frequency quite a bit. Also the measuring stick that you grab out of the tool box to measure the ... Yes you get the idea and we certainly enjoyed the humor in this part of the video! Also enjoyed the smores very much so.

[@stevegladden1364]

I think I'm craving a toroidal snack.

[@theCodyReeder]

I’m going to make my own measurement system in which distance units are defined by how far light travels in one hyperfine transition of a ceasium atom.

[@DrBecky]

You can name the unit the Cody 👍

[@ozzymandius666]

@@DrBecky I'll stick with c=G=h=1. I'm into theoretical physics.

[@EnglishMike]

I prefer marshmallows...

[@Gene_the_OG_Raver]

Cody, you are TOO funny... in still trying to find out where the light travels the slowest in our universe. I'm sure there are places where it drops down to just a few kilometers per hour, or even slower.

[@ozzymandius666]

@@Gene_the_OG_Raver Light slows down like that in some Bose-Einstein condensates.

[@issamedoot2864]

Thank you for having this video and this channel. You appeared in my recommendations list and your videos are just amazing. I have learned so much in such a short amount of time! I am currently in university and I became excited when I saw how the equations and math I am learning are being applied in this video. I look forward to seeing more of your videos!

[@DrBecky]

Thanks Eric 🤗👍 good luck with all your studies

[@andrewmullen4003]

Thanks Dr Becky, for the quote of the century, "the meter is like a yardstick" love it, works on many levels!

[@BenjaminGib]

I’m an editor and fascinated with the speed of light and love the study of it, and I’ve realized that Hell after death is actually editing this video.

[@Locut0s]

I’m amazed that Rømer was able to measure the speed of light as accurately as he was able to given the state of knowledge of the universe in the 17th century. To say nothing of the tools available at the time.

[@ericsonnen5248]

Locut0s truly amazing

[@kdog3908]

Yes! Much like Eratosthenes and Al-Biruni and their measurements of earth circmuference and radius. Very accurate given the tools at their disposal.

[@XrisD147]

These people have very logical minds.

[@Yora21]

That parallax animation is amazing. I didn't expect the effect to be that big.

[@manipunation]

Yes it was amazing! And unbelievable, as that means that when you see a star chart, you aren't even seeing an accurate representation of the positions of stars in the sky. You are only seeing an approximation. Either that or you are seeing an accurate representation of the stars in the sky, but for only one particular day of the year.

[@leapdrive]

Parallax angles are also used to measure the distance of stars in parsecs. A parallax second or parsec is equal to 3.26 lightyears if measured from the two edges of the earth orbit.

[@thesinistermobs1564]

Greatly exaggerated

[@jpdemer5]

@@thesinistermobs1564 Greatly indeed - it takes close examination of photos to see the parallax at all.

[@DrHealth4Life]

The music you were hearing in the background is from the Opera Carmen. You did a wonderful job singing and it it was highly recognizable.

[@meow75714]

Great video Becky! Thanks for sharing the knowledge.

[@ccchhhrrriiisss100]

This is an amazing video! Thank you for doing such a wonderful job explaining science at an easily understood level. I've been sharing these videos with family members (including my nephews and nieces).

[@Gam3B0y23r0]

Finally! youtube recommended an excellent channel! I even clicked the bell

[@theoysterman1]

Please take a look at PBS spacetime. kzbin.info/www/bejne/o6S5pnZ7qJ2MmtE

[@alwaysdisputin9930]

@@theoysterman1 oh god PBS Spacetime is often confusing with their explanations. DrPhysicsA is the best IMO. I like Dr Becky. I like ScienceClic. Everyone makes mistakes however. We must replace them all with robots ASAP. +++ REPLACE ++++

[@harshininarayanan5277]

Amazing and such a great reference video as we are studying a small part in the measurement of speed of light in optics. Our syllabus only covers Fizeau's method(which I couldn't comprehend at first but this helped me plenty😀)..and it was mindblowing to know that they've made so many observations and conducted so many experiments to reach this defined value. Great video as always, Dr.Becky! 😁

[@IVANHOECHAPUT]

I was riveted to my computer screen! Great presentation, but what impressed me most were the outtakes at the end. You are very intelligent, human and adorable!

[@dalexdba1]

Thanks so much for translating such complex subjects into readily-digestible English! I really admire your style!

[@4legdfishman]

WOW, I never knew until today, what humans went through to figure out the speed of light! Thanks for that. New subscriber!

[@LancePhillip212]

Yes, for all our self-made problems, we have some things to be proud of as a species too.

[@lorrinrodrigueshs-bcp192]

Yes, it was much easier for the non-humans to calculate the speed of those rascally little photons & neutrinos etc.

[@bengreatorex502]

This is a brilliant channel on YT. Rebecca explains these concepts as clearly as anyone could, in my opinion.

[@jimsandoval7633]

So much history in this video, I had to seat, watch and enjoy every Usec of it. It is amazing how resourceful those guys from way back them were. Thank your for putting all this information together for us. Your video makes astrophysics so amazing and interesting. We just witness over 300 years of history all the way to TODAY!

[@tadbeer3090]

Wish I had you as my Physics Professor, my major would be completely different now! Your videos are motivating & informative. Keep it up!

[@manifold1476]

Wanker

[@StasiSLG]

Hi Dr. Becky, happy to see another video from you. After I saw the previous one about a day of yours, I'm even more impressed you find the time to make these videos for us. So on behalf of everyone 'I hope': Thank you for your time and dedication, we greatly appreciate you.

[@wormfighter1]

I love how excited you are about your topic.

[@douglasstrother6584]

"The Michelson-Morley Experiment" ~ The Mechanical Universe #41 kzbin.info/www/bejne/f6HCm5eZdp2Zmdk A tour d'force in Experimental Physics!

[@followthetrawler]

"We as humans defined a metre as a 'yard' stick" - Dr Becky, made me laugh - hope it was intentional.

[@mrmarmellow563]

LOL Yea but yea BUT Were That yard Measured wit 'an YardFoot or an YARD-ARM LAD?? AYE;🙄 ... 😆😇🤓😎🇳🇿

[@fivish]

A Meter is approximately a Yard. The Kilometer is short of a Mile by 666 Yards!

[@georgeemil3618]

"Yard-stick" is used as a common phrase or figure of speech. If someone asked what the car's mileage was and the answer was 10km/l, it'd be awkward but still understandable.

[@chrisspere4836]

@@fivish hi, a yard is approximately 3inches longer than a metre

[@allangibson8494]

Particularly as a yard has been defined by the meter since 1959 (when the length of an inch was redefined to exactly 25.4mm).

[@captphobos7764]

Great video... you packed it all in so well that my head hurts in a good way, looking forward to part 2, thanks editing Becky :^)

[@oldkid8811]

excellent video Dr. Becky. one of your best IMO

[@tonyr.5111]

Thank you for the clear, concise explanation... Very ENLIGHTENING!!! 👍👏👏👍

[@frankies.canvas]

I love your videos! You are very concise and explain things so clearly. It's almost like you studied the physics of space to highest degree haha. You definitely earned the degree and a subscriber!

[@nimbusnation9584]

I really really love your sense of humor... And your video they are so straight forward

[@gazzyal9371]

I really enjoy watching your video's.. and your easy on the eyes too keep up the good work Dr.

[@Lusa_Iceheart]

Me: Makes an unclean-able mess in the microwave My excuse: A Physicist told me it was science.

[@lyrimetacurl0]

Did you do the plasma thing?

[@macgyveratlarge2133]

I've microwaved a considerable number of potatoes in microwaves, and never once poked a hole. Never had one explode, either.

[@thelopergizer2682]

Lusaceheart, I literally laughed out loud reading your post!

[@ImAllInNow]

I was today years old when I realized that Diagon Alley was a pun on diagonally. 9:03

[@keohi2539]

I was today years old when I realized they were saying diagon alley instead of diagonally

[@ProfRonconi]

@@keohi2539 As a non-native speaker, the difference between dʌɪˈaɡ(ə)nəli and dʌɪˈaɡnˈalʌɪ' was enough to make miss the joke for a few seconds. And in any case the joke was rather lame. When I explained it to my daughter she she just said a perfunctory "OK". Think of he potential of "magically" "incorporeally", "optionally", etc.

[@itsklein73]

my life is a lie

[@Timberwolf69]

@@ProfRonconi It's probably because we as non-native speakers don't have the same amount of slop in our english as a native speaker. If you speak Diagon Alley fast enough, it gets quite similar to diagonally, as the pronounciation tends to get less pronounced with increased speed.

[@blameitondanny]

I'm a non-native speaker and I quickly noticed the pun. Especially after seeing Knockturn Alley sounds a lot like Nocturnal.

[@coeur8042]

Thank you for presenting all the history about the measuring of the speed of light!

[@AC-bi3bz]

A very informative, entertaining video … your bubbly, enthusiastic presentation make it fun to watch! I am continually humbled by what some very bright people have discovered / researched / deducted in the past … before computers, internet & co. … standing on the shoulders of giants …

[@skinnyTheCat]

Thanks Dr for a wonderful and great video tutorial! You certainly explain difficult subject quite easily! :)

[@Magickaplays]

The Focus seems to like the PHD Certificate

[@theCodyReeder]

It’s a lot easier for the camera to pick out something with high contrast (text) to auto focus on. I use a page of text when focusing my high speed camera.

[@PieterPatrick]

@@theCodyReeder CODY!!! ...Great seeing you here. :-)

[@PieterPatrick]

I first thought it was in low resolution. :-) But than I saw the certificate.

[@sundhaug92]

She's slightly too far forward, so something behind her is appearing focused

[@tucatnev123]

Wow, Cody's is actually suggesting to have Kiss Band like make up for Becky and zebra onesie? Wow, just wow. So... ...let's see that, only for the observation of the autofocus , OC

[@mechmania8450]

Very good I like to know how they struggled in the old days to come up with things we now take for granted, great explanation.

[@isellburritos123]

Great explanation! Clear and concise. Funny edit error at 13:10, great to see your performance is solid in the outtakes ;) Love your channel!

[@attiajos]

I watched another video about the same subject, narrated by a man, and it was almost the same, word for word, including the rain analogy. Not sure who gets the credit here.

[@pythondrink]

​@@attiajosjust check who's video is older

[@uprightape100]

Thank you so much, Becky. That was lovely.....had to watch it twice.

[@jackjones3657]

This makes you realize how successive generations stand on the ground laid by those who have gone before.

[@Hal2718]

"If I have seen further it is by standing on ye sholders of Giants" - Isaac Newton

[@megaxenu753]

That's not necessarily true. The astrophysical methods of measuring light are very different to the electromagnetic ones. If you ask me zipping a laser around in a lab is typical lazy microwave meal 20th and 21st century cop outs.

[@megaxenu753]

@nuff sed the fact is they are measuring the speed of a laser and not the speed of photons emitted by the sun or any other star for that matter.

[@megaxenu753]

​@nuff sed you don't know that. you can't know that. one day people will look back at your belief and deem it as crazy as light being generated by your eyes.

[@megaxenu753]

@nuff sed as far as i understand relativity doesn't work well with quantum theory. there are lots of problems around subatomic particles that can't be explain because people can't accept that some rules we have created for ourselves might not apply. in the next few decades people will have to let go of those kinds of assumptions.

[@blessedheavyelements8544]

Thank you Doctor for the video/information and hard work! Best Regards and Best Wishes!

[@thatboyken]

I don't really understand the phsyics thing and I am probably too old to learn it but I love listening to your videos , keep it up because I am quite sure that you are giving some of the younger generation a reason to get involved .

[@michaelbauers8800]

Never to old to learn. I have been teaching my mom, who is over 80, how to play kenken. How to factor numbers. How to eliminate possibilities. She had the benefit of knowing Sudoku, but she's learning new ideas

[@ddsoco1]

I recently discovered your channel and just want to say how much I enjoy it, especially these historical overview episodes. They really are fantastic, even to a piano teacher with arithmophobia like me. Congratulations. I also like how you avoid using a soundtrack. Makes it easier to focus on the content. Cheers.

[@robertlozyniak3661]

Piano teacher? Arithmophobia? I thought music was just math in disguise.

[@ddsoco1]

Robert Lozyniak Yes it is, but it sounds *way* better.

[@jppitman1]

@@ddsoco1 Even the early talkie film makers began to use music to fill up the empty spaces. But, Dr. Becky is so animated and engaging that any underlying music would be distracting to her videos because she leaves no empty spaces whatsoever in her dialogue. Her abundant enthusiasm for her subjects is her "music".

[@TomLeg]

Brighter light, higher number f-stop, wider depth of field ... Dr Becky in focus wherever she sits. It's a concept to explore.

[@geoffgwyther7269]

Not ``wider``. .... ``deeper.

[@TomLeg]

@@geoffgwyther7269 is right, strictly speaking. Since the in-focus plane is a constant from the camera, I imagined myself with the camera on one side and the subject on the other, so the D.O.F. would be a width.

[@sergiowybo3662]

Lovely, fantastic, well explained, deeply grateful !

[@cherylmurten4202]

I really want to thank you for your explanations. I can follow what you are explaining very well. Nice! 😊💜😊🌌

[@davidmitchell9003]

I find it a little sad that guys hundreds of years ago were/are light years ahead of me in physics

[@wirenutt57]

Apparently so - a light year is a measure of distance, an incredibly huge distance, while you seem to be referring to units of time.

[@firstlast-fr1le]

@@wirenutt57 He is not wrong though, other than the accuracy of the statement. These guys were wicked smart. Way ahead of me or any of my friends / co workers. I try to have simple conversations with some folk and they can't even grasp the most basic concepts. I myself am not advance in mathematics by any means but i understand the basics - many folk even today just don't get it at all. With everything we know to be true most people are still science illiterate and all the info is there if ya just even bother to look.

[@KennethSorling]

Check out old Archimedes! He was the monster mind of the ancient Greek world-

[@whiteshadow1771]

ain't it the truth

[@christianheichel]

Those were the Einsteins and Coca-Cola's of the day we're just the off-brand dollar stores/John and Jane Doe's. lol.

[@jamesanderton8817]

13:11 Oooo Editing Becky is going to be mad.

[@Legen_Terry]

For a minute there I thought that I was in the matrix and just experienced some déjà vu.

[@wlan246]

@@Legen_Terry For a minute there I thought that I was in the matrix and just experienced some red déjà vu.

[@mutototokatanga]

Thank you Dr Becky, enlightenment much appreciated going beyond gravitational forces!

[@timblack6422]

I love that tune on the piano! Also, very fine lesson, Doc!

[@philipmartin2622]

Back in the 1960s I remember using a new ruby red laser, a spinning mirror, a tuning fork and a meter stick to measure the speed of light. We used the tuning fork to match with the frequency of the motor spinning the mirror and we used the meter stick to measure how far the reflected light shifted as the mirror was brought up to the tuning fork frequency speed in cycles/second. Some simple math gave us the speed of light which was remarkably close to the accepted value at the time. We then took a six inch machinist's ruler and the meter stick to calculate the wavelength of the ruby red laser. The machinist's ruler acted as a diffraction grating and we measured the first, second and third order diffraction lines on the laboratory wall. Crude experiments that gave very accurate results. I'm sure every physics major over the years has done some version of these experiments.

[@CybranM]

Oh, I never realized you have a youtube channel, you're one of the best guests on DeepSkyVideos! Thanks for making great content

[@billszentagotay8819]

This lady knows her stuff, and is very bright as a good teacher. Keep it up!!!

[@reymundoguiquing6054]

You are very brilliant Dr. Becky

[@AstroQuest1]

Thanks for explaining Romer's method in more detail. Your explanation is the first one I have seen that made sense to me. Cheers Kurt

[@dcterr1]

I recall measuring the speed of light with a laser and a rotating mirror in my physics lab in college. We obtained what I thought at the time was a very accurate measurement of 2.998 * 10^8 m/s. Pretty cool to know you can do as well with a microwave oven!

[@GREGGRCO]

Becky ! OMG, WE GET TO SEE HIS HAND WRITING ! AND WHEN TROUBLESHOOTING I HAVE TO TAKE NOTES LIKE THIS ! HIS NOTES LOOK LIKE THEY COULD HAVE BEEN WRITTEN TODAY !! THIS WAS SO COOL DR. SMETHURST ! THAT WAS REALLY STUNNING FOR ME. THANK YOU ! AND HIS SOLUTION ! PRETTY GOOD THINKING ! MATH IS TIMELESS. OR ALL ENCOMPASSING, ONE; OR BOTH.

[@tomklein3718]

The videos are fantastic. Keep them coming. I've often said if I win the lottery, I would return to college and get a degree in Physics. It is fascinating but the chance of making a living at it seems pretty small. I chose software. Back in 1980 there were few women in my Physics 101 classes. Glad to see you've made it your passion. Doubt you were alive in 1980, LOL. If you make it to the USA, Washington University in St. Louis is great place to lecture. I'll be the old guy slipping into the back of the hall to listen. It is rare that someone of your intellect can take the difficult concepts easy to understand. Thanks again.

[@JobBouwman]

It is so nice that Sophie Ellis-Bextor is now into Astronomy.

[@spyrilleedinlarge5319]

She had to. There was blood on the dance floor

[@I_Don_t_want_a_handle]

nah, can't be SEB as this lass can sing.

[@pa1275]

omg, I never thought I would be so engaged in a video. Thank you!!!

[@MichaelBristow137]

I really enjoyed this summary; thank you for producing it... 😃

[@runtrls]

I loved this video - really informative, with a lot more information about Roemer and others were doing than I've seen elsewhere, all the way through the increasingly fine measurements of the value (and my favorite part, the fact that the value comes from vacuum permissivity and permeability). I did get a chuckle at about 18:30 where Dr Becky is talking about the definition of a meter, about how us (sic) as humans had invented it as 'sort of like a *yardstick*, to say "this is a meter"'. :-)

[@EtzEchad]

I love things like this. The history of science is often as interesting as the science itself.

[@Smonserratm]

Why didn't we do ourselves a favor by defining it as 300000 km/s and have a slightly shorter metre?

[@eatthisvr6]

Because the whole metric system was based around water. 1m cubed is 1 metric ton of water

[@condorboss3339]

It would have been great if the SI measurements had been created from that in the beginning, but now so many other measurements derive from the original standard that we would have to reconstruct our entire system. (It isn't possible to 'fudge in' the old derivations because the tolerances were already finer than the ratio of 299,792,458/300,000,000.)

[@theCodyReeder]

Sergi Monserrat Mascaró because then property boundaries would have weird values. A square kilometer would now be slightly less. That would upset a lot of people.

[@denmaroca2584]

Because it would cost an absolute fortune and cause years of chaos as we recalibrated everything to fit the new meter, especially when it came to precision engineering.

[@tncorgi92]

@Gordon Bird and don't forget Smoots.

[@Buddythunder1]

What a delightful history! One of your best.

[@roadie3124]

Thanks for this, Dr. Becky. Very interesting and very well presented. When I did physics A level in the mid 1960s it all seemed obvious. I found it very easy. That was a long time ago and mostly forgotten. I'm having fun reminding myself of things I used to know and learning about how things have changed. In defining the exact speed of light we had to change the definition of a second and the definition of a metre. The speed of light hasn't changed, just the definitions of the units used to measure the speed of light. I'll have to buy new rulers and my Rolex Oyster will have to be re-calibrated, but it's nice to have an agreed number for the speed of light. 😀 An Imperial inch used to be 25.399977 mm. It was changed to exactly 25.4 mm in 1930 and adopted by different countries over time. People who had been used to the inch being defined as 1/36 of an Imperial Standard Yard had to get used to it being defined as a number of mm. Some people I know couldn't understand the significance of the difference.

[@byrnemeister2008]

Great video. So much information. I can’t believe you can knock these out in your spare time. Parker solar probe results coming soon??

[@aarontyler9569]

I bet she doesn’t get too many negative comments, her personality is perfect for explaining complex concepts

[@psyclobe]

You are so full of life and curiosity, its contagious

[@MrImmers]

Your enthusiasm is very captivating 🤩🤩🤩🤩

[@Ray_Sellner]

I have no idea how this ended up in my feed. I went from a physics lesson to melting marshmallows. I'm good.

[@clayz1]

In my city, 25 miles per hour is the defined universal speed limit.

[@johnabbottphotography]

Love your videos. I especially feel so much better watching your videos, for the endings. Because I mess up pronouncing a words I know while making mine.

[@darrenatkins8073]

I love your videos! Thank you for making this

[@johnneuman9470]

YAAAAAAAAAAAAAAAAAY! It’s science time with my favorite astrophysicist Dr. B!!!!!!!! 😀❤️

[@DrBecky]

👋👋👋👋

[@rhoddryice5412]

Congratulations to beeing first at 9/10 LS

[@erictaylor5462]

Actually, it's Doctor S. But if I ever meet her, it might be Dr. T.

[@johnneuman9470]

Rhoddry Ice you could see the red and blue shifts of my fingers upon the keyboard 😬

[@robertlawrence9000]

@@erictaylor5462 competition? 😂

[@calyodelphi124]

Your next door neighbor was playing "L'amour est un oiseau rebelle", the infamous "Habanera" song from Carmen! :D

[@goober5713]

I am so happy I found this channel.

[@Ukraine-is-Corrupt]

I applaud you for stating clearly & upfront that the Speed of Light is nowadays, a definition & not a measurement any more; most other people on KZbin seem to miss that point - & it's a very important point

[@Alex-Lay]

The idea of the meter being a yard stick is surprisingly amusing.

[@hannanpakthini7221]

Why???

[@TheMarrethiel]

@@hannanpakthini7221 a yard is three feet? I am guessing it is the play on words

[@lesliefranklin1870]

@@TheMarrethiel In that case, the phrase "being a yard stick" refers to a generic measuring device. In other words, it is being used as a metaphor. However, a meter and a yard have so much in common that the phrase can also literally be used as a very rough estimate. The humor derives from the double wordplay.

[@macgyveratlarge2133]

Three inches of difference is noticeable enough to affect the speed greatly. Yardsticks were an arbitrary measurement by the will of a king, meters are based on the measument of the planet itself. Anyone care to wager on the reliability of a king compared to a planet?

[@HAL-nt6vy]

@@macgyveratlarge2133 Which king are you sloppily nattering about? A yard is three feet. Our foot is identical to the foot used at the beginning of western civilization in Ancient Greece. The length of the foot was established for the first Olympic games in 776 B.C.

[@PolioVitruvius]

Thanks! What I find most interesting about light is that it is invisible unless it enters your eye. A room could be filled with light as bright as the sun and you wouldn’t see any of it by looking through the room. The idea of light being invisible to the eye is fascinating.

[@hurri7720]

We can only see the visible part of light, sort of obvious, but there is also for us an invisible part of light.

[@PolioVitruvius]

@@hurri7720 How can you SEE ‘visible spectrum light’ that passes from point A to point B in front of your eyes? You can’t. That is the irony. You only see the light that reflects off something and then enters your eye.

[@hurri7720]

@@PolioVitruvius , tried to look at the sun, seen nothing.

[@PolioVitruvius]

@@hurri7720 Well you will only see the light that goes from the sun to your eyes. You cannot see all the other light that goes past you until it bounces off something and then enters you eyes.

[@hurri7720]

@@PolioVitruvius , yes, that is true, like when you look at the moon at night.

[@loganpe427]

Dr Becky you must be the most humble really smart person I've ever seen & heard. Very cool! 😁👋🏻.

[@FabianoCores]

Hey... use lots of quality light, and put a higher F number (smaller apperture)... you'll have less problems with focus! The video is awesome by the way!

[@mardanananak]

Maybe somebody heard that f5. 6 was a sweet spot.

[@tubastud06]

25 minutes of me not being productive at work. Thank you for the break :D

[@abrisvegas]

I really should comment on more of your videos, I love your channel!

[@arthurgarcialucero704]

Dr. Becky great video I just wish I was further along in my understanding of science, but I am trying to hang in there. I think you try to layout your topics with the understanding that you have people like me out there.Thank you

[@Morbacounet]

Editing Becky : what have you done, filming Becky ? Everything is blurry ! Filming Becky : well, you forgot to remove one of the two "red flags" at 13:10.

[@ahooper99]

Citations on a science video, YAY! and to papers from the 17th century!

[@nautblue4284]

That parallax deal is freaking cool as hell. I thought I was going crazy when I looked up to notice the big dipper wasn't pointing as closely to the North Star as compared to other times I had looked at it.

[@SpinWave]

Beautiful explanation. Thanks

[@Valdagast]

It's amazing how accurate they were considering the instruments they were using. Clearly we need to measure everything in Planck units.

[@karlgru3n654]

ARGH. ;) you beat me to the punch.