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

Dr. Becky
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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? ruvid.net/video/video-jRLNVvsxfFo.html
You can pre-order my book ' Space: 10 Things You Should Know' as a hardback (UK only) or e-book (worldwide): bit.ly/SpaceDrBecky
My phases of the Moon necklace is from Eclectic Eccentricity, who have a great selection of space-themed jewellery: eclecticeccentricity.co.uk/products/roam-moon-phase-necklace?_pos=1&_sid=cd742dc82&_ss=r
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More details on measuring the speed of light (or really, the length of a meter!) with your microwave: physicamechanica.wordpress.com/2013/12/04/speed-of-light-in-a-microwave/
Here are links to the papers mentioned:
Rømer (1667) - gallica.bnf.fr/ark:/12148/bpt6k77856x.image.f39.langEN#
Huygens (1677) - gallica.bnf.fr/ark:/12148/bpt6k77856x.image.f37.langEN
Newton (1704) - gallica.bnf.fr/ark:/12148/bpt6k3362k.image.f235.vignettesnaviguer
Bradley (1729) - royalsocietypublishing.org/doi/pdf/10.1098/rstl.1727.0064
Fizeau (1849) - onlinelibrary.wiley.com/doi/abs/10.1002/andp.18501550113
Weber & Kohlrausch (1856) - onlinelibrary.wiley.com/doi/epdf/10.1002/andp.18561750903
Maxwell (1864) - upload.wikimedia.org/wikipedia/commons/1/19/A_Dynamical_Theory_of_the_Electromagnetic_Field.pdf
Essen (1950) - www.jstor.org/stable/pdf/98433.pdf?refreqid=excelsior:117b745e1aa593a54098a1332f856030
Evenson et al. (1972) - journals.aps.org/prl/pdf/10.1103/PhysRevLett.29.1346
Dr Becky Smethurst is an astrophysicist researching galaxies and supermassive black holes at Christ Church at the University of Oxford.

Science & Technology

Published on


Dec 4, 2019




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Comments 2 603
Dr. Becky
Dr. Becky Month ago
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!
J Burr
J Burr 12 days ago
@Will Shank of course but muppets spouting what they learned is silly.
J Burr
J Burr 12 days ago
@Will Shank well yes , you show me without telling me what you have been taught. all the billion possible reasons for everything.
C. Rasool Curry
C. Rasool Curry 16 days ago
At what speed does something travel before it can NO LONGER BE SEEN BY THE HUMAN EYE
Louise Paisley
Louise Paisley 18 days ago
@Payge You are a fecking moron, wake the fuck up you backward dickwad
Xanthan Gum
Xanthan Gum 19 days ago
john underwood your really fuckin annoying. Go do something with yourself.
Jianu Flavian
Jianu Flavian 2 days ago
way the camera is focus on the diploma in sted of your face?
Viha Shah
Viha Shah 49 minutes ago
Jianu Flavian she put a note on the beginning of the video that she was sitting too close to the camera and so the focus was off
mountaingoat1003 4 days ago
Thank you for this. So many people put out junk on youtube.
Cole Wilson
Cole Wilson 6 days ago
I'm glad i'm not the only one who can't remember these darn constants. #nice video #new subscriber!
Svemirski Modulator
How can someone get PhD (in physics, math, astro... and similar) and not know about Ptolemy??
Dr. Becky
Dr. Becky 8 days ago
Exactly. Because I do know about him. I just have never heard his name pronounced out loud!
Jean-Marc Ayotte
Jean-Marc Ayotte 8 days ago
Ruler calibration... that what we do when we make the microwave experiment
markleyg 9 days ago
Whenever I feel like a know it all I come to this channel.
Robert Langford
Robert Langford 10 days ago
Ptolemy had a silent P.
Robert Langford
Robert Langford 10 days ago
Does it matter which colour marshmallows are and is their size important?
KK B 11 days ago
Hi Dr. Becky, great video but the camera focus isn't on you, makes it a bit difficult to watch.
TheBlueArcher 12 days ago
So the question now is, now that the the meter is defined by the speed of light, what's the error of which we can accurately produce a meter?
David Minicz
David Minicz 13 days ago
Thanks Dr for a wonderful and great video tutorial! You certainly explain difficult subject quite easily! :)
Dennis Tucker
Dennis Tucker 14 days ago
Love the video. I struggle with some of the content in this video. I am familiar with the subject matter though. For example, when visible light passes from air to water it is said that light slows down due to the change in medium. If this is true, then the speed of light(aka em) depends on the medium. So the thought that goes through my head is that light might travel infinitely fast but the speed we measure/observe might be the result of it passing through another medium(the vacuum of space which is just another medium). Your thoughts please. I also have problems with the statement that nothing can travel faster that light. Supposedly, the observable universe is limited due to distant galaxies(and stars) moving away from the Earth at....speeds faster than light. what the hell?
Mantis_Riger 14 days ago
I like a challenge.
xyckriz 14 days ago
why is light always in a hurry? where is it going?
aowi7280 14 days ago
All these comments and no one has mentioned how pretty she is. And fascinating to listen to.
wayne thomas
wayne thomas 15 days ago
Well, I'll admit, I only got as far as, "Romer only used basic trigonometry" that's where I stopped watching...for a minute anyway. I'm just struck by how valuable Trig has been and for how long it's been used. I learned trig in college by a navy woman....actually I think she was a Michigan maritime or coast guard person. Everything she taught, initially anyway, was referenced to navigation. When they didn't have GPS and only had lighthouses, Trig was extremely valuable and amazingly useful! all we needed to do was scale it down to apply to angles as they related to machining practices and bam...we were hooked up. I actually used it because I liked it. And once the the people I worked with and for found out that I was able to calculate angles easily they started asking me to do so for them. I worked for a fabrication related shop and the actual angle was rarely really needed for bending a bar or tube. But as the fabricator figured out, knowing the angle for setup, just to get close faster was very useful. It wasn't long before I was the chief engineer and it was kind of a Cinderella story. I had no experience and started there as a floor sweeper/toilet cleaner.
Inevitable Defeat
Inevitable Defeat 15 days ago
The piano was playing habanera
GySGT German
GySGT German 15 days ago
If light is ""infinite"" then why cant we see instantly stars millions or billions of "light years" away¿¿¿ So whatever light we see from these distant stars means they are millions or billions of years older then we see right now... dammmn
Matthew Bennett
Matthew Bennett 16 days ago
In one word, Maths.
Perhaps 16 days ago
Two people: -- Person #1 is standing still, pointing his flashlight to the south. -- Person #2 is traveling north, at 185,999 MPH, and pointing his flashlight south. When person #2 reaches person #1, both of them will be shining their flashlights, in the same direction, right next to each other (for an instant). Will the light from both of their flashlights run parallel to each other? Or, will person #2's light seem to be almost standing still / moving at 1 MPH? And if person #2 shines his flashlight north, will his light travel (from his perspective) at 1 MPH? Thank you.
Claudia Juarez
Claudia Juarez 16 days ago
So much nonsense.
Teitur Ardal Rosengren
@Dr. Becky very nice video! I have a question, how is it that the relative speed between two photons traveling in opposite directions are moving away from each other at the speed of light and not the speed of light times 2. I found this explanation but tbh I dont get it. ( www.physicscentral.com/experiment/askaphysicist/physics-answer.cfm?uid=20130130105151 ) thank you for your videos!! :)
Nov Prezime
Nov Prezime 16 days ago
Your flaws make you even more attractive, here’s a brain teaser for your next video, if you pass light through water it slows down however if you were to have that same beam of light exit that body of water would it travel at the slower pace or return to this universe constant? And if you were to repeat the experiment through multiple bodies of water would you be able to slow light to a walking pace? Also if light is a particle how is it that they are still traveling at the same speed after 13 billion years without any loss of energy?
bogen broom
bogen broom 16 days ago
How does our Atmosphere lay alongside the vacuum of Space.......? .....it’s impossible! Space is Fake
Simon Green
Simon Green 13 days ago
@bogen broom I don't really need to. And you appeared to make an assumptions (on what data?) that I don't understand what a vacuum is. So, continue with your explination. That's how sci/research works. If you present a claim you need to support it with evidence. You're already raising a lot of red flags that show you don't seem to understand basic principles of sci/research methodology. That doesn't bode well for what ever your how your original claim is going to turn out.
bogen broom
bogen broom 13 days ago
Simon Green Do some research on Vacuums........
Simon Green
Simon Green 14 days ago
I guess you will be able to furnish us with a bit more information as to why that is impossible?
5610winston 16 days ago
There was an episode of the old TV western BONANZA in which Michelson (who actually lived in Virginia City in his teen years) spooked every horse in town by n experiment to determine the speed of light.
Hemant Gaikwad
Hemant Gaikwad 16 days ago
Unrelated question : 1) Is the sun rotating on its axis ? Why not ? 2) Why does the earth rotate on its axis ?
EZZA 15 days ago
2...…...THE Earth is Flat and Motionless
Onyourknees Beforeme
I love how the one picture in the video is her plaque that says she's not only a doctor but a PhD holder as well. What an ego...
xyckriz 14 days ago
and it is redundant. either put dr before the name or phd after the name. not both
C. Rasool Curry
C. Rasool Curry 16 days ago
Does anyone here know, What speed does something travel before it can No Longer Be Seen By The Human Eye
Jason Brooks
Jason Brooks 17 days ago
Light doesn’t travel at all, it is an ether perturbation.
SpicyShiba 17 days ago
You have an awesome way of explaining things! Cool video!
dalexdba1 17 days ago
Thanks so much for translating such complex subjects into readily-digestible English! I really admire your style!
John King
John King 17 days ago
GRB makes Light look like it’s frozen in Spacetime. There was a moment in History when 100 mph was the “Speed Limit”...
Ramon Preston
Ramon Preston 17 days ago
The fastest time ever measured? From the time the stop light turned green until the guy behind you started blowing his horn.
Shaunn Hartmann
Shaunn Hartmann 17 days ago
Uh, Superman goes faster than the speed of light... 🤔
Sarratt 18 days ago
I honestly tried to 5 mis but I can't follow
Austin Page
Austin Page 18 days ago
and still our second is off to the tune of 4 (supposed to be 6 thanks fred) hours a year... something like a 4 1/2 (1.0006849315 seconds with sorrected figures above) ten-thousandths too short.
Austin Page
Austin Page 14 days ago
@ffggddss totally. and either way its still arbitrary measurement applied to the natural world. elliptical s being what they are and all. next we'll start talking about Keplarian time measurement for area swept. It's been a pleasure Fred and i appreciate the non -critical nature of this discussion especially since it wasn't a serious subject or original statement.
ffggddss 14 days ago
@Austin Page Here's the real point. There's one thing in this situation which is *not* of our making, and cannot be sidestepped - the average tropical year (year of the seasons), counted by mean solar days (average time from the Sun crossing your celestial meridian to the next), is 365.24219... So you can either have a system of timekeeping that is totally "nice and tidy" and has no connection with days and years, or one that does have a connection with days and years and is full of complicated rules and exceptions. There simply is no getting around that. It's a fact of nature. Fred
Austin Page
Austin Page 14 days ago
@ffggddss absolutely the point is we have the power if we can overcome the past to make everything all nice and tidy since theyre all arbitrary definitions we use to measure and catalogue.
ffggddss 15 days ago
@Austin Page "That make more sense?" Actually, no. I don't think you've stopped to consider what "nice-ifying" the calendar this way, would do to your clock!! Time of day would no longer agree with the time on a clock. The noonday Sun, when it occurs at 12:00 N one day, will happen at 6:00 am a year later; 12:00 M two years later; 6:00 pm three years later, then finally, back to 12:00 N after four years. And at all the times in between those, during that four-year period. And then that cycle repeats ad infinitum. But if you don't mind that, then let's *really* tidy things up, by lengthening the second enough to make the year 336 days, consisting of 12 months of exactly 28 days = 4 weeks. No more need to consult a calendar to know what day of the week a given month and day will be; the 1st of every month will be the same weekday; the 2nd of every month will be the same weekday; etc., all the way out, every single year. Fred
Austin Page
Austin Page 15 days ago
@ffggddss its just one of those things. If we get to define it in order to measure and catalogue we could make it nice and neat. Tidy up the calender etc.
Stoner 18 days ago
ANOTHER 1 TIMOTHY 2:12 KJV Woman Teacher Going To Hell
NorthernChev 18 days ago
My jaw SLAMMED against the ground at the end when I heard my favorite Doctor of Astrophysics stumble over how to pronounce 'Ptolemy'... No, really. I was nearly knocked unconscious from doing a double take so fast when I heard that. Shame, Dr. Becky. Shame. I am carving a notch out of the corner of your degree for one.
M Detlef
M Detlef 18 days ago
NorthernChev DUMBASS!!! You NEVER heard Carl Sagan pronounce his name??? You’re a LOSER!!!!
Barrie McCockinher
Barrie McCockinher 18 days ago
How fast is a fart?
Bosted Tap
Bosted Tap 18 days ago
Tantalising science history, many thanks for sharing, new subscriber.
Dominic MacCuaig
Dominic MacCuaig 18 days ago
AT 10:55 she proves the earth is flat. He shines a light through a cog wheel (guess they had lasers back then) 8 miles away onto a mirror 🤦‍♂️. Then reflects that light back 8 miles with perfection...perfectly back through the cog wheel. Wake up. Using earth curvature calculator. With 6ft observer height, the mirror would be hidden by 16ft of curvature. But let’s say it was all done on a table at 3ft off the ground. Then there would be 23ft of curvature. There’s no way he could see that mirror unless the earth is flat. I’m order for this to work, everything has to be lined up perfectly level.
Dominic MacCuaig
Dominic MacCuaig 18 days ago
LIGHT DOES NOT TRAVEL. ITS ALL A LIE. ruvid.net/video/video-YcoYRofMtyk.html
Brian Wind Grøndahl
when yo reach the speed of light time slows down, so a Photon from the sun takes ca.11 min to reach us from our time reference, but in the eyes of the Photon it is instant...
Daniel Goleš
Daniel Goleš 19 days ago
Love the camera focus...
Jaxxon Balboa
Jaxxon Balboa 19 days ago
Honey have you seen my keys?....and she breaks out into the explanation of the speed of light!
Onkel Tschimi
Onkel Tschimi 19 days ago
Jaxxon Balboa
Jaxxon Balboa 19 days ago
The reason the speed of light doesn't go faster is because it has reached it maximum ability to move through the "resistance" of space just as an object's ability to move through air or water has a limit! The faster you go the more the resistance. Once motion and travel medium are at equilibrium top speed is established. Space is not empty and its "resistance" is slight but at great speeds it becomes relatively dense! Actually, a more interesting question is what propels photons in the first place?
Jaxxon Balboa
Jaxxon Balboa 19 days ago
"You can't go faster than the speed of light."...I beg to differ young lass, have you ever seen tourist at a free buffet?
Björn Johansson
Björn Johansson 19 days ago
I believe you heard the Habanera song aria, from the opera Carmen from the piano. :)
MrBenski81 19 days ago
You would think that someone with a PHD in Astrophysics, would know how to focus a camera properly 🙄
Dale D'Angelo
Dale D'Angelo 19 days ago
Thank you Dr. Becky! Both enjoyable to listen to and look to. :) My education is on aerodynamic/aerospace levels and this kept me focused. Excellent discussion and personality.
B AD 19 days ago
Quantum entanglement is actually faster than the speed of light as it has no distance limitation.
Dale Martin
Dale Martin 19 days ago
Not bad for old school.
A Hooper
A Hooper 19 days ago
Citations on a science video, YAY! and to papers from the 17th century!
Craig Lenhard - rvrguyd
All very interesting. When you go out on a date, what do you talk about?
Jose Montalvo
Jose Montalvo 20 days ago
I think Time and Light are a convention connected to each other. Using time to measure Light or vice versa is like measuring the age of rocks by C-14, then turn around and measure C-14 by the age of rocks. Does Light slow down by distance? Was Light faster many ages ago? When you bend Light, it does slow down. We can slow down Light, does that mean we can speed up Light? We must consider other facts before we shout from our Ivory Towers!
Tomáš Konkoľ
Tomáš Konkoľ 20 days ago
Hi Becky subscriber earned, You have some flaties infestation in comments 😂
Magnus Kesselmark
Magnus Kesselmark 20 days ago
There was a description of a ”simple” set-up using a light source (led or laser) and mirrors, a little like the Michelson-Morley set-up, described in Am hobby electronics journal Nuts&Volts in 2005 or 2006. A bit more complicated than marshmallows in the micro, and not so tasty, but cool enuff.
Don Moore
Don Moore 20 days ago
Think of all the science going on in the 17th century - Romer's method relied on Kepler's theory of planetary motion. Very well done video. The fingernail polish is interesting, but it is distracting as you move your hands frequently.
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