Why No One Has Measured The Speed Of Light

31 okt 2020
6 502 217 Áhorf

Physics students learn the speed of light, c, is the same for all inertial observers but no one has ever actually measured it in one direction. Thanks to Kiwico for sponsoring this video. For 50% off your first month of any crate, go to kiwico.com/veritasium50
Huge thanks to Destin from Smarter Every Day for always being open and willing to engage in new ideas. If you haven't subscribed already, what are you waiting for: ve42.co/SED
For an overview of the one-way speed of light check out the wiki page: ve42.co/wiki1way
The script was written in consultation with subject matter experts:
Prof. Geraint Lewis, University of Sydney ve42.co/gfl
Prof. Emeritus Allen Janis, University of Pittsburgh
Prof. Clifford M. Will, University of Florida ve42.co/cmw
The stuff that's correct is theirs. Any errors are mine.
Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der physik, 17(10), 891-921.
(English) ve42.co/E1905 (German) ve42.co/G1905
Greaves, E. D., Rodríguez, A. M., & Ruiz-Camacho, J. (2009). A one-way speed of light experiment. American Journal of Physics, 77(10), 894-896. ve42.co/Greaves09
Response to Greaves et al. paper - arxiv.org/abs/0911.3616
Finkelstein, J. (2009). One-way speed of light?. arXiv, arXiv-0911.
The Philosophy of Space and Time - Reichenbach, H. (2012). Courier Corporation.
Anderson, R., Vetharaniam, I., & Stedman, G. E. (1998). Conventionality of synchronisation, gauge dependence and test theories of relativity. Physics reports, 295(3-4), 93-180. ve42.co/Anderson98
A review article about simultaneity - Janis, Allen, "Conventionality of Simultaneity", The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.) ve42.co/janis
Will, C. M. (1992). Clock synchronization and isotropy of the one-way speed of light. Physical Review D, 45(2), 403. ve42.co/Will92
Zhang, Y. Z. (1995). Test theories of special relativity. General Relativity and Gravitation, 27(5), 475-493. ve42.co/Zhang95
Mansouri, R., & Sexl, R. U. (1977). A test theory of special relativity: I. Simultaneity and clock synchronization. General relativity and Gravitation, 8(7), 497-513. ve42.co/Sexl
Research and writing by Derek Muller and Petr Lebedev
Animations by Ivàn Tello
VFX, music, and space animations by Jonny Hyman
Filmed by Raquel Nuno
Special thanks for reviewing earlier drafts of this video to:
Dominic Walliman, Domain of Science: ve42.co/DoS
Henry Reich, Minutephysics: ve42.co/MP
My Patreon supporters
Additional music from epidemicsound.com "Observations 2"

  • So if you use multiple mirrors and have the lightbeam travel various different directions, will it always result in c when it returns to you?

    michalskimichalski9 mínútum síðan
  • The equation allows for this... because it cannot define/measure it. That does not mean that light can physically do this... only theoretically. The universe experiences the present at the same time. Now = Now. Our observation of the present is dictated on the distance and light refraction between the points we are looking at. Time is relative, but it's not as complicated as it sounds... measuring it on the other hand... well, that is complicated because... due to light and the distance, "now" for point "A" appears different from point "B" and vice-versa. If there was a way to manipulate an instantaneous photon delivery method, trust me, the telecom industry would have found it. If there was a faster method for observation than light, we would maybe have a chance at finally solidifying the speed of light... but today, we don't know of one.

    Longmatey 80Longmatey 8051 mínútu síðan
  • 4:20 this IS clickbait

    purgruvpurgruv59 mínútum síðan
  • This video just reminded me that our Solar system and Galaxy are moving all the time, so light speed measured in directions could be different. And, we could know which direction we are actually moving towards if there is a spherical equipment to measure one way light speed in all direction at the same moment.

    AK1314AK1314Klukkustund síðan
  • Basic speed of light is 3>3333333333 faster than speed of photon. 1 John 2:22 Who is the liar? It is whoever denies that Jesus is the Christ. Such a person is the antichrist--denying the Father and the Son.

    IAM HEIAM HEKlukkustund síðan
  • Awesome video, thank you so much.

    Carlos Fernando Castañeda OlanoCarlos Fernando Castañeda OlanoKlukkustund síðan
  • What if we put a light source perpendicular to a spinning object and spin it fast enough to bend the light, if the speed of light is instantaneous in a certain direction the light shouldn't bend when it comes to that particular direction.

    Sami LodhiSami LodhiKlukkustund síðan
  • why moving clocks moves slow with stationary observer ??

    adarsh07adarsh074 klukkustundum síðan
  • I'm too dumb for this.

    Nelson BaiettiNelson Baietti4 klukkustundum síðan
  • The question itself requires examination as to whether or not significant. At at any point in space, there are many photons moving in all directions. For any one photon, what is its status, i.e., is it a "returning" photon or is it an "outgoing photon"? If there is a difference in the speeds, shouldn't you see the difference? More absurd (as a means to prove that the whole premise is incorrect) is the fact that a photon is somehow aware of the difference. Let me give an example. Suppose there is a pair of mirrors perfectly aligned so that photons bounce between the two many times, both mirros 99% reflective so that 1% of the bouncing photons are detectable at both sides. Using this, one can measure time of flight of both "outgoing" and "returning" photons before the photon density between the mirrors falls below the detectable limit. What will you measure? Clearly you should measure two different kinds of photons, one associated with "outgoing" and one associated with "returning" at both mirrors! For the photons that bounce many times, are they switching "status" at every reflection or once bounced off of one mirror is it permanently a "returning" photon? Einstein has told us that the speed of light is a constant at C. Occam's razor should have answered this question before being posed and videoed.

    James KimJames Kim4 klukkustundum síðan
  • Believe in Jesus Christ and you will be saved, John 3:16, KJV....

    Eagle775Eagle7755 klukkustundum síðan
  • What if you did the simultaneous synchronization method, but rather than have it based on the shortestdistance, you instead have the wires equally long in both directions, so the one way speed being different wouldn't impact the signal time so it would take the same time to get to both even if the one way speed was different

    Isak OhmanIsak Ohman6 klukkustundum síðan
  • Could we measure the 2-way speed of light in 3-orthogonal directions of 3D space, compare the results and triangulate any differences? Is the amount of time it takes for a mirror to reflect light known to be 0 or this stipulated/assumed?

    RoyRoy6 klukkustundum síðan
  • Can't they figure it out by comparing the clock of the astronaut with mission control when he comes back?

    Alex MobiiAlex Mobii6 klukkustundum síðan
  • How doe we measure? What determines when the reflection returns? Is it when we detect a single cycle of some frequancy? If so the trip time will change with frequency.

    John GilmourJohn Gilmour6 klukkustundum síðan
  • What if light doesnt have a speed and is instant but when moving at the speed of light, light experiences time dilation making the machines used to measure it inaccurate

    LaktosfrittLaktosfritt7 klukkustundum síðan
  • what if we used quantum entanglement to sync the two clocks via electron spin?

    Dardan GjokaDardan Gjoka7 klukkustundum síðan
    • Let's do it

      Tim UrenTim Uren7 klukkustundum síðan
  • Would light travelling a circle around a deep gravity well be considered a one-way or round trip?

    xamantoxamanto7 klukkustundum síðan
  • What if you have an astronaught on mars with a watch perfectly synced to the time zone that the message is being sent. So if they send a message at 12pm and the astronaut receives it at 12:10pm then he sends a message back saying it was received at 12:10pm and they can go from there? Will that work?

    Dani HernandezDani Hernandez8 klukkustundum síðan
    • @Dani Hernandez the problem is like the clock at the begining. Impossible to sychronize clock at distance. So in your exemple your clock are already synchronized. And that is all the problem synchronize them at first

      Idris ghalebIdris ghaleb6 klukkustundum síðan
    • @Idris ghaleb perhaps it might cause say the station send its a at 12:00pm and the astronaut gets it at 12:07pm and he sends a reply saying he received it at 12:07pm and then say the station gets that message at 12:20pm. Now they now it took 7 minutes to reach the astronaut and 13 to go back to the station. Is what im trying to say but maybe I'm missing something.

      Dani HernandezDani Hernandez8 klukkustundum síðan
    • Nah wont work cause it would always 20 min. So if its instant from mars then it will take 20 min from earth to mars. If its 2 min from mars then it will be 18 min from mars. T

      Idris ghalebIdris ghaleb8 klukkustundum síðan
  • But what if we have realy good telescope whitch can se what someone is doing on mars and then writing it down and then sendjng it so then if he is doing it in the right that time he would send us that yes he is dooing that and even looking at clock when ha is doing that so when the massage reaches him then he would know how mutch time it took for light to travel from earth to mars so he could send back to earth the message of saying ether from earth the light speed was instantainius or it had some time delay? I hope my theory kinga helped out

    nightmare_glitchesnightmare_glitches9 klukkustundum síðan
    • Yea bot working sadly. Because imagine. In the first case : it take 10 min to travel from earth to mars and 10 min from mars to earth for the light. Then if you are looking the guy in the telescoop then you will always see what he was doing 10 min earlier. And then you will say you are doing this. He will always always receive a massage that indicate what he was doing 20 min earlier. Because case 2 : its instant from mars then it take 20 min from earth ( always take 20 min cause we know the speed of light only when she comeback) Then you will see instant what he is doing but he will receive it 20 min latee.

      Idris ghalebIdris ghaleb8 klukkustundum síðan
  • This was a cool discovery,,Lemme thibk about it well

    rick mohamedrick mohamed9 klukkustundum síðan
  • How about synchronizing the clocks before starting them

    rick mohamedrick mohamed9 klukkustundum síðan
  • how abouth lightning and tunder? cant we mesure the time diference between light and sound pulse at different distances in different direction from it to compare it?

    Étienne BeaudoinÉtienne Beaudoin9 klukkustundum síðan
  • why not have the two clocks on opposite ends and a lazer in the middle that fires one beam in both directions measure the 2 speeds

    Nich FroddleNich Froddle9 klukkustundum síðan
    • He explain it in the video

      Idris ghalebIdris ghaleb8 klukkustundum síðan
  • So, we live in a simulation

    Marc GeurensMarc Geurens9 klukkustundum síðan
  • The fastest is= mind energy.. the dark energy.. white energy.. spiritual energy dark matter 4 things around in exists world that is 2+2=5 that is truth.. 5 is real energy of life.. 5×5×5 =1:25 125 100_25 100 is century life.. century life is=1 1 is human 25 is B.B Big-Bang create every thing now... Bsquare proofs.. DIMOND BOY.. B.B=BELIEVE- 3 STARS IN SKY 7 STARS TO SEVEN WONDER.. 7 IS LUCKY 3 IS THREE TIMES COVERT EVERYTHING.. 3 IS LIGHT BLOOD AIR SOLID LIQUID GAS NEGETIVE POSITIVE NEUTRAL TIME SPEED EXIST...

    HYDRA GamingHYDRA Gaming9 klukkustundum síðan
  • I suspect that if C varied based on direction in space, there would be some kind of observable variation, directly or indirectly, that would reveal the differential speeds. And everything that's discussed in the video relies on the ideal environment of a vacuum, which we know is not what space is. For example, if all light traveled instantaneously (or nearly so) in one direction, then it likely would affect the matter it encounters (such as the odd hydrogen atom) somewhat differently at different speeds. We still might not be able to measure the speed of a photon directly, but we might be able to observe the differences in the things that light interacts with and how they are affected. Based on everything we _have_ been able to observe to this point, I suspect that Einstein was right - the speed is the same no matter the direction of travel. And it still _does_ seem like an academic issue - the question of whether "now" here is the same as "now" on Mars is in fact a nonsensical question, as Einstein's theories showed - because "now" is relative to the observer's frame of reference. The only "now/then" that matters is what can be observed, so if the observations are _consistent_, that's all we need to know. The only interesting thing that could come from this discussion is the discovery that there's a meaningful _inconsistency_ in what we observe, which would then make it worth our while to continue exploring the question. Otherwise, I'm with the majority of physicists - apply Occam's razor and move on.

    MemeticsMemetics10 klukkustundum síðan
  • How about moving the clocks apart from the middle slowly. And then just doing the whole experiment again but turning the whole thing around? Meaning sending the light from the other direction. If the same result keeps coming out doesn't that at least help prove that light doesn't travel at different speeds in different directions? Unless you are saying light somehow "knows" from where you are measuring it?

    greenningreennin10 klukkustundum síðan
  • Can you constantly rotate the experiment around a center point? If the speed of light depends on some strange universe direction, you would see compression/dilatation of time.

    escainescain10 klukkustundum síðan
    • Observation depends on lightspeed being equal in all directions...

      xamantoxamanto7 klukkustundum síðan
  • Time wasting

    John HensonJohn Henson10 klukkustundum síðan
  • Except for the fact that the speed of light has been measured... 670,616,629 mph.... ISworlds will let any moron post a video.

    Bmddoe sntmatterBmddoe sntmatter11 klukkustundum síðan
    • You have to watch video before talking lol

      Idris ghalebIdris ghaleb8 klukkustundum síðan
    • Guess you only made it five seconds into the video xD

      tramar thomastramar thomas11 klukkustundum síðan
  • What about 1 light beam that makes a strate path and 1 that takes 2 legs of a triangle (with a meroe). You know how long after the 1st one hit before the 2nd one hit and you know the 2 disticest with a little pathagris or mesuring.

    Ben ChermsideBen Chermside11 klukkustundum síðan
  • 16:13 - finally those flies left you alone! They were so annoying.

    Ravi RajyaguruRavi Rajyaguru12 klukkustundum síðan
  • Okay, I watched until the end and I think I found something where I think you're confused or I'm confused about your position. "Light moves at the same speed in all directions." To me, this is a misrepresentation of special relativity. Special relativity states that light moves at the same speed in all directions WHILE IN A VACUUM. The speed of visible light and other forms of radiation can be interrupted and distorted just like any other instance of matter in motion.

    PALEX XXPALEX XX12 klukkustundum síðan
  • I wasn't able to watch the entire video, but can't you just apply this concept to.... well, everything? Who cares if it's light that you're measuring, the point is that YOU'RE measuring it. Right? Greater point I took from this video was the flip of Einstein's relativity claims. We have no universal, objective reference when it comes to measuring how quickly matter moves. What do you guys think?

    PALEX XXPALEX XX12 klukkustundum síðan
    • @Idris ghaleb You can use identical methods to measure the speed of a bullet and the speed of light particles. All the issues this video brings up when trying to measure the speed of light, also apply to measuring the speed of any other instance of matter.

      PALEX XXPALEX XX5 klukkustundum síðan
    • Well o you cant apply this to everything lol. I can calculate the speed of a bullet for example

      Idris ghalebIdris ghaleb7 klukkustundum síðan
  • But it may it mark set the clock on time even if it is 10 minutes late and came on earth and measure time difference. By this way calculating time difference. And create same situation for earth and calculator the difference in delay if they are different the speed of light may be different for both ways.

    Shubham TodkarShubham Todkar13 klukkustundum síðan
  • Our body’s can react in that time

    yodkagodyodkagod13 klukkustundum síðan
  • You do not need to synchronize clocks, only make clear agreements. Mars and Earth agree that they will each send two consecutive signals one minute apart, or some other duration, according to their own local clocks. The first signal is to start the timer. You know then that one minute later, according to the clock of the sender, a second signal will be sent. It then becomes a simple comparison of all the data. The only problem is to distinguish the first signal from random ones. But that shouldn't be too difficult. All the difficulties reside in the idea that Einstein also could not let go off, that there is some absolute time that somehow keeps track of everything. Once we realize that time measurement can only be a (local) convention the problem disappears.

    OdalOdal13 klukkustundum síðan
    • @Idris ghaleb It is not important. They can agree on the procedure before the Mars observer leaves Earth.

      OdalOdal26 mínútum síðan
    • But your theory not working lol how do they make the agreement?

      Idris ghalebIdris ghaleb7 klukkustundum síðan
  • Put two synchronising devices at equal distance and send signals two times to start and stop and the reading will be different in both the clocks that'll mean speed of light is different in both the directions.

    TARUN SHARMATARUN SHARMA14 klukkustundum síðan
  • Wait!? If you shoot a light beam at a mirror on Mars and then move in that direction to a closer spot before it gets back to you, you will intercept it at a different distance and get all the details about the returning light speed.

    Slobodan MilovanovićSlobodan Milovanović14 klukkustundum síðan
  • In this video you explain so much about light speed but i think the main issue is time synchronisation like you said in video its not possible to sync clocks;😄lol. I think i have watched a video of our great science which describes that time is synchronised by 1/627272.... Sec. If that's true, then may be we can measure exact speed of light in each direction and effect of gravity on light as well. Just an idea and i know i am not a scientist🙃.

    shail patelshail patel15 klukkustundum síðan
  • The last thing was a clear indication to the question - what if quantum entanglement works BECAUSE speed of light is C/2 and Infinity based on direction?

    Raj ‎Raj ‎15 klukkustundum síðan
  • Hey Veritasium, I just saw on QI that they measured light traveling through -260° sodium at 38 mph. Could this relatively sensible speed be used to reduce the theoretical problems with directional measurements? At 38mph you could stand at the other end of a large room and use a stopwatch, right?

    Anthony ReidAnthony Reid15 klukkustundum síðan
  • What If we shot out an astronaut in every direction and told them to return at a specific point in their journey. They would all get back at different points in time... Right? Somehow use this to discover times speed?

    Truth BeToldTruth BeTold15 klukkustundum síðan
  • What if you had two beams of light going in opposite directions triggering two clocks?

    Bowen CreerBowen Creer15 klukkustundum síðan
  • Always felt like there is more to the speed of light. Awesome thinking and looking in new directions. “How about light speed from a moving source ?” “Maybe there is a positive and negative light “ Like magnets. Black holes are maybe negative speed of light, and opposites attract. Maybe I know too little 😁 But you are a great mind

    Babak VaeziBabak Vaezi16 klukkustundum síðan
  • And what if you take one timer, move it away from the other, look at the error, take a third timer on which the error of the second is increased by 2 and move it in the same direction at the same speed, and at the time of connection, these timers are synchronized with 1😊

    †РаБоТяГа_ ek_FaStbik††РаБоТяГа_ ek_FaStbik†17 klukkustundum síðan
  • one of those videos that you get it but you still dont know crap

    The_AnimalThe_Animal17 klukkustundum síðan
  • What if Marc and an observer on earth had some very powerful telescopes 🔭 ? If the speed of light is different depending the way light is travelling, wouldn’t they see that their watches are out of sync?

    Vincent TrototVincent Trotot17 klukkustundum síðan
  • Our inability to measure the one-way speed of light sounds eerily similar to our inability to properly observe whether light is a particle or a wave, so my thoughts are that the two problems are probably related. Since light's round-trip time is not actually about the speed of light, but the speed of causality, perhaps there's a way we can cheat by measuring other mass-less particles (which also travel at the speed of causality) to measure the one-way speed of light?

    wefinishthis nowwefinishthis now17 klukkustundum síðan
  • Could gravitational forces be influencing the one way speed of light?

    Rishi AchaibersingRishi Achaibersing18 klukkustundum síðan
  • wouldn't light travel through the path which takes the minimum time ?

    praharshitha josyulapraharshitha josyula18 klukkustundum síðan
  • Out of the box thinking ❤️

    ARUNBAL VICKYARUNBAL VICKY19 klukkustundum síðan
  • Could it also be, that light is less than 1/2 c in one direction and more than infinite c in the other direction, so ist travels back in time?

    RonnieRonnie19 klukkustundum síðan
  • Dumb question: What if you then wait, until Mars is at the other side of the earth and send the signal again. Let's say Mars is 10 Minutes back and the signal is infinitely fast in one way. It's 12:50 at earth and 12:40 on Mars. They send the signal, "hey, it's 12:50", and this signal instantly reaches Mark, then he would recognize the error. Only thing I'm not sure about: He travelled faster than the earth to reach the other side of our solar system. But doesn't that mean, his time went even slower than the earths?

    Hirnriss LpHirnriss Lp20 klukkustundum síðan
  • We should do multiple calibration of time from earth-mars. You calibrate once and then verify your theory...send a message from earth to mars, calibrate and respond from mars, then immediately send a second mesaage with the time when the message was received .. We should know if einstein was right or wrong

    Sg ///Sg ///22 klukkustundum síðan
  • this is why I will always trust and have faith in Jesus Christ

    johnny hartjohnny hart22 klukkustundum síðan
  • The problem with your argument is simple: If light isn't consistent in both directions, then it violates the principle of relativity. The speed of light HAS to be constant in any direction in order for special relativity to work.

    Elliot LinElliot Lin23 klukkustundum síðan
  • Question... Is there not a way to use a T layout with a neutral clock? Lets call the top left of the T (A), and the top right (B). While the bottom will be N for neutral. Break the line from A to B exactly in the middle and put your mirrors to reflect to N and sync both clocks for A and B to N. A and B's one way trip to N and or vise versa should be identical. From that point just continue the experiment, Removing N and only going from A to B and B to A. I really have no idea here... but that popped up into my head.

    Nick SmithNick Smith23 klukkustundum síðan
  • Tell me if this is dumb but it's just an idea get a really long plank of wood or whatever can stay straight for a long time and put one small square of glass at the start of the plank and one at the end then then shine a laser through one end and line it up with the other piece of glass and put some sort of sensor in the glass then goes off when I senses light you could measure the amount of time that it takes between One sensor to go off and the other and determine how fast it's going

    Mario PortilloMario Portillo23 klukkustundum síðan
  • This is all nonsense. c = c. (regardless of the direction of travel) If c changed depending on direction, you would have blue-shift in one direction, and red-shift in the opposite direction. It could be measured and quantified.

    Kara0kiKara0ki23 klukkustundum síðan
  • I think that while in theory you could have an infinite number of models for how light moves in the universe, it is more useful to use a simpler and more consistent model, so long as it remains consistent with all our observations of the universe. Most things that we recognize as proven fact could in theory be completely fade perceptions, but so long as they are consistent with all observations of reality, it remains practical to assume that we’ve proven them true.

    Stephen PaulStephen PaulDegi Síðan síðan
  • So those that think that light might be traveling instantaneously on its return path, do they think this is due to some quantum entanglement principle at work?

    radioduransradioduransDegi Síðan síðan
  • why not put in a third mirror? and let the light in turn from different sides.

    Ivan SushkovIvan SushkovDegi Síðan síðan
  • Couldn't you connect the switches mechanicly to start both timers at the same time?

    Isaiah CutshallIsaiah CutshallDegi Síðan síðan
  • I can think of two ways: 1) Two stop watches are set to start with a delay after x amount of time. Once they are in place then shoot the light beam with perfect synchrony. 2) Two stop watches are started at the same time/same place. We move one watch at a time at a constant velocity. Once the odometer reads a certain value the watch is stopped. Then this process is repeated in the opposite direction. Both times their face times are matched. If C is constant in both direction then both these watches should have the same difference in time. This method would require extremely high precision in distance and time measurement.

    I Love PizzaI Love PizzaDegi Síðan síðan
  • The fact that earth is moving through space actually makes it impossible if we're measuring a reflected signal. It just won't be travelling at the same speed in both directions. We'd have to find a way to stand still in space.

    Del ScovilleDel ScovilleDegi Síðan síðan
  • At 10:15 he talks about having a center clock to send the pulses out. What if you sent 1 pulse to start and a 2nd to stop it and record the difference. If light travels the same speed both directions then they would be the same when brought back together

    Void XVoid XDegi Síðan síðan
  • If you have two clocks in the first scenario, you could send a light pulse to the second clock, then use like something premeasured like electricity speed to send a signal back. Subtract the time it took for the electricity, and you get the one way speed of light, at least in that direction. Right? The electricity isn't affected by the speed of light, so it should work? I'm probably wrong though

    HastorHastorDegi Síðan síðan
  • What if you set off an explosion really far away and measure the difference between when you see it and hear it, then do it again on the other side... surely that would work?

    I don't know any good namesI don't know any good namesDegi Síðan síðan
  • I was so much interested in science, but after watching video multiple times. Me: why should I care about speed of light😁

    AVINASH GAVINASH GDegi Síðan síðan
  • round way trip speed = 299792 km/s, then One way speed= could be 599584km/s🤣🤣🤣

    AVINASH GAVINASH GDegi Síðan síðan
  • Two clocks. They start in the center, centered on the equator with the 1km path running perpendicular to the equator. Then use mechanical linkages to move them BOTH from the center to the ends of the 1km at a fixed, matching speed. Have a light source at both ends. Fire them off, bring clocks back to center, times should match.

    Nick FoxNick FoxDegi Síðan síðan
  • What if we put a atomic clock on the moon? It would look like the clock slows down and then speed up...

    Alvaro NavarroAlvaro NavarroDegi Síðan síðan
  • I love the mark watney reference amazing movie

    Dominic GindoffDominic GindoffDegi Síðan síðan
  • I duno, use Sundials instead of digital clocks? lol

    Champa PowerChampa PowerDegi Síðan síðan
  • I have a thought, in order to find out the speed of light in one direction. A to B, Try measuring from B? To A and C? Distance B to A and B to C = B as centre For example: Like you said if we have a satillite in centre of earth and mars shoots a signal to both sides Time taken to earth is different to mars then it proves light travels same speed in all directions?

    Siddharth ReddySiddharth ReddyDegi Síðan síðan
  • Can;t be measured , because it is defined, and it defines length.

    lynn rathbunlynn rathbunDegi Síðan síðan
  • even if u find way to measure u cant say speed of light is this or this, because it depends on light, not every light are same not every light go so far and so fast , battery light cant move fsater than light from sun :D so all it depends on some power of light, use so low energy to a small light and there u can get slowest speed of light

    Drift SimulatorDrift SimulatorDegi Síðan síðan
  • Si ponemos 3 lásers que disparen al mismo tiempo, y para asegurarnos ponemos un medidor por cada láser a una distancia conocida, el láser A activará un segundo medidor a distancia de x, el láser B activará el medidor de inicio mediante rebote en un espejo y el láser c activará un medidor a 3x. Si c=3a b=2a indicaría que no existe diferencia entre una dirección y otra.

    TRONCO0013TRONCO0013Degi Síðan síðan
  • Put a light source in between a circle of clocks if the clocks show different times for when light hits them we could know if the speed of light is different depending on direction if they all show the same time this means the speed of light is constant no matter the direction

    abdul_wasay 06abdul_wasay 06Degi Síðan síðan
  • Wow so we could theoretically be seeing everything In the present rather than the past? The agreed upon speed of light could be wrong?

    Kelton CamposKelton CamposDegi Síðan síðan
  • What if you use 2 syncron clocks in the middle and send them to opposit sides with the same speed. The clocks have agreed tgat the light will be sendt on a spesifik time like 10s. So the clocks will count the time between

    Filip HoggenFilip HoggenDegi Síðan síðan
  • what if people on earth have really strong telescope and can watch the person on mars from earth? if light takes 10 minutes to get to mars, the people on earth would see the person on mars write down the time at 12:10, but if light takes 20 minutes to get to mars, wouldn't the people on earth see the person on mars mark the time 12:10 when the time on earth is 12:20 and know to correct it? or does that fall into the same problem? idk my brain feels like mush right now

    J BJ BDegi Síðan síðan
  • That's just visable light though right? The rest of the spectrums travel at different wave lengths this different speeds I would presume.

    Russian BotRussian BotDegi Síðan síðan
  • Does anyone know if measuring two quantum entangled particles that changed polarity when a light beam hit them would work? Since the one at the end would instantaneously trigger the one on earth and let them know when the beam had hit Mars in real time?

    Banks KayeBanks KayeDegi Síðan síðan
  • "how do u know FORSURE that the earth is round"

    jackson schmittjackson schmittDegi Síðan síðan
  • What about an experiment where you are in a vacuum have an observer in the middle and two people on either side both equally far away with lights. Both of the light bearers have come to an agreement on 2 oclock for when they turn on the lights and the observer records the difference. Please tell me if this is an unreliable experiment or what doesn't work.

    joel rogersjoel rogersDegi Síðan síðan
  • When the astronaut comes back to Earth, wouldn't they notice the time isn't in sync? Following that logic: there should be a 10 minutes difference between Houston time and the synchronized time?

    ZealotZealotDegi Síðan síðan
  • A mirror may suffer some latency

    kevin ncubekevin ncubeDegi Síðan síðan
  • I cant even imagine what the conversation would be like between Einstein (if he would be alive today) and Veritasium 😂

    Yuriy KryvoruchkoYuriy KryvoruchkoDegi Síðan síðan
  • i might have misunderstood the WHOLE video altogether but could this mean... 1. Events that happen in Earth and in space eg an eclipse or the celestial alignment of planets could have happened before we perceived them because if light doesnt travel at the same speed in different directions, this means that our perception of events might be totally different than our estimated or 'accurate' times. 2. If we technologically advance, we can measure the speed of light by trapping it in a black hole and building mirrors that have sensors built in around it, and, from the point of light entering the BH(black hole), record the time it takes for a ray to hit the sensor mirror 3. Since our very eyes rely on two way light or that for us to see, light has to go on an object and return to the eyes, we are unfortunately uncapable of coming up with a way to measure the speed of light and thus base future experiments on possible animals' eyes or machine life...though i must admit this is quite dark

    WolferDub JazzWolferDub JazzDegi Síðan síðan
  • I have news for you. The speed of light has indeed been determined using one way measurements. In fact the first estimates of the speed of light were made using one way measurements. The moons of Jupiter are often eclipsed by the planet. If the distance between the Earth and Jupiter was constant the intervals between successive eclipses should be fixed but, since the Earth and Jupiter are in different orbits around the sun, the distance between the planets is constantly changing. This means that eclipse times, as measured on Earth, are sometimes early and sometimes late. The orbit of Jupiter can be determined using Newtonian dynamics together with parallax measurements of objects such as asteroids that pass close to the Earth. Given the orbits of Jupiter and the Earth the distance between Jupiter and the Earth can be determined. The speed of light can then be determined from the eclipse timing data. Note that different eclipse times are measured for different Earth to Jupiter directions, but the different directions no bearing on the results so this is a demonstration that space is isotropic. This was the way in which the speed of light was first estimated. The accuracy of this method will be limited and certainly a lot worse than modern methods. It is, however, a very strong pointer to the speed of sound being the same irrespective of whether a one way or two way path is used and also to the isotropy of space.

    Roger PhelpsRoger PhelpsDegi Síðan síðan
  • I have an thought ( in laymen term) the light move faster towards the object have higher mass, resulting in high gravitational force ( exp. Black holes) so when we sent the signals to mark at mars the time required was high due to lower mass of mars but when her replay back it will come faster due to Earth's mass, also if we add the sun gravitational force the message will receive faster , this just an idea i can wrong

    Himanshu AgrawalHimanshu AgrawalDegi Síðan síðan
  • This is probably stupid, but couldn't you have a set up like that at 2:14, so 2 clocks a distance apart but both start when hit by the laser? Then the difference between the time on the clocks would be the time it takes for the laser to travel the distance. The clocks would continually count up, but at any time you could still take the difference (using cameras, etc). Or program the clocks to stop via a sound wave that starts at the middle so they stop at the same time. Or I'm sure there are many other clever ways to get them to stop without light being involved, but then you could directly get at the time the light took.

    Robert GruenerRobert GruenerDegi Síðan síðan
  • @veritasium if c in one direction was c/2 we would see half of night sky shining becouse of all of those stars which light didnt come to us yet.

    AdamGa92AdamGa92Degi Síðan síðan
  • I am wondering, you mentioned to sync the 2 clock in the middle [kind of end of video], but I am pretty sure you could try to sync them lets say 3/4 vs 1/4 way [so not in the middle] and do a measurement, if what you measure using the syncing in middle and what you measure when syncing not in middle gives different results, it is a proof that the speed there and back is different, if you measure the same result it is a proof the speed is the same, I think.

    Andrej CelkoAndrej CelkoDegi Síðan síðan
  • Use fibre create big loop you can measure one way

    Dawid KaminskiDawid KaminskiDegi Síðan síðan
  • cant we use the problem of time dialation itself to solve the problem? okay i was gonna propose another solution when i wrote the first sentence, but now i thought of something else, when we start two clocks at the same place at the same time like at 2:43. cant we start moving clock B, and since B is moving towards the other side at a constant speed, (or that we just know how much time its going to take to get there) lets say for example 1 minute. and we stop both clocks. so we sync them up start moving clock B to the right knowing that its going to stop after 1 minute, (each clock with relative to itself, since one minute is one minute (right?) even if clock A still sees clock B as moving it stops after 1 minute anyways (because of time dialation). stop both after 1 minute (when B finishes traveling) and we get the clocks back together to see how different is the time, and using that we can tell what is the speed of light in that direction. the only thing that im not sure of is that both of them have to stop after 1 minute of being separated, which im trying to say is "absolute time" when each of them counts it in their own local time, and without relation to its perspective. as in from a 3rd perspective, looking at both clocks from far away, would they stop at the same time? idk

    RaniRaniDegi Síðan síðan
    • okay nvm i remembered that then they would both have the same time on them because the minute itself would run differently in such a way that at the end they end up with the same time

      RaniRaniDegi Síðan síðan
  • What if we synchronize the clocks using (when it finally becomes possible) quantum entanglement?

    Anubis __117Anubis __117Degi Síðan síðan