Why No One Has Measured The Speed Of Light

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"


  • Bernardo Dias
    Bernardo Dias8 minuta më parë

    What if they test sending the hours from different angles to mars and if the mars clock is not synchronized in any of those cases then we have a proof?

  • Aliashar Cdiongkat
    Aliashar Cdiongkat11 minuta më parë

    nothing is impossible 😁

  • P F Sloan
    P F Sloan35 minuta më parë

    Albert A Michelson was awarded a Nobel Prize in 1907 for measuring the speed of light. Yes, it was a two-way.

  • Jesse Groves
    Jesse Groves37 minuta më parë

    If we do the thing that you were talking about in the synchronisation video where we have two metronomes some distance apart and eventually they synchronise by putting them on the same swaying board, couldn’t we say “if it takes some specified number of tocks to sync (say 20) then on the 50th tock one end will have a laser fire and then you count how many tocks it takes for the beam to reach the other side?” Because in theory the clocks would be in sync right? Please tell me if this makes any sort of sense or if i’m just downright insane

  • Asasi Sarangthem
    Asasi Sarangthem50 minuta më parë

    Even if the speed of light is measurable , it cannot be express in "numbers" , all we do is "assumption" and "commence" everything in this universe, that's just how "nature" works.

  • alexkha
    alexkha54 minuta më parë

    What about Doppler effect? If speed of light were simultaneous in any direction, we would have no Doppler shift from objects in that part of skies, no?

    SOMDEB SAR54 minuta më parë

    We can do one thing, that we know a property which measures momentum of light called ''photoelectric effect'', so if we get infinite speed, so definitely my potential would be ∞ but if not then certainly normal photoelectric effect, and this should be on both sides to measure the momentum of it on both sides.

  • Leon
    Leon56 minuta më parë

    I do really love this videos

  • Jack Tsai
    Jack TsaiOrë më parë

    Wait, wouldn't inverting the clocks solve the problem? For example you fire up two experiments. First you sync A and B together, then you move clock B to, let's say, west, and measure the light. Then you so it again, but this time you move clock A to east, and measure. And if you're in the mood, you can repeat this paired measurements in all axis you care about. If light travel speed is indeed different depending on the directions, wouldn't this pairing measurements detect it?

  • Ryan Jones
    Ryan JonesOrë më parë

    Why couldn't you just have two clocks that start the timer once the laser fires? Then rather than receiving the bounce back you have the first laser firing into the receiver of the second clock and the second clock firing into the first clocks receiver?

  • joe l
    joe lOrë më parë

    To determine the % difference you could but two clock controlled in-phase lazers (maybe a mazer or razer if they exsist) next to each other. Move them away at the same speed until they are 1 period of the wavelength apart. turn them on at the same time, measure the amplitude of the wave to determine which is faster.

  • Raisul Raj
    Raisul RajOrë më parë

    Your one-way limited speed other-way infinity speed doesn't makes any sense .

  • Chris Chatfield
    Chris ChatfieldOrë më parë

    When light gets to the mirror it would have to know at what speed to travel back to get an average of c. It would need some internal definition of ‘away from’ and ‘back to’. For any direction, in all the universe. (I get that this is one of the points the video is making.) If the source starts moving away from the mirror while the light is heading towards the mirror then the light has no way of knowing that it needs to adjust its outward speed to give the correct average of c on its return. The video is sort of saying - what if when we add 1 and 1 we are actually adding 1.5 and 0.5.

  • Denis Goloubev
    Denis GoloubevOrë më parë

    Prove me wrong I'm not smart or anything, but what about the microwave experiment? To remove the spinning disk in the microwave and place a chocolate bar inside, the constructive and destructive interference seen from the melted spots and normal spots should prove that light travels at the same speed. If they were at different speeds in the two-way trip, wouldn't the interferences vary since the velocity of a wave is dependent on the frequency and wavelength. Let's say one way the wave is c/2, then either the wavelength or the frequency would be halved, and when that reflects off the microwave walls, for the way back if the speed was faster then it would have a larger wavelength or frequency; in which if these scenarios were true, uneven constructive and destructive interferences would occur.

  • Simply.Daniel


    Orë më parë


  • joe l
    joe lOrë më parë

    I'm confused how this isn't just normal relativity, except for the infinitely fast return trip. As things increase in speed relative to an inertial observer they shrink. So a light beam would take less time to travel that distance, but time would be moving slower so it feels the same. obviously infinite almost always doesn't make sense. For it to be infinite the 'prefered' direction of the universe would have to be c which would require infinite energy.

  • Mervin Buenaflor
    Mervin BuenaflorOrë më parë

    Do not try and measure the speed of light in one direction. That's impossible. Instead... only try to realize the truth.

  • Nzeenzi
    NzeenziOrë më parë

    2way speed of light ÷ 2 = one way speed of light. Done!

  • Martinart_x
    Martinart_xOrë më parë

    What if we use quantum entanglement? If it's possible idk

  • Martinart_x
    Martinart_xOrë më parë

    What if we use quantum entanglement? If it's possible idk

  • Ivan Davidov
    Ivan DavidovOrë më parë

    start with two clocks synchronizing in the middle, then move both clocks away from each other at the same speed and position them accordingly at the start and at the finish line ?

  • pfm
    pfmOrë më parë

    just wow! The animated diagram at 13:50 makes the concept so easy to understand. Perfection. So hard to do that. Thank you.

  • Hills and Gills
    Hills and GillsOrë më parë

    could you apply quantum entanglement in some way to measure the one way speed?

  • Kai Arnold
    Kai Arnold2 orë më parë

    Wouldn't the cosmic microwave background be anisotrop if the one way speed of light is different for different directions?

  • reedofwater
    reedofwater2 orë më parë

    @Veritasium The Earth-Mars time sync analogy isn't accurate since there is no return trip, Earth uses a one way signal to send, and Mars is using another one way signal to reply, there is no automatic light bounce back.

  • Yash Gupta
    Yash Gupta2 orë më parë

    Okay, IF we create some explosion from moon which can be seen from Earth and at the exact same time send a wired signal from moon to Earth which is at 300,000,000m/s(approximate speed of light)....and if we see the explosion and receive signal at the exact microsecond then we can prove that light travels at the same speed in both the directions.....maybe...🤔🤔

  • electro grim
    electro grim2 orë më parë

    Doesn't the fact that GPS does work using the assumption prove the assumption? Otherwise it would only be correct if the tracked satellites were in certain orientations wrt a receiver.

  • ninjabagel01
    ninjabagel012 orë më parë

    I'll be honest I don't understand the math or science behind it well enough to know if this is a dumb idea but theoretically, if somehow we figured out how to teleport or figured out warp bubbles to be able to travel faster than the speed of light, those could be feasible ways to measure the one way speed of light possible right? Because then we would be able to as in your mars example get from earth to mars before the estimated 10 minutes it should take the message to arrive, so if you get to mars from earth in 7 minutes, and the message takes another 3 minutes to arrive, since you are the one that sent it you would know it took C to get there, or conversely you travel from mars to earth in 7 minutes and the light beat you there you would know it was going faster than C by at least some amount. If my simple little theory is right in principle then figuring out a feasible warp drive could become even cooler. please do feel free to correct me if this hypothesis is off though :)

  • parth Pandey
    parth Pandey2 orë më parë

    Tell me if i am wrong, Consider 2 points A and B and their distance of 1km, then choose a 3 point which is equidistance from point A and B but A,B and C aren't colinear but A and B are . Make A source of light and B light detector and a clock and C be the comand area. We send signal toh both A and B, the light from A will hit the detector on B and the clock will stop and Suprise Suprise you got speed of light in single round

  • Jack Von Gogh
    Jack Von Gogh2 orë më parë

    what if we used two clocks linked via quantum entanglement? therefore the transportation of information would be instant.

  • Kuba Swiatek
    Kuba Swiatek2 orë më parë

    I still don't really understand how the camera thing wouldn't work. If you send a really quick pulse of light bouncing off a mirror, it would show how fast the light travels both ways irregardless of the light travelling to the camera. Right? If it was instantaneous on the way back from the mirror then the camera would show that pulse just disappearing rather than bouncing back? Please someone explain.

  • Ekansh Mishra
    Ekansh Mishra2 orë më parë

    Well i will be more interested if i could know why would light prefer going in one direction rather than other when it does not get affected by any other force except for curvature of space time.

  • Anna Frolova
    Anna Frolova2 orë më parë

    So non of the laws of electromagnetism care about c being non isotropic? 🤔 and how can you define forward and backwards direction? In relation to what?

  • Sees F All
    Sees F All2 orë më parë

    What about speed light travelling through different magnetic wave pulses Bends light different magnetic charge speed up light or combine light within magnetic wave bubble theory bend space time have no resistance.. Even Nikola Tesla said Einstein was wrong on sync convention and don't need algebra figure out speed light get back observation based science first principles ignor previous papers studies

  • Sees F All
    Sees F All3 orë më parë

    Actually wrong can measure speed light need use first principle throw out all Einstein's equations and probabilities theories he's wrong . Measure it without clocks only one clock or by firing two beams light towards each other mirror theory is wrong too.

  • Raazz404
    Raazz4043 orë më parë

    Question: Video Title Answer: *Thumbnail!!*

  • Joshua Okemiri
    Joshua Okemiri3 orë më parë

    Place two clocks in the middle of the km and have them trigger their independent light sources on opposite ends and record the times on the two clocks.

    IT IS EASY3 orë më parë

    Initially, I thought I am learning something knew than I asked to myself " how long light take to reach earth from sun? and Vola!.

  • Mircea Pintelie
    Mircea Pintelie3 orë më parë

    you could start with 2 clocks at halfway the distance and move them at the same speed towards the beginning and the end of the distance no?that way the relativistic effects work the same way

  • Delton Baker
    Delton Baker3 orë më parë

    I remember someone dug a deep pit in the ground and measured the speed the light traveled across the bottom of the pit to determine the circumference of the earth. Would it not be proof by associative properties to use several other methods to determine the circumference of the earth. Then solve the pit measurement in reverse to know long it should take. I think there a solstice or lunar eclipse involved as well.

  • Danny Viunov
    Danny Viunov3 orë më parë

    very interesting but i think its wrong and its easy to check... ask the astronaut to to send a one way signal after some time... later ask when did he send it. and even if the direction is not the main factor but the stage of the light(outgoing vs incoming) you can project a ray at a mirror that connected to a device that projects a new ray back and then measure the arrival time of the two way ray vs the one way ray.

  • Danny Viunov

    Danny Viunov

    3 orë më parë

    p.s. if the speed is direction dependent you will have different receiving times at earth because earth and mars spins around the sun. (in case of a repetition of the first scenario)

  • Delton Baker
    Delton Baker3 orë më parë

    This as bad as click bate in my web browser. You got all worked up till I actually watched you episode.

  • Zelmorph Sacay
    Zelmorph Sacay3 orë më parë

    Is this also the reason why my analog watch wont sync with the time on my desktop or phone after a few days?

  • ROBIK 100
    ROBIK 1003 orë më parë

    What if you put a satellite (or several) between Earth and Mars? 🤔 And what do you mean saying a “direction”? Like the direction to the center of the universe and out?

  • FaffyWaffles
    FaffyWaffles4 orë më parë

    I can't explain why, but this feels like the ultraviolet catastrophe for some reason.

  • bohba13
    bohba134 orë më parë

    Could quantum entanglement help?

  • psychocyan
    psychocyan4 orë më parë

    "or have they" **vsauce music starts playing**

  • Daniel Blue
    Daniel Blue4 orë më parë

    Where do I send the bill for the damage done to my brain?

  • FaffyWaffles
    FaffyWaffles4 orë më parë

    1: Get a Super Powerful Laser 2: Find a Black Hole 3: Stand near the Black Hole in a way that when you shine the laser, it will bend around it, such that it will reach the observer 4: Profit

  • Winston Smith
    Winston Smith4 orë më parë

    MlIles per second

  • Anders Borg
    Anders Borg4 orë më parë

    There's a way around this, at least in theory: Consider that the universe would be created simultaneously, so whatever direction you are looking the universe is more or less uniform. If speed of light would be largely different in different directions we would see very different distant things in those directions.

  • D.A.R.K
    D.A.R.K4 orë më parë

    i know one importance to know... if one way is infinitely faster, meaning the cap of light speed is infinite and we can be faster than light one way. Also why not sync the clock close together and stay there rather than putting the other one 1km away, sure the numbers may be ultra small but i believe its doable to measure

  • geovanni lacaste
    geovanni lacaste4 orë më parë

    I wont be surprised if this guy, one day, accepts the earth-flat theory.

  • Craig Gleason
    Craig Gleason4 orë më parë

    Just curious why or if gravity is part of your equation? Also what about the reflective source is as true as the original emission of light? As a lighting person in the film industry we will bounce light off of many different surfaces. We actually take advantage of it. Even if we use a mirror I know depending on the glass that we do not get an exact replication of light in return.

  • hobo0ralph
    hobo0ralph4 orë më parë

    if it's not true for every direction, wouldn't it be likely we'd see differences in lightspeed if we measure and then angle slightly differently and measure again?

  • zaf2917
    zaf29174 orë më parë

    Ok, I didn't even take science in school, but here is my harebrained idea: Quantum entanglement. You'll need some quantum particle that gets activated by light (i.e. flips states when hit by a photon). Have the one particle at the start position and the entangled particle with the clock. The clock needs to be activated be a flip in the quantum state of the particle. Two parallel light beams are required that are activated at the same time. One hits the particle at the start, which flips state as does the entangled partner (which also activates the clock). The second beam travels to the clock, and hits the particle there, flipping it and stopping the clock.

  • WhyBecause
    WhyBecause5 orë më parë

    But why would the returning time from B to A be instant? What really changes once the speed of light reaches B that the speed for going back would change?? Light has the same conditions when traveling AB and BA. Only thing that changes is that the connection is established once it first touches B. So would that in any plausable theory be enough for speed of light to change going “back”??

  • Peter Adams
    Peter Adams5 orë më parë

    Use a quantum computer to teleport information, problem solved. TBH i seriously hope that the speed of light is different, because everything about what we think to know about the universe, including the Big Bang, falls apart.

  • Abhinav Bhatnagar
    Abhinav Bhatnagar5 orë më parë

    Let's talk I think I know a way to measure c

  • Joe Perry
    Joe Perry5 orë më parë

    DISCLAIMER: I AM A MECHANICAL ENGINEER MAJOR IN COLLEGE, I AM WATCHING A ALthe VIDEO ABOUT THIS, I AM OBVIOUSLY NOT AN EXPERT IN PHYSICS SO THIS MAY HAVE SOME FALTS OR MAY HAVE TO MANY VARIABLES TO BE CONSIDERED AS A WAY TO MEASURE IF TIME HAS A PREFERED DIRECTION OR NOT. Thank you Let the area that you are using be flat, let the area you are using be a vacuum, let the area you are using have no variation gravity. This is not made to measure time but to see if there is a difference What if you do the middle clock thing, moving both clocks away from each other from the middle and measure said dilation. Once you have that number, repeat it to see the average. After you have done that, do it again, shooting from clock (A) to clock (B). This would not be done using time to measure but distance, use two wheels that are the same size to measure the distance, as they roll along the ground to measure how far you are. (it doesn't matter as long as you know where the clocks are) Once you have the time that it takes to get from (A) to (B) then you have the amount of time it takes for light to move in that one way, you can do this more than once, however, less is more as the more you move the clocks around the more likely it is that you introduced variables. Now do the same for the way back, moving the clocks to make sure that the dilation is the same again so that everything is even, then measure the time it takes for light to go from (B) to (A), take away the average you set in the beginning and now you have a number that should be the same as the one from (A) to (B). If these numbers are not the same then light may have a preferred way to go in the universe, if they are the same then more than likely, light is a constant speed. OPINION TIME I do not think that light has a preferred direction, though it may have varying speeds depending on gravity and other variables, I think that it is constant as long as the conditions that it is going through is constant, this is because light is energy, energy can not be destroyed or lost because it is going one way, unless light photons are always spinning in one way. but that is another discussion for someone smarter than me.

  • Flare Corona
    Flare Corona5 orë më parë

    From common sense perspective you could just use three points over a very long distance. First is to send the light and receive it. Second is to block or allow light through depending on expected time. Third would be to reflect the light back. You run this experiment a few times, blocking the second point of passage at different times. If the return of the light is instant, then it should be stopped within this point if it is closed. For 18 minutes it would be open, for the last two closed. If the light still reaches both mars and back through that point, the return of the light is not instant If it does not, then it is significantly faster on the return trip. Then you finetune that, until you find that border. If nothing else it would give an idea of the speed of light relative to that particular distance. ( TL;DR Just block the passage of the light at near the expected return to the original source using a point between the two. )

  • אברהם אנסבכר
    אברהם אנסבכר5 orë më parë

    i dont realy understand how the speed of light can be different in different directions, but if it so, what is the meaning of the speed of light? there will be no one speed of light, we will have a spectrum of speeds... is the Sound behave the same way? because if the voice speed is permanent maybe we can calculate the speed of light by calculating the remainder between the light and the sound...

  • DaFragileTomato
    DaFragileTomato6 orë më parë

    the intro gave me a stroke

  • Dave
    Dave6 orë më parë

    That is easily measurable if you just take laser stick in smogy area and point it to the mirror on the ground under 45° and use extremly high speed camera to measure the time it reaches the mirror and the time light reaches exactly same heigh the laser is on the other side. (\/). Then if there is any difference we can asume it travels in different speed in different direction.

  • Marcin Gosiewski
    Marcin Gosiewski6 orë më parë

    Clickbait title and the whole reasoning is actually wrong. The mirror method of mesuring speed of light is actually fully valid. Using this reasoning from the movie you can easily get to conclusion that you cannot measure the speed of anything. Even the ball. The same problem with clocks synchronisation applies to the ball speed measurement. The problem is only less obvious. Additionally one can argue that the ball slowas down during flight so you are measuring only AVERAGE speed over distance. The same with the light. You measure average speed over back and forth trip. Even more: the clock synchronisation problem can be avoided using one clock placed like the camera is placed - outside the light path, taking 2 measurements with different distances of the ‚calock/camera’ and using some geometry and math to eliminate signal time to camera problem.

  • Gillie Siut
    Gillie Siut6 orë më parë

    What if there is some sort of dark mater current that can effect the speed of light

  • Thomas Arter
    Thomas Arter6 orë më parë

    Aren't all astronomical observations based on the speed of light being the same in all directions from which they are received? If that isn't true then which celestial bodies are possibly being observed in real time and which are being possibly being observed billions of years after the light left?

  • J Gamer
    J Gamer6 orë më parë

    Can mark like bring a clock from Earth to Mars and when people on Earth sent a message to mark at 12:20 and he received it on 12:30 and replied he received it on 12:30 the reply reached Earth on 12:40 so it took 10mins for the message to reach

  • J Gamer

    J Gamer

    6 orë më parë

    This is breakin my brain

  • Es Jay
    Es Jay6 orë më parë

    I haven't completed this video while writing this so please forgive me if you answered this question. You could maybe use quantum entanglement to synchronize the clocks. Possibly using two coupled cells of a quantum processor to both change states at the exact same time. The state could change when the light is detected at the first clock, change again when it reaches the second clock, and you could add a mirror to repeat the process in the other direction. I'm not sure what distance we're currently able to study quantum entanglement, or if we might be able to use that to change states of quantum cells, but if we CAN do that at a known distance, we should be able to test whether or not light moves faster in a specific direction and accurately measure one-way speed of light in a given direction. The question then becomes is the speed of light in any direction constant? If not, at what distance (if any) would the average speed be measurable? If a distance exists in which the average speed is measurable, is it the same in all directions? To answer that we would need quantum entangled clocks at different distances, light sources closer and further away from the clocks (as well as mirrors), and beyond that, we would have to decide whether or not the speed of light in a vacuum tube is the same as that in the vacuum of space. (Would an enclosed environment change the way in which light travels?)

  • Mukul Namagiri
    Mukul Namagiri6 orë më parë

    Its kinda fishy man the man on the mars feels thst he got the speed 20 mins late 🤔

  • Joe Nah Shin
    Joe Nah Shin6 orë më parë

    This might be kinda stupid, but *WHAT IF,* the speed of light in one direction is _greater_ than c/2, and the speed of light in the other is _negative._ The math still works. The two-way speed of light will still be 2c, but there will be some weird time stuff happening if the speed of light in one direction is greater than c/2.

  • Ian Barone
    Ian Barone7 orë më parë

    Okay riddle me this: You are on an infinite flat plane, you (A) and your friend (B) are three feet apart. Behind (A) is a Star minus the heat but all the light. (B) is waving his arms at you up and down. If (A) were to have unimaginable vision and could focus on (B) no matter the distance. If you slowly moved (B) from (A) to a distance of 1 Light year, would there be a moment where you no longer see him waving? Would it slow down? I ask because of the long told “if you look at a star you’re seeing it for what it was thousands of years ago. Which makes sense granted light does have a speed. So if (B) is 1 light year away from (A) you would HAVE to be seeing him in the past...? I struggle to comprehend that hypothesis. I believe (A) would be seeing (B) in real time even 1 light year away. But all science disagrees with that because what I believe would mean that light is instantaneous and has no speed. Anyone agree??? Or am i crazy?

  • Caleb Clunie
    Caleb Clunie7 orë më parë

    Sono-Luminescence is a thing. How many Beats Per Minute (BPM)? Did I just earn an Honorary Doctorates Degree?

  • Scotty
    Scotty7 orë më parë

    Bit late but what if you set up a rig that released a reflective panel that will intersect the path of a lazer an exact time after release? Fired at a specific time after observing the release of the panel, you could measure the distance between the source and the panel to find when you should fire the beam. If the speed of light is actually 299,792,458m/s, then you can predict when to fire the lazer beam. If it reflects the beam back at the source, then you know that you were correct, however if it misses it entirely, then you could say that the beam needed to be fired half as early because light is actually travelling at only 149,896,229m/s. Maybe there's a hole in the logic somewhere but can't it be measured in such a way?

  • VideoMem
    VideoMem7 orë më parë

    If there is no way to measure it we cannot say there is an error. However...

  • Riot Viktor
    Riot Viktor7 orë më parë

    The simulation doesnt want us to know the truth

  • Gabestew
    Gabestew7 orë më parë

    i like the martian reference to mark watney. extremely interesting, I'm in honors physics and these videos help my understanding of some complicated aspects of it alot.

  • dheer prasad
    dheer prasad7 orë më parë

    well if light travels at different speeds in different direction refraction through a prism the refracted ray in the prism would have a slight bend in the prism wouldn't it. I am no expert

  • Zar 15
    Zar 158 orë më parë

    Put a Highly accurate strobe light on the moon, 0 out on any pulse now measures the pulses from earth , if there's any shift in the timing then light isn't the same speed.

  • Jaggith
    Jaggith8 orë më parë

    Are we not able to measure this with entangled particles?

  • Templarfreak
    Templarfreak8 orë më parë

    Here's a weird thought experiment I had that this video lead me on that I felt like sharing: 1. Assume an otherwise totally blank universe. 2. Two objects conveniently at the same starting position. We inconveniently don't know exactly where that starting position is. 3. They travel in opposite directions. 4. They have the same period of time to travel in. At which point, they instantly come to a stop and will conveniently lock into place so we can't accidentally bump them. 5. we don't know how fast they are travelling. Is it possible to tell if one object has travelled a further distance than the other or not? Technically no. Hell, one of the objects could still be *at* the starting position. With nothing else to go off of, the only thing we can do is measure the distance from one of the objects to the other. We'd then just assume somewhere at half that distance is the middle point between the two and that the middle point could be the starting position. We have no way to actually "prove" that was the starting position, though. We can probably prove where their starting position is if we knew how fast they were travelling, but we don't. We lack this information and have no way to get it. This is kinda how this problem seems like to me, but in this case it's a matter of distance instead of time.

  • Stanley Dang
    Stanley Dang8 orë më parë

    For the first example, what if instead of the second clock stopping the first clock which causes a time delay. Why can't the light just start the second clock when it gets there. Thus if you find the difference between the two clocks it would be the time it took for the light to travel from the first clock to the second clock. start clock 1 when light is fired, the light gets clock 2 then clock 2 starts counting. so if clock 1 is at 10 seconds and clock 2 is at 2 seconds, then it took 8 seconds to travel whatever it was. Thus not needing to sync anything and no time delay.

  • Johnnyboy
    Johnnyboy8 orë më parë

    Because of Newton’s third law of motion and light also exerts radiation pressure.

  • MrFreeGman
    MrFreeGman8 orë më parë

    Maybe I'm missing something, but why would the question as to whether light travels faster in any particular direction even arise to begin with? What property of spacetime could exist for this to be the case? Correct me if I'm wrong, but this seems like nothing more than a pointless thought experiment. Can you prove that every time you blink a little fairy that's only visible to you doesn't appear and disappear from your field of vision while your eyes are closed? Why is that possibility any less likely than the possibility that light travels faster in certain directions? If the physics works the same one way or another, what value is there in pondering it? Why this recent trend where we must constantly question our basic understanding of the nature of reality to the point where people have stopped believing in the concept of truth altogether? Science need not be an exercise in nihilism.

  • Monil Gunjaria
    Monil Gunjaria8 orë më parë

    Why would the message sent from the Mars would reach instantaneously? They are using the same devices to transmit!

  • Jeff Osborne
    Jeff Osborne8 orë më parë

    You would think that the intensity of the light would have some sort of factor in its speed...Like a really bright, high intensity light would be faster than say a candle light. A friend told me about this channel today...now I'm binge watching.

  • B_Rad
    B_Rad8 orë më parë

    prism in a vacuum with a light sensitive sensor at the end that records independently from the laser, and start 1 clock at the very end that only measures until the light hits the sensor?

  • Takuro_Spirit
    Takuro_Spirit9 orë më parë

    Is there any evidence to suggest why the speed of light would be different in any given direction? Is there any significance to any given direction that would indicate light would go faster? I feel like physically it has to move the same way in any given direction. Take a star radiating light in a spherical shape around itself. Photons are ejected with roughly the same energy all around the surface of the sphere. What inherently about directionaity could change the physical properties of a photon to make it go any faster or slower than it would have normally?

  • Nicolas Pacheco Monroy
    Nicolas Pacheco Monroy9 orë më parë


  • Harlan Gilbert
    Harlan Gilbert9 orë më parë

    The very first measurement of the speed of light, by Olaus Romer, measured the speed in a single direction: from Jupiter’s moons to the Earth. The principle could be used today with greater accuracy than Romer achieved.

  • GetMoGaming
    GetMoGaming9 orë më parë

    If light was faster in one direction, wouldn't we have observed a difference in the back ground radiation reflecting those differences? I.e. the universe would be larger and so more red-shifted in one direction.

  • Kabss10
    Kabss109 orë më parë

    If speed of light isn’t constant then wouldn’t that mean the expansion of the universe would be different in all directions? And if it’s different then wouldn’t we be able to observe the difference of expansion in the universe to know for a fact that C is different in any direction? But if the universe expands equally in all directions would that mean the speed of light is equal in all directions or are we actually observing it in one direction which then means how do we know which direction is the speed of light ? I would say logically the speed of light has to be equal in all directions otherwise we would have been able to observe it as the universe has been expanding for billions of years so even a slight difference in speed would have been obvious

  • Eli Deaver
    Eli Deaver9 orë më parë

    Wouldn’t stars in one direction not have any light delay? So a telescope looking in the far distance left would see stars that can be identified cosmologicaly as from a different time period? This kind of seems like moving two clocks away from each other, but the way I’ve always understood the expansion of the universe is that through math magic the rug of space is being stretched underneath everything, and I don’t think that would count? I guess whatever bit of math explains how the universe expands also probably depends on the speed of light.

  • Zedekai
    Zedekai9 orë më parë

    If light is potentially instantaneous in one direction, what does that say for near lightspeed travel in that direction?

  • Zach Morgan
    Zach Morgan9 orë më parë

    But how & why would the arbitrary direction change the speed and how would it be instantaneous

  • SavageGreywolf
    SavageGreywolf9 orë më parë

    why not have two beams of light? Clock A fires Beam a at Clock B and Clock B fires Beam b at Clock A. Have clock A measure beam b when it arrives as well as reflect it back to clock B and vice versa. You cannot ensure the clocks are synchronized, but you _can_ determine when each 'foreign' beam arrived relative to the 'home' beam. Then you compare the two measurements and you should be able to determine, at the very least, whether there is a directional bias in the speed of light. Or is there something about special relativity I am not considering?

  • cardo deloro
    cardo deloro9 orë më parë

    this video is the best medicine for insomnia!! it cured mine..everytime i want to sleep, all i do is to listen to these two conversing..👍 try it..

  • Zach Morgan
    Zach Morgan9 orë më parë

    I feel like your making this harder than it has to be

  • LastAvailableAlias
    LastAvailableAlias9 orë më parë

    You can do this reflection experiment in any direction and see the result is the same. A difference in directions would should a violation of universal frames of reference which has never been seen.

  • Aidan Roberts
    Aidan Roberts9 orë më parë

    So one way we could potentially measure this would be quantum entanglement (not a well versed person on this topic so i am probably missing things), at the moment we cannot use entanglement to transfer information, but should we be able to in future you could sync the clocks and measure the speed of light in x direction. That opens up a whole bag of worms on how, and where to measure to remove all doubt, as spacetime/relativity could affect results. Also that then results in people trying to measure entanglements speeds in different directions in the same way, thus transfering the problem up?

  • LastAvailableAlias
    LastAvailableAlias9 orë më parë

    This is also why sci-fi movies showing someone reacting to a laser blast are impossible. You'd have to see the light before it reached you to react to it. You'd be hit by it at the same time you detected it.

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