Why The Speed Of Light Is Unmeasurable

Birt 31 okt 2020
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"


  • After this video, it seems to me that every physics video (and school) is lying about these things. Like how far away objects appear to us as they were in the past, which is frequently used to argue in favor of theories about the structure and the age of the universe. It is really disturbing that our current science is based so heavily on assumptions that the truth could be basically whatever. If there even is one.

  • Allow two people to be a kilometer apart. Program a laser to shoot at the top of the hour. Program the clocks to start at the top of the hour. There may be a small delay but you may be able to control the delay

  • What if you synchronized the clocks with a pair of entangled particles (wave function collapse)? Would that be possible?

    • You can't use entanglement to send information faster than light, so you can't use it to synchronise the clocks like that. Otherwise it would violate causality.

  • Simple, if you cannot trust light then use sound to sync the clocks. If you say "the speed of light will change how sound moves" then at least you could prove that the speed of light is not infinite in either direction because the speed of sound is known to not be infinite. It is much easier to observe sound waves than light waves so this is easily proven. Also, the bottle with the visible light pulse should also be a one-way measurement because rather than measuring a return path the measurement direction is orthogonal to the path of the light being measured and thus is normalized to that direction. Thus, you are measuring the one-way speed of light. The alternate explanation is that the light moved instantly through the bottle but the light from the bottle decided to change its speed to move quickly from the bottom and slowly at the top. Basically meaning that even in a single direction the speed of light changes but is centered and defined by a water bottle kept in your basement. With the minimum speed being C/2, you could easily disprove this with a bottle that is long enough such that this increasing slowdown would exceed that minimum.

  • My thoughts as soon as he started talking about the roundtrip of light. "He has a point but this makes no sense!" DX

  • What about a quantum entangled clock set? A laser hits the first clock, triggering both at the same time and stops when it hits the second?

  • They have measured it using photons.

  • Well what you define 2 way might just be 1 way , like you need to see it as a whole

  • Is it possible to lay light as long as possible? instead as we travel thru light itself?, we travel in inside light = instantaneously? Have anyone try it before? I think it this way. But I have no science base theory in head. So please don't toxic me or scold me. I drop out from school at the start secondary 3/ high school 3rd year, and my school teaches me how to stand on a school court for several hours daily, due to always late in class. Is a habit that I can't change, I feel like I doesn't lives 24hr a day as per normal human, my body clock always lives in a 36hrs clock than usual.

  • We are observing the sun and the sun has movement on it. It spews plasma and does all kind of things we can observe. If the speed of light is faster in one direction, we would observe the suns natural ground movements to speed up and slow down as we move around it. But we do not observe this effect at all. Therefore the speed of light must be the same in all directions, or at least extremely close to the same speed.

  • Hey bro can you figure out life? I think its a virus or a type of after corona this year

  • Wouldn't the stars have less heavy elements in one direction than another if the speed was different? Spectroscopy on one side of the world would have a statistical increase of heavy elements. Unless my reasoning is off.

  • If the speed of light is variable, it will not be a constant anymore. If it is defined as a constant, in both way the speed should be the same. Or are you saying that the speed of light is distracted by some external matters, like frictional forces? I dont understand the meaning the speed of light is not the same in different direction.

  • may i remind you speed of light is constant.So why would it travel at different speeds at different direction?

    • No, pay attention, it is *_constant_* *_in_* *_a_* *_2_* *_way_* *_trip_* . A 1 way trip left may be different from a 1 way trip right. Think of it this way. The speed of light is AB. AB is constant. But is A and B the same?

  • Michaelson Morley proved light is the same speed in both directions😀

  • The cosmological constant bro. Eaaaasy

  • The comment SECTION is full of relatively FUNNY' jokes or is it?

  • What's about a detector placed at one corner of a equilateral triangle , with the beam traveling on the opposite edges ? 2 small mirror sample the light pulse and direct it to the detector , one clock mesure the time in between the 2 signal . et voila , the light return path is 60 degrees instead of 180 , surely this would expose some contradiction with other measurement?

  • Laser on each end. Camera on each end. Difference between the two will determine whether the speed of light is the same. Unfortunately your problem is easy to solve. The real problem is that, after you use that solution, are you seeing the difference in speed of light in each direction, or are you just seeing how fast we are moving in one direction?

    • How would you synchronize the clocks on either side? He talked through this in the video, it doesn't work.

  • Would we be able to test the 1 way speed of light in our distant future, if we could come up with some craft that could collapse a star, and send it out knowing exactly how long it will take to reach it's destination from our reference frame. Then whoever is alive when it arrives, if they still know about the test, could determine if the light from the star disappears immediately, or if it stays present for a longer period of time, proportional to the number of light years away it is and the speed of light, than we would expect if C is the one way speed. Interestingly enough, if this proposition is not flawed (which it probably is, as I am no physicist), it would be measuring by turning a light off instead of turning one on.

  • Are you implying that all combinations of speeds that are valid occur in some universe? This is quite intriguing. Indeed, it is known the speed of light is the average (group) velocity of photons. And that hypothesis agrees with that of John Smith's: ischats.info/fun/fNibin99aV2Ln3w/v-deo . Smith says that the sum over all Feynman paths in the universe must remain constant.

  • What about synchronizing the clocks with entanglement? I don't have the physics background to know if that's a sentence that even makes sense.

    • Entanglement cannot transmit information (at least not without the help of some classical bits as well but that defeats the purpose here) so you can't use it to synchronize the clocks, sadly.

  • Your mars example demonstrates that it must be the same, due to symmetry. Just reverse the experiment. Let the guy on Mars send the pulse. The return signal comes from Earth.

  • Please read me! There's no way to measure the speed of light directly because there's no way to send information faster than the speed of light. If we used a phenomena clock like "start the clocks when you see/detect the moon explode", that perception of the moon event is still based on light. If we had a better way to send information than light, then we could measure light with that other thing. Like Gravity Waves or something might be useful. "When you detect gravity waves from the moon exploding, start the clock". But so long as our ability to measure light also depends on a synchronization event, you can't design a syncing event without light itself being the unifying aspect. The only other possibility is a set of clocks all along the path which are stationary, and all synced to each other to begin with. Just use stopwatches and set all to zero. Have one switch that turns all of them on with one signal. Then after the clocks are started, shoot the light. As the light passes through each "gate", that clock stops. The detection of light in each gate would be a constant you can subtract from all values, and the time of clock 0 (so the time between starting all clocks and when the light is first emitted) can be subtracted from every value. If the pre-emission time is removed as a constant, and the detection value is removed as a constant (on average), then the leftover value should be the time it takes for the beam of light to go from Gate A to Gate B. And if this is a long thing that goes in each directions, accounting for the time it takes to reflect at various points, you could, at the very least, predict whether the speed of light is constant or if it varies.

  • The speed of light in nature isnt a constant. And we know that. Just like the meter in nature isnt a constant. We just chose to use the speed of light as a constant, just like the french chose to use one 10 million-th of the distance from the equator to the pole as a constant of the metric system. Its all focus points in order to build a system around. If we have no constant, no settled points, no references, theres no system to build. But if we use constances, we can build a system. And no matter how right, wrong or partual we were about them,the system will still be mostly,if not fully, true. Physics, maths, measurements. Ultimatively it all us only an approximation to reality. And thats all it ever will be. We can get ever more exact. But we will neber find The One True value.

    • The speed of light IN A VACUUM is constant, or measured as such via two-way experimentation. "c" does not fluctuate

    • In some way, religions are right. Science doesnt tell you the truth. Because it doesnt. It only tells you the closest approximation to the truth they, until the moment they tell you about it, know. Doesnt mean religions are more correct or exact with their truths though. Ultimatively, they only approximate the true nature reality too.

  • There IS a way to tell the difference between a 50/50 and a 100/0 round trip. You setup multiple differen forms of measurements and sync them up. Assuming you account for unknowns with tolerances, you can get a pretty good idea what reality is like. Yes. That doesnt allow you to measure the exact speed of light either. But lets be honest. Measurements, physics and math are all approximations to reality. All we can do is get more exact. But never hit THE true value.

  • Just wait a few more years and you should be able to use quantum entanglement to synchronize the clocks. Then they will be exactly the same no matter the distance. Unless quantum entanglement is a farce. Then you may have to wait a few billion more years to watch the universe start to dim. If it dims equally all around you, then the speed of light is fairly constant. It's a long wait for an answer though.

  • Hey Derek! I saw this video about free energy, I know perpetual motion machines are impossible but this one seems half legit, what do you think? ischats.info/fun/kbCTeXmAqYemf2s/v-deo "Free Energy Generator Using Armature Method"

  • A thrid point in space at the same distance to the measure point as well as to the starting point of the light beam would solve this problem when used as a "trafic light" as a reference for when the light beam is triggered and a light beam is shot from both spots and measured on both spots. You're welcome physics.

  • Let the length of a timer be 1km, i.e., the start button is 1km away from the stop button(of course the buttons are light sensitive) Now set up a laser at the start button and send light. As soon as the light hits the start button, the timer starts. It travels 1km distance and then hits the stop button. Now the speed of light c = 1km/time recorded Did I solve the problem... Or am I missing something?

  • 1. Fire a photon exactly at 5 seconds. 2. The first fire starts a timer at the other end. 3. At another 5 seconds fire a second photon. 4. The second photon stops the timer. Iterate to confirm. That way, you can measure the difference and see what offset the 5 second interval has respect the distance. And that way we can measure the speed of light in one way.

  • Can't you synchronize the 2 clocks with something in the middle using a conductive independent of light? Also, what if the light between here and Mars takes a million years and then goes back in time a million years minus 20 minutes?

  • A sphere of clocks

  • Use 2 pulses of light instead of one. First pulse turns the clock on (on detection) and the second one turns it off. The clock reading should say how long it took the second pulse to be read by the clock, repeat for all directions. All pulses must be set to the same interval. The readings must differ for this theory to be correct.

  • Remember when this guy had 100000 subs?

  • Split the beam to two paths of different lengths, the difference can be used to calculated the speed however, does the speed of light remain the same outside of a gravitational field and in a vacuum.

  • What if we used two gears, or a chain of connected gears and repeated the original experiment? Shouldn't there be a way to measure when the light actually returned back because it would have to pass through multiple gaps at a certain speed in order to be seen returning. I didn't think this through very much, my Drew is lazy, but it seems like there should be a way to use precise nanogears with different gear ratios to determine whether light is moving instantaneously.

  • I wonder if quantum coupling became measurable, could it be used to synchronize the clocks as described in this video in order to obrain a one-way speed of light?

  • The universe appears to be isotropic and to change over time. For example we see more numerous and power quasars at greater distances, thus at younger ages, in all directions. If the one-way speed of light, call it c1, is constant in all directions, then this appearance is a consequence of the universe BEING isotropic and changing over time. If c1 is not constant, then the universe needs to be anisotropic in just the right fine-tuned way to cancel out the anisotropy of c1. It is in a sense another Occam's Razor argument, but one "bigger" than just the local physics. On another note, if you allow c1 to vary with direction, what even constrains it to be positive? Why not slow light one way, and the other way the light goes backwards in time and arrives BEFORE it leaves?

  • What if theres an observer on the moon, and earth sends a massive laser beam to mars, witch can be seen front the observer perspective and them 5 minutes later mars shoot back to earth, wound’t the guy on the moon be able to measure it ??

    • But then light has to reach him anyway, so i dont know

  • What if we take the two clocks and start both of them with a beam of light. Then they have the inherent delay of the light going in one direction but not the other A->B=X. So if we shoot a beam of light from the other side there can be two options the light zeros out the clocks or there is some time left on the other clock B->A=Y. Either X=Y or X≠Y.

  • Doesn't this mean that a meter is also unmeasurable? Isn't it defined by the one-way speed of light??

  • wait, what if you use constant pulses of light from 2 different points to synchronize with a point in the middle so that you can sync the clocks together, at that point you fire another pulse (different light from the pulses because the pulses are still going to make sure the clocks are still synchronized) from each side then record when the lasers are fired and when they are received. The only problem is that if the clocks are ticking at different rates due to gravity then accurate measurements wouldn't work. Basically if you get both pulses sync'd at the center clock then you can sync the 2 outer clocks. Getting the pulses to sync properly would be a pain but as long as you can get the pulses to sync up in the center with the same number of pulses (pulse 10 from point A reaches the center at the same moment as pulse 10 from point B and so on) then the clocks can be sync'd. Once all three clocks are synchronized then recording the times that lasers are fired from each side and received on the other side the time difference should no longer be based on the speed of light but rather how accurately the clocks are ticking the time.

  • funny guys talking about physics studying on facebook.... Dude if they have synchronized clocks WHY DO YOU CUT 10 MINS from astronaut s reply?? And if they have to synchronize it why not saying as reply “got the time, replied after X minutes!!!”

  • Why will the two clocks not work? We cannot use the two clocks to calculate the absolute speed of light, but we can measure the speed in both directions separately using the same setting and see if they are different? The error due to non synchronisation will cancel right?

  • It isn't the speed of light that matters. It is a mathematical constant but can't be accurately measured because the curvature of space time makes that impossible. But your proposition that light may travel faster in one direction than in the other is easily solved by running identical experiments in different directions.

  • Have two two-way trip light timers facing away from each other. For the light shot towards or from the mirrors, slow it down with some transparent material. There might be a time difference in trips now?

  • Lightspeed kind of defines time ("right now" is when I can see it -- there's no better definition) so maybe the problem is we're trying to disentangle these two things because we grew up thinking they must be separate but then reality goes NOPE.

  • The method that Ole Roemer used in 1676 also measures the speed of light in one direction only. It's based on the time between eclipses of Jupiter's moon Io. When the Earth is closest to Jupiter the eclipses happen 11 minutes sooner than average, but when Jupiter is farthest from Earth, the eclipses happen 11 minutes later than average. Thus (given the size of the Earth's orbit) the speed of light can be calculated with only a one way directional measurement.

  • Maybe you could measure the speed of light putting a clock at each end of the tunnel and then comparing the readings of the 2 clocks

  • I dont think there is a reason to change speed of light depend on the direction.

  • What if you use a monochromatic light of a known frequency, make some standing waves with interference and measure the distance between standing waves. Does this involve any assumptions? If the speed of light is different in different directions - you'll have different standing waves picture depending on the direction

  • One direction method: start only one clock at the receiving end, it records the moment light hits a sensor and detects for the following sound of a gunshot. On the sending end, you have a machine which synchronizes a gunshot sound with a beam of light. Your receiver determines the start time by using the 2 knowns: distance between the sensors and the speed of sound. Now obviously compare the derived start time with the recorded finish time and you’re there.

    • In order for this to work, you’re going to need to measure the speed of sound to an insane precision, a precision which will be unobtainable due to the synchronisation issues discussed in this video.

    • I was wondering this as well. If you know the exact distance, and synchronize the sound with the light pulse, then you should be able to calculate exactly how much time has passed since the light left. I imagine there's some very basic logic error here.

  • Ok so - we can look out and see thing that we know happened a long time ago, as they coincide with the timeline associated with the Big Bang. And we can see these in every direction, so ... how is this still valid?

  • 11:32 "just how differently the world works if c is not the same in all directions". Wrong. By the definition if there's now way to prove theres a difference, then there's no difference.

  • What if we use a 1 km clock???

  • Waaaaiiit a minute.... Wouldn't you be able to see farther away in one direction than the other if that were the case??? Aka you would see more stars in one side of the universe than the other.

  • You said after the light hits the mirror, the speed it returns could be infinite? So is there a theory the return speed is instant? Like it travels to an object when it hits it, it's entangled already where the observer is viewing from where it left?

  • Sincroniza os relogios exatamente na metade do tubo de vacuo... nos 500mts ... dai separe os 2 simultaneamente na mesma velocidade referente a um observador estacionario no meio do tubo ... quando eles chegarem nas extremidades do tubo se a velocidade de ambos foi exatamente a mesma referente ao observador no meio do tubo...ambos sofreram a mesma ''dilatação temporal'' então voce pode medir com 2 relogios... cade meu nobel''???

  • Ok, but if we look into the deep field Hubble photos in different directions do they look the same? wouldn't one direction look different if light traveled faster in one direction?

    • Also, I just want to add I just think that time is a byproduct of movement.

  • Okay, so what if we use a clock with a device that receives the signal and communicates it back, using a pair of tangled electrons, and sends a quantum bit to signal the precise moment of being hit by light at the end of trip? (of course assuming we have technology to do that)

  • Speaking of the speed of light.. I learned of Anatoli Bugorski recently. In 1978, he had an accident when a high energy proton beam (moving near the speed of light) from a particle accelerator passed through his brain. He lived then, and he's still alive today. Fascinating.

  • First off, my education is more or less self-taught. Not that it's a bad thing, but I'll have a few missing pieces at times. When dealing with this experiment, the video shows light going in one direction for testing all the time. You can't do that for at least one of them. I liked the idea of having a pulse in the center to verify time placement. Although, I'm not sure that would work. You can't have light come from one end or the other when your control is in the center. The light needs to come from there too. With that way of testing, there still needs to be two timers at a predetermined distances from the center. However, I would also put a timer and light sensor in the center as well. In the center, there should be a split mirror, to shine the light into two different directions. You'd need a light sensor and timer at said mirror, to start the test from the start. By having two paths, you can also have a control and a variable. This gives you an opportunity to do a proper Mars to NASA test. Light is traveling in both a vacuum and two different atmospheres in that scenario. So you could easily accomplish that in one or two goes. Also, you could do the Double Slit, "Photon Test," to see if speed has a factor. To know how fast and the amount of photons that show up at the speed of light. By having two paths, you're given a control and one to experiment with at the same time. There are so many possibilities you can do with this... I hope.

  • -You generate 2 entangled particles. -You keep one at clock 1 close to you. -You send one at clock 2 1Km away. -You "trigger" particle one so that particle two is also "triggered" simultaneously. -You use the "trigger" to reset clocks when you shoot the light beam. -Bam! Synced clocks at 1Km distance.

    • Manipulating entangled particles collapses the wave function and causes them to disentangle.

  • If you look to very distant stars and compare they age with the age of the universe, we should see very young stars in one direction and very old stars in the opposite. Maybe that wouldn't tell us the speed of light but it would tell us something about it's preferred direction. I'm not a physicist is just an idea...

  • What if you use another "known” speed - like the speed of sound - to synchronize both clocks ?

    • What the video didn’t really touch on is that the one-way speed of anything is inherently ill defined, it just so happens that for slow moving things, the uncertainty is very small. Unfortunately to measure light speed you need very high precision, which is simply impossible to obtain.

    • To effectively measure the light it needs to be in a vacuum so as to not have any interference, and acoustic sound waves ONLY exist when there is a medium

  • Snell's law states that the refraction of light, as it moves from one substance to another, depends on its speed in each substance. If the speed of light is different depending on direction, shouldnt refraction also be different? If this is the case, couldn't we use this to measure the one way speed of light.

  • Couldn't you just use a physical control variable such as an object going the same direction as the beams to stop the timers? That way the instant the beam is released the object is in a physical, set, measurable velocity and then you can compare both directions with the time of the beams. That way you know the object will hit the timer at X time due to it's velocity being set instead of using signals that are going at the speed of light.

  • Let's say that between Mars and the Earth there is onother planet that could catch the signal and then send it immediately to Mars, then re-catch it when it comes back to Earth and re-send it immediately to Earth. If we knew that the signal should take 20 minutes to come back here, then it would arrive at the central planet in 5 minutes, then a pause of 10 minutes, then the signal coming back to the central planet and then, in 5 minutes, coming back here. If light's speed took actually 20 minutes to arrive to Mars and 0 seconds to come back to Earth, then the central planet would catch the signal after 10 minutes, then after other 10 minutes it would re-catch it and then in 0 minutes it would arrive to Earth. If the central planet told Earth that he caught it, then we would know that light's speed isn't the same everywhere

  • what if as u said in your videowe use 2 clock which are conected to eachother but instead of using a pulse signal. we use a mechanical option. so as we know mechanical movment moves at the spead of sound in that material so then we can subtract that.

  • Derek, sorry i lied to you. Please upload once per 2 weeks. I really miss your videos aaaaaaaaa....

  • Does the speed of light change when it is reflected? Surely stopping motion and changing direction should take some energy.

  • OK, here is one mental thought. Let's suppose that speed of light is quicker in one direction and slower in opposite. And we put a light bulb that emits some light on some simple constant frequency. We call this place B. On the left of the B is spot A where we put one observer. On the right of B (opposite of A) is C where we put another observer. And now A and C meassure the frequency of the light from B. And let's say that speed in direction ABC is quicker and direction CBA is slower. If speed is not same in both direction then A should detect higher frequency then B measure on the source (Blueshift) since more waves are in the same amount of space. C on the other hand should detect lower frequency of light than B (Redshift) since less waves are in the same amount of space. Unless time does not pass differently for A, B and C, this had to occur and as I understand, this does not happen. So, does this qualify as a proof that speed of light is the same in both direction? Or am I missing something here?

    • @Max oobbxxx I'm not talking about measuring one-way speed of light. This experiment is about to determine if speed of light is the same in each direction. In my experiment, A, B and C are stationary relative to each other. And they does not need to be far away. They can be two meters from each others, doesn't matter. Actually I alone should be able just by circling around the source of light to see different collors of the light bulb depending of the direction in which I look into it. Or maybe not, my brain hurts currently. :) I need to think more about this.

    • No, it does not. What you're describing is a typical "aether rest frame" experiment, and Lorenzian aether model cancels out the effects you describe via the Lorenzian length contraction and time dilation (also used in the special theory of relativity). Basically, and attempt to measure the one-way speed of light is essentially trying to detect the global rest frame.

  • Thank you. I have been waiting so long for this video. Now my friends dont think Im crazy anymore.

  • Let me add another dilemma. Our eyes are just a sensor what is the speed of sensing of our eyes and are they the same for all people ? Computers with data bus of fiber optics was not possible because we couldn't make a light sensor that could change states quick enough

  • Could you not just set up two lasers and two clocks one kilometer apart and when the laser closest to its respective clock starts the clock is started and the light from the other laser stops it on both sides it would measure in both directions at the same time

  • The answer is quantum entanglement. It's faster than light. We only need to do it over a longer distance than some nanometers

  • Have you ever measured an equilateral triangle ABC from side a? We place ourselves in C and we make the light pass from A to B and in turn we send a signal when it goes from A to C and when it goes from B to C, in this way we would have the time with the same changes, then you go to B and do the same with A and C and the same from A, if the times are the same the speed is the same.

  • What would happen if you shot light through a vacuum and somehow bounced it off a mirror and back through a medium we know the speed of light for? If the speed is a constant we should be able to figure out what the total time should be. If the speed is different for different directions it will be a different answer to if it is the same.


  • What about femtophotography?

  • What If we take the help from optical fiber of the internet to measure the speed of light coz it has speed of almost 2,00,000km/sec ?? Edited: I saw you both talking about optical fiber later ..... I commented this when I was seeing Einstein's synchronisation convention!!!

  • When the Astronaut will return to Earth then the time can also be calculated and corrected

    • Nope. The distortion will fix his clock on the way back so it matches Earth time, within a predictable margin of error. (Especially if set manually as the example implies.) The hypothetical spacetime distortion will change the time his travel takes by the same offset as the signal beamed through space.

  • Perhaps humanity is ready to perceive the light's center/source is in each [thing]. Thereby the observer is immeasurable relative to the whole. An observer implies separation when there is no separation in pure light/energy.

  • Why not simply use 2 rapidly spinning gears (different rotation speeds) at different locations? Both of them rotating with the correct speed?

  • Say you have 2 clocks have them tic with the inversion of 2 entangled particles. Every nanosecond you flip the rotation on one which flips the rotation of the other instantly and the clock adds 1 ns. would this work?

  • wait so, think about this: quantum entanglementnt

  • This almost sounds like a quantum problem.

  • In curved spacetime: along some geodesics light will get back to us and yet technically go straight all the way.

  • why not sync the clocks, start them ticking then move them apart by 1km. Knowing that the light signal will be sent at a certain time. then when the second clock gets the light signal. measure the time difference. TaDa.....

    • Whatever you "know" is undone by moving through spacetime. Any distortions that change the speed of light also change the speed of everything else, including observance of time. Opposite direction of movement, opposite effect. In one direction the laser might be slow, then the recording is slow in the other direction. The result looks the same on both sides, as if there was no distortion. (Point of the video is that we can't observe any distortion because we're inside it.)

  • Hey @Veritasium, A thought came to me. Do you think much of the universe is dark because there could exist forces and energies that our limited human mind cannot sense? We can see (photoreceptors), hear (vibrating medium), touch (electromagnetic field), smell & taste (chemical interaction) - but maybe there are forces and energy that exists that we possibly cannot comprehend which comprise the dark energy of the universe and interacting with matter that we CAN "sense" in a way that causes the universe to expand?

    • I would imagine a subset of intelligent life residing within this un-sensable (can't find a word for this) realm of energy

  • Is it possible to calculate speed of one way light on event horizon of black hole

  • This shows, that its impossible to measure a system, you as an observer are within. I always believe humans are locked to the earth, and we have no way of knowing the answers to questions we are looking for. This reminds me, of when you spit into something, you literally get a brief taste of what your spitting at or into. Meaning there is a two way connection. What ever the spit lands on, you briefly get a taste of what your spit lands on. ie spit lands on something, and although you see no real spit in air, there for some reason is a brief moment you can taste what you spat on. ^^ I have no idea if anyone else has found this, but it always make me careful of when i need to spit out mucus out of my mouth. This video proves that we can only understand what we know by observation. I wonder when you design super computers, will they too be like us, inhibited by these observation laws, as we created them, and they have to operate within the earth we were made for. What you said about how you may be watching the stars in real time, made me think about the spitting analogy above, how you throw out a force(ie light), but you get an instant reflection back. So we will never know, as all we know is what we are born to observe. We cannot observe what is totally real, as we are within a system, that we were created for. ie life on earth. Neil armstrong went to the moon, and when he came back, he told people he could not trust his brain, ie his perceptions. ^^ That should tell you something, about how earth energies, are shaping our reality we see in our brain. Its how we are infact locked to the earth.

  • I have a question: suppose for the sake of contradiction that the speed of light was c/2 in one direction and instantaneous in the other. Then, if we looked in the direction so that the light we see is instantaneous, shouldn't the observable universe be infinite in that direction? If we developed tech to see that far, would we be able to tell if the speed of light is truly the same in all directions, if only because we could see a lot more stars etc. in that direction?

    • I think that what you are saying is analogous to what he says at 10:40 (we can't be sure that time dilation is equal in both directions, therefore when we look at distant galaxies in opposite directions, we cant assume their clocks are in sync (one galaxy could appear young because it is, and the light travels instantly to us, whilst the other could appear young, even though it is very old, because the light has taken a long time to reach us)) So no, but good thinking.

    • We only see light that moves toward us. So instead of thinking directionally in terms of "this side of the sky and that side of the sky" try thinking directionally in terms of "toward the observer and away from the observer."

  • What if we put checkpoints between mars and and the earth, and check when it reached mars, and when it came back

    • for that, we need the clocks on earth and mars to be synchronized to find the difference. Which again is the main reason we can't find the 1-way direction - synchronizing clocks are practically impossible as shown in the video.

  • veritesium have you read any scientific test or experiment before making such video .Means do you collect all information about them before making the video

  • I'm only a student, but couldn't the Mars thought experiment - or any experiment where there's a signal being sent from one clock to another over vast distances - be solved by simply bringing the clocks together? Assuming there is indeed a preferential direction, the planets' orbits - and even the solar system itself - would have shifted in such a way that the return trip would be on a different axis than when it received the initial synchronization signal from Earth. The time it showed would either be explainable by the current convention that c is constant (and all that that entails) or it wouldn't. So while we would not be able to measure the one-way speed of light, we would at least be able to prove if light travels in different speeds along different directions, which would be a big first step.

    • Unfortunately no. The clocks have to travel through spacetime just like the light. They will self-correct as they're brought together. (Not including offsets from relative speed slowing time as the objects move quickly.) Even a big loop or any convoluted shape, still flows through whatever distortion might change c. Teleportation or quantum entanglement are about the only ways you might get around this obstacle. Lots of comments are throwing out the idea of quantum entangled particles...but I'm not sure we have the tech to reliably move particles and measure their "heartbeats," while ensuring they're not still subject to different time dilation. The distortion ALWAYS self-corrects on the return trip.

  • Hi veritasium Can you answer my question that i had from very long time Can i press a water until it become solid matter?

    • I'm not from Veritassium, but yes. At around 10 times the pressure at the bottom of the ocean, a different kind of solid ice would form. It wouldn't have the same crystalline structure as ice frozen by low temperature, and some forms of ice aren't even crystals. We call water non-compressible, but it's just extremely resistant, as most non-gasses are. Even rubber doesn't compress much if you don't let the sides expand. There are at present 18 known phases of ice, detailed with ideas that are deeper than I care to learn, in this article: en.wikipedia.org/wiki/Ice#Phases See forms VI, VII, X, and XI, which form at higher pressures. The higher the pressure, the lower the dependence on temperature for ice to form.

  • Use mirror matter once we get it lol

  • Why not set up a clock and a laser on each side of the 1km trip and and have each clock stop when the laser from the other side hits it. The signals that show the time that it took should then be sent in 2 directions away from the line of fire. That way you could even tell if the data travel had different speeds. My idea while watching the video. I would love to hear whats wrong with it. 😃