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
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:
For an overview of the one-way speed of light check out the wiki page:
The script was written in consultation with subject matter experts:
Prof. Geraint Lewis, University of Sydney
Prof. Emeritus Allen Janis, University of Pittsburgh
Prof. Clifford M. Will, University of Florida
The stuff that’s correct is theirs. Any errors are mine.
References:
Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der physik, 17(10), 891-921.
(English) (German)
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.
Response to Greaves et al. paper —
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.
A review article about simultaneity — Janis, Allen, “Conventionality of Simultaneity”, The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.)
Will, C. M. (1992). Clock synchronization and isotropy of the one-way speed of light. Physical Review D, 45(2), 403.
Zhang, Y. Z. (1995). Test theories of special relativity. General Relativity and Gravitation, 27(5), 475-493.
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.
Research and writing by Derek Muller and Petr Lebedev
Animations by Ivy 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:
Henry Reich, Minutephysics:
My Patreon supporters
Additional music from “Observations 2”
What if… We used a "stick" to connect the clocks. First calculate the compression of the body so then we could calculate the delay that it caused between the two clocks and just subtract it from the end result? Would that be viable (at least theoretical) option?
8:16 if the time the light bounced back was 2 miliseconds. then just devide it with 2 but also somehow calculate how much is needed if the angle was straight up 90° about 1.005 miliseconds😅
Why can’t we repeat the same test of measuring the speed of light bidirectionally perpendicular to the beam between A and B? Then we may be able to know whether there is an actual change in velocity relative to the direction.
How does this affect objects with mass? I can never accelerate an object with mass to the speed of light as I would need infinite energy. But 90% (and way above) of C is certainly possible and has been done (protons in lhc). Why not take a proton and accelerate it with a known amount of energy, then measure it's speed the same way the speed of the baseball has been measured. Repeat the experient while shooting the proton in a different direction, but use the same amount of energy to accelerate it. Assuming c is equal in all directions, the speed of the proton should be the same as well.
What about the age of the universe: if c was faster in one direction, how come we see galaxies of "equal" age no matter what direction we look? If C was infinite in one direction, it wouldn't be possible to see "old" galaxies.
My teacher told me the speed of light, welp im gonna show this to my teacher
You know that all them star distance and such are all irrelevant now eh captain.
This makes total sense. I saw this other video showing full galaxies at the end of the universe pictured by James Webb telescope. People argued that the Big Bang can not have created them unless light from these galaxies reaches us instantaneously. Thus, the whole concept of "looking into the past" when seeing the nightsky was questioned.
If information cannot exceed the speed of light then we cannot expect to receive an ordinance for information before light has been requested, max the journey and returned. It is no different than messengers of old.
As we knew frequency remains same it just depends on source only lets assume reflected light has different speed that means its wavelength will change . another thing is its momentum will change if speed of light is different so that same photon will have different momentum when it reflect and return to the same clock that means it will exert different force. these are some logics which can easily prove that light speed will be in both direction . what you think ????????????????????
I may be over simplifying things but how come you can’t just measure the light beam from left to right, while simultaneously measuring a light beam from right to left and see if they are the same? I can only see the possibility of the light being different if it were dependent on the sun to see but in a dark vacuum I would think that would be ok.
This explains why Star Wars ships can get so far so fast at light speed, they are just choosing different directions to go!
Maybe entangled qubits can be used somehow. Those are definitely synchronized.
I have a question: Can’t you just have two clocks at 1km distance and then shine the light two times? Once for starting the clocks and the second time for stopping them. Then we subtract the different times on the clock and divide them by 2. we shoot 2 light beams but in the same direction so why would that not count as one way light measuring?
Derek, I've had an exciting thought inspired by this video. I want to present it to you and get your thoughts about it. I saw your video some time ago, and it has been on my mind off and on since then.
What if we are entirely wrong about how light propagates through space and time? Instead of thinking of light traveling through space and time as a photon at the "speed of light," a light waveform is created that is instantly present everywhere (and everywhen!) the moment it comes into existence. (After all, a waveform is a mathematical construct independent of time.) What we have previously interpreted as the "speed of light" in this view is actually the speed of the collapse of the waveform. The farther away from the source of the waveform, the longer it takes for the waveform to collapse into a point-like photon.
Would this help to resolve some of the quirkiness of quantum physics? For example, in double-split experiments wherein one of the "light paths" is "delayed," there would be no actual delay in either path, so, even though it takes time for a photon to be detected, the waveform is present everywhere and instantly available to interfere with itself, even for a "single photon wave."
This point of view also helps to resolve Einstien's spooky action at a distance for entangled particles.
I'm not sure that I like this idea very much. The idea of something existing everywhere and everywhen instantly after creation seems absurdly contradictory. Still, I don't know that it is more disturbing than other ideas, especially regarding quantum mechanics.
Not to be mean but I totally don’t agree
I have an idea that I'm pretty sure doesn't count, but I can't figure out why. Have a light and a clock with a sensor, and have a button exactly equidistant to either piece connected (with the same quality of wire obviously) to both appliances– The button simultaneously starts the clock and turns on the light, and the clock will stop when the light hits it.
It's possible that you wouldn't use electricity with something like this.
In that case what about CMB? It’s about the same distance in every direction. If the speed of light would be different depending on direction the CMB would be more distant in the direction from whitch it travels faster to us. And if the speed of light would be infinite in one dorection we wouldn’t see the CMB there at all.
Can’t you just send a message from a satellite too two different clocks and have a person at each then see if they match up. Then you would know if the speed of light is the same in both directions or not
Never bought all of this
Speed of light thing
What about if you look at light from distant galaxies. For example, assume light is infinite one way and c/2 the other way. If you used JWST to look really far in opposite directions, you should see protogalaxies and young stars in one (half speed) direction, but modern galaxies and old stars in the other (infinite speed) direction. Since we see protogalaxies in all directions, doesn't that mean light speed is the same in all directions?
what about michleson morley exp.
Using sensors that detect light you can put a lumen sensor at one end of a dark tunnel and turn on a light at the opposite end of the tunnel . . . Measure the distance between the light and the sensor, measure how long it took the sensor to detect the light and you have the time it takes the light to travel the distance of the tunnel to reach the sensor
Light travels at roughly 680 million miles per hour . . . . If you can measure how far light travels over a set rate of standardized time (eg. secs, hours or one year) then you can break down THE THEORY of light years into miles per hour . . . . if you claim the measurement of speed is flawed based not on variables but hypothesis then the entire THEORY of light years is flawed in itself . . . . If someone told you they built a car that could travel 6 trillion miles in one year then you could calculate its miles per hour sec or minute without fail
Can't we use gravity to 'bend' the path that light will take so that it will return back to our starting point? from the light's perspective, it'll still be travelling in a straight line. We can then use 1 timer to measure both the start and stop times.
Ok. So you have 2 clocks measuring a laser from A-B. But add a second laser going from B-A that is triggered by the B clock as it stops. That should verify the speed. If light is traveling at different (slower) speeds, The A clock will be more than double B clocks time. And if traveling at different (faster) speeds. It will be less than.
Dear Veritasium,
I would like to propose an experiment to disprove the theory of the speed of light changing based on direction.
It is simple, merely constructing an interferometer and placing it on a rotating plate. Due to the nature of interference, when the plate rotates, uf the speed of light is different in one direction, the interference pattern changes.