Holy smokes it's another masterpiece. Light is such a tricky subject to understand.

The overly clever setup of the beam splitters, detectors and the clock reminds me of how designers of perpetual motion machines try to hide the energetic symmetry of their devices behind complex mechanics. The end result is the same: you just can't cheat your way around conservation laws and symmetries.

Somewhere, probably on PBS Spacetime, I learned that if I relabel the 'speed of light' as 'speed of causality', and light just going 'full speed', that makes the intuition slightly better. After a few years of mulling it over, I agree.

your videos have repeatedly inspired me to re-engage with scientific learning. you make scientific understanding feel within reach of anyone, not just labs with big grants. also you are precise, but also humble. thanks for publishing your videos.

Nothing but garbage on TV tonight, what a brilliant timing!

This is a great a great video! I want to point out that you missed a simple (yet arguably not obvious) way to make opposing anisotropy locally: make the spring-mass setup MOVE! Or equivalently, make the wave generator move, so that it excites a different mass at each time step. From the resting frame of the wave generator the wave propagation is then opposing anisotropic. You might say that this makes no sense in the setting of the EM field because the field cannot have absolute motion. However, this is exactly the ether interpretation of the EM field (and spacetime itself): opposing anisotropy in the speed of light can be explained by assuming an absolute rest frame (the rest frame of the ether aka the rest frame of the CMB) and anisotropy resulting from motion relative to it.

The best explanation of 0ermittivity and permeability ever, and the analogy with mechanics made so clear the concepts of isotopi and anisotropy of the material... literally wow, You deserve an honorary Professorship

i think people are too afraid of "unsolvable" problems like this. if our current model does not predict any effects from this hypothetical and we do not observe any effects to prove our model wrong in that regard, it truly does not matter what the answer is. its like trying to prove the existence of an unobservable god with science. i appreciate the insight on how a closely related problem is much more approachable

Fascinating! The examination of the asymmetric spring analogy was very intuitive

Very good video as usual. Thanks for the time spent on the videos of this channel, we all appreciate it.

A couple points: 1) the expansion of space has the effect of slowing light in that the frequency is reduced and energy isn’t conserved. 2) the moving mass you mentioned in the end of the video you say we would not be able to detect, we do detect with LIGO. You are describing gravitational waves. I must say, I thoroughly enjoy your videos sir. Thank you and I always look forward to seeing the next! 👍🏻👍🏻

once again, what a brilliant video. one way speed of light is such a complex thing to wrap my head around, when I'm thinking about it i always get lightheaded.

I'm glad I've already watched the videos you referenced.. what a great time to be on the internet!

Fantastic video, I'm glad KZfaq recommended your channel!

Well done. It's one thing to argue that we cannot measure the speed of light in a single direction (always requiring a reflection), but quite another to assert that the speed of light in one direction could be different from the speed of light in the other. While the E=cp explanation is much simpler, I very much enjoyed the full exploration of the possibilities of anisotropy. Recall that it was Maxwell's equations that eventually led to special relativity, where only the Lorentz transformation could account for a constant speed of light. So if Maxwell's equations are fundamentally true (omitting quantum perturbations), then even if you can postulate the speed of light being different in opposing directions, it's impossible to model it in a self-consistent way.

Loved the explanation in this one, so eloquently put forth. Thank you so much for making these video's, always look forward to them! Makes me wanna go back to school and study light.

And this, ladies and gentlemen, is how you explain in a easy way something hard to understand. What a great video!!!

I love this approach! Finding contradictions in conservation laws and the inconsistent consequences if one assumes anisotropic speed of light.

Wow! I love hove how concise yet clearly and detailed you describe such complex topics. You are great educator.

I thought about this some time and realized that no matter what I try, I always end up with a two-way speed of light in the end. And I had some really crazy ideas that would be practically impossible to do even if we had access to materials that literally have unrealistic properties.

Now I am imagining that little green fempto-pulsed laser blip surfing a gravity wave and I am very happy with that. Thank you for the spring mass analogy. Love when EE concepts match up pretty well to mechanical concepts. Gives more intuition about the more abstract concepts which can be powerful. Love your videos by the way. Optics was one of my favorite courses in college and these videos solidify fundamental teaching while at the same time not shying away from more advanced concepts and topics. At the end of our course, our professor talked some about non-linear optics but treated it as dark magic. Would be awesome to explore some experiments or topics in that realm if your equipment would allow.

Huygens Optics Video: My favourite kind of photons 🤗

The moment you said a spring with a different constant on one side than the other, I instantly realized as it vibrated, it would grow in vibration in a single direction and I was like: “Free energy machine!!!” 😂

I haven't been this excited for a video in a long time

I enjoyed quite a lot this video and the way this problem is presented.

The equivalence principal tells us if we do these experiments while in freefall we will always get the same results. This is even true if the experiment is done in free fall towards the event horizon of a black hole (one large enough to ignore tidal effects). However, observers in different reference frames may see things very differently. Light will always move at the same velocity, but clocks in different reference frames won't agree. Since the velocity is constant, the wavelength will appear different.

When digging down, it's conservation of energy, which forces the speeds to be equal. Conservation of energy is one way to state the system is symmetric in terms of Noether's theorem.

wow i’m so glad i didn’t click away after the first half lmao first half was amazing (for the viewer that’s not familiar w the conclusion) just excellent motivational context-building but the second half was an amazing next layer deeper of thought experiment / pre-analysis

Thanks for actually adding to the discussion.

This was cool even if in the end I'm not convinced. Love this channel!

The speed of light being different in different directions breaking the conservation of energy explains why a lot of people very desperately want this to be the case since it would allow for infinite energy devices.

Very interesting thought process!

There's an experiment that came to me today, just before this video came out, that I would love to see, but don't have the means to perform. I just recently learned that Neodymium-doped Lithium Niobate can be used as a lasing medium. I am incredibly interested in the interference and diffraction patterns produced by it, specifically when it is being used as a laser medium while also piezoelectrically oscillating. Lazing Lithium Niobate, even when not doped, is a parametric down conversion process too, producing more than typical entangled photon pairs! If you're interested at all I can link you to a few papers, and a source for Neodymium-doped Lithium Niobate on substrate. Love you so much!!

Outstanding content!

19:21 you're such a man of culture!

21:00 That's how free energy devices is made scientific way.

The only issue with the conclusion is if we take the reference of an object in motion, let’s say traveling at say 1/2C then the speed of light forward of the object should appear to be slower (1/2C) while the speed of light aft of the object should appear to be faster (3/2C). This simple thought experiment seems to suggest that there MUST be a difference in 1 way speed of light for any object in motion.

My day just got a whole lot better!

Well, if we did measure different times for different directions than it would mean that there was some additional moving gravitational potential somewhere in the vicinity of the light's trajectory and in fact we do use it cosmology when we measure the mass of black holes in a galaxy using long term gravitational lensing observations - recently covered by Anton Petrov on his channel discussing results from a Warsaw, Poland study on this exact subject. There's really no good reason to assume that light travels with different speeds in opposite directions when there's no change with respect to time in gravitational potential. If there was a difference we would not be able to infer anything about masses from any measurements, which would be contrary to experiments and day to day perception.

I’ve often heard that the speed of light “pops out” of Maxwell’s equations, but this is the first time I’ve seen an explanation of how. Thanks!

such an awesome channel for someone who did a bit of physics like me

I understood just enough of this to be entertained.

What a prank! 🤣🤣🤣 "This is the end of the video" This scared me a lot😬

I would say that the combination of the 2-way measurement and the fact that a measurable frequency/wavelength calculation both give the same value is pretty convincing.

Yes, light propagating through space will be the same in any direction; but while the light is travelling, the receiver can move, which makes the detection at the detectors register different speeds. Your initial setup is indeed a two-way experiment... but you can change the setup: 1) 2 detectors in close vicinity, in the center, with a high precision clock that records the time when the detectors on left and right are triggered. 2) Put two emitters that have a stable clock, that will always tick at the same rate (more detail later). Each emitter will fire a short pulse (milliseconds are fine, you just mark the leading edge detection). Amazon has lasers that are good for up to 2 miles (10,000ft)... (light is approximately 1 foot per nanosecond, so over 10000ft is 10000ns or 10us.) Over the 10us, because of our motion through the universe towards VIrgo according to the redshift in the CMB, we move 370km/s or .0012 ft/ns... so it will move 12ft over 10us, and register a different speed from one side vs the other of +/-12ns... or a total difference in the speed of light of 24ns. (in the perfect arrangement).... so this would need to be aligned with the constellation virgo/cetus(opposite side constellation), so that there is a best-case... it's aligned to approximately +9degrees north; can't really just make this go any direction, or you can end up with a near null result, the orbit around the sun is 10% of the speed through the universe, and the rotation of earth is 1% of that... so +/-1.2ft or +/-0.012ft from those effects... the motion through the universe is much more on point. Mind you - the speed of light does not change based on the speed of the emitter... just the speed of the receiver - such speed is then c+v and c-v..... There may be additional skews to the stable clocks.... that once deployed they are not in exactly the same gravitational field... but this will be a constant effect, and the constant drift can be factored out. Air pressure is an insignificant factor; and since it's likely that the 4 miles the experiment covers (2 miles from one side of center and 2 miles from the other side) will likely be the same it ends up being non-measurable... and any change under like 100,000atm is barely notable... a few millibar is not going to change the experiment...same with humidity - the same amount of humidity is likely experienced across the apparatus. Another way to consider this is say you're playing catch with someone else, and every second they throw a ball at you at the same speed, if you move towards them, then the throw that happens while you are moving will be caught by you in a slightly shorter time. If you continue to stand in place, at this new distance per second, the ball will be registered as every 1 second. If you walk away during a throw, then the time it takes to catch the ball is slightly longer, again, until you stop. If the experimental apparatus is perpendicular to the velocity, that's basically initial conditions - and every pulse is received at 1 second intervals from both sides... as the apparatus aligns with the direction of motion, it's like the center detector is able to take a few steps forward, and pulses from one side will arrive in slightly shorter time or slightly longer time, whether the detector is moving towards or away from the emitters respectively. Once it reaches the maximum alignment, the pulses will still be every 1 second, but will be skewed from each other compared to where they started... if they start on every integer second, then it would be at +12ns and -12ns from the original state along a timeline. The times between each impulse registered from each side are subtracted from each subsequent sample, leaving a small delta change between each received pulse... at the end you'll have a net bias (probably) from clock skew, this can be removed by subtracting the final value from the initial value, and subtracting that sloped line from the result, biasing the beginning and and to 0. (This would mean a complete 24 hour cycle should be run... it would be less meaningful to do only 6 hours or even 12 hours - because at the 12 hour mark you're not necessarily in the same arrangement, since the apparatus is aligned with a specific point on the horizon, at 180 degrees of earths rotation, then the device is no longer in the same alignment as it started, but is tipped in a counter direction (that's not the right word but maybe you get the idea). I have been working on setting up this experiment, and if I can get the apparatus built, I would take it to the great salt lake, there are very few areas where there are 4 miles in an arbitrary direction that are entirely clear and flat, and wouldn't interfere with the line of site - could built something in the ocean maybe, but it would have to be tied to the ground. The two emitters and the center detector MUST be rigidly arranged - floating them, or launching them into space will not help. Anyway - I did setup a program for an FPGA that has a high speed clock, and two registers internally that can be latched when a signal from a light detector is received... the latched clocks can be read more slowly and stored in a computer over the next second between each pulse. The FPGA though is actually pretty slow, and although I'm almost able to get a 600ps clock, it's unstable, and a temperature corrected crystal oscillator that is more than a Ghz is expensive... That, and I don't know how many photons the photodetectors have to see, or how bright the intensity is - but I would expect actually quite a bit of noise from that receiver - plus, I don't know a good way to gate a laser pulse - for the same reason, propagation of all the voltage regulators, plus time to build up a signal to transmit are likely going to cause more noise in the experiment than +/-12ns. I've somewhat settled on using a synchronous AC motor that rotates 60RPM (once per second) and put a wheel with a 1mm slot on it - but even then, as that is actually pretty slow, it's going to uncover a fraction of the beam, and then the whole beam, and then start covering a fraction of the bean as it passes... which makes the signal leading edge not very concise... and is another point of noise. There are high precision frequency generators/counters but I find they are $16,000+ to be within a range of desired resolution, and then that is a bulky external thing, which introduces nanoseconds of delay with propagation of those signals... I just don't see it being done with off-the-shelf components. Probably have to make an ASIC with dedicated silicon for the clock and 2 latch registers (a super simple thing, though you do need about 52 bits of precision, which is quite a lot of bits, and a long chain to update.... which potentially makes the clock have grey bits in the middle while it's still counting a clock tick... the clock edge at one end will change even while the middle of the counter is still updating from the previous tick. That and the light detectors need to be pretty precise, and the module I got, the sensor is actually not JUST the sensor but has a transisitor the detector is attached to - which, again, signal jitter/noise. I don't know if maybe I got a DLP(?) sort of chip used in projectors that could gate small mirrors in two directions pretty fast... I would think those are still on the scale of 100s of microseconds... and far from the nano/picosecond gating I would want. Speaking of space, changes in space of the gravitational field only propagate at the speed of light also, which means ahead of our planet the field is somewhat compressed, and slightly more dense and is slightly weaker, so satellites will orbit just slightly further away (1 meter per kilometer roughly), and on the trailing side, the field will be elongated, and have slightly greater effect, making satellites on that side orbit slightly closer... This makes GPS signals always take the same amount of time to arrive. There is a thing in the solar system the 'Axis of evil' which is an overall alignment of the elliptical orbits of things that is always in the same direction. One could argue that GPS (multiple emitters some fixed distance away from a central detector that registers the time in the same location with 0 propagation time of the clocks signal) is the same as the above; that is, the same, except for the specification of being rigidly attached. On my channel the last few videos I did were on simulations I built (open source, links in descriptions to the demos and/or more information documents in the github repo) that are based on the one-way constant speed of light... and indeed, any two-way measurement will always be a constant time for a distance, in any frame, moving in any direction, while the individual one-way paths are neither the speed of light (as registered by a moving receiver). Light, once emitted no longer has anything to do with the source that emitted it, and it propagates in space regardless of what the source does afterward... Though the net combined effect does result in light aberration, both on transmission, and on reception of signals (see synchrotron radiation beaming effect for more about this aberration for electrons travelling close to the speed of light - the direction the light is detected is mostly all directed forward.

Best time resolution in a time-of-flight experiment with electronic triggers may be around 50 picoseconds. A 1 ppm error of c consequently can be achieved at a measuring distance of the light path of more than 15 km. Getting a stable signal transport over e.g. a coax cable of the same length looks almost impossible. However, if it may be feasible, rotating the whole setup 180 degrees might answer the question of whether the vacuum in space is anisotropic, of course only at the precision level of 1 ppm..

I will repost my comment from Veritasium's video HOW TO MEASURE SPEED OF LIGHT We can measure the time it takes light to go from point A to B in vaccum, bounce of the mirror and go back from B to A, BUT inside a glass Then repeat the experiment but in the opposite way. If the speed of light decreases by the same % going from vacuum to glass regardless of the direction, we should be able to see the difference in travel time. Furthermore, we can make 2 additional experiments (both ways in vaccum and both ways in glass), so that we have 4 measurements of time and 4 unknown velocities - easy to solve equations Great video btw :D

Was just writing a comment about the AlphaPhoenix video when you mentioned it. Another under-rated channel.

Honestly I think that it doesn't feel like that's possible, because if it were, then wouldn't we see a little bit of redshifting if we look at something in one direction vs a different direction? Since, if the speed of light is different, that would mean the waves would get a little doppler shift if you look at them from a different direction.

If in a Frame of Reference there is a shift in time as a function of shift in space, then light would effectively travel at different speeds depending on the direction it travels. This is exactly what happens in the equation t’ = gamma(t-vx/cc). In a moving Frame of Reference, time is shifted proportional to distance. It’s the x in the special relativity time dilation equation. Therefore the speed of light is different in the 2 directions depending on the Frame of Reference.

*Advanced Tinkering* channel just mentioned you in an absolutely lovely way in the video "Creating Ultra-Fine Details in Titanium - 20 Micron Resolution" ! 🤗

Why isn't it setup so the sensor detects the light twice? First the light directly from the source and second the light off the mirror? Distances and time between sensor detections are known

Well, you convinced me. But there again I'm more content to accept "common sense" explanations than any mathematician, or most scientists, would. So, for now, I'm going to ignore any "special pleading" attempt to introduce a possibility of there being a difference in light speed with a change of direction, while in a vacuum.

That wave animation was excellent

Good video. You compare the situation with an elastic medium that has different spring constants for the two directions, which leads to violation of conservation laws. But that is not the only way to achieve anisotropic propagation speed. The medium itself could be isotropic, but simply have a velocity in one direction. That would not lead to violation of conservation laws. I think the more profound observation is that there is a delicate link between the one-way speed of light, and how simultaneity of distant events is defined. The definition of simultaneity makes it theoretically impossible to measure the one way speed of light. You might want to do another video on that. Ever since I discovered this myself, I have marvelled at the following simple sentence in the introduction of Einstein's 1905 SR paper: _"We have not defined a common “time” for A and B, for the latter cannot be defined at all unless we establish _*_by definition_*_ that the “time” required by light to travel from A to B equals the “time” it requires to travel from B to A"_ Einstein already knew that what you have discovered as well: that the one-way speed of light is a matter of definition, not of measurement.

Now i might be missing something but we know the speed of sound is constant in air so could one not use spund to trigger the clock and the delay is the distance divided by speed of sound. Thus avoiding measuing the two way speed of light.

now I know how the anisotropic filters works in video games. nice!

Huygens Optics there is a channel called Dialect - do you know them? They are talking the same problem that you do - but they don't take for granted that the one way speed is 1/2 the 2 way speed... They allow it to be any value - even the one way being faster than the 2nd way... Thus they are trying to explain a different interpretation of relativity! I wish you could watch their videos and tell us what you think of them!

Like you said at the beginning, when you start mixing the timing in wires or fiberoptic cables to send back the information to the clock, you've just added yet another variable and compounded the problems and then made the problem what's the transmission inside of a cable. The fact that there's some kind of refractive indexing in space really boggles my mind!

As someone who studies physics and electronics my brain got constant error 404 with the spring. It's a spring not a resistor, It's a spring not a resistor, It's a spring not a resistor...

Damn I love this channel

The elephant in the room is that distance D is defined by the speed of light. The speed of light and distance are not independent. Look, the distance from Da to Db may not be the same as the distance from Db to Da but we define them as being the same to make the speed of light constant.

Armand Hyppolyte Fizeau is one of the best names I've ever heard, holy crap P.S. how do you pronounce "Huygens"? Is part of that silent like the Vietnamese Nguyen (or are my white eyes just seeing the same letters and assuming a connection lol)?

an anisotropy you forgot to consider was if the grid of springs was moving in some particular direction with respect to the observer

This is a random thought that hit me while watching: 1) We sync up two clocks very close to each other so we know the speed of light difference will have an extremely minimal effect 2) Both clocks are taken away from each other to some meaningful distance that will allow us to measure a difference in the speed of light 3) Based on a predetermined time in the future both clocks will cause a flash of light at the same instant 4) Both sides of the setup record the exact moment based on these clocks when they witness the light from the other end of the setup getting to them The problem this solves is the relative nature of light which we are able to subvert due to us taking a higher perspective. We are looking at a 'global' time for this context & creating two independent light sources & measurements based on this 'global' time. This allows us to bypass the issue of relativity completely & get a measurement of the speed of light in one direction.

We could fire a beam of light through an incredibly long internally reflective looped tube. Light is fired in at the opening and a timer starts, it then loops back to the starting point where a sensor stops the timer. This is arguementably a one way trip.

fascinating !!

Very nice - thank you! My vague understanding was that at a microscopic level, the speed of the collective light wavefront was dependent on the time delay/phase shift of individual photons when they were absorbed and re-emitted when colliding with the atoms in the medium. So any directionality would have to come from a difference in that mechanism?

This is the most fundamental idea I've seen in years. THANK YOU!!!

To me it seems that as long as you measure it localy, it HAS TO be different in one direction than the other because light reaches its speed limit instantly and its speed isn't affected by where it came from. Except the place where it came from isn't standing still, it's moving through the universe. Meaning that relative to where it came from, light has to travel at different speed in different directions. If you were to hypothetically travel at the speed of light and projected two light beams, one in front of yourself and one behind, relative to you the beams couldn't travel at the same speed otherwise they would travel at different speeds relative to "the universe" (because you're already traveling at the speed of light relative to "the universe" and light clearly can neither travel at twice the speed of light, nor stand still). Unless I'm misunderstanding something, this would tell me that as long as I'm moving relative to "the universe", as planets, solar systems and galaxies clearly do, relative to where I am light would be slower in on direction by the speed at which I'm moving in the other, since light doesn't accelerate and its speed doesn't change based on where it originated from. "The universe" because it is unclear what is it as a frame of reference.

You don’t make mention of length contraction/time dilation in your proposal which depends on the speed of light in a given direction. Given that these depend on c, it’s likely that these effects would cancel out the infinite energy and there would be no observable difference if the speed of light was different in different directions. It would be interesting to see a follow up taking these into account.

Very good video and I think you are correct. Light speed is the same in any direction - HOWEVER what we really want to look at is if it takes the same time for light to go the same distance one way or the other. You may say that if we measure the same length both ways then of cause it should take the same time if light speed is the same. I then say but what if the whole setup is moving? That is really the thing I believe as we all know that we are moving and as the speed of that movement is unknown it has been said that any reference point is valid. I believe that if you put a movement into the setup it is clear that the time taken one way must be different to the other way but to find those time differences is THE REAL PROBLEM. Remember the distance scale is moving with the observer. When we become able to check the time taken for light to go in any direction we should then be able to calculate time dilation for every observer correctly. The way I understand that it is at the moment is that an observer traveling near the speed of light can claim to be the reference point and there fore standing still. If this observer sends a light beam in his own travel direction it obviously can't travel very fast COMPARED TO THIS OBSERVER as other ways it would break the speed of light. My view point is that all observers are probably moving at different speeds and in different directions BUT then there must be an ABSOLUTE STAND STILL. This will allow all observers to see the speed of light at their point AND THEIR TIME SPEED to calculate every body else's time speed and actual speed as well I believe. Time dilation is real I believe but that is really not much different to that we freeze meat. The frozen meat lasts longer (stays young longer) before it gets too old to eat.

I agree with all your points, but the real reason for measuring the one way speed of light is to establish if the speed of light is the same for all observers, regardless of their speed. So, if we were measuring the speed of sound in air, the one way speed would be different in the directions in the direction of motion as compared to the other direction. If the only measure possible is a 2 way measure, then you could not measure this difference due to the motion of the observer. It is important to measure this because the independence of the speed of light from the speed of the observer is the central assumption of Special Relativity.

If the clock and the sensor is the same, i.e. a quickly rotating disc of photosensitive material, and the disc at both ends of the race track is coupled with a long axle that spins at a constant rotational speed, will this device show a different speed of light one way (angle between dots) if it is oriented horizontally and vertically in a deep shaft? I suppose the measured rotating speed of the disc at the bottom of the shaft will be higher than that of the top one, even if they sit on the same axle.

Compare speed of light in opposiite directions by doing 2 simultaneous 1-direction measurements. Make a rigid shaft 1 uSec (at C) long. At each end L/R, have a disk-like object with 4 quadrants (1,2,3,4) 1, 2, and 3 are solid, with a very skinny radial slit at middle of 2 to let light pulse pass throgh at a speciic angle. Call that angle 0. 4 is empty, with a very skinny radial pin which can block a light pulse at a specific angle (180) At 0 rotation speed, align disks so that position 2 (slits) are lined up both at -0. Place light source at each end, beyond the disks, Now rotate the rod at increasing rate, until light passing through each slit is blocked by the pin at the opposide end. You don't need to measure the time that light is received. You don't pay a priice in time to report the results back to the middle. All you need is to observe that the light is blocked by the pin at the same RPM in each direction, giving cnostant dark at each end. Note that the speed of light is the primary way to measure distance. But since the same shaft is being used in each direction, and since (I assume) the rod has the same length in opposite directions at the same time, it seems that the only way to have light obstructed by the pin at each end at the same RPM is ffor light to take the same time in each direction. Single-direction measurements, not sum of delays in 2 opposite directions.

Could we separate two clocks, and using calibrate the clocks assuming the speed of light was similar both ways so they would show the same time in this case. Then send a light signal from one to the other and check if the light arrived on the predicted time or not?

i see that the end credits song is "Floating - Earlybirds" is there any way i can find this somewhere? i am having no luck so far

I think the speed of light is the same in both directions. Because if speed of light in one way was different even by a tiny amount, the universe would look completely different in different directions. Also, if speed of light was different in different directions, things like GPS would break because the time dilation would be different than expected.

Clever set up. I always thought that a refractive medium with a secondary reflective surface would demonstrate the incident speed and the reflected speed would have to be the same. If they were different it wouldn't follow the law of reflection in the medium after a while. If it changed speed it would alter the secondary oscillation of the electrons in the medium, as it is time dependent, and thus change the summed slower electric field oscillation creating a different reflection angle about the normal axis.

I really like the concrete visualisation of what would happen if the speed of light was directionally anisotropic. It brings home the fact that in dealing with reality, you can't just arbitrarily change/question fundamental properties without it having collateral effects on everything. You have to step back and think what your proposition actually means in the greater context. Directionally anisotropic c won't just mess with your measurements, it implies a wholly different universe.

Their are other t` variables you forgot to consider.. t`a being the left path after the beam splitter, and t`b to the right. these variables will cancel each of the respective vectors associated with t`1-4 which makes the directional vector Naught and is equivalent to measuring the 2-way speed of light.

If the speed of light varied based on direction, then the observable universe would not be spherical and we would not be at its center. We'd see farther in some directions than others and thus see more stuff in those directions. We could (roughly) measure the relative speeds of light coming from different directions by counting the number of galaxies visible in those patches of the sky. I think it would also show up as an anisotropy in the size of the temperature fluctuations in the CMB. The relation between redshift and distance would be anisotropic. And probably other stuff too that I can't think of at 3 in the morning.

If the speed were different in different directions, then you are correct it would cause uneven dispersal, but we could not observe that unevenness unless we were standing at some third perspective independent of those. Perhaps though we would see sections of the night sky that would be apparently more bright or darker.

If you were flowing with a river (but didn't know it), could you measure the one-way speed of sound in the water? Is this a valid analogy?

Very thought provoking. In no particular order… 1. I think the effect of moving mass in the return leg is somewhat related to how we detect gravitational waves, so perhaps not impossible to measure. 2. I wonder whether setting up a dual comb fs laser system, with the two periods walking past each other at a low (Hz or kHz rate) would enable you to decouple the effective speed of light in optical space from the effective speed of light in message passing (the wires to the clock). Instead of the time of arrival of the laser pulse being when the fs pulse appears, the time of the event is when the two fs laser pulses align at the detector. This has the effect of slowing down the speed of light by the mean repetition rate of the lasers divided by the difference in their repetition rates. For a fairly simple fs pair of lasers this might be 50MHz/500Hz, or a factor of 100,000, so instead of 300mm/ns the apparent speed of ligh would be 30mm/ms. Your wires would still be communicating at 300mm/ns, since they are only passing info that the correlation has happened. Effectively one of your two terms in your pair of equations will each have a time stretch factor in it and the other will remain purely related to c or c’. By changing the relative length of optical path and electrical path you can set up situations which should pick up if there are different speeds in the two different directions. 3. For the reasons you set out at the end of your video this would be an experiment with a very dull result. Thanks for the excellent work you do!

can't we use some way to transmit the sensor signal that it is not related to the speed of light? like sound? On the other hand, if we can't assume that the speed of light is invariant for direction, can we actually say that the speed of ANYTHING is invariant for direction?

Around the year 2002 an interesting star was observed. It's V838 Monocerotis star which is surrounded by a thin nebula. After its "eruption" during which it brightened considerably, "an echo of light" on the nebula was observed. Since it expanded equally in all direction it is hard for me to imagine how the speed of light could be different depending on the direction. Also, in 1676 Rømer roughly measured the time the light takes to travel the Earth's orbit diameter and as far as I can understand his method, it's a one way determination of the speed of light which is the same, regardless of the Earth-Jupiter orientation in space (granted, only along roughly the plane of the ecliptic but I fail to see how that particular plane would be in any way special in the cosmic scale).

Nice! But now you have me mentally attempting to prove 1 way == 2 way using a combination of wave and velocity measurements.

Isnt the michelson moreley experiment aimed at proving the two way speed is the same as the one way speed? The basis of their experiment was that an aether would cause light to be faster in one direction than the other as we moved through it.

I can't find the end music anywhere, the name "floating earlybirds" isn't really searchable

It's actually quite easy, technically you just have to take advantage of the non-linear nature of time dilatation. Basically you take some atomic clocks, move them rapidly in two different directions and graph the change in time. The time dilation on each clock should fit the curve t₀/√(1-v²/c²) where t₀ is the time for the 'rest' reference. If they fit to the same curve for each direction, then the one way speed of light is the same. In the Veritasium video Derek seemingly forgets that time dilation is nonlinear and doesn't get 'canceled out' when you bring the clocks back together to check their differences. It's annoying. The other way is that you just use a particle accelerator to spin a proton or whatever at like 99.9999% C. If the speed of light were different then the particle would have to slow down on one side of the ring in order to maintain it's mass. (or it's mass would fluctuate on each side) in either case you'd need to adjust the timing or strength of the magnets to maintain the particle in the ring - in other words every time the large hadron collider is use the one-way speed of light is verified to be equal.

I'm studying Electromagnetic Fields rn and this was simply put fantastic

wait isnt this difference in propagation speed caused by local gravitational fluctuations the working principe behind a gravitational wave detector?

There is another way to have a difference in the opposite direction speeds. You can check the last couple videos of Dialect

I was wondering if you could use a very distant time signal say from a pulsar to synchronise two atomic clocks, one at each detector and thus defuse the measurement problem?

can we use doppler effect to say if the light speed differed directionally then we would measure some things bluer or redder depending on whether we are facing the source from right or left?

I'm no scientologist but I always heard that read as "C1" and "C1 prime", not dash.

Love your videos. Could you not create a metamaterial where the permittivity or permeability can be influenced by a change in magnetic field or electric field, thereby allowing the experimenter to change the speed of light after one bounce, or even on every even bounce and use multiple round trips?

One possibility for a different speed of light in opposite directions is when one assumes the substrate for the light propagation, the "ether", moves in a specified direction relative to the apparatus. So if we cannot measure the speed difference according to direction what about the Michelson-Morley result ?

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