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.

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.

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!

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

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!!!” 😂

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

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 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

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.

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.

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.

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.

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.

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! 👍🏻👍🏻

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

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

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

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

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

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.

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

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

Huygens Optics Video: My favourite kind of photons 🤗

Thanks for actually adding to the discussion.

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.

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

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

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

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

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.

If light was biased, half the galaxies on one side of the sky would looker younger and closer than the other half. Otherwise the question becomes "...within a sphere small enough to ruin your experiment idea"

Very interesting thought process!

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

Outstanding content!

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

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.

I love it when people push against physical boundaries! I wish someone could violate entropy laws one day or better yet create energy out of nowhere

This video should be mandatory for anyone about to study special relativity.

The Global Positioning System (GPS) is predicated on the assumption that the one way speed of light (actually microwaves） is constant - so job done!

I have an idea to measure the oneway speed of light, which is hard to verify and maybe wrong. I hope one clever commenter can explain it to me. the problem: for both ways most experiments use the electromagnetic force. My idea: use for one way the gravitational force: Hang a mass with photosensor at the bottom in a specific height (measured by an physical object and not with a laser :)). Make mechanism that releases the ball if the photosensor senses the light from the bottom. Now measure the time from start of light (at the bottom) till the mass drops to the bottom. If you are careful with the wires of the release mechanism the EMForce travel just in mostly one direction. So the time is time of drop + time of release+ time of detection + time of light with oneway-speed. Just the speed of light would change, if you do the experiment on an perfectly round homogeneos earth (or a black hole if you have one) when you vary langitude and latitude :) At least that what i think in the moment.

I think when Ole Romer measured speed of light from the delay in appearance of the Jupiter's moons, he actually did measure the one-way speed of light. Because the delay corresponds to light traveling from the moon to the Earth and did not depend on how long light took to go from the Sun to that moon.

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!!

When I watched the Veritasium video, I thought of 2 experiments: 1. Examining deep field astronomical observations to see if the universe looks younger in any particular direction. 2. Take a device consisting of a laser facing an array of detectors, spin it at a constant rate, and measure the change of the deflection. Then twist the axis of rotation so as to check every 3D direction, and measure the change of the change in deflection of the beam. If the deflection remains the same for a system in constant rotation, then the 1-way speed of light is the same in all directions. Do these experiments fail, and if so how?

My day just got a whole lot better!

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?

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.

Thank you for your informative and highly educational videos. I myself have done research on the viabily of Einstein’s theories for almost 40 years now, and since this video of yours is somewhat relativity-related, I dare to give some uncommon insights into the matter. In the course of developing his theory, Einstein ran into the most blatant-seeming contradiction. How can it happen that the speed of light relative to an observer cannot be increased or decreased if that observer moves towards or away from a light beam? Einstein states that he wrestled with this problem over a lengthy period of time, to the point of despair. But then it suddenly dawned on Einstein that this problem was based on certain assumptions about the nature of time. He “solved” the problem by adopting the view that time and space must be regarded as components of spacetime. In his extraordinary confidence in the result of the Michelson-Morley experiment and driven by his ambitious urge to explain it, he threw overboard the fundamental tools of scientific understanding, the independent concepts of space and time. In retrospect, it is easy to see that it was Einstein’s philosophically reckless attempt to “explain” the MM-experipent that started the madness of modern physics and cosmology. Here is how Einstein stated the premise of all his reasoning (in his own words): “Whenever we talk about the motion of a body, we always mean by the very concept of motion relative motion … These conditions are really quite trivial … All this goes without saying and does not need any further discussion.” This is Einstein's basic statement behind his theories. He firmly believed that only relative motion of two or more moving objects is real. Then we have Einstein's statement from 1925: “If [the speed of the earth with regard to the ether] should be confirmed, then the special relativity theory, and with it the general theory in its present form, fails. Experiment is the supreme judge.” So, according to Einstein's own criteria, to refute his theory it is only necessary to succesfully measure the Earth’s motion relative to the space rest frame, and this is without question the CMB rest frame. The first confirmation feared by Einstein was accomplished by Dr. E. W. Silvertooth 35 years ago. With his (modified) Sagnac interferometer he revealed an ‘ether wind’ of 378 km/s ±5% in the direction of constellation Leo. It must be emphasized that he predicted the outcome of the later COBE- measurements. The original article is here: spirit-science.fr/ArchivesScientifiques/1989Silvertooth.pdf Why was Einstein led astray in the first place? Here is the answer: In 1902 W. M. Hicks, a British mathematician and physicist (a student of Maxwell’s), made a thorough criticism of the Michelson-Morley experiment in the article “On the Michelson-Morley Experiment relating to the Drift of the Aether”, Philos. Mag., 3, 9. (1902). In it he stated that "The theory is not so simple as it may appear at first sight owing to the changes produced by actual reflexion at a moving surface. The correction due to alteration in the angle of reflexion was first introduced by Lorentz, and was taken account of in the joint paper by Michelson & Morley in 1887. But reflexion produces also a change in the wavelength of the reflected light. Further, when the source of light moves with the apparatus, the light incident at any instant on a plate does not come from the position occupied by the source at that instant, but from a point which it occupied at some interval before..." In other words: both the Doppler effect and aberration must be taken into account. A peer reviewed and published analysis in line with requirements by W. M. Hicks can be seen here: www.researchgate.net/publication/347975401_QED-based_analysis_of_the_Michelson-Morley_experiment. The article also introduces the Silvertooth experiment. The Michelson interferometer is insensitive to its velocity in space. This is the core reason for the present rotten state of physics and cosmology.

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.

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.

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!

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.

That wave animation was excellent

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

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.

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!

When I first noticed this KZfaq, I said to myself "A Daniel come to judgement !" Then I checked the source: William Shakespeare. But Shylock was being used to show a weakness in human nature: the difficulty of avoiding a difference between mercy and legal justice. The problem here is restriction to a single clock. Often one encounters the "clock hypothesis", that acceleration doesn't alter the rate of an ideal clock. I think that, just as there is no ideal person, so is there no ideal clock. We need many clocks, so that we need to be empirical about synchronising them. Why does the Global Positioning System (GPS) lead to a difference between east/west and north/south one-way light speed comparisons? I think it is because the GPS uses a master clock located on the earth’s axis of rotation. The master clock synchronises several remotely located and transportable clocks, not using Einstein synchronisation. We now have atomic clocks, which Einstein didn't. My conclusion is that there is a privileged frame of reference, defined by the ether, in which Maxwell's theory works directly.

When moving through a medium, like with a speaker and microphone that is not stationary in air, the one-way speeds can be different relative to your apparatus (but consistent relative to the medium, in the case of air). We now know that there is no aether and the speed of light is independent of the frame of reference, but this had to be established. So I would think the one-way speed of light has more to do with moving reference frames than with anisotropic physics.

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.

I'm about to ask a question, and I'm guessing the answer to this depends upon the difficulties with defining "now" across large distances. However- Couldn't we use more refined measurement of the Jovian eclipses, like Ole Roemer did? It seems we can use Keplerian mechanics to predict when an eclipse or occultation of one of the Jovian moons will occur, measure the difference between that time and when we actually observe the eclipse by telescope, and use that to calculate the speed of light in the Jupiter -> Earth direction.

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

Nice detailed explanation of what you've defined as direction, but I feel like 'toward' and 'away' from an observer are conceptually different from directions like 'left' and 'right'.

I thought that the one way speed of light CAN be measured, or at least shown to be either equal or not equal to in opposite directions. The set up is simple but it doesn't use photons, but two muon guns, calibrated to eject muons at a known velocity. The velocities don't have to be equal but for simplicity, let's say it is. Below, M is the muon gun and D is a detector. M.................... D......................M Here, we don't care how long it takes for the muon to reach the detector. We simply measure IF the muon is detected from both sides. The decay time of a muon is very well studied. We repeat this experiment by changing the distances between the detector and the muon guns until there is a statistically significant difference in muon detections, or they both fall off at the same rate. If there is a difference, then there's a difference in speed of light in different directions, if they fall off the same way, then there is no difference.

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!

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.

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.

The direction and distance of Jupiter from Earth can be predicted very accurately using Newtonian mechanics. Furthermore the orbits of the Galilean moons around Jupiter can also be predicted very accurately. The moons are periodically occulted by Jupiter and the occultation times can also be predicted very accurately. The apparent times of the occultations as seen from are delayed because of the light propagation time from Jupiter to Earth. The delays are always given by the distance divided by the accepted speed of light and this is regardless of the actual direction of Jupiter from Earth. This is very strong evidence for the one way speed of light being constant regardless of propagation direction. I believe the occultation timing delays were first observed by Romer and led to an estimate of the speed of light in 1676.

If the one way speed of light is the same for all observers, relativity has a new paradox. If the observer on Einstein's train kicks his light clock over, in the direction of travel, the calculation for gamma requires (c + v) and (c - v) to get gamma^2. Length contraction then reduces that to gamma. If the speed of light remains constant in each direction, relative to the clock, there would be no need for length contraction because there would be no time dilation.

17:50 Well but the point here is not that the speed of light doesn't change. It's only saying that the 2 way speed is identical for both A and B. Try moving A and B closer to the mirrors, without moving the mirrors. Then the signal from B will arrive earlier than A. We can do for this an interference experiment in the vertical direction, aligned with the gradient of the gravitational field :) this experiment has been done and the result is called the shapiro delay. The reason you don't see interference normally is because the gravitational field is pretty much identical in every point of the room you're in. 21:50 well we DO have gravitational redshift, so energy conservation does not really apply here...

fascinating !!

I would like to see science point its efforts into the development of the everlasting gobstopper. I would appreciate that more than knowing if light travels differently in opposite direction.

Damn I love this channel

Thank, great. As one of the commenters noticed - if we cannot measure it - who cares - Russel’s teapot. But from the other side it somehow looks similar to me to neutrino masses mix problem (we can measure only squared sums of pairs of masses)

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

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.

Even in local you can have anisotropy in direction, not because of the medium but because of the observer travelling with respect to medium and all the calculations, will still result in speed being c

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?

I am familiar with the reasoning in the Veritasium video, which is repeated (and added onto) in yours. Yet, in principal, I think the one-way speed of light can be measured. But it may be difficult in practice. Say we have two lasers opposite and parallel to each other. If we give both an extremely short and equal amount of time to reach the other side before they are occluded by some device in the middle, it will then be clear to see whether both lasers make it to the other side in time or only one of them. The device in the middle makes sure the timing is equal by way of mechanics, so without a clock. One such device may be two opaque rotating discs, each with a small hole close to the outer edge. The discs rotate extremely quickly in opposite directions, giving an extremely short amount of time where the holes align, allowing light at c to pass through but not anything slower. The holes will align at two places so the lasers can each use one of those. Because the lasers are both subject to the exact same device at the same time, equal time to pass though both holes is guaranteed as long as the discs spin at a constant rate which will be helped by the inertia of the discs. The alignment time of the holes will have to be tuned to the distance between them to require a speed of c for light to pass through. Although we could also simply keep increasing the speed of the discs until one or both lasers no longer come through. And we will have experimental evidence. Sadly this may be very hard in practice because the discs may have to rotate so fast they would be torn apart at distances we can manage on earth.

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 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.

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

This must be a mirror universe where port and starboard are flipped... 😂

Great video, but I'm still waiting for the Vacuum Tube video.

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?

My attempt: find a very short-lived particle, which decays into something easy to measure. Accelerate such particles to a very high percentage of the speed of light, and measure the _distance_ from the source where the particles decay. In my mind (which has been fooled by counterintuitive quirks of relativity before, feel free to point out if/what I'm missing), the distance should vary based on the direction of the particle accelerator, if and only if the speed of light varies with direction..?

Great video, but I think the head scratcher for many is what happens if the 'ether' is moving...

An idea about wave power increasing in one direction and decreasing in the opposing direction: perhaps that *is* how things work. How would you check that the wave amplitude is equal in both directions? You would want to "sample" the wave amplitude at, say, x=-d and x=+d and compare at the midpoint of x=0. But if wave amplitude rises and falls in opposing directions, attempting to compare the wave amplitudes at different positions (than they were sampled) would reverse the very effect you are trying to measure and amplitudes would appear to be equal at the midpoint. I think there maybe something more fundamental and profound about the inability to do these experiments. I appreciate the explanation involving the permittivity and permeability, and it does seem like the existing equations do not accommodate opposing anisotropy, but perhaps there is a more convoluted mathematical structure that allows for such a behavior.

Could astronomical observations of "light echoes" from events such as supernova explosions also demonstrate the equality of speed of light in opposite directions? Great video, as always!

Your mom is so big she can measure the one way speed of light with how far her eyes are apart

But if there is some global effect so that light travels at different speeds in opposite directions then the effect will also affect the signals (electronic or optical) going to the timing device canceling out any observable difference. A solution is to have two clocks that are placed together and synced. Then they are moved to the detection position, do their work, and then placed together again to check the results and any drift.

I think the spring discussion misses the question of when you're measuring the speed while you're moving relative to the springs. The problem isn't that the light is going the same speed northward and eastward. The problem is that it's going to same speed in both directions regardless of how fast you're moving relative to the propagation of the waves. Your last comment in that segment sounds a lot like Doppler shift to me.

Or ... one can actually trace the evolution of the constant evolved for denoting the speed of electromagnetic propagation, in a given medium, called "the speed of light" to Wilhelm Weber, read the old scientific publications, and literally understand that the speed depends on the inverse product of the magnetic and dielectric (permeability) constants in a give medium. Ones armed with such an "archaic" (and seems now days unconventional) knowledge one can ask the simple question, does any medium show dependency of the magnetic or the dielectric constants on its geometrical structure ( different properties based on the material spacial orientation). If that were to be the case, then it seems that either the magnetic or the dielectric permeability will have different values when an electromagnetic vibration propagates in space so the "speed" of ( light ) that wave propagation will be different in the different dimensions, if not, then clearly there will be no difference, exactly based on the physical (and formulated) definition of what the speed of light is.

Hi, very nice video. I was thinking that you only considered a restricted class of anisotropy. I think it should be possible to have non equal speed in two directions without breaking energy conservation: you would do it with a chiral material. Left handed and right handed have different propagation speed. At the mirror you convert one into the other

I think occam's hazor can help with this. If there is no experiment to prove either case, we stick to the simpler one.

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...

Put 2 atomic clocks in sinc together, and move them apart in opposite directions with the same speed and acceleration. Now, shoot a laser from point A and mark the time. Mark the time of detection at point B. Bring the 2 clocks together and compare the 2.

Arguments about energy already assumed same speed of light in both direction. Just replace c by 2c1 c1'/(c1+c1') in all formulas to get the two way version.

I don't see why the "one way" speed of light is an issue of such concern to so many people. We fundamentally do not believe that there is any such thing as a "special direction" in space. Why WOULD the speed of light a long a path depend on which way it's going? The natural, obvious Occam's Razor assumption is that it travels the same speed both ways. And by the way - even if it didn't, it WOULDN'T MATTER, because if it did, then we could use that as a way to MEASURE the one-way speed of light. There very fact that we can't measure it means it's entirely irrelevant to us. People just like to "pick at things."

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?