Knowing the one-way speed of light

So you think you have a brilliant idea about how to measure the one-way speed of light, only to discover that it contains a very trivial oversight. Luckily this initial failure was the start of a few new insights on the relationship between one-and two-way speed of light. Well, at least new to me...
What do you think: is there a way that the one-way speed of light is not identical to the two-way speed of light?
Content:
0:00 Introduction
0:30 Origin of the two-way speed of light definition
1:40 The Fizeau speed of light experiment
3:27 Trying to measure the one way speed of light (and fail)
12:07 Speed of light from the wave perspective
18:24 Problems related to opposing anisotropy in vacuum
21:23 Violation conservation laws (abstract)
22:23 But... when spatial anisotropy changes with time...
The video contains 2 short clips of the following videos:
Veritasium: • Why No One Has Measure...
Alpha Phoenix: • Measuring the speed of...
Thumbnail in light of Yoda's Light Sabre from Star Wars
For the simulations, I used a Python script supplied by ‪@DiffractionLimited‬
End music: Floating - Earlybirds
Special thanks to Physbuzz for interesting views and discussions. If you enjoy coding waves, check out this page: www.mathandcode.com/2024/04/2...

[@TheCaphits]

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

[@defeatSpace]

photons are downright fucking weird

[@MrFujinko]

the kid is a natural

[@armandaneshjoo]

@@defeatSpace Photons don't exist

[@NeroDefogger]

no it is not

[@raulkaap]

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.

[@NeroDefogger]

false, read my comment

[@thepathablaze]

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.

[@SplendidKunoichi]

learning is what makes it unique, whereas any of the many edutainment-type science channels can ultimately only offer a form of teaching or lecturing, the distinct ability this channel has is to just offer learning instead

[@Shokkwavez]

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

[@farhanrejwan]

you still watch TV?

[@Shokkwavez]

@@farhanrejwan It's my excuse for laying on the couch.

[@farhanrejwan]

@@Shokkwavez i think i'd still prefer my phone or my pc on a couch 😂

[@photoelectron]

@@farhanrejwan surprinsingly, many ppl still enjoy oldtimes TV bc they don't need to choose something, just the channel and watch watever they give youl; I have a tiny amt of doubts and also no evidence, but I believe this could be cause by mental exhaustion,(which can come from anywhere), at least for ppl who do it for long periods of time......... also could be they don't want to bother looking for a youtube channel or whattever that plays random shit, like simpsons episodes or so,, due to ignorance or also lack of mental energy, there is a possible third scenario, which involves a third party showing the subject how to look for and/or setup such tvlike experience on phone, pc or whatever, but the subject will refuse, possibly due to lack of mental energy.... yea, i know, it's the root of all problems, even mine..... what am i even doing here, all coked up, writing pretentious nonsense..... ah.....

[@TheOneMaddin]

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.

[@isaacamante4633]

Like a wave on a river

[@entcraft44]

But while this possibility doesn't suffer from the energy conservation problem, the aether has other problems, and all the old aether theories have been disproven, e.g. by the movement of the earth trough the aether. My takeaway is the following: The current theories describe most of our universe extremely well, so we often try to make small changes only. This video convincingly shows that an anisotropic one-way speed of light is not possible by making only small changes to our theories. But your comment hints that we can never consider every possibility. That is, unless we employ a much more systematic approach to the assumptions underlying our modern understanding of the universe. There are a lot of interesting ideas out there, but nowhere near enough manpower and funding to check every single one with all the consequences individually.

[@potaatobaked7013]

@@entcraft44the key thing here is that the *old* aether theories have been disproven. There is a way to have an aether field in such a way where you get the exact same equations that you get from relativity, where the speed of light is constant in the reference frame of the aether, and objects moving relative to the aether get length contracted and time dilated. This formulation gives identical math as special relativity so it is not possible to distinguish which interpretation is closer to reality through measurements

[@spliter88]

@@entcraft44 Aether has been disproved but an absolute rest frame has not. In big part because of the two-way speed of light. Once you assume that speed of light is the speed of transfer of information from one segment of space to another then you can have both the relativistic effects as well as an absolute reference frame, where the constant speed of light in any reference frame is just a result of slower processing of the thing moving close to the speed of light. (ie: imaging you have a clock that measures the passage of time by sending a short ray of light towards a mirror and waiting for a response, that clock would indeed slow down the closed you go to the speed of light, and stop completely once you reach the speed of light, since light would have to move perpendicularly to the mirror, the exact same thing happens when one particle sends our information and waits for it to come back) All the other space warping effects are also accounted for as being just a mirage caused by difference in processing speeds of information( photons in this case) that arrives perpendicular or and paralel to the motion as well as what just happens when light travels at a fixed speed.. First from the object's PoV the entire universe shrinks along direction of motion because its own processing slows (so everything just starts becoming very bright, and very blueshifted) Additionally the faster the object moves the more perpendicular to its movement the light has to be to actually hit it. I did the math and some simulations on this and it all checks out, tho I'm still unsure as to where the extra mass is coming from when you accelerate things close to the speed of light.

[@nikolaiturcan6963]

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

[@SupernovaSpence]

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!!!” 😂

[@aethelredtheready1739]

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

[@jotch_7627]

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

[@YuriyKrivosheyev]

Exactly, Russel’s teapot

[@jotch_7627]

@@YuriyKrivosheyev not quite. russel's teapot places some arbitrary "burden of proof" on the "positive claims". in reality, neither claim inherently has more merit than the other. the proper way to look at it is that whether this teapot "exists" or "doesnt exist" cannot mean anything unless it has observable effects (which it does not). embrace the fact that there is no useful answer rather than arguing one way and demanding proof from the one arguing the other way. otherwise its like trying to prove a god *doesnt* exist with science.

[@musiqtee]

Yes, but… not really? Science has its dialectic in pure idealism, the imaginary, intuitive and creative. These “brain functions” are our pathway to pure empirical knowledge - ontology and epistemology, subject and object. More culturally, we depict someone getting a bright idea (lightbulb moment in cartoons), and then the “labour” to prove or disprove, calculate vs experiment, theory vs observation. As this video also shows, Einstein knew his Maxwell, and “imagined” how time, space, mass and energy interacted - and, well it still works, empiricism backwards and forwards to now. Einstein also put a nail in time travel, so the only way forwards is - unknown and undefined. Perfect for those “unscientific” traits we have, besides logic or reasoning. This is also how “wrong ideas” emerge, but are not empirically sound if they can be disproved. There are always “ideas” in a sort of superposition - before we are able to both prove and disprove. Both are necessary. If I’m wrong in this position, we have invented or discovered “everything”, there is no forward empirical path - nothing to imagine into the field. It’s a gloomy world at this time, but not _that_ gloomy?

[@Holobrine]

@@jotch_7627Ultimately you can’t prove about a generic god with science, but many religions make enough additional specific claims about their gods that the gods they describe can be assessed

[@deipalladium8362]

@@Holobrine and no proof for their gods

[@terapode]

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

[@douginorlando6260]

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.

[@mikelord93]

Does that mean we could trick conservation of energy with a clever application of relativity?

[@douginorlando6260]

@@mikelord93no

[@applechocolate4U]

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

[@erikziak1249]

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.

[@takanara7]

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. 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.

[@renedekker9806]

@@takanara7 _"If they fit to the same curve for each direction, then the one way speed of light is the same. "_ - they will always fit the same curve for each direction, independent of what the one-way speed of light really is. _"in either case you'd need to adjust the timing or strength of the magnets"_ - you wouldn't need to, because the timing and current in the electromagnets would be subject to the same isotropic speed of the light. The one-way speed of light is theoretically impossible to measure. Trying to come up with mechanisms that could measure it, is like coming up with mechanisms for a perpetuum mobile. It is just a matter of determining why it fails.

[@rogerphelps9939]

@@renedekker9806 Romer's experiment!

[@renedekker9806]

@@rogerphelps9939 :"Romer's experiment!"_ - That's a difficult one. The mathematics for why Rømer's experiment is not a one-way speed of light measurement is described by L. Karlov in “Does Roemer's method yield a unidirectional speed of light?” Australian Journal of Physics 23, 243-258 (1970) I have a hard time interpreting what the correct interpretation of the math is, but I think it may be the following: Rømer's experiment measured the time between eclipses of Jupiter's moon Io. When the Earth in its orbit is moving away from Jupiter, there should be more time between those eclipses than when the Earth is moving towards Jupiter. Assuming the speed of the Earth wrt. Jupiter is the same in both case (but in opposite direction), you can calculate the (one-way) speed of the light from that difference. But how do we know that those speeds are the same? We measure that by measuring angles towards the Sun. Those measurements are based on the lines of sight to the Sun and Jupiter, that is, based on the (one-way) speed of light from the Sun to the Earth. Due to that, with an anisotropic speed of light, the Earth's speed would appear to be larger on one side of the Earth's orbit than on the other. That is, the orbit of Earth would appear to be more elliptical. The difference in travel time of the light between both side of the Earth's orbit would be attributed to that difference, rather than to the lower/higher speed of light.

[@renedekker9806]

@@rogerphelps9939 That is a difficult one. The mathematics for why Rømer's experiment is not a one-way speed of light measurement is described by L. Karlov, in “Does Roemer's method yield a unidirectional speed of light?” Australian Journal of Physics 23, 243-258 (1970) I have a hard time interpreting the math, but I think it may be the following. Rømer's experiment measures the apparent time between eclipses of the moon Io of Jupiter. It then compares those times at a part of the orbit of Earth when it is moving away from Jupiter, with the part where Earth is moving towards Jupiter. Effectively, it is a comparison of the Doppler shift in two directions. But the calculations assume the speed of the Earth wrt Jupiter is the same for both directions. The result depends on that, but that is merely a convention. To measure the "real" speed of the Earth, you would need to measure the distance Earth travels between two points in time. But how much you measure for that speed, depends on the one-way speed of light again. That is, you are measuring the one-way speed of light in relation to the one-way speed of light.

[@BreadboardBasics]

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.

[@nicouxgwendal]

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

[@maxceppi7189]

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

[@lorc]

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

[@uumlau]

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.

[@Argoneui]

If we can't measure the one-way speed, does it even make sense to consider to question what the one-way speed really is? In SR one-way speed of light is just a choice of coordinates. The problem with the argument from Maxwells equation is that in their standard form you have already chosen an isotropic, Einstein synchronized coordinate system. If you choose another coordinate system, you will have equations with anistropic light speeds (and hence not the standard wavequation with c = 1/√(ε₀μ₀))

[@renedekker9806]

_"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."_ - it is actually the other way around. Any anisotropic model of the light speed is self-consistent, as along as the two-way speed is c. The one-way speed is theoretically impossible to determine and is a matter of definition. You can choose any one-way speed you like for your model, and you will still end up with the current consistent physics model.

[@tsraikage]

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.

[@NeroDefogger]

exactly, that is how they do it, they try to " complex thing to wrap my head around" their way so that you don't try to put yourself on it, to try to understand it, to fight it, to bother with it, and to just accept what they say, they scare you, they make you think you just can't understand it, make you feel dumb, fight that, you are not dumb, put them in their place

[@tsraikage]

@@NeroDefogger they don't make me feel dumb. they're making me smarter by making my brain work until topic makes sense. whatever you are saying is not the way to learn anything. my comment had an intended pun.

[@NeroDefogger]

@@tsraikage "is such a complex thing to wrap my head around" and "when I'm thinking about it i always get lightheaded" does not "make me feel dumb"? whatever you say... "they're making me smarter" no they are not. "until topic makes sense" the topics never make sense. "whatever you are saying" I'm pretty clear on what I'm saying, did you not understand what I said yet you claim to understand the topic of the video? "is not the way to learn anything" what? you really did not understand anything I said... "had an intended pun" ... what?

[@zane003]

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

[@user-lz1yb6qk3f]

Thanks for actually adding to the discussion.

[@QuanrumPresence]

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.

[@dodokgp]

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

[@duggydo]

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! 👍🏻👍🏻

[@Holobrine]

Does a mass at constant velocity produce gravitational waves? I thought it was only masses in orbits with angular acceleration

[@entcraft44]

@@Holobrine You are correct, only accelerated masses in an asymmetric configuration (like two orbiting masses, but not one spinning mass) cause gravitational waves. But while a mass moving linearly with constant speed wouldn't radiate waves, it would still have a varying field that could in theory be detected locally with a LIGO-like system. In practice this particular case (e.g. caused by the sun moving relative to the earth) is absolutely impossible to detect with current designs, because the gradients in the field would be much lower, and several sources of noise like seismic noise and thermal noise, that are small in the region of interest for gravitational waves, explode in the low-frequency limit.

[@takanara7]

@@entcraft44 It would be a 'wave' but it would just have an extremely low 'frequency' If a large black hole zipped right past the earth you would probably be able to see it with ligo, lol. (and the 'frequency' would be higher if it were closer as well, if you think about it)

[@takanara7]

LIGO measures the 2-way length of a laser bouncing multiple times between two mirrors, it's the 2-way speed of light that determines the result. It uses interferometry so the laser has to go out and come back in order to work. So LIGO wouldn't be able to detect changes in the one-way thing.

[@duggydo]

@@takanara7 you are mistaken

[@JoseSilveira-newhandleforYT]

Very interesting thought process!

[@alba.1978]

Outstanding content!

[@eagledee7753]

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

[@harriehausenman8623]

Huygens Optics Video: My favourite kind of photons 🤗

[@NeinStein]

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.

[@rockapedra1130]

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

[@hamjudo]

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.

[@henrikskott]

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.

[@renedekker9806]

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.

[@laraaleatoria]

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

[@cylosgarage]

My day just got a whole lot better!

[@mezzanoon]

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!!

[@GustavoPinho89]

23:19 I was waiting for Light my Fire to kick in...

[@MarcStollmeyer]

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.

[@abrikos1100]

One-way speed of light: "immeasurable" EM equations: hold my waves

[@vaakdemandante8772]

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.

[@cowgoesmoo2]

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

[@DJR000]

That wave animation was excellent

[@manuelorrego3314]

Damn I love this channel

[@3zdayz]

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.

[@3zdayz]

Mind you this does not measure the one-way speed of light. It only measures the differences in the one way speed of light

[@carlbrenninkmeijer8925]

fascinating !!

[@ImTheReal]

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

[@clairecelestin8437]

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.

[@1337w0n]

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?

[@ozzymandius666]

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.

[@13lacle]

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.

[@WideCuriosity]

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.

[@nweston5070]

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!

[@entcraft44]

Regarding number 1: LIGO and other gravitational wave detectors are only sensitive enough in a small frequency band, limited from below by thermal and seismic noise and above by quantum shot noise. They would not be able to detect a mass in uniform motion since that is in the low-frequency limit. Another approach would be to use a pulsed laser system, but with my limited knowledge I doubt that the necessary precision can be achieved with current technology.

[@raymitchell9736]

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!

[@tukan1652]

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

[@riffzifnab9254]

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

[@TheMeccio]

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

[@leonhardtkristensen4093]

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.

[@dariusherick7001]

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.

[@-_Nuke_-]

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!

[@whig01]

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.

[@crawkn]

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?

[@samgage2715]

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'.

[@grayishcolors]

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.

[@blahfasel2000]

@grayishcolors The problem is that the two synchronized clocks experience time dilation when you move them, even if you move them only slowly. And if the speed of light really is anisotropic they would experience different amounts of time dilation even if you move them at the exact same speed over the exact same distance in opposite directions. Edit: And there would be no way to measure the difference in time dilation unless you bring the clocks back together again, at which point you're back to having done a two-way measurement rather than a one-way.

[@grayishcolors]

@@blahfasel2000 I was aware of the time dilation aspect, but the thought was that it has almost no effect unless we are talking about really fast speeds. All we care about in this hypothetically is to prove that light moves at different speeds, not necessarily have a perfect measurement. I suppose you could have a perfect measurement like you said if you brought the clocks back together. I still don't see how that breaks the results though. If the time dilation is different in direction then the measurements will also see the difference which would add to the results. Perhaps there's something I am not seeing as an issue here?

[@grayishcolors]

To add to this: We have a CLOCK A & CLOCK B Let us assume that time dilation somehow made CLOCK A run a whole 10 seconds slower by the time it gets to position for the experiment. Light always goes at C from all reference frames meaning the time the light takes to travel won't be affected be affected. So the light will be released by CLOCK B 10 seconds faster than CLOCK A & arrive at CLOCK A 10 seconds sooner than it will create its own light. This is of course assuming light moves at a constant speed & not two separate speeds. The only way for this not to be the case is if the speed of light moves at a different speed as to exactly cancel out the time dilation. That doesn't make sense, though, because time dilation is relative & based on speed whereas the speed of light is always C from all perspectives.

[@aniksamiurrahman6365]

​@@grayishcolorsUnfortunately, for something as fast as light, even such minute difference is very important. U can't ignore the time dilation.

[@entcraft44]

@@grayishcolors "time dilation is relative & based on speed" this is true only as long as you look at uniformly moving clocks. As soon as you accelerate the clocks to start and stop them that is no longer true. This is the famous twins paradox: If twin A flies to Andromeda and back at high speed, will she be elder or younger than twin B when she returns? During flight, both twins will insist that the other twin experiences the time dilation, so both should be younger? No, because twin A had to turn around and this changes things completely. Twin A will be younger when she gets back. I have not done detailed calculations for your scenario with the two clocks, but I am convinced the effects will cancel out exactly.

[@joshmyer9]

Would the difference in c result in different wavelengths for the same frequency propagating in opposing directions? Those quantities feel tractable to measure, and are a lot more local of a measurement, avoiding the whole loop problem. It felt like the video was about to go there a couple of times, so maybe I missed where the idea was dismissed as not even wrong 😅

[@samb443]

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

[@HuygensOptics]

Indeed all examples assume the setup and observer are within the same inertial frame of reference.

[@niekiejooste4637]

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.

[@user-bh6ey1ke4n]

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

[@WEPayne]

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

[@dcorp80]

Every reflection in the setup changes a direction of a beam

[@ruebenmikoch1828]

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.

[@raulkaap]

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.

[@3141minecraft]

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.

[@lllbutcher]

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.

[@lyuboslavilov]

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

[@glitchyglitch1235]

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

[@FunkyDexter]

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...

[@jamiekawabata7101]

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.

[@takanara7]

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.

[@andreasboe4509]

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.

[@frizzarazz]

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.

[@frizzarazz]

Strictly speaking, if one laser were exactly half or a fourth (etc) as fast as the other, both would still pass through. But that seems like enough margin to rule out most difference in speed.

[@Takyodor2]

Wouldn't the lasers have to fire synchronously for this to work?

[@Takyodor2]

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..?

[@kantanlabs3859]

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 ?

[@pradyuman9151]

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

[@mishun]

13:06 well, properties of SI system of units

[@Iron44You]

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

[@dodokgp]

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.

[@HuygensOptics]

The measurement does make assumptions about the distance from io to jupiter and Jupiter to earth and also assumes that the speed of light is equal in both cases.

[@dziban303]

this channel rules

[@terrypitt-brooke8367]

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!

[@ryborg123456]

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

[@YuriyKrivosheyev]

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)

[@darrennew8211]

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.

[@tatianatub]

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

[@tatianatub]

I might be misunderstood but that sounds to me like we can measure it

[@HuygensOptics]

Yes but because it is based on interference, always in a two-way way.

[@gabri41200]

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

[@DestroManiak]

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.

[@entcraft44]

You can easily convert the wave amplitude into a power measurement by time averaging. Unlike timestamps, power measurements are absolute. Imagine the center oscillator as a radio antenna. We would obviously notice if radios could only pick up the signal to the east of the antenna but not the west. How much signal is picked up by your radio is not relative.

[@LaserFur]

If the spring rate was different in the two directions then the inertia would also be different in the two directions. And that makes it cancel out. So the wave increasing in power does not happen.

[@tolkienfan1972]

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?

[@jorenboulanger4347]

Can you have two accurate clocks, sync them up and then move them apart? That way you can emit a light pulse from one clock to the other and measure the delay directlly.

[@minxythemerciless]

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.

[@martinhsl68hw]

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?

[@chopper3lw]

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

[@realcygnus]

Nifty !