Why No One Has Measured The Speed Of Light
소스 코드
- 게시일 2024. 04. 26.
- Physics students learn the speed of light, c, is the same for all inertial observers but no one has ever actually measured it in one direction. Thanks to Kiwico for sponsoring this video. For 50% off your first month of any crate, go to kiwico.com/veritasium50
Huge thanks to Destin from Smarter Every Day for always being open and willing to engage in new ideas. If you haven't subscribed already, what are you waiting for: ve42.co/SED
For an overview of the one-way speed of light check out the wiki page: ve42.co/wiki1way
The script was written in consultation with subject matter experts:
Prof. Geraint Lewis, University of Sydney ve42.co/gfl
Prof. Emeritus Allen Janis, University of Pittsburgh
Prof. Clifford M. Will, University of Florida ve42.co/cmw
The stuff that's correct is theirs. Any errors are mine.
References:
Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der physik, 17(10), 891-921.
(English) ve42.co/E1905 (German) ve42.co/G1905
Greaves, E. D., Rodríguez, A. M., & Ruiz-Camacho, J. (2009). A one-way speed of light experiment. American Journal of Physics, 77(10), 894-896. ve42.co/Greaves09
Response to Greaves et al. paper - arxiv.org/abs/0911.3616
Finkelstein, J. (2009). One-way speed of light?. arXiv, arXiv-0911.
The Philosophy of Space and Time - Reichenbach, H. (2012). Courier Corporation.
Anderson, R., Vetharaniam, I., & Stedman, G. E. (1998). Conventionality of synchronisation, gauge dependence and test theories of relativity. Physics reports, 295(3-4), 93-180. ve42.co/Anderson98
A review article about simultaneity - Janis, Allen, "Conventionality of Simultaneity", The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.) ve42.co/janis
Will, C. M. (1992). Clock synchronization and isotropy of the one-way speed of light. Physical Review D, 45(2), 403. ve42.co/Will92
Zhang, Y. Z. (1995). Test theories of special relativity. General Relativity and Gravitation, 27(5), 475-493. ve42.co/Zhang95
Mansouri, R., & Sexl, R. U. (1977). A test theory of special relativity: I. Simultaneity and clock synchronization. General relativity and Gravitation, 8(7), 497-513. ve42.co/Sexl
Research and writing by Derek Muller and Petr Lebedev
Animations by Ivy Tello
VFX, music, and space animations by Jonny Hyman
Filmed by Raquel Nuno
Special thanks for reviewing earlier drafts of this video to:
Dominic Walliman, Domain of Science: ve42.co/DoS
Henry Reich, Minutephysics: ve42.co/MP
My Patreon supporters
Additional music from epidemicsound.com "Observations 2"
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My bank uses the same theory, but vice versa. When the money leaves my debit card, it goes really fast. When something is to be repaid, it takes much longer.
XD
Underrated comment lol
Oh, same here
It is, for bank, a convenient model to embrace. You'd be a banker, you'd do the same !
Now , of course, you don't believe Einstein really had any clue what was the speed of light.
The number just fell into his hat. Actually Morley and Michaelson were trying in 1887 to measure the speed of light. But the 'ether" screwed up everything.
Einstein just took M&M experiment result and declared that "ether" does not exist, and that froze everything in place including the speed of light.
LOL my bank is involved in this inverse of equities and is complicit as far as I'm concerned....I speak into existence and impose the maximum penalty for their impetulance with the application of the converse of consequence to the algorithms restricting my transactions and unleash the acholaids of irreverence to expand and proliferate the funds available to be unlimited everyday and to exponentially grow... please and thank you :)
Light: "My speed is immeasurable, and my time is ruined"
That's..... actually quite brilliant 😐
Underrated comment!
@@sinpi314 yes it is
Lmao 🤣😂🤣
You made me laugh so much 😂😂😂
WOW
On another note, I enjoy your videos immensely! You explain things in a way that I can understand and I find myself spending more time watching your content. Your videos satisfy that itch to learn.
me too
Learning is fun as long as it's not forced...
He did PhD in physics on how to make physics more approachable to general public using media. So, he knows thing or two about presenting in a way that everyone can get along.
@@hawavideouploader That's so sick
I really never looked at that this way and you're right its facinating. Keep creating videos like this
This was a very fun present to unwrap. When you called me and told me to turn the camera on I knew something weird was going to happen and you certainly delivered. As long as I’ve known you Derek you’ve been destroying assumptions. Thank you for this friendship. It’s certainly enjoyable from my perspective.
Aaand subscribed
First
13 seconds ago
hey Dustin!
Use quantum entanglement
"So someone has measured the speed of light...or have they?"
Huge Vsauce moment right there
They both never really were the same after the "Is anything Random?" collab.
Or were they?
*vibrophone intensifies*
what about quantum entangle ment to start the two clocks on both sides
@Windigo Jones that is why you watch flat earth videos lol
What about using quantum entanglement to measure the speed of light?
Fire the light, it sets off the entangled particle, and when the light reaches the end, the entangled particle there would immediately send the info back to the first one.
No information is exchanged when entangled photons decohere.
@@MagruderSpoots what about before they decohere? There is information transmitted while entangled...
@@SpartanFilms1997 None. Entangled particles can't be used for communication.
What if we get 3 timer-clocks that have an internal mechanism that ticks every second, with extreme accuracy.
They start synchronized, but it doesn't matter if they get slightly desinchronized. as long as the mechanism inside of all 3 keep independently cycling exactly every second
We move two of them 1km on each side, and leave one of the center.
The ones on the sides are programmed to start counting passed time after a given period of time (an hour?).
The one of the center is programmed to emit a light beam a few milliseconds before that same amount of time (enough to cover for distance but still trigger first) and then and then keeps cycling every second.
The clocks at the two sides start counting time every second after that and stop every time they receive a light beam/signal, the timer never resets, keeps starting and stopping, building up cumulative delay.
If there was any desyncrhonization due to moving the clocks it doesn't matter, because that will only count once, while the repeated cycle will continue adding up and stacking possible delays caused by possibly different speed of lights.
We then move the clocks back together and confront the amount of total delay built up.
The difference caused by desyncrhonization will count once, but the the difference caused by possible different speed of light will be cumulative, so we can still spot differences...?
I don't genuinely believe i solved a century-old problem in 10 minutes, but can someone help me undertand why this wouldn't work?
@@SpartanFilms1997when two particles decohere, if you only have one, you have no way of knowing if or when it decohered.
Derek, you have blowed my mind today. Great video and even greater attitude into science mystery.
"so someone has measured the speed of light... or have they?" Hey, Vsauce... Michael here
Hahaha exactly!
You look like vsauce but mexican
Lol
xD
VSalsa
Great video. Despite getting a physics degree and teaching physics for years, I never came across this or thought about it. I was treating the video mostly as a 'fun to think about' sort of video, but your point at the end is really intriguing.
Even after watching the video, I have a few questions. What terrifies me the most about the questions, isn't that I think that they'll find a way to solve the one way speed of light; but the fact that if I am thinking about these questions, someone else likely has already, and there is a reason these questions don't answer it, and when I try thinking of the reasons, it makes the whole concept seem even more bizarre than it already is.
For instance, we are trying to measure the speed of light in a vacuum. But we could also measure the speed of light in a medium; intuitively there should be a relation between them. But the intuition must be wrong right? Or at least unverifiable. Which means even with an instantaneous vacuum speed of light one way, and a 0.5c vacuum speed of light the other way, there is some very strong asymmetrical physics going on when light goes through a medium.
Even if I have a medium that slows light down to a crawl, there has to be a reason it doesn't show the asymmetricity in the speed of light.
There also has to be a problem with colliding objects at relativistic speeds, due to the vastly changed special relativity formula. Two objects with the same insane kinetic energy relative to their stationary mass, can be travelling at two vastly different speeds depending on which direction they are traveling. One could be moving near instantaneously, while the other can be moving just below half c. Intuitively there must be some way you could use this information to solve the problem; but the intuition must be wrong, otherwise it wouldn't be an open ended problem.
Probably the reason things act so asymmetrically weird if the speed of light in a vacuum is asymmetric, is because that isn't "just the speed of light", it is the speed of causality. It means cause and effect acts different speeds in different directions; and there is no experiment you can do that can get past the limitations of cause and effect. All physics basically goes bonkers such that the asymmetrical speeds will always work out.
----------------------------------------------
Anyways, other than my mind breaking, I do agree that the end of the video is very intriguing. A solution to figuring out if the speed of causality is asymmetrical or not, could exist in a unifying theory. So the mind breaking isn't all for not.
Or perhaps the concept turns out to be pointless. As what does it mean if the speed of causality is different in two different directions? What is differences in time and space even mean if causality is different in two directions, aren't time and space dependent on causality. Perhaps the entire paradox of asymmetric speed of light is dependent on our own ignorantly rigid view of space and time? And thus unifying theory will have nothing to do with answering our fallacy of a question?
Ugh, my head. Anyways, I can always find solace in that Hexagons are the bestagons.
Hexagon = Bestagon
I made a separate comment, but no one replied so ...here it goes:
"I honestly have no idea what I am talking about, but ... can you use quantum entanglement to measure the speed of light somehow?
The entangled particles are "already synced", so "hit" the one "far away" with "something" that changes it's state and observe it/measure the time on the one "near" you... and do the same speed of light test from/in all "directions", then just compare the times to see if it's the same.
Only objection I could find to this not working is that I have no idea what breaks entanglement, so stuff like lasers, photons, whatever ... might not break it.
In rest, it seems like a good idea. Obviously I am wrong, or else it would of been tried by now, but I would really like an answer for this, if someone could educate me. Like I said I have no idea what I am talking about, so don't jump me. :)"
Okay, I have a question is the solution of 10:00 in multiple ways correct to verify de one-way speed of light okay hear me out.
1. If you do this and film both the clocks you can see which one turned on the fastest. Or which one is further. Once again you need to time it perfectly by turning on the cameras at the same time. But this one could be possible
2. If you move the middle clock to the left or the right you would get another result out of it if the speed of the light is different if not you have done it. To this correct you nee to set the clock on both sides at 300 meters away from the middle
I hope my English wasn’t that bad and that you understood my brain thoughts
Think one more time: "are distances AB and BA the same or they are measured in terms of light traveling time?" and you will get your sanity back. You can easily simulate the entire special relativity universe defining your (name A) causal boundary as now. It looks like "c0 towards you is \inf", and "c1 away from you is c/2" and for every BA synchronization event all time travel distances pointing to you are just zero, and still (c0 dt0)^2 = dr^2 = (c1 dt1)^2 the metric invariant your coordinates must obey.
This kind of "absolute" distance independent from your speed of light choice came from you actually postulated the object B being at the same location for AB and BA synchronization events but how can you define "the same point" within the experiment? Observer from Pluto will surely note your signals were sent and received at different points of space.
And here comes the answer: how can you measure any kind of "directional" speed of light if you can not provide the same distances in different directions?
Your videos are amazing, I don´t speak english but I'm learning as fast as I can cuz I don´t want to miss your videos in both english or spanish. Thanks Derek.
Love the video and the way you explain things making it easy to understand.
Alright here is my idea, assuming our nomber for c is correct then given a long enough test area we could set up a test with a gate programed to close on a percice delay to be some distanc light travles over that delay. Could we then wait back at the start for the reflection and adjust the timed gate untill we know exactly what timing it takes for the light to be blocked?
In my mind you could do this in any direction and since you are blocking it before it hits the mirror then your getting the one way messurment
For me it seems you replaced second clock with gate, but the problem with synchronizing them still occurs
"We've invented an FTL drive but you can only turn left."
Lol
If you turn right you go backwards.
It's not an ambi-turner!
Douglas Adams Likes this comment!!!
NASCAR is going to have to build bigger tracks...
When a physicist comes to an engineer with a question: "OH you're gonna do something weird arent ya?"
+Science Revolution I see but the whole religion thing is better than science is defunct
Thing this deep makes me question the existence of this very video. Really.
@Science Revolution , You list SO many things which are not true here that I won't bother to point them out. You might as well be traveling instantaneously.
@Science Revolution If you write an article and prove that mathematically, you could actually become a reputed scientist. Go ahead and do that. That's the beauty of science, all scientist have that in their minds, a sentence that says: "we could be wrong, and we probably are". We have like 3 centuries of science and look around you, look what they've already done! 300 hundred years is nothing compared to the time that our species is in the planet and absolutely nothing compared to the age of Earth itself. Stop comparing Science to Religions, they have nothing to do with each other.
Guys don't reply to that revolution guy,he/she literally mass spams this exact paragraph on all science related channel nowadays along with bunch of his flat earther friends,
Well he's/she's literally questioning the very science which is allowing him/her to watch this video on his/her device, it's like if Elon Musk's son starts saying neuralink is fake.
I was watching a video where a psychiatrist explains why these kinda people exists who claim the earth that it's flat or all the theories are bs , it's like they want to feel special as if they possess a knowledge which is hidden from the general public,it's like the film 2012 where only few people knew about what will happen actually in the start ,these want to get a feeling like that forgetting the difference between real life and Christopher Nolan's fiction scriptures , however this is also a state of mental illness which must be treated and not like back in 1700s when if someone started seeing ghosts , people started excorism or drowned him/her in the water lol
My kids and I love your channel and I love the conversations induced afterwards. This video covers a topic that I’ve wondered about for quite some time. How can an object like the andromeda galaxy that is 200,000 light years across “appear” as a single object instead of a smear? Especially since it is also moving through space? These are the things my sons and I ponder. Thanks again for the great content!
@veritasium I love your videos! I have a comment on this one in particular. If the speed of light returning to us was immediate, we couldn't then explain the cosmic microwave background or the red/blue shifting from stars right? As you explained, we cannot measure the speed of light directly, but evidence shows that it may be the same in both directions. What do you think? Thank you again for your wonderful videos!
He did really address this issue, albeit in an offhand way. He simply said, paraphrasing, 'Of course we can always apply Occam's razor' - which is what we do in so much of science.
There is no 'evidence' as such. Our models have been created based on the convention that light moves at the same speed regardless of direction. If this convention is wrong, then our models will almost certainly be wrong, and, at the very least, how we view the Universe, its structure and evolution will need a major rethink.
It isn't that our models support the convention, it is that the convention creates the models.
Should have wrote this in my physics exams, "It is neither a supposition, nor a hypothesis, but a stipulation that I can make of my own free will"
i still believe that 1 is prime
@@gasun1274 0 is odd
Definitely would have gotten your word count up
0 is positive
Lets be honest or make a bet... when the time comes, it will be proven that the speed of light is same for both directions, its pretty obvious. Right now "Veritasium" got the free hall pass for making wild assumptions since speed of light can't be measured with synced clocks.
I love how he called it right off the bat, “oh you’re talking relativity, you’re gonna something weird aren’t you?”
"You're gonna something weird"
This sentence makes me confused and or scared.
So is the speed of shadow the same as the speed of light?
Quay
@@Chadwicktrumpet No
@@Kristian-ql8zw but why is it not that speed. That is the question
Check a paper published by Andrzej Dragan and Artur Ekert "Quantum principle of relativity". Reading it you can see that if you would have a observer who travels faster than light (in your case this would be the information carrier about the time on Mars if you consider return speed as nearly infinite) from his perspective reality would be much different than for observes that are traveling with the speed
In the AB mirror lets add a c mirror so we have -- means light so then A ---------------------------------------------B but we will do now then where the light is sent out from A and hits b, and we have a C in the middle which is a mirror and has a like time detector, which is calculate from the third dimension, when we look at it through quantam terms
4:06 - "Or have they?"
I feel like this was a missed chance to put the vsauce theme on
Also noticed VSause referenceh
Here
krplus.net/bidio/lLWvZqdriZ68lII
this is depressing here the right url krplus.net/bidio/qNl7gWdfdoOnknI
Moon Men by Jake Chudnow
(Vsauce theme) - krplus.net/bidio/hLJqZ5eanZ3LgnI
krplus.net/bidio/icuhq4d5q4HHepQ
Destin was probably having a perfectly fine, normal day and then the phone rings. Now he has a broken brain.
Have a rocket exactly half way between earth and Mars. The ship will send one message to earth and the other the Mars and then instantly back to earth. You would expect that the message that was sent to Mars first would take exactly three times as long to reach earth as the one sent straight there. If this does happen then the speed of light is the same in both directions.#big🧠
@@jontisaurusrex9851 In order to measure that, the clocks on the ship and on earth would have to be perfectly synchronized. How are you going to do that?
And that makes it an even better day.
@@Anton-cv2ti Maybe one way would be to accurately calculate at what time exactly the ship would be in a specific place between mars and earth and when the ship knows it's in the correct spot, just set the time to the predetermined on earth time, then proceed with the experiment suggested above.
Rotate a tube or a some type of gear with slits on opposite sides and have a detector to see if light made it through. At a certain velocity of rotation light won't be able to traverse it. That speed and however large the slit is (the distance it would need to rotate to stop the light from making it through) gives you the time. Use that time and distance between the slits to get the speed of light
Great video as always! I had one question which I could not wrap my mind around. What if we put two light sources inside an object pointing at each other and mark where both sources point at. Then we accelerate the object and measure the distance of deviation from the original point they were pointing at. Couldn’t we see if the deviation is different between the two oppositely directed light sources, and by extent measure the speed of light using the deviation?
Thank you in advance to anyone who sees this and answers!
As far as I can tell this should work.
The problem then would be that you would have to fire both the sources at the same time, which would not be possible due to having 2 clocks be synchronized and separated as he stated in the video
You don't need to measure the speed of light in every direction, in order to check if it travels with the same speed in every direction.
There is a method to prove that, you can check the Michelson-Morley experiment in which they were trying to prove that the "aether" exists. They ended up proving that the "aether" doesn't exist and that the speed of light is the same in every direction. They developed an instrument that was named after Michelson, called the Michelson interferometer.
I really love most of his videos and I am a huge fan, but this video is straight misinformation. What @veritasium is saying in this video is simply wrong. It's shocking to me, given the reputation of his channel. :/
Great video, really got me thinking
My real takeaway is that two clocks, regardless of precision, will never be truly synchronized. This explains why I am frequently late.
@Steven Moore
Since I am the moving observer, it's my timepiece that runs slower. Only when traffic is unusually light can these relativistic effects be mitigated. It's just physics.
That’s what I told my boss for getting late at work, and I got fired...
Would like but your count fits into 8 bits exactly, don't wanna be the one to change that :)
*Edit* Damn someone changed it, oh well, added the like now it no longer fits into a perfect 0xFF
@@markm8188 ...but are you...or are you the observed standing still...
I think about that all the time, how two things can’t be happening at the same time, EXCEPT for two things touching each other.
That’s why going to work feels like a drag and coming home feels quick.
Lol
Hahaha
I know this is a witty joke but there's a video about this kinda time where u feel vs time that is real in vsauce
The answer is easy, if u wanna measure the time delay in one direction, u send an impulse between this two clock's in each of this two directions and u will see if one starts later ....
Quick maths
@@babylebron6119 I'll take it you didnt watch the whole video when you wrote this. He explains that the problem is that it remains unknown if light travels the same speed in all directions.
Ooo,ooo! Idea! Start with two clocks that are synchronized. Move them apart REALLY FAST so the timeline dilation is visible. Then move back together REALLY SLOWLY so time dilation is (almost) negligible if the speed of light is different in different directions the clocks won't be synchronized anymore no matter what time they show. If the clocks show the same time then reset them and measure the speed of light with almost any of the ideas in this video.
All other methods are using light as a mode to transfer information, however if you use sound which we do have a one way direction for, we could base the speed of light on its comparisons so the speed of sound
“We don’t you reply quicker?”
“Sorry babe I’m at Mars rn”
That's the way to measure the one-way speed of light. Send babe to Mars.
@@majidaskari8306 it doesn't matter; no matter if light is same or different in all directions, it will take 20 mins.
I swear this channel is a gold mine for educational and entertaining content
Indeed
True
Very much so
Why are you watching this on the same day I am
@@autoclockk the comment man works in mysterious ways
feel like since light seems pretty consistent you could use light to trigger a clock to get rid of the latency.
Certain audio cables have essentially an unused control line that detects interference and compares it to the signal with the desired "noise" and deletes anything they have in common. Which makes for a pretty clean signal
2 synchronized clocks starting at the same point.
Move 1 of them in a direction towards the sun for 1km (or something with a more stable position in the space like a far galaxy/star).
Wait ~12 hrs so earth rotates. (something close to 12hrs, calculating all the planetary rotations, just be sure now the same direction is inverted relative to the space, this is the hardest part since you may need to wait a lot to be sure its the same direction again when moving back)
Then move it back to the initial position. (now it actually moves on the same direction that it moved at the first trip)
Once they are together again check the difference between clocks.
Divide it by 2 and now you know that direction's time dilation for 1km.
Next time synchronize the clocks again and do all again but with a completely different target, so completely different direction. (another far star, preferably in the opposite direction)
Measure the second difference of the clocks once they are back together.
If they don't have the same difference with the first test, you proved that the direction matters.
If not you are close to prove that direction doesn't matter. Just do it again with different directions to be sure.
Good luck
I know for a fact that the speed of light depends on direction, because sometimes when I sit at a traffic light I can see the BMW driver behind me flashing before I see the light turning green.
I don't think the difference can be felt/measured through humanly senses. That bmw guy is flashing before it turns green.
@@yogi30303 that is the joke.
@@glinchdk yeah I guess among all the serious comments I took this seriously too.
@@yogi30303 r/swooosh
@@Majesticbro r/slamdunk
"So some one has measured speed of light"
"Or have they??"
*Vsause theme plays*
I kinda expect there's a theme song... But left disappointed
*vibrophone intensifies*
Is he even alive
Vsauce must have run out of topics. Loved his channel.
yes what a garbage clickbait channel this has become....
In cmb,we see almost same picture in all directions that mean the light speed is same in all directions.
(If that is not true,we will see different picture in all directions because the time of different direction's photos will be different that means,we see stars in one direction and cmb in another direction)
That was my thought too. Weird that Veritasium did not address that.
NOO
In my pov
Considering the situation if we take a picture say a panoramic photo in whole 360
You must start from north then to back again to same
But the time you will take to click that whole pic would be considered
Universe is not state for matter of space it is even a space-time fabric ❤
Please if any correction ❤❤😊
As a lay person (i.e., as a retired guy fascinated by science in general and Veritasium in particular but with no formal education in physics), can we bring entanglement into this discussion? Can we imagine, in some future time, a scientific instrument based on entanglement that we could use in order to synchronize two clocks at two different locations, or that we could use to undertake some of the other experiments you described for learning more about the true nature of c? As I understand entanglement, it is instantaneous, i.e. faster than light. Perhaps my limited understanding of "spooky action at a distance" may be revealing a woeful lack of knowledge about entanglement and about physics in general, etc. On the other hand, perhaps my comment here will inspire some undergrad physics student who follows Veritasium to pursue the line of inquiry I've described above and end up becoming the Nobel laureate in Physics in, oh, let's say 2030. Remember folks: You heard it here first!
This is the best explanation of "WHEN WILL THEN BE NOW?" I've ever seen.
Isnt it the best 'lack of explanation' of 'WHEN WILL THEN BE NOW'?
**i really like your point
Soon.
Get two clocks that are 100 meters away from each other. Start them at the same time. Shoot a light across from one clock to the other. When the light reaches the first clock it will stop. When the light reaches the second clock it will stop. You have 2 times and you subtract them to find one. And convert to the larger scale
@@jordanammons4851 How do you start them at the same time?
@@jordanammons4851 2:07
I like the extra effort you put into the short acting parts to visualize the concepts.
Have a big clock display of a super accurate clock suspended on a roof or tower. Aim two cameras at this display. Let the clock run, its start time doesn.t matter. Have the first camera triggered instantly, the second camera triggered by a beam of light activated the same time as the first camera. Compare the photos to see yhe time difference. Depending on how powerful the camera, depends on how far away you could feasibly place the static clock and second camera
@@Raythe But how both cameras will activate in the same time?
@@Raythe but because the speed of light is different for different directions, two cameras will see differently delayed images so that the time difference you see in the images will be c.
We can use two clocks
When the beam hits the first clock timer start
And when it hits the second clock the another timer stars
Clock are individual
And we can get the diff between the clock
Hi mr veritasium, I think that it is key to understand what the medium that light travels through is made of.
It could be that space and light and all kinds of matter are essentially made of the same thing.
Just like sound waves are the vibrations happening in different kinds of materials, like solids and liquids and gases.
These vibrations mean there is energy causing them to happen. Maybe, it is the same (the energy) that we see happening with space, light and matter.
“No Officer. I was not speeding. You see, the speed of light different depending on the direction.”
Officer:
“lol. Here’s your ticket.”
Actually this argument has held up in court. Google it, it’s pretty awesome.
@@epicvillain8308 I tried to but couldn't find it. Mind sharing?
@@epicvillain8308 you are lying
@@lilsabin 🤣
@@epicvillain8308 No but the dopler effect has, even though it works in the driver's favor. Far as I know this argument does not work anymore because of that factor (I just drive a truck though 😅).
Lol when Destin realizes Derek is about to drag him into relativity
yeah, that was hilarious.
That got me good when he realized what was about to come up XD
Can we use some ohter signal than light to reset the 2. clock? Use sound signal to clear the clocks and then send the laser light after a predefined timedelay eg. 1s, then measure the time it took light to travel between the clocks. We know the speed af sound, we know the delay, isn't the rest a simpel calculation.?
We are trying to measure speed of light in vacuum. Sound doesn’t travel in vacuum.
Yes, but it is a vacuum inside a cylinder, and sound do travel in the material the cylinder is made of.
@@danbroe5966 Even if so, the speed of light is actually the speed of information. The speed of sound is determined by the speed of light anyway. It doesn't matter what "other speed" you wish to use to synchronize clocks, because the speed of every wave is based and limited by the speed of light anyway.
Wouldn't redshift and blueshift disprove the c/2 and ∞. I mean sure we can't know if it's the same in both directions but I guess we can know that it cannot be instantaneous. Or if it doesn't work this way please explain why.
4:04 "So someone have measured time... or have they?"
You're making me feel like you're actually Vsauce's cousin
Anything to churn out another vid, my friend, just anything......
..or is he?
Hey Veritasium, Derek here
@Alexis Degryse If the light from the nuke has reached us then it already happened an hour ago.
(Not sure what you mean by an ‘hour behind’)
He said "or have they?" I was kinda sad no vsauce music played
yea
If the pulse to start the second clock us traveling at the speed of light(2:27), then why can't you just subtract the number of seconds the second clock has been running for from the first? I feel like this is a stupid question and I'm missing something...
I know, right?! At 4:05 I thought I was watching a Vsauce video. lol
@@mahirshyam4127 because if the speed of light is potentially different in both directions, then how could they know how much to subtract it by?
The experiment would be trying to measure the speed of light, but you’d need to know the speed of light in order to know how much to subtract the second clock by.
For example, imagine trying to solve for the length of the hypotenuse of a right triangle, but the only information you’re given is the length of the bottom leg.
Asking why you can’t just subtract the speed of light from the clocks to get the answer would be kind of like saying “why not use the length of the bottom leg and *the length of the hypotenuse* to find the other leg? Then from there, just use this other leg to find *the length of the hypotenuse.”*
Hopefully this example makes sense, I’m not the best at explaining things.
yeahhhh
Love your videos :) but I really need an explanation for the part where you showed an equation for different speeds of light in different directions...
we can use two clock but their distance from each other will be only 1 meter, put both clock in running position at 0.5m and then gradually move towards opposite ends this way both clocks will remain perfectly synchronized and also they will reach at our desird points. after that, we can through the beam of light from 1 end where 1 clock is present, and running to the other where 2nd clock is perfectly synchronised. this way we can calculate speed of light.
thanks for giving me another thought I can't talk to most people about cause they'll just say I'm crazy.
yeah xD also with the gravity video
They probably don't even have the intelligence to understand what you are telling them so it's easier for their little brains to consider you crazy than to accept that they are stupid.
Toooo true
This is just him wanting to think is so smart conjecturing that c is different one way than the other
@@r3kpwner303 Not true. People have different things to focus. No one has the capability to be smart in every thing.
Debunk this: There is no way to measure a people's intelligence.
"Speed of light is the universe's refresh rate." -Stephan Wolfram
Has that got to do with his physics project?
speed = distance/time
refresh rate = 1/time
YES LOL
I haven’t got to the end yet but can you use some kind of quantum entanglement trigger in the future.
Are we living in a simulator?😢
Puts an interesting spin on astronomy where we deem distances relate to time, as in the “early universe” due to time it would take light to travel.
Here's an idea how to measure the one way speed of light:
Put a photosensitive material at the other end of the light's path. When the light touches the material, it will start a reaction (similar to developing a photo). That's one way. Now, I know that in order for us to notice that reaction we either need to observe it (which means light would have to bounce back for us to see it, which creates a return travel for light) or measure the time in which the photo reaction occurred (but here we arrive at the two clocks problem). BUT! What if we create a sort of a tube made of entirely photo sensitive material, that would be constantly monitored by ultra high speed cameras and we measure the "speed" of photo reaction progressing towards the end of the tube?
I have a feeling that what I just described is based on a cartoon logic, but... I don't know. Maybe it will inspire someone :D
"Or have they?"
**strong flashback of Vsauce*
Why didn't he queue the song?! haha
@@KingR787 He'd need a colab and permission to use the song =)
It's a shame Vsauce doesn't really do these types of videos anymore
Us to aliens: "We measure a meter as the distance a light takes to travel in 1/299 792 458 seconds"
Aliens: "Which way monkeys?"
Ok
The worst part about me watching this video, is that I understand everything he says, but at the same time, I understand nothing.
Dude is too smart
@@abrahamsanchez7455 indeed
Understand the words, but not the meaning, it's what happens
He does speak English pretty well
Same here...
This reminds me of the Jason Lisle articles on AiG trying to rationalize the creation story. Big difference in Lisle's view from the one way is presuming a self-centric zero-tick perspective on the directionality of light.
I just have one solution to this(a thought): c = displacement / time. You said we can not synchronize the time for far distant location(which you think distance is constant) referring wrt measuring speed of light. Now at that light speed what if i say we can never measure distance precisely at different time instants ( mind it we are working at light speed ). Time dilation says space ( length) changes time which is significant at light speed, so at that speed how we define distance is important. Similarly if time changes ( which is very fast at c) length should also change significantly, if time dilation follows reciprocity. So what if c=kd/kt=d/t,where k is just constant which just shows if time reduces, length reduces or vice versa. So at the end c remains fixed. Basically if direction changes your speed, so should the distance!!! Now the only question is how correct is c value, well know that depends on how correctly we have captured the time to distance proportional constant. So how correctly we have captured c will tell how correct we are at calculations which involves "c". Either if it's incorrect only the definition changes not the meaning. It's just like saying instead of 1-meter now my new reference is 0.80-meter. I might have 20% error in value but it's just my new reference now. As long as my all observations / calculations are at far lower values wrt to my new reference the delta error i generate is very small in new observation. The same is the case with c, more close my observation reach to c, more error i would encounter.
New thought: in your earth/mars spatial diagram you said c= infinity = distance/Zero ( as per your case), it also hold true for my explanation above which says c=0/0, distance is zero here which still makes c infinite!!!
"Ok, let's synchronize our watches!" - 2070 Nobel Prize winner
SEND THE REINFORCEMENTS THIS GUY IS LOW ON LIKES
Also there were two nobel prizes awarded for breaking the assumption scientists had about the charge parity and time simmetries so this wouldn't be a first but would probably be one of the best
You could just move both clocks at the same speed for half a mile then measure the exact time the light was turned on, and then the exact time it reached the destination and do the math
i meant move them In opposite directions
I thought this said witches... Happy Halloween!
“So someone has measured the speed of light......or HAVE they?”
*Vsauce intro starts
I heard that music start...
Im your 69th like
Agreed - thought this felt like a Vsauce video the whole time.
It is easy - set 2 sensors that send pulse when light passes thru them to perpendicular single clock, so there is no need for synchronization. Path from each sensor to clock and path direction should be same, so delay betwedn pulses will express exzctly time that took light to travel from first sensor to second - that's it, you will measure one direction light speed. Can make a such measurment in several dirdctions just to verify that result is same
you don't need to know when exactly they send it - you just need to measure delay between 2 pulses. Since each sensor sends its pulse in same direction for same distance , so time of travel of each pulse to clock will be same and delay measured by clock will express exactly time that took light to travel from sensor to sensor
I think we should take a straight (line) path and take 3 equidistant points A,B&C on the path and synchronize both the clocks at point B and then move both of the clocks to each's opposite directions at same relatively speed at points A & C
He explains why that won't work at 10:45
@@hydrogencyanide4999 At that time I had only watched the first quarter of the video
This is similar to the problem I have regarding the speed of gravity effect: if gravity acts instantaneously there’s no issue but if it acts at c - or any speed - the effect on an orbit of another body surely must be at some angle other than the instantaneous direction between the bodies?
Anyway, another thought provoking video from V - well done!
No gravity is constant in every where
"Stars look exactly as they are right this instant." Gave goosebumps.
krplus.net/bidio/nrZ_dYqaZ3ichWU
We saw past of star coz it take millions of year to reach star's light to earth
@@ABHEEeeee Watch the video
@@ABHEEeeee how do you know? Are you saying you can prove the return trip isn't instantaneous?
Have a rocket exactly half way between earth and Mars. The ship will send one message to earth and the other the Mars and then instantly back to earth. You would expect that the message that was sent to Mars first would take exactly three times as long to reach earth as the one sent straight there. If this does happen then the speed of light is the same in both directions.#big🧠
4:07 *Vsauce music starts* and im anticipating a round head will pop from the bottom of the screen.
Hey, just to let you know someone else in this comment section named Nihab Khan copied your exact comment word for word soon after you posted. Control + F to find the bastard. Go give him hell lmao.
I’m glad someone else thought this😂
IKR
Or is it,,😂😂
yeah my brain stopped working.
I long for the days of before I saw this video
So in basic school we've learned, that light "moves" like waves, but reatcs like photons, as in atoms.
Why don't we send a laser beam through a wheel which has many flaps and it spins fast like Fizeau's, but the light would be blocked when the top flap is on top dead center, and it would let light through while the next flap is coming up to top dead center (when the flaps are making a V shape instead of a horizontal line). Then, you put a phometer on the other side, and u check all 6 directions, and compare them.
If the light doesn't go with the same speed in all directions, then when it's going "against" the flow, the photometer would measure less light.
if the light is traveling slower then so is the time so the rate of the spin of the photometer would still line up to give the same amount of light through in all directions. Unfortunately the speed of light affects the speed of time so no setup like this could possibly solve the problem
This video was so good! I understand so much More about the speed of light. But I propose a way to measure the one_way speed. So you have 2 clocks, one at the start and one at the 1km mark, and you use a 1km pole and you stand in the middle of thật pole and then you push ít. Thật way your clocks will bé synchronize and then you cần measure the one way.
Us to aliens: "We measure a meter as the distance a light takes to travel in 1/299 792 458 seconds"
Aliens: "Which way monkeys?"
Lol
But the aliens can't know what a monkey is and they probably won't be able to speak English (I know it is just a joke)
@@shivannapv4262 nasa would like to hire you
@@omniyambot9876 why (I still know its a joke)
@@shivannapv4262 because biden is gonna be the new president😄😄😄😄
The moment when Destin realized "Oh snap, Derek is going relativity• is Golden!
Where does the quotation end?
I am experiencing 299,792,458 meters of life every second, and no one else gets to experience it from my perspective. I dunno about you, but I'd call that relatively beautiful:)
Consider a long length of evacuated glass tubing. At one end is a light source. At the other a mirror. Along the length of the tube are light detectors capable of detecting the intensity of the beam as it travels down the tube. Light pulse 1 is sent with the expectation that it will return back at the source in time T. If a second pulse is fired with a delay of T/4, if the speed of the first pulse is precisely the same in both directions, the expectation would be that they would cross paths 1/2 way down the length of the tube. The event being picked up by the light detectors; registering a sudden peaking of the intensity at that position. If there were any asymmetry in the speed of the light in pulse 1, the detectors should register a shifted cross over point ?
To measure the speed of light in the way you present it. Is take the distance of the timer 1 and timer 1 and put that distance between the light and timer 1 then make the wires for timer 1 and 2 is to do the length of the lights wire plus the length/ distance of timer 1 and 2
This video really changed the way I look at “simultaneously”
haven’t seen the video yet, but based off psychedelic experience i know exactly what you mean
@@vast5853 could you explain ?
Simultaneity*
Now when i do simultaneous equations, i do the calculation for one, and i instantly get the other one.
This is an awesome, very thought inducing video. A follow up video on what could be causing this potential asymmetry would be fascinating as well. However the centered clock synchronizer at 9:50 doesn't exactly equate to the GPS example he introduces next because in the first example, the A/B clocks agree (correctly) that they are stationary relative to the other. In GPS, the clocks are in relative motion. This creates a larger problem when trying to keep the satellite clocks running at the same "rate" as the ground clocks. They in fact have to run at the same ongoing rate for the system to function accurately but this (like it or not) is in contradiction with special relativity. I cover this in my twin paradox video.
Thank you for putting a camera on Destin when you had this conversation with him. I wanted to see his pondering/puzzled face so much! 😁😁😁
We* wanted
in one of your examples light can travel at c/2 from Earth to Mars and infinite from M to E...if this were the case, shouldnt the reverse speed also be applicable from M to E and vice versa? Love this channel and your work...also huge fan of @smartereverday
God I love your channel so much. Thank you for all the effort you make at putting them out, I've spent countless hours enjoying your content and the quality of it, you really seem to enjoy what you're doing and it feels so nice to watch topics I'm interested in presented by someone who really seems to share that interest. I really appreciate you.
"so someone has measured the speed of light? or have they " sounds like vsauce
Veritasium, Derek here...
*Vsauce music starts playing..*
**Camera zooms to michael**
Rip vsauce
@@debo325 is michael dead????
This was the most elaborate, philosophical, mind-boggling, brain bending, insightful, confusing way of saying: We Can’t.
What about starting with two syncronized clocks next to each other, then move them a set distance away from eachother, moving them precisely at the same speed. Then, have a light sensor precisely in the middle of the two clocks. Each clock fires a laser at a set time. If the clocks remained in sync whilst being moved, both lasers would hit the light sensor at precisely the same time. Add additional light sensors in between the clocks and the central light sensor to ensure that these also detected the light at the same time as one another to ensure the "instaneous" movement of light in one direction did not occur.
Conceptual Experiment Using Gravitational Waves or Cosmic Events
Objective: To measure the one-way speed of light using the timing of gravitational waves or cosmic events, which can be detected simultaneously across different points in space without relying on electromagnetic signals.
Materials:
• Two highly sensitive and synchronized detectors placed at points A and B, a known distance apart. These detectors could be designed to capture gravitational waves or observe specific cosmic events (like neutrino detections from a supernova).
• High-precision clocks at both locations, synchronized using the detection of the gravitational wave or cosmic event as a common reference point.
Procedure:
1. Event Detection: Wait for a significant gravitational wave event or a cosmic event (e.g., a supernova) that emits both gravitational waves and light. The event itself serves as a synchronization signal because gravitational waves travel at the speed of light, and their detection should be almost simultaneous across short distances on Earth, allowing for precise clock synchronization.
2. Synchronization: Use the exact moment the gravitational wave or cosmic event is detected by both detectors to synchronize the clocks at points A and B. This method assumes that gravitational waves travel at the speed of light, an assertion supported by General Relativity and confirmed by observations.
3. Light Speed Measurement: After synchronization, use a controlled light source at point A to send a light signal to point B. Measure the time it takes for the light to travel from A to B using the synchronized clocks.
4. Calculation: Calculate the one-way speed of light by dividing the known distance between points A and B by the time difference recorded by the clocks.
You still synchronize the clocks using the speed of light, which was responded to in the video. You can't do that.
Everyone gangsta until someone calculated the speed of shadow
Heyyyy vSauce, Michael here
Its istantaneous because it doesnt carry information (so it could go above c)
Hey Michael, vsauce here
@@giacomogroppi5768 No, the shadow is not instantaneous. The light that passed the object before the rest is blocked will still be visible until the last unblocked light reaches the surface of the observation.
@@ozciva but it’s still the same thing... how do you measure the speed of the shadow accurately?
Mark the Astronaut sends a message.
Me, on mission control: "Oh hi, Mark..."
You're tearing me apart, expansion of the universe!
"Oh, hi Mark. What did you have for dinner last night? Potatoes!"
"So, how's your research life?"
ha ha ha what a story Mark.
I thought he was going to say, it comes back slower, and then he said returns instantaneous.
Very thought provoking
Take three clocks,
For example Clock-a, clock-b and clock-c.
Take points, point-1 and point-2.
(The distance 1km)
Clock-a and clock-c will stay at the point-1 and clock-b will stay at the point-2.
Start clock-b and clock-c ( at the same time) before clock-a.
Shot the light and start the clock-a.
When the light will hit the clock-b the clock-b will stop.
And now, do some calculation and you will get one-way speed of light.
"so someone has measured the speed of light... or have they"
Vsauce right there caught in 4k stealing Derek's skin
*Vsauce was an imposter*
0 imposter remains
*vsauce music starts*
Old Derek: makes people on the street feel stupid
Present Derek: make rocket scientists feel stupid
True
only by asking a dumb philosophical question that reality refutes on every level... This is why philosophy is garbage. Possabilities dont matter, reasonable probabilities do.
@@terryfuldsgaming7995 bro my mans said Einstein himself did the two way measurements and he has many points in this video based on relativity not philosophy
Past derek and present derek is meaningless according to this video
@@terryfuldsgaming7995 Reasonable? Take a good, hard look at our current understanding of physics and come back when you're ready to call it reasonable. The very reason philosophy _is_ interesting is precisely because there are possibilities about reality that sound completely bonkers to us, and yet there's strictly nothing explicitely fordidding those possibilities... so what rational reason could you possibly have to dismiss them entirely? Symmetry? That sounds like a good argument, but there are plenty of situations in the real world where symmetries are broken. You could say that, on a purely a priori basis, more symmetrical options are more likely, but because it's a continuum between being completely symmetrical and being completely asymmetrical, without changing anything about causality, physics or your everyday experience (since if there was a difference you could measure it), the probability that the situation is _perfectly_ symmetrical would be zero. Even as a thought experiment, this entire thing is fascinating because it completely breaks our intuitions.
I think the problem with measuring the 1 way speed of light is the fact that we are thinking about time and space being mutually exclusive when in fact they are the same exact thing
Would be curious if you could theoretically measure the speed of light using 3 things, since we know photons are the basis of light, the directional speed of the photon, the frequency of the photons energy and the effects of gravitational interactions showing light moves at different speeds based on how gravity warps not only matter but energy if we consider gravity has correlation to the magnetic fields of different objects, and if we consider gravity can warp time and space then we should assume that light is not constant it should be correlated to the specific circumstance in which that single beam of light is traveling
So someone has measured the speed of light.
Or have they?
**Vsauce Music intensifies*
What is light? This feather is light.
Feathers are used by birds to fly.
*Flips you the bird* But what about this bird?
Why is this called "the bird"?
@@MatthewHughes811 accurate
Hey guys Micheal here
Thief
@@MatthewHughes811 lol this is gold hahaha
I love when Destin is presented with something he genuinely didn't know/understand before. His face lights up with extreme excitement and intrigue.
That moment led me to evaluate my whole existence on whether I could share something interesting enough to impress Destin that much.
What about a laser a mirror and a receptor the mirror reflects and as soon as it reflects its caught by the receptor? But also how would you activate both the timer and the receptor at the same time? Via the phone as soon as a noise is made person 1 or p1 will wait 1 second then will turn on the timer and p2 will also turn on the receptor so it wil befome vise versa
Use a clock and the same setup, but with another clock at the end point that syncs with a pulse from the first clock. Put the clock at the point of origin and send the signal and light to the end point and they will get there at the same time. Then move the clock some ways between them. Again run the experiment using a third clock, one at each end and one at some point in the middle and if there is a difference in the speed of light in one direction versus another the delay should at this point become noticable. By moving the one clock up and down the experimental line and syncing it to the other clock via light pulse the difference should become measurable.
"someone has measured the speed of light....
...or have they?"
Don't you VSauce us like that
Hahaha exactly! I expected Michael to pop up out of nowhere
@Florentin sound happens because of pressure waves, which happen because of the fluid (air?) Moving, which is made of various particles, that interact with each other by means of electrical forces. And those are linked to electrical fields, which propagate.. at the speed of light... 🙈 Therefore I guess it's checkmate again
Brow if they send massage to Mars at 12:00 and send another massage to Mars after 20 earth minutes. So the man in the Mars kan take the first massage at 12:10 and he can take time and he can se how long it will take to get the other massage so he can find the light speed.
It is to easy.
Believe me
@@tthozan7632 Stop spamming this.
Veritasium basically is VSauce nowadays
A micro millisecond: The time between when you shut, and lock your car door, and realize you left your keys inside.
What if the realization happens before the door shuts, but too late to stop it?
@@davidpowers746 We are talking reverse space time continuum. Way past my realm of comprehension.
@@treeguyable The physics isn't ripe yet.
Or perhaps 15 years because she told you she was 18.
The onosecond
Take two clocks on the midpoint of a theoretical line, move clock A at a defined speed for a certain distance, shoot a beam of light to a sensor on clock B then record the time difference which will be the speed of light over that difference minus dilation. Then move clock B that same distance at the same speed away from clock A and do the same experiment to get the difference equal to the speed of light plus dilation. If the two dilations affecting the speed of light are the same then it goes the same speed in both directions, but if not then the speed changes with direction through spacetime. I'm thinking about testing this but want feedback before I do any Tom Foolery.
(1:02) How to measure speed:
Formula: Speed = distance ÷ time
I just laughed so hard on "Oh! You're going relativity! You're gonna do something weird, aren't you?" hahahah
They're both great channels.
"Oh you're going relativity aren't you, you're gonna do something weird"
Physicists: "He knows"
I think i cracked the code:
A lazers shoots a beam of light in 2 opposit dirrections, on each side you have a thing that starts moving towards the lazer at a set speed for example(100m/h). if the speed of light is different in both dirrections 1 of the 2 things will reccieve the light earlier wich makes it start earlier wich result in one of the 2 things ariving faster than the other.
It could be that im wrong if thats the case please tell me what i did wrong.
@@korneelverbeke9746 You would probably run into the same issue of different amounts of time dilation in each direction as he mentioned in the video, causing the two clocks to become desynchronized.
@@Hahalol663 I think @Korneel is right because he is shooting two "beams". Time dilation can not affect light itself. Light does not have mass.
01:47 In line with the example alternatives provided in this video... What if you start with two detectors in the middle with synchronized atomic clocks (or whatever most accurate clock to-date), then separate them by moving one to the left 1/4 km and the other to the right 1/4 km (or any distance but must be the same for both), at the same time and speed, then compare the detectors timestamps? I feel like that was left out on purpose cause you guys are smart and that should've been one of the options IMHO...
Wouldn't that result in a reliable timestamp on both sides since both detectors wouldv'e been moved equally and therefore any time dilation would be equal?
I would even add a light source on both sides that trigger at the same time according to the clocks and end with two results that should be identical?
Lastly, to validate, I would repeat the test many times, then bring both detectors back to the center, again moving them at the same speed to confirm they are still synchronized.
Any flaw with my logic here?
Also, keep in mind that the two-way mirror experiment uses LIGHT REFLECTION, NOT SIGNAL TRANSMISSION... The light doesn't disappear and reappear, it reflects off the mirror and actually travels back, you can easily test this by using a flashlight and a mirror with smoke in between so its easier to observe like car headlights through fog.
place the middle switch in the part of a right triangle, that is where the side fo the triangle, and the base of it meet. Then remove the hypotenuse, and send a signal out from the middle switch/clock. The signal should then be immediately sent back when a device at the end of the wire detects it. When the signal leaves the middle clock, it should start a timer. And when a signal is detected by the clock, it should switch off the timer. So if in one direction c is slower than the other, the clock should stop, and then start again a short amount of time after. The difference cannot be nulified, because the signals do not travel in opposite directions. It would be almost impossible to get two pieces of wire that are the excact same length, but in theory it would work. BTW, this will not tell us what the speed of light is. It will only verify/refute the theory that c is different in different directions. If it is different (meaning the clock both stops and starts), then we can't measure it yet. If it isn't, then we have the speed of light already. Just measure the two way speed of light the way you did with the mirror, and divide it by two.
17:01
"And we'll wonder, why we didn't c it before?"
Amazing!
g, I'm not sure...
Even if we figured out the exact speed of light and KNEW what it was, it doesn’t change the fact that I have to go to work tomorrow
and that's facts
the speed of work is always c
@@rsolsjo unless it's Monday. On Monday it is C/2
@@michaeldivens7570 bruh in a maths lecture its c/5
When's the lay off?
Haven't finished the video yet, but could you set up the same mirror set up, but also set up a reciever at the mirror so you can compare the time it was recieved at the mirror and the time the reflection was recieved at the source?
Edit: I now realize the two clocks wouldn't be synchronized and we're back where we stared. If the speed of electrons through wire is a constant then we could use a single computer between both end points that was responsible for both timers but I don't think that electricty moves at a constant speed.
Knowing what we believe to be a symmetrical c, could we use a mirror to measure the two way speed of light, but add a second mirror in another location to seeming triangulate and measure the difference in speed?
In the case of interplanetary time synchronization, shouldn't you be able to measure variance by having a third planet provide time from another location? I feel like this would address the question of relative speed by direction. If c is symmetrical, light bounced through points A, B, C, A through a trapezoid should be equal to the time it takes light to pass through A, D, C, A.