Solution to Veritasium Speed of Light test

This video offers a simple solution to the problems with determining if the speed of light is the same for all directions. If you haven't already, watch the Veritasium "Why No One Has Measured The Speed Of Light" video first. AFTER THIS VIDEO please watch my new Einstein Twin Paradox video.

[TheOneMaddin]

This does not work for the following reason: if the speed of light is different in different directions, then acceleration works differently in different directions. Your two rockets do accelerate differently, even though they think they accelerate equally. So they speed up to different terminal velocities. This comes out just right so that they experience the same time dilation. Their clock photos will match in either case, and you will have learned nothing.

[watchman]

Trust me if you could measure the one way speed of light you would win a noble prize

[alvaromartinortega]

As I posted in the Veritaserum video: the reason we know that speed of light is constant is not because we measured it, but because we can obtain a wave equation from Maxwell equations that descries electromagnetic waves (light) with a constant speed. This constant speed is a function of other constants that we can measure in the lab without messing around with lasers and mirrors.

While this kind of problems can be interesting puzzles to solve (and you came up with a good and original one), it not addresses the problem that they can be a disinformation tool, by omitting the facts that could truly disprove their claims. I would have liked to see a reference to all those facts that led to scientist one hundred years ago to the conclusion that the speed of light is indeed a constant in all directions, one-way, two-way and anyway trip.

[bradey]

Couldn’t you use the expansion of the universe to solve this problem. If the speed of light is constant in all directions, light coming from all directions from distant galaxies would be equally redshifted and the observable universe would be spherical shaped (if the universe is large enough). Whereas if the speed of light is different in different directions far away galaxies would be redshifted by different amounts depending on their orientation from earth and the observable universe would possibly sized so that we could see further in the universe in one direction.

[chriskennedy]

Ignacio Gamboa also had a pretty good idea that he left as a comment on Veritasium's video page. He suggested setting up a LASER with careful measurements of gravity and seeing how much light bending of the LASER would be observed, suggesting that the amount of bend would be related (inversely) to the speed of the beam. I think that would work too.

[kywill]

In your experiment, wouldn’t space dilation coincide with time dilation, resulting in the same measurement of time for each spaceship regardless of whether c is the same in both directions or not. For example, if light traveled at 2c in one direction, space would contract 2 times the amount as it would contract if light traveled at c in that direction. The time to travel a set distance would be the same for clocks on the spaceship regardless of the speed of light in that direction. Please provide additional insight if I am missing something.

[hellrazr]

My thought is this: have two clocks and one emitter and exactly in between them. You stand at that spot, and press the button to sync the clocks. Then, you click the emitters. As soon as the emitters hit the clocks, they stop. If the speed of light is different in one direction, it will be c/4, since it had to go in that direction twice.

[alexmipego]

What about ditching the sync clock requirement? Wouldn't that solve the major problems?

Each side has both a detector and emitter of photons. When a signal is detected, it emits a photon as fast a possible. The devices will then use a frequency counter to show us how many times it's activating per second — a frequency meter. This means we can measure it at close distances to know the lag of our devices to re-emit a photon, and with that baseline, we can calculate 1 way photon trips… and compare directions easily.

[suareduj]

Hi Chris, I've just watched the Veritasium's viedeo and YouTube suggested me yours.
To be honest my first thought after watching Derek's video was the same, so to move both clocks in opposite directions and check the time dilation on both. However I'm afraid it wouldn't work. Let me explain why.
The assumption that speed of light is not the same in every direction means that time dilation would also be different in these directions (this is the idea behind your solution). However we are not talking about the speed of light per se, but about the maximum possible speed of information propagation in this direction. So the time would dilate differently not because there is speed of light (c) in the formula of time dialtaion but due to the fact that the information propagation is also different (I guess that you could agree that "c" in the formula for dilation is actually the maximum speed of information propagation, which just happens to also be speed of light, which travels at the maximum "allowed" speed).
Having said that, how could you make sure that both rockets (with clocks) would travel with the same speed? So the clock running slower would be carried by the rocket that also travels slower (by the same portion). Therefore you would see the same time on both photos, which wouldn't mean that speed of light is the same in both directions.

Let me know how you see it.
Thanks!

[ozmutazbuckshank]

Thank you for taking the time to make this video, I'm not a physicist but the way you have presented the information was concise and understandable. Very interesting content

[andreii]

just a note actual "solution" starts at 11:00

but moving two rocket ships apart is no different than having two clocks move apart first, you're just adding an extra step of having it on a space ship - because of different time dilations, it'll take different amount of time to reach the same "finish line" so your "photo finish" will give you the same time on both clocks (assuming anisotropic speed of light).

[haroldnowak]

Reply to @tramvaj12: All the errors add up but the difference in speed measurement may be far less than the errors. I would suggest that every time you do the experiment, you will come up with a radically different answer.

[bumbixp]

How would you know what speed the rockets are travelling at? Since v=dx/dt and the time dilation is unknown and therefore dt is unknown. If you have a known distance and measure the time it took to travel that distance, you have an unknown time dilation (which you are trying to measure). The same thing applies even if you measure distance travelled in an infinitesimally small amount of time (dt).

Even if you had a perfect rocket which when lit always produces the same amount of force, would this not also introduce the same issue since F=ma + vdm/dt. Both the acceleration (a) and the velocity (v) would be affected by the time dilation?

[haroldnowak]

To determine the distances from the centre point C to A and from C to B you use what? You can only use light for the long distances you need. I would contend your method builds in the speed of light so is not varifying the speed of light.

[JoeMicroscope]

If the speed of light was different, then you would see totally different views of the cosmos in different directions. In one direction you would be looking back at the stars as they were at the start of the universe (13.8 billion years ago). The other direction you would be seeing the distant stars as they are now. Therefore the composition of elements in the stars would would be substantially different. As the composition of stars (by their light spectrum) is the same in both directions looking into space, the speed of light must be the same.

[albertorasa]

Chris, you suggest to solve the issue, raised in some comments, of exactly measuring the two ships' velocity by "... begin accelerating the ship from behind the center line so that when it reaches the center start line it is traveling the desired velocity confirmed by sensors at the rear of the ship". But his is not how velocity actually is defined, that is as ∆s/∆t where ∆t is the interval of time measured by two clocks which are still, at distance ∆s, in an inertial frame of reference. Certainly there is a relation between the two different types of velocities, but they are not the same: only the first is "the velocity". Can this difference be sent to 0 reducing the acceleration time? But the error in measuring this time amplifies the error in time dilation during the travel at constant speed.

[thatdumbdemonchild]

Would taking a photo not work, as, similaire to your eyes, you receive the information after the light has reflected then reached you camera. So taking a photo is less accurate, because it will be delayed by the speed of light, which is unknown.

[thurstenmoore]

I don't know... It seems that the proposal to take a picture of each clock at a defined time is assuming that there is a way to synchronize the exact time to take each of the pictures. But since the idea is that there may be time dilation in one direction more than in the other direction, then the ability to synchronize the time to take the picture is the same as trying to synchronize the clocks themselves. The difference in time dilation will impact the observed time to take the picture, and so it will result in the same time when both are taken.

[m_m_m_m_m_m_m_m_m_m_m]

It's the time dilatiom issue all again, no?

I mean, the perception of time might be the same for both ships. Aren't you assuming in that case that time dilation only afects clocks and that somehow the "phitographer" in the ship is "immune" to it (along with everything else that is not the clock)?

[AizenMD]

the question is, how do you know the rockets traveled the mentioned distance?
the rockets can't measure it because if time dealation is affected by the speed of light so is the distance dealation, if you put some sort of inertial observer in each way point they will observe the size of the rockets differently (assuming different speed of light in each direction).
Maybe I'm not taking into account the effects correctly but it does look like a problem, I think.