Why we cant measure the speed of light

This is pretty cool. At the 3:30 mark she explains what Einstein said about light very well.

a reply to: FauxMulder

Nice i like it no this was created do to an argument i was having. I caught myself explaining why we know an aether does not exist. So i of course brought up the Michelson–Morley experiment. And he said well if we are wrong about the speed of light then it proves nothing. So i proceeded to tell him how we know the speed of light. And then he said it he said well what if light doesnt travel the same speed all the time. And my argument came to a screeching halt because we assume the speed is constqnt but we have never proved it to be correct. So needless to say i started looking through papers and realized this is unknowable.

a reply to: bluemooone2

Ive seen it all now a physisist that makes music videos.

In this sense, we could be seeing starlight instantaneously, rather than the theorized time delay due to light having to travel. One of the possibilities given the conundrum in the OP is that light travels instantaneously in one direction, and c/2 in the other.

My only guess how this could be solved is at the hadron collider. If they are able to make black holes, even though very miniscule, they may be able to bend spacetime enough to have the light emitter also be the receiver, and therefore it would be just one clock that is measuring the one way directionality of the light.

You can calculate by frames per picosecond.

Please don't let this sink into "is Ether real or not" (regarding some none-scalar values of the speed of light, of running "against" the ether or "with" the ether).

This was closed in the beginning of the 19th century.

Light speed can be measured. Light and its very well known speed is (a) the SI-basis of the meter and (b) is used in highest resolution in many, many applications like lidar etc.


This thread should be closed, as there cannot come any new knowledge from this.

originally posted by: ManFromEurope

Light speed can be measured. Light and its very well known speed is (a) the SI-basis of the meter and (b) is used in highest resolution in many, many applications like lidar etc.


This thread should be closed, as there cannot come any new knowledge from this.

The average speed of light for simplicity's sake is 2.99x10^8 m/s. But the speed in which it travels in either direction is unknowable at the moment. For example, It could be instantaneous in one direction, and c/2 in the other direction. Einstein himself made the actual speed of light equation involve the average of the speed in one direction and the other direction divided by 2. I don't remember which paper he used this formula though.

what does direction of its travel have anything to do with its speed?

One little detail you haven't covered yet is the Doppler shift that the light would undergo as Earth and Mars are constantly changing position to each other. To a viewer on Mars wouldn't the light actually move faster when Earth is closing in on Mars in their orbits and slow a touch as Earth pulls ahead? And the color of the light specifically would change a bit as the planets change positions.

Just saying the speed of light could be a variable when dealing with movable objects.

a reply to: FauxMulder

I watched the video a few days ago, it's pretty obvious light has no preferred direction even if we cannot verify that. Why would it have a preferred direction, is there some sort of ether light moves through? Because I was pretty sure the Michelson–Morley experiment debunked any notion that such an ether exists, and it pretty much proves light moves the same speed in all directions. I don't know how Veritasium could overlook this experiment, surely has has heard of it, so maybe I'm missing something.

The Michelson–Morley experiment was an attempt to detect the existence of the luminiferous aether, a supposed medium permeating space that was thought to be the carrier of light waves. The experiment was performed between April and July 1887 by American physicists Albert A. Michelson and Edward W. Morley at what is now Case Western Reserve University in Cleveland, Ohio, and published in November of the same year.[1]

The experiment compared the speed of light in perpendicular directions in an attempt to detect the relative motion of matter through the stationary luminiferous aether ("aether wind"). The result was negative, in that Michelson and Morley found no significant difference between the speed of light in the direction of movement through the presumed aether, and the speed at right angles.

Michelson–Morley experiment

a reply to: ChaoticOrder

I don't remember anything about an aether. I think the point is no one has ever measured light in one direction. Its next to impossible. The only way I can think of, would be some sort of quantum entangled clocks. Its sort of just a paradox. Kind of like Zeno's paradox.

I'll rewatch the video later and see if he mentioned anything about what you said. Or if I too missed something.

originally posted by: ManFromEurope
Please don't let this sink into "is Ether real or not" (regarding some none-scalar values of the speed of light, of running "against" the ether or "with" the ether).

This was closed in the beginning of the 19th century.

Light speed can be measured. Light and its very well known speed is (a) the SI-basis of the meter and (b) is used in highest resolution in many, many applications like lidar etc.


This thread should be closed, as there cannot come any new knowledge from this.

You cannot measure light without assuming light moves the same speed everywhere. We have not devised an experiment yet that does not cause light to move in two directions. And we cannot assume it moves the same speed what we can do is know the total being C. As Of today there is not one singlw experiment that measures light in only 1 direction.

Im sorry if this is to difficult for you to wrap your head around but we have had to make asumptions when it comes to the speed of light. Even Einstien admitted that to do his equations he said light would have to move the same in all directions for the sake of his paper. But he could not prove it to be true.

a reply to: ChaoticOrder

you are i really want to avoid aether because i believe its not neccesary. However having said that are proof that it does not exist assumes light always travels at the same speed in a vacuume. If their is directionality and the total of 2 directions give us c then the whole experiment is a failure. It would also say no two incidents in the universe could ever be proved to occur at the same time. Takes Einstienes refference frames to all new levels.

For example as i said its impossible to measure the one-way speed of light. Because of the effects of relativity, clocks cannot be moved apart without changing the rate at which they run, and therefore you can't just shoot a beam of light from one clock to another and record the start and stop times. All any conceivable experiment can do is provide you with the round-trip speed of light between two points.

It is, however, possible (as in, 'the math can be made to work') to state that for any observer, light traveling toward the observer travels at greater than c, and light traveling away from the observer moves slower than c, such that the average speed over the whole round trip is exactly c. This exchanges time dialation for another set of weirdness. And it would not effect our calculations in any way. Now the argument could be made well if both are the same go with the simpler solution. But what if that is the reason we cant come up with the theory of everything?

Now im not trying to prove anything just pointing out that something i assumed to be true may not be because we cant prove it.

originally posted by: FauxMulder
a reply to: ChaoticOrder

I don't remember anything about an aether. I think the point is no one has ever measured light in one direction. Its next to impossible. The only way I can think of, would be some sort of quantum entangled clocks. Its sort of just a paradox. Kind of like Zeno's paradox.

I'll rewatch the video later and see if he mentioned anything about what you said. Or if I too missed something.

Well yes this could in theory work if quantum entanglement is indeed instantaneous. Then we can have two clocks set to the same time and just take a one way measurement. Problem is we dont know if quantum entanglement is.

I'm a thicko really, but I read a bit ago that there is a panel of world renowned scientist that meet to discuss this issue and accordingly they have altered the speed at which light travels a couple of times over the years. I don't know where I read it, but?

Imagine a gigantic dual strand fiber optic wire made into a loop. A pulse of light is shot through each strand at the same time in opposite directions. All along the loop are devices to measure the gap of time between the light pulses. The devices don't know when the pulses started, just the difference in time. The difference in time should be the same on each timer near the halfway point. If there was any acceleration/deceleration of the light the timers would be different at their opposite point on the loop.

You could perform this experiment with every loop shape imaginable, timers that could measure more than one loop series, loops that started in different places but ran through several common timing devices along the route, thousands of loops at once... I am not a physicist but this seems like something that should be measurable.

I thought the accepted measure for the speed of light is 186,000 m/s (186,000 miles a second)........?

I would also think that light moving on a horizontal plane with the Sun and the planets would move faster that light moving in a vertical plane when compared to the Sun and the planets.

originally posted by: dragonridr

originally posted by: Akragon
a reply to: dragonridr

Wouldn't a laser be able to measure light traveling in one direction?

No again you need 2 clocks you have to measure when it left and when it arrived at its destination. Relativity tells us the difficulty in syncing two clocks. This is why they used a mirror and only one clock to calculate c. If light is effected by directionality no matter how we try to figure out the speed of C will always use 2 directions. Lets say we use your laser and a camera. And we see how long it took light to cgo through a vacuume. The problem is we have to take in to account the time the light took to reach the camera. Meaning we are still measuring it in two directions.

You are ignoring the Michelson-Morley experiment and those that followed, namely the Trouton–Noble experiment (1903) and the experiments of Rayleigh and Brace (1902–1904). If you know the distance to a mirror, you only need one clock, located at the light emitter source. It can measure the time of transmission and the time of return. As an example, you don't need 2 clocks to measure the speed of a Nascar car at Daytona. If the 1 clock at the start/finish line shows 1 minute for 1 lap, then the speed was 150 mph. This is the basis for laser interferometry experiments.

What always fascinated me about light isn't just how fast it is, its that photons have mass yet still travel at the speed of light - something nothing with mass is supposed to do. Light also moves in wave form like ripples on water. Only fluid mass can do that.