Accurate measurement of the speed of light

[TonyYuan]

[Mentors' note: Split from https://www.physicsforums.com/threa...w-that-the-speed-of-light-is-constant.986641/]

I know the amazing thought experiment by Albert Einstein with the two light clocks.
(The observer at the train station has a light clock and the person in the train.)

It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes.

But this experiment requires the knowledge that the speed of light is constant.
(So I personally would never have come to this thought experiment.)

I wonder if there is another thought experiment that shows that the speed of light is constant?

If I could provide you with a device to measure the speed of light, and the accuracy can reach 0.01 m/s, what would you do?

 

[Ibix]

If I could provide you with a device to measure the speed of light, and the accuracy can reach 0.01 m/s, what would you do?

I would point out that you need to update your device to modern standards. The speed of light is now a defined constant, so measuring it is either using an older standard or contains some circular logic.

 

[Orodruin]

I would point out that you need to update your device to modern standards. The speed of light is now a defined constant, so measuring it is either using an older standard or contains some circular logic.

As an undergrad I had a lab measuring the time delay of the signals between the photons from e+e- annihilation hitting two opposite detectors before and after moving one of the detectors. The conversation with the professor went something like this:
Professor: - So, what have you measured now?
Me: - Nothing, I have calibrated your ruler.

 

[TonyYuan]

I would point out that you need to update your device to modern standards. The speed of light is now a defined constant, so measuring it is either using an older standard or contains some circular logic.

My device is definitely not only used to output 300000000.0 m / s, it is the most primitive method applied to modern program technology to achieve the measurement of the speed of light.

 

[Nugatory]

My device is definitely not only used to output 300000000.0 m / s, it is the most primitive method applied to modern program technology to achieve the measurement of the speed of light.

The meter is defined to be the distance that light in vacuum travels in 1/299792458 seconds, so any device that purports to be measuring the speed of light in vacuum is in fact just calibrating itself.

 

[PeterDonis]

If I could provide you with a device to measure the speed of light, and the accuracy can reach 0.01 m/s, what would you do?

Ask you what the point of your question is.

 

[Ibix]

My device is definitely not only used to output 300000000.0 m / s, it is the most primitive method applied to modern program technology to achieve the measurement of the speed of light.

As pointed out already, this is not correct.

The simplest way to measure the speed of light in vacuum is to time how long it takes to travel a known distance. The problem is that distances are measured in meters, or can be converted to meters, and one meter is defined as the distance light travels in 1/299792458 seconds. So your distance measure depends on the defined speed of light, and will return exactly the defined value to your available precision.

Older systems, including earlier SI schemes, define distance in terms of multiples of the length of some object. You can measure the speed of light in such a system, but it is not the modern approach. Such schemes have been supplanted by the modern SI approach because it can be made more precise.

 

[TonyYuan]

Ask you what the point of your question is.

With a good light speed meter, you can be in the United States, in France, in the Antarctic, at rest, in motion. . . Both can be used to measure the speed of light, and even use it to detect gravitational waves, just like LIGO does.

 

[TonyYuan]

As pointed out already, this is not correct.

The simplest way to measure the speed of light in vacuum is to time how long it takes to travel a known distance. The problem is that distances are measured in meters, or can be converted to meters, and one meter is defined as the distance light travels in 1/299792458 seconds. So your distance measure depends on the defined speed of light, and will return exactly the defined value to your available precision.

Older systems, including earlier SI schemes, define distance in terms of multiples of the length of some object. You can measure the speed of light in such a system, but it is not the modern approach. Such schemes have been supplanted by the modern SI approach because it can be made more precise.

The measurement of the speed of light is a function that everyone can see. But the impact of changes in external conditions on the speed of light is where we focus. Many people tell us that the speed of light is constant. Why not measure it yourself, let the light run with you, and drive with you. Let us see if the speed of light will change.
Some people may say that you already know the speed of light, why do you need to do such a boring thing? Do you know 1 + 1 = 2? Many mathematicians are proving it.

 

[PeterDonis]

With a good light speed meter

As has already been pointed out, with the current SI definition of the meter, there is no such thing; the speed of light is a defined constant number.

even use it to detect gravitational waves, just like LIGO does.

I have no idea where you are getting this from, but you need to review the PF rules on personal speculations. They are not allowed here.

the impact of changes in external conditions on the speed of light is where we focus. Many people tell us that the speed of light is constant. Why not measure it yourself, let the light run with you, and drive with you. Let us see if the speed of light will change.

Even under a system of units in which the speed of light is not a defined constant (like SI before the latest changes), asking whether the speed of light can change is a meaningless question because the speed of light is not a dimensionless physical quantity. The correct question to ask is whether dimensionless physical quantities--the relevant one for light is the fine structure constant--can change. All the evidence we have to date shows that it hasn't. You will find plenty of previous threads on PF discussing this.

 

[PeterDonis]

The thread topic has been sufficiently addressed. Thread closed.