# We cannot measure the speed of light

## Main Question or Discussion Point

we cannot measure the speed of light in a vacuum only in space, and since we are told that space is expanding it could be that the speed light varies through time.

## Answers and Replies

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SteamKing
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Wait, what?

Nugatory
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since we are told that space is expanding it could be that the speed light varies through time.
We measure the speed of light locally, so that expanding space doesn't affect the speed of light although it may, while the light is in flight, move around the endpoints of the light's journey.

There's an analogy that you'll see sometimes: Imagine a snail crawling along an elastic band that we're stretching. There's no question about what the speed of the snail is relative to the rubber underneath it, and stretching the rubber doesn't change that speed. However, depending on where the snail started and how quickly we're stretching the rubber, the time it takes the snail to get from point A to point B may not have much to do with the distance between A and B at any particular moment.

Nugatory
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we cannot measure the speed of light in a vacuum only in space,

Mmmmm.... How is a vacuum (aka "empty space") not space? In any case, we measure the speed of light by look at the relationship between the emission and detection of light signals (and there are some important subtleties about the difference between one-way and round-trip measurements).

I am saying that since we cannot measure the speed of light without space we do not know if the speed is dependent on it. I believe that space something, not a vacuum in the true sense of the word, and if it is expanding it is either becoming less "dense" or more space is coming from somewhere to maintain the "density".

I am saying that since we cannot measure the speed of light without space we do not know if the speed is dependent on it.
Can you measure any speed at all without space? What does "meters/second" mean?

davenn
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I am saying that since we cannot measure the speed of light without space we do not know if the speed is dependent on it. I believe that space something, not a vacuum in the true sense of the word, and if it is expanding it is either becoming less "dense" or more space is coming from somewhere to maintain the "density".
I think you are referring to space as in outer space ?

this is not correct, as others have been trying to tell you
You are still holding onto a mistaken belief

just google "how to measure the speed of light"
there's hundreds of references for you to read.

Speed of light can easily be measured in a laboratory with the appropriate equip.
You don't need outer space. The ONLY "space" you need is a distance between the measuring equip.

Dave

Drakkith
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I am saying that since we cannot measure the speed of light without space we do not know if the speed is dependent on it.
Well, depending on what you mean by "speed", it is dependent on space. In cosmology there are multiple ways to define the speed of something since expansion is moving things away from us. The speed of light "through" space appears to be c at all times, where c is approximately 300,000 km/s. However, the expansion of space can make it appear that light is traveling slower than it really is over very very long distances or time. For example, one might expect light emitted 1 billion light years from us to take 1 billion years to reach us. However, the expansion of space actually causes it to take much longer. (I don't know exactly how much longer)

I believe that space something, not a vacuum in the true sense of the word, and if it is expanding it is either becoming less "dense" or more space is coming from somewhere to maintain the "density".
While you may believe whatever you want, it may help to realize that our best understanding of the expansion of space is that it is geometrical in nature and does not involve "new" space coming from somewhere or that space is becoming less dense. Note that this is built off of real, modern observations and math, and it fits all observations extremely accurately.

ZapperZ
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Closed, pending moderation.

Zz.