I find myself wondering where this is supposed to lead. Are you trying to say that the universe is 6000 years old (the topic of the thread) -- by way of trying to prove that we cannot measure the speed of light?

Einstein may well be right about us not being able to measure things with infinite precision. I don't see how this links up with your point though, unless you are hoping that this means we are way off base in our attempts to measure the speed of light, which doesn't appear to be the case as people have been doing this in a creative variety of ways for decades and their results are in agreement. I've read claims myself that the speed of light may not be a constant, though no scientist has actually proposed that this would mean the universe is only a few thousand years old.

Here is an interesting article I found on the subject. It explains why it can be meaningless to attempt to measure things that possibly fluctuate when the measures that are used could possibly fluctuate as well. It's better to look at a ratio and investigate whether the ratio has changed over time. In this article, the ratio used is represented as alpha, which is a measure of the relationship between electromagnetic effects and quantum effects and thus has no units (the speed at which an electron in a hydrogen atom orbits the proton, divided by the speed of light). It then goes on to look at evidence which could prove or disprove that this ratio has changed since the beginning of the universe. The conclusion is that is may have changed a little in the very beginning.

Thank you CTD for inspiring me to learn more about this. I can't see how it would support a 6000 year old universe, but it's truly fascinating.