# Tag Info

### How many significant figures does the speed of light have?

The meter and the second are defined to be the units of time such that $c = 299792458$ m/s exactly. This number can be thought of as having an infinite number of significant figures, since it is an ...
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Accepted

### General Relativity an inevitable consequence of the constancy of light-speed and the equivalence principle: Is this really true?

No, the equivalence principle and special relativity are not enough to uniquely determine a theory of gravity. Misner Thorne and Wheeler have a whole chapter (Chapter 39) on alternative theories of ...
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The units of space and time are tightly linked by the speed of light. In the framework of general relativity it makes a lot of sense to associate the differences in time $\Delta t$ with the quantity $... 3 votes ### Two Photons are Traveling$A$doesn't have a perspective, there are no coordinate frames moving at$c$. Nevertheless, you can naively use the velocity addition fromual:$$v_{21} = \frac{v_1+v_2}{1+v_1v_2/c^2}=(c-c)/(1+(c)(-c)/... • 36k 3 votes ### Why is Special Relativity preferred over Lorentz Ether Theory? About the question of choosing between special relativity and Lorentz Ether theory. Historically there have been cases with a parallel to that question. I will first describe one such parallel case, ... • 21.8k 3 votes ### How many significant figures does the speed of light have? Physical quantities don't have significant figures. Measurements have significant figures. If you perform a measurement of the speed of light, and you're confident that the first five digits of your ... • 8,863 3 votes Accepted ### Can we measure$c=1$? With the current definitions of the SI units, any proposal to "measure$c$" is tautological. You build (or buy) an atomic clock to establish a time/frequency reference, then use light ... • 91.7k 3 votes ### Can we measure$c=1\$?

Imagine a place where gravity affects pretty much all aspects of life. People there think of vertical distance as a completely different physical quantity than horizontal distance. They can measure ...
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### Why is Special Relativity preferred over Lorentz Ether Theory?

The current dominant interpretation of special relativity isn't actually Einstein's original formulation, but rather Minkowski's geometrical formulation in terms of spacetime. This has a number of ...
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### Why would time distorsion be greater than space distorsion?

In the case of propagating light the distribution is about 50/50. Here I mean by 'distribution' how much time effect there is, and how much spatial effect Around 1907 Einstein explored (and published)...
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1 vote

### Did Einstein say the one-way speed-of-light is "not a fact of nature"?

I concur with the comment stating that this is a question for the History of Science and Mathematics stackexchange. In the specific case of Einstein: There is the Einstein Papers project. Not only is ...
• 21.8k
1 vote

### Would an anisotropic speed of light possibly allow light to escape a black hole?

The one way speed of light (or of anything) is really just a matter of convention, namely how the two clocks involved are synchronized. That's why it cannot be measured: you can choose coordinates to ...
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