I have read that very high precision attempts to confirm the constant speed of light in all directions have been successful. For example in 2009, Stephan Schiller's lab was able to achieve a precision level that was one hundred millions times more precise than the original Michelson Morley experiment. Here is the link to the article: http://physicsworld.com/cws/article/news/2009/sep/14/michelson-morley-experiment-is-best-yet I am trying to understand the level of precision the article talks about. I am assuming that improvements have been made in the last six years, but I wanted to convert the numbers into velocity. For example, "Schiller's experiment is sensitive to eight of these parameters and the team was able to show that four are zero to about two parts in 10^17; one is zero to about one part in 10^16; and three are zero to about two parts in 10^13. According to Schiller, this represents a factor of more than 10 improvement over previous measurements of these parameters and a factor of about 100 million better than Michelson and Morley's original experiment" According to the article, such precision is important to test some quantum gravity theories and to detect dark energy. The parameters talked about in the above quote refer to nineteen measurable Lorentz symmetry parameters (not that I know what those are). So here are my questions: Is it a simple matter of dividing the speed of light by the factors such as 10^17 to get the precision in terms of velocity? Schiller's apparatus was suspended on air, so I assume that both light rays remained parallel to the Earth's surface during rotation. Have similar attempts at precision been made for light rays that rotate between horizontal and vertical? If so, what precision was attained in those experiments? I know that the orientation shouldn't matter. I have seen a Youtube video that purports to show a change in the speed of light for a vertical orientation using off the shelf equipment, but another Youtube video showed that the flimsiness of the equipment was to blame. So the lesson there is that only professionals should make the attempt. However, it just seems that to be thorough, both orientations should be tested.