# Speed of Light, when the medium is Cesium

A Professor of astro physics once told me that the speed of light is actually higher when light passes through cesium as its medium as opposed to vacuum or worse mediums.

I dont really find a consensus that this is true, apparently there where some experiments in around 2000, but it doesnt seem widely accepted - OR it doesnt matter. The implications of FTL even in such a restricted way, especially with regards to causality should be incredible. But apparently that is not the case.

How is light moving in a medium like cesium, at > c, not the same as without cesium and not a big deal?

• Welcome to Physics Stack Exchange! This related question may help you. – HDE 226868 Mar 11 '17 at 18:03
• In the view of General Relativity light travels on a geodesic; there should be no surprise if some geodesics are shorter than others. There can be more geodesics that connect 2 points. But the speed of light will still be c during the time of the travel, only that the road will be shorter. This if truly we talk about shorter geodesics and not about the effect that @HDE226868 linked you with. – Mihai B. Mar 11 '17 at 18:12
• Also, "In 1967, acting on Einstein's proof that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. Since then, caesium has been widely used in highly accurate atomic clocks." en.wikipedia.org/wiki/Caesium – Mihai B. Mar 11 '17 at 18:19
• Well I was under the impression that the length of geodesics are about the curvature of spacetime, meaning gravity. I dont see how moving in a different medium would change the length of the geodesic. I will read the linked discussion. – DRXO Mar 11 '17 at 18:25
• In Einstein's view the geodesic is not just the result gravity but of the non-symmetric field, the unified field of gravity and of electromagnetism. If you want I can quote you Einstein directly. – Mihai B. Mar 11 '17 at 18:29

When you look closely at the waves in a medium other than vacuum (which is "no medium"), a packet of finite duration can be thought of as having a mixture of frequencies present. If the medium is dispersive, not all those frequencies travel at the same speed. When you put them all back together, it looks as though there is a wave traveling faster than $c$. However, when you look at the envelope, it always travels at a speed $\le c$.