Information can't travel faster than $c$

When we say that information cant travel faster than light, we mean that phase velocity can go beyond $c$ but the group velocity can't. Hence, we say that a plane wave carries no information. What I don't understand is what we mean by information here, is it supposed to be energy? Because plane waves do possess energy.

• – ACuriousMind Feb 12 '16 at 21:44
• I think the best way to think about this is that we'll never find any physically measureable quantity, measured at time $t$ and point $x$ whose value depends on properties of the system at a space-time point outside the reverse light cone of $(t, x)$. I realize that's annoyingly formal and convoluted language, but I'm trying to be precise. Does it make sense? – DanielSank Feb 12 '16 at 21:45
• Answered here: What do physicists mean by "information"? – Mark H Feb 13 '16 at 7:20