1

I was reading John Rennie's answer here, Can light travel faster?

I now know that the sentence ''the speed of light is always constant in vacuum'' is just an oversimplification, for example light travel at the speed greater than the speed of light (non-locally) sometime and also can travel at the speed less than the speed of light (non-locally).

Its just when in special relativity the metric tensor take its simple form light travel at the speed of light.(SR is local). [sir John Rennie's example: if you were hovering just outside the event horizon you'd observe the speed of light to be less than $c$ everywhere nearer the black hole than you, but faster than $c$ everywhere farther from the black hole than you.]

Doesn't it mean information travelling faster than light (at least non-locally)? Is it possible? And also how much local is local?

Community
  • 1
Paul
  • 3,475

3 Answers3

2

The local speed of light is always $c$. Local in this sense could mean that for each observer there exists a neigborhood of that observer such that, if we call $v_c$ the "observed speed of light",

$|v_c - c| < a$

where $a$ is arbitrarily small.

However $v_c$ is a slightly nebulous concept as beyond the inertial frames in special relativity, there isn't a general way to define the observed speed of light for a given observer. $v_c$ will depend entirely on how we choose to define $v_c$ for a given observer in a given spacetime and the above limitation on $v_c$ is merely expression of that, whatever the definition of $v_c$, we want it to match the local measurements of the observer.

John Davis
  • 1,133
1

What matters in principle is not if information can travel faster than c, rather if this leads to causality violations. As pointed out in this article, the Scharnhorst effect is the only known effect where light is expected to travel faster than c. However, in that case you cannot use this to create a causality paradox.

Count Iblis
  • 10,396
  • 1
  • 25
  • 49
-3

If light(to be taken here) travels @ a velocity >c, you won't be able to see or apprehend it. So, you need an observer having a relative velocity =

Abhijeet Bhardwaj
  • 1