# can light go faster than light speed? [duplicate]

Foreword.

Since light can't escape from a black hole, we can assume that light is influenced by gravity.

So I can imagine that light that would otherwise escape from a black hole is in fact restrained/slowed from the gravity in the black hole.

A light ray/photon that is going in direction of a black hole can also be accelerated from black hole gravity? In this case is light going faster that light speed?

Argue each point if i'm wrong.

## marked as duplicate by Kyle Kanos, John Rennie, user108787, heather, Jon CusterNov 21 '16 at 19:14

• – Kyle Kanos Nov 21 '16 at 16:21
• I'm not sure the reason for the downvote, it's a perfectly reasonable question to ask (even if it's been asked before) – Kyle Kanos Nov 21 '16 at 16:23
• Hi Rudi, I added in the word otherwise to try and clarify your question and reduce down votes based on that misreading . If it changes the meaning of your question, please edit it out. – user108787 Nov 21 '16 at 16:56
• @JohnRennie in your answer i found this, that clarify me:" dr/dt=(1−(2GM/rc^2))c So in this case the speed of light is not cc. In fact it's less than cc for any value of rr less then infinity." so i can assume that light never goes at light speed apart in vacuum? So, for example here on earth light speed tends at maximum light speed. exatly? – RudiDudi Nov 21 '16 at 17:03
• @RudiDudi: If you measure the speed of light at your location then you'll always get the value $c$ regardless of where you are. If you measure the speed of light at a distance location you may get a value different from $c$, as I describe in the linked question. This speed is known as the coordinate speed to distinguish it from the local speed. Assuming you are on the Earth's surface then as measured by you the coordinate speed increases with altitude. So for example you would measure the coordinate speed of light at the altitude of a GPS satellite to be faster than $c$. – John Rennie Nov 21 '16 at 17:07