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Any object which reaches close to a black hole accelerates towards it just like an apple accelerates towards earth when released from a height.

My question is that what happens when light reaches close a black hole does it accelerates and if it accelerates then its speed would become greater than 299,792,458 m / s which is a speed limit of anything in the universe.

So should it accelerate or not.

The Mathemagician
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  • Your question contains an incorrect assumption in the first sentence. Free-fall is specifically not even called acceleration in General Relativity. – m4r35n357 Jun 29 '18 at 13:59
  • If you do mean motion on the horizon then this is not a dpulicate. Least time principle + $dt=0$ on the surface – user76568 Jun 29 '18 at 14:19
  • Light slows down near and stops at the event horizon. The energy of light becomes zero at the event horizon, so light ceases to exist. Nothing ever crosses the event horizon. It is not a spacelike surface that something could cross. The event horizon is lightlike and impossible to cross. Furthermore, there is nowhere to cross, because there is no such a place in our universe as "inside a black hole". There is no singularity "inside" (since there is no inside). All energy of the black hole is outside the event horizon. The energy of the infalling light is converted to the gravitational energy. – safesphere Jun 29 '18 at 15:02
  • The event horizon is not impossible to cross (particles can fall in). – m4r35n357 Jun 29 '18 at 15:06
  • @safesphere Please post that as an answer so people can up/down vote and/or accept. Also, do you mean your answer in the context of a reference frame outside the black hole? Presumably you're not saying that an object which moves towards a black hole's event horizon cannot cross from its own reference frame? – JBentley Jun 29 '18 at 15:21
  • @JBentley Questions marked as a duplicate do not accept answers. – safesphere Jun 29 '18 at 15:24
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    @safesphere you mean you will not observe particles ever reaching the horizon. However you can go about it yourself. The horizon is like a one way valve of world lines. Yet light should prefer to stay on the horizon instead of wasting time going through. Unless there is something to counter this. ? – user76568 Jun 29 '18 at 15:29
  • @JBentley The mass of the infalling observer becomes zero at the horizon, so he ceases to exist with nothing left to "cross". There also is nowhere to "cross", because the "inside" of the black hole is not a place in our universe. Crossing the event horizon is a super popular urban legend, but it's a myth, not a reality. – safesphere Jun 29 '18 at 15:39
  • @safesphere That is a very unorthodox view, not supported by mainstream science (assuming you are referring to what happens from the point of view of the in-falling object, and not merely from the point of view of an outside observer). See for example: "Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon ... The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time". – JBentley Jun 29 '18 at 15:43
  • @JBentley This quote is a silly interpretation that for starters violates the local energy conservation. – safesphere Jun 29 '18 at 15:49
  • @safesphere the horizon/s are two dimentional without time. Just a spherical shell. On the other side the energy switches roles with the momentum :). And it is not singular. It is just a coordinate singularity. The actual aleged singularity is at $r=0$. – user76568 Jun 29 '18 at 15:54
  • @safesphere conservawhat? :D – user76568 Jun 29 '18 at 15:58

2 Answers2

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No, light does not accelarate. Light redshifts (or blueshifts, depending on observers location) in gravitational fields.

Kosm
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First off, note that acceleration does not direction correlate with speed. It is related to velocity, so light can in fact be accelerated by any gravitational source (gravitational lensing).

However, these effects just change the velocity vector direction, they do not change the speed of the light and they cannot. Light must never move faster than the physical limit of $299,792,458 m/s$.

So what happens when light is released near a black hole and travels directly towards it with not change in direction? As the photon falls into the gravitational well, the frequency changes, thereby changing it's energy proportionally to it's change in gravitational energy. This is what we see as "red shift" and "blue shift" when photos move away from or towards a gravitational source.

fhorrobin
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