
Ask HN: Do black holes violate the equivalence principle of general relativity? - ijidak
The equivalence principle in General Relativity says there is no way to devise an experiment to determine if I am in a craft accelerating due to thrust from &quot;engines&quot; of some g or in that same craft on the ground being accelerated by the same g due to gravity. (Correct me if I&#x27;m wrong.)<p>But wouldn&#x27;t this break down inside or at the event horizon of a black hole?<p>Imagine I fire a beam of light directly at a black hole. It would never be seen to come out on the other side, because it would become trapped in that black hole.<p>But if I fired that same beam of light normal to the path of my craft, accelerating at the same g as that black hole, wouldn&#x27;t the beam of light be able to pass right through my craft?<p>I would then know that I&#x27;m not seeing gravity, but I must be under some non-gravity (non-spacetime-curvature) related acceleration.<p>So doesn&#x27;t that break the equivalence principle?
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db48x
I'm no expert, but I don't think you've got two equivalent experiments there.

Imagine two spacecraft, one that is accelerating in intergalactic space, and
another that is spiralling into a black hole, and is in fact already inside
the event horizon. In both cases, a beam of light from outside the craft
passes into one window and out another on the opposite side. The passengers of
both craft can measure the path of the beam, and both discover that the beam
is slightly bent. The amount of bending is the same in both cases, even though
one is in a gravitational field and the other is accelerating under its own
power. The degree of bending is proportional to the strength of that field and
to the amount of acceleration, and neither passenger can tell from this
experiment which circumstance they are in.

There are, of course, some differences. Inside the black hole, all paths end
at the singularity. This means that the light beam is spiralling in towards
the singularity just like the spacecraft, but at a different angle. Pretty
neat, but the passenger on the ship can't tell that this is happening, because
they can't catch up with the beam.

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T-A
[https://en.wikipedia.org/wiki/Equivalence_principle#The_Eins...](https://en.wikipedia.org/wiki/Equivalence_principle#The_Einstein_equivalence_principle)

Note in particular the clarification of what "local" means in this context.

