0
$\begingroup$

If one were to drop, say, a pebble, from a sufficient distance, into a black hole. The pebble would continue to accelerate until it were just short of the speed of light. In the next moment it's speed should exceed the speed of light. Why not?

$\endgroup$
2
  • 1
    $\begingroup$ Also astronomy.stackexchange.com/questions/2441/… $\endgroup$
    – James K
    Feb 27 '19 at 6:33
  • 1
    $\begingroup$ Why do you specify a black hole? Do you think a pebble accelerates differently towards a black hole than it would towards an equivalent mass such as a massive star? In other words, is your question about black holes or about constant acceleration? If it's the latter, you might find this Wikipedia page interesting. Anyway, I recommend you edit your question if you're asking about something the two proposed duplicates don't answer. :-) $\endgroup$ Feb 27 '19 at 10:38
0
$\begingroup$

Gravity is no different from any other force. Doesn't matter it comes from a black hole, a star, a planet, or a lump of cheese floating in outer space. It's just a force, like any other.

Put a very powerful rocket engine on the pebble. In a short time it would accelerate the pebble near the speed of light. In the next moment it should exceed the speed of light. Why not?

Because that's not how it works. As you accelerate more and more, time dilation and space contraction also get stronger, so it gets harder and harder to get even closer to the speed of light, no matter what's pushing or pulling the pebble, be that a giant rocket engine or the enormous gravity of a large black hole. Same thing. The faster the pebble goes, the more warped space and time become, the harder it is to increase the speed.

It cannot even reach the speed of light because then time dilation, etc., would become infinite, effectively preventing it from reaching speed of light.

You cannot cheat relativity.


Note: This is not a rigorous explanation but it's close enough and should provide some intuitive understanding of what's going on.

$\endgroup$
1
  • $\begingroup$ "I am fine" with the PA splitting as mechanism "for a BH that radiates". I am less with PA popping around but this is another story. Back to the Q/A, shouldn't the BH grow if mass/energy enter in itself? How is then that they evaporate, whatever the time for evaporation is? Do you have a worded explanation? Thx. $\endgroup$
    – Alchimista
    Feb 27 '19 at 12:44

Not the answer you're looking for? Browse other questions tagged or ask your own question.