0

If you make an effort to escape a black hole, would your free fall velocity be slowing down? It is known that you can't escape once you passed event horizon. But is it possible to slow down? Or should I make a sample numbers. The escape velocity where you stand is $1,5 c$, your free fall speed at the moment is $0,75 c$. If the pilot start to begin escape attempt with $0,6 c$ in the normal world, how much is the free fall speed now? Or perhaps instead of slowing down, the free fall become faster now, because it turns out that there is only one direction in the black hole??

Kenneth Kho
  • 153
  • 1
  • 8
  • Could you give us some idea of your level of background in physics and math? Your question is ambiguous, because there is more than one way to define velocity here. If you define your velocity as the derivative dr/dt of the Schwarzschild radius with respect to the Schwarzschild time, then $v\rightarrow0$ as you approach the event horizon. –  Jun 14 '18 at 12:01
  • Possible duplicate of Why can't you escape a black hole? – sammy gerbil Jun 14 '18 at 12:03
  • 2
    How is the word effort compatible with the word free fall?? – Qmechanic Jun 14 '18 at 17:39

1 Answers1

2

There is no such thing as an "escape velocity" inside the black hole, because inside the coordinates $r$ and $t$ are interchanged. This means that the flow of time is represented by decreasing $r$. So whatever rocket thrust you have, you can't even hover at $r = const.$

You can however optimize your survival time to a certain degree:

https://arxiv.org/abs/0705.1029v1

No Way Back: Maximizing survival time below the Schwarzschild event horizon Authors: Geraint F. Lewis, Juliana Kwan Abstract: It has long been known that once you cross the event horizon of a black hole, your destiny lies at the central singularity, irrespective of what you do. Furthermore, your demise will occur in a finite amount of proper time. In this paper, the use of rockets in extending the amount of time before the collision with the central singularity is examined. In general, the use of such rockets can increase your remaining time, but only up to a maximum value; this is at odds with the ''more you struggle, the less time you have'' statement that is sometimes discussed in relation to black holes. The derived equations are simple to solve numerically and the framework can be employed as a teaching tool for general relativity.

timm
  • 1,525
  • 6
  • 16
  • Wow, thanks.. I've read the paper.. What I've got is that the best way to fall is having 0 velocity (e=0) at the event horizon. One can maximize their survival time by boosting their rocket to e=0 if e>0, which practically is always the case, you are falling with >0 velocity that you can't resist and that's why you fall into that black hole. – Kenneth Kho Jun 14 '18 at 14:09
  • One more question though, a few second after someone fall into a black hole, he would still be able to walk, talk, breathe, think, and live normally for some moment, right? Enjoying the last moment of his life before the spaghettification.. – Kenneth Kho Jun 14 '18 at 14:12
  • 2
    Indeed, if the black hole was large enough spaghettification wouldn't be a problem for a while, because the tidal force goes with 1/M² at the horizon. For a supermassive black hole of 3 billion solar masses it takes 13 minutes to reach the singularity. So there are good chances to cook a 5 minute egg. – timm Jun 14 '18 at 18:56