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Q & A: falling into black hole

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Most recent answer: 03/07/2013
Q:
Suppose (just suppose) that the tmoasphere from where effect of gravity can be felt is increased to infinity and a particle having mass is released from infity to free fall then it will take 353 days to reach it to the speed of light under the effect of gravity(10m/s^2).What happens to it when it reaches and crosses the speed of light? Will einstein's relativity fails there?
- Gourav (age 17)
Sanawad,madhya pradesh,india
A:
The situation you're describing is basically what happens when things fall into black holes. Roughly speaking, the classical escape velocity would be bigger than c, so classically something falling in from rest would go faster than c. The real world doesn't work that way.

The large mass distorts spacetime, so that the effects are different. There are conventional ways to divide up that effect into effects on space and on time, and I'll follow the most common (Schwarzschild) way.

As the particle falls, it reaches lower regions where all clocks run slower than they do far away. That's called the gravitational redshift, and it's even measurable between the top and bottom floors of buildings on earth. That effect slows the particle's motion, as judged from the outside. It gets stronger and stronger as the particle falls farther. The effect gets so strong that it actually takes the particle an infinite amount of outside time to reach the horizon of the black hole.

So does general relativity break down here? Not really, although the smooth extension of our outside coordinate system across the black hole horizon breaks down. However, as the black hole starts to form the slowing affects things falling into the not-quite black hole, It turns out to take an infinite time to form the black hole, so that viewed from the outside it never quite forms. We believe that the almost-black-hole evaporates by quantum processes without ever completely forming.

Things would look completely different from the point of view of someone falling in, keeping time in their own coordinates. There's currently a great debate about how to reconcile these different points of view in a consistent physical picture. See

Whoa- it looks like the latest major development in this topic was published in the last few hours. . or 


Mike W.

(published on 03/07/2013)

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