What you see if you observe a star collapse to form black holes
Amazing na watching a star from your home from your roof collapsing, in order to understand what you would see if you were watching a star collapse to form a black hole, one has to remember that in the theory of relativity there is no absolute time. Each observer has his own measure of time. The time for someone at a distance, because of the gravitational field of the star. This effect has been measured in an experiment on earth with clocks at the top and bottom of the water tower. Suppose an intrepid astronaut on the surface of the collapsing star sent a signal every second, according to his watch, to his spaceship orbiting about the star. At some time on his watch, say ten o’clock, the star would shrink below the critical radius at which the gravitational field becomes so strong that the signals would no longer reach the spaceship.
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| Watching Stars |
His companions watching from the spaceship would find the intervals between successive signals from the astronaut getting longer and longer as ten o’clock approached. However, the effect would be very small before 9:59:59. They would have to wait only very slightly more than a second between the astronaut’s 9:59:58, signal and the one that he sent when his watch read 9:59:59, but they would have to wait forever for the ten o’clock signal. The light wave emitted from the surface of the star between 9:59:59 and ten o’clock, by the astronaut’s watch, would be spread out over an infinite period of time, as seen from the spaceship.
The time interval between the arrivals of successive waves at the spaceship would get longer and longer, and so the light from the star would appear redder and fainter and fainter. Eventually, the star would be so dim that it could no longer be seen from the spaceship. All that would be left would a black hole in space.
The star would, however, continue to exert the same gravitational force on the spaceship. This is because the star is still visible to the spaceship, at least in principle. It is just that the light from the surface is so red-shifted by the gravitational field of the star that it cannot be seen. However, the red-shift does not affect the gravitational field of the star itself. Thus, the spaceship would continue to orbit the black hole.
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