# What is the speed for an object that travel close to the speed of light?

I have some questions regarding Einsteins theory of Relativity that should be fairly easy to answer. Lets say we make an experiment where we have a rocket (with an astronaut inside) that travels very close to the speed of light and it is heading away from Earth. Also back on earth I have an observer that makes measurements regarding the rocket that travels very close to the speed of light.

Question 1: What is the speed of the rocket that the earth observer would measure?

Lets change the above experiment and place the rocket very close to the event horizon of a black hole. According to what I have heard and read the rocket would seem to stand still as our observer see it back on earth.

Question 2: What is the speed that the earth observer would make about the rocket in this situation?

Question 3: Are the two experiments the same or are they somehow fundamentally different? Would an object seem standing still while traveling very close to the speed of light?

-

## 1 Answer

Q1: At near the speed of light--- this is the condition you gave. Q2: At near the speed of light, but with time-freezing

The regions objects stop near a black hole horizon as measured from outside is not because they are moving slower, but because they run out of time--- the time outside is finite. So you see the rocket time get stopped, and the rocket freeze and redshift away to blackness, you don't see the rocket slow down in the sense of slower speed. relative to the stationary t-axis, you see it speed up as it falls in to the black hole.

-
Hi Ron and thanks for responding. After reading your answer I believe I need to rephrase my questions. The way I understand it is that speed is the value of the distance travel by an object divided by a time interval, correct? My real question is how do I measure the speed of the rocket if the object appears to get freeze in either experiment? Thanks – mpc Jul 30 '12 at 17:54
@mpc: The problem is the notion of "time interval" is shrinking near the black hole, going to zero at the horizon. The definition you give is not appropriate for curved space, you want the tilt of the rocket relative to the notion of "not moving" defined by the time-axis. – Ron Maimon Jul 30 '12 at 21:01
Thanks, I believe that now I understand it better. – mpc Jul 31 '12 at 11:30