View Full Version : Time, the event horizon and relativity.

2007-Feb-22, 10:00 AM
Hi folks,

After some clarification if possible.

Under general relativity, an external observer would see an infalling "victim" essentially freeze at the event horizon of a black hole. My first question is does the victim then see time for our external observer move infinitly fast, or does the infalling victim see the external observer freeze also?

My second question is under the principle of equivelance, does the answer to Q1 apply to motion under special relativity?

Lastly, I've read that under general relativity, the time and radial (or distance?) vectors "swap" once inside the event horizon, in other words, the faster one tries to escape the black hole the faster you actually reach the singularity. Could someone please explain this in a little more detail.

I understand that in many cases one needs to understand the math to appreciate these things, but I haven't done calculus in 20 years, so the more conceptual the answer the better (if possible).

Thanks in advance.


2007-Feb-22, 01:23 PM
Garvs- there's an active thread over in Astonomy, under "do black holes exist?", which is covering this ground in great detail.

Kaptain K
2007-Feb-22, 06:23 PM
Garvs- there's an active thread over in Astonomy, under "do black holes exist?", which is covering this ground in great detail.
Too great? :)

Ken G
2007-Feb-22, 06:37 PM
The answer to your question is there are really three different ways to talk about time. One is the time you measure on your own clock, but that only measures time where you are, and this form of time acts perfectly normally in all reference frames (the central postulate of relativity), including for someone falling toward an event horizon. A second one is the way you take your proper time and convert it into a conceptualization of someone else's proper time. The conceptualization needs to be consistent, so that you can apply physics to it, but it does not need to be unique, it's kind of up to you-- hence it is not an "objectively real" way to talk about what time is doing, it's more a mathematical contrivance, like using latitude and longitude to identify a spot on Earth (and what is the longitude at the North pole?). The third way is to use light to see how time appears to be proceeding for someone else, but this is the least "real" way to think about time, because it it fraught with all kinds of time-of-flight illusions and is generally corrected for to avoid unnecessary confusion. It sounded like you were phrasing your question in terms of this last type of time, in which case people on Earth would "see" time as stopping for the person falling into the black hole, but the person falling into the black hole would not "see" the entire future of the universe as though time were speeding up infinitely outside the black hole. That's because it in effect takes an infinite time for the light to reach the event horizon if we use this particular conceptualization of time, so the future would still take an infinite time to see.