On Sun, Jan 26, 2014 at 4:31 PM, Edgar L. Owen <[email protected]> wrote:
> Brent, Liz and Jesse, > > OK, now I understand the effect you guys are referencing... > > I thought Jesse had been saying that things don't ACTUALLY fall into black > holes, they just pile up on the event horizon surface, because their motion > actually slows down as they approach the surface BECAUSE their clocks slow > down from the intense gravity. That of course is incorrect. > > But of course things actually DO fall into black holes continuously > accelerating as they do so. Otherwise black holes could never form, could > not exist, and we would not be observing them.. > > But I see now what you guys are referencing is just how it appears to an > outside observer as the falling object approaches c as it nears the event > horizon. > What do you mean "approaches c"? Again, if we are talking about what the external observers *sees* visually (and ignoring the difficulty of seeing highly redshifted light), he sees the falling observer getting closer and closer to the horizon but never quite reaching it--so visually the falling observer would seem to slow down, inching closer and closer to the horizon but forever appearing to remain outside it, from the visual POV of the external observer. But of course the falling observer doesn't experience it taking forever to reach the horizon, he crosses it at some finite proper time and then passes into the interior. If we're talking about velocities rather than just visual rates of movement as seen by some far away observer, hopefully you understand that all statements about the velocity of anything depend on the choice of coordinate system you use to assign position and time coordinates to events. In the "waterfall" coordinates that Brent mentioned (described at http://jila.colorado.edu/~ajsh/insidebh/waterfall.html ), it is true that if an observer starts out at rest at infinity and falls into the black hole, his velocity reaches c at the horizon. In other commonly-used coordinate systems for a black hole this isn't the case though--in Schwarzschild coordinates the coordinate velocity of anything falling into the hole (even light) always approaches 0 at the event horizon, and it takes an infinite coordinate time to cross the horizon (but still a finite proper time). With Kruskal-Szekeres coordinates which I mentioned earlier in a post to Richard, the coordinate system has the nice property that light has a constant coordinate speed just like in an inertial frame in special relativity (which isn't true of some other commonly-used coordinate systems like Schwarzschild coordinates and waterfall coordinates, where the coordinate speed of light rays varies), while all massive objects have a coordinate speed less than light, and in these coordinates the horizon itself expands outward at the speed of light. So this gives another way of thinking about why nothing can escape the horizon once it's entered it--it's the same reason you can't "escape" the future light cone of some event once you've entered it. Jesse -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
