leaving black holes
in [Relativity]
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From: Bernhard Kuemel on 1 Oct 2009 06:09 Hi spr, sa! http://en.wikipedia.org/wiki/Spaghettification says: "The point at which these tidal forces kill depends on the black hole's size. For a supermassive black hole, such as those found at a galaxy's center, this point lies within the event horizon, so an astronaut may cross the event horizon without noticing any squashing and pulling (although it's only a matter of time, because once inside an event horizon, falling towards the center is inevitable)." Now if an astronaut orbiting just outside the event horizon of such a supermassive black hole stuck his hand into the event horizon, would he not feel any squashing and pulling? Could he still see his hand? Could he retract it from the event horizon? AFAIK the escape velocity at the event horizon is c. So an object travelling up at the event horizon at c should slow down and come to a halt at distance infinity. If it was faster (impossible for c) it would keep some finit speed at infinity. If it was slower it would eventually stop and fall back (unless it would gain some more momentum). But someone told me, objects (including light) within the event horizon always travel down and never up. So light from a flashlight pointing up would still travel down. And the astronauts hand within the event horizon would have to travel down, too, and he could not see his hand. But that must feel like pulling and contradict the paragraph in wikipedia. Or since his hand, including nerves will travel down it will become detached from his body and he might no longer feel it. But he would start bleeding and his space suit will leak atmosphere. Still not the picture I got from the wikipedia article. Also, suppose a flashlight was decended slowly from an orbit into the event horizon. At what speed would the flashlight and the light (pointing up) travel? Will the flashlight suddenly travel at c? Or, if it decends only slowly, why will it's light not cross the event horizon? Or will it? Thanks, Bernhard
From: Androcles on 1 Oct 2009 08:55 "Bernhard Kuemel" <bernhard(a)bksys.at> wrote in message news:63977$4ac47fc5$557f726a$23713(a)news.inode.at... > Hi spr, sa! > > http://en.wikipedia.org/wiki/Spaghettification says: > > "The point at which these tidal forces kill depends on the black hole's > size. For a supermassive black hole, such as those found at a galaxy's > center, this point lies within the event horizon, so an astronaut may > cross the event horizon without noticing any squashing and pulling Someone should ask if the dawn tsunami that hit Samoa was connected with any squishing and pulling of the Earth's crust by the Sun and Moon and killed anybody. Water moves easily with tidal forces, rock tends to stick and break. And it breaks at the weakest points. Maybe the Moon is a black hole.
From: dlzc on 1 Oct 2009 10:49 Dear Bernhard Kuemel: On Oct 1, 3:09 am, Bernhard Kuemel <bernh...(a)bksys.at> wrote: > Hi spr, sa! > > http://en.wikipedia.org/wiki/Spaghettification >says: > > "The point at which these tidal forces kill depends > on the black hole's size. For a supermassive black > hole, such as those found at a galaxy's center, this > point lies within the event horizon, so an astronaut > may cross the event horizon without noticing any > squashing and pulling (although it's only a matter > of time, because once inside an event horizon, > falling towards the center is inevitable)." > > Now if an astronaut orbiting just outside the event > horizon of such a supermassive black hole The event horizon is at 2M. The lowest stable orbit for matter is something like 6M. > stuck his hand into the event horizon, would he > not feel any squashing and pulling? Depends on the size of the hole. If we squashed the Earth into a black hole (to satisfy guskz), it would have an event hoizon that was something like 4 inches in diameter. At our current distance from the center, we'd fall normally, not noticing a difference in pull between feet and head. > Could he still see his hand? Could > he retract it from the event horizon? He'd have fallen in long before the signal to pull his hand from the "horizon", which is nothing special to him... not some sort of mirror or dark surface, made it from his brain to his hand. > Also, suppose a flashlight was decended slowly > from an orbit into the event horizon. At what > speed would the flashlight and the light > (pointing up) travel? If it fell from infinity, it would be going pretty close to c when it reach 2M. And we'd never see the light fall in, since light will spend a lot of time climbing out of that place to tell us it is gone. > Will the flashlight suddenly travel at c? Suddenly, no. > Or, if it decends only slowly, why will it's > light not cross the event horizon? From below the event horizon, there is no "up" that includes striking eyes or instruments far from the 2M surface. > Or will it? If the eyes are close, it is briefly possible. And those eyes will be in too in microseconds. David A. Smith
From: Bernhard Kuemel on 1 Oct 2009 11:47 dlzc wrote: >> Now if an astronaut orbiting just outside the event >> horizon of such a supermassive black hole > > The event horizon is at 2M. The lowest stable orbit for matter is > something like 6M. I don't know what M is, but ok. So let's assume a tough sphere (maybe iron), that withstands the low tidal forces at the event horizon, passes a black hole (on a hyperbolic or similar applicable trajectory) so close, that a small part of it dips into the event horizon. Is that possbile? What would happen to the part that dipped into the event horizon? > Depends on the size of the hole. If we squashed the Earth into a > black hole (to satisfy guskz), it would have an event hoizon that was > something like 4 inches in diameter. At our current distance from the > center, we'd fall normally, not noticing a difference in pull between > feet and head. A size like the one mentioned in the spaghettification article such that the tidal forces at the event horizon are not noticable to humans. Bernhard
From: Tom Roberts on 1 Oct 2009 12:04
Bernhard Kuemel wrote: > Now if an astronaut orbiting just outside the event horizon of such a > supermassive black hole stuck his hand into the event horizon, would he > not feel any squashing and pulling? Could he still see his hand? Could > he retract it from the event horizon? [Note: I consider only the case of a Schwarzschild black hole.] There is no freefalling orbit near the event horizon, but if the astronaut's spacecraft has VERY powerful thrusters he could pilot it to follow a circular trajectory just outside the horizon, with his thrusters aimed downward. In such a trajectory, for a supermassive black hole he would not feel significant tidal forces, but would feel ENORMOUS forces from his thrusters (far too great for a human to survive, but I ignore that). If he reached his hand into the horizon, it would be ripped from his body, because it would necessarily be outside his ship and its powerful thrusters are more than enough to sever his hand. > AFAIK the escape velocity at the event horizon is c. So an object > travelling up at the event horizon at c should slow down and come to a > halt at distance infinity. "escape velocity" is almost irrelevant in GR. But yes, just outside the horizon, if one fired a bullet outward at 0.999 c or so, it would slow down and essentially halt at distance infinity. But a light beam emitted outward there would not slow down (as measured locally, anywhere along its trajectory); it would be greatly redshifted, however. Note that "traveling up" is not possible at the event horizon -- once an object is at or inside the horizon, it can never approach it from inside, it can only go downward toward the singularity. Indeed this is also true of a light ray, either emitted from inside the horizon or incoming from outside. Except for the special case of a light pulse emitted from an infalling source precisely at the horizon and precisely aimed outward -- such a light pulse could in principle remain at the horizon, but this is an unstable situation and the slightest perturbation will make it fall inward. Note that any timelike object always crosses the horizon with relative speed c (regardless of how carefully and slowly the crossing is attempted with powerful thrusters). > But someone told me, objects (including light) within the event horizon > always travel down and never up. So light from a flashlight pointing up > would still travel down. Yes. > And the astronauts hand within the event > horizon would have to travel down, too, and he could not see his hand. > But that must feel like pulling and contradict the paragraph in > wikipedia. Different situations. The Wikipedia article apparently considered the astronaut inside his spaceship, and you put his hand outside. The TIDAL forces can be small, but the GRAVITATIONAL force cannot be small for a spaceship HOVERING just outside the horizon. Stated somewhat differently, a doomed astronaut whose spaceship falls through the horizon would experience negligible tidal forces and zero gravitational forces at the horizon. Indeed, he could not determine where the horizon is located by measurements within his ship, he could only locate it by looking at light from distant objects. > Also, suppose a flashlight was decended slowly from an orbit into the > event horizon. At what speed would the flashlight and the light > (pointing up) travel? Will the flashlight suddenly travel at c? Or, if > it decends only slowly, why will it's light not cross the event horizon? > Or will it? A flashlight aimed upward and lowered by a cable from a spaceship hovering just outside the horizon will break the cable before reaching the horizon, no matter how strong the cable is. It will fall inward, and cross the horizon with local speed c. The light from the flashlight will always travel with local speed c, but will be increasingly redshifted as it approaches the horizon, with the redshift becoming infinite as it crosses. Tom Roberts |