Fun with black holes
in [Relativity]
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From: Edward Green on 12 May 2010 17:50 Consider an ordinary Schwarzschild black hole: (1) Let an object be dropped from an arbitrary value of the radial coordinate above the event horizon. (2) Is there ever a case where the object has not reached the event horizon where it is _not_ possible to reverse its trajectory and escape back to its starting point, by means of a sufficiently powerful thrust? (3) Assuming the answer to (2) is "no", let T be the greatest lower bound on the time to make a return trip. (4) Does T tend to infinity as the distance from the event horizon tends to zero? (5) Assuming the answer to (4) is "yes", in what sense does a distant object ever finish falling into the event horizon?
From: BURT on 12 May 2010 17:54 On May 12, 2:50 pm, Edward Green <spamspamsp...(a)netzero.com> wrote: > Consider an ordinary Schwarzschild black hole: > > (1) Let an object be dropped from an arbitrary value of the radial > coordinate above the event horizon. > > (2) Is there ever a case where the object has not reached the event > horizon where it is _not_ possible to reverse its trajectory and > escape back to its starting point, by means of a sufficiently powerful > thrust? > > (3) Assuming the answer to (2) is "no", let T be the greatest lower > bound on the time to make a return trip. > > (4) Does T tend to infinity as the distance from the event horizon > tends to zero? > > (5) Assuming the answer to (4) is "yes", in what sense does a distant > object ever finish falling into the event horizon? The red shifts and blue shifts related to light by Pound Reabka show that light will break the energy laws by going to zero energy and infinite energy at the event horizon depending on what direction it is emitted in. Mitch Raemsch
From: Thomas Heger on 12 May 2010 18:53 Edward Green schrieb: > Consider an ordinary Schwarzschild black hole: > > (1) Let an object be dropped from an arbitrary value of the radial > coordinate above the event horizon. > > (2) Is there ever a case where the object has not reached the event > horizon where it is _not_ possible to reverse its trajectory and > escape back to its starting point, by means of a sufficiently powerful > thrust? > > (3) Assuming the answer to (2) is "no", let T be the greatest lower > bound on the time to make a return trip. > > (4) Does T tend to infinity as the distance from the event horizon > tends to zero? > > (5) Assuming the answer to (4) is "yes", in what sense does a distant > object ever finish falling into the event horizon? My idea about black holes is different to 'standard' It's based on GR and curved spacetime. This means, that the timelike 'axis' has a direction (in spacetime) and that can be curved. Since space denotes actually past events, we see into the past, if we look into the sky. More distant means longer ago. But this requires an axis pointing in a direction, where the future light cone of an event matches our past lightcone. Guess this axis would not, than we could loose sight of such events, because they would not radiate in our direction (hence things get black). Since a spaceship is an object, following its timeline, that ship will eventually drop into such a black hole, because time-reversal is generally regarded as impossible and no engine could be strong enough to accelerate into the past. A black hole looks pointlike, because of what SR calls length contraction. That could be understood, if velocity is interpreted as an angle and c would refer to 45�. Once this angle is reached, the space observed from that ship gets pointlike, because its future in not anymore ours. So, in short, black holes would be an optical illusion, because seen from there, the Earth would vanish in a black hole. Greetings TH
From: eric gisse on 12 May 2010 11:26 Edward Green wrote: > Consider an ordinary Schwarzschild black hole: > > (1) Let an object be dropped from an arbitrary value of the radial > coordinate above the event horizon. > > (2) Is there ever a case where the object has not reached the event > horizon where it is _not_ possible to reverse its trajectory and > escape back to its starting point, by means of a sufficiently powerful > thrust? No. What do you think an event horizon 'is' ? > > (3) Assuming the answer to (2) is "no", let T be the greatest lower > bound on the time to make a return trip. > > (4) Does T tend to infinity as the distance from the event horizon > tends to zero? Yes, by trivial inspection of the expressions for coordinate time as a function of distance in all of the black hole metrics. > > (5) Assuming the answer to (4) is "yes", in what sense does a distant > object ever finish falling into the event horizon? Mazeltov! You have re-discovered the basic fact that an observer outside the event horizon of a black hole can never see an object pass through the event horizon in finite amounts of proper time.
From: eric gisse on 12 May 2010 11:27
Thomas Heger wrote: > Edward Green schrieb: >> Consider an ordinary Schwarzschild black hole: >> >> (1) Let an object be dropped from an arbitrary value of the radial >> coordinate above the event horizon. >> >> (2) Is there ever a case where the object has not reached the event >> horizon where it is _not_ possible to reverse its trajectory and >> escape back to its starting point, by means of a sufficiently powerful >> thrust? >> >> (3) Assuming the answer to (2) is "no", let T be the greatest lower >> bound on the time to make a return trip. >> >> (4) Does T tend to infinity as the distance from the event horizon >> tends to zero? >> >> (5) Assuming the answer to (4) is "yes", in what sense does a distant >> object ever finish falling into the event horizon? > > > > My idea about black holes is different to 'standard' It's based on GR > and curved spacetime. This means, that the timelike 'axis' has a > direction (in spacetime) and that can be curved. Since space denotes > actually past events, we see into the past, if we look into the sky. > More distant means longer ago. But this requires an axis pointing in a > direction, where the future light cone of an event matches our past > lightcone. Guess this axis would not, than we could loose sight of such > events, because they would not radiate in our direction (hence things > get black). > Since a spaceship is an object, following its timeline, that ship will > eventually drop into such a black hole, because time-reversal is > generally regarded as impossible and no engine could be strong enough to > accelerate into the past. > A black hole looks pointlike, because of what SR calls length > contraction. That could be understood, if velocity is interpreted as an > angle and c would refer to 45�. Once this angle is reached, the space > observed from that ship gets pointlike, because its future in not > anymore ours. > So, in short, black holes would be an optical illusion, because seen > from there, the Earth would vanish in a black hole. > > Greetings > TH Yep, you are an idiot. Thanks for letting us know. |