Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 30

Thread: Black Hole Question

  1. Top | #11
    Fair dinkum thinkum bilby's Avatar
    Join Date
    Mar 2007
    Location
    The Sunshine State: The one with Crocs, not Gators
    Posts
    22,759
    Archived
    10,477
    Total Posts
    33,236
    Rep Power
    87
    Quote Originally Posted by SLD View Post
    I was always taught that if an astronaut fell into a black hole and you observed him waving good bye, he would never really disappear. His waving hand would just slow down and turn redder as time progressed. From the observer perspective of course.

    But is the Schwarzschild radius not determined by the mass of the black hole? And if so, wouldn’t the radius then expand when the astronaut fell in and thus swallow the image of the astronaut? But when?

    SLD
    No, it wouldn't, because the 'image' is of the astronaut approaching the event horizon. Over time, it gets closer and closer, but at an ever decreasing rate; and the increasing radius due to accretion (including the addition of the mass of the astronaut) doesn't make the radius arbitrarily larger - it never gets so big that it includes all of the path the astronaut took to reach it. So there's always a point where light was reflected off the astronaut when he was just outside the radius, even if that radius is increasing.

    To use an analogy, imagine a car racing away from you along a highway. At some point, it passes a stationary black truck; And there's a moment when the back of the car and the front of the truck are exactly the same distance from the observer. Now, if the truck isn't parked, but is instead moving toward you, does that mean that there's no longer a point when they are observed to be the same distance away? Of course not - it just means that that point is closer to you than it would be if the truck was stationary.
    Last edited by bilby; 06-28-2020 at 06:45 PM.

  2. Top | #12
    Contributor skepticalbip's Avatar
    Join Date
    Apr 2004
    Location
    Searching for reality along the long and winding road
    Posts
    5,529
    Archived
    12,976
    Total Posts
    18,505
    Rep Power
    66
    Quote Originally Posted by repoman View Post
    Is there some sort of handwaving similar to Zeno's Paradox about never crossing the black hole horizon?

    Because black holes get bigger and things fully enter them.
    Black holes are weird critters. Can objects ever actually cross the event horizon given time dilation and other relativistic effects?

    Many say the answer is no... that matter falling into the gravity well will be accelerated closer and closer to c, asymptotically increasing the time dilation. This would mean that the infalling matter ends up as degenerate matter (because of tidal effects) at the radius of the event horizon. Matter falling through the event horizon would have been accelerated to c, which Uncle Albert says can't happen. This really weird critter they have named a collapser which is, in effect, mathematically identical to a black hole when dealing with space time outside the event horizon. Not to say they deny that there are black holes, they allow for black holes to have been created at the 'big bang'.

  3. Top | #13
    Veteran Member
    Join Date
    Sep 2004
    Location
    California
    Posts
    3,975
    Archived
    4,797
    Total Posts
    8,772
    Rep Power
    62
    Quote Originally Posted by SLD View Post
    I was always taught that if an astronaut fell into a black hole and you observed him waving good bye, he would never really disappear. His waving hand would just slow down and turn redder as time progressed. From the observer perspective of course.
    Something to keep in mind about that whole scenario is that when the waving hand appears to slow down, it doesn't make the universe dump any more light energy per second than usual onto the hand as the astronaut makes final approach into the black hole. So the total light reflected from his hand in a second is getting stretched out over minutes and weeks and so forth. So the light doesn't just get redder; it also gets fainter. But light is quantized. It can't get fainter than one photon and still be there. From the astronaut's perspective only a finite number of photons hit him before he enters the black hole; that means there's going to be a last photon. So whenever that one finally makes it out, he really disappears.

  4. Top | #14
    Super Moderator
    Join Date
    Sep 2000
    Location
    Nevada
    Posts
    27,603
    Archived
    96,752
    Total Posts
    124,355
    Rep Power
    100
    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by repoman View Post
    Is there some sort of handwaving similar to Zeno's Paradox about never crossing the black hole horizon?

    Because black holes get bigger and things fully enter them.
    Black holes are weird critters. Can objects ever actually cross the event horizon given time dilation and other relativistic effects?

    Many say the answer is no... that matter falling into the gravity well will be accelerated closer and closer to c, asymptotically increasing the time dilation. This would mean that the infalling matter ends up as degenerate matter (because of tidal effects) at the radius of the event horizon. Matter falling through the event horizon would have been accelerated to c, which Uncle Albert says can't happen. This really weird critter they have named a collapser which is, in effect, mathematically identical to a black hole when dealing with space time outside the event horizon. Not to say they deny that there are black holes, they allow for black holes to have been created at the 'big bang'.
    The falling astronaut doesn't see the slowing, while they can't quite reach lightspeed they're moving very close to it by the time they cross the event horizon. It's only the outside observer that sees the astronaut slow down--because the image is being slowed down.

  5. Top | #15
    Contributor skepticalbip's Avatar
    Join Date
    Apr 2004
    Location
    Searching for reality along the long and winding road
    Posts
    5,529
    Archived
    12,976
    Total Posts
    18,505
    Rep Power
    66
    Quote Originally Posted by Loren Pechtel View Post
    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by repoman View Post
    Is there some sort of handwaving similar to Zeno's Paradox about never crossing the black hole horizon?

    Because black holes get bigger and things fully enter them.
    Black holes are weird critters. Can objects ever actually cross the event horizon given time dilation and other relativistic effects?

    Many say the answer is no... that matter falling into the gravity well will be accelerated closer and closer to c, asymptotically increasing the time dilation. This would mean that the infalling matter ends up as degenerate matter (because of tidal effects) at the radius of the event horizon. Matter falling through the event horizon would have been accelerated to c, which Uncle Albert says can't happen. This really weird critter they have named a collapser which is, in effect, mathematically identical to a black hole when dealing with space time outside the event horizon. Not to say they deny that there are black holes, they allow for black holes to have been created at the 'big bang'.
    The falling astronaut doesn't see the slowing, while they can't quite reach lightspeed they're moving very close to it by the time they cross the event horizon. It's only the outside observer that sees the astronaut slow down--because the image is being slowed down.
    Of course the astronaut doesn't see his time frame as slowing. That isn't the problem that many physicists question. The question they address is that of matter being able to cross the event horizon because of being limited by relativistic effects.

  6. Top | #16
    Veteran Member Tigers!'s Avatar
    Join Date
    Sep 2005
    Location
    On the wing waiting for a kick.
    Posts
    1,965
    Archived
    2,558
    Total Posts
    4,523
    Rep Power
    56
    Reminds me that i should not stand too close to any hole but especially a black one.
    NOTE: No trees were killed in the sending of this message, but a large number of electrons were terribly inconvenienced.

  7. Top | #17
    Veteran Member Lion IRC's Avatar
    Join Date
    Feb 2016
    Location
    Australia
    Posts
    4,096
    Rep Power
    21
    Quote Originally Posted by Bomb#20 View Post
    ...light is quantized. It can't get fainter than one photon and still be there.


    From the astronaut's perspective only a finite number of photons hit him before he enters the black hole; that means there's going to be a last photon. So whenever that one finally makes it out, he really disappears.
    Ceases to exist?

  8. Top | #18
    Fair dinkum thinkum bilby's Avatar
    Join Date
    Mar 2007
    Location
    The Sunshine State: The one with Crocs, not Gators
    Posts
    22,759
    Archived
    10,477
    Total Posts
    33,236
    Rep Power
    87
    Quote Originally Posted by Lion IRC View Post
    Quote Originally Posted by Bomb#20 View Post
    ...light is quantized. It can't get fainter than one photon and still be there.


    From the astronaut's perspective only a finite number of photons hit him before he enters the black hole; that means there's going to be a last photon. So whenever that one finally makes it out, he really disappears.
    Ceases to exist?
    If somebody walks behind a wall so that you can no longer see them, have they ceased to exist?

    If you turn the lights out in a deep slate mine, so no photons bounce off your hands, have your hands ceased to exist?

  9. Top | #19
    Veteran Member Lion IRC's Avatar
    Join Date
    Feb 2016
    Location
    Australia
    Posts
    4,096
    Rep Power
    21

    I see your point

    "He alone is immortal and dwells in unapproachable light. No one has ever seen Him, nor can anyone see Him."

  10. Top | #20
    Fair dinkum thinkum bilby's Avatar
    Join Date
    Mar 2007
    Location
    The Sunshine State: The one with Crocs, not Gators
    Posts
    22,759
    Archived
    10,477
    Total Posts
    33,236
    Rep Power
    87
    He is only immortal in its mass 3 and 4 isotopes. The longest lived unstable He isotope has a half life of about 800ms (6He).

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •