Black Hole Question

[SLD]

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

 

[Bronzeage]

An interesting question. How close to a black hole's center does an object with mass have to be. before the light that allows us to see it, cannot escape the gravity of the black hole?

 

[Wiploc]

An interesting question. How close to a black hole's center does an object with mass have to be. before the light that allows us to see it, cannot escape the gravity of the black hole?

The more massive the black hole, the farther the event horizon is from the center.

 

[repoman]

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.

 

[SLD]

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.

From the perspective of the astronaut going into the black hole, he just goes into the hole. The light leaving him though is stretched to the red because of the gravitational field. It also take longer to get to the observer and time appears to slow down.

 

[Elixir]

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.

From the perspective of the astronaut going into the black hole, he just goes into the hole. The light leaving him though is stretched to the red because of the gravitational field. It also take longer to get to the observer and time appears to slow down.

Do neutrons have an event horizon?

 

[Elixir]

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.

From the perspective of the astronaut going into the black hole, he just goes into the hole.

Until he gets so close that tidal forces physically rip him apart...

 

[Shadowy Man]

The tidal forces will depend on the mass of the black hole. A supermassive black hole won’t have nearly as large a tidal force as a small one.

 

[Elixir]

The tidal forces will depend on the mass of the black hole. A supermassive black hole won’t have nearly as large a tidal force as a small one.

As long as you're falling "straight in" you might survive a while ... but otherwise you'll go into close orbit, accelerate to near lightspeed and become part of the superheated glowing aura around the supermassive black hole where magnetic fields are so strong that the very atoms of your body are ripped into their constituent subatomic particles... fun times!

 

[Wiploc]

As long as you're falling "straight in" you might survive a while ... but otherwise you'll go into close orbit, accelerate to near lightspeed ...

Why would going into orbit speed you up more than falling straight down a gravity well?

 

[bilby]

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.

 

[skepticalbip]

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'.

 

[Bomb#20]

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.

 

[Loren Pechtel]

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.

 

[skepticalbip]

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.

 

[Tigers!]

Reminds me that i should not stand too close to any hole but especially a black one.

 

[Lion IRC]

...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?

 

[bilby]

...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?

 

[Lion IRC]

I see your point

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

 

[bilby]

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).