Black Holes

[redwithpurpleflashes]

This may be a naive question but it is something that I have been wondering for a while now.
The explanations that I’ve read say that if an astronaut falls into a black hole, you would see them fall slower and slower as it takes the light longer to get to your eyes. Then, at the point they hit the event horizon they would appear frozen since it will take forever for the light to get to you.
My question is, if a star is collapsing, wouldn’t the last bit of light that comes from the star be frozen at the event horizon making it visible (to someone observing it from the outside)?

[Silas]

It's actually the other way around...

If we, at a safe distance, watch someone fall into a black hole, we will see them get ripped into shreds by the tidal effects. Then they will disappear in a flare of energy. As far as the timing, it wouldn't be any different than if they had fallen into an ordinary hole.

If we, however, *are* the poor souls falling into the black hole, *then* time will seem to slow, from our point of view.

So, good question, but based on (I think?) a reversal of the theoretical effects.

(Not having a good-sized black hole handy, and a strange paucity of volunteer subjects, I am unable to test the issue experimentally...)

Silas

[Wiley]

On 2002-09-12 19:05, Silas wrote:
It's actually the other way around...

If we, at a safe distance, watch someone fall into a black hole, we will see them get ripped into shreds by the tidal effects. Then they will disappear in a flare of energy. As far as the timing, it wouldn't be any different than if they had fallen into an ordinary hole.

If we, however, *are* the poor souls falling into the black hole, *then* time will seem to slow, from our point of view.

Sorry, Silas. But I think you got it backwards.

Imagine that we are observing some poor sap falling into a black hole, and we, being the savvy observers that we are, stayed far away from the black hole (i.e. negligible curvature due to the hole). The time measured by the poor sap, who is r from the center of black hole, is

dt' = (1 - r_s/r)^(1/2) dt

where r_s is the Schwarzchild radius or the location of the event horizon, and where dt is the time interval we savvy observers measure. Thus we will observe the sap's clock to run slow by

(1 - r_s/r)^(1/2)

and as the sap's position r approaches the event horizon, r_s, the clock will stop completely. Our unlucky falling fellow will not notice any of this. Time would pass normally in his frame, as really by definition. An observer's local time is his "normal" time.

[ILeaTHS]

As to what the guy falling into the whole looks like, as we would observe it, would he appear frozen at the event horizon not because the light would take forever to reach us, but because we wouldn't be getting any "new" light once he crosses because the light itself can't cross either? So we'd never know what happened to him past that point.

I don't know if I made my thought clear, or if it makes any sense anyway, but I'd like to see if I understand this.

[Hale_Bopp]

Yes, as you approach the event horizon, the light an external observer sees would be increasingly redshifted, all the way to infinity. So no, you could not see something "frozen" at the event horizon.

To tell you the truth, this confused my to no end when I read an old book called "Black Holes, Quasars and the Universe" back in 7th grade and couldn't get anyone to give me a satisfactory explanation for years. Very small Iowa town...no local college or anything for a resource.

Rob

[ILeaTHS]

Ok, I'm still trying to understand (sorry).
Does that mean that once someone crosses the even horizon they wouldn't appear frozen to us, but would disappear from our view completely?

And can you recommend a good book that about black holes that wouldn't be too hard to wrap my mind around?

[DStahl]

Yes, I think you've got it: any light emitted or reflected from the doomed astronaut fades to red, then infrared, then radio waves, and essentially fades to nothing--just as the light from the collapsing star does at the formation of a stellar black hole.

My favorite black-hole book is the one by Kip Thorne, Black Holes and Time Warps: Einstein's Outrageous Legacy. Your local library probably has a copy or can conjure one up on inter-library loan.

[mallen]

On 2002-09-12 21:44, ILeaTHS wrote:
Ok, I'm still trying to understand (sorry).
Does that mean that once someone crosses the even horizon they wouldn't appear frozen to us, but would disappear from our view completely?

They would not just disappear; They would literally be outside our Universe. Since nothing can escape a black hole, communication from the inside to the outside would be impossible, and therefore, the inside of the black hole is not in our Universe.

As I understand it, this is what the two observers would see (if we ignore the tidal forces which would the spaceship apart):

FROM THE OUTSIDE (I'm pretty sure of this)

As the unfortunate astronaut falls into the black hole, we would see his clock start to run slower and his light to redshift. As he approaches the event horizon, his clock will run slower and slower (but never quite stop). You wouldn't actually see this though, because as his light redshifts more and more, it eventually becomes impossible to detect. Then, after a long, long time (sometime around the heat-death of the universe), the black hole will have evaporated.

FROM THE INSIDE (not quite as sure)

As the astronaut approaches the event horizon, his clock will start running slower relative to the rest of the universe (almost stopping as he approaches the event horizon). Of course, he doesn't notice this because he is running slower too. What he would see is the universe speeding up and blue-shifting. This has two consequences: 1) he'd be completely baked by extremely high-energy (blue-shifted) radiation and 2) if he can handle that, he would get to see the end of the universe. Again, the hole process would end when the black hole evaporates.

WHAT I DON'T UNDERSTAND

The big question that I have no idea about is what happens when the black hole evaporates. There are two ideas that I cannot really reconcile:

1) Since it takes forever to reach the event horizon, the black hole evaporates before he gets there.

2) But, if he is allowed to not fall into the black hole (because he reaches it after it is gone), all the matter that "fell" in would still be there, and it couldn't have evaporated.

If anyone can straighten out my headache for me, I would be most grateful.

[ILeaTHS]

"My favorite black-hole book is the one by Kip Thorne, Black Holes and Time Warps: Einstein's Outrageous Legacy. Your local library probably has a copy or can conjure one up on inter-library loan."

thanx, I've seen it there and will take a look.

"2) if he can handle that, he would get to see the end of the universe."

I thought that he'd be crushed into the singularity.

[Ring]

From the physics FAQ

"What about Hawking radiation? Won't the black hole evaporate before you get there?"

http://antwrp.gsfc.nasa.gov/htmltest...h_pub_faq.html

[redwithpurpleflashes]

It was the red shifting that I was missing. I would guess this is why they are invisible after they collapse. I was wondering why they wouldn't be visible due to the light being frozen at the event horizon.

Can we detect this red shifted light or is it too "stretched out" to detect.

Well, since my "In Basket" seems to be permanently frozen at "full" I should probably get to work. If we had a black hole, I could throw it in and see what happens.