The Event Horizon Questions

[KryptaRax]

A probe sent to the event horizon would supposedly stop just before it entered because time slows down right? Well if that is true, then why aren't black holes covered in compressed hydrogen, stars, plasma, and anything you fin din space that gets sucked in? It seems to me that black holes should be completely concealed by all the stuff and yet, despite the idea that time slows around it, suns get completely sucked in at a fairly normal pace. At least that's the way I see it. So why is that?

[mathematic]

It depends on the reference frame. In its own frame the probe will simply enter the black hole. In the frame of a distant observer, the probe will slow down and any light it gives off will be red shifted to the point of being invisible.

[ACG52]

 

 

A probe sent to the event horizon would supposedly stop just before it entered because time slows down right?

No, time does not slow down in the frame of reference of the probe. It enters the EH and is never seen or heard from again.

 

From a distant observational frame, we see the probe appearing to slow down, while it's photons become increasing red-shifted, until the last photon emitted before the probe enters the EH red shifts its way to zero energy and we see the probe vanish.

[KryptaRax]

It depends on the reference frame. In its own frame the probe will simply enter the black hole. In the frame of a distant observer, the probe will slow down and any light it gives off will be red shifted to the point of being invisible.

I understand the first part, but why would the probe shift to red and then invisible? And why aren't other objects invisible because of their color?

[Delta1212]

Color is based on frequency. As time slows down, frequency decreases. Red is at the low end of the light spectrum, so decreasing frequency due to relative speed (or time) is called red shift. A corresponding increase in frequency is called blue shift.

 

As objects blue shift, their brightness increases. As they red shift, the brightness decreases. If the light red shifts far enough, the brightness will be effectively zero and you will see only blackness. Since black holes don't emit light and absorb any light that passes through the event horizon, the event horizon appears as a solid black area (hence the name). Any object falling in would red-shift until it appeared solid black... against the black hole. Thus, it would not be detectable by light and you would be unable to see it.

[imatfaal]

No, time does not slow down in the frame of reference of the probe. It enters the EH and is never seen or heard from again.

 

From a distant observational frame, we see the probe appearing to slow down, while it's photons become increasing red-shifted, until the last photon emitted before the probe enters the EH red shifts its way to zero energy and we see the probe vanish.

"...until the last photon..." Agreed completely up to this point. Surely with very high time dilation (although not infinite if we do the coordinate system right and avoid S'child) we do not get to see the last photon. For any finite life observer in a finite life universe - the observer sees the probe slow and red shift till the signal is so red shifted it has so little resolution that the signal could come from any part of the EH - but the probe will still be o/s the eh for any distant accelerated observer in a finite universe

[KryptaRax]

Interesting, so from our point of view a block hole is basically a bunch of black stars and hydrogen in a circle?

[ACG52]

 

 

but the probe will still be o/s the eh for any distant accelerated observer in a finite universe

Yeah, but in it's own frame, the probe is gone and toast.

[ACG52]

Again, some basic reading would do you good.

 

http://en.wikipedia.org/wiki/Black_hole

[Delbert]

A probe sent to the event horizon would supposedly stop just before it entered because time slows down right?

Not quite. Time for the probe as seen by the observer (say back on earth) will appear to slow as it approached the black hole, such that it will never be seen to enter or pass through the event horizon. Even if you waited long enough to the death of the universe. But if you were piloting said probe (ignoring the crushing and stretching!) you wouldn't sense time slowing at all, but rather upon looking back (say at earth) you'd see things speeding up. Presumably the conclusion might be that you'd see the universe speed up so fast (and shrink) that entering the event horizon would coincide with the future end (death) of the universe.

 

That's what relative or relativity means, time is relative to us wherever we are. In other words, our time will always be correct for us, with others or elsewhere apparently being wrong (slowing or speeding up). Go to where it was wrong and it will now be right, leaving back were you came from now being wrong! Time is relative as per Einstein.

 

As for why we should see all the matter (stuff) that's entered the black hole actually hovering at the event horizon, it probably is hovering. But its visibility would at least be tempered by frequency shifting. Couldn't see it because as matter approaches the event horizon it also gets fainter, light frequency (or any radiation) becomes ever lower (Doppler) to the point that it would disappear to us back on earth. I also think there are other aspects that make it disappear to us back on earth.

 

Although he didn't seem to venture too far into the subject, one interviewee mentioned the above apparent bizarre fact during the last Sky at Night (UK) TV program.