About the black holes

1. Aug 11, 2012

valentin mano

the time for crossing the event horizon is infinite,so no singularity,no evaporation and no loss of information could be observed?

2. Aug 11, 2012

julian

Somebody said , I forget who (sorry - while ago), that in the ideal fluid interior model the event horizon starts at the center and expands to encompass the matter within finite time as measured by an outside observer.

Plus, (according to Hawking I think) if you have a massive star on the verge of having an event horizon, matter surounding the star will give out a burst of radiation and fall below the event horizon within finite external time, that way forming a black hole. Hawking found out to his suprise that the black hole continued to give out radiation - this is how he discovered `Hawking radiation'...I think something like that.

3. Aug 11, 2012

QuantumJon

The time it takes for you to fall into the Black Hole is not more than an hour, before hitting the event horizon. What it looks like to someone watching it is that you do not fall. You basicly stops there forever in time. Information goes down the Black Hole, but information is not lost (while Stephen Hawking tried to explain that the conversation of information was wrong) when it goes down a Black Hole. That is against all nature.

Did that answer it? I couldn't really understand the question fully.

4. Aug 11, 2012

julian

Classically the person falling into the black will pass the event horizon in a finite time as measured by themselves. But matter does fall into the black hole within a finite time as seen by an outside observer if you take into account quantum field theory on curved spacetime.

5. Aug 11, 2012

bcrowell

Staff Emeritus
The time you're referring to as infinite is the time measured by a distant observer who is receiving signals from the region of the black hole. If a transmitter at A, near the black hole, sends out flashes of light at an interval of 1 second, a distant observer at B sees these flashes as being more than 1 second apart. If B sees them 387 seconds apart, B says that time is passing for A at 1/387 the normal rate. This ratio approaches infinity as A approaches the horizon.

This makes it easy to answer the part of your question about the singularity. The singularity lies inside the event horizon, so no signal can ever get from it to a distant observer. That means that the notion of time measured by a distant observer doesn't make any sense when you apply it to the singularity. (In technical terminology, the notion of gravitational time dilation requires a static spacetime, so that we can define a gravitational potential. The Schwartzschild spacetime is static outside the event horizon, but not inside it.)

You didn't ask about the time required for formation of the event horizon, but that's not infinite either, even as seen by a distant observer. The statement about infinite time for infalling matter to cross the event horizon refers to a permanent black hole, but we're describing a black hole that formed at some time, so it wasn't permanent. There are solutions to the Einstein field equations, such as the Vaidya solution, in which formation takes finite time.

The part of your question about evaporation is answered here: http://cosmology.berkeley.edu/Education/BHfaq.html#q9

Last edited: Aug 11, 2012
6. Aug 12, 2012

Warp

Where is that arbitrary time coming from?

7. Aug 12, 2012

Naty1

Hi Valentin:
The time you measure via a clock you carry is always YOUR [local] proper time. You may see other distant clocks run at different rates relative to yours, but even though they will might see your clock run at a different rate relative to theirs, your's retains it's steady, fixed tick rate.

For an excellent recent discussion about black holes, see:
Black Holes- Two points of view.
https://www.physicsforums.com/showthread.php?t=626874

A key thing to remember in relativity is that different observers see [measure] different things....everything is RELATIVE.

A simple analogy: You stand next to a house and friend observes it from a distance, say a mile. You say 'the house is much bigger than a person;
your friend observes: 'that house sure looks tiny from here". Who is right? Both. But to
recognize such observational differences in relativity, as well as real differences, takes some
time to understand.

'time for crossing is infinite'....nope, depends on your frame of reference

Crowell:
I would add: that is based on an idealized model, not our lumpy galaxy. Crowell has a nice explanation which relates to that:

'so no singularity'....not in the sense you probably mean; are you referring to the apparent time 'singularity' at the horizon, or the divergent curvature at the center of the BH?

'no loss of information'.....Hawking has conceded that point. But that doesn't mean we can read [retrieve] all the information in practice....

Last edited: Aug 12, 2012
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