Celestia Forums

I recently watched a show about black holes. It revisited Hawkings assertion that data is lost. Then, some other dude, whos name escapes me, set out to prove Hawkings wrong and eventually got a moderate admission of possible error, maybe. Anyway during the show this other dude described what an observer would see should a companion fall into a black hole. He maintained that as the person got nearer, the observer would notice the person slowing down due to time dilation to the point that the falling companion would appear to stop completly to the observer. I let that sink in for a moment, then I began to consider a star doing the same thing. This brought me to my conundrum. If a star (or a falling companion) is fozen in time, how can I see it at all? Hasn't the photon rate, the apparent brightness of the star, been affected by the time dilation, to the point that I can't see it at all anymore?

That's one of the reasons it's called a black hole (in English, anyhow). Light cannot escape from below the event horizon.

However, as a practical matter, there's usually plenty of stuff still above the event horizon, and it's radiating like crazy.

Yes, I know, and I expected someone would point that out to me, but this implies that the reason that light cannot escape is due to time dilation, not imense gravity. In the latter case the light is still radiating but being bent 180 degrees and absorbed. In the time dilation scenario, the light isn't radiating at all. The conclusion I reach is that what would be viewed as the event horizon is actually matter suspended in time blocking our view of what lies behind it, not just a gravitational threshhold.

Time depends on gravity, thus it has the same effect.
Some time ago we discussed similar question about time-phenomena here:
http://shatters.net/forum/viewtopic.php?f=7&t=16125
Maybe you are interested...

Michael

I understand that, but if the event horizon is actually matter that someone could theoreticaly touch, versus just a boundary like an invisible fence, that challenges the current black hole model.

The event horizon is not made of matter. It is only a boundary, and locally everything behaves normally there. A person passing through it would notice nothing unusual.

jeffmack wrote:The event horizon is not made of matter. It is only a boundary, and locally everything behaves normally there. A person passing through it would notice nothing unusual.

Perhaps I am not understanding here...
I thought that a person passing through the event horizon would notice spagetification?
No?

Thanks, Brain-Dead

The singularity is the limit of our (not mine I add) understanding.

BobHegwood wrote:Perhaps I am not understanding here...
I thought that a person passing through the event horizon would notice spagetification?
No?

As you get further from a celestial body, it takes less energy to get away (escape velocity). The event horizon is simply the altitude above the singularity where the escape velocity is the speed of light.

The "spaghettification" would come in once the object is close enough to the singularity for the gravitational tides across your body to tear you apart. It's similar to the tidal disruption of small moons that get too close to their planet. The altitude from the black hole that this would occur at is the Roche limit.

Where the Roche limit is in relation to the event horizon depends on a number of factors, including the mass of the black hole, the size of your body (being small gives you an advantage), and so on.

For smaller black holes, tidal disruption may occur near or before the event horizon, but for black holes of much higher mass, it could happen well after one crosses the event horizon.

In summary, the two altitudes (event horizon, Roche limit) aren't well correlated.

Edit:

Think of it this way. The event horizon is where the gravitational attraction between you and the black hole reaches a critical value (determined by the speed of light).

The Roche limit is where the slope of that gravitational attraction with decreasing altitude reaches a critical value (depending on black hole mass, your radius, how well you're put together so as to resist the tidal effects, and so on).

Hmmm.... haven't made spag bol for a while... this has got me thinking!

Yes, Hungry4info pretty much said it. When I said that a person crossing the event horizon wouldn't notice anything unusual, it was, of course, under the assumption that he hadn't already been torn apart by tidal forces, or killed by intense radiation, etc.

The event horizon has more relevance to an outside observer than to a local observer. A local observer crossing the event horizon would simply not be aware of anything changing - it would be a "non-event" for him. An outside observer can never see anything actually reach the event horizon for two reasons: 1) It would take an infinite amount of time (from his perspective), and 2) The light from the object would be too red-shifted and dim to see.

As Hungry4info said, the event horizon is not correlated to the Roche limit. Indeed, one would be ripped apart by tidal forces well before reaching a neutron star (or even less massive object) - and it doesn't even have an event horizon. Crossing the Roche limit would definitely be "eventful!"

Okay now all of that has been said I can reinterate my postulate. If, to the outside observer, all the matter that is captured by the BH:
a) takes an infinte amount of time to pass into the event horizon
b) has had its radiation too red shifted/dimmed to be seen
Doesn't it follow that what we might see is a black area of space that is actually matter frozen in time, and not a hole?

Thanks all very much for the education.
Much appreciated by the Brain-Dead...

http://forum.celestialmatters.org/viewtopic.php?t=402

Fridger

I grow weary of closed mindedness. 5 years ago everybody was sure that the universe expansion was slowing down. Now we know differntly. I am not at all comfortable with the current BH model. It may be exactly correct, but I still have questions. Currently the model allows for atomic nucleus sized, (and samller) BH's. I question that as well. Feel free to hold onto your beliefs, I'll hold on to my questions.

Unfortunately, in order to be able to discuss the topic usefully, you have to understand and be able to use the mathematics that describes what's happening. Trying to discuss it using concepts based on poorly understood, misleading or mistaken English analogies simply isn't particularly useful. While such discussions can be interesting in their own right, you have to remember that they're really just fantasies. Conclusions drawn from such discussions are likely to have nothing in common with reality.

mburley wrote:Doesn't it follow that what we might see is a black area of space that is actually matter frozen in time, and not a hole?

Not quite. Remember that to see something, light from it has to reach your eye. A photon at the event horizon cannot escape the event horizon and cannot be received by the eye.

mburley wrote:I grow weary of closed mindedness.

You're in luck! Most of us here are fairly scientifically minded, so we understand and appreciate investigating alternatives, testing hypotheses, and going with what the evidence suggests.

mburley wrote:5 years ago everybody was sure that the universe expansion was slowing down. Now we know differntly.

The accelerating expansion of the Universe has been known since 1998.

mburley wrote:I am not at all comfortable with the current BH model

What about it bothers you?

mburley wrote:I question that as well. Feel free to hold onto your beliefs, I'll hold on to my questions.

I can't speak for others, but me personally, it isn't much a matter of belief. The available evidence points toward a conclusion. Do hold onto your questions, continue to ask them and learn. How ever would we learn without them?

edit: selden makes a very good point. You can't realistically claim to have a problem with something if you don't understand it, and you can't understand black holes very well outside the mathematics of it.

There might be a better definition of what a black hole is on the quantum level.
Fuzzball (string theory) http://en.wikipedia.org/wiki/Fuzzball_% ... _theory%29
In a nut shell it propose to solve the both the information paradox and the singularly problem by calming that for all intents and purposes its a star (used loosely) that is composed of strings, hence fuzzball and follow the mass-density inverse-square rule.

Form what i know and have read this seem to be a better description. it still doesn't change the fact that once you fall in you cant get out though, or not in one piece, only as hawking radiation.