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Post by speakertoanimals on Dec 8, 2010 19:00:58 GMT 1
We know from observations that there are some very dense, very energetic objects out there: en.wikipedia.org/wiki/Black_hole#Observational_evidenceOkay, the accretion disks of black holes, but given that even if they are real black holes, we wouldn't expect to see anything beyind the event horizon anyway, they are the best candidiates we are going to find. ANd even if we did travel to one, and a brave explorer dropped through the event horizon and discovered what was REALLY going on in there, they wouldn't be able to send out what information they had gathered. A point I should have made earlier -- for a really big black hole, it doesn't have to that dense before it disappears behind its own event horizon, the density of water, for example. SO perhaps we should instead be asking: Is it possible for gravitating objects in our universe to form event horizons, and what behaviour would we expect to see if they do? rather than getting in a tizzy about the predictions of infinite density from a purely classical theory of gravity. The previous question is actually quite benign, given that we already know that gravity can effect light, so all we need to do then is consider an object whose escape velocity is lightspeed in effect, a consideration that comes in even if you just use simple newtonian gravity. Either there is some unknown cut-off that prevents us from piling enough matter up to form such an object (which would seem odd), or the finiteness of lightspeed and the fact that light is effected by gravity inevitably lead to the concept of objects becoming massive or dense enough that they can create event horizons around themselves, and stop even light escaping. Far from being a fallacy, it is instead unavoidable under even very mild assumptions, which is why the basic idea of black holes predates relativity.
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Post by speakertoanimals on Dec 9, 2010 14:32:00 GMT 1
Wrong.
The error is in the basic maths.
Lets take a function that falls off as 1/r. Which means at a distance 1, it takes the value 1, and distance 2 the value 1/2 and so on.
At distance 0, we get infinity. Does that now mean that we get infinity at distance 1? Of course NOT!
We could also take the function 2/r. It is also infinite at r=0, but different to 1/r at any finite distance.
In gravitational terms, we have 1/r^2 rather than 1/r. The difference between 1 and 2 is the total mass of the gravitating object.If we arange the mass as a sphere, for gravitational fields outside the sphere, it doesn't depend what SIZE the sphere is (leaving event horizons aside for the moment), because the TOTAL mass remains the same. Hence even if the sphere goes to zero size and infinite density, that DOESN'T somehow create infinite gravity all around it, as you mistakenly seem to think!
In fact, the original statement is soooooooooooooo daft that I suspect that even missus isn't dim enough to mean it seriously.......................
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Post by Progenitor A on Dec 9, 2010 14:54:00 GMT 1
Wrong. The error is in the basic maths. Lets take a function that falls off as 1/r. Which means at a distance 1, it takes the value 1, and distance 2 the value 1/2 and so on. At distance 0, we get infinity. Does that now mean that we get infinity at distance 1? Of course NOT! We could also take the function 2/r. It is also infinite at r=0, but different to 1/r at any finite distance. In gravitational terms, we have 1/r^2 rather than 1/r. The difference between 1 and 2 is the total mass of the gravitating object.If we arange the mass as a sphere, for gravitational fields outside the sphere, it doesn't depend what SIZE the sphere is (leaving event horizons aside for the moment), because the TOTAL mass remains the same. Hence even if the sphere goes to zero size and infinite density, that DOESN'T somehow create infinite gravity all around it, as you mistakenly seem to think! In fact, the original statement is soooooooooooooo daft that I suspect that even missus isn't dim enough to mean it seriously....................... Read what I said you dim clown! You will find that the concept 1/0 = infinity is a mathematical concept that has no counterpart in the real world That was the point of the post ALL real physicists are acutely embarrassed by physical concepts such as infinity. That you , someone that can hardly express her thought processes articulately, and can barely construct a grammatical sentence, bandy indinity around as if it is second nature shows you to be fraud that you are Physicist! Bah Humbug
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Post by speakertoanimals on Dec 9, 2010 18:49:16 GMT 1
The only point of your post, as far as I can see, was to have a mathematically inept and incorrect series of statements (such as the supposed gravity at earth caused by a black hole elsewhere), trying to show that black holes make no sense.
Just because a simple infinity cropped up in the naive, classical maths. Not that anyone thinks that it matters that much because quantum gravity will alter those results at the smallest scales anyway.
I read what you wrote, and it is nonsense. You said it, don't try to deny it. You are claiming that since 1/r equals infinity at 0, it equals infinity at finite r, which is obvious nonsense.
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