infinite density in black holes?

[foofighter]

when we talk about a black hole as being infinitely dense, is that not a figure of speech for "very very dense more than any other object we know of yet?" if it was really TRULY infinitely dense, would not the event horizon match the outer boundaries of the universe?

[ajb]

Well, the equations that describe gravity within the framework of general relativity say that the curvature is infinite.

 

But it is generally believed that quantum mechanics would smooth out this infinity.

 

So my answer to your question is what do you mean by TRULY? In general relativity yes, in nature probably not.

[foofighter]

isn't that one of the shortcomings of relativity, what makes it an incomplete theory, that there exists singularities? because in nature, even the start of the big bang, assuming that the universe is finite in space and material, the density and pressure had to be finite (although VERY high), by common sense at least this would seem to have to be, even though GR's equations might point otherwise. how much more so black holes, which form from stars of finite matter and don't encompass the entire universe.

[ajb]

I would interpret the presence of singularities as the failure of general relativity under "extreme conditions". This suggest that general relativity needs modification on these scales and that maybe quantising gravity would resolve the issues.

[Martin]

when we talk about a black hole as being infinitely dense, is that not a figure of speech for "very very dense more than any other object we know of yet?" if it was really TRULY infinitely dense, would not the event horizon match the outer boundaries of the universe?

 

I agree with AJB. I think he has given you correct responses to your questions as far as I can understand your questions.

 

another way to respond would be to stop you right away when you say

"when we talk about a black hole as being infinitely dense," and say

we should not be talking about a black hole being infinitely dense because that is bull that they put in books to sell the public

so lets forget all about that.

 

So then what is your question? Do you have a question about black holes or something else that we can try to answer?

 

I think it was respectable that you came to the conclusion by yourself that there was something fishy about the pop-sci description of the blackhole singularity

[foofighter]

now that i have been assured that it is not truly infinitely dense, my question dissolves. weird how i read that misconception everywhere, including "a briefer history of time."

[Martin]

now that i have been assured that it is not truly infinitely dense, my question dissolves. weird how i read that misconception everywhere, including "a briefer history of time."

 

about "briefer history", get rid of it but don't give it to anybody you like.

I'd say landfill but then it might turn into methane which is worse than CO2,

given what we now know about greenhouse gasses the best thing to do with the book is burn it.

my mother-in-law gave me "brief history of time" and it went to landfill long ago, since there was nobody I despised enough to give it to

 

foo, we have to find some real science reading to start you on. I need to know your educational level.

reading pop sci cråp just makes things worse.

 

Friends and fellow SFNies!

All here who love Physics, Cosmology, Astronomy!

 

give us your suggestions of things on the web that FOO could look at!

 

I am getting foofighter into focus and I think he/she is a college grad with major in ELEMENTARY EDUCATION, who had some calculus in highschool.

 

the thing about foo is he/she ASKS QUESTIONS and seems to think critically (foo figured out that there really isn't infinite density in Nature, just in theories where they break down, nobody had to tell foo this)

 

let's call foo a HE for convenience.

 

WHAT WE NEED to serve foo properly is a list of source material that is NOT POPULARIZATION POTBOILERS that people write for the mass market.

but which someone who has forgotten highschool calculus can understand!

 

this kind of stuff is very very hard to find, maybe it doesnt exist.

 

but I have a suggestion.

 

In November 2005 Roger Penrose gave a talk to FELLOW MATHEMATICIANS AND PHYSICISTS AT THE NEWTON INSTITUTE AT CAMBRIDGE which was entirely illustrated by funny cartoons which he had drawn himself.

(I've seen several of his talks including in person and he always seems to communicate visually with these cartoons that he draws with different color felt-tip pens0

He puts up a slide and then he says very perceptive things about it. His lectures are MATHEMATICALLY DEEP BUT DONT HAVE MUCH EQUATIONS.

 

this is not pop sci. this is for other math/phys people

 

I think some of Penrose ideas are wrong, but SO WHAT?, it is better to have someone who communicates and who is attacking real basic problems, so at least you get a whiff of what the questions are. If his answer is cock-eyed that is just a minor drawback.

 

So I will get a link to that online slide/audio lecture and see if Foo likes it.

Any other suggestions?

 

here is the talk Penrose gave

 

 

http://www.newton.cam.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/

 

at the Isaac Newton Institute at Cambridge U, in November 2005

suggest watching it even if you think you arent understanding anything because the pictures will percolate into brain and begin to think for themselves

[foofighter]

martin - i am male lol. and i don't mind relearning calculus as the first step if that would give me a better way to comprehend things before i tackle physics. its not like i have a deadline to learn these things so if i am able to commit the time to learn calc do u suggest doing so before approaching physics - or should i jump into physics and calc simultaneously? i'm still in college doing elem ed, btw - i haven't graduated yet.

[Martin]

...or should i jump into physics and calc simultaneously? ...

 

that sounds like a plan.

 

get a college physics book you like, with lots of interesting photographs and diagrams and exercises

 

and work on the chapter that you like----reviewing the calculus that you need for that chapter as you go along.

 

or likewise get a GENERAL ASTRONOMY textbook that you like and read the chapters that interest you the most and review the physics and math you need for that particular chapter, as it comes up.

 

the most important equation in cosmology is called the Friedmann equation(s)

 

it is actually two, but for some reason people always refer to it in the singular.

 

It is a really simple ordinary differential equation that determines a simple function of time a(t) called the "scale factor"

a(t) is the size of the universe

the friedmann equation determines how the size of the universe grows.

 

you have to be comfortable with things like a'(t) the first derivative of a(t)

 

and equations explaining how a'(t) changes with time.

 

a'(t)/a(t) is a fraction which is called the Hubble parameter H(t).

 

you should realize easily that this fraction is the reciprocal of a time.

that is, you need a basic understanding of units of measurment

 

this is very basic and not hard to learn, but it shows why Freshman college courses are so useful. they teach you way more than you need, but when you walk into Cosmology class and see a simple thing like a'(t)/a(t) and the friedmann equation, which is THING ONE, then it doesnt frighten or confuse you. you realize that it is one over a time----the reciprocal of a time.

 

 

because a' has the dimensions of a speed, and a has dimension of distance

 

and any speed divided by any distance is the reciprocal of time. right?

 

but in a popular book about cosmology, if you said that in the first chapter and showed the friedmann equations----90 percent of the people would leave immediately, or so we think (I dont know if it has ever been tried)

 

You have to do what it is you feel like doing. jumping into Freshman physics and calculus simultaneously sounds like a good idea if that is what you want to do

 

you could always try basic astronomy and cosmology later, and the math wouldnt be so apt to freak you out

[foofighter]

thanks for the info