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Question about gravity

Question about gravity - Physics Forum

Question about gravity - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 01-18-2005, 02:22 AM
Greg Merideth
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Default Question about gravity



I've only recently started reading about string's and black holes and
have only a basic introduction to math (just college calc) so I cannot
provide any math to explain what I'm asking but I'll try like hell to
follow along with any responses.

After reading several papers about black holes I had a question about
gravity itself. Aside from the "what is" gravity part, if black holes
have at their center an infinite point of gravity so that not even
photons can escape to a large distance then:

a) How can you apply a formula like gravitational force against an
object using an infinite value like the black holes singularity?
Wouldn't black holes pull with an infinite force against all matter
regardless of distance? It seems that using a gravitational force of
infinite in a formula wouldn't work so on to point b.

b) If the exact center of a black hole has a singularity of infinite
gravity then what is the gravitational pull one inch away from the
singularity? How do you take a value of infinite and subtract one inch
from it to get a measureable value?

If "just" the singularity has infinite volume + gravity then the space
next to it must have some form of measureable gravitational force(?)

c) From reading, I would take it that gravity has no effect on itself?
If photons cannot escape from an event horizon, yet, gravity can, then
not even the tidal forces of a black hole can prevent gravity itself
from escaping? It would seem that gravity almost exists outside of what
we experience yet acts on us locally?

I've searched through google to find some answers and found a few "ask
the scientist" pages but none have answered what I'm sure are common
questions which hopefully someone can shed some light (no pun) on.

Thanks.
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  #2  
Old 01-18-2005, 03:01 AM
tadchem
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Default Question about gravity


"Greg Merideth" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...


The gravity of a black hole is not infinite.

A black hole occurs when the escape velocity V of a gravitational field
exceeds the speed of light in a vacuum c. If M is the mass of any body and G
is the gravitational constant, then at any distance R the escape velocity is

V = sqrt(2*G*M/R)

for this velocity to become greater than c, both M must become very large
and R mus become very small, so that

M/R > c^2/(2*G)


Again, the gravity of a black hole is not infinite; it is simply *VERY*
large. Also the singularity occurs only at a distance of 0. If R>0 then
you are no longer 'at' the singularity.


In calculus, the practice is not to try to move *away from* the singuarity
but towards it. At the singularity there are quantities that are
*undefined*. You can't take an undefined quantity anywhere. Try
approaching the singularity from a point where all quantities are defined
and all functions are continuous and differentiable.

From *outside* the black hole we can observe the effect the intense gravity
has on matter and energy near it, as a function of distance from its center.
That allows us to estimate M in the gravitation equations.


Correct.


You are evidently trying to visualize gravity as a particle (gravitons?) or
an energy field (gravity waves?).

So far the most accurate model we have for gravity is a scalar field (has
your calculus class touched upon that concept yet). The scalar field that
works best is one that applies a scalar quantity to every point in space
that we call a curvature (a variable that affects how space itself and the
measurement of distance changes from point to point). As a property of the
space per se, it *affects* matter and energy, but it is not matter OR
energy, so it does not affect itself.

"Tidal forces" are an entirely different problem from simple gravitation,
and you would be well-advised to try to avoid using the term until you can
handle the concepts of differential gravitation, such as how the gravitation
of the sun and moon are different at different points on the earth's
surface. In regards to a black hole, tidal forces are relevant to when,
where and how objects that are drawn in to a powerful gravitational 'well'
are ripped asunder into component particles.


Tom Davidson
Richmond, VA


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  #3  
Old 01-18-2005, 03:20 AM
N:dlzc D:aol T:com \(dlzc\)
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Default Question about gravity

Dear Greg Merideth:

"Greg Merideth" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

This is the central singularity... once matter falls in, we can't say
*anything* about the distribution of matter inside.


This phenomenon occurs at the event horizon... doesn't require a
singularity to occur.


Because it is not requied to be a singularity.


Gravity is what gravity is, it falls off by about 1/r^2. The answer is
still that the Universe sees only so much matter attributed to the black
hole, and treats it so.


Since the singularity is a problem only for the inside of the black hole,
you don't need to worry about it.


Our space (the one inch) doesn't apply inside the event horizon.


Why would it matter?


No.


Space does not curve space. Correct.


The space that is a product of the matter in a black hole, never falls into
the hole.


Gravity does not propagate, right. Once you get into quantum gravity, you
have the graviton which is a virtual particle. Virtual particles are not
limited to c...


I probably didn't help.

David A. Smith


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  #4  
Old 01-18-2005, 04:34 AM
Greg Merideth
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Default Question about gravity

Actually both your post and Tom's helped a great deal, now I have a much
better understanding of what I was asking and where I was asking using
invalid points.

Thank you both.

N:dlzc D:aol T:com (dlzc) wrote:
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  #5  
Old 01-18-2005, 06:03 AM
N:dlzc D:aol T:com \(dlzc\)
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Default Question about gravity

Dear Greg Merideth:

"Greg Merideth" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

Tom rocks!

David A. Smith


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  #6  
Old 01-18-2005, 06:16 AM
ošin
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Default Question about gravity

>> Actually both your post and Tom's helped a great deal, now I have a much

Yes, he does seem to rock much of the time. But I have heard rumors that he
is a petulant, ill-mannered rocker.


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  #7  
Old 01-18-2005, 11:01 AM
tadchem
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Default Question about gravity


"ošin" <oš[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

he

I an *NOT* petulant!

I'm NOT.
I'm NOT!
I'm NOT!!

<wink>

As I have written elsewhere...
"If you think the shoe fits, then by all means feel free to try it
on, but don't complain to the shoemaker if you think its ugly. He didn't
make it just for you."

I've been called a lot worse than 'petulant' and 'ill-mannered,' and with
more accuracy.



Tom Davidson
Richmond, VA


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  #8  
Old 01-18-2005, 03:43 PM
Theo
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Default Question about gravity



Just a question.
Doesn't a gravitational field contain energy
which also generates a garvitational field.

Wasn't the precession of mercury explained by adding the gravitational
force increase from the Sun.




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  #9  
Old 01-18-2005, 06:25 PM
Morituri-|-Max
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Default Question about gravity

Theo wrote:

As he said in his example, gravity is not a field, it is more a scalar.
Since it isn't a field there is no energy involved as such.



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  #10  
Old 01-18-2005, 06:32 PM
TechMind
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Default Question about gravity

Actually the gravitation formula does not fit the "center" of the black
hole.
It is a result from Einstein's theory, which he thinks is wrong.

gravity can be taken similiar to the curvature of space-time.
So it cannot itself.


"Greg Merideth" <[Only registered users see links. ]> ???
news:[Only registered users see links. ] ???...


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