Beginner question about gravity

I know I'm getting into uncharted territory here, but:

I've been doing some reading over the years about black holes and
detecting extra-solar planets by looking for the doppler shift from
the wobble around a center of gravity.

I understand that a black hole (theoretical as it might be) is an
object that has a mass so high that the escape velocity exceeds the
speed of light.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

What makes this even tougher on my brain, is that one of two
conditions appear available to me, when you consider two bodies, Like
a big planet (Jupiter) and a sun, separated by a significant distance
(say 5-10 AU's):
1. The objects are attracted to where each other is, which means
gravity is instantaneous?
2. The objects are attracted to where each other was, which means
gravity has a speed?

I'm sure I'm missing a key point, but I can't quite figure it.

Any suggested reading is appreciated.

Thanks,

Ed L.

Ed L. wrote:
I know I'm getting into uncharted territory here, but:

Not at all. This is a somewhat hot topic. The April 2003 issue of
Sky and Telescope has a News Notes about it.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

The best formulation we have of gravity is general relativity. It is
a classical theory. By that we don't mean that it's simple or very
old--only that it doesn't contain anything about quantum mechanics in
it. One of the holy grails of physics is the unification of general
relativity with quantum mechanics.

Although that unification hasn't been achieved yet, there are sound
theoretical reasons to believe that there is a particle called the
graviton which is the so-called exchange particle of gravity, that it
is massless, and that it has a number of other properties which I won't
get into.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

According to the most common forms of general relativity, gravity travels
exactly at the speed of light. One reason this makes sense with our
notions of the graviton is that the graviton is supposed to be a massless
particle (like the photon), and massless objects "ought" to travel at the
speed of light.

So, if the graviton only travels at the speed of light, and light can't
escape beyond the event horizon of a black hole, how can the black hole
exert any gravitational influence? I'm just speculating, but in general
relativity, gravity isn't a point-to-point force, as it is in Newton's
theory of gravitation, but is instead a curvature in space-time.
Gravitons just outside the event horizon can escape the black hole, and
maybe it is they that convey the intense space-time curvature.

What makes this even tougher on my brain, is that one of two
conditions appear available to me, when you consider two bodies, Like
a big planet (Jupiter) and a sun, separated by a significant distance
(say 5-10 AU's):
1. The objects are attracted to where each other is, which means
gravity is instantaneous?
2. The objects are attracted to where each other was, which means
gravity has a speed?

The good money is on 2, but it's not been proven. There was an experiment
recently where the bending of a beam of starlight by Jupiter was measured
precisely. The experimenters discovered that the bending of the light
beam was retarded from what it would have been if the speed of gravity
were infinite. From the amount of the delay, they concluded that the speed
of gravity and the speed of light are equal, to a loose precision of about
one part in four or so.

Or so they thought. Other physicists think they've only derived a
measurement of the speed of light, albeit by a novel means. There's
apparently a debate being exchanged in journal communications over what
the experiment has actually measured. The rest of us will probably just
have to wait.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

I'm just starting "The Elegant Universe" by Brian Greene and would recommend
it for a layman's explanation of such questions below...



"Ed L." wrote in message
m...
I know I'm getting into uncharted territory here, but:

I've been doing some reading over the years about black holes and
detecting extra-solar planets by looking for the doppler shift from
the wobble around a center of gravity.

I understand that a black hole (theoretical as it might be) is an
object that has a mass so high that the escape velocity exceeds the
speed of light.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

What makes this even tougher on my brain, is that one of two
conditions appear available to me, when you consider two bodies, Like
a big planet (Jupiter) and a sun, separated by a significant distance
(say 5-10 AU's):
1. The objects are attracted to where each other is, which means
gravity is instantaneous?
2. The objects are attracted to where each other was, which means
gravity has a speed?

I'm sure I'm missing a key point, but I can't quite figure it.

Any suggested reading is appreciated.

Thanks,

Ed L.

"Ed L." wrote:

I know I'm getting into uncharted territory here, but:

I've been doing some reading over the years about black holes and
detecting extra-solar planets by looking for the doppler shift from
the wobble around a center of gravity.

I understand that a black hole (theoretical as it might be) is an
object that has a mass so high that the escape velocity exceeds the
speed of light.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.


Sorry to overwhelm you, but there is a wealth of wonderful information
on the world wide web for those interested in finding out more about
gravitation. Here is some of the good stuff.

Fundamental Forces
http://scienceworld.wolfram.com/phys...talForces.html

Gravitation
http://scienceworld.wolfram.com/physics/Gravity.html
http://scienceworld.wolfram.com/phys...elativity.html

Ref: Hartle, "Gravity: An Introduction to Einstein's General Relativity", Addison
Wesley (2003)

"A few properties of the gravitational interaction that help explain when
gravity is important can already be seen from the gravitational force law

F_grav = G m_1 m_2 / r_12^2

o Gravity is a universal interaction in Newtonian theory between all mass, and,
since E = mc^2, in relativistic gravity between all forms of energy.

o Gravity is unscreened. There are no negative gravitational charges to cancel
positive ones, and therefore it is not possible to shield (screen) the gravitational
interaction. Gravity is always attractive.

o Gravity is a long-range interaction. The Newtonian force law ia a 1/r^2
interaction. There is no length scale that sets a range for gravitational
interactions as there is for the strong and weak interactions.

o Gravity is the weakest of the four fundamental interactions acting between
individual elementary particles at accessible energy scales. The ratio of
the gravitational attraction to the electromagnetic repulsion between two
protons separated by a distance r is

F_grav G m_p^2 / r^2 G m_p^2
-------- = -------------------- = ------------- ~ 10^-36
F_elec e^2 / (4 pi e_0 r^2) (e^2/4pi e_0)

where m_p is the mass of the proton and e is its charge.

These four facts explain a great deal about the role gravity plays in physical
phenomena. They explain, for example, why, although it is the weakest force,
gravity governs the organization of the universe on the largest distance
scales of astrophysics and cosmology. These distance scales are far beyond
the subatomic ranges of the strong and the weak interactions. Electromagnetic
interactions COULD be long range were there any large-scale objects with net
electric charge. But the universe is electrically neutral, and electromagnetic
forces are so much stronger than gravitational forces that any large-scale net
charge is quickly neutralized. Gravity is left to govern the structure of the
universe on the largest scales.

Background:
http://scienceworld.wolfram.com/phys...wtonsLaws.html
http://scienceworld.wolfram.com/physics/Gravity.html
http://scienceworld.wolfram.com/biography/Newton.html

The theory of general relativity describes the phenomenon of gravity very differently:
http://scienceworld.wolfram.com/phys...elativity.html

Observational and Experimental Evidence Bearing on General Relativity
http://math.ucr.edu/home/baez/RelWWW/tests.html

General Relativity Tutorial
John Baez
http://math.ucr.edu/home/baez/gr/gr.html

Relativity on the World Wide Web
http://math.ucr.edu/home/baez/relativity.html

General Relativity and Cosmology FAQs
http://math.ucr.edu/home/baez/physics/

Developments in General Relativity: Black Hole Singularity and Beyond
http://arxiv.org/abs/gr-qc/0304052

Improved Test of General Relativity with Radio Doppler Data from the Cassini Spacecraft
http://arxiv.org/abs/gr-qc/0308010

What is the experimental basis of Special Relativity?
http://math.ucr.edu/home/baez/physic...periments.html

Physics is an experimental science, and as such the experimental basis for
any physical theory is extremely important. The relationship between
theory and experiments in modern science is a multi-edged sword:

1. It is required that the theory not be refuted by any experiment within
the theory's domain of applicability.
2. It is expected that the theory be confirmed by a number of
experiments which cover a significant fraction of the theory's
domain of applicability.
3. It is expected that the theory be confirmed by a number of
experiments which examine a significant fraction of the theory's
predictions.

Special Relativity (SR) meets all of these requirements and expectations.
There are literally hundreds of experiments which have tested SR, with
an enormous range and diversity, and the agreement between theory and
experiment is excellent. There is a lot of redundancy in these experimental
tests. There are also a lot of indirect tests of SR which are not included
here. This list of experiments is by no means complete!

Other than their sheer numbers, the most striking thing about these
experimental tests of SR is their remarkable breadth and diversity. An
important aspect of SR is its universality - it applies to all known physical
phenomena and not just to the electromagnetic phenomena it was
originally invented to explain. In these experiments you will find tests
using electromagnetic and nuclear measurements (including both strong
and weak interactions); gravitational tests are the province of General
Relativity, and are not considered here, see Experimental Tests of GR.

There are several useful surveys of the experimental basis of SR:

Y.Z.Zhang, Special Relativity and its Experimental Foundations,
World Scientific (1997).
G.Holton, "Resource Letter SRT-1 on Special Relativity Theory",
Am. J. Phys., 30 (1962), p462.
D.I.Blotkhintsev, "Basis for Special Relativity Theory Provided by
Experiments in High Energy Physics", Sov. Phys. Uspekhi, 9 (1966),
p405.
Newman et al. Phys. Rev. Lett. 40 no. 21 (1978), p1355.

Zhang's book is especially comprehensive.

Textbooks which have good summaries of the experimental basis of
relativity a

M.Born, Einstein's theory of Relativity.
Bergmann, Introduction to the Theory of Relativity.
Moller, The Theory of Relativity.
M. von Laue, Die relativitätstheorie (in German).

Guys,

Thankyou very much for the great replies.

I will check out the reading links you've provided and see if I can't draw
further understanding from it.

Thanks

Ed L.

"Ed L." eladner@y wrote in message
m...
I know I'm getting into uncharted territory here, but:

I've been doing some reading over the years about black holes and
detecting extra-solar planets by looking for the doppler shift from
the wobble around a center of gravity.

I understand that a black hole (theoretical as it might be) is an
object that has a mass so high that the escape velocity exceeds the
speed of light.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

What makes this even tougher on my brain, is that one of two
conditions appear available to me, when you consider two bodies, Like
a big planet (Jupiter) and a sun, separated by a significant distance
(say 5-10 AU's):
1. The objects are attracted to where each other is, which means
gravity is instantaneous?
2. The objects are attracted to where each other was, which means
gravity has a speed?

I'm sure I'm missing a key point, but I can't quite figure it.

Any suggested reading is appreciated.

Thanks,

Ed L.

"Ed L." wrote in message
m...
I understand that a black hole (theoretical as it might be) is an
object that has a mass so high that the escape velocity exceeds the
speed of light.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

Ok a layman's answer to a layman's question (a good one which got me
thinking and searching the net this evening).

From what I understand the forces (EM, Weak, Strong and most probably
gravity) occurs from the exchange of virtual particles.

Now thoughout a vacuum particle/antiparticle pairs are created for a short
time as a result of Heisenberg's uncertainty principle. Now if these pairs
ar created close to the event horizon of a black hole one can fall into the
black hole and the other could escape.

I assume that like the photon the anti particle of a graviton is a graviton.
So could it be that the gravitons which carry the gravitational force from
the black hole do not come from inside the black hole but from these
graviton pairs produced close to the events horizon.

Anyone can point me to anything that confirms or denies this theory?

Stan

"Ed L." wrote:

I know I'm getting into uncharted territory here, but:

Nope, it's been chewed over for a long time.

What I'm having trouble with is: what is the speed of gravity.

Newton assumed gravity acted instantly at all distances.
In Einstein's models, gravity has a speed, and it's the
same as the speed of light. Both are due to the way
spacetime works. Experiments so far support the Einstein
model.

How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

This is where I think it gets interesting. As I understand it,
this is a great example of why black holes are important.
They're one of the places where relativity and quantum mechanics
both apply, but when you try to make gravitons follow the
relativistic model of gravity, the experiment jumps the tracks.
We need that unification of the two theories to give a full answer.

One short answer: even if gravitons can't escape the
black hole, the hole clearly makes a massive warp in spacetime.
Things outside the black hole can clearly see that warp long
before they pass inside the event horizon.

I second the suggestion to read "The Elegant Universe."
Or watch the PBS adaptation, if your local station is replaying
it soon.

--
Glenn Holliday

Sam Wormley wrote a wonderful reply

Leaving out only the aberation of light or conversely the
non-aberation of gravity. The aberation of light means that
sunlight 'looks' like it came from the position the sun was
8 minutes ago (hint it takes 8 minutes for sunlight to reach
the earth from the sun.) However, in the Keplerian sense,
planetary orbits would not be stable if the gravitational
vector of the sun pointed towards where the sun was 8 minutes
ago. Planetary orbits are only stable when gravity 'looks'
like it eminates from where the source is now! But this is the
nonreletivistic way to view the problem. When orbits are
computed from the space time distortion of the sun, stability
remains. E.g. SR solves this kind of problem--too.

Clear skies

Mitch

"Ed L." wrote in message
m...
I know I'm getting into uncharted territory here, but:

I've been doing some reading over the years about black holes and
detecting extra-solar planets by looking for the doppler shift from
the wobble around a center of gravity.

I understand that a black hole (theoretical as it might be) is an
object that has a mass so high that the escape velocity exceeds the
speed of light.

I've even heard of some ideas that there might be a particle that
transmits gravity, light a photon transmits electro-magnetism, that
I've heard called a 'Graviton'.

What I'm having trouble with is: what is the speed of gravity. How
can a black hole effect objects around it, if light can't escape, does
that mean that gravity travels faster than light? I'm not ready to
accept that.

What makes this even tougher on my brain, is that one of two
conditions appear available to me, when you consider two bodies, Like
a big planet (Jupiter) and a sun, separated by a significant distance
(say 5-10 AU's):
1. The objects are attracted to where each other is, which means
gravity is instantaneous?
2. The objects are attracted to where each other was, which means
gravity has a speed?

I'm sure I'm missing a key point, but I can't quite figure it.

Any suggested reading is appreciated.

Thanks,

Ed L.


Ed,

Great questions! Very insightful. If you aren't planning to
study physics for a living, you should reconsider.

Cheers, (and I don't have the answers either.)
Larry G.

The key is that gravity itself is a product of mass, but has no mass itself.
The black hole
(or any mass) warps space/time, and this probably would be at the speed of
light. But the mass causes gravity, it does NOT attract it.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)