What exactly is gravity?

[Thurlor]

Whenever I ask this question of somebody it always sounds dumb, but never the less no ones yet managed to answer it.
I (and almost everyone else) knows of the effects of gravity (something probably inevitable whyen one lives in a gravity well), but I dodn't know what actually causes it beyond an interaction of mass. A couple of specific questions I've often wondered about are;

1) Does gravity pull or push 'down'? More specifically is gravity am attractive forces between two or more bodies of mass, or is it a repulsive force of 'empty space' limited by the density of the mass beneath us?

2) What is the speed of gravity? I once came up with a silly little thought experiment, in which the earth's moon instantly dissapears (for whatever reason). Would we notice the gravitational changes instantly, or would there be the equivalent lag of the speed of light?

3) If one were standing on scales directly beneath the moon (at the equator) would one weigh less?

4) What exactly is gravity?

I know this will all seem pretty obvious to someone, but hopefully then they can answer my questions.

Thanks in advance.

[Cham]

Thurlor,

according to the modern theory of gravity (General Relativity), gravity isn't a force. It doesn't "pull" or "push". Gravity is the geometry of space-time (vacuum) itself. It just happens that matter (the Earth for example) curves the geometry of space AND time around. And moving objects (you) just feel and "follow" the geometry of that space-time. You are moving along the "straightest" path possible, permitted by that deformend geometry. There is no "force". Just geometry.

2) Speed of gravity is the same as for light waves in vacuo.

3) Yes, you'll be weighting a bit less. The moon "pull" (or "attract") the water on Earth (ocean tides), and it act the same on you.

It just happens that, in "normal" circumstences, it may be usefull to interpret gravity as a "pulling force". This was the interpretation in Newtonian physics. But actually, gravity is a deformation of the space-time geometry caused by the presence of matter.

[Thurlor]

Thanks that explains a lot.

So in regards to my second question, would there be a lag in the effects of the 'new' gravity system?

And I just realized that my third question is a little bit tricky, as wouldn't the scales also feel the same 'pull' (which isn't really a pull ) thus balancing out the difference.

[Cham]

So in regards to my second question, would there be a lag in the effects of the 'new' gravity system?

Yes. If the sun could just disappear right now, the Earth will still continue to move on its elliptical path for about 8 minutes before it start to fly straight in space. On the cosmological scale, gravity is "slow".

[GlobeMaker]

Hello Thurlor, Cham, and forum readers.
Here are some comments on gravity.

Cham said, "Gravity is the geometry of space-time (vacuum) itself.
It just happens that matter curves the geometry of space AND time around".

I agree. However, the word "curves" is used too much in explanations.
An alternative is to say that space is changing. Time is the essence of change.
Explanations are written about gravity bending the space-time continuum.
This is true, but our languages are not limited to the words, "curves, bends,
and warps" space-time. As an alternative wording, please consider the phrase,
"matter is changing space". This is accurate, since "change" is about "time passing"
in this context. I want to give the impression of motion for the colorless space.
People should not visualize a stationary bent 3 dimensional grid
and that imaginary time is somehow bent without visibility. Imagine a flowing
3D grid of space, being sucked into the Earth as a cost-free supply
of frames of reference. Time is not seen in this visualization method,
it is experienced as motion.
_____________________________________________________________

The gravitational force is a force. Units of kg*meter/(sec*sec)
The gravitational field is an acceleration. Units of meter/(sec*sec)
_____________________________________________________________

Thurlor said, "I know of the effects of gravity, but I don't know what
actually causes it beyond an interaction of mass". You and Einstein and Newton
share the same conclusion on this point. Newton's genius produced the famous
force field equation for gravity. Before Newton, there were no field equations, only
Kepler's Laws, which are not field equations. Newton theorized that mass causes
gravity, but only experiments told him how much gravity there is. The experiments
gave him the gravitational constant G.

G = 67 pico (cubic meters per (kilogram x second^2))

Einstein made improved field equations, but he made no advancement on how
to derive the gravitational constant G. His equations are derived from basic
principles, but G was not derived from basic principles. G is based on
experiments. Those scientists defined how gravity acts, but not why gravity
acts upon masses.
_______________________________________________________________

Between the time of Newton and Einstein, significant thought was put into
masses and their interactions by someone called Ernst Mach. Mach tried to
understand "why" mass behaves as it does. This link about Ernst Mach:
http://www.johnkharms.com/reference.htm
should not be read. Please do not go to this link. But if you do, please
disregard everything it says. Mach was a philosopher, he wondered, "Why"?

Thurlor asked, "What exactly is gravity? I don't know what actually causes it."
Mach also wanted to know. He never found out the answer.

[Malenfant]

I agree. However, the word "curves" is used too much in explanations. An alternative is to say that space is changing. Time is the essence of change. Explanations are written about gravity bending the space-time continuum. This is true, but our languages are not limited to the words, "curves, bends, and warps" space-time. As an alternative wording, please consider the phrase, "matter is changing space".

Why? To say that masses 'curve' or 'warp' space-time (not just space) is just as valid as saying they 'change' or 'affect' it. The rubber grid analogy may not be totally realistic but it's an effective visualisation nonetheless, and one that most people can get their heads around.

As for whether gravity is attractive or repulsive - while it is actually a manifestation of the curvature of spacetime, to all intents and purposes it appears as an attractive force; objects are attracted to large masses. It is never repulsive, though some other antigravity forces have been theorised (as yet unproven) to explain the expansion of the universe.

[Thurlor]

My specific question about gravity being 'attractive' or 'repulsive' was probably a little confusing (and now irrelevent, as it has been pointed out to me that gravity does not involve particles - gravitons). What I actually meant to ask if I can adequately explain this is;

Attractive gravity - Some force draws two masses together. This force is a localized occurance. For example we are drawn to the centre of the Earth's mass.

Repulsive gravity - Some force pusshes on everything from every direction with mass/density able to block this force. Thus we aren't drawn to the Earth's centre of mass, rather we are pushed towards it because it blocks the force from acting on us from beneath.

How do we know gravity is 'attractive' rather than 'repulsive' in regards to the above context?

[Cham]

Thurlor,

what you are talking about isn't physics. It's philosophy, or metaphysics. Both interpretations are completely equivalent, and they cannot be stated as physicaly relevent.

In the special case of gravity, there is no "pushing" or "pulling". There is no gravitational forces. However, you may ask why Newton's constant G is a positive number. This is a physically meaningfull question. Modern physics doesn't have the response, yet.

[Thurlor]

Thanks.

So what would happen if we changed G to a negative number in regards to other theories?

[Cham]

Thanks.

So what would happen if we changed G to a negative number in regards to other theories?

Then gravity would become "repulsive" (in presence of normal matter).

There's actually a "selection effect" occuring. If gravity was repulsive, we wouldn't be there to talk about it (no planets and stars could form). Because we are obviously here, the constant G had to be positive. This doesn't explain why G is a positive number, it is just philosophy.

[fsgregs]

Cham:

You suggested that gravity propagates at light speed. Remove the mass of the sun magically, and the warpage of space would not change at Earth for at least 8 minutes. However, I've encountered no report or reference yet that claims to have actually measured the "speed" of propagation of a gravitational "wave" or warpage of space. That is because we've found nothing that has suddenly appeared in the universe for us to measure its gravitational influence on. Has someone actually measured the speed of propagation of gravity, or is that still just a theoretical conjecture? Just curious!!

Frank

[Cham]

fsgregs,

there has been several indirect confirmations that gravity does propagates at the "c" velocity. Gravity waves have not been directly detected yet, but some pulsar binaries are showing things that are in full agreement with General Relativity (GR) and supports the theoretical claim that gravity propagates at the speed of light. However, this is a moot point, as Special Relativity (SR) doesn't allow any signal to propagate faster than light in vacuo. GR his built on SR, so gravity signals (waves) cannot go faster than "c", or else there would be some contradictions in the theory. If really gravitationnnal waves could go faster than light, we (in principle) could use them to violate causality (temporal communication, etc...).

[selden]

One discussion of the differences between Newtonian gravity and General Relativity can be found at http://math.ucr.edu/home/baez/physics/R ... speed.html

So far the only measurements seem to be indirect: the slowdowns of certain pulsars are predicted by GR, finding a transmission speed that's within about 1% of the speed of light.

[proregulus]

Hi all

I am new to this forum - and not a professional physicist (or even trained beyond 16 years exams = 'O' Levels).

However, I have very serious misgivings about the generally accepted ideas about gravity (amongst other things).

I believe there must be a much simpler and more logical explanation to how gravity works. One that also explains the things 'explained' by, for example. quantum mechanics.

I also think it will be amazingly simple, straightforward and logical - and will not call for multi dimentional universes, superstring theories, branes, etc.

I certainly do not believe in any conventional form of 'warped time-space continuum', time dilation, etc.

Still, that's just me - and I am sure someone out there will suggest that I just don't understand the theories!!?

Cheers

[selden]

How physics works isn't subject to belief. What matters is how well the various theories predict what we observe. So far as I know, there are no quantum-mechanical theories which can be used to accurately predict observed gravitational effects. Einstein's theory of General Relativity is still the most accurate tool for doing that.

Belief does affect the kinds of theories that physicists choose to try to develop, of course. And also results in emotional attachments to particular ways of describing events.

[t00fri]

Hi all

I am new to this forum - and not a professional physicist (or even trained beyond 16 years exams = 'O' Levels).

However, I have very serious misgivings about the generally accepted ideas about gravity (amongst other things).

I believe there must be a much simpler and more logical explanation to how gravity works. One that also explains the things 'explained' by, for example. quantum mechanics.

I also think it will be amazingly simple, straightforward and logical - and will not call for multi dimentional universes, superstring theories, branes, etc.

I certainly do not believe in any conventional form of 'warped time-space continuum', time dilation, etc.

Still, that's just me - and I am sure someone out there will suggest that I just don't understand the theories!!?

Cheers

to reinforce Selden's response from the professional side:

GR, extra dimensions, string theory with branes and all that are highly complex theoretical physics subjects. Given your background, it is perfectly understandable that these concepts appear contrived and untransparent to you. They are simply /impossible/ to grasp in any depth from reading popular science reviews, with the writers usually not understanding either... Just keep in mind that a research physicist receives highly specialized training until he/she is 27-28 (PhD) . Postdocs are then typically spending another 3 x 2 years (!) at leading international institutes to receive their "final polish" as scientists before they are considered "grown-ups" .

The concepts you mentioned then appear in a completely different light due to a MUCH deeper understanding of their intrinsic beauty and elegance ...Unfortunately, much much intensive study is necessary before one has a chance to see light at the "end of the tunnel".

Bye Fridger

[Vincent]

You can still read Brian R. Greene's Books "The elegant universe" and "The fabric of the cosmos" to slightly feel the intrinsic beauty and elegance of the string theory with branes even if you don't have the necessary intensive study requested by Dr Fridger...

[t00fri]

You can still read Brian R. Greene's Books "The elegant universe" and "The fabric of the cosmos" to slightly feel the intrinsic beauty and elegance of the string theory with branes even if you don't have the necessary intensive study requested by Dr Fridger...

Certainly, Vincent,

my reasoning is not "necessary and sufficient"!

My explanations above may serve more to make the critical minds understand a bit that the attractiveness of basic theories may well be hidden behind a host of apparent "complexity" that inevitably requires training to be surmounted...

Bye Fridger

[Vincent]

I completely agree with you Fridger.
And one of the reasons that explain this complexity is that since Einstein and the "Theory of relativity", physics deals with events that people can't really feel in their everyday life. That was not true with Newton's theory of "Universal Gravitation". So this is quite new, and a lot of time will be needed so that people can adapt their conception of the universe to the latest theory of physics.

[t00fri]

I completely agree with you Fridger.
And one of the reasons that explain this complexity is that since Einstein and the "Theory of relativity", physics deals with events that people can't really feel in their everyday life. That was not true with Newton's theory of "Universal Gravitation". So this is quite new, and a lot of time will be needed so that people can adapt their conception of the universe to the latest theory of physics.

True.

Bye Fridger