Question about gravity

JollyRoger · 26th January 2008, 08:17 PM

On Earth when an object is affected by gravity in that it falls to the ground.
In space an object produces gravity depending on the size and density.
When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space.

What Produces gravity? why does it exist? How does it work. Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

Zeuzzz · 26th January 2008, 08:30 PM

Originally Posted by JollyRoger

What Produces gravity? why does it exist? How does it work. Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

Gravity is though to be caused exclusively by the attraction of mass. How this attraction occurs is still not known, mainly due to the weakness of gravity compared to other forces which makes experiments testing its exact nature very hard without EM forces interferring with the experiment. Millikan's oil drop experiment is a fine example of this, where you are able to make an drop float by applying an electric field. When this is done the entire mass of the earth is pulling down on the drop, yet an electrical effect can counter it with ease, which shows how hard it is to directly test the mechanism by which gravity works.

Maxwell tried to explain gravity with electromagnetic effects, but with no avail, and it has since been considered to be a completely separate force from EM effects.

It doesn't help that the definition of mass is not very precise, it is explained in term of gravity, and gravity is explained in terms of mass. Wiki says; http://en.wikipedia.org/wiki/Mass

Quote:

Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object".

Hardly definitive to say the least, what exactly 'mass' is, is another question that is quite hard to answer too

steve s · 26th January 2008, 08:32 PM

Originally Posted by JollyRoger

...and the opposite is true in space.

Not sure I understand that part. Are you talking about creating artificial gravity by rotating a spacecraft? The same principle is at work. But if you're inside the spacecraft, you'll stick to the outer walls.

Quote:

What Produces gravity? why does it exist? How does it work.

Matter warps the fabric of spacetime. I'm sure you've seen the analogy of a heavy ball on a large sheet of rubber. Try googling "general relativity."

Quote:

Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

They do attract each other, but other forces (such as friction) are greater than any attraction due to gravity.

Steve S.

JollyRoger · 26th January 2008, 09:14 PM

Originally Posted by steve s

Not sure I understand that part. Are you talking about creating artificial gravity by rotating a spacecraft? The same principle is at work. But if you're inside the spacecraft, you'll stick to the outer walls.

I understand sentrifical force for the most part I was wondering the mechanism of Gravity what causes and it?

Quote:

Matter warps the fabric of space-time. I'm sure you've seen the analogy of a heavy ball on a large sheet of rubber. Try googling "general relativity."

yes I have but how does warping space time produce gravity

Quote:

They do attract each other, but other forces (such as friction) are greater than any attraction due to gravity.
Steve S.

not sure on this one but how would you have friction between to objects without gravity pulling the objects close enough to each other to produce friction

JollyRoger · 26th January 2008, 09:17 PM

Originally Posted by Zeuzzz

Gravity is though to be caused exclusively by the attraction of mass. How this attraction occurs is still not known,

I often wondered because they always seem to dance around the specific subject of its mechanism

MG1962 · 27th January 2008, 12:03 AM

Gravity is a myth

The Earth sucks

bobhope2112 · 27th January 2008, 12:25 AM

Originally Posted by JollyRoger

I understand sentrifical force for the most part I was wondering the mechanism of Gravity what causes and it?

There really is no such thing as a "centrifical"/centrifugal force. Just to be clear what we're talking about: People will describe the tendency for you to fly off of a fast moving merry-go-round as being acted upon by a centrifugal force. The reality is that it's just inertia at work. The merry-go-round is turning away from you, and if there is no force on you to make you turn with it, you'll keep going straight and fall off.

What causes gravity? Excellent question, but one that need not be addressed to have a coherent theory of gravity. As long as the theory describes our observations of gravity, it's of use. If a Grand Unification Theory ever pans out to describe all of nature's forces, en masse, I suspect we'll have a bit more of an answer as to what /causes/ gravity, but it may be too abstract to satisfy your practical sense.

Quote:

Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

Objects do attract each other here on Earth. In fact, it's the principal reason you don't go flying off this merry-go-round/planet as it spins. It is a relatively week force, though, so you won't see two billiard balls slamming together, like a couple of magnets, from their gravitational attraction. A subtler interaction comes to mind: If you hold a plumb bob next to a mountain, it will actually deflect a bit instead of pointing "straight down." The mountain's mass attracts the plumb bob, but the far dominant force will be the massive Earth.

Fredrik · 27th January 2008, 01:49 AM

Originally Posted by JollyRoger

On Earth when an object is affected by gravity in that it falls to the ground.
In space an object produces gravity depending on the size and density.

Gravity works the same on Earth as it does in space.

Originally Posted by JollyRoger

When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space.

Uh, no. Objects move in straight lines unless a force is acting on them. The gravitational attraction between you and the Earth is what's keeping you from moving in a straight line (i.e. from being "thrown" into space).

Originally Posted by JollyRoger

What Produces gravity? why does it exist? How does it work. Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

The same is true on Earth. What makes you think it isn't?

No one has a complete answer to the first two questions. There are however two good theories of gravity: Newton's and Einstein's. Einstein's is better in the sense that it agrees with the experimental results to a higher degree.

Newton's theory simply states that the magnitude of the force between two massive point particles with masses m₁ and m₂ a distance r apart, is m₁m₂/r² (in units chosen to simplify the formula), and that the force is attractive.

Einstein's theory is much more difficult to understand. It describes gravity as a geometric effect. Matter tells space how to curve, and space tells matter how to move. It's impossible to really understand Einstein's theory without learning differential geometry.

LordoftheLeftHand · 27th January 2008, 05:49 AM

Originally Posted by JollyRoger

I often wondered because they always seem to dance around the specific subject of its mechanism

This "mechanism of mass" is an interesting concept, it is sometimes called the Higgs Mechanism. The ATLAS experiment should be going online sometime this year. It is supposed investigate the Higgs Mechanism (which to this date has never been observed). You might be hearing a lot about this in the near future.

Here is a nice video about it:
http://atlasexperiment.org/multimedi...ure_atlas.html

LLH

sol invictus · 27th January 2008, 07:19 AM

Originally Posted by JollyRoger

When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space.

As Fredrik already pointed out, that's not true.

Quote:

What Produces gravity? why does it exist? How does it work.

One answer is that energy curves space (the rubber sheet with a bowling ball on it), and objects just move in straight lines as usual - but on that curved sheet. If you didn't know the sheet was warped, you'd say there must be a force acting to make the object move in a curve, and that's the thing we call the force of gravity.

Another answer is that the force is caused by the exchange of gravitons, which are a hypothetical type of elementary particle. Gravitons are emitted and absorbed continuously by anything with mass/energy, and this happens more often and more gravitons are involved if the object is more massive, which is why the force is stronger.

Quote:

Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

The same is true on earth.

robinson · 27th January 2008, 08:03 AM

Originally Posted by JollyRoger

What Produces gravity?
why does it exist?

Well, we are sure matter produces gravity. Because gravity doesn't exist without matter. Why and how this is so, is one of the greatest mysteries.

sol invictus · 27th January 2008, 08:19 AM

Originally Posted by robinson

Well, we are sure matter produces gravity. Because gravity doesn't exist without matter. Why and how this is so, is one of the greatest mysteries.

Why do people keep saying this? It's really not any more of a mystery than why electric charge produces an electric field. We have extremely precise theories to describe both. Of course you can always keep asking why why why and come to a point where there's no answer - but you can do that with anything.

I would say gravity is understood better than almost anything else in the world. Certainly we understand it far better than we do prosaic things like wind, the human body, or the mixing of cream as it's added to coffee.

69dodge · 27th January 2008, 08:46 AM

Originally Posted by JollyRoger

On Earth when an object is affected by gravity in that it falls to the ground.

On Earth, objects fall towards the Earth, because the Earth is massive.

Far from Earth, objects also fall towards any massive body that happens to be nearby.

Originally Posted by JollyRoger

In space an object produces gravity depending on the size and density.

Yes, and the same occurs on Earth.

Originally Posted by JollyRoger

When you spin a sphere on an axes on earth everything on the surface of the object is repelled,

Yes.

Originally Posted by JollyRoger

and the opposite is true in space.

No. Where did you hear that?

Originally Posted by JollyRoger

What Produces gravity? why does it exist? How does it work.

Those are hard questions.

Originally Posted by JollyRoger

Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

But the same is true here on Earth. The gravity field we're in, in fact, is just the Earth attracting other objects.

robinson · 27th January 2008, 08:50 AM

Originally Posted by sol invictus

Why do people keep saying this? It's really not any more of a mystery than why electric charge produces an electric field. We have extremely precise theories to describe both.

Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why. There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb.

Originally Posted by sol invictus

I would say gravity is understood better than almost anything else in the world.

Again, the question isn't about understanding, it is about explaining.

sol invictus · 27th January 2008, 08:55 AM

Originally Posted by robinson

Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why. There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb.

Last time I checked we had a perfectly good theory of gravity, just as we do for the other fundamental forces (it's called general relativity). We're not sure how to quantize it, but the uncertainties in calculations due to that are MUCH smaller than uncertainties in calculations involving the other forces (which stem from our ignorance of physics at high energies).

Our theory of gravity is much more precise than any other theory we have, at least in a certain technical sense.

Zeuzzz · 27th January 2008, 09:01 AM

Just for some fun;

Q1 Where is gravity of the Earth the strongest?

A - On the surface of the Earth
B - In the centre of the Earth
C – Somewhere about halfway down to the centre of the Earth

Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity?

A. - Increases definitely.
B. - Stays more or less constant.
C. - Decreases definitely.

There is more to it than you might think.

Southwind17 · 27th January 2008, 09:31 AM

Originally Posted by sol invictus

I would say gravity is understood better than almost anything else in the world. Certainly we understand it far better than we do prosaic things like wind, the human body, or the mixing of cream as it's added to coffee.

Really? Please go ahead and explain it, then, and satisfy the majority of the Earth's collective unfulfilled wonderment!

sol invictus · 27th January 2008, 09:33 AM

Originally Posted by Southwind17

Really? Please go ahead and explain it, then, and satisfy the majority of the Earth's collective unfulfilled wonderment!

Einstein already did, nearly a century ago.

I can use his theory to compute the orbits of planets to very high precision (and the accuracy is not limited by the theory, but by uncertainties in the positions of other gravitating bodies). I can compute the scattering of two low-energy gravity waves to a theoretical uncertainty less than for the scattering of two electrons at the same energy.

There is no gravitational experiment I can't predict the result of with precision greater than the experimental error using GR (apart from chaotic systems, but the difficulty there has nothing do with a lack of understanding of the theory of gravity).

Southwind17 · 27th January 2008, 09:37 AM

Originally Posted by Zeuzzz

Just for some fun;

Q1 Where is gravity of the Earth the strongest?

A - On the surface of the Earth
B - In the centre of the Earth
C – Somewhere about halfway down to the centre of the Earth

Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity?

A. - Increases definitely.
B. - Stays more or less constant.
C. - Decreases definitely.

There is more to it than you might think.

Simplistically:

Q1 - None of those. The effects of gravity would be more apparent on the surface of the Earth.
Q2 - None of those. The effects of gravity would decrease.

Southwind17 · 27th January 2008, 09:38 AM

Originally Posted by sol invictus

Einstein already did, nearly a century ago.

... and he said what?

Zeuzzz · 27th January 2008, 09:42 AM

Originally Posted by sol invictus

Einstein already did, nearly a century ago.

How does matter 'bend' space-time then?

Southwind17 · 27th January 2008, 09:53 AM

Originally Posted by Zeuzzz

How does matter 'bend' space-time then?

I'm sure that when Sol imparts his understanding, which surely must be simple and easily conveyed, given that it usurps the degree of understanding of almost anything else in the World, then we'll all ... well ... simply understand, I guess!

Ziggurat · 27th January 2008, 10:11 AM

Originally Posted by Zeuzzz

How does matter 'bend' space-time then?

The foundations of any physical theory cannot have a "how" answer. They just are. If you ever come up with an answer to a "how" question about the foundations, that just means you've pushed the foundations of your theory to a new location, which will have new unanswered "how"s. So there is no "how" answer, and there never was. There is only an answer to the "what", and that answer is
$G_{ab} = \kappa T_{ab}$

Southwind17 · 27th January 2008, 10:14 AM

Originally Posted by Ziggurat

The foundations of any physical theory cannot have a "how" answer. They just are. If you ever come up with an answer to a "how" question about the foundations, that just means you've pushed the foundations of your theory to a new location, which will have new unanswered "how"s. So there is no "how" answer, and there never was. There is only an answer to the "what", and that answer is
$G_{ab} = \kappa T_{ab}$

Excuse me; what's the question?

Yllanes · 27th January 2008, 10:20 AM

Originally Posted by Southwind17

Really? Please go ahead and explain it, then, and satisfy the majority of the Earth's collective unfulfilled wonderment!

What Ziggurat and sol invictus said. But why don't you or Robinson explain why you feel gravitation is not understood, while the other three forces are?

Southwind17 · 27th January 2008, 10:24 AM

Originally Posted by Yllanes

What Ziggurat and sol invictus said. But why don't you or Robinson explain why you feel gravitation is not understood, while the other three forces are?

Why don't you just go ahead and explain it, simply, if it's so well understood?

Yllanes · 27th January 2008, 10:24 AM

Originally Posted by Zeuzzz

How does matter 'bend' space-time then?

Exactly like this:

$ \[ R_{ab} - \frac12 R g_{ab} = 8\pi G T_{ab} \] $

If what you are asking is why this happens, then the same question can be posed for any other force. As in: why do electrons exchange virtual photons?

Southwind17 · 27th January 2008, 10:30 AM

Originally Posted by Yllanes

Exactly like this:

$ \[ R_{ab} - \frac12 R g_{ab} = 8\pi G T_{ab} \] $

If what you are asking is why this happens, then the same question can be posed for any other force. As in: why do electrons exchange virtual photons?

I don't see how citing a formula satisfactorily addresses either the "how" or the "why". As such it helps little in promoting understanding.

Southwind17 · 27th January 2008, 10:36 AM

Incidentally, not wishing to plagiarize Einstein, but are we talking "general" gravity here or "specific" gravity? If the latter then my father held an inimitable understanding, but then he was pretty adept in the home-brew department!

sol invictus · 27th January 2008, 10:56 AM

All I said was that we understand gravity better than just about anything else in the world. If you don't agree, why not give some examples of things you think we understand better?

Yllanes · 27th January 2008, 11:16 AM

Originally Posted by Southwind17

I don't see how citing a formula satisfactorily addresses either the "how" or the "why". As such it helps little in promoting understanding.

That formula explains exactly how energy bends spacetime. If you don't understand it, that's your problem, not the theory's.

When we say that gravity is understood just as well if not better than the other interactions, we mean exactly that. We don't mean it can be explained in a forum post.

Zeuzzz · 27th January 2008, 11:59 AM

Originally Posted by Southwind17

Simplistically:

Q1 - None of those. The effects of gravity would be more apparent on the surface of the Earth.
Q2 - None of those. The effects of gravity would decrease.

Technically, if ur being pedantic, your answer of 'none of those', is correct, i should have worded it better

The answer is not what i expected,

Well, the way I figured:

For a homogenouos sphere it can be determined that g(r)=GM(r)/r2. M(r) being the mass within of the sphere with radius r. - G gravitational constant and rho density

The total mass outside that radius is cancelling each other. Since M(r)=4/3 * rho * pi *r^3 it follows that g(r)=4/3 * G * rho * pi * r. In other words the gravity inside a homogenouos sphere is directly proportional to the distance to the centre.

Now assuming that the density rho is linear decreasing from value a with increment b, we replace rho with rho(r)=a-br. Substitute it in the first formula to get only a simple square formula, so intergrating is basic.

However we have five discontinuing layers: inner core, outer core, lower mantle, upper mantle, lithosphere. Each with their own numbers r-min, r-max, rho-a and b. I used 100 km increments for each shell.

Now, the answers are clear albeit a bit surprising. Gravity remains more or less constant initially. Why? We still can neglect the mass outside the radius when heading for the centre - this is decreasing the gravity. But we also come closer and closer to the very dense core - this is increasing the gravity. Both are of about the same magnitude initially, cancelling each other. (note that in the initial stage, entering the (relatively very light) crust the gravity increases sharply from 9.81 to 9.96 ms-2)

Coming closer to the the core however, the increasing factor wins and my model indicates a maximum value of 10,62 ms-2 at the core mantle boundary versus 9,81 at the Earth surface.

Inside the cores the behaviour is more or less approaching the homogeneous sphere - lineair proportional to the radius. Now isns't that a nice to know for discussions. So concluding:

Q1 Where is gravity of the Earth the strongest?

A - On the surface of the Earth
B - In the centre of the Earth
x C – SOMEWHERE ABOUT HALFWAY DOWN TO THE CENTRE OF THE EARTH. 10,62 ms-2 at a depth of approximately 2890 km

Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity?

A. - Increases definitely.
X B. - STAYS MORE OR LESS CONSTANT. (fluctuates actually between 9.96 and 10.04 ms-2)
C. - Decreases definitely.

There is more to it than the eye meets.

portlandatheist · 27th January 2008, 12:05 PM

Originally Posted by Zeuzzz

Just for some fun;

Q1 Where is gravity of the Earth the strongest?

A - On the surface of the Earth
B - In the centre of the Earth
C – Somewhere about halfway down to the centre of the Earth

Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity?

A. - Increases definitely.
B. - Stays more or less constant.
C. - Decreases definitely.

There is more to it than you might think.

Q1: C
Q2: A (at some point after a thousand miles, it would begin to decrease)

If you dug a hole all the way through the earth, and took away the atmosphere, you could drop an object and it would bounce back and forth, essentially being a highly elliptical orbit.

ETA: missed your latest post

Southwind17 · 27th January 2008, 10:12 PM

Originally Posted by Yllanes

That formula explains exactly how energy bends spacetime. If you don't understand it, that's your problem, not the theory's.

So, when my kids become old enough to start to try to assimilate this wonderful formula what do you suggest, that they persist in staring at it until it makes sense, or might it help if somebody really "explains" it to them?

Originally Posted by Yllanes

When we say that gravity is understood just as well if not better than the other interactions, we mean exactly that. We don't mean it can be explained in a forum post.

"We" being who, exactly? If one properly understands something then one should be able to explain it to others in a comprehensible way. If you can't explain gravity here than I posit your understanding is not as complete as you clearly seem to think it is.

Southwind17 · 27th January 2008, 10:19 PM

Originally Posted by sol invictus

All I said was that we understand gravity better than just about anything else in the world. If you don't agree, why not give some examples of things you think we understand better?

Wouldn't it be more sensible for you simply to posit your understanding? If you insist, however, I believe our understanding of how and why rain falls, for example, is pretty much complete. Or how and why the plasma screen I'm looking at works.

shadron · 27th January 2008, 11:08 PM

Originally Posted by robinson

Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why.

I'm not really sure I should stick my oar in this water, but the question about how, and particularly why, gravity is like it is is at least philosophical and metaphysical, and perhaps downright theological (pardon my French). *How* does gravity "suck"? Who knows? No one likely to answer on this forum, certainly. If you had the answer, I predict that it will be at least an order of magnitude harder to understand than gravity. *Why* does it suck? Perhaps because if it didn't we couldn't have asked the question? Ask the creator, if you can find him.

Sometimes *why* is a valid scientific question: why does almost all the mass in the universe seem to be matter as opposed to anti-matter? Good question - it might lead to a new discovery. But why does gravity vary directly with mass and inversely with distance squared? It just does. The difference, as was pointed out above, is that you can ask why when there is some basic, underlying principle that can be measured and observed. With gravity, there is none such, so asking why is scientifically futile.

In most basic science description is the best we have. With a suitable description you can predict what gravity will do to Jupiter next Wednesday, or where it was on Dec 25, 1BC. Sol reminds us that we can do that to within a whisker's width. The description is as complete as we can make it without any new discrepancies to explain away, and that is why Sol says we understand it.

Quote:

There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb.

Oh? Please explain the electromagnetic force, then. Why does it expel, as well as attract, unlike gravity? How does the strong force overcome electromagnetic repulsion only out to about the radius of a Uranium nucleus? Why that far and no farther? What theory have we got to cover these sorts of questions? None.

Quote:

Again, the question isn't about understanding, it is about explaining.

Some basic things have to be simply accepted as observed. Sorry - no deeper scientific explanation is currently possible. Perhaps it will be so forever.

Cold one · 27th January 2008, 11:45 PM

Originally Posted by Southwind17

If you insist, however, I believe our understanding of how and why rain falls, for example, is pretty much complete.

The way I understand it rain falls due to gravity and since you dont understand gravity then your understanding of water falling from the sky is therefore incomplete.

Fredrik · 28th January 2008, 12:31 AM

I agree with the claim that gravity is at least as well understood as anything else. It's not at all correct to say that there's no theory of gravity as there is for the other interactions. There's no quantum theory of gravity yet, but that's another matter. (See my comments at the end of this post).

It isn't possible to really understand Einstein's equation (the one posted by several people earlier in this thread) without studying differential geometry, but you can get a rough idea.

The right-hand side represents all the matter and energy in the universe. The two indices go from 0 to 3, so it has 16 components, but it is symmetric (T_ab=T_ba), so only 10 of the components are independent. One of those components is the density of matter/energy as a function of position. The others are stuff like the pressure, and internal stresses. (Take a piece of rubber and hold on to one end while twisting the other end. You don't change the density much, but you introduce forces between different parts of the rubber, and that changes its contribution to those "other" components of the stress-energy tensor).

The left hand side represents the geometric properties of space-time. The quantities on the left (the G, or equivalently the Rs) can be constructed from another symmetric tensor with two indices called the metric tensor. The metric tensor tells us the "distance" between any two points in space-time, and it also tells us which lines are to be considered "straight". I put "distance" in quotes because it's defined in a way that means it can be negative, and I put "straight" in quotes because it's not what we're used to either. The path through space-time that a planet takes when it's orbiting a star is straight in this geometry. The distance between the event where a laser beam is emitted and the event where it hits its target is always zero in this geometry (and this path is straight too).

If there had been no matter or energy in the universe whatsoever, the components of the metric tensor would have been

$\begin{pmatrix}-1 & 0 & 0 & 0\\0 & 1 & 0 & 0\\0 & 0 & 1 & 0\\0 & 0 & 0 & 1\end{pmatrix}$

The space-time with this metric tensor is called Minkowski space. This is the space-time of special relativity, the theory that Einstein came up with in 1905. Special relativity is nothing more than the claim that space and time can be represented mathematically by Minkowski space. This means it doesn't describe gravity, but it's a suitable framework for e.g. both classical and quantum mechanical theories of electromagnetism. General relativity is the claim that the relationship between the metric tensor and the matter/energy content of the universe is described by Einstein's equation.

Famous quote by John Wheeler: "Matter tells space how to curve. Space tells matter how to move".

It might seem that we understand gravity less than the other interactions since we've been able to find quantum theories describing those, but that conclusion would be incorrect. The piece of information that you would need to understand why, is that all of those quantum theories are formulated in the framework of special relativity, which we already know is a much less accurate representation of space-time than general relativity.

Southwind17 · 28th January 2008, 01:09 AM

Originally Posted by Cold one

The way I understand it rain falls due to gravity and since you dont understand gravity then your understanding of water falling from the sky is therefore incomplete.

That's why I wrote "pretty much complete" and not "entirely complete". The gravity aspect of rain physically "falling" is essentially incidental to the wider question of why rain "happens". I didn't think "rain happens" sounded very eloquent, though. I'm sure you got all that, though, the first time around!

Yllanes · 28th January 2008, 01:12 AM

Originally Posted by Southwind17

So, when my kids become old enough to start to try to assimilate this wonderful formula what do you suggest, that they persist in staring at it until it makes sense, or might it help if somebody really "explains" it to them?

It was claimed in this thread that our knowledge of gravity was more imperfect than our knowledge of the other interactions. Someone said we lack a theory for it such as we have for the other fundamental forces. And that is incorrect because we do have a theory, based on a couple of postulates (one of which is the equation I posted earlier), which lets us predict the physics of gravitation with astonishing precision.

To really understand gravitation one must be able to carry out such calculations. You can't understand it at that level with just a couple of paragraphs in English. What you can do with a couple of paragraphs is to say something such as Fedrik's post. But the same thing is true for the other interactions. You cannot explain the Standard Model in a forum post so that you audience can then go and compute, say, the half life of the Higgs boson at tree level.

So, my question to you is: do you believe gravity is less understood than, say, the strong nuclear force? If you don't then we agree and there's nothing else to say. If you do, please tell us why.

26th January 2008, 08:17 PM	#1
JollyRoger Muse Join Date: Apr 2006 Location: Buggery's Island Posts: 502	Question about gravity On Earth when an object is affected by gravity in that it falls to the ground. In space an object produces gravity depending on the size and density. When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space. What Produces gravity? why does it exist? How does it work. Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.
	__________________ The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed. Albert Einstein

26th January 2008, 08:30 PM	#2
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268	Originally Posted by JollyRoger What Produces gravity? why does it exist? How does it work. Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field. Gravity is though to be caused exclusively by the attraction of mass. How this attraction occurs is still not known, mainly due to the weakness of gravity compared to other forces which makes experiments testing its exact nature very hard without EM forces interferring with the experiment. Millikan's oil drop experiment is a fine example of this, where you are able to make an drop float by applying an electric field. When this is done the entire mass of the earth is pulling down on the drop, yet an electrical effect can counter it with ease, which shows how hard it is to directly test the mechanism by which gravity works. Maxwell tried to explain gravity with electromagnetic effects, but with no avail, and it has since been considered to be a completely separate force from EM effects. It doesn't help that the definition of mass is not very precise, it is explained in term of gravity, and gravity is explained in terms of mass. Wiki says; http://en.wikipedia.org/wiki/Mass Quote: Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object". Hardly definitive to say the least, what exactly 'mass' is, is another question that is quite hard to answer too
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268	Last edited by Zeuzzz; 26th January 2008 at 08:31 PM.

26th January 2008, 08:32 PM	#3
steve s Graduate Poster Join Date: Feb 2006 Posts: 1,892	Originally Posted by JollyRoger ...and the opposite is true in space. Not sure I understand that part. Are you talking about creating artificial gravity by rotating a spacecraft? The same principle is at work. But if you're inside the spacecraft, you'll stick to the outer walls. Quote: What Produces gravity? why does it exist? How does it work. Matter warps the fabric of spacetime. I'm sure you've seen the analogy of a heavy ball on a large sheet of rubber. Try googling "general relativity." Quote: Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field. They do attract each other, but other forces (such as friction) are greater than any attraction due to gravity. Steve S.
steve s Graduate Poster Join Date: Feb 2006 Posts: 1,892	__________________ "Nature abhors a moron." -- H. L. Mencken

26th January 2008, 09:14 PM	#4
JollyRoger Muse Join Date: Apr 2006 Location: Buggery's Island Posts: 502	Originally Posted by steve s Not sure I understand that part. Are you talking about creating artificial gravity by rotating a spacecraft? The same principle is at work. But if you're inside the spacecraft, you'll stick to the outer walls. I understand sentrifical force for the most part I was wondering the mechanism of Gravity what causes and it? Quote: Matter warps the fabric of space-time. I'm sure you've seen the analogy of a heavy ball on a large sheet of rubber. Try googling "general relativity." yes I have but how does warping space time produce gravity Quote: They do attract each other, but other forces (such as friction) are greater than any attraction due to gravity. Steve S. not sure on this one but how would you have friction between to objects without gravity pulling the objects close enough to each other to produce friction
	__________________ The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed. Albert Einstein

26th January 2008, 09:17 PM	#5
JollyRoger Muse Join Date: Apr 2006 Location: Buggery's Island Posts: 502	Originally Posted by Zeuzzz Gravity is though to be caused exclusively by the attraction of mass. How this attraction occurs is still not known, I often wondered because they always seem to dance around the specific subject of its mechanism
	__________________ The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed. Albert Einstein

27th January 2008, 12:03 AM	#6
MG1962 Philosopher Join Date: Sep 2006 Posts: 5,236	Gravity is a myth The Earth sucks
MG1962 Philosopher Join Date: Sep 2006 Posts: 5,236

27th January 2008, 12:25 AM	#7
bobhope2112 Scholar Join Date: Jun 2007 Posts: 93	Originally Posted by JollyRoger I understand sentrifical force for the most part I was wondering the mechanism of Gravity what causes and it? There really is no such thing as a "centrifical"/centrifugal force. Just to be clear what we're talking about: People will describe the tendency for you to fly off of a fast moving merry-go-round as being acted upon by a centrifugal force. The reality is that it's just inertia at work. The merry-go-round is turning away from you, and if there is no force on you to make you turn with it, you'll keep going straight and fall off. What causes gravity? Excellent question, but one that need not be addressed to have a coherent theory of gravity. As long as the theory describes our observations of gravity, it's of use. If a Grand Unification Theory ever pans out to describe all of nature's forces, en masse, I suspect we'll have a bit more of an answer as to what /causes/ gravity, but it may be too abstract to satisfy your practical sense. Quote: Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field. Objects do attract each other here on Earth. In fact, it's the principal reason you don't go flying off this merry-go-round/planet as it spins. It is a relatively week force, though, so you won't see two billiard balls slamming together, like a couple of magnets, from their gravitational attraction. A subtler interaction comes to mind: If you hold a plumb bob next to a mountain, it will actually deflect a bit instead of pointing "straight down." The mountain's mass attracts the plumb bob, but the far dominant force will be the massive Earth.
bobhope2112 Scholar Join Date: Jun 2007 Posts: 93	Last edited by bobhope2112; 27th January 2008 at 12:30 AM.

27th January 2008, 01:49 AM	#8
Fredrik Graduate Poster Join Date: Jun 2004 Posts: 1,176	Originally Posted by JollyRoger On Earth when an object is affected by gravity in that it falls to the ground. In space an object produces gravity depending on the size and density. Gravity works the same on Earth as it does in space. Originally Posted by JollyRoger When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space. Uh, no. Objects move in straight lines unless a force is acting on them. The gravitational attraction between you and the Earth is what's keeping you from moving in a straight line (i.e. from being "thrown" into space). Originally Posted by JollyRoger What Produces gravity? why does it exist? How does it work. Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field. The same is true on Earth. What makes you think it isn't? No one has a complete answer to the first two questions. There are however two good theories of gravity: Newton's and Einstein's. Einstein's is better in the sense that it agrees with the experimental results to a higher degree. Newton's theory simply states that the magnitude of the force between two massive point particles with masses m₁ and m₂ a distance r apart, is m₁m₂/r² (in units chosen to simplify the formula), and that the force is attractive. Einstein's theory is much more difficult to understand. It describes gravity as a geometric effect. Matter tells space how to curve, and space tells matter how to move. It's impossible to really understand Einstein's theory without learning differential geometry.
Fredrik Graduate Poster Join Date: Jun 2004 Posts: 1,176

27th January 2008, 05:49 AM	#9
LordoftheLeftHand Dominus Sinistrae Join Date: Jun 2005 Location: Las Vegas Posts: 1,161	Originally Posted by JollyRoger I often wondered because they always seem to dance around the specific subject of its mechanism This "mechanism of mass" is an interesting concept, it is sometimes called the Higgs Mechanism. The ATLAS experiment should be going online sometime this year. It is supposed investigate the Higgs Mechanism (which to this date has never been observed). You might be hearing a lot about this in the near future. Here is a nice video about it: http://atlasexperiment.org/multimedi...ure_atlas.html LLH
	__________________ What though the field be lost? All is not lost—the unconquerable will, And study of revenge, immortal hate, And courage never to submit or yield - Milton, Paradise Lost Last edited by LordoftheLeftHand; 27th January 2008 at 05:49 AM.

27th January 2008, 07:19 AM	#10
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 5,761	Originally Posted by JollyRoger When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space. As Fredrik already pointed out, that's not true. Quote: What Produces gravity? why does it exist? How does it work. One answer is that energy curves space (the rubber sheet with a bowling ball on it), and objects just move in straight lines as usual - but on that curved sheet. If you didn't know the sheet was warped, you'd say there must be a force acting to make the object move in a curve, and that's the thing we call the force of gravity. Another answer is that the force is caused by the exchange of gravitons, which are a hypothetical type of elementary particle. Gravitons are emitted and absorbed continuously by anything with mass/energy, and this happens more often and more gravitons are involved if the object is more massive, which is why the force is stronger. Quote: Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field. The same is true on earth.
	Last edited by sol invictus; 27th January 2008 at 07:21 AM.

27th January 2008, 08:03 AM	#11
robinson Banned Join Date: Aug 2006 Location: Washington D.C. Posts: 6,136	Originally Posted by JollyRoger What Produces gravity? why does it exist? Well, we are sure matter produces gravity. Because gravity doesn't exist without matter. Why and how this is so, is one of the greatest mysteries.

27th January 2008, 08:19 AM	#12
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 5,761	Originally Posted by robinson Well, we are sure matter produces gravity. Because gravity doesn't exist without matter. Why and how this is so, is one of the greatest mysteries. Why do people keep saying this? It's really not any more of a mystery than why electric charge produces an electric field. We have extremely precise theories to describe both. Of course you can always keep asking why why why and come to a point where there's no answer - but you can do that with anything. I would say gravity is understood better than almost anything else in the world. Certainly we understand it far better than we do prosaic things like wind, the human body, or the mixing of cream as it's added to coffee.

27th January 2008, 08:46 AM	#13
69dodge Illuminator Join Date: Nov 2002 Posts: 3,144	Originally Posted by JollyRoger On Earth when an object is affected by gravity in that it falls to the ground. On Earth, objects fall towards the Earth, because the Earth is massive. Far from Earth, objects also fall towards any massive body that happens to be nearby. Originally Posted by JollyRoger In space an object produces gravity depending on the size and density. Yes, and the same occurs on Earth. Originally Posted by JollyRoger When you spin a sphere on an axes on earth everything on the surface of the object is repelled, Yes. Originally Posted by JollyRoger and the opposite is true in space. No. Where did you hear that? Originally Posted by JollyRoger What Produces gravity? why does it exist? How does it work. Those are hard questions. Originally Posted by JollyRoger Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field. But the same is true here on Earth. The gravity field we're in, in fact, is just the Earth attracting other objects.
69dodge Illuminator Join Date: Nov 2002 Posts: 3,144

27th January 2008, 08:50 AM	#14
robinson Banned Join Date: Aug 2006 Location: Washington D.C. Posts: 6,136	Originally Posted by sol invictus Why do people keep saying this? It's really not any more of a mystery than why electric charge produces an electric field. We have extremely precise theories to describe both. Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why. There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb. Originally Posted by sol invictus I would say gravity is understood better than almost anything else in the world. Again, the question isn't about understanding, it is about explaining.

27th January 2008, 08:55 AM	#15
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 5,761	Originally Posted by robinson Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why. There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb. Last time I checked we had a perfectly good theory of gravity, just as we do for the other fundamental forces (it's called general relativity). We're not sure how to quantize it, but the uncertainties in calculations due to that are MUCH smaller than uncertainties in calculations involving the other forces (which stem from our ignorance of physics at high energies). Our theory of gravity is much more precise than any other theory we have, at least in a certain technical sense.

27th January 2008, 09:01 AM	#16
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268	Just for some fun; Q1 Where is gravity of the Earth the strongest? A - On the surface of the Earth B - In the centre of the Earth C – Somewhere about halfway down to the centre of the Earth Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity? A. - Increases definitely. B. - Stays more or less constant. C. - Decreases definitely. There is more to it than you might think.
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268

27th January 2008, 09:31 AM	#17
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by sol invictus I would say gravity is understood better than almost anything else in the world. Certainly we understand it far better than we do prosaic things like wind, the human body, or the mixing of cream as it's added to coffee. Really? Please go ahead and explain it, then, and satisfy the majority of the Earth's collective unfulfilled wonderment!
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 09:33 AM	#18
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 5,761	Originally Posted by Southwind17 Really? Please go ahead and explain it, then, and satisfy the majority of the Earth's collective unfulfilled wonderment! Einstein already did, nearly a century ago. I can use his theory to compute the orbits of planets to very high precision (and the accuracy is not limited by the theory, but by uncertainties in the positions of other gravitating bodies). I can compute the scattering of two low-energy gravity waves to a theoretical uncertainty less than for the scattering of two electrons at the same energy. There is no gravitational experiment I can't predict the result of with precision greater than the experimental error using GR (apart from chaotic systems, but the difficulty there has nothing do with a lack of understanding of the theory of gravity).
	Last edited by sol invictus; 27th January 2008 at 09:37 AM.

27th January 2008, 09:37 AM	#19
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by Zeuzzz Just for some fun; Q1 Where is gravity of the Earth the strongest? A - On the surface of the Earth B - In the centre of the Earth C – Somewhere about halfway down to the centre of the Earth Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity? A. - Increases definitely. B. - Stays more or less constant. C. - Decreases definitely. There is more to it than you might think. Simplistically: Q1 - None of those. The *effects* of gravity would be more apparent on the surface of the Earth. Q2 - None of those. The *effects* of gravity would decrease.
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 09:38 AM	#20
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by sol invictus Einstein already did, nearly a century ago. ... and he said what?
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 09:42 AM	#21
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268	Originally Posted by sol invictus Einstein already did, nearly a century ago. How does matter 'bend' space-time then?
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268

27th January 2008, 09:53 AM	#22
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by Zeuzzz How does matter 'bend' space-time then? I'm sure that when Sol imparts his understanding, which surely must be simple and easily conveyed, given that it usurps the degree of understanding of almost anything else in the World, then we'll all ... well ... simply understand, I guess!
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17) Last edited by Southwind17; 27th January 2008 at 09:54 AM.

27th January 2008, 10:11 AM	#23
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 15,520	Originally Posted by Zeuzzz How does matter 'bend' space-time then? The foundations of any physical theory cannot have a "how" answer. They just are. If you ever come up with an answer to a "how" question about the foundations, that just means you've pushed the foundations of your theory to a new location, which will have new unanswered "how"s. So there is no "how" answer, and there never was. There is only an answer to the "what", and that answer is $G_{ab} = \kappa T_{ab}$
	__________________ "There is certainly not one government in Europe but is now watching the war in this country, with the ardent prayer that the united States may be effectually split, crippled, and dismember'd by it... We are all too prone to wander from ourselves, to affect Europe, and watch her frowns and smiles. We need this hot lesson of general hatred, and henceforth must never forget it. Never again will we trust the moral sense nor abstract friendliness of a single government of the world." - Walt Whitman, 1864 Last edited by Ziggurat; 27th January 2008 at 10:11 AM.

27th January 2008, 10:14 AM	#24
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by Ziggurat The foundations of any physical theory cannot have a "how" answer. They just are. If you ever come up with an answer to a "how" question about the foundations, that just means you've pushed the foundations of your theory to a new location, which will have new unanswered "how"s. So there is no "how" answer, and there never was. There is only an answer to the "what", and that answer is $G_{ab} = \kappa T_{ab}$ Excuse me; what's the question?
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 10:20 AM	#25
Yllanes Muse Join Date: Sep 2005 Location: Madrid Posts: 826	Originally Posted by Southwind17 Really? Please go ahead and explain it, then, and satisfy the majority of the Earth's collective unfulfilled wonderment! What Ziggurat and sol invictus said. But why don't you or Robinson explain why you feel gravitation is not understood, while the other three forces are?

27th January 2008, 10:24 AM	#26
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by Yllanes What Ziggurat and sol invictus said. But why don't you or Robinson explain why you feel gravitation is not understood, while the other three forces are? Why don't you just go ahead and explain it, simply, if it's so well understood?
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 10:30 AM	#28
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by Yllanes Exactly like this: $<br /> \[<br /> R_{ab} - \frac12 R g_{ab} = 8\pi G T_{ab}<br /> \]<br />$ If what you are asking is why this happens, then the same question can be posed for any other force. As in: why do electrons exchange virtual photons? I don't see how citing a formula satisfactorily addresses either the "how" or the "why". As such it helps little in promoting understanding.
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17) Last edited by Southwind17; 27th January 2008 at 10:31 AM.

27th January 2008, 10:36 AM	#29
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Incidentally, not wishing to plagiarize Einstein, but are we talking "general" gravity here or "specific" gravity? If the latter then my father held an inimitable understanding, but then he was pretty adept in the home-brew department!
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 10:56 AM	#30
sol invictus Philosopher Join Date: Oct 2007 Location: Nova Roma Posts: 5,761	All I said was that we understand gravity better than just about anything else in the world. If you don't agree, why not give some examples of things you think we understand better?

27th January 2008, 11:16 AM	#31
Yllanes Muse Join Date: Sep 2005 Location: Madrid Posts: 826	Originally Posted by Southwind17 I don't see how citing a formula satisfactorily addresses either the "how" or the "why". As such it helps little in promoting understanding. That formula explains exactly how energy bends spacetime. If you don't understand it, that's your problem, not the theory's. When we say that gravity is understood just as well if not better than the other interactions, we mean exactly that. We don't mean it can be explained in a forum post.

27th January 2008, 11:59 AM	#32
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268	Originally Posted by Southwind17 Simplistically: Q1 - None of those. The *effects* of gravity would be more apparent on the surface of the Earth. Q2 - None of those. The *effects* of gravity would decrease. Technically, if ur being pedantic, your answer of 'none of those', is correct, i should have worded it better The answer is not what i expected, Well, the way I figured: For a homogenouos sphere it can be determined that g(r)=GM(r)/r2. M(r) being the mass within of the sphere with radius r. - G gravitational constant and rho density The total mass outside that radius is cancelling each other. Since M(r)=4/3 * rho * pi r^3 it follows that g(r)=4/3 G * rho * pi * r. In other words the gravity inside a homogenouos sphere is directly proportional to the distance to the centre. Now assuming that the density rho is linear decreasing from value a with increment b, we replace rho with rho(r)=a-br. Substitute it in the first formula to get only a simple square formula, so intergrating is basic. However we have five discontinuing layers: inner core, outer core, lower mantle, upper mantle, lithosphere. Each with their own numbers r-min, r-max, rho-a and b. I used 100 km increments for each shell. Now, the answers are clear albeit a bit surprising. Gravity remains more or less constant initially. Why? We still can neglect the mass outside the radius when heading for the centre - this is decreasing the gravity. But we also come closer and closer to the very dense core - this is increasing the gravity. Both are of about the same magnitude initially, cancelling each other. (note that in the initial stage, entering the (relatively very light) crust the gravity increases sharply from 9.81 to 9.96 ms-2) Coming closer to the the core however, the increasing factor wins and my model indicates a maximum value of 10,62 ms-2 at the core mantle boundary versus 9,81 at the Earth surface. Inside the cores the behaviour is more or less approaching the homogeneous sphere - lineair proportional to the radius. Now isns't that a nice to know for discussions. So concluding: Q1 Where is gravity of the Earth the strongest? A - On the surface of the Earth B - In the centre of the Earth x C – SOMEWHERE ABOUT HALFWAY DOWN TO THE CENTRE OF THE EARTH. 10,62 ms-2 at a depth of approximately 2890 km Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity? A. - Increases definitely. X B. - STAYS MORE OR LESS CONSTANT. (fluctuates actually between 9.96 and 10.04 ms-2) C. - Decreases definitely. There is more to it than the eye meets.
Zeuzzz Master Poster Join Date: Dec 2007 Posts: 2,268

27th January 2008, 12:05 PM	#33
portlandatheist Graduate Poster Join Date: Jun 2007 Posts: 1,381	Originally Posted by Zeuzzz Just for some fun; Q1 Where is gravity of the Earth the strongest? A - On the surface of the Earth B - In the centre of the Earth C – Somewhere about halfway down to the centre of the Earth Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity? A. - Increases definitely. B. - Stays more or less constant. C. - Decreases definitely. There is more to it than you might think. Q1: C Q2: A (at some point after a thousand miles, it would begin to decrease) If you dug a hole all the way through the earth, and took away the atmosphere, you could drop an object and it would bounce back and forth, essentially being a highly elliptical orbit. ETA: missed your latest post
	Last edited by portlandatheist; 27th January 2008 at 12:07 PM. Reason: ETA

27th January 2008, 10:12 PM	#34
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by Yllanes That formula explains exactly how energy bends spacetime. If you don't understand it, that's your problem, not the theory's. So, when my kids become old enough to start to try to assimilate this wonderful formula what do you suggest, that they persist in staring at it until it makes sense, or might it help if somebody really "explains" it to them? Originally Posted by Yllanes When we say that gravity is understood just as well if not better than the other interactions, we mean exactly that. We don't mean it can be explained in a forum post. "We" being who, exactly? If one properly understands something then one should be able to explain it to others in a comprehensible way. If you can't explain gravity here than I posit your understanding is not as complete as you clearly seem to think it is.
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 10:19 PM	#35
Southwind17 Illuminator Join Date: Sep 2007 Location: Location: Location Posts: 3,650	Originally Posted by sol invictus All I said was that we understand gravity better than just about anything else in the world. If you don't agree, why not give some examples of things you think we understand better? Wouldn't it be more sensible for you simply to posit your understanding? If you insist, however, I believe our understanding of how and why rain falls, for example, is pretty much complete. Or how and why the plasma screen I'm looking at works.
	__________________ *How very dare you!* "England and America are two countries divided by a common language." - George Bernard Shaw (seconded by Southwind17)

27th January 2008, 11:08 PM	#36
shadron Illuminator Join Date: Sep 2005 Location: Colorado Posts: 4,475	Originally Posted by robinson Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why. I'm not really sure I should stick my oar in this water, but the question about how, and particularly why, gravity is like it is is at least philosophical and metaphysical, and perhaps downright theological (pardon my French). How does gravity "suck"? Who knows? No one likely to answer on this forum, certainly. If you had the answer, I predict that it will be at least an order of magnitude harder to understand than gravity. Why does it suck? Perhaps because if it didn't we couldn't have asked the question? Ask the creator, if you can find him. Sometimes why is a valid scientific question: why does almost all the mass in the universe seem to be matter as opposed to anti-matter? Good question - it might lead to a new discovery. But why does gravity vary directly with mass and inversely with distance squared? It just does. The difference, as was pointed out above, is that you can ask why when there is some basic, underlying principle that can be measured and observed. With gravity, there is none such, so asking why is scientifically futile. In most basic science description is the best we have. With a suitable description you can predict what gravity will do to Jupiter next Wednesday, or where it was on Dec 25, 1BC. Sol reminds us that we can do that to within a whisker's width. The description is as complete as we can make it without any new discrepancies to explain away, and that is why Sol says we understand it. Quote: There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb. Oh? Please explain the electromagnetic force, then. Why does it expel, as well as attract, unlike gravity? How does the strong force overcome electromagnetic repulsion only out to about the radius of a Uranium nucleus? Why that far and no farther? What theory have we got to cover these sorts of questions? None. Quote: Again, the question isn't about understanding, it is about explaining. Some basic things have to be simply accepted as observed. Sorry - no deeper scientific explanation is currently possible. Perhaps it will be so forever.