Gravity is Bunk!!!

[Dorfl]

"Bended / deformed space" does produce a force on an object moving in it. Mass curves ("deforms") spacetime so that movement along a straight line in spacetime becomes a curve. This curvature in movement is a change in direction of the velocity of an object. This is an acceleration. An acceleration on a mass is described as a force (F=ma). We call that force gravity.

Is there any simple way of visualising this, or is it best just treated mathematically?

 

[Reality Check]

Is there any simple way of visualising this, or is it best just treated mathematically?

The simplest way to visualize the curvature of spacetime is the rubber sheet model.

You can use the same model to see that a uniform contraction of a rubber sheet containing a straight line leaves the line as straight, i.e. there is no force.
I think that a non-uniform contraction would be the same as a curvature.

 

[sol invictus]

Here in the Earth’s orbit around the Sun, - space bend / deforms
The Sun (matter) is the cause to that happen.
Billion km. away from the Sun the same thing happen: Space bend / deform because of the Sun.

The earth is 200 million km from the sun, so a billion km is not that much further out. But anyway, the farther out you go, the less of an effect there is.

We can say that the Sun (matter) have some kind of “contact // connection” with space right?
I mean because of matter causes space to: “bend” / “deforms”

All forms of energy do that, yes.

How can matter billion of km away from the source (matter) bend space?
What happen?

It's not so different from how a magnetic field can extend beyond a magnet, or a heavy weight can stretch a rubber sheet away from the weight.

Does space have some kind of “density” that can deform / bend / “””contract”””” ?

Sort of, yes. Spacetime is not a rigid, fixed thing, it's flexible. For example one can send ripples of stretching and contraction through it by wiggling a mass.

What the Sun (or matter does) is to contract space.

Does that not sound acceptable?

No, it's really not a contraction. If anything it's more of a stretching. But these words are imprecise, to really see what's going on you need equations.

"Bended / deformed space" does produce a force on an object moving in it. Mass curves ("deforms") spacetime so that movement along a straight line in spacetime becomes a curve. This curvature in movement is a change in direction of the velocity of an object. This is an acceleration. An acceleration on a mass is described as a force (F=ma). We call that force gravity.

That's not quite right, although it's not far off either. In general relativity there isn't really a force of gravity. Objects simply follow "straight lines", meaning the shortest distance between two points, but those paths don't look straight if you believe that the space is flat. It's like plotting the path of a jet making a long international flight. On a flat map the path is curved, but on the globe it's the shortest distance between the origin and destination it's a "straight line".

Still, you're correct that at least when the spacetime is close to being flat, one can approximate it as such and add a force of gravity to account for the curvature of those paths (that's the Newtonian approximation to general relativity).

 

[Roboramma]

Sol, quick question:

Your post made me wonder: in the rubber sheet when you place a mass on it, the mass pushes the sheet down, stretching it. When the area around it gets stretched, the sheet itself pulls on the areas further out, stretching them.

Does that analogy hold up in this way? Ie. I know that gravity propogates at the speed of light, but does it do this by the mass having a local effect, and that local deformation causing deformation further out, etc.?

Hope I explained my question well...

 

[Bjarne]

No it does not.
Contracting space does not produce any force on an object moving in it. .

No space does not, But what about matter?
How can space bend / deform without a force is involved
By the way how can space expand without a force is involved? No deformation of space is “free” right?

Newton tells us that there is no force acting on them. .

Wrong. Newton believed a force was involved in gravity, but Einstein did not.

"Bended / deformed space" does produce a force on an object moving in it. .

Really ? - Where is the source to that force `

Mass curves ("deforms") spacetime so that movement …………..

So a force (to bend space) must be involved. Newton was right and Einstein was wrong, since Einstein claimed that no force is involved in gravity?

In general relativity there isn't really a force of gravity.

It depend what you mean by gravity.
If you mean space bends without force / energy you could very well be wrong.
No smoke without fire, and no bended space (or expanding space) without force / energy right?

BUT
What is really deformed / bended space?
And how can mass (for ex. The Sun) bend space billion and billion and billion away from it self?
WHAT is the Sun “doing” to space so long away?
How does the Sun connect with space billion and billion and billion km away from it self?
How can the Sun Reach INFINETY and affect spaces (infinity) away from it self?

Well we have no answers, - and even a attemp to understand seems to be beyond common sense.
But NOT if you allow you self the thought, that bended space = contracted space.
Nothing prevents us from such understanding.
Contracted space could very well be exactly the same as “bended space”.

The next “mystery” (as already mention): how can matter bend (contract) space without energy or a FORCE is involved?
A Force must be involved.

Why should it not require energy / force to bend space?
We have no right to claim that .
Nothing happen without a reason.

 

[Roboramma]

No space does not, But what about matter?
How can space bend / deform without a force is involved
By the way how can space expand without a force is involved? No deformation of space is “free” right?

Energy (or mass) is present, but none is converted in the deformation of space.

And, while I don't know so much about all this, it seems to me that F=ma means that force acts on mass, not on space-time.

How can the Sun Reach INFINETY and affect spaces (infinity) away from it self?

Gravity propagates at the speed of light, so unless I'm missing something, lacking infinite time, it can't.

 

[Reality Check]

No space does not, But what about matter?

What about it? Matter causes the curvature of spacetime.

How can space bend / deform without a force is involved

Matter causes it without a force being needed. Forces act on material things. Spacetime is not material. It is what material things exist in.

By the way how can space expand without a force is involved? No deformation of space is “free” right?

Expanding space is though to be a result of the inflationary period of the Big Bang.
Free of what? Free from what?

Wrong. Newton believed a force was involved in gravity, but Einstein did not.

I did not say that. Newtons F=ma is a statement about all forces. But I really should have just mentioned his first law (see below), i.e. no force on an object moving in a straight line.
Newton believed that gravity was a force between 2 masses. This was confirmed by observations for a couple of centuries until it was noticed that it did not predict the precession of the perihelion of the orbits of the planets, especially of planet Mercury.
Einstein demonstrated that it was better described as the result of curvature of spacetime. This fits the observations a lot better (tests of General Relativity).

Really ? - Where is the source to that force `

The curvature of space.

I assume that you know about Newton's laws of motion.
Basically:
The first law is "In the absence of force, a body either is at rest or moves in a straight line with constant speed."
In a curved spacetime the corresponding notion of a straight line in flat spacetime is a geodesic.
An object in a curved spacetime follows a curved path when compared to an object in flat spacetime (see sol victus's analogy).
From Newton's first law, there must be a force acting on the object to produce the curved path.
This is the gravitational force.

I suggest that you read something about General Relativity rather than relying on my basic knowledge.

So a force (to bend space) must be involved. Newton was right and Einstein was wrong, since Einstein claimed that no force is involved in gravity?

No force is needed to bend space.
All that is needed is mass (and energy)
Newton was correct for weak gravitational fields and low velocities (compared to the speed of light).
Einstein is more correct since General Relativity is also correct for strong gravitational fields and high velocities (GR includes Special Relativity).

 

[Reality Check]

How can the Sun Reach INFINETY and affect spaces (infinity) away from it self?

A small point: Infinity is not a number. See for example "Infinity is NOT a number".
Thus the gravitational force from a massive object cannot "Reach INFINETY" or "affect spaces (infinity) away from it self".

However the curvature of spacetime caused by a mass extends without limit from the position of the object.

 

[athon]

No. "Deformed space" simply means space that isn't flat. If that's hard to imagine, think about a stretchy, flexible surface (the infamous rubber sheet). Start with it stretched across a drum head, and cover it with a regular pattern of dots. Now picture what happens to those dots if you tighten the screws on the drum (they move apart), or loosen them (they move together), or if you poke your finger into the surface (they do something more complex).

How far should this analogy be taken? In other words, a two dimensional sheet deforms when pressed because it dimples in the third dimension, right? You need the third dimension to explain what is happening. Is this analogous to what is happening to spacetime? Is it the third dimension dimpling (for lack of a better word) into the fourth?

Or am I taking the analogy too literally?

Athon

 

[sol invictus]

I know that gravity propogates at the speed of light, but does it do this by the mass having a local effect, and that local deformation causing deformation further out, etc.?

Yes.

Mass/energy cannot suddenly appear (that violates one of the equations of GR, so you can't ask about it within GR). But if you grab a mass and wiggle it back and forth, that affects the spacetime locally at first, and the effects propagate out in waves moving at the speed of light. That happens because spacetime satisfies a local wave equation, much like the rubber sheet would (although the equation is also different in several very important aspects).

And how can mass (for ex. The Sun) bend space billion and billion and billion away from it self?
WHAT is the Sun “doing” to space so long away?
How does the Sun connect with space billion and billion and billion km away from it self?

I gave you several ways to think about that. You seem to have ignored them.

How can the Sun Reach INFINETY and affect spaces (infinity) away from it self?

It doesn't.

Well we have no answers, - and even a attemp to understand seems to be beyond common sense.

Not at all.

But NOT if you allow you self the thought, that bended space = contracted space.
Nothing prevents us from such understanding.
Contracted space could very well be exactly the same as “bended space”.

Except that it isn't.

How far should this analogy be taken? In other words, a two dimensional sheet deforms when pressed because it dimples in the third dimension, right? You need the third dimension to explain what is happening. Is this analogous to what is happening to spacetime? Is it the third dimension dimpling (for lack of a better word) into the fourth?

Since it's spacetime that deforms in GR, you'd actually need (at least) a fifth dimension. But it turns out it's not necessary to think of curved surfaces (or higher D analogues of surfaces) as embedded in higher dimensional volumes (or higher D analogues of volumes). You always can, but it's not necessary mathematically.

And in fact the number of extra dimensions necessary to embed a surface in higher D depends on the surface. For example a 2-sphere (the surface of a spherical ball) can obviously be embedded in ordinary flat 3D space. But its negatively curved analogue - called by various names, such as hyperbolic 2-space, the Poincare disk, the Lobachevsky plane - cannot be embedded in 3 (Euclidean) dimensions (I think it takes at least 6 to do it smoothly!). Yet both spheres and hyperbolic surfaces are perfectly valid surfaces that differ only in the sign of their curvatures; either would be allowed by Einstein's equations.

 

[Bjarne]

-

Bjarne
How can space bend / deform without a force is involved
Reality Check
Matter causes it without a force being needed. Forces act on material things. Spacetime is not material.

Does that statement express: something you know or something you believe?

Expanding space is though to be a result of the inflationary period

Think about the expression: Dark Energy.
Where you have energy you also have a force right?
Gravity resists the expansion of the Universe, right ?
Hence only a fool will deny that it doesn’t require energy / force to expand the Universe.
So as we easy can see: a force can very well be involved in the process and property of the (expanding) deformation of space, and not only limited to matter. Sorry Reality Check,- reality shows your conclusion very well could be wrong. Nothing proves that energy / force is limited to matter.

Reality Check
Newton believed that gravity was a force between 2 masses.

And Einstein believed that a force was not involved in gravity.
BUT - We can not use: what people believes to anything?

Is the question: whether it requires energy/force to bend space (or not) really definitive settled, and if so where is the proof for such a great a discovery?

Reality Check
Bjarne
How does the Sun connect with space billion and billion and billion km away from it self?
Reality Check
I gave you several ways to think about that. You seem to have ignored them

Sorry I can not see the decisive argument.
Tell me exactly what do we know concerning the relation between matter and the result: bended space.
How does matter bend space, and even billion km away from the matter that causes it to happen?
If we really are able to conclude: that no force is involved, we should at least be able to answer: How does matter bend space. (without a force / energy)
Do you really know how that happens?
So fare I understand we do not know anything about why and how this happen ? Right?
Also not why and how that happens billion and billion miles way from the Sun, - and that’s it.

Bjarne
But NOT if you allow you self the thought, that bended space = contracted space.
Nothing prevents us from such understanding.
Contracted space could very well be exactly the same as “bended space”.
sol invictus
Except that it isn't.

And how do you know that ?
-

 

[Dorfl]

We do not know anything for certain. Nor do we have proof for anything except mathematical theorems. We do have observations which correspond very closely to what General Relativity predicts we should observe, which is why we believe it is an accurate description of reality. And GR says that gravity isn't a force, as such, and that not all bent space is contracted space.

 

[Dorfl]

Bjarne, would you like to start another thread discussing this?

This thread is a zombie with an uncertain subject, it would be nice to have a clear OP where you detail exactly what you're asking about.

 

[shadron]

Yeah, This question like all that seem simple, is just too far over my head I think. I just can't picture in my head what exactly causes gravity. Does each planet have its own gravity and if so what controls their gravity (sun??).

Gravity is an intrinsic property of matter. If it has mass, it has gravity, in direct proportion. The earth not only attracts apples, but the apples also attract Earth, in direct proportion of their masses. There is no known way to shield or "redirect" gravitational attraction.

Gravity is therefore self-controlling. Any two or more bodies in space will attract each other and therefore orbit each other endlessly. Unfortunately, these bodies are not true point sources as are depicted in digital simulations (see orbitsimulator.com, for example), but are bodies that actually take up space. As some orbits bring the bodies close enough together to collide, those orbits then cease to exist in this "reality" model. What you have left, after billions of years, are the orbits that are stable enough to last a very long time. That is the control you seek, the ability to winnow out unstable orbits over (very long) time and be left with only those that are mutually stable.

The solar system is about 5 billion years old, and we still worry about collisions with some of its denizens upon our Earth. Eventually the Earth will absorb them all (all the ones that intersect the Earth's orbit), though it may not always go well for that pond scum on that body's surface. That too is control of a sort.

 

[shadron]

BTW: Why are there two high-tides in a day?

INRM, this has been done to death in a couple of threads just a month or two gone by. Imagine that the parts of the earth were not held together by gravity. In orbit around the sun, the parts closest to the sun (where it is noon, natch) will be in slightly shallower, faster orbits than the rest; those on the far side are in slower orbit. However, the parts do hold together and resist this orbital smearing and are pulled/pushed into the same single orbit. What remains is effectively a residual force that pushes loose parts (the oceans) of the earth's surface both towards and away from the sun. It amounts to about a foot of water height on each side that effectively travels around the world once each sidereal day (due to the Earth's spin). The moon, being closer, causes another, somewhat higher tide (pair of bulges) that follow the moon on approximately a sidereal day plus one hour per day cycle. The solar tides cycle twice every 365/366 days, while the lunar tides cycle twice in about 29/28 days.

 

[Bjarne]

We do not know anything for certain. Nor do we have proof for anything except mathematical theorems. We do have observations which correspond very closely to what General Relativity predicts we should observe, which is why we believe it is an accurate description of reality.

Agree.

And GR says that gravity isn't a force

This is not what Isaac Newton thought, but Yes Einstein did.
Even if we assume that should Einstein was wrong according to that, it doesn’t mean the rest of GR is wrong.
Einstein just thought a force didn’t seem to be necessary to pull down a stone on Earth, but notice he did not try to explain why space bends. To bend space could very well require force.

Bjarne, would you like to start another thread discussing this?
This thread is a zombie with an uncertain subject, it would be nice to have a clear OP where you detail exactly what you're asking about.

Yes why not.

 

[Bjarne]

Sory double..

 

[Dorfl]

INRM, this has been done to death in a couple of threads just a month or two gone by.

Uh... You're aware that you're responding to a post made in 2008?

 

[Reality Check]

-Think about the expression: Dark Energy.
Where you have energy you also have a force right?
Gravity resists the expansion of the Universe, right ?
Hence only a fool will deny that it doesn’t require energy / force to expand the Universe.
So as we easy can see: a force can very well be involved in the process and property of the (expanding) deformation of space, and not only limited to matter. Sorry Reality Check,- reality shows your conclusion very well could be wrong. Nothing proves that energy / force is limited to matter.

Dark energy is called energy because it provides a gravitational force. In GR gravity is a result of mass or energy.
And yes it has an effect on the expansion of the universe.
So you are right - I am wrong.

-
And Einstein believed that a force was not involved in gravity.
BUT - We can not use: what people believes to anything?

Einstein believed that gravitational force is the result of spacetime curvature.
Einstein created a scientific theory where gravitational force is the result of spacetime curvature.
And for some strange reason General Relativity works!
No one thinks that GR is correct because Einstein believed that it was correct.
Everyone trusts that GR is correct beause it works (so far). When we find a better theory that fits more data then we will throw away GR.

-
Is the question: whether it requires energy/force to bend space (or not) really definitive settled, and if so where is the proof for such a great a discovery?

Yes it definitely settled (until a better theory is devised): Tests of general relativity.

-
...snipped...
And how do you know that ?
-

We do not know it.
All we know is what we know about any scientific theory - General relativity works. That is the way science operates - no theory is proved to be true or "known". It is just correct for now.
When a better scientific theory comes about that matches all of the observations that GR matches and explains other observations we will discard GR. This theory will have to reduce to GR in some approximation so that it can also reduce to Newtonian mechanics and gravitation.

 

[Dorfl]

Bjarne's started a new thread here. Let the poor zombie rest.