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What is gravity?
Posted: 24 October 2011 01:56 PM   [ Ignore ]   [ # 31 ]
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George - 24 October 2011 01:49 PM

No, Stephen, I have never wondered about that. Sorry. You know how it usually goes when I put on my philosophy hat, right? grin

I think you’ll find this one makes free will look like a walk in the park, it’s even had me considering taking theism seriously grin

Stephen

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Posted: 24 October 2011 11:41 PM   [ Ignore ]   [ # 32 ]
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kkwan - 24 October 2011 09:48 AM
GdB - 24 October 2011 03:09 AM

It makes no sense to ask what is prior: gravity, mass or spacetime. They go together. Mass does not cause gravity, or gravity does not cause spacetime. When there is spacetime, then there is gravity. When there is gravity there is spacetime. Same with mass and gravity.

Quite so. However, this is circular.

What is gravity is not resolved at all.

In some sense you are right, it is circular. But is that a problem? It is just a consequence of repeating the question ‘How can X be explained?’ again and again. Say X can be explained by Y. Then at first every scientist rejoices the new result. But after a while, they will start to look for an explanation of Y. Now there are 3 possibilities:

1. We do not succeed in explaining Y
2. We find an explanation for Y, namely Z
3. X is an explanation for Y.

Take as simple example Newton’s 3rd law:
F = m.a (Force equals mass multiplied with acceleration).
Now how do you define force? Can you do it without reference to mass? Or do we explain mass with the concept of force? If you like it or not, it is circular. But that does not mean Newton’s law is useless. Same for natural selection as ‘survival of the fittest’. Just have a glance at it, you will see that it is circular too. But still, it is the basis of evolutionary biology.

Now how with ‘entropical gravity’? It could be an explanation in the sense of 2), and as Z (informational entropy) is already a known factor, we possibly need less basic concepts to understand the universe. That would be a clear gain. But you present ‘entropical gravity’ as an already established scientific proven fact. And that it is not, not yet at least. And it is a scientific question, not a philosophical one. It is a highly interesting question, that is true. But it is still speculative, but speculative physics is not the same as philosophy. So let’s follow what comes out of this business.

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Posted: 25 October 2011 08:10 AM   [ Ignore ]   [ # 33 ]
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GdB - 24 October 2011 11:41 PM

Take as simple example Newton’s 3rd law:
F = m.a (Force equals mass multiplied with acceleration).
Now how do you define force? Can you do it without reference to mass? Or do we explain mass with the concept of force? If you like it or not, it is circular. But that does not mean Newton’s law is useless. Same for natural selection as ‘survival of the fittest’. Just have a glance at it, you will see that it is circular too. But still, it is the basis of evolutionary biology.

From the wiki on force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape.

Defining force as such is not circular. F=ma is the amount of force needed to move an object of mass m with an acceleration a.

As for natural selection, defining it as “survival of the fittest” is circular but it is not so if it is defined
as in the wiki on natural selection

Natural selection is the nonrandom process by which biologic traits become more or less common in a population as a function of differential reproduction of their bearers.

As for gravity, from the wiki on gravitation

Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass.

Defined as such, it is not circular.

OTOH, in Einsteinian physics:

Modern physics describes gravitation using the general theory of relativity by Einstein, in which it is a consequence of the curvature of spacetime governing the motion of inertial objects.

Without absolute time, space and no force, we have a situation whereby mass/gravity/space-time are intimately interrelated.

Hence, bodies have mass with gravitational fields that are manifested as space-time. This is circular.

But you present ‘entropical gravity’ as an already established scientific proven fact. And that it is not, not yet at least. And it is a scientific question, not a philosophical one. It is a highly interesting question, that is true. But it is still speculative, but speculative physics is not the same as philosophy. So let’s follow what comes out of this business.

The quote from the wiki on entropic gravity which I cited in my first post clearly stated “it is a hypothesis in modern physics”.

Just because gravity relates to physics does not mean it is only scientific question. What is gravity is a metaphysical, epistemological and ontological question as well.

Surely, science has no justifiable absolute claim on gravity, time, space etc. Speculative physics with its concepts and hypotheses verge into the realm of philosophy.

In reality, science is infused with philosophy.

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Posted: 25 October 2011 10:40 AM   [ Ignore ]   [ # 34 ]
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kkwan - 25 October 2011 08:10 AM

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape.

Defining force as such is not circular. F=ma is the amount of force needed to move an object of mass m with an acceleration a.

As for gravity, from the wiki on gravitation

Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass.

Defined as such, it is not circular.

OTOH, in Einsteinian physics:

Modern physics describes gravitation using the general theory of relativity by Einstein, in which it is a consequence of the curvature of spacetime governing the motion of inertial objects.

Without absolute time, space and no force, we have a situation whereby mass/gravity/space-time are intimately interrelated.

Dear kkwan, don’t you see for one moment that your citations just say what I am saying? Force is defined by using mass. Now please lookup what mass is in Wikipedia…

kkwan - 25 October 2011 08:10 AM

In reality, science is infused with philosophy.

Yes and no. That is a topic on its own.

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Posted: 25 October 2011 09:18 PM   [ Ignore ]   [ # 35 ]
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GdB - 25 October 2011 10:40 AM

Dear kkwan, don’t you see for one moment that your citations just say what I am saying? Force is defined by using mass. Now please lookup what mass is in Wikipedia…

Force is defined in the wiki as:

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape.

and not as F=ma.

F=ma is the amount of force needed to move an object of mass m with an acceleration a. That is not the definition of force per se. It is only the relation between force, mass and acceleration in mathematical terms.

From the wiki on mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.

Defined as such, there is no circularity between force and mass in the sense that force is not defined wrt mass and vice versa.

OTOH, in Einsteinian physics:

1. If an object has mass there is a gravitational field and space-time exist.

2. If there is a gravitational field, there is an object with mass and space-time exist.

3. If space-time exist, there is an object with mass and a gravitational field.

In other words, mass, gravity and space-time are all interdependent and can only be defined as such, which is circular.

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Posted: 25 October 2011 10:52 PM   [ Ignore ]   [ # 36 ]
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kkwan - 25 October 2011 09:18 PM

Force is defined in the wiki as:

In physics, a force is any influence that causes an object to undergo a change in speed (1), a change in direction (2), or a change in shape (3).

and not as F=ma.

F=ma is the amount of force needed to move an object of mass m with an acceleration a. That is not the definition of force per se. It is only the relation between force, mass and acceleration in mathematical terms.

No. I put references in your definitions:
1. Is in words what F=ma says.
2. Is also what F=ma says
3. Is about elasticity. Now try to define what elasticity is without referring to a force.

Exactly the same with:

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.

That again is exactly what F=ma says. You cannot define mass without reference to a force. You cannot define force without reference to mass (or elasticity). You are missing the most fundamental knowledge of mechanics.

kkwan - 25 October 2011 09:18 PM

OTOH, in Einsteinian physics:

1. If an object has mass there is a gravitational field and space-time exist.
2. If there is a gravitational field, there is an object with mass and space-time exist.
3. If space-time exist, there is an object with mass and a gravitational field.

In other words, mass, gravity and space-time are all interdependent and can only be defined as such, which is circular.

So there we are, exactly what I said, in classical mechanics and in relativity. Definitions can be circular, and useful at the same time.

Again kkwan, thinking, or even knowledge, is more than chaining citations together.

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Posted: 26 October 2011 08:20 AM   [ Ignore ]   [ # 37 ]
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GdB - 25 October 2011 10:52 PM

No. I put references in your definitions:
1. Is in words what F=ma says.
2. Is also what F=ma says
3. Is about elasticity. Now try to define what elasticity is without referring to a force.

You don’t get it. F=ma is not the definition of force.

Again, from the wiki on force

Definition of force:

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape.

Where is mass in the above definition of force?

Specifically, F=ma is Newton’s second law of motion, not the definition of force.

From the same wiki on force:

The use of Newton’s second law as a definition of force has been disparaged in some of the more rigorous textbooks, because it is essentially a mathematical truism. Notable physicists, philosophers and mathematicians who have sought a more explicit definition of the concept of force include Ernst Mach, Clifford Truesdell and Walter Noll.

Again, from the wiki on mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.

Where is force in the above definition of mass?

Also, note that:

Mass must be distinguished from matter in physics, because matter is a poorly-defined concept, and although all types of agreed-upon matter exhibit mass, it is also the case that many types of energy which are not matter—such as potential energy, kinetic energy, and trapped electromagnetic radiation (photons)—also exhibit mass. Thus, all matter has the property of mass, but not all mass is associated with identifiable matter.

The authors of the wiki on force and mass defined both force and mass carefully to avoid circularity.

[ Edited: 26 October 2011 09:42 AM by kkwan ]
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Posted: 26 October 2011 07:00 PM   [ Ignore ]   [ # 38 ]
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In our ongoing philosophical investigation of gravity, let’s now consider:

If gravity is not a force, how is gravity modeled in physics?

As a gravitational field. From the wiki HERE

The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses.

Gravity is a “fictitious force”:

In such a model one states that matter moves in certain ways in response to the curvature of spacetime, and that there is either no gravitational force, or that gravity is a fictitious force.

The field is not real:

In classical mechanics as in physics, the field is not real, but merely a model describing the effects of gravity.

If the field is only a model and it is not real, then what is it?

Generally accepted fundamental hypothesis?

Dr. Jesse L. Greenstein of the California Institute of Technology wrote:

  The detection of gravitational waves bears directly on the question of whether there is any such thing as a “gravitational field,” which can act as an independent entity. … this fundamental field hypothesis has been generally accepted without observational support. Such credulity among scientists occurs only in relation to the deepest and most fundamental hypotheses for which they lack the facility to think differently in a comparably detailed and consistent way. In the nineteenth century a similar attitude led to a general acceptance of the ether ….

It appears that scientists are at a loss wrt whether there is any observable gravitational field at all.

However:

Most scientists believe that the gravitational field and its gravitational waves are the physical interpretations of Einstein’s equations of general relativity.

If the field is only a model and it is not real, why is there a necessity to believe it exist and observe it at all?

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Posted: 26 October 2011 09:32 PM   [ Ignore ]   [ # 39 ]
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kkwan - 26 October 2011 07:00 PM

In our ongoing philosophical investigation of gravity, let’s now consider:

No, no, no. This is no philosophical investigation in gravity. On one side I am trying to explain simple mechanics to you, on the other side I want you to explain what present state of science about gravity is.

The correct version of F = ma is:

F = dp/dt. 

Bold to express that F and p (impulse)  are vectors (i.e. they have a direction). This is how Newton wrote it, and interestingly, this version is also valid in special relativity, i.e. also for light and for changing mass. Further it is historical that Newton had problems to define what mass is. He was aware of the circularity, and only solve it by defining mass as the product of volume and density, which of course just shifts the problem.

kkwan - 25 October 2011 09:18 PM

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape.

Where is mass in the above definition of force?

The word I made bold. The essential property of physical objects in general is that they have energy and impulse.

kkwan - 25 October 2011 09:18 PM

The use of Newton’s second law as a definition of force has been disparaged in some of the more rigorous textbooks, because it is essentially a mathematical truism. Notable physicists, philosophers and mathematicians who have sought a more explicit definition of the concept of force include Ernst Mach, Clifford Truesdell and Walter Noll.

Yes, they have sought, but not succeeded.

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.

Again, one can define a force with help of mass, or the other way round. Change of velocity, remember, is what a force does, see your citation above.

The authors of the wiki on force and mass defined both force and mass carefully to avoid circularity.

No, They did not succeed. They cannot succeed because the definitions are circular.

There is indirect proof of gravitational waves in binary neutron star systems.

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Posted: 27 October 2011 08:07 AM   [ Ignore ]   [ # 40 ]
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GdB - 26 October 2011 09:32 PM

Further it is historical that Newton had problems to define what mass is. He was aware of the circularity, and only solve it by defining mass as the product of volume and density, which of course just shifts the problem.

From this paper
Newton on mass and force

Newton’s definition of mass:

Newton’s definition explicitly is one of “quantitas materiae”. This quantity of matter is the subject of Newton’s definition, and the definition he gives for it is a quantitative, not a semantic one: “Quantity of matter” is defined through a quantitative, mathematical term: it is the quantity that is represented by the product of “density times volume”. And Newton makes this definition compulsory, saying “I mean this quantity whenever I use the term ‘body’ or ‘mass’ in the following pages.”

Density time volume is only a formula, not the definition of mass:

So, contrary to nearly everything that has ever been written on Newton’s definitio 1, this definition is not one of “mass”, nor is it circular, as some have opined; rather it is a formula for the quantitative determination of an even experimentally measurable quantity of a body’s material contents. The term “mass”, as it appears in Newton’s explanation of the definition, is clearly meant as only another name for this “quantity of matter”, as well as “body” (according to Newton) is such another name. It is nothing but a different semantic expression for the subject of Newton’s definition.

Consequently, if we ask for the meaning of “mass” in Newton’s theory, we must infer that it precisely means “quantity of matter”, and nothing else.

Newton’s concept of force and his Second Law of motion:

However, for the sake of completeness an additional word must be said about Newton’s authentic concept of “force”, as it is present in his second law of motion. I will concentrate on this second law because it is generally but erroneously held to be an equivalent of the formula F = ma.

Newton’s authentic second law:

Now, Newton’s authentic second law in its very different central message reads:

The impressed motive force is proportional to the change in motion.

Quite obviously this statement is not synonymous with its common rendering into F = ma, and consequently our common use of “force”, F, must differ dimensionally from Newton’s force, Fnewton (in the following: Fn). Even though it has sometimes been noticed that Newton’s law is one not on acceleration, but on change of momentum (Latin “mutatio motus”), i.e. Fn µ D(mv), little attention has been paid so far to the fact that Newton puts the force not equal to its effect on a body’s motion, but proportional to it.

Do read the whole paper in full to get the whole perspective.

[ Edited: 27 October 2011 08:13 AM by kkwan ]
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Posted: 27 October 2011 02:33 PM   [ Ignore ]   [ # 41 ]
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Force is a potential of spacetime fabric, a metaphysical concept of a latent excellence and causal potential which is acquired in reality when a massive object is in motion.  When a massive object acquires speed it also acquires additional mass and its potential force is a result of both speed and mass. Even then this force is latent until it meets resistance. At that time its acquired speedmass exerts a force on the resistance.

If we have “spacetime” then things within spacetime relate and must act as objects with “massspeed”, which is measured as Force.

Force is the acquired energetic potential of a massive body in motion (masspeed). The mass is in relation to its coordinate in space, the motion is in relation to its direction in time. To keep mass and speed as seperate qualities is the same as keeping space and time as seperate qualities.

[ Edited: 27 October 2011 02:40 PM by Write4U ]
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Posted: 28 October 2011 08:39 AM   [ Ignore ]   [ # 42 ]
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kkwan - 27 October 2011 08:07 AM

Consequently, if we ask for the meaning of “mass” in Newton’s theory, we must infer that it precisely means “quantity of matter”, and nothing else.

The meaning, yes, but pity enough we cannot use it if we do not make it physically operative. To compare: most people know what a unicorn is (a silver coloured horse like animal with a beautiful spiraled horn on its forehead), which means: the semantics is clear. So literally your source is right. But it has no physical meaning, like the unicorn.

kkwan - 27 October 2011 08:07 AM

Newton’s concept of force and his Second Law of motion:

However, for the sake of completeness an additional word must be said about Newton’s authentic concept of “force”, as it is present in his second law of motion. I will concentrate on this second law because it is generally but erroneously held to be an equivalent of the formula F = ma.

Exactly, but here I must make a small linguistical excuse: I wrote impulse, but in English it is called momentum. If you would understand something of physics you would have immediately recognised my error, because I wrote F = dp/dt as original formulation of Newtons 2nd law. (p stands for momentum (not impulse…)). For non-relativistic mechanics this is equivalent to F = ma (But notice the bold: F, a and p are vectors).

The way Dellian interprets it seems a little far-fetched for me, especially when he identifies Fn : d(mv) = X : Y with the velocity of light. That is not Newtonian at all. But it is an interesting article. I will read it completely, and eventually comment on it.

kkwan - 27 October 2011 08:07 AM

Do read the whole paper in full to get the whole perspective.

Do understand physics, before you write about it.

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Posted: 28 October 2011 12:52 PM   [ Ignore ]   [ # 43 ]
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GdB - 28 October 2011 08:39 AM

The meaning, yes, but pity enough we cannot use it if we do not make it physically operative. To compare: most people know what a unicorn is (a silver coloured horse like animal with a beautiful spiraled horn on its forehead), which means: the semantics is clear. So literally your source is right. But it has no physical meaning, like the unicorn.

To compare Newton’s concept of mass to a unicorn is ludicrous. A unicorn is pure fiction, but mass is real, has physical meaning, properties and effect. Newton will turn in his grave because you compare his fundamental concept of mass to trivial fiction, GdB.  cheese

Do you understand what mass is?

To elaborate on Newton’s concept of mass, from this paper HERE

At that point it is appropriate to summarize the properties of mass in Newtonian mechanics:

1. Mass is a measure of the amount of matter.
2. Mass of a body is a measure of its inertia.
3. Masses of bodies are sources of their gravitational attraction to each other.
4. Mass of a composite body is equal to the sum of masses of the bodies that constitute it; mathematically that means that mass is additive.
5. Mass of an isolated body or isolated system of bodies is conserved: it does not change with time.
6. Mass of a body does not change in the transition from one reference frame to another.

Hence, Newton’s concept of mass is neither fictitious nor trivial and it is “physically operative”. LOL

[ Edited: 28 October 2011 12:55 PM by kkwan ]
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Posted: 28 October 2011 03:02 PM   [ Ignore ]   [ # 44 ]
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kkwan - 20 October 2011 12:55 PM

The effect of gravity i.e. the attraction between two macro physical objects is a fact. On the earth, all free falling objects is due to gravity. Newton characterized gravity as a force and derived his law of gravitation.  However, he had reservations wrt how there could be “action at a distance”.

From the wiki on Newton’s law of gravitation

“That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it.”

Einstein came up with a solution:

In general relativity, the gravitational force is a fictitious force due to the curvature of spacetime, because the gravitational acceleration of a body in free fall is due to its world line being a geodesic of spacetime.

However, even though if one can conceive of massive objects like planets or stars doing that, it still remains quite ridiculous for small objects like apples to fall just like that, notwithstanding Einsteinian physics. In that respect, Newton’s gravitational force makes more sense but “action at a distance” is clearly absurd. How about mediating gravitons, the Higgs boson etc. Very esoteric indeed.  cheese

OTOH, in describing micro quantum objects, QM do not consider gravity at all. 

Apparently, QM is incompatible with general relativity because one intractable issue is gravity.

Does quantum gravity exist or is there no gravity at the quantum scale and if so, why is it so?

What is gravity and why does it do what it does in the macro realm?

According to Erik Verlinde, gravity is an emerging “entropic force” on the macro scale.

From the wiki on entropic gravity

Entropic gravity is a hypothesis in modern physics that describes gravity as an entropic force; not a fundamental interaction mediated by a particle, but a probabilistic consequence of physical systems’ tendency to increase their entropy.

Also, from this article at the
NYT

What is gravity?

Forget curved space or the spooky attraction at a distance described by Isaac Newton’s equations well enough to let us navigate the rings of Saturn, the force we call gravity is simply a byproduct of nature’s propensity to maximize disorder.

Einstein’s equations of general relativity are fundamentally about thermodynamics:

In a provocative calculation in 1995, Ted Jacobson, a theorist from the University of Maryland, showed that given a few of these holographic ideas, Einstein’s equations of general relativity are just a another way of stating the laws of thermodynamics.

His concept of gravity is interesting and controversial. Nevertheless, from the wiki HERE

It appears that Verlinde’s approach to explaining gravity leads naturally to the correct observed strength of dark energy. In June 2011, he was awarded the prestigious Spinoza Prize with a 2.5 million euro grant for this work, including his paper.

If gravity is an emerging macro entropic force and is deterministic, the question which arises is, at the micro level where quantum objects interact randomly and indeterministically, is there any quantum gravity at all?

You question was answered about a century ago.

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Posted: 28 October 2011 06:11 PM   [ Ignore ]   [ # 45 ]
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kkwan,

Forget curved space or the spooky attraction at a distance described by Isaac Newton’s equations well enough to let us navigate the rings of Saturn, the force we call gravity is simply a byproduct of nature’s propensity to maximize disorder.

How is that possible? We started from chaos (disorder) and due to gravitational forces we see an orderly development and evolution of the universe. How can chaos be related to gravity?

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