Why Does Gravity Pull Objects Together?

[chantelhammer]

I am wondering what would go against gravity? I was thinking, if gravity pulls us down, how could we get an object to pull against gravity to make it stand straight up. Like for someone in a wheel chair. Could it be possible to make something that would pull that person up in an upright position?

 

[CDCraig123]

I love how everyone seems to know so much, you ask a question and they throw you a rule of this or according to this it has to be this. This simple question needs to be used by everyone to keep in mind that we just don't know. We have been wrong fare more times then right about most things. At one point in our history we thought the world was flat, that the Earth was the center of all things. Almost nothing is truly known for a fact and can be proven when compared to the almost infinite unknowns. I just spent a lot money on a software program that was suppose to simulate how gravity works as NASA sees it, its suppose to be a copy of the program NASA uses to project orbits with some tweaks. I ran the pre loaded version of are solar system and let it run with in 3 years Venus was flying past Saturn with the latest RK4 (Runge Katta) calculations. I have been in my free time trying to find a simple formula or formulas to show how energy is transferred from different bodies in motion. If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity. What carries this energy? Look at the theory of expansion (assuming that the red shift we use is right) what we know about gravity tells us the universe should be slowing down not speeding up. If that is right where is all this energy coming from to expand everything. Sorry for being long winded.

In short I don't have a freaking clue why there is gravity, but I can come up with a lot of reasons there should be gravity.

 

[Dale]

If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity.

No it doesn't. What would make you say that.

 

[Edward Solomo]

Then, why don't you try asking questions in the domain of interest of physics?

As a TA in 127 calculus, your attitude towards students and others who wish to learn and inquire about physics does not belong on these forums.

 

[Dickfore]

As a TA in 127 calculus, your attitude towards students and others who wish to learn and inquire about physics does not belong on these forums.

How do you know if I am a TA in '127 calculus'?

 

[OnlyMe]

No it doesn't. What would make you say that.

Actually Dale, would not that position be very Machian?

It is not phrased such directly, but for inertia to be as Mach believed, the essence is there. The inertia of each and every gravitational body must be accounted for in the essential nature of inertia itself.

How, that happens is yet to be discovered. Still it is consistent with how things appear to be.

 

[Dickfore]

Actually Dale, would not that position be very Machian?

It is not phrased such directly, but for inertia to be as Mach believed, the essence is there. The inertia of each and every gravitational body must be accounted for in the essential nature of inertia itself.

How, that happens is yet to be discovered. Still it is consistent with how things appear to be.

I was right when I said this is a Physics forum and not a Philosophy one.

 

[Dale]

Actually Dale, would not that position be very Machian?.

I don't see the relevance at all. He made incorrect assertion that there is energy transfer involved in a situation where velocity is changing but not speed. I don't think that Mach would make the same assertion.

 

[Edward Solomo]

I was right when I said this is a Physics forum and not a Philosophy one.

Except there is a theory in physics (although highly debated) that does provide an explanation for inertia and the constant G itself. The link is to a thread on these forums. Read the second post. So, unless this theory is discredited, you are wrong.

Also, if this theory were right, it would answer the OP's "PHILOSOPHICAL" question, which is: Why does gravity exist?

A question that pertains to physics or philosophy, whose answer is unknown, cannot be determined to be a question of either. If the answer is known, than it pertains to physics. Therefore who are you to judge which questions pertain to physics and which pertain to philosophy?

https://www.physicsforums.com/showthread.php?t=440707

Re: The Origin of Inertia
Originally Posted by Charlie G View Post

I was wondering if anyone is familiar with D.W. Sciama's theory of inertia based on Mach's principle? I came across the theory in a book or selected writings on motion I found at a used book sale and found it very compelling.

Just curious to hear any fallacies in his reasoning or negative experimental results from anyone who is familiar with the work.

Thanks.

Sciama's explanation of inertia is beautifully neat, but it is based on a simple analogy between gravity and electromagnetism which is of the same order of accuracy as Newtonian theory combined with Special Relativity, so it is more of an illustrative idea than a full-fledged theory.

If the same idea is applied to General Relativity, it suggests that inertia would simply be caused by the linear frame-dragging effect of the entire universe, and this means that the gravitational "constant" G would actually be given by an expression of the form

G=k∑imi/ric2where k is a simple constant (probably 1/4 for maximum compatibility with GR,
at least according to Nordtvedt) and the sum is for all masses and their distances from the observation point.
There are various alternative gravity theories which are to some extent based on Mach's
Principle (such as Brans-Dicke theory) and these typically require at least some effective variation in G as in the above expression.

If this idea of a varying G were true, it would conflict with GR, which assumes that G is constant. It would mean that G could vary both with location in space and with time, but experiments have placed very severe constraints on any such variation.

Variation with space is not necessarily ruled out by experiment, because in the simplest form of Sciama's idea, the variation in G due to local masses manifests as the varying gravitational potential, and when the potential is converted back to Newton's form, the G which appears describes the effect of all non-local masses, which is effectively constant.

Variation with time is more severely constrained, in that if there is any variation occurring at present, it appears to be on a time scale greater than the age of the universe, which seems to rule out any simple model based on Sciama's idea.

Another closely related aspect of Sciama's idea is that rotation is relative to the rotational frame-dragging effect of all the masses in the universe. This too apparently requires G to be variable in a similar way. This effect is described as the "sum for inertia" and is mentioned for example in MTW "Gravitation".

Personally, I find Sciama's ideas compelling, and I consider it very disappointing that GR appears to be provably incompatible with them, even though there appears to be a surprisingly strong coincidence that both the linear and rotational frame-dragging effects of the whole universe appear to be around the right order of magnitude.

Given that GR is having a lot of problems explaining experimental observations (galaxy rotation curves requiring "dark matter" with increasingly implausible properties, weak lensing results, anomalous redshifts), I personally suspect that GR itself needs some fixing, and I think that Sciama's Machian ideas may provide some useful foundations for a more successful theory.

 

[Dickfore]

Also, if this theory were right, it would answer the OP's "PHILOSOPHICAL" question, which is: Why does gravity exist?

A question that pertains to physics or philosophy, whose answer is unknown, cannot be determined to be a question of either. If the answer is known, than it pertains to physics. Therefore who are you to judge which questions pertain to physics and which pertain to philosophy?

https://www.physicsforums.com/showthread.php?t=440707

Why is Mach's Principle valid?

 

[Edward Solomo]

Why is Mach's Principle valid?

It's more likely that Mach has an incomplete theory and has laid the foundation for the next chapter in the history of physics. I came across that thread because I was convinced of a similar result of my own and I believe I will be orientating my career towards the completion of Mach's Principle (I'm also interested in the recent Dark Flow observations).

Was Democritus's theory of atoms valid? No it wasn't, but it also laid the foundation or the next evolutionary chapter in physics nearly two thousand years later.

We are more often wrong than we are right, but we must fail in order to succeed.

 

[romsofia]

As a TA in 127 calculus, your attitude towards students and others who wish to learn and inquire about physics does not belong on these forums.

I think what dickfore is saying, if you're interested in physics know that it won't always tell you WHY something happens. Which is good because if people are looking for the WHYs of the world, then physics isn't really for them.

 

[Dickfore]

It's more likely that Mach has an incomplete theory and has laid the foundation for the next chapter in the history of physics. I came across that thread because I was convinced of a similar result of my own and I believe I will be orientating my career towards the completion of Mach's Principle (I'm also interested in the recent Dark Flow observations).

Was Democritus's theory of atoms valid? No it wasn't, but it also laid the foundation or the next evolutionary chapter in physics nearly two thousand years later.

We are more often wrong than we are right, but we must fail in order to succeed.

My point is that you should have seen the video I linked.

When we try to explain the explanation behind a series of phenomena by invoking a principle that is more 'fundamental' than the previous one, we are always at the danger of forming a backward cascade of causal relationships, which we must end at some point. According to our current understanding of Nature, there is nothing more fundamental than the explanation given by GR.

Furthermore, a same person (such as Mach or Democritus) might be working both as a Philosopher or a Physicist. When Mach worked on the problems of propagation of sound waves, he was surely a physicist. When he was speculating about Cosmology, such as in the case of the Mach's conjecture, he was more of a philosopher than a physicist. At that time, Cosmology was not a branch of physics, as it is considered now after the Big Bang Theory. The same goes for Democritus. If his model was aimed at explaining observable facts about matter, then we would consider it physics. If it was a matter of a principle or a general idea without the regards for the details, then I would say it was a philosophy.

Finally, and this is the most crucial part, all these 'alternative theories' give predictions which are quite different than the accepted theory. This means they are not equivalent and experiment can distinguish between them. Until these theories spit out verifiable predictions that can be put to the scrutiny of experiment, they are philosophical considerations with no necessary connection to the world that surrounds us.

 

[Edward Solomo]

I think what dickfore is saying, if you're interested in physics know that it won't always tell you WHY something happens. Which is good because if people are looking for the WHYs of the world, then physics isn't really for them.

This isn't a Philosophy, but a Physics forum.

This is his first response to the OP's question. The attitude wreaks of rudeness. Not to mention several people said it in much more respectable way before the entered scene.

If he is allowed to be rude to new comers, I'm certainly allowed to call it out.

 

[Dickfore]

Oh, go whine somewhere else, will you?

 

[CDCraig123]

I don't see the relevance at all. He made incorrect assertion that there is energy transfer involved in a situation where velocity is changing but not speed. I don't think that Mach would make the same assertion.

Velocity is a distance per time in a certain a direction(vector quantity). Speed is a distance per time. Because Earth is orbiting a body which is in turn orbiting another body the speed will at some point have to change, and velocity is always changing. Earths speed is not a constant.

 

[Dickfore]

Velocity is a distance per time in a certain a direction(vector quantity). Speed is a distance per time. Because Earth is orbiting a body which is in turn orbiting another body the speed will at some point have to change, and velocity is always changing. Earths speed is not a constant.

lol, lrn2uniformrotation

 

[CDCraig123]

lol, lrn2uniformrotation

Uniform rotation? I apologize I only read and write English.

 

[Drakkith]

Velocity is a distance per time in a certain a direction(vector quantity). Speed is a distance per time. Because Earth is orbiting a body which is in turn orbiting another body the speed will at some point have to change, and velocity is always changing. Earths speed is not a constant.

What Dalespam means is that, other than small variations due to the orbital shape of the Earth, the speed of the Earth compared to the sun is always the same while the velocity changes. The angular momentum of the Earth in June is almost exactly the same as it is in December. No momentum or energy has been lost or gained even though the direction of motion is constantly changing.

 

[harrylin]

I was right when I said this is a Physics forum and not a Philosophy one.

Mach vs. Newton is very much physics

[..] if people are looking for the WHYs of the world, then physics isn't really for them.

"Why" is often a question about the physical cause; and that is a typical physics topic.

 

[WannabeNewton]

"Why" is often a question about the physical cause; and that is a typical physics topic.

Couldn't agree more my friend.

 

[Dale]

According to our current understanding of Nature, there is nothing more fundamental than the explanation given by GR.

I think that this is a key point. In science there is always a fundamental explanation beyond which we can only say "because it fits the data", i.e. that's just the way things are. As you mention, that is the position held by GR. We can explain many less fundamental things in terms of GR's Einstein Field Equations, but if you ask "why the EFE" the only answer is that it fits the data.

 

[Dale]

Velocity is a distance per time in a certain a direction(vector quantity). Speed is a distance per time. Because Earth is orbiting a body which is in turn orbiting another body the speed will at some point have to change, and velocity is always changing. Earths speed is not a constant.

A circular orbit has no change in speed and therefore no change in KE. An elliptical orbit simply changes KE for PE, there is no energy transfer.

The only way to get an energy transfer is via a hyperbolic trajectory "gravitational slingshot" or via tidal effects. Neither of those are applicable to the Earth orbiting the sun.

 

[OnlyMe]

If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity.

No it doesn't. What would make you say that.

Actually Dale, would not that position be very Machian?
It is not phrased such directly, but for inertia to be as Mach believed, the essence is there. The inertia of each and every gravitational body must be accounted for in the essential nature of inertia itself.

How, that happens is yet to be discovered. Still it is consistent with how things appear to be.

For my part in the above exchange, which seems to have ignited some controversy, I can only say that my post was little more than a comment on what I saw as the larger context within CDCraig123's post. Generally I try to read posts, even published papers with an ear for the larger context. I should have probably explained my own context more fully at the time.

From the context of a discussion of gravity, the energy exchange between bodies suggested by CDCraig123's original post, can easily be associated with gravitational interaction, which does represent an exchange of energy, resulting in a balanced inertial system. Within that larger context, it just seemed that CDCraig123 was describing a Machian perspective of inertia. It was really initially no more than an observation, of the similarity. The energy transfer being the universal influence of gravitational interaction between all objects and the solar system model an inertial representation of that "exchange", locally.

While it is true, that to a significant extent Mach's views, as they relate to this topic, remain largely philosophical, Einstein was influenced sufficiently by Mach that he worked hard in an attempt, to try and incorporate Mach's ideas into his field equations.., unsuccessfully.

Even so, generally.., the concepts attributed to Mach involving the origin of inertia, continue to play a central role in many cosmological models involving inertia. And no we do not as yet have a definitive answer to its origin. Still it is difficult to deny the similarities and connections between what we experience as inertia and what we experience as the force of gravitation.

A circular orbit has no change in speed and therefore no change in KE. An elliptical orbit simply changes KE for PE, there is no energy transfer.

The only way to get an energy transfer is via a hyperbolic trajectory "gravitational slingshot" or via tidal effects. Neither of those are applicable to the Earth orbiting the sun.

This would seem to me, to assume that the involved gravitational center of mass is "at rest" or that at least the frame of reference from which it is true, is one at rest relative to the involved inertial system. The orbit of a planet, moon, star or even galaxy winds up more complex than the simple model suggested. While the Earth's motion/speed, in orbit relative to the sun appears to be constant, when compared to the galaxy as a whole it is constantly changing. Kinetic energy and speed are both relativistic and dependent upon one's frame of reference.

Though it was obviously not clear, my intent had not and is not to be contradicting the issues raised in Dale's posts. I only meant to be broadening the frame of reference.

With respect to the science vs philosophy argument, I don't believe it has any merit. When the discussion involves subjects or phenomena which are beyond the explanation of current scientific theory and/or models. The advancement of science in such cases has often evolved from a logical examination of a philosophical approach to those unanswered questions.

Science at present gives us no, unflawed explanation of inertia. Nether does Mach's ideas on the subject. On the other hand Mach's "philosophical" perspective does give science a valid underlying frame of reference from which to explore the subject.

 

[baywax]

I love how everyone seems to know so much, you ask a question and they throw you a rule of this or according to this it has to be this. This simple question needs to be used by everyone to keep in mind that we just don't know. We have been wrong fare more times then right about most things. At one point in our history we thought the world was flat, that the Earth was the center of all things. Almost nothing is truly known for a fact and can be proven when compared to the almost infinite unknowns. I just spent a lot money on a software program that was suppose to simulate how gravity works as NASA sees it, its suppose to be a copy of the program NASA uses to project orbits with some tweaks. I ran the pre loaded version of are solar system and let it run with in 3 years Venus was flying past Saturn with the latest RK4 (Runge Katta) calculations. I have been in my free time trying to find a simple formula or formulas to show how energy is transferred from different bodies in motion. If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity. What carries this energy? Look at the theory of expansion (assuming that the red shift we use is right) what we know about gravity tells us the universe should be slowing down not speeding up. If that is right where is all this energy coming from to expand everything. Sorry for being long winded.

In short I don't have a freaking clue why there is gravity, but I can come up with a lot of reasons there should be gravity.

It may be a little like saying, "why is the sky blue?" then answering "because it is." But, there is a "reason" why there is gravity and that is because there is mass.

 

[Drakkith]

From the context of a discussion of gravity, the energy exchange between bodies suggested by CDCraig123's original post, can easily be associated with gravitational interaction, which does represent an exchange of energy, resulting in a balanced inertial system. Within that larger context, it just seemed that CDCraig123 was describing a Machian perspective of inertia.

I disagree. I think it shows a lack of understanding of relativity, energy, and several other topics.

This would seem to me, to assume that the involved gravitational center of mass is "at rest" or that at least the frame of reference from which it is true, is one at rest relative to the involved inertial system. The orbit of a planet, moon, star or even galaxy winds up more complex than the simple model suggested. While the Earth's motion/speed, in orbit relative to the sun appears to be constant, when compared to the galaxy as a whole it is constantly changing. Kinetic energy and speed are both relativistic and dependent upon one's frame of reference.

The key here is that talking about an orbit necessitates identifying what is orbiting what. In Dalespams case it was the Earth orbiting the Sun. You cannot ignore that and still talk about the Earths kinetic energy because the Earth is not an inertial frame.

As you said, kinetic energy and speed are both relative to a frame. This is why it is so important to identify what frame you are observing from or comparing something to. CDCraig is NOT using the Sun as the frame for the Earths orbit, but is instead using multiple other frames. It is a given that according to SOME frame the Earth's speed is not constant.

 

[harrylin]

[..] In science there is always a fundamental explanation beyond which we can only say "because it fits the data", i.e. that's just the way things are. As you mention, that is the position held by GR. [..]

GR doesn't have such a position; if it did, then it would not be Einstein's theory as his position was different - see #28.

 

[Dale]

GR doesn't have such a position; if it did, then it would not be Einstein's theory as his position was different - see #28.

By "position" I meant that amongst the successful theories of gravity, GR is the fundamental theory. I.e. You can derive Newtonian gravity from GR, but not vice versa. Therefore, if you ask "why Newtons law of universal gravitation" you can answer in terms of the EFE, but is you ask "why the EFE" all you can do is answer in terms of experimental results. It has nothing to do with the Einstein's later thoughts on the aether.

 

[harrylin]

By "position" I meant that amongst the successful theories of gravity, GR is the fundamental theory. [..] is you ask "why the EFE" all you can do is answer in terms of experimental results. [..]

Thanks for the clarification; it sounded as if you meant the contrary!
Indeed, GR itself doesn't have the position that "there is nothing more fundamental than the explanation given by GR"; as a matter of fact, it doesn't even try to explain the "why" in the sense of the question.


Harald

 

[OnlyMe]

If conservation of energy is right that means every time Earth orbits the sun energy has to be moved from Earth to the sun and to the surrounding starts and Milky way galaxy and to the local cluster and then back to Earth to keep it in a stable orbit because of its change velocity. What carries this energy?

From the context of a discussion of gravity, the energy exchange between bodies suggested by CDCraig123's original post, can easily be associated with gravitational interaction, which does represent an exchange of energy, resulting in a balanced inertial system. Within that larger context, it just seemed that CDCraig123 was describing a Machian perspective of inertia.

I disagree. I think it shows a lack of understanding of relativity, energy, and several other topics.

If we assume that the solar system, our solar system, is an isolated gravitational system, when the Earth orbits the sun the Earth is accelerated toward the Sun and the Sun is accelerated toward the earth. The center of mass for the system as a whole is not the center of mass of the sun. It is the barycenter.

Are you saying that the kinetic changes in the motions of the planets and the Sun, which result from their gravitational interaction, do not represent an exchange of kinetic energy? And that CDCraig's post cannot be viewed as an exchange or transfer of kinetic energy resulting from the dynamic gravitational relationship involved?