Why Does Gravity Pull Objects Together?

[gkangelexa]

so, I know the gravitational force is F = (Gm1m2)/r²... but what makes objects attract each other?

what is it about a big body such as the Earth that makes it have gravity?

 

[tenchotomic]

I think answer to your question is provided by general relativity:

"[URL
http://en.wikipedia.org/wiki/General_relativity

"en.wikipedia.org/wiki/Introduction_to_general_relativity"[/URL]

 

[Drakkith]

Are you looking for WHY, or for HOW?

 

[abhishekpant]

it is due to "graviton particles".

 

[Drakkith]

it is due to "graviton particles".

No, it is not. We have seen no evidence for a graviton yet.

 

[abhishekpant]

ya you are right graviton is a hypothetical elementary particle , but we can't deny it.
see this link,
http://en.wikipedia.org/wiki/Graviton
thanks

 

[Drakkith]

ya you are right graviton is a hypothetical elementary particle , but we can't deny it.
see this link,
http://en.wikipedia.org/wiki/Graviton
thanks

Yes we can. And we must deny it until sufficient evidence is gathered to prove it's existence. If you want to ask specifically about how a graviton is thought to work, then feel free. But we cannot say that gravitons are the carrier of gravity if we have don't have sufficient reason to say they exist yet. Note that I am talking about answering the question "How does gravity work" with the claim that it is because of gravitons. It simply is not the accepted way of how it works.

I don't think I explained it very well, but I hope you understand what I'm saying.

 

[SteveL27]

so, I know the gravitational force is F = (Gm1m2)/r²... but what makes objects attract each other?

what is it about a big body such as the Earth that makes it have gravity?

Isaac Newton was asked the very same question in 1676. He said, famously: "I frame no hypotheses." Meaning, that Newton well understood that his theory of gravity was descriptive, not explanatory. Newton was providing an equation that would allow one to calculate the gravitational attraction between two bodies; but he was not putting forth any explanation of the underlying cause.

The fuller quote is:


Hitherto I have not been able to discover the cause of those properties of gravity from the phenomena, and I frame no hypothesis; for whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy.

http://www.thenagain.info/Classes/Sources/Newton.html

I think it's well to keep Newton's wisdom in mind. If tomorrow morning they discover the Higgs particle, and the newspapers go on about the "God particle" and announce that now, finally, the physicists have unlocked the secret of the universe; the truth is that they would have done nothing of the kind. We would know that there is a Higgs particle. But we would not know why there is a Higgs particle.

All science can do is describe nature. The underlying causes -- the "why?" -- are beyond science, by definition. Newton knew this. I believe it's still true.

Just my humble perspective on this ... science describes, it doesn't explain.

 

[Adam Strange]

Here's an explanation I like:

http://www.science20.com/hammock_physicist/it_bit_entropic_gravity_pedestrians-66244

 

[Dale]

Just my humble perspective on this ... science describes, it doesn't explain.

Fundamentally, I agree.

 

[tenchotomic]

All science can do is describe nature. The underlying causes -- the "why?" -- are beyond science, by definition. Newton knew this. I believe it's still true.

Just my humble perspective on this ... science describes, it doesn't explain.

Although Iam a rookie when it comes to these philosophical arguements, I would like to quote a famous physicist here:

The purpose of theoretical physics is not just to describe the world as we find it,but to explain -in terms of few fundamental principles- why is the world the way it is.
Steven Weinberg

 

[OnlyMe]

Comparing the following two quotes, highlights Newton's real genius. The comparison should be extended even further. So much of what we think we know today would fall into the same category, of describing rather than explaining. We have a great deal of knowledge about how things interact (the how here being descriptive of the interaction) and very little about the underlying how and why (the how and why here representing the fundamental cause, reason and origin).

Thanks Steve, for the reminder.

Isaac Newton was asked the very same question in 1676. He said, famously: "I frame no hypotheses." Meaning, that Newton well understood that his theory of gravity was descriptive, not explanatory. Newton was providing an equation that would allow one to calculate the gravitational attraction between two bodies; but he was not putting forth any explanation of the underlying cause.

The fuller quote is:


Hitherto I have not been able to discover the cause of those properties of gravity from the phenomena, and I frame no hypothesis; for whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy.

http://www.thenagain.info/Classes/Sources/Newton.html

The purpose of theoretical physics is not just to describe the world as we find it,but to explain -in terms of few fundamental principles- why is the world the way it is.
Steven Weinberg

 

[Nabeshin]

what is it about a big body such as the Earth that makes it have gravity?

Just to clarify something lurking in the OP's post... There's nothing special about the Earth that gives it gravity -- the bigness does not come into it. Of course, any object will produce a gravitational field, however big or small.

I generally agree with the sentiment that science cannot answer the why question. Newton posited no hypothesis for why gravity behaved as it did. Einstein came along and said "Mass warps the geometry of spacetime", but he didn't really answer why this is the case (He produced an equation which perfectly describes how it works, but as to why, who knows?). Maybe in the future someone will come and say "Well obviously mass warps the geometry of spacetime because of X", but then naturally we'll ask "well why does X do that?". At some point you have to just accept some axiom as true and work your way up from there, otherwise there is no ground to stand on, so to speak.

 

[Dickfore]

so, I know the gravitational force is F = (Gm1m2)/r²... but what makes objects attract each other?

what is it about a big body such as the Earth that makes it have gravity?

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

 

[DaveC426913]

Just my humble perspective on this ... science describes, it doesn't explain.

Agreed. But there are two types of 'why' question. One is the philosophical' why' as in what is the purpose or meaning. But the physics one is simply 'what causes this? What is the deeper phenomenon?'

(Why does water fall over a cliff? Because of gravity's action. Why does a rainbow have colours. Becuae of diffraction.)

So, while science is meant to describe, really what the OP may be asking is for us to merely describe gravity on a deeper level. 'What property of matter results in it manifesting gravity?'

 

[gkangelexa]

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

I know i wanted a physics answer...

 

[Dickfore]

I know i wanted a physics answer...

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

 

[gkangelexa]

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

This is a physics forum. gravity pertains to physics. I can ask what i want as long as it pertains to physics. I did not know if the explanation to my question would be philosophical or mathematical.

And how is this not in the "domain of interest of physics"?

 

[WannabeNewton]

The best explanation of that is probably the one given by the field equations of general relativity which post #2 gave a Wikipedia link to so you could start there. Yours was a perfectly valid physics question.

 

[Drakkith]

This is a physics forum. gravity pertains to physics. I can ask what i want as long as it pertains to physics. I did not know if the explanation to my question would be philosophical or mathematical.

And how is this not in the "domain of interest of physics"?

Don't get too worked up. Your question was fine; it just needed a little clarification. Has it been answered already?

 

[Dale]

Although Iam a rookie when it comes to these philosophical arguements, I would like to quote a famous physicist here:

The purpose of theoretical physics is not just to describe the world as we find it,but to explain -in terms of few fundamental principles- why is the world the way it is.
Steven Weinberg

Sure, but the fundamental principles are only described, not explained. There is always a last layer that is simply experimentally observed and has no theoretical justification.

 

[DaveC426913]

This is a physics forum. gravity pertains to physics. I can ask what i want as long as it pertains to physics. I did not know if the explanation to my question would be philosophical or mathematical.

And how is this not in the "domain of interest of physics"?

Yes, this is not your fault.

IMO, science-minded people (read: PFers) often interpret "why" too literally.

See my clarification, post 15.

Personally, I think this should be the default assumption, to-wit: "Hi gkangelexa, I'm going to assume when you say why, you really mean how..."

 

[Oldfart]

Why does a rainbow have colours. Becuae of diffraction.)

You mean refraction, me thinks...

 

[baywax]

Don't get too worked up. Your question was fine; it just needed a little clarification. Has it been answered already?

This is a little like my question "is gravity an emergent phenomenon?" (and, of course, I had to correct myself and ask if gravity is an "emergent property".

Check out my thread...

https://www.physicsforums.com/showthread.php?t=474303

 

[Dickfore]

This is a physics forum. gravity pertains to physics. I can ask what i want as long as it pertains to physics. I did not know if the explanation to my question would be philosophical or mathematical.

And how is this not in the "domain of interest of physics"?

https://www.youtube.com/watch?v=wMFPe-DwULM​

 

[Phrak]

Personally, I think this should be the default assumption, to-wit: "Hi gkangelexa, I'm going to assume when you say why, you really mean how..."

Yes! and a nice example with the waterfall. We might ask ourselves why, and with physics we end up answering 'how', only you put it much better.

 

[Phrak]

To ask gkangelexa's question in another way, "How is displacement inextricably linked to energy density?"

This is something that shows up in like fashion in quantum mechanics. E,p :: omega,k.

 

[harrylin]

I know i wanted a physics answer...

Modern physics (free from philosophy or models of the unseen), has not yet found an answer on that question. However, physics does traditionally include not only mathematics but also models or concepts of the unseen ("philosophy"). For example, "atoms" were for many centuries the object of philosophical debate.

As several "philosophical" physics suggestions have been given already, here's one more by Einstein (see in particular p.19-21):
http://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity

Cheers,
Harald

 

[DaveC426913]

You mean refraction, me thinks...

Doh. Too much typing too fast. Thx.

 

[Phrak]

I know i wanted a physics answer...

I'll try to answer.

Special relativity came about because Einstein thought that all inertial frames where physically indistinguishable and equivalent. This was successful and has had much experimental support.

Next, general relativity came about because Einstein thought that all frames, in any state of relative motion, were equivalent. This includes frames of reference accelerating with respect to one another. This idea is called general covariance.

We could say that gravity exists because the laws of nature are independent of an imposed coordinate system.

 

[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.