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 arguments, 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 arguments, 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.