So.. we don't know what causes gravity?

It seems so weird to me... that we know so much about the world, but the simplest of things, the one thing that has been since the beginning of forever... we don't fully understand.

One of my science teachers once told me that if we placed two pens a few feet apart in space that wasn't influenced by any object's gravity, that they would draw themselves closer together. He couldn't tell us why, he just said that that's the way it is...

But seriously, we don't know why gravity... occurs?

[Erasmus00]

Originally Posted by kyle8921

It seems so weird to me... that we know so much about the world, but the simplest of things, the one thing that has been since the beginning of forever... we don't fully understand.

One of my science teachers once told me that if we placed two pens a few feet apart in space that wasn't influenced by any object's gravity, that they would draw themselves closer together. He couldn't tell us why, he just said that that's the way it is...

But seriously, we don't know why gravity... occurs?

The best theory of gravity to date is Einstein's theory of general relativity. This theory suggests that gravity is caused by curved space-time. It sounds more complicated then the basic idea (or maybe it does not). To use an oft quoted analogy that I think is Hawking's (if it is not, someone please correct me) consider a bed sheet stretch very tightly. If you roll marbles across the bed sheet they go straight. Now, put a basketball in the middle of the sheet. As you roll marbles across, they'll curve toward the basketball, the reason being they follow the curvature of the sheet.

Einstein's theory is crudely similar to the bedsheet. However, with the bedsheet, the curvature is of a two dimensional surfact. Real gravity seems to be the product of not 2 or even 3 dimensional curvature, but a 4 dimensional one (because the earlier theory of special relativity added the dimension of time to our spacial dimensions)
-Will

[Aki]

Here's a page for general relativity: http://en.wikipedia.org/wiki/General_relativity

[Tormod]

Originally Posted by kyle8921

But seriously, we don't know why gravity... occurs?

"Why" it occurs is not so important. We don't know the "why" of a lot of things.

But yes, it is correct, we don't really know what gravity is nor what causes it. Curved spacetime is Einstein's interpretation and it does work very well as an explanation. It is still a mystery - very exciting stuff.

[Qfwfq]

Originally Posted by Tormod

But yes, it is correct, we don't really know what gravity is nor what causes it. Curved spacetime is Einstein's interpretation and it does work very well as an explanation. It is still a mystery - very exciting stuff.

I quite agree, the geometric interpretation still leaves the question of "why" a mass causes the curvature. Apart from the fact that, I believe, we aren't yet near to distinguishing experimentally between the GI and other interpretations.

[Erasmus00]

Originally Posted by Qfwfq

I quite agree, the geometric interpretation still leaves the question of "why" a mass causes the curvature. Apart from the fact that, I believe, we aren't yet near to distinguishing experimentally between the GI and other interpretations.

The precession of mercury leads some support to GR, and the shapiro time delay experiments (which calculates the time delay in a light beam traveling from here to mars an back). The deflection of light due to gravity has also been observed and is as GR calculates. Also, observations of binary stars support the idea of gravitational waves, although waves are a feature of any non-instantaneous theory of gravity. The post newtonian parameters have been measured to some ridiculous degree of accuracy, though, so if a theory of gravity differs from GR, it must differ third or fourth order in the PPN. I even think the gravitational time dilation has been measured. Lots and lots of verification, and as measurements of the PPN get better and better, most of the other candidates seem to be dropping out.
-Will

[UncleAl]

http://www.fourmilab.ch/gravitation/foobar/

Annalen der Physik 4 XVII 891-921 (1905)
Annalen der Physik 4 XLIX 769-822 (1916)

http://relativity.livingreviews.org/...1-4/index.html
http://www.weburbia.demon.co.uk/phys...periments.html
Experimental constraints on General Relativity

Reality is not Galilean, Newtonian, or classical. As much as approximating

lightspeed = infinity
Planck's constant = zero
Newton's G = G

makes for a simple, comfortable physics model that works beautifully under common circumstances, it is wrong. The world cannot be accurately modeled with simple theory using simple maths. If you wish to have some empirically useful understanding of what really occurs - to be able to accurately predict - you must invest in the best real answers we have. It is difficult. The alternative is to be one of the mob going and doing what it is told. Look up "fungible."

Uncle Al says, 'The future is created by those who show up to do it."

[Qfwfq]

Note Erasmus, that I wasn't challenging GR, I only commented on the geometric interpretation which is harder to confirm indisputabely. I saw most of those things in my GR course, years ago. I also remember Weingerg, in "Gravitation and cosmology" giving the GI as not being uniquely prooved by GR, not currently. A few months ago there was a bit of discussion on these boards about the GI vs. the optical interpretation, VSL and PV.

If you truely understand GR you must realize that the considerations can be much more subtle than the precession of Mercury's perihelion and other things that don't prove the global topology of space-time. The very fact that GR is all about general coordinate transformations makes it subtle to experimentally prove that the geometrical interpretation isn't just an interpretation. Large scale cosmology is still fraught with difficulties and unknowns.

[Erasmus00]

Originally Posted by Qfwfq

Note Erasmus, that I wasn't challenging GR, I only commented on the geometric interpretation which is harder to confirm indisputabely. I saw most of those things in my GR course, years ago. I also remember Weingerg, in "Gravitation and cosmology" giving the GI as not being uniquely prooved by GR, not currently. A few months ago there was a bit of discussion on these boards about the GI vs. the optical interpretation, VSL and PV.

If you truely understand GR you must realize that the considerations can be much more subtle than the precession of Mercury's perihelion and other things that don't prove the global topology of space-time. The very fact that GR is all about general coordinate transformations makes it subtle to experimentally prove that the geometrical interpretation isn't just an interpretation. Large scale cosmology is still fraught with difficulties and unknowns.

My apologies, I wasn't reading clearly. I must have hit the g and my brain filled in general relativity instead of geometrical interpretation. I may go digging for that thread, as I'm only actually familiar with the geometrical and the optical theory. I've never heard a variable speed of light interpretation of GR, and honestly have never heard of PV.

And, like all theories, interpretations are nearly impossible to test. I'm personally partial to the geometric interpretation, if only because I find it makes the math behind the theory particularly elegant.
-Will

[CraigD]

Your teacher is correct. Although Newton’s Law of Universal Gravitation describes it pretty well, and Einstein’s General Relativity seems to describe it well enough to account for everything we can observe (including the precession of the orbits of the planets, which Newton’s law failed at), both are just descriptions, not satisfying explanations, of gravity.

What’s worse, there’s strong reason to suspect that even GR will fail to describe it enough to explain phenomena like the interior of black holes.

A couple of generations of scientists have been as bothered by this as you appear to be. Most science folk are hoping that we’ll be able to expand The Standard Model of Particle Physics to include gravity, so much so that there’s even a name for it in that theory – “graviton” – and a family – the bosons – with a bunch of relatives - the photon, gluon (the “strong nuclear” force that keeps atomic nuclei together), W, Z, and maybe several “Higgs bosons”. Nobody’s managed to actually observe a Higgs yet, but most people are confident one will be found at least by 2007, when then next biggest particle collider, CERN’s LHC, is scheduled to begin work.

The Standard Model has done a beautiful job of explaining 3 out of the 4 fundamental forces, so you can understand why people are hopeful it’ll do the same for gravity.

If all goes according to plan, then, we’ll know if the Higgs exists by 2010. We may understand gravity in the next 10 or 20 years – it’s hard to say. Then we’ll be ready for an even bigger question: why is the Standard Model the way it is? Nobody has a satisfying explanation for exactly why there are the fundamental particles it describes, or why each particle has the precise characteristics it does.

My personal favorite for the next question is “Quantum Graph Theory”. It’s still so speculative, though, that the best explanations of it I’ve encountered are in a SF novels (

"Schild's Ladder"
"Schild's Ladder"

by Greg Egan), and even there it’s hard to grasp.

We live in exciting times - Physics is just starting to get difficult.