What exactly is gravity?

[proregulus]

Thanks for your feedback folks - I am pleased to have stirred up some response.

Selden
I agree that what matters is how well a particular theory predicts what is observed. However, I do get the feeling that the 'physics establishment' have too much invested in particular current theories to give 'due diligence' to the consideration of other therories that might, just might?, be even better at predicting what we observe: for example a simple theory that would have utility in gravity as well as the other fundemental forces. That is as opposed to what appears to be the current trend to try and bolt together two disparate theories (that admittedly appear to have considerable utility in their specific areas) - by 'inventing' another (highly complex) theory to 'paper over the cracks'.

I don't think I suggested that there were any quantum-mechanical theories that accurately predict gravitational affects - what I was refering to was an alternative theory or model that might predict ALL the effects of the fundemantal forces. As to whether Einstein's theories are the most accurate - I can;'t comment, because I have not examined a range of other ideas in enough detail (though I don't dispute that this is accepted wisdom).

How the universe works isn't subject to beliefs (at least I don't believe so - I don't think I could change how it works by what I believe). However, I would suggest that physics, science or natural philosophy as it was once called - is entirely the subject of belief. C.f. the 'geo-centric' model of the universe - this was (apparently) the best model for predicting observations for a good many years - hence many people (at least those aware of it) 'believed' in it. That was the 'accepted' science of the time - it did not mean to say that was how the universe worked - any more than today's 'accepted' theories are necessarily the way the universe works.

Cheers P

[proregulus]

Dear t00fri

Thanks for that vote of confidence! ... my 'background', by the way is: a good science degree, postgraduate qualifications in education and management, and over 20 years experience as a professional scientist (though as I said previously - not a physicist).

I assume from your comments that you do not believe in relativity? After all how could someone without "highly specialist training until 27-28" followed by a load more "years at leading international institues" be "polished" enough to have a "grown-up" understanding to make such a concept non-"contrived" and "transparent"?

Wasn't much of Einstein's famous work at odds with established physics at the time?

I would suggest that people should not always assume that just because someone does not believe accepted theory does not mean to say they do not understand it.

Cheers P

[proregulus]

Hi Vincent

I understand where you are coming from about understanding complexity with regards to events that people can't really feel in ther everyday life.

However, I feel that it is an extraordinary coincidence that the 'laws' of physics, thoeries, concepts, etc, should suddenly change in such a way as to make them that much more complex that we have to invent new ways of thinking, etc - just at the limit that we cannot experience them directly!

- perhaps there is a god or omnipotent entity out there with a weird malevolent sense of humour?


With regard to your last sentence: surely we should be adapting our theories of physics to the conception (observations?) of the universe - not the other way around?


Fridger

How does a "MUCH deeper understanding ... of the intrinsic beauty and elegence" of a theory or a model help it explain / predict our observations of the universe?

As to your view that "much, much intensive study is necessary before one has a chance to see the light at the end of the tunnel": I think it is just as likely that someone outside the establishment, that can bring an open mind and novel ideas, will bring about the next 'quantum leap' (sorry couldn't resist it) in fundemental physics theory. Perhaps a single simple model that can predict our observation of gravity and all other forces (without the need to 'bolt together' other ideas)?

Cheers P

[Malenfant]

The trouble is, your beliefs or feelings about how science should work are completely and utterly irrelevant.

Maybe you're just spoiled by Einstein who can sum up something complex in a simple "E=mc^2", but the universe ain't a simple place. It tends to be elegant, but that doesn't necessarily mean 'simple'. The theories we've come up with to describe things like gravity, dark matter etc are basically the simplest that we can come up with to match the observations. And yet to a layman they're pretty obtuse and complex-sounding. We're not just complicating things for the hell of it, fact is it HAS to be complicated to explain what we see.

That's just the way it is. We've done the simple stuff, now we're getting into the realm of the counter-intuitive, esoteric, and just plain bizarre. And we need explanations and theories that can handle all that.

Usually if people don't 'believe' an established theory then it's pretty likely that they don't understand it. It's one thing to claim that you have doubts about an established theory, but in doing so you have an obligation to provide evidence that supports those doubts. If others can examine that evidence and come to the similar conclusions, then you have a valid argument that deserves further examination. But if you have nothing beyond "I have a gut feeling that you're wrong, because I think things should be simpler" then that's not going to get you any credibility at all.

The thing you miss about the old models of the universe is that back then, people really didn't have the means to rigorously test any of their ideas. You couldn't even call what the likes of Ptolemy was doing "science" in the sense that it is today, because it was riddled with unfounded assumptions based on belief (eg that the Earth was the most important thing in the universe and therefore everything revolved around it). It was only when people like Copernicus and Galileo came along with instruments and observations to show that this wasn't the case that practical science as we know it today was really born.

The difference between the ptolemaic view and our current view of the universe is that we have the tools and the methodology to KNOW that we are getting at least a large majority of the available observational data to formulate our theories with. A thousand years ago we had pretty much no means to do this, and the only thing we had to base our views on reality were bibles and holy books and assumptions about how great and important we were - and you certainly shouldn't mistake that for science.

[t00fri]

Hi Vincent

...
However, I feel that it is an extraordinary coincidence that the 'laws' of physics, thoeries, concepts, etc, should suddenly change in such a way as to make them that much more complex that we have to invent new ways of thinking, etc - just at the limit that we cannot experience them directly!

It is actually a basic "design" principle in theoretical physics that physics does not change abruptly! It is part of the internal consistency of our present theories that they exhibit a /smooth/ correspondence between the appropriate descriptions in different regimes of Nature. In quantum mechanics, the "correspondence principle" precisely states that, and there is even a "duality" regime between the classical mechanics description and quantum mechanics ("particle - wave duality"). A quantum state may be understood as a /linear/ superposition of solutions of the /classical/ equations of motion!

This very basic feature continues to hold between Newtonian gravity and general relativity, classical and quantum field theory etc.

In quantum physics, the deterministic property of classical mechanics is lost necessarily. This causes extra complications. While the classical equations of motion determine the position of a particle at some time precisely, in quantum physics we can only quote probabilities to find the particle at a particular location. This is intrinsically coupled to Heisenberg's uncertainty principle, of course.

By making statements like the one above you demonstrate a fair amount of ignorance of the essence of contempory physics...On the one hand, that's all fine: we cannot and should not all be professional physicists . There are many other important tasks in life! But once you start making such public statements about the laws of physics, it's good to know about the basics.

Fridger
...
As to your view that "much, much intensive study is necessary before one has a chance to see the light at the end of the tunnel": I think it is just as likely that someone outside the establishment, that can bring an open mind and novel ideas, will bring about the next 'quantum leap' (sorry couldn't resist it) in fundemental physics theory.

What do you mean precisely with "outside the establishment" here? I suppose you mean even someone /without/ proper theoretical physics training??

Under this premise, the above statement of yours is either utterly naive or extreme nonsense. Sorry. The most crucial aspect about a thorough training in fundamental physics is to learn asking crucial questions that have a chance to lead us further on. It is essential to understand the difference between conceptual simplicity and calculational simplicity! The formulation of general relativity, or even string theory is very simple conceptually, yet when it comes down to calculations of observable effects a high amount of labour and know-how is required. The latter is certainly of no principal concern, since our world is complicated. We have to face this fact.

The composition of a human body out of quarks, gluons and electrons is a most complex "compound", yet the underlying laws according to the Standard Model of electroweak and strong interactions are very simple and beautiful. While the agreement of our theoretical predictions with these laws have been successfully confirmed with many thousands of experiments over decades, partly with >10 digit precision (!), the case with precision tests of the gravitational force at large distances and early time scales is way harder. For tests of gravity we need BIG masses, i.e. the Universe as a laboratory. Yet, since a few years, we do have spectacular confirmation of our view about the early, inflationary Universe from precision measurements of the cosmic microwave radiation (KOBE, BOOMERANG, WMAP,...) , the light curves of distant supernovae, large scale structure surveys, high z gamma ray bursts, gravitational lensing etc.

As an illustration of the power of the present theoretical predictions within the Standard Model of electroweak and strong interactions, let me pick just two examples, the anomalous magnetic moment of the muon and electron :

Code: Select all

Theory:   a_mu = (11659182.8 +-7.3)* 10^(-10)
Experiment: a_mu = (11659208+-5+-4)*10^(-10)


Using the value of the electromagnetic fine structure constant as measured from the Quantum Hall effect,

Code: Select all

alpha =1/137.03599911(46)


the anomalous magnetic moment of the electron is prediced
within Quantum-Electrodynamics as

Code: Select all

a_e theor. = 0.0011596521535(240)


in incredibly good agreement with the experimental precision value

Code: Select all

a_e_exp. = 0.0011596521859(38)


To arrive at these theoretical predictions, takes years of very complicated calculations. Also, the experimental measurements of such fundamental properties of muons and electrons to this breathtaking accuracy is an amazing achievement! Only such demanding theoretical and experimental work about basic quantities, can most significally contribute to our conviction that the underlying theory is RIGHT.

On the other end, the validity of Newtons law has not yet been precision tested at very short distances, below ~ 1 mu, say. Any deviations would directly signal the existence of extra spacial dimensions.

Bye Fridger

[t00fri]

Dear t00fri

Thanks for that vote of confidence! ... my 'background', by the way is: a good science degree, postgraduate qualifications in education and management, and over 20 years experience as a professional scientist (though as I said previously - not a physicist).

Yet it surprises me even more that --as a trained scientist-- you tend to make depreciative|unfavourable statements about the world's most promising theoretical approaches without really being able to grasp their full content and implications...

I assume from your comments that you do not believe in relativity? After all how could someone without "highly specialist training until 27-28" followed by a load more "years at leading international institues" be "polished" enough to have a "grown-up" understanding to make such a concept non-"contrived" and "transparent"?

The essence of my comments was that for a trained theoretical physicist many aspects of our fundamental theory will not appear untransparent or contrived, quite unlike "amateur" scientists.

Wasn't much of Einstein's famous work at odds with established physics at the time?

I don't know whether you have ever read some of Einstein's original scientific publications. I certainly have. By any standards this is a most impressive experience.

Einsteins CV was admittedly unconventional at the beginning, and he was probably a "bad" student since his abilities as a physicist were far superior to those of his teachers . But clearly his scientific work left no doubt about his extremely high level of understanding of physics and unique gut feeling about "scientific beauty" ...For many non-physicists Einstein's work is synonymous with E = m*c^2, but of course his actual scientific work is vast, very deep and always brilliant.

One perhaps more significant reason why Einstein was at odds with the German scientific establishment was the political situation of the 30's and the NAZI association of a number of his German colleagues.

After all he made it to Princeton, the world's most prestigeous University institute...

+++++++++++++++++++++++++
A man with little or no training would not have the slightest chance of achieving what Einstein did!! Unlike you, a man of Einstein's scientific profile had ALL QUALIFICATIONS to judge the mainstream physics views of the time and to try an entirely new and unconventional approach himself.
+++++++++++++++++++++++++

I would suggest that people should not always assume that just because someone does not believe accepted theory does not mean to say they do not understand it.

Cheers P

There is a big difference if a colleague of mine (whose scientific work I respect) tells me that he does not believe in String theory, say. Then I can be sure that he knows what he is talking about!

Bye Fridger

[delmarco]

My specific question about gravity being 'attractive' or 'repulsive' was probably a little confusing (and now irrelevent, as it has been pointed out to me that gravity does not involve particles - gravitons). What I actually meant to ask if I can adequately explain this is;

Attractive gravity - Some force draws two masses together. This force is a localized occurance. For example we are drawn to the centre of the Earth's mass.

Repulsive gravity - Some force pushes on everything from every direction with mass/density able to block this force. Thus we aren't drawn to the Earth's centre of mass, rather we are pushed towards it because it blocks the force from acting on us from beneath.

How do we know gravity is 'attractive' rather than 'repulsive' in regards to the above context?

Dude,
I joined this forums after reading this to say you are describing the equal sign in the gravitation equation in the fanciest way I can think of. When we are standing on the planet or any body that is not in motion in any gravitational field is under the effect of both the pull of gravity and the pushing force of something else...being friction or the surface of the planet...that holds us or confines us from falling into the center of the planet.

When we stand on the planet we ARE indeed gravitationally attracted to the "center" of earth and not the surface because the center is prettier than the surface. ha ha ha. jking. read on.

(this fact is blatantly stated in the Gravitation Equation and why we take radius of the gravitationally attracted bodies). However, because the earth is thick and has an outer core, mantle and crust between us and its central core we don't go flying into the center of the planet.

It is the"pushing up" force of the mantle and crust that keeps earth's gravity from sucking everything on it's surface inside.
Compare earth to a neutron star right before it becomes a black hole. The star's mass at it's center is so strong and dense that it sucks its own "core","mantle" and "crust" inside the central core's singularity.
I have a personal theory/idea that gravity is the only variable of space-time that is NOT dependant in anyway on space or time. Meaning the source of gravity is inside singularities or spaces so small the may be wrapped up in another dimension. So gravity can be seen as a " prime force" that is acting on all frames of reference inside our reality from OUTSIDE our reality. Or rather gravity acts on us from it's own non-visible frame of reference. And the bridge into these other "gravity" wells is found on the event horizon of black holes.

I'm not quite sure, but i think that is one of the tenets behind M-Theory and the suggestions that other dimensions SHOULD exist all around and within our universe.

heck, don't take my word I'm just an Environmental Engineering student...this is my hobby...especial Gravity...I love gravity, dark energy, dark matter, Lambda...what else.

but dude..i think you are trying to understand why the gravitational equation, or any KE=PE equation has an equal sign in it....because equilibrium can be achieved in the shifting of the forces, the forces can be transformed, but always conserved inside the system, so we give equal signs to equations to state this. Gravity pulls but it's pull can be negated or limited by another equal (but non-gravity) force such as friction or the mass/matter of the earths crust that hold us up. You cannot call that force anti-gravity....unless your name is TESLA ftw!


good luck...
also there is NO repulsive gravity.....yet. Only Magnets and Electricity have dual forces &/or multiple charges.