Question about gravity

[robinson]

The cloud seeding thing is just a distraction. Nobody who knows about cloud seeding is confused about how it works. The questions are about does it cause more rain or not, not about how it works, or if it does.

It's a silly sidetrack.

 

[DrBaltar]

...why is it so much weaker than other forces? Why can I pick up a brick when the entire gravitational force of the Earth is pulling it down?

I'm by no means a physicist, but I've read a little about M-theory (and I stayed at a Holiday Inn Express?). From what I remember, one of the current theories about gravity is that it originates in another membrane - another universe - and leaks into ours. Thats a bit bizarre for me to wrap my head around, but it speaks to the lack of understanding of gravity being discussed here.

I believe according to M-theory, the gravitational sources we know of are in this membrane, not another one. But that the effects of gravity do not just curve space in this membrane but in others too. So our membrane sees only a 4D slice of the total higher dimensional gravitational field, and therefore only a fraction of its strength. Experimental evidence for M-theory could be in the form of a gravity field centered around nothing that we can detect in our universe because the source is actually in another membrane.

Someone asked why gravity works the way it does. The left side of this equation describes the curvature of spacetime. Rather than any random curvature, the Ricci tensor and scalar are used, which relate certain components of the Riemann tensor to T (the energy/momentum tensor).

[latex]
\[
R_{ab} - \frac12 R g_{ab} = 8\pi G T_{ab}
\]
[/latex]
The right side describes the distribution of energy/momentum. The two sides are set equal to enforce what is at the core of any good physics equation: the conservation of energy. So mass/energy tells spacetime how to curve, and the curvature of spacetime tells mass/energy how to propagate.

 

[patnray]

Why do objects in space attract one another and why is the same not true here on Earth where we are within a gravity field.

Simply not true. In fact, the gravitational attraction between two masses can be easily and precisely measured, even on the earth's surface in the presence of the earth's much stronger gravitational pull. Look up "Cavendish balance". I did this in a freshman physics lab 40 years ago. It's how the gravitational constant in Newton's equations was measured.

 

[ponderingturtle]

The cloud seeding thing is just a distraction. Nobody who knows about cloud seeding is confused about how it works. The questions are about does it cause more rain or not, not about how it works, or if it does.

It's a silly sidetrack.

I said does cloud seeding work, not how does it work. If we understood rain as well as we understand gravity we could answer such a question.

 

[ponderingturtle]

Maybe I am misunderstanding something. Is there something that says we are at a limit in understanding the fundamental forces? So that there is nothing to find when we try to look deeper?

No one is arguing that we can't understand things better than we do now, but that the question of why is the universe like it is can not truly be answered by science in any other way that "That is just how it is".

Sure we might be able to say that because of some specific new theory that is why X is the way it is, but then we just moved the "why?" question on stage back. You can never get rid of it.

 

[Fredrik]

Yes, but I can say the very same about my digital watch, but be no closer to understanding how it works.

That isn't exactly true, and even if you just meant that you, personally, can't do that, wouldn't you agree that someone who can do it has a very solid understanding of digital watches?

As for the water/rain analogy I'm not sure it adds anything to the discussion started by the OP: "What Produces gravity? why does it exist? How does it work".

The OP believes that gravity on earth behaves differently from gravity in space, and isn't interested enough to comment on the replies he got. There's no way someone like that could understand a good answer to that question. And the question he should be asking at this point isn't "why", but rather "what does Newton's theory of gravity say, and how do we know that gravity behaves that way?"

Stating that we know more about gravity than anything else necessarily pushes us into a subjective argument about "more" rather than allowing a discussion about the whys and hows of gravity.

That was a response to the absurd statement that we don't even have a theory of gravity, and the (also incorrect) claim that gravity is not well understood. A person who makes those claims either doesn't know what is known about gravity, or doesn't know what it means to understand something. In this case, the real issue seems to be the latter. Hence the discussion about understanding.

To simply state "it works, live with it" intentionally sidesteps the question...

But that's not what we're saying. What we're saying is that if you ask "why?", the answer is just another reason to ask "why?" again.

The answer to almost all gravity-related "why-questions", such as "why does a rock fall to the ground when I drop it?", have the same answer: "because the geometry of space-time affects the matter it contains, and vice versa, as specified by Einstein's equation". This is the answer to almost all questions of that sort. But now we have another "why" to answer: Why does Einstein's equation hold? The answer is that no one knows. It's important to understand that, but it's even more important to understand that no matter what answer future scientists will come up with, it will lead to another "why?". There's no way that chain could ever end. (Edit: OK, maybe it could, but we would have no way of knowing that we have reached the end).

 

[gannon]

but then we just moved the "why?" question on stage back

so whats wrong with that? That does bring up an interesting question. Can there be a way of knowing that something is fundamental as opposed to brought about by or composed by something else?

I have no problem with people saying we don't know it just seems some people are saying to the why question why bother asking if it just leads to more questions. People need to say we don't know that question but here is what we do know and any answer to the question must agree with these observations.

 

[ponderingturtle]

so whats wrong with that? That does bring up an interesting question. Can there be a way of knowing that something is fundamental as opposed to brought about by or composed by something else?

I don't think so. If something is composed of smaller parts, but it is impossible to observe any effects from these smaller parts, then how could you discover them?

There needs to be some observable effect from the level beyond our current understanding to be able to examine it and thus describe and understand it.

I have no problem with people saying we don't know it just seems some people are saying to the why question why bother asking if it just leads to more questions. People need to say we don't know that question but here is what we do know and any answer to the question must agree with these observations.

That is not really the case. But holding up a lack of understanding why something is the way it is as a problem with the understanding of how it functions is of very limited if any value.

So for rain vs gravity, we might have answers to why does it rain because we have levels of understanding deeper than that it rains, while with gravity it represents a fundamental current understanding. But we understand gravity better than rain because the theory is very accurate at describing it, while for a case as simple as does cloud seeding work we can not really answer the question.

 

[robinson]

This weaseling about over "why" is damn funny. Why? Because if there is one thing scientist like to know, and always ask, it is "why?". The other is "how?".

 

[Zeuzzz]

Anyone interested in this who lives in the UK should watch tomorrows episode of Horizon on the 29th; http://www.bbc.co.uk/sn/tvradio/programmes/horizon/broadband/tx/gravity/

What on Earth is wrong with gravity?

Tuesday 29th January 2008, 9pm, BBC Two

Particle physicist and ex D:Ream keyboard player Dr Brian Cox wants to know why the Universe is built the way it is. He believes the answers lie in the force of gravity. But Newton thought gravity was powered by God, and even Einstein failed to completely solve it. Heading out with his film crew on a road trip across the USA, Brian fires lasers at the moon in Texas, goes mad in the desert in Arizona, encounters the bending of space and time at a maximum security military base, tries to detect ripples in our reality in the swamps of Louisiana and searches for hidden dimensions just outside Chicago.

He's a particle physicist, and so will probably try to exlain it with the invention of some new mysterious particle or 'graviton' which functions in a metaphysical darkmatter 11 dimensional multi-continuum spacefabric, etc, etc, but despite that, it shoud be an interesting watch. I've always been a fan of maxwells ideas that gravity is an unknown consequence of EM forces caused by the geometric properties of atoms. But lacking any concrete idea of how this would work, i'm going to keep my mouth shut on that concept.

 

[Fredrik]

so whats wrong with that? That does bring up an interesting question. Can there be a way of knowing that something is fundamental as opposed to brought about by or composed by something else?

There's no way to prove that a theory is fundamental in the sense that it's the last step in a series of "why" questions. However, it's not hard to see that every theory that exists today is not fundamental. General relativity includes a classical description of matter, but we already know that matter behaves in a way that can only be described by quantum mechanics. So GR can't be fundamental. The standard model, which describes the other particles and interactions, is a quantum field theory formulated in the framework of special relativity, which basically means that it really just explains how matter would interact in a fictional universe where Einstein's equation doesn't hold. So the standard model can't be fundamental either.

 

[Fredrik]

This weaseling about over "why" is damn funny. Why? Because if there is one thing scientist like to know, and always ask, it is "why?". The other is "how?".

Who do you think is weaseling? I have answered the question as thoroughly as it can be answered without resorting to highly speculative ideas that aren't developed enough to be called "theories" yet.

 

[gannon]

Well in the case of gravity I think why and how are the same question.

That is not really the case. But holding up a lack of understanding why something is the way it is as a problem with the understanding of how it functions is of very limited if any value.

People are spending tons of money on this thing you say is very limited if any value. Asking why and how and finding out ways of testing there answers.

As for pushing the question back that is what science always does. Science gives our current understanding, not certainty so there will always be some unanswered questions.

There's no way to prove that a theory is fundamental in the sense that it's the last step in a series of "why" questions. However, it's not hard to see that every theory that exists today is not fundamental. General relativity includes a classical description of matter, but we already know that matter behaves in a way that can only be described by quantum mechanics. So GR can't be fundamental. The standard model, which describes the other particles and interactions, is a quantum field theory formulated in the framework of special relativity, which basically means that it really just explains how matter would interact in a fictional universe where Einstein's equation doesn't hold. So the standard model can't be fundamental either.

interesting but wouldn't some part still have to hold even if a theory changes? Like can a new theory get rid of uncertainty or does that have to stay? thanks for explaining that I find that much more informative

 

[trvlr2]

My thanks to the 'physics geeks' for the detailed workings of gravity.
Especially for the "atlas" link. I await the preliminary results in July!

It seems that there is always one more 'why?'; especially at the fundamental levels of the universe.
And cloud seeding definitely works!sometimes.

 

[Cold one]

Ok let me see if I can explain somethings.

Why does Gravity work?
Gravity works because everything is made up of matter/energy and all matter/energy is not the same.

Why is everything made up of different types of matter/energy?
Because if everything was made up of the same type of matter/energy then nothing would be indistinguishable.

Then the next question could be - Why does everything need to be different?
Everything doesn't but if everything was the same then none of the questions would be asked because noone would be around to ask them.

That is not to say that god put us here to ask the questions or that everything was put here for us to be able to ask the question but rather that we can ask the question because we are here.

 

[sol invictus]

I understand how all the others are calculated but this one is beyond what I know about orbits. Why does it deviate from being perfectly elliptical?

Everything people said about the GR corrections modifying elliptical orbits was absolutely correct, but it's also worth pointing out that corrections due to other bodies in the solar system are important too. Even in Newtonian gravity, it's only a two body system for which orbits are elliptical. So the planets affect each other's orbits a bit, and most of the deviation from ellipticity is actually due to that rather than to GR.

 

[Fredrik]

Why does Gravity work?
Gravity works because everything is made up of matter/energy and all matter/energy is not the same matter/energy shapes the geometry of space-time and is also influenced by it in the way that's described by Einstein's equation.

...and some extra text so I can post this.

 

[Southwind17]

Ok let me see if I can explain somethings.

Why does Gravity work?
Gravity works because everything is made up of matter/energy and all matter/energy is not the same.

You see, this doesn't qualify as an explanation to any meaningful extent. The fact that everything is made up of matter/energy and all matter/energy is not the same goes no way whatsoever to explaining gravity. It affords no better an explanation than: "Objects with mass attract each other. We call this phenomenon "gravity"." It is premature, therefore, to drill down to the next level of "why/how".

Fredrik's expansion of this "quasi-explanation", however, does seem to begin to constitute a proper explanation:

Gravity works because everything is made up of matter/energy and matter/energy shapes the geometry of space-time and is also influenced by it in the way that's described by Einstein's equation.

For this "explanation" to demonstrate understanding, however, presupposes a prior understanding and comprehension of the "geometry of space-time" and "Einstein's equation". Now, who here can demonstrate such understanding, as opposed to simply "explaining" them, that is?

 

[sol invictus]

For this "explanation" to demonstrate understanding, however, presupposes a prior understanding and comprehension of the "geometry of space-time" and "Einstein's equation". Now, who here can demonstrate such understanding, as opposed to simply "explaining" them, that is?

I think there are a number of posters here that understand those quite well, but I for one am not willing to spend more time trying to determine what you are after. Do you want one of us to write a book on GR in a series of forum posts?

Understanding those things requires learning some background, just as understanding rain at the same level of detail would (particularly if you started off not knowing what evaporation was).

Look - everybody knows water evaporates and condenses because everybody's seen it happen for themselves. So they feel they "understand" rain. Most people have not noticed two objects attracting each other due to gravity in any way that made it obvious what was happening, so they don't feel they "understand" gravity. But fortunately the human mind is capable of going beyond what we can see easily with our eyes, and that allows us to observe and understand gravity just as well as - actually better than - rain. But you have to do a little work if you want to get there.

 

[ponderingturtle]

For this "explanation" to demonstrate understanding, however, presupposes a prior understanding and comprehension of the "geometry of space-time" and "Einstein's equation". Now, who here can demonstrate such understanding, as opposed to simply "explaining" them, that is?

Ah I see your problem, you don't understand it, so that means it is not understood. This is not really true you know.