Question about gravity

[sol invictus]

Had a rather drunken discussion on this at the pub last month, if I remember correctly we all agreed that the speed of propagation was finite, but myself and another were of the opinion that it was fractionally greater than the speed of light, whilst others stated it was at c

It's c.

IMO, understanding the process of rainfall does not extend to predicting when it will happen, just like understanding the rules of a soccer match don't extend to predicting the final score.

But we know the rules of gravity very precisely. We only know the rules that govern rainfall in relatively vague and general terms.

I don't deny this. It does not, however, demonstrate your or any level of understanding of gravity. I could calculate the Internal Rate of Return (IRR) for two competing real estate projects to identify which offers the best investment by application of an Excel spreadsheet function. That doesn't necessarily mean I understand IRR (which I do, incidentally).

I'm not sure that demonstrated exactly what you wanted it to.

I think it could be something to do with nobody here being capable of explaining it in terms that are easily related to and comprehensible. That suggests a lack of understanding.

rain:

water evaporates from the sea, forms clouds, and later condenses and falls as water again.

gravity:

massive objects attract each other.

Is one really so much more comprehensible? The real difference lies in the fact that we have a very precise theory of gravity, and we don't of rain.

Yes, and as soon as this is done, the Nobel Prize committee will be interested as well. But don't hold your breath.

I'm getting the distinct impression you have no idea what you're talking about. As everybody keeps telling you, we already have a theory of gravity, and it appears to be correct to an extremely precise level. If a more precise experiment reveals that it's not quite right there may be a prize, but not otherwise.

Do you understand why rain falls?

Yes. Water is heavy, and things that are heavier than air fall. Everybody understands this.

Right - because of gravity.

 

[Southwind17]

But we know the rules of gravity very precisely. We only know the rules that govern rainfall in relatively vague and general terms.

Would you care to cite the respective '"rules"?

rain:

water evaporates from the sea, forms clouds, and later condenses and falls as water again.

This is an explanation, albeit brief, suggesting understanding.

gravity:

massive objects attract each other.

This is simply an observation; nothing more; nothing less. Incidentally, don't all objects attract each other?

Is one really so much more comprehensible? The real difference lies in the fact that we have a very precise theory of gravity, and we don't of rain.

Seems odd, then, that we seem to have an explanation for rainfall, but not for gravity.

 

[SirPhilip]

On Earth when an object is affected by gravity in that it falls to the ground. In space an object produces gravity depending on the size and density. When you spin a sphere on an axes on earth everything on the surface of the object is repelled, and the opposite is true in space. What Produces gravity? why does it exist? How does it work. 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.

Nothing had to cause everything to generally suck, thereby becoming something by implication.

 

[ponderingturtle]

This is an explanation, albeit brief, suggesting understanding.
.

Not really, it suggests a possible vague understanding. If we understood rain as well as we understand gravity we could answer the question of does cloud seeding work.

 

[Skeptic Guy]

IMO, understanding the process of rainfall does not extend to predicting when it will happen, just like understanding the rules of a soccer match don't extend to predicting the final score. The final score is a product of the application of the rules taking account of all the variable factors that can arise, just like the timing of rainfall is a consequence of the very variable factors within the process.



I don't deny this. It does not, however, demonstrate your or any level of understanding of gravity. I could calculate the Internal Rate of Return (IRR) for two competing real estate projects to identify which offers the best investment by application of an Excel spreadsheet function. That doesn't necessarily mean I understand IRR (which I do, incidentally).

That's because you have had the education and training by which to understand IRR.

Understanding the process of precipitation and using a GPS are not really capable of comparison. What, exactly, are you comparing?



I think it could be something to do with nobody here being capable of explaining it in terms that are easily related to and comprehensible. That suggests a lack of understanding.

It does, but by whom?

 

[sol invictus]

Would you care to cite the respective '"rules"?

For gravity, they were written down by Einstein in 1915 or so, and later in this thread. For rain, I don't know what they are (at least not precisely).

This is an explanation, albeit brief, suggesting understanding.

This is simply an observation; nothing more; nothing less.

What's the difference? Seriously - I'm really not getting your point. Maybe you can answer this: what would you regard as an explanation of gravity?

Incidentally, don't all objects attract each other?

Yes, with a force proportional to the product of their masses (or more accurately, their energy).

 

[Yllanes]

It was also claimed that our understanding of gravity is greater than our understanding of almost everything else in the World. I think that's both an incorrect and condescending comment, even if made by somebody who probably does understand gravity better than the average Joe.

OK, we have made some progress. My and others initial claims were aimed specifically at this remark:

Describing gravity is easy, and was understood long ago. Describing something is not the same as understanding how, or why. There is no theory for gravity like there is for the other fundamental forces. Saying there is, is dumb.

That is wrong. We have a theory for gravitation and we understand it better than, for example, the strong interaction.

Unfortunately, I am not qualified to comment on our actual or perceived level of understanding of the strong nuclear force. I do maintain, however, that our understanding of gravity does not compare with our level of understanding of many, many aspects of the World, which is the point that led to my challenge.

This is a different discussion. At a fundamental level, there are four interactions (gravitation, electromagnetism and the strong and weak nuclear forces). For simple systems exhibiting those interactions (such as fundamental particle) we can attempt to achieve an understanding at this fundamental level. It is very difficult and it requires several years of full time work to grok it, but at the end you can compute many things with an incredible precision.

If we want to study a more complex system, such as the wheather, we definitely cannot think in terms of fundamental interactions. We have to start from a different layer, with effective models and simulations, etc. The result is that we cannot achieve the same precision as with simple, microscopic systems, but also that one can get a feeling of a greater part of the theory without being a professional physicist, because the starting point is easier to comprehend than the postulates of quantum field theory or general relativity.

Saying we have an explanation for rainfall and not for gravitation is not fair, because your standards of what constitutes an explanation are not the same. Robinson said that rain falls because it is heavier than air and heavy things fall and you accept that as an explanation. You don't demand him to explain why heavy things fall. But if I say that gravitation works because energy bends spacetime, you do demand to know why energy bends spacetime.

 

[Ziggurat]

Seems odd, then, that we seem to have an explanation for rainfall, but not for gravity.

Ever talk to a toddler who keeps asking "why?" You'll tell them something, they ask why, you explain it, they ask why again, and on and on. There's no end to it, at some point you have to say "just because", or something to that effect.

Well, you're like that toddler, whether you intend to be or not. What are you after? An explanation for why gravity exists at all? There is none. There's no explanation for why any of the fundamental properties of the universe are the way they are. Nor can there be. They just are. You can only get explanations about "why" for non-fundamental properties when those explanations are given in terms of the fundamental properties. We can know everything there is to know about the how, but we will never, ever know the why about those fundamentals. Rain is not fundamental, and so an explanation can be given in terms of more fundamental properties. Gravity is a fundamental property of the universe, there's nothing you can fall back on to explain it in terms of. You've already been given an answer about the how. If you are not satisfied with what's already been provided, well, you'll have to be a lot more specific about what you want, and what will satisfy you, or people are going to start doing what they do to toddlers in the end: ignore you.

 

[madurobob]

That is wrong. We have a theory for gravitation and we understand it better than, for example, the strong interaction.

Yes, we have a fairly complete theory in terms of predicting gravity's effects. But, where our understanding falters - and I think what Southwind may be getting at - is that there is much we don't know about gravity. For instance, 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.

 

[ponderingturtle]

Yes, we have a fairly complete theory in terms of predicting gravity's effects. But, where our understanding falters - and I think what Southwind may be getting at - is that there is much we don't know about gravity. For instance, 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?

So? His discussion about the rain would lead to an equivelant question, Why is there Water?

 

[robinson]

We actually can explain why there is water.

 

[madurobob]

So? His discussion about the rain would lead to an equivelant question, Why is there Water?

No doubt. But thats not really the question. The question is not "why is there gravity" but "why does gravity behave as it does". We have this pretty well nailed down for water compared to what we know about gravity.

 

[ponderingturtle]

No doubt. But thats not really the question. The question is not "why is there gravity" but "why does gravity behave as it does". We have this pretty well nailed down for water compared to what we know about gravity.

Not really. For both we have good ideas how they behave, why they behave that way is not as clear(well you might need to go a level or two more for water)

But if we understood water and rain so well we could answer the question of does cloud seeding work. We can answer a question like that for gravity, but not for rain so we know less about rain than gravity.

 

[Fredrik]

The question is not "why is there gravity" but "why does gravity behave as it does". We have this pretty well nailed down for water compared to what we know about gravity.

Yes, but only because when we explain the behavior of water, we do it in terms of things that are less understood than what we are trying to explain. I can't fathom why anyone would consider that a higher degree of understanding than we have of gravity, where we have exact definitions of all the terms we're using, where we can write down all the rules on one line, and where those rules can be used to make almost absurdly accurate predictions about a very wide range of phenomena, including some that were previously unknown.

 

[madurobob]

...where we have exact definitions of all the terms we're using, where we can write down all the rules on one line, and where those rules can be used to make almost absurdly accurate predictions about a very wide range of phenomena, including some that were previously unknown.

Yes, but I can say the very same about my digital watch, but be no closer to understanding how it works. 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". To simply state "it works, live with it" intentionally sidesteps the question, as does a discussion about understanding water and chaotic weather systems.

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.

 

[Fredrik]

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?

It's been a while. This is how I remember it: Newton's theory says that the force is proportional to 1/r², or that the potential is proportional to 1/r. When you use GR to calculate the potential, the result you get is a series. The first term is the Newtonian 1/r. Then there are other terms, 1/r² and so on. Elliptical orbits is what you get when the potential is 1/r, so with those other terms present, the orbits can't be exactly elliptical. I don't know exactly what the effects of the additional terms are. I know that the exact location of the perihelion is shifted a bit each year, but I don't know if there are any other effects that matter.

 

[Ziggurat]

It's been a while. This is how I remember it: Newton's theory says that the force is proportional to 1/r², or that the potential is proportional to 1/r. When you use GR to calculate the potential, the result you get is a series. The first term is the Newtonian 1/r. Then there are other terms, 1/r² and so on. Elliptical orbits is what you get when the potential is 1/r, so with those other terms present, the orbits can't be exactly elliptical. I don't know exactly what the effects of the additional terms are. I know that the exact location of the perihelion is shifted a bit each year, but I don't know if there are any other effects that matter.

There's an extremely important and very noticeable effect of those additional terms: the force diverges to infinity at non-zero r, namely at the event horizon.

 

[Yllanes]

It's been a while. This is how I remember it: Newton's theory says that the force is proportional to 1/r², or that the potential is proportional to 1/r. When you use GR to calculate the potential, the result you get is a series. The first term is the Newtonian 1/r. Then there are other terms, 1/r² and so on. Elliptical orbits is what you get when the potential is 1/r, so with those other terms present, the orbits can't be exactly elliptical. I don't know exactly what the effects of the additional terms are. I know that the exact location of the perihelion is shifted a bit each year, but I don't know if there are any other effects that matter.

Yes, that's how it is. The expression for the force, once you have applied the first correction due to GR, which is velocity independent, can be written as

[latex]
\footnotesize
\[
\vec F \simeq -\frac{G M_\odot M}{r^2} \left(1-\frac\alpha r\right)\frac{\vec r}{r}
\]
[/latex]

where M is the mass of the planet and alpha = 3.8·10^-8 AU.

This has the effect that the orbits are no longer closed, so that the point of maximum approach to the Sun (the perihelion) advances with each turn.

 

[ponderingturtle]

No doubt. But thats not really the question. The question is not "why is there gravity" but "why does gravity behave as it does". We have this pretty well nailed down for water compared to what we know about gravity.

Not really. For both we have good ideas how they behave, why they behave that way is not as clear(well you might need to go a level or two more for water)

But if we understood water and rain so well we could answer the question of does cloud seeding work. We can answer a question like that for gravity, but not for rain so we know less about rain than gravity.

 

[gannon]

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?
It was my understanding that people where still trying to make theories on such stuff like why different partials have different masses or why different forces have different strengths.
(I don't really care about our understanding of water in this context)