Question about gravity

[sol invictus]

A wave on the surface of a body of water spreads out in two dimensions, not three. I'd guess that the energy density in such a wave falls off as 1/r, not 1/r².

Let's take this example a little further and see where it goes. Imagine for a moment that the earth is completely covered with ocean - no land or ice, just water everywhere. Now an asteroid comes flying in and hits the ocean somewhere. Like a stone thrown in a pond, that sets up a circular wave that expands out from the impact point. Let's suppose the impact was the south pole, so every part of the wave is moving due north.

In the case of a stone thrown into a flat pond, 69dodge is (roughly) correct about the energy density in the ripples falling off like 1/r (ignoring friction and a few other effects). Let's see what happens instead on the curved surface of the earth.

The wave expands and eventually gets to the equator. As it keeps moving north, it actually contracts, and its energy density increases. After a while longer it will contract to zero at the north pole and smack into itself, sending up a huge plume of water.

So you can see the 1/r law is totally wrong in this case, because the earth's surface is curved. While it's harder to picture, three dimensional space (or four dimensional spacetime) can be curved too, and, speaking a bit loosely, that's what causes the deviations from Newton's law.

 

[Southwind17]

What about photosynthesis? Comparing apples to oranges.

Do you understand the nature of a "thread"? It's often like a story, which usually has a beginning, a middle and an end, requiring one to read from the beginning, or back somewhat if jumping in at the middle, in order to appreciate the context. Lazy people tend not to do this, then wonder why they don't understand and/or follow the story/argument. Here's an important extract from an earlier "chapter", that you seem to have skipped over:

I would say gravity is understood better than almost anything else in the world. Certainly we understand it far better than we do prosaic things like wind, the human body, or the mixing of cream as it's added to coffee. [emphasis added]

Apples and oranges? Gravity, wind, the human body, coffee & cream? Does this help to partially fill your knowledge gap?

 

[Southwind17]

Can you explain photosynthesis without mentioning light, or chemical reactions? (The answer is no). So how do you explain light? How do you explain chemistry?

Oh, I see, in order to claim to be able to explain something one has to be able to explain absolutely everything associated with it, regardless of how remote, right down to the molecular/atomic level, and beyond? On that basis we can't explain anything then, including gravity. If we can't explain gravity how can we claim to understand it?

I could try explaining to my kids how an internal combustion engine works, but I guess I'd be wasting my time, because I'd struggle to explain the chemical composition of fuel. Oh well, I'll just put their futile educations on hold; save a shedload in school fees! Until the F1 Ferrari engineers really understand all there is to know about the chemical make up of the fuel they use there's no point in trying to develop their engines, because they don't "understand" how they work. How can they hope to improve them if they don't even "understand" them? But hang on, engines have improved over time. How can that be? Do you have any theories, or even formulae to explain this most-mysterious phenomenon?

 

[Southwind17]

Given the level of "understanding" you expect with gravity, that simply won't cut it. To say you "understand" photosynthesis you also need to explain how photons exist, how carbon exists, how oxygen exists, how hydrogen exists, how the spacetime that photon must travel through to to reach the matter exists, how a photon can travel through spacetime, how it is possible for matter to absorb a photon, how that absorption can result in a chemical reaction, and you must explain all similar questions which will undoubtedly arise from your explanations.

Are you for real? See above.

You might be willing to admit that you understand nothing (which, frankly, seems plausible, given your views!). I, however, am not, because I do.

 

[Southwind17]

It appears from the examples you've given that what you want is a mechanism - an explanation for gravity in terms of some process involving other ingredients of some kind. Is that correct?

If so, general relativity provides such a mechanism.

Don't you mean "The Theory of General Relativity"?

Gravity is understood as the curvature of spacetime.

"Spacetime"! What's that then, exactly?

The presence of energy in a spacetime bends it in a particular way (given by the equation that was posted above).

I'm sorry, "presence of energy"? Can you be more specific; maybe give an example?

"in a spacetime"? Hopefully your answer to my previous question will help with this.

Bends what, the "spacetime"? Bends it in what sort of way, and how?

As in Newtonian mechanics, particles follow "straight" lines at constant velocity unless acted on by some external force (which in this case does not include gravity).

Ah, now you're talking!

However straight lines on a curved space look curved - for example if you plot the flightpath of transatlantic flights on a flat map, they look curved (but are actually the shortest paths).

"curved space"? What's that then, exactly?

At this point you should picture the sun as a bowling ball deforming a taught rubber sheet, and the earth as a marble rolling around and around in circles in the well created by the sun (like one of those coin donation things you see sometimes at airports).

Yes, I can picture this, a little like the black hole analogy.

OK?

Er, that's it? I should be able to claim to "understand" gravity now? Methinks you have more work to do!

 

[This is The End]

exact - characterized by, requiring, or capable of accuracy of detail; very accurate; methodical; correct

precise - strictly defined; accurately stated; definite

perfect - complete in all respects; without defect or omission; sound; flawless


A theory can be exact, yet not as exact as another, and still stay exact.
A theory can be precise, yet not as precise as another, and still stay precise.

I'm not touching perfect.

 

[The Man]

Having just found this thread and not reading through its entirety, I am confronted with the current discord concerning “theories” verses “laws”. Theories attempt to explain the relationship between observed results and current observationally determined contributing factors. Laws dictate what must happen given certain precursors. Although certain physical principles are identified as “laws”, science is not based on “laws”. Science is based on theories; those theories are supported by the current observational evidence, refuted by it or shown to require some modification due to that evidence. Laws are only the basis of criminal or civil adjudication and pseudoscience. Once one considers a perceived physical “law” to be immutable, un-modifiable and without possible violation (or other then a well established theory supported by significant and current observational evidence) they are outside the realm of science, no longer inquiring as to the relationships of how things may work but now tiring to dictate how those “laws” presume things must work. The unmitigated adherence to perceived physical “laws” is the path of pseudoscience. The acceptance of a significantly observationally established theory until some other significant observations require a modification or replacement of that theory is the defining aspect of science. Science does not dictate how the universe must work but only tries to explain how it does work.

 

[Fredrik]

Oh, I see, in order to claim to be able to explain something one has to be able to explain absolutely everything associated with it, regardless of how remote, right down to the molecular/atomic level, and beyond?

The point is just that our understanding of photosynthesis isn't complete either.

I'll try to explain this better. I'm not saying that photosynthesis isn't well understood, only that it isn't better understood than gravity. I actually agree with you that photosynthesis is very well understood. In both cases (photosynthesis and gravity) we have a theory that agrees with experiments, and that's a prerequisite of understanding. No understanding is possible without a theory. In both cases, the theory is understood perfectly by the best scientists in the respective fields. Both theories contain things that seem "magical" because the theory offers no explanation for them. Those things are the axioms of the theories. (Every theory has to start with some set of statements that are just taken for granted). In the case of gravity, the only axiom is that space-time can be represented by a real four-dimensional smooth manifold with a metric tensor of Lorentzian signature that satisfies Einsteins equation. In the case of photosynthesis, every single chemical reaction is a separate axiom, and so is the existence of light, the existence of the chemicals, and so on.

I actually changed my mind a bit while I was writing this. I was thinking that photosynthesis is much less understood than gravity, because of the its dependence on chemistry and even quantum electrodynamics (absorption of photons), but it isn't. When we're talking about photosynthesis, those underlying causes should be thought of as the axioms of the theory, and not as an indication of poor understanding. I still think that gravity is better understood than photosynthesis because the theory covers such a huge range of phenomena and agrees with experiments to such a ridiculous degree of accuracy, and also because it only has one axiom instead of a long list.

Now the question is, why do you think that gravity is not well understood?


You wrote the stuff below in a reply to Sol, but I'm going to answer it anyway.

Don't you mean "The Theory of General Relativity"?

General relativity is a theory, so your way of saying it doesn't add anything. It's like saying "I saw it with my own eyes". Well of course you did. It's not like you could have seen it with someone else's eyes.

"Spacetime"! What's that then, exactly?

A real four-dimensional smooth manifold with a metric tensor of Lorentzian signature that satisfies Einsteins equation.

I'm sorry, "presence of energy"? Can you be more specific; maybe give an example?

More specifically, what he means by "presence of energy" is the stress-energy tensor, the T thingy on the right-hand side of Einstein's equation. I explained it to a degree that's appropriate for a forum post earlier in this thread. If you want more details, you're going to have to pick up a book on GR. "General Relativity" by Robert Wald is one of the standard texts.

Bends what, the "spacetime"? Bends it in what sort of way, and how?

Exactly as described by Einstein's equation. If you want more information, read Wald's book.

"curved space"? What's that then, exactly?

This can't be explained in a forum post. Read Wald's book or a textbook on differential geometry.

Er, that's it? I should be able to claim to "understand" gravity now? Methinks you have more work to do!

No, you have more work to do. If you want to understand gravity you have to understand differential geometry first. Other prerequisites include linear algebra, calculus, classical mechanics and special relativity.

 

[Yllanes]

What that means, is that on earth, Newtons law and Einsteins equations are the same results.

Almost the same results. They are effetively the same for most practical purposes.

Except that Newton's equations is used to actually design and build things, not Einsteins.

What about GPS?

Newton's theory is just an approximation to GR, although a very good approximation many times.

 

[Fredrik]

If you are disagreeing with any of the comments, please just say what you think is wrong. Everything is from current scientific publications or University courses. By all means, if you can disprove any of them, you have come up with something nobody else has.

Saying I don't know what I am saying is pretty funny. Everything I am saying is from famous scientist and well known scientific texts. I'm not doing anything but repeating well known scientific facts. But by all means, keep going.

Newton's Law of Gravity is valid, it is used all the time, and it is exact.

You really have no idea what you're talking about. Nothing that I've been saying in this thread is new.

If Newton's law had been true, then a planet orbiting a star would have an elliptical orbit (with corrections for the gravitational pull from the other planets), but it has been known for at least a century that the orbits aren't exactly elliptical. This disproves the inverse square law. Get over it.

Einsteins Law of Gravity (which uses curved spacetime) is used all the time as well, and better describes gravity outside of Earth's gravity. Einstein's field equations reduce to Newton's law of gravity in the limiting cases of a weak gravitational field and slow speed relative to the speed of light.

What that means, is that on earth, Newtons law and Einsteins equations are the same results.

No, they're not. Einstein's results are still a better approximation than Newton's, but neither is exactly true.

Except that Newton's equations is used to actually design and build things, not Einsteins.

Yes, because it's much easier to use, and because it's predictions are accurate enough for those purposes.

The inverse square law is a description of how nature is, it isn't a made up theory.

No, it's an (approximate) description of nature, because it's a theory, and because it agrees with experiments, as long as you don't measure things with too much precision. (All theories are "made up" by the way).

It applies to electromagnetic waves of all kinds, sound waves and water waves and gravity and is a fundamental factor in physics. Like most principles, laws and formulas used by science, they describe reality,...

Like any good theory, it provides an approximate description or reality, or equivalently, an exact description of a fictional universe that's a useful model of our own.

Don't take my word for any of this. But whatever you do, don't listen to anyone who tries to tell you they can break the laws of physics, or have somehow discovered they are wrong. That is crazy talk.

You keep showing that you're willing to hold on to your delusions like a creationist on crack. I can't believe I wasted time writing another answer for you. I'll probably just ignore you from now on.

 

[edd]

Those two statements contradict each other.

Well, technically not. Newton's gravity makes exact predictions. Which are wrong.

robinson - you were misguided to mention the luminosity inverse square relation. It's quite wrong on cosmological scales. Funnily enough, if you work it out with general relativity... you get it right. (Exception being that astronomers define a quantity called the luminosity distance, which is a contrived measure of distance set so that the luminosity relation still works)

Please, listen to sol invictus.

 

[sol invictus]

Er, that's it? I should be able to claim to "understand" gravity now? Methinks you have more work to do!

I have lots of work to do, but it's got nothing to do with you.

If you're genuinely interested, I'd recommend reading a book. This wouldn't be a bad place to start:

http://www.bartleby.com/173/

 

[ponderingturtle]

And kids, that was all gentle mockery, a reasonable response to extreme nonsense.

Newton's Law of Gravity is valid, it is used all the time, and it is exact. Einsteins Law of Gravity (which uses curved spacetime) is used all the time as well, and better describes gravity outside of Earth's gravity. It doesn't work at small distances however.

Wrong, Newton's laws are useful aproximations as long as certain strong inequalities hold. That does not mean that the inverse square law is correct.

The inverse square law is a description of how nature is, it isn't a made up theory. It applies to electromagnetic waves of all kinds, sound waves and water waves and gravity and is a fundamental factor in physics. Like most principles, laws and formulas used by science, they describe reality, the best the collective knowledge can, at the current time.

Not really it is a mathmatical relationship, sure so the law works fine in math but not in all physical systems.

 

[Cuddles]

I always find it amusing when people with no clue what they are talking about try to argue with professionals.

 

[ponderingturtle]

I always find it amusing when people with no clue what they are talking about try to argue with professionals.

Haven't studies shown that the less someone knows about something they are often more confident in their knowledge of it?

 

[Cuddles]

Haven't studies shown that the less someone knows about something they are often more confident in their knowledge of it?

Yes. Definitely.
OK, I have no idea.

 

[robinson]

Do you understand the nature of a "thread"? It's often like a story, which usually has a beginning, a middle and an end, requiring one to read from the beginning, or back somewhat if jumping in at the middle, in order to appreciate the context. Lazy people tend not to do this, then wonder why they don't understand and/or follow the story/argument.

Good point. The sidetrack about gravity (in that case about understanding why rain falls, cloud seeding, etc, was started early on). If you don't know about that, you missed part of the story. But time is limited, and this thread is long. We should be forgiving of those who came late to the party.

 

[robinson]

Let's take this example a little further and see where it goes. Imagine for a moment that the earth is completely covered with ocean - no land or ice, just water everywhere. Now an asteroid comes flying in and hits the ocean somewhere. Like a stone thrown in a pond, that sets up a circular wave that expands out from the impact point. Let's suppose the impact was the south pole, so every part of the wave is moving due north.

In the case of a stone thrown into a flat pond, 69dodge is (roughly) correct about the energy density in the ripples falling off like 1/r (ignoring friction and a few other effects). Let's see what happens instead on the curved surface of the earth.

The wave expands and eventually gets to the equator. As it keeps moving north, it actually contracts, and its energy density increases. After a while longer it will contract to zero at the north pole and smack into itself, sending up a huge plume of water.

That is not correct. It isn't even close. Such examples make it difficult to not resort to mockery. Your lack of understanding of oceans, and physical laws of motion, have led you to a completely wrong conclusion.

It would be an entire new thread to explain why.

Please, listen to sol invictus.

After watching him/her go so completely wrong, I am now starting to doubt his/her claim to be an expert.

The claim that the inverse square law is "wrong" was bad enough, but that example is beyond wrong.

 

[robinson]

exact - characterized by, requiring, or capable of accuracy of detail; very accurate; methodical; correct

precise - strictly defined; accurately stated; definite

perfect - complete in all respects; without defect or omission; sound; flawless


A theory can be exact, yet not as exact as another, and still stay exact.
A theory can be precise, yet not as precise as another, and still stay precise.

I'm not touching perfect.

Good points. I think the problem here is that most people chiming in (who are wrong) don't have an engineering or physics background. Newton's inventing Calculus was a direct result of his experiments with gravity. One could claim Calculus is wrong, if you apply the same level of perfection to the results of using it. But nobody is going to stop using Calculus, or listen to you.

 

[robinson]

Although certain physical principles are identified as “laws”, science is not based on “laws”. Science is based on theories; those theories are supported by the current observational evidence, refuted by it or shown to require some modification due to that evidence.

That is an outstanding claim. Extraordinary. Can you provide us with the source of this extraordinary claim? Because if you can't, I am going to mock you too.

The unmitigated adherence to perceived physical “laws” is the path of pseudoscience. The acceptance of a significantly observationally established theory until some other significant observations require a modification or replacement of that theory is the defining aspect of science. Science does not dictate how the universe must work but only tries to explain how it does work.

The unmitigated adherence to perceived physical “laws” is the path of pseudoscience.

In the real world we call adherence to the laws of physics things like, science, engineering, and reality. I don't know what planet you live on, but it can't be Earth. Here we have to obey the laws of physics, and depend on them to get things done.

Violations (or rather, attempts to violate) the laws of physics, results in much suffering.