Question about gravity

[Ziggurat]

Have you ever heard of the "inverse square law"? It isn't considered "the inverse square theory", and if you were to talk about it like that, you would be considered uneducated.

Laws only express the existence of relationships, they do not indicate the origins of that relationship. That's what theories are for. And laws are not intrinsically more certain or exact than theories. Newton's second law (F=ma), for example, is actually wrong. Classical electrodynamics is a theory, and it includes the inverse square law and Gauss's law, but it's far more comprehensive than either law alone.

A law alone, without a theory to use it, is frequently of little use in understanding anything. A prime example is Ohm's law. Now, many materials happen to obey Ohm's law, but it's actually far from universal. And nothing about the law itself indicates when it does or doesn't hold, how well it holds, or why R takes on the value it does for a given object. If you want to understand that, you need to resort to a theory, not a law.

 

[robinson]

A law alone, without a theory to use it, is frequently of little use in understanding anything.

Like the law of gravity?

 

[Fredrik]

Have you ever heard of the "inverse square law"? It isn't considered "the inverse square theory", and if you were to talk about it like that, you would be considered uneducated.

The inverse square law is just a part of Newton's theory of gravity, which has been proven wrong. And guess what, they are still called "the inverse square law" and "Newton's theory of gravity". No amount of evidence can ever promote a theory to a law, or make it lose its status as a theory. That's just a common misunderstanding.

sometimes they are called "equations", "principles", or "functions". But nobody who is educated in science would try to tell you that they are only theories,

No one who is educated in science would put the word "only" before the word "theories", so in a way you're right, but not for the reasons you think.

The naive belief that we are never certain about anything, is sometimes trotted out to make some kind of bizarre point, usually that we can never know anything, we only have theories and stuff. It just isn't true.

You got that wrong too. Yes, we can't ever be 100% certain of anything, but if we demanded that level of certainty we might as well stop thinking altogether. We can be sure enough about some things to say that we "know" them. For example, I know that I'm not a giraffe.

When it comes to theories in physics, there are some things that we can know, and other things we can't know. The things we can know are mostly statements like "prediction X of theory Y agrees with experiment Z to within W percent", because that's the sort of thing we can prove (to a reasonable degree of certainty). Theories can't be proven however, because that would require that we perform an infinitely accurate measurement at every point in space-time.

The most important part of science is the experiment, not the theory.

One without the other would be useless, so I'm not going to say that one of them is more important than the other in the scientific process. But when it comes to understanding, there's no contest. The theories are more important.

One thing you seem to be overlooking is that it's the theory that defines all the relevant concepts. Without a theory, physicists wouldn't even know if they mean the same thing when they say something like "photon".

I've had this discussion before, and if you want to argue that there are no laws, only theories, go right ahead. But after this attempt to enlighten you, I'm only going to mock you, not try and educate you.

I think you've had it with me actually. I can see that you're clinging to your delusions like a creationist, so I have probably waisted my time trying to explain things to you.

 

[robinson]

Yes, we can't ever be 100% certain of anything, but if we demanded that level of certainty we might as well stop thinking altogether.

That doesn't make any sense. I was going to mock you, but you beat me to it.

We can be sure enough about some things to say that we "know" them. For example, I know that I'm not a giraffe.

That is better than what I was going to say.

Yes, we can't ever be 100% certain of anything, ... I know that I'm not a giraffe.

 

[robinson]

The inverse square law is just a part of Newton's theory of gravity, which has been proven wrong.

Really? The inverse square law has been proven wrong? Oh noes! We need to call somebody.

 

[Fredrik]

Now you're just being a troll. What I said does make sense, and what you've been saying is still wrong.

 

[Fredrik]

Really? The inverse square law has been proven wrong? Oh noes! We need to call somebody.

Yes, it has. See posts #76-78 in this thread. And if you had understood anything that we've been saying in this thread, you'd know that this is no big deal.

 

[tsig]

How about photosynthesis?

What about photosynthesis? Comparing apples to oranges.

 

[sol invictus]

Have you ever heard of the "inverse square law"? It isn't considered "the inverse square theory", and if you were to talk about it like that, you would be considered uneducated.

I love it when people say something ignorant and completely wrong, and then claim that anyone that doesn't agree is "uneducated". Classic.

As Fredrik pointed out, Newton's inverse square law for gravity is false. Or rather, it's not exactly right. It was a theory - and quite a good one - but it does not describe the world correctly. It has been supplanted by GR, which is another theory, and also not quite right (but closer). Someday someone will find another theory, better than GR, but probably still not quite right.

 

[tsig]

Now you're just being a troll. What I said does make sense, and what you've been saying is still wrong.

There are some on this forum who will disagree with anything that is said.

 

[robinson]

The inverse square law is just a part of Newton's theory of gravity, which has been proven wrong.

Newton's inverse square law for gravity is false.

There are some on this forum who will disagree with anything that is said.

Yep. They will even claim they know more than anybody else in the world. Like the above statements about Newton, which are so unique, so special, they have never appeared anywhere on the world wide web before. They are brand new statements! Never heard before.

Newton's inverse square law for gravity is false.

http://www.google.com/search?hl=en&...+law+for+gravity+is+false"&btnG=Google+Search
See? Nobody has ever said that before (at least according to Google). Amazing. Don't take my word for it, just look for yourself.

How about

The inverse square law is just a part of Newton's theory of gravity, which has been proven wrong.

http://www.google.com/search?hl=en&...ity,+which+has+been+proven+wrong"&btnG=Search
Again, such a huge scientific discovery has never been uttered before! Wow.

Lets try part of it

The inverse square law is just a part of Newton's theory of gravity

http://www.google.com/search?hl=en&...ewton's+theory+of+gravity"&btnG=Google+Search
Nope. Nobody has ever said that before. Fascinating.

How about the essence of the claim

Newton's theory of gravity has been proven wrong

http://www.google.com/search?hl=en&...ity+has+been+proven+wrong"&btnG=Google+Search
Whoops! We have one hit! Lets see who said that before...

Gravity is only a theory, not fact. Newton's theory of gravity has been proven wrong. Many scientists believe that Einstein's relativity describes it better -- some do not.

Theory does not mean "crackpot hypothesis" it means something that scientists have come to as a conclusion by extensive research. The scientific process never allows anything to be proven true.

Georules 11-28-2005, 08:39 PM
http://www.hardcoreware.net/forum/cnn-creationism-piece-last-night-t25285p2.html?

So you have a supporter! One person has said that, according to Google. And there you have the statement I mentioned earlier!
The scientific process never allows anything to be proven true.


There it is!

Wow. It must be true! So let us look at that statement: "The scientific process never allows anything to be proven true."
http://www.google.com/search?hl=en&...lows+anything+to+be+proven+true."&btnG=Search

Whoops. The only place that appears, is in the source of it. Nobody else has ever said that on the web! (Of course, all of these will soon appear in a link to my post, but that won't matter)

So according to this logic, this scientific breakthrough, Gravity isn't real, it is just a theory.

Oh wait, earlier it was claimed we know more about gravity than almost anything.

So nothing can be known to be true, yet we understand almost everything about gravity, and we can explain it with a theory, better than almost anything.

Or something.

And some people say this stuff is hard to understand. I think it is all clear now.

Newton was wrong, Einstein was wrong, and we don't really know if gravity is real, and we also understand gravity better than anything, and the theory explains it.

And if you can't understand this, it is all your fault for being dumb.

I think that should clear up any questions.

 

[robinson]

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.

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.

A full explanation of these things is beyond the scope here, but you can study these things yourself.

Laws can be updated, if experimental data leads to a better description. As was pointed out, some laws only describe something, they don't explain why.

But the simple inverse square law can actually be explained, and you can do an experiment yourself to see both how and why it is accepted as a scientific law. And I can't believe I am having to explain this on a skeptics forum in the math and science section.

 

[sol invictus]

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.

robinson... I don't know how to say this politely, and I don't mean it as an attack - but you have absolutely no idea what you are talking about (you also have the world's worst googling skills, but that's another matter). It would be better if you didn't post false statements on topics you are clearly completely ignorant of - someone reading them might be mislead.

Newtonian gravity is the weak-field limit of general relativity. What that means is that when the gravitational fields involved are not very strong, the predictions of general relativity are very close to those of Newton. Because the corrections are small, no one noticed that Newton's law of gravity was wrong until the orbits of the planets had been measured with great precision in the late 19th century (and even then it wasn't immediately clear what was wrong).

I happen to be an expert in this field, and I assure you I do know what I'm talking about. But since you're probably not going to believe that, why not try some other sources. Search for precession perihelion mercury for yourself; probably any hit from that search would do. Or read this, written by the greatest authority on the subject that ever lived. It's so perfectly topical and well-written that I will quote it at length (my emphasis; you can read the rest at http://www.bartleby.com/173/a3.html):

As soon as a science has emerged from its initial stages, theoretical advances are no longer achieved merely by a process of arrangement. Guided by empirical data, the investigator rather develops a system of thought which, in general, is built up logically from a small number of fundamental assumptions, the so-called axioms. We call such a system of thought a theory. The theory finds the justification for its existence in the fact that it correlates a large number of single observations, and it is just here that the “truth” of the theory lies.

Corresponding to the same complex of empirical data, there may be several theories, which differ from one another to a considerable extent. But as regards the deductions from the theories which are capable of being tested, the agreement between the theories may be so complete, that it becomes difficult to find such deductions in which the two theories differ from each other. As an example, a case of general interest is available in the province of biology, in the Darwinian theory of the development of species by selection in the struggle for existence, and in the theory of development which is based on the hypothesis of the hereditary transmission of acquired characters.

We have another instance of far-reaching agreement between the deductions from two theories in Newtonian mechanics on the one hand, and the general theory of relativity on the other. This agreement goes so far, that up to the present we have been able to find only a few deductions from the general theory of relativity which are capable of investigation, and to which the physics of pre-relativity days does not also lead, and this despite the profound difference in the fundamental assumptions of the two theories. In what follows, we shall again consider these important deductions, and we shall also discuss the empirical evidence appertaining to them which has hitherto been obtained.

(a) MOTION OF THE PERIHELION OF MERCURY

According to Newtonian mechanics and Newton’s law of gravitation, a planet which is revolving round the sun would describe an ellipse round the latter, or, more correctly, round the common centre of gravity of the sun and the planet. In such a system, the sun, or the common centre of gravity, lies in one of the foci of the orbital ellipse in such a manner that, in the course of a planet-year, the distance sun-planet grows from a minimum to a maximum, and then decreases again to a minimum. If instead of Newton’s law we insert a somewhat different law of attraction into the calculation, we find that, according to this new law, the motion would still take place in such a manner that the distance sun-planet exhibits periodic variations; but in this case the angle described by the line joining sun and planet during such a period (from perihelion—closest proximity to the sun—to perihelion) would differ from 360°. The line of the orbit would not then be a closed one, but in the course of time it would fill up an annular part of the orbital plane, viz. between the circle of least and the circle of greatest distance of the planet from the sun.

According also to the general theory of relativity, which differs of course from the theory of Newton, a small variation from the Newton-Kepler motion of a planet in its orbit should take place, and in such a way, that the angle described by the radius sun-planet between one perihelion and the next should exceed that corresponding to one complete revolution by an amount given by...

 

[robinson]

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.

Newton's law of universal gravitation (Sir I. Newton)
Two bodies attract each other with equal and opposite forces; the magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centers of mass of the two bodies; mathematically,

F = (G m M/r²) e,

where m and M are the masses of the two bodies, r is the distance between. the two, and e is a unit vector directed from the test mass to the second.

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

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.

Laws can be updated, if experimental data leads to a better description. As was pointed out, some laws only describe something, they don't explain why.

But the simple inverse square law can actually be explained, and you can do an experiment yourself to see both how and why it is accepted as a scientific law. And I can't believe I am having to explain this on a skeptics forum in the math and science section.

Here is an extensive list of Laws (and other cool stuff)
http://www.alcyone.com/max/physics/laws/

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.

 

[sol invictus]

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.

Those two statements contradict each other.

What that means, is that on earth, Newtons law and Einsteins equations are the same results. Except that Newton's equations is used to actually design and build things, not Einsteins.

Two more contradictory statements.

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.

Nope. The inverse square law is a description of how things work in three flat spatial dimensions. The idea that the world consists of three flat spatial dimensions plus time is a theory - and it's wrong. We've known that for nearly a century, but evidently that wasn't long enough for you to find out about it.

And I can't believe I am having to explain this on a skeptics forum in the math and science section.

And I can't believe you keep saying that.

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.

Did you read the quote from Einstein above? Ever heard of him?

 

[robinson]

I happen to be an expert in this field,

Great! You can explain it to us dumb people. Start with how the inverse square law was shown to be wrong. That should be interesting.

Here is a simple example of the inverse square law.
http://csep10.phys.utk.edu/astr162/lect/light/intensity.html

Here is another one.
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/estatics/u8l3c.html

Explain why it isn't a law, how there are no laws, how we don't know if it is true.

Here is a good place to start as well.
http://hyperphysics.phy-astr.gsu.edu/Hbase/forces/isq.html

There are thousands more examples, but I just can't wait. It's thrilling to know you have discovered that Newton was wrong. You will be getting a Nobel prize, and we got to see it happen.

Newton's inverse square law for gravity is false.

Oh this is going to be great. All those engineers and scientist are wrong, and you are right.

 

[69dodge]

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.

I don't understand what you're trying to say here. If Newton's law of gravity is exact, how can Einstein's law of gravity be better?

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.

Water waves? Are you sure? 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.

But that's a side issue. Let's talk about gravity.

Electromagnetic waves and sound waves carry energy away from their source. And energy is conserved. So, if they spread out in three dimensions, their energy density follows an inverse square law. But gravity, in Newton's theory, is not something that is emitted from its source and travels away from it, carrying along any energy. The gravity of a massive body doesn't ever get used up, for example, whereas a flashlight or a portable radio soon exhausts its batteries. So, why does gravity need to follow an inverse square law? I don't know of any reason.

 

[69dodge]

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.

It means that the weaker the gravitational field is, the closer the predictions of Einstein's theory are to those of Newton's theory. It doesn't mean they're ever exactly the same.

In any case, if the inverse square law of gravity is so inviolable, how can Newton's theory ever fail to hold, even for strong gravitational fields?

 

[sol invictus]

Great! You can explain it to us dumb people. Start with how the inverse square law was shown to be wrong. That should be interesting.

I just did - see above. It's because spacetime is curved.

There are thousands more examples, but I just can't wait. It's thrilling to know you have discovered that Newton was wrong. You will be getting a Nobel prize, and we got to see it happen.

Seriously - have you ever heard of Einstein? It's as if the 20th century has passed you by completely...

Earth to robinson: the most famous scientist in history (and a Nobel prize winner*) did that that 90 years ago!!

*And he would have won the prize for it if he hadn't already won it for something else. He deserved about four Nobels.

 

[robinson]

I don't understand what you're trying to say here. If Newton's law of gravity is exact, how can Einstein's law of gravity be better?

That is an excellent question. Since we have an expert here, he might be able to explain it in great detail. Anything I say will just be repeating physics texts and writings of Einstein and others. I don't understand the math, but I trust they are correct.

Sadly, while some people claim to be "experts", they can't seem to explain what they say they know so well. What can one do? If no knowledge is forthcoming from our resident expert, I can link you to several papers that explain it in great detail. But it is awful slow going.

Water waves? Are you sure? 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.

But that's a side issue. Let's talk about gravity.

Sure, but that is an excellent observation. The surface of a body of water is a boundary between two fluids, and energy is transmitted to the air, according to the same laws, but the boundary acts as a reflection, so we immediately run into one of the realities of the laws of physics. They are exact for closed systems, ideal situations. In reality there are reflections and refractions and other factors. In regards to the inverse square law, a shockwave or other energy source in water does obey the inverse square law. But just like sounds waves, or even light, it is only perfect when there are no reflections or other variables to alter the energy, which radiates out in all directions. Which brings us to one reason some may be loathe to delve in to a deep discussion of these matters.

Electromagnetic waves and sound waves carry energy away from their source. And energy is conserved. So, if they spread out in three dimensions, their energy density follows an inverse square law. But gravity, in Newton's theory, is not something that is emitted from its source and travels away from it, carrying along any energy. The gravity of a massive body doesn't ever get used up, for example, whereas a flashlight or a portable radio soon exhausts its batteries. So, why does gravity need to follow an inverse square law? I don't know of any reason.

Our expert may explain all that. My understanding, from reading literature on this, is that mass is lost from the energy of gravity. This is, unlike some things, theory theory theory, but backed up by observation. Experiments on this are extremly difficult to do, but according to sources available on the net, research is being done.

Variations or violations of the inverse square law in regards to gravity is a contentious topic, from what I can see.
http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

I am no expert on this. I just steal from the Giants.