Question about gravity

[Southwind17]

The description is as complete as we can make it without any new discrepancies to explain away, and that is why Sol says we understand it.

The "description" might be, but to claim that we understand gravity better than just about anything in the World ...? C'mon man!

Some basic things have to be simply accepted as observed. Sorry - no deeper scientific explanation is currently possible. Perhaps it will be so forever.

A force that we experience every day, can observe in action and measure, but will never be able to fully explain? Are you serious?

 

[Southwind17]

It was claimed in this thread that our knowledge of gravity was more imperfect than our knowledge of the other interactions.

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.

Someone said we lack a theory for it such as we have for the other fundamental forces. And that is incorrect because we do have a theory, based on a couple of postulates (one of which is the equation I posted earlier), which lets us predict the physics of gravitation with astonishing precision.

"Theory" and "understanding" are poles apart. Our ability to predict accurately does not necessarily infer understanding. I can predict with almost certainty what will happen IF I HIT THE "CAPS LOCK" KEY ON MY KEYBOARD. I would not profess to understand how it happens, though.

To really understand gravitation one must be able to carry out such calculations. You can't understand it at that level with just a couple of paragraphs in English. What you can do with a couple of paragraphs is to say something such as Fedrik's post. But the same thing is true for the other interactions. You cannot explain the Standard Model in a forum post so that you audience can then go and compute, say, the half life of the Higgs boson at tree level.

I enjoyed Fredrik's post, but with great respect it only serves to explain the formula, and then only briefly. It does not necessarily aid understanding. I feel no better placed to explain gravity than I did yesterday.

So, my question to you is: do you believe gravity is less understood than, say, the strong nuclear force? If you don't then we agree and there's nothing else to say. If you do, please tell us why.

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.

 

[Ladewig]

My knowledge of physics is very limited, so please forgive me if this question is too basic. Does our knowledge of gravity allow us to answer the question: if we could dramatically and instantaneously change the mass of the Sun, would all the planets change their orbits at the same time or would Mercury's orbit change well before Earth's orbit changed?

 

[Fredrik]

My knowledge of physics is very limited, so please forgive me if this question is too basic. Does our knowledge of gravity allow us to answer the question: if we could dramatically and instantaneously change the mass of the Sun, would all the planets change their orbits at the same time or would Mercury's orbit change well before Earth's orbit changed?

Nothing wrong with that question. Mercury's orbit would change first, Earth's orbit later. Newton's theory says the effect is instantaneous, but this is one of those things it's wrong about. Einsteins theory says the effect would propagate as a "gravitational wave" at the speed of light. Gravitational waves have not been measured directly, but they have been measured indirectly. Two stars that orbit each other lose energy by emission of gravitational waves, and this affects their orbit. That effect has been measured directly.

 

[Cold one]

Southwind

What I am basically trying to say is that your reasoning that we understand less about gravity than anything else such as say rain is off a bit.
Using your example to understand Subject A (rain) one must first have full understanding of Subject B (gravity)
Therefore understanding of A cannot and will never exceed understanding of B.
You can substitute any complex function into A (rain, rockets, the keyboard) and any basic principle of that function into B (gravity, electricity, magnetism).
Also the question of how rain falls and how rain happens are still answered by the same statement that I gave earlier.
And as has been explained earlier the whole gravity theory is widely understood if not to your satisfaction, but at the same time the why of gravity may not be fully realized similarly to the why of evolution, or the why of just about anything for that matter.

 

[sol invictus]

If you insist, however, I believe our understanding of how and why rain falls, for example, is pretty much complete.

Really? Let's examine that one. Part of understanding rain, I would say, is knowing whether it will rain next week. Or tomorrow. Or even in five minutes. Sometimes and in some places you might be able to answer those questions, but very often you can't (and neither can any one else), even given supercomputers and data from weather stations across the world.

On the other hand I can tell you almost precisely where Venus will be next week, and when and where we can see it in the sky, given the back of an envelope and a pencil. With a computer and some more data orbits can be computed with very good precision. Even 2000 years ago and more people were predicting astronomical events accurately.

Or think of the GPS system for a second. You've got a swarm of satellites orbiting the earth so precisely you can use them to pin down the location of any point on the surface to around a meter. That's true even though the earth is not exactly round, even though Newtonian gravity wouldn't be precise enough (you need GR corrections), etc. Imagine trying to do something equivalent with weather - forget it! Totally impossible.

I honestly don't see where you're coming from. What is it about gravity you find so mysterious?

 

[sol invictus]

Sometimes *why* is a valid scientific question: why does almost all the mass in the universe seem to be matter as opposed to anti-matter? Good question - it might lead to a new discovery. But why does gravity vary directly with mass and inversely with distance squared? It just does. The difference, as was pointed out above, is that you can ask why when there is some basic, underlying principle that can be measured and observed. With gravity, there is none such, so asking why is scientifically futile.

Actually, I can answer both of those in terms of our models for gravity. If you think like a particle physicist, gravitational forces are created by the exchange of gravitons. Think of the sun has emitting and absorbing streams of gravitons flowing radially outwards and inwards all the time. The number of gravitons produced by an object is proportional to its mass (that follows from the action principle). The fact that the force falls off as inverse square follows immediately from two things: that the gravitons in those streams don't get absorbed by something, and that there are three (spatial) dimensions. (If you prefer thinking of field lines, you'll get the same answer.) So gravity falls off as inverse square for the same reason electric fields from a point charge or the energy of sound waves in free space from a localized source do.

I'm glossing over a few subtleties here, but in essence both those "why" questions can be answered. Obviously you can now ask why three dimensions and why that action principle.

By the way, several experiments are probing gravity at short distances (around a millimeter) to see if the inverse square law is modified. One way that can happen is if there are more than three dimensions, but the extra ones are curled up to around a millimeter (or smaller). Then the argument I gave tells you the force will go from inverse square to -(2+n) at that distance (n the number of extra dimensions).

 

[Fredrik]

Southwind17, your claims are very strange to us physics geeks. Can you please give us an example of something that you think is well understood, and then explain it?

Also consider this:

* If you drop your keys, they fall to the floor.
* Our planet is in a near elliptical orbit around the sun.
* Light from distant stars is deflected slightly when it passes close to the sun.
* A clock on the fourth floor of a building runs at a different rate than an identical clock on the third floor in the same building.
* The decay rate of unstable particles depends on their speed.

Einstein's theory says that all of those things and a lot more are explained by the fact that there is a relationship between the geometry of the universe and the matter content of the universe, that can be expressed in the form of an equation. That equation also gives us all the details: It tells us how fast your keys will fall, how our planet's orbit will deviate from perfectly elliptical, and so on.

How is that not "understanding"?? And if this is not understanding, then what is?

 

[sol invictus]

Southwind17, your claims are very strange to us physics geeks.

Maybe it's because rain is a relatively complex process, and so when someone asks why it rains you can go through a long list of reasons? You could talk about water evaporating, clouds, sun, rivers and the ocean, etc. If you were explaining it to a child, you could survive many layers of "why" questions before getting stuck. That might make it seem like you understand it better.

Gravity on the other hand is extremely simple, probably because it's fundamental - it is what it is, and we understand quite well what it is, but even one or two well-placed "why" questions can get us stuck. To a physicist, that means we understand it much better than rain - rain is something tha depends on many fundamental laws (such as gravity), so our understanding of it is at best as good as our understanding of those laws, and in fact considerably worse - but maybe it doesn't strike everyone that way?

Not everyone thinks like a reductionist, I guess...

 

[Fredrik]

Yes, this is my impression as well. He seems to think that if there are several layers of explanations, we have a better understanding than if there's only one.

 

[Ladewig]

how our planet's orbit will deviate from perfectly elliptical,

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?

 

[69dodge]

Here's what Feynman had to say about the question of explanation in physics (The Feynman Lectures on Physics, vol. 2, sec. 1-5). He was talking about electromagnetism, not gravity, but the general idea is the same.

1-5 What are the fields?

We now make a few remarks on our way of looking at this subject. You may be saying: "All this business of fluxes and circulations is pretty abstract. There are electric fields at every point in space; then there are these 'laws.' But what is actually happening? Why can't you explain it, for instance, by whatever it is that goes between the charges." Well, it depends on your prejudices. Many physicists used to say that direct action with nothing in between was inconceivable. (How could they find an idea inconceivable when it had already been conceived?) They would say: "Look, the only forces we know are the direct action of one piece of matter on another. It is impossible that there can be a force with nothing to transmit it." But what really happens when we study the "direct action" of one piece of matter right against another? We discover that it is not one piece right against the other; they are slightly separated, and there are electrical forces acting on a tiny scale. Thus we find that we are going to explain so-called direct-contact action in terms of the picture for electrical forces. It is certainly not sensible to try to insist that an electrical force has to look like the old, familiar, muscular push or pull, when it will turn out that the muscular pushes and pulls are going to be interpreted as electrical forces! The only sensible question is what is the most convenient way to look at electrical effects. Some people prefer to represent them as the interaction at a distance of charges, and to use a complicated law. Other love the field lines. They draw field lines all the time, and feel that writing E's and B's is too abstract. [...]

The best way is to use the abstract field idea. That it is abstract is unfortunate, but necessary. The attempts to try to represent the electric field as the motion of some kind of gear wheels, or in terms of lines, or of stresses in some kind of material have used up more effort of physicists than it would have taken simply to get the right answers about electrodynamics. It is interesting that the correct equations for the behavior of light in crystals were worked out by McCullough in 1843. But people said to him: "Yes, but there is no real material whose mechanical properties could possibly satisfy those equations, and since light is an oscillation that must vibrate in something, we cannot believe this abstract equation business." If people had been more open-minded, they might have believed in the right equations for the behavior of light a lot earlier than they did.​

So, Southwind17, when you ask for an explanation of gravity, in terms of what do you want it explained? And why doesn't that, whatever it is, need its own explanation?

 

[Furi]

Nothing wrong with that question. Mercury's orbit would change first, Earth's orbit later. Newton's theory says the effect is instantaneous, but this is one of those things it's wrong about. Einsteins theory says the effect would propagate as a "gravitational wave" at the speed of light. Gravitational waves have not been measured directly, but they have been measured indirectly. Two stars that orbit each other lose energy by emission of gravitational waves, and this affects their orbit. That effect has been measured directly.

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,

I have read and that I know it has not been measured, and that there will need to be some serious flux availabe to measure the speed of propogation (maybe a massive supernova or other Mass Energy conversion event) is there any model that dictates what the theoretical speed is required to be, as I am sure this is why I got the idea that gravity had to exceed c.

 

[Southwind17]

Really? Let's examine that one. Part of understanding rain, I would say, is knowing whether it will rain next week. Or tomorrow. Or even in five minutes. Sometimes and in some places you might be able to answer those questions, but very often you can't (and neither can any one else), even given supercomputers and data from weather stations across the world.

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.

On the other hand I can tell you almost precisely where Venus will be next week, and when and where we can see it in the sky, given the back of an envelope and a pencil. With a computer and some more data orbits can be computed with very good precision. Even 2000 years ago and more people were predicting astronomical events accurately.

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).

Or think of the GPS system for a second. You've got a swarm of satellites orbiting the earth so precisely you can use them to pin down the location of any point on the surface to around a meter. That's true even though the earth is not exactly round, even though Newtonian gravity wouldn't be precise enough (you need GR corrections), etc. Imagine trying to do something equivalent with weather - forget it! Totally impossible.

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

I honestly don't see where you're coming from. What is it about gravity you find so mysterious?

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.

 

[ponderingturtle]

Just for some fun;

Q1 Where is gravity of the Earth the strongest?

A - On the surface of the Earth
B - In the centre of the Earth
C – Somewhere about halfway down to the centre of the Earth

Well can we make the assumption that the earth is a non rotating perfect sphere of uniform density? Then it would be the surface, as a perfect sphere can be thought of as a point particle for forces that fall off with the square of the distance with its total mass, and mass increases with the cube of the distance while the force drops off with the square.

Q2 If you dig yourself about one thousand of miles into the Earth, what happens to gravity?

A. - Increases definitely.
B. - Stays more or less constant.
C. - Decreases definitely.

There is more to it than you might think.

D. - You burn up and die
E. - You break throught into the hollow earth
F. - You find yourself in Hell

 

[ponderingturtle]

I don't see how citing a formula satisfactorily addresses either the "how" or the "why". As such it helps little in promoting understanding.

It expresses how perfectly because it perfectly describes the nature of the effect. It does not answer why.

 

[ponderingturtle]

Really? Let's examine that one. Part of understanding rain, I would say, is knowing whether it will rain next week. Or tomorrow. Or even in five minutes. Sometimes and in some places you might be able to answer those questions, but very often you can't (and neither can any one else), even given supercomputers and data from weather stations across the world.

On the other hand I can tell you almost precisely where Venus will be next week, and when and where we can see it in the sky, given the back of an envelope and a pencil. With a computer and some more data orbits can be computed with very good precision. Even 2000 years ago and more people were predicting astronomical events accurately.

Or think of the GPS system for a second. You've got a swarm of satellites orbiting the earth so precisely you can use them to pin down the location of any point on the surface to around a meter. That's true even though the earth is not exactly round, even though Newtonian gravity wouldn't be precise enough (you need GR corrections), etc. Imagine trying to do something equivalent with weather - forget it! Totally impossible.

I honestly don't see where you're coming from. What is it about gravity you find so mysterious?

GPS has been used to measure the uplift of the Himalayas, so it is more than just 1 meter of accuracy.

 

[ponderingturtle]

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.

So you claim that the process of rainfall is so well understood, well try this, does cloud seeding work?

If we really understood rain that well we could answer that question easily.

 

[robinson]

All I said was that we understand gravity better than just about anything else in the world. If you don't agree, why not give some examples of things you think we understand better?

That formula explains exactly how energy bends spacetime. If you don't understand it, that's your problem, not the theory's.

Wouldn't it be more sensible for you simply to posit your understanding? If you insist, however, I believe our understanding of how and why rain falls, for example, is pretty much complete. Or how and why the plasma screen I'm looking at works.

The conversation is becoming convoluted, like a snake consuming a jackass while discussing rat droppings.

All I said was that we understand gravity better than just about anything else in the world.

Yes, but like many statements, it means nothing, unless you think you know something. We understand why rain falls, because we understand that water is heavy and falls down.

That formula explains exactly how energy bends spacetime. If you don't understand it, that's your problem, not the theory's.

That is one of the most absurd claims I have ever come across. The woos should be proud.

Wouldn't it be more sensible for you simply to posit your understanding?

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

 

[robinson]

Do you understand why rain falls?

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

Do you understand why large objects attract every other object in the Universe? But small objects don't attract each other? But they are attracted by large objects. Please explain this.

Yes, because ... uh ... hmm ... you go first.