View Full Version : gravitational field, force at a distance

stitt29

2008-Dec-19, 01:31 PM

Hi

I've often read that the trouble with Newton's gravity was that it was force at a distance. the force of gravity is invisible. When Einstein described it as bending space and time, it became acceptable. I've never heard anyone complain about magnetic field, which is similarly invisible. Is Magnetic field also better described as bending space, time or other dimensions for metals.

Also gravitional pull was measured by "someone famous" with a meter next to a mountain from which the Gravitational constant was derived. Has Einstein proved these guys wrong and obsolete with bending spacetime?

NEOWatcher

2008-Dec-19, 02:45 PM

Hi

Hi.

I would be interested in seeing that information in context. I've never heard the "invisible" references before.

I've never heard this meter next to a mountain reference either.

The only thing I can say about the difference with EM is the fact that it can be measured independent of mass and varies with the atomic structure.

stitt29

2008-Dec-19, 03:55 PM

the mountain reference is me misremembering the "Cavendish experiment" which was when Gravity was measured using a torsion balance. I'm sure this torsion balance held up to a mountain moves enabling someone to work out its mass and density and therefore G.

Invisible force simply means that the Earth is in orbit around the Sun but there is no pole from the Sun to the Earth enabling a force to be exerted. Thus it is sometimes called invisible, or said to be action at a distance.

DrRocket

2008-Dec-19, 04:18 PM

Hi

I've often read that the trouble with Newton's gravity was that it was force at a distance. the force of gravity is invisible. When Einstein described it as bending space and time, it became acceptable. I've never heard anyone complain about magnetic field, which is similarly invisible. Is Magnetic field also better described as bending space, time or other dimensions for metals.

Also gravitional pull was measured by "someone famous" with a meter next to a mountain from which the Gravitational constant was derived. Has Einstein proved these guys wrong and obsolete with bending spacetime?

I think the problem is more akin to force at a distance that occurs instantaneously with no propagating field. The electromagnetic force is associated with an electromagnetic field or photon that propagates at the speed of light. In Newtonian gravity, the gravitational force is felt at all distances from the mass that generates it instantaneously.

In general relativity the curvature that generates gravity propagates at light speed.

There is an approach to explaining electromagnetism as a geometric effect in a unified theory withe general relativity. It is known as the Kaluza Klein theory, and requires 5 dimensions. I don't know that I would describe it as "better", particularly since the best available theory of electromagnetism is quantum electrodynamics which is not formulated so as to be compatible with general relativity, and Kaluza Klein does require a (compactified) dimension that is not observed. Einstein spent much of his effort in the latter part of his life unsuccessfully trying to find a unified theory of electromagnetism and general relativity. Unification of quantum electrodynamics with general relativity is an active area of research, and such a theory, with the inclusion of the weak and strong forces is pursued under the heading of a Theory of Everything (TOE)http://en.wikipedia.org/wiki/Kaluza-Klein_theory

grant hutchison

2008-Dec-19, 04:34 PM

the mountain reference is me misremembering the "Cavendish experiment" which was when Gravity was measured using a torsion balance. I'm sure this torsion balance held up to a mountain moves enabling someone to work out its mass and density and therefore G.You're probably thinking of Nevil Maskelyne measuring the mass of the Earth, by the deviation of a pair of plumb lines from the local normal to the Earth's ellipsoid.

He set them up on either side of Schiehallion (http://www.undiscoveredscotland.co.uk/rannoch/schiehallion/), a Scottish mountain of pleasing symmetry. The deviation of the plumb lines let him estimate the mass of the Earth in "Schiehallion masses", and the symmetry and geology let him estimate the mass of Schiehallion with reasonably accuracy, and so work his way to the mass of the Earth.

Grant Hutchison

Tim Thompson

2008-Dec-21, 06:21 AM

I've often read that the trouble with Newton's gravity was that it was force at a distance.

I think the problem is more akin to force at a distance that occurs instantaneously with no propagating field.

That's what so vexed Newton. He thought that instantaneous action at a distance was absurd and said so. But he could also find no other satisfactory description of gravity. Newton was acutely aware of the problems with his own theory. The popular concept of Newton as genius based on what he accomplished fails to capture the true level of his genius, in my opinion, because it overlooks his own understanding of the shortcomings of what he did. Indeed Newton was aware of the basic problem of general relativity, and tried to solve it himself, but could not. When Einstein finally got the job done a few hundred years later, he used Ernst Mach (http://en.wikipedia.org/wiki/Ernst_Mach)'s interpretation of Newton's spinning bucket problem (http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Newton_bucket.html) to guide him. Newton would have been Einstein if he were born a few hundred years later.

OldGazer

2008-Dec-21, 07:24 PM

This is coming from a layman, so please bear with me.

I have a rudimentary understanding of how gravity, light, space, and time "appear" to work on a cosmic level, and I have a similar understanding of how these things "appear" to fail to work at a sub-atomic or quantum level.

The "holy grail" of physics is the successful melding of the cosmic and sub-atomic into one elegant construct that will increase our understanding and perhaps help us solve the mysteries surrounding the creation of what we call "The Universe."

As I see it, the real problem we are having involves our observational perspective and our math.

From a cosmological perspective, the properties of gravity, time, and space "behave" in a predictable manner, with those predictions being based on our direct and experimental observations. Galileo dropped two different masses and observed that they stuck the ground at the same time. Galileo asked a question, i.e. "What will happen if I drop these from a tower?" Newton took that data and using math that Galileo did not have, Newton was able to describe why Galileo's experiment produced the results that it did.

On the other hand, although quantum entities like electrons, photons, and other particles also appear to behave in a predictable fashion, these entities do not appear to adhere to, or be affected by Newtonian physics. Additionally, quantum entities exhibit some rather odd behaviors that necessitate the use of probabilities and uncertainties to describe those behaviors.

What this boils down to is that the behavior of cosmic entities can be directly observed, quantified, and using that data their future behaviors can be predicted with remarkable accuracy, but when we are dealing with quantum entities, merely observing a particle's behavior some how alters the outcome to the extent that we cannot say with certainty where in space-time a particular particle is, was, or will be.

So then, the question becomes, "Why is this so?"

Galileo asked this question, but because he lacked the necessary math and because Galileo did not approach the problem from the correct perspective, he couldn't come up with an answer. Newton was able to answer Galileo's question, and in doing so Newton paved the way, or at the very least Newton opened the door, to words our gaining a better understanding of the PHYSICAL universe as defined by the interactions of masses existing within space-time. In affect, thanks to the work of Galileo and Newton, we could now observe the heavens as well as objects on our planet and through the use of mathematics we could predict what those objects would do in the future.

For example, a satellite's orbit around a planet is determined by the mass of the planet, the mass of the satellite, the angular velocity of the satellite and do forth. In contrast, the orbit of an electron around the nucleus of an atom appears to be a function of the energy level of the electron.

If you increase the velocity of a satellite, the distance between the surface of the "parent" body and the satellite will increase. This is analogous to increasing the energy level of an electron. If you decrease a satellite's orbital velocity, the orbital distance decreases. But, if you cause an electron to move from one orbital shell to a lower energy shell, not only does the orbital distance decrease, the electron sheds energy in the form of a photon.

To me, the question then becomes, "If the cosmic and the quantum exist in the same space-time continuum that we call "The Universe", why don't they obey the same set of physical "laws"? I submit the answer is simple: Cosmic and quantum entities do not behave in the same or similar fashions because they exist within different dimensional subsets of what we call "The Universe".

To help me illustrate this hypothesis, let me use the "Cat in the box" (Schrödinger's cat) experiment. In this classic thought problem, a cat is placed in a closed box along with a radioactive source, a Geiger counter, a flask of poison and a hammer that is triggered by the counter. When the counter detects any radiation, it triggers the hammer, breaking the flask. The poison in the flask is released, killing the cat. The crux of this "problem" is predicting with any certainty when will the cat die.

From quantum theory we "know" that quantum systems are capable of having "quantum superpositions" which (broadly stated) implies that any quantum particle or system of quantum particles can have more than one discreet state of existence. Additionally, the Copenhagen Interpretation of superpositions says that a quantum system will collapse into one state or another ONLY at the time of quantum measurement. By extrapolation we can then say that in this case, until someone actually looks into the box, the cat can be alive and dead at the same time, and therein lies the paradox, at least when viewed from a cosmic perspective. The issue under examination is the effect of a seemingly "random" quantum event on a predominantely cosmic universe.

What this demonstrates is that when we try to define quantum behavior based on observations made from a cosmic perspective (and vice versa), strange things seem to happen, and those strange things will continue to happen until such time as we are able to observe these actions from a different perspective. That pretty much covers our trouble with observation, but what about the math?

What I believe is that the math itself is sound. After all, 1+1 does still equal 2. Where I feel the real problems lie is with the units of measure we are using. For example, how long is a foot or a meter? How much mass actually makes up a pound or a kilogram? How long is a second of time? The truth is, there are no such things as feet, kilograms, pounds, or seconds. These units of measure are totally arbitrary and are based on observations made from our view of the universe, and in some cases, measurements taken under one set of circumstances may or may not be equal to measurements taken under a different set of circumstances. A simple example of this is gas pressure. If we measure the pressure of a given volume of gas at sea level we get a specific pressure. If we move up in elevation to 20,000 feet ASL, we get an entirely different pressure for the simple fact that our zero pressure reference point has changed. In fact, in this example the zero point actually went negative.

We created our various units of measure as a way of giving meaning and order to the apparent chaos of the universe around us. It there fore stands to reason that the only way to "see" what is really going on is to find a vantage point from which we can observe the universe in its totality, and the only way to understand the universe is to measure or quantify our observations using units of measure that apply to both the cosmic and quantum dimensional subsets that together (with perhaps other subsets of dimensions) make up "The Universe."

kleindoofy

2008-Dec-21, 07:36 PM

... Newton's gravity was ... invisible. When Einstein described it ... it became acceptable. ...

Interesting, to date I've never heard anybody describe Newton's thoeries as inacceptable. Incomplete, perhaps, but never inacceptable. :confused:

hhEb09'1

2008-Dec-21, 07:55 PM

Interesting, to date I've never heard anybody describe Newton's thoeries as inacceptable. Me, I've never heard anything described as inacceptable (that I can remember, google only returns a million hits, dictionary.com none). :)

kleindoofy

2008-Dec-21, 08:00 PM

^^^^

Sorry, I'll have to blaim that one on my internal bilingual translation matrix.

German 'inakzeptabel' = English 'unacceptable' :whistle:

hhEb09'1

2008-Dec-21, 08:08 PM

I apologize for that, I should have known better--I'm going to go into the next room and berate myself. I was just so impressed that google had a million and a half hits that I had to share.

DrRocket

2008-Dec-22, 03:20 AM

....

The "holy grail" of physics is the successful melding of the cosmic and sub-atomic into one elegant construct that will increase our understanding and perhaps help us solve the mysteries surrounding the creation of what we call "The Universe."

As I see it, the real problem we are having involves our observational perspective and our math.

.... Edited for brevity."

A few observations:

1. Things on a cosmic scale and things on an atomic do obey the same physical laws. The difference lies in the relative importance of the various forces and in the degree to which some phenomena are the result of the behavior of large numbers of smaller particles or subsystems.

1a. There is indeed a major research push to develop a "Theory of Everything" that will unify the most fundamental physical theories -- quantum field theories of the electroweak force and the strong force with general relativity which is our best theory of gravity. But even if such a unified theory is achieved, it will not explain all that is observed, not be a long shot. That is because much of what we experience is the result of the complex interactions of many particles. and those complex interactions are not readily explained even if the fundamental laws are understood.

We already have a very accurate description of the electromagnetic force, quantum electrodynamics (QED). QED is sufficient to explain all everyday phenomena not involving gravity. It provides the fundamental rules for chemistry and biology. But no one can actually explain chemistry and biology from that basic theory. We simply cannot handle the complexities of nature from just the fundamental laws -- the equations are too hard to solve and the dynamics of many particle systems are too complicated. We cannot even explain turbulent fluid flow.

Knowing the rules of chess does not make one a Grand Master. So even if a Theory of Everything is successfully formulated, there will still be a LOT of physics remaining to be discovered and explained.

2. The atom is not a miniature solar system. Electrons do not orbit the nucleus in the same manner that satellites orbit the Earth. Although that analogy was used early in the development of quantum mechanics and is still used today in very elementary explanations of the atom it is just wrong.

3. There is no such thing as a dimensional subset. Dimensions don't work that way. It may be, or may not be, that there really are more dimensions that what we experience in everyday life, but if that is so then everything has those dimensions. They may not be important for many purposes, but they apply to everything or they apply to nothing. If the compactified dimensions of string theory exist they apply as much to a D9 Caterpiller as they do to a quark. But they are not very important in the specification of the position of the D9 Cat -- the uncertainty in its position is too small to be important in moving a ton of dirt.

4. The problem that we are having is that we have developed physical laws and the mathematics to go with them, but we have observations that we cannot explain in terms of those laws, and we don't yet have the understanding and the mathematics to formulate better laws. That may require new perspective, but the questions are not at all simple from the perspective that we do have.

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