Do we really know how gravity and magnetism operate?

It depends entirely on what you mean by knowledge.

Our theories for gravity and magnetism allow us to describe the essential physics of systems from nearly the size of the universe, to events at a scale of nearly 1 million times smaller than the nucleus of an atom. The latter phenomenon is explained by quantum electrodynamics which routinely makes predictions correct to 10 decimal places. For gravity, we can accurately describe systems as vast as the universe and its evolution, to the surfaces of black holes.

From a practical point of view we understand these two forces almost perfectly, and to our best current ability to measure.

A charged particle sitting in your reference frame motionless produces a pure electric field like the figure above.As you get farther from the center, the number of imaaginary field lines is conserved through each spherical surface and so the strength of the field decreases as the reciprocal of te surface area of the sphere. This is the relationship if space is 3-dimensional and explains the inverse-square law.

Where does this electric field come from? It comes from the electromagnetic force which is a quantum phenomenon explained in great detail by a theory called Quantum Electrodynamics.Every charged particle is surrounded by a cloud of virtual photons that are exchanged between other charged particles to produce the familiar electrostatic force. Here is a diagram showing the exchange of just one of these virtual particles (the wavy line) exchanged between two electrons.

A magnetic field is what we observe in our frame of reference as an electric field passes by traveling at a speed relative to us. The movement of these charges is called an electric current, and all electric currents produce magnetic fields so long as the speed of the current is not zero. But a magnetic field is not fundimentally a new field in nature, its just the familiar electric field seen in a different reference frame. Our understanding of 'electromagnetic' fields is virtually perfect to the extent that its bassis in quantum mechanics is a solid foundation.

Gravitational fields are very different. We call them 'fields' because that is a left-over description from Newtonian physics and it serves us well in most things that we come into contact with in our solar system and local universe. But this is not the correct way to think about gravity. It is neither a field nor is it a force, although it resembles both of these in rough terms.

General relativity is our most sucessful theory for describing gravity, but it is a purely geometric theory and not one that looks anything like quantum electrodynamics. Instead, 4-dimensional spacetime is the 'field' that describes gravity and distortions in it lead to the experiences of accellerations as particles try to travel in straight lines through the geometry. At the same time, matter (and energy) also creates these geometric distortions. General relativity is a theory of the geometry of worldlines and not some mythical background space into which gravity is embedded. The biggest flaw in general relativity is that it does not tell us exactly how matter creates gravity. The only other theory that we have that describes how forces are produced is called Quantum Field Theory of which quantum electrodynamics is the most successful one.

There is no similar theory for gravity, so we do not really know how gravity is created by matter. Because the field for gravity is called spacetime, what we have to imagine is a theory that describes where spacetime 'comes from' in other words how matter produces space and time!!!

If you are asking a more metaphysical question about our knowledge, then we don't really understand ANYTHING about gravity at the deepest level, such as why does gravity exist? Is it a quantum field? What is the nature of space-time?

In terms of the human sphere of activity, we understand enough about these two fields that we will never have much practical need for better theories than the ones we now have.

Return to Dr. Odenwald's Gravity page at the Astronomy Cafe Blog.