Q & A: gravity without fields?

This question is actually from my uncle who thinks that I have some sort of insight because I volunteer at a science museum. I don't, and I apologize in advance. Here is his question: For generations, physicists have been uneasy with the concept of "action at a distance" as they called it. They didn't like to explain gravity or magnetism. Even though they talked about "fields" and could predict with accuracy how objects would behave in such "fields", the idea was, and is, a mathematical convenience. Then along came Einstein. His first wife was a Mathematician and she and her friends came up with the equations on his behalf and at his request, and then he could manipulate them and read them. He would, of course, explain to them how objects behaved, and then they would put it in mathematical form. (Pure Mathematicians are strange folks.) In these formulas, spacetime was born, and furthermore, terms in the formulas suggested that massive objects "warp" spacetime so that another object passing through the "warped" spacetime area can curve around the massive object but still be following a straight line. Physicists were ecstatic. No more "action at a distance". Gravity was nothing more than local geometrical shapes. Objects still traveled in straight lines, and if spacetime itself bent, the straight lines would also have to bend to appear straight, and no force was involved. OK, fine. I can see their argument, and even appreciate it. But, envision a universe with nothing but one sun in it. There would be no space. The term spacetime would be meaningless. Would spacetime physics break down? What would happen to our sun? Or, consider a black hole. The gravity is so massive that time stops. There is no time, and physicists admit that physics breaks down there. And at the core of my gripe is the argument that objects warp spacetime is no more explained than the field concept. Can you find me an explanation of gravity that doesn't involve fields, or spacetime?
- Chanin (age 44)
Fort Worth, Texas, USA

Let me start with the physics, then work back to the history. No, we have nothing to say about gravity without fields or spacetime, a sort of field itself. All of physics, not just gravity, ultimately faces the question of being "unexplained", since at best what we are aiming to do is get a self-consistent, complete and accurate set of laws.* Even if we got that, it would be in some sense just a bunch of unexplained math.

The black-hole stuff is a bit over my head. We don't say that physics breaks down at the singularity, just the physics that we know to this point. From the outside perspective, not even that happens. We think that the in-falling stuff forms a thin sheet just outside the would-be horizon and ultimately evaporates by Hawking-like radiation.  A universe that was empty except for a star could have a perfectly ordinary spacetime, which would soon have lots of photons whizzing through it.

On fields- sure they started as a mathematical convenience. At this point, they're all we've got. Physics has no ingredients other than fields. 

Your description of the development of relativity sounds fishy. The basic math of Special Relativity was so simple even Einstein could handle it. The 4-D version was due to Minkowski, not to either of the Einsteins.  The General Relativistic math was harder. I've heard that the first successful formulation was due to Hilbert, who may have been just barely ahead of Einstein.


Mike W.

*For the in-crowd, no, that needn't violate Gödel's theroem, since not all formal statements need to map to ones with physical significance.

(published on 07/06/2011)