“the (gravitational) field equations are derived . . . from an analysis of tidal forces.” Thus sayeth p. xii of the preface to “The Geometry of Spacetime” by James J. Callahan. This kept me from passing over pp. 174 –> on tides, despite a deep dive back into differentiating complicated functions, Taylor series etc. etc. Hard thinking about tidal forces finally gave me a glimpse of what general relativity is all about.

Start with a lemma. Given a large object (say the sun) and a single small object (say a satellite, or a spacecraft), the path traced out by the spacecraft will lie in a plane. Why?

All gravitational force is directed toward the sun (which can be considered as a point mass – it is my recollection that it took Newton 20 years to prove this delaying the publication of the Principia , but I can’t find the source). This makes the gravitational force radially symmetric.

Now consider an object orbiting the sun (falling toward the sun as it orbits, but not hitting the sun because its angular momentum carries away). Look at two close by points in the orbit and the sun, forming a triangle. The long arms of the triangle point toward the sun. Now imagine in the next instant that the object goes to a fourth point out of the plane formed by the first 3. Such a shift in direction requires a force to produce it, but in the model there is only gravity, so this is impossible meaning that all points of the orbit lie in a plane containing the sun considered as a point mass.

You are weightless when falling, even though you are responding to the gravitational force (paradoxic but true). An astronaut in a space capsule is weightless because he or she is falling but the conservation of angular momentum keeps them going around the sun.

Well if the sun can be considered a point mass, so can the space capsule. Call the local coordinate of the point representing the capsule point mass x. The orbit of x around the sun is in the x — sun plane.

Next put two objects 1 foot above and below the x — sun plane.

object1

x ——————————sun point mass

object2

Objects 1 and 2 orbit in a plane containing the sun point mass as well. They do not orbit parallel to x (but very close to parallel).

What happens with the passage of time? The objects approach each other. To an astronaut inside the capsule it looks like a force (similar to gravity) is pushing them together. These are the tidal forces Callahan is talking about.

Essentially the tidal are produced by gravitational force of the sun even though everything in the capsule floats around feeling no gravity at all.

Consider a great circle on a sphere — a longitudinal circle on the earth will do. Two different longitudinal great circles will eventually meet each other. No force is producing this, but the curvature of the surface in which they are embedded.

I think that what appear to be tidal forces in general relativity are really due to the intrinsic curvature of spacetime. So gravity disappears into the curvature of spacetime produced by mass. I’ll have to go through the rest of the book to find out. But I think this is pretty close, and likely why Callahan put the above quote into the preface.

If you are studying on your own, a wonderful explanation of just what is going on under the algebraic sheets of the Taylor series is to be found pp. 255 –> of Fundamentals of Physics by R. Shankar. In addition to being clear, he’s funny. Example: Nowadays we worry about publish and perish, but in the old days it was publish *and* perish.