In the September "Einstein" issue of Scientific American, audiences are given the impression that gravity is caused by curvature of space-time. For instance, on the 1st page of that segment, we read "gravity ... is the by-product of a curving universe", on p. 43 we discover that "the Einstein tensor G describes how the geometry of space-time is warped and curved by massive objects", and on p. 56 there is a reference to "Albert Einstein's explanation of how gravity emerges from the bending of space and time".
In reality, many physicist today emphasize "curvature" as the definition for gravity. As Stephen Hawking penned in A Brief History of Time, "Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a consequence of the fact that space-time is not flat, as had been previously assumed: it is curved, or warped.".
The issue is, that's NOT what Einstein said. Einstein made it rather clear that gravity is a force like other forces, along with (obviously) specific distinctions. In the actual paper cited by Scientific American ("The foundation of the general theory of relativity", 1916) he wrote," [there is] a field of force, namely the gravitational field, which possesses the remarkable property of imparting the same acceleration to all bodies". The G tensor, said Einstein "describes the gravitational field." The term "gravitational field" or just "field" occurs 58 times in this article, while the word "curvature" doesn't turn up at all (except in relation to "curvature of a ray of light"). And Einstein is not the only physicist who believes that. For example Sean Carroll, a prominent physicist of today, wrote:.
Einstein's general relativity describes gravity in terms of a field that is defined at every point in space ... The world is really made out of fields ... deep down it's really fields ... The fields themselves aren't "made of" anything-- fields are what the world is made of ... Einstein's ... "metric tensor"... can be thought of as a collection of ten independent numbers at every point.-- Sean Carroll.
To suppress the field concept and emphasize "curvature" not only misstates Einstein's perspective; it likewise offers folks a false or deceptive understanding of basic relativity.
So where does "curvature" originated from? According to Einstein (in the cited paper), the gravitational field causes physical adjustments in the length of measuring rods (just like temperature can cause such changes) and it is these changes that produce a non-Euclidean metric of space. Actually, as Einstein indicated, these changes can take place even in a space which is without gravitational fields-- i.e., a rotating system. He then showed that this non-Euclidean geometry is mathematically equal to the geometry on a curved surface, which had been developed by Gauss and extended (mathematically) to any number of dimensions by Riemann. That this is a mathematical equivalence is clearly stated by Einstein in a later paper: "mathematicians long ago solved the formal problems to which we are led by the general postulate of relativity".
For the full article visit the blog at Fields of Color.
