Lots of folks think that Einstein’s theory of general relativity claims that gravity is due to curvature in fourth dimension. As an example, here’s a recent question uploaded on Quora: “”Einstein tells us that gravity is motion in curved space-time, so why do scientists still refer to it a force?”.
Forgive me for yelling, but this one genuinely makes me crazy. EINSTEIN DIDN’T SAY THAT! In his theory of general relativity, developed in 1915, gravity is a force field, not much different from the electromagnetic (EM) field. It is CERTAINLY NOT four-dimensional curvature. But first a bit of history.
The field idea was presented into physics in 1845 by Michael Faraday through his studies of electric and magnetic phenomena. When James Maxell provided formulas for Faraday’s field in 1864, the field view of EM forces was generally accepted. But Isaac Newton’s theory of gravity, which included “action at a distance”, stayed the same. Newton’s theory was extremely successful, and is still taught in elementary physics classes today, but Newton was not satisfied with the concept of “action-at-a-distance”, stating “That one body may act upon another at a distance, through a vacuum, without the mediation of anything else … is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it.”.
This altered, obviously, when Einstein offered his principle of general relativity in 1915. In Einstein’s words: “As a result of the more careful study of electromagnetic phenomena, we have come to regard action at a distance as a process impossible without the intervention of some intermediary medium … The effects of gravitation also are regarded in an analogous manner … The action of the earth on the stone takes place indirectly. The earth produces in its surroundings a gravitational field, which acts on the stone and produces its motion of fall … [T] he intensity and direction of the field at points farther removed from the body are thence determined by the law which governs the properties in space of the gravitational fields themselves.”.
Keep in mind that Einstein mentioned not a thing about “curvature”. By creating formulas for the gravitational field, as Maxwell had performed for EM forces, Einstein answered Newton’s complaint about action-at-a-distance and brought the gravitational field into physics on a par with the EM field.
For people who have no idea just what a field is, I deliver this basic definition: A field is a property of space. There is no such thing as vacant space. You can’t have space without fields. These fields follow laws (i.e., equations) that define how a change at one point impacts the field at adjacent points, and additionally how one field affects other fields….
Find out more on the blog at Fields of Color…!