
Originally Posted by
brianp
Farsight seems to believe that Einstein did not accept curved spacetime. However, the analogy Einstein used in 1922 in conversation with his son suggests otherwise.
In 1922, 12 year-old Eduard Einstein asked his father why he was famous. Einstein replied: "When a blind beetle crawls over the surface of a curved branch, it doesn't notice that the track it has covered is indeed curved. I was lucky enough to notice what the beetle didn't notice."
I'm pleased that somebody is offering evidence in an attempt to counter the evidence I offer. But the "suggestion" is wishful thinking, your evidence isn't adequate. Einstein talked a great deal about curvilinear motion, and the above is more of the same. What's important is that the "modern interpretation" of general relativity ascribes the cause of this curvilinear motion to curved spacetime, whilst Einstein did not. Curved spacetime is the effect. Paraphrasing, Einstein said a photon travels in a curved path through space over time because the matter/energy of a planet has "conditioned" the local space it travels through, creating a gradient in the speed of light which causes the photon to veer. This is usually dismissed without examination of the original material, see post 22. For further reading I again refer you to Pete Brown's paper Einstein's gravitational field. It's well worth reading. Here's an excerpt:
It is widely assumed that, according to Einstein’s general theory of relativity, gravitation is a curvature in space-time. There is a well accepted definition of
space-time curvature. As stated by Thorne:
space-time curvature and tidal gravity are the same thing expressed in different languages, the former in the language of relativity, the later in the language of Newtonian gravity.
However one of the main tenets of general relativity is the Principle of Equivalence: A uniform gravitational field is equivalent to a uniformly accelerating frame of reference. This implies that one can create a uniform gravitational field simply by changing one’s frame of reference from an inertial frame of reference to an accelerating frame, which is rather difficult idea to accept. A uniform gravitational field has, by definition, no tidal forces and thus no space-time curvature. Thus according to the interpretation of gravity as a curvature in spacetime a uniform gravitational field becomes a contradiction in terms (i.e. no tidal forces where there are tidal forces). This apparent contradiction is obviously quite confusing and can certainly be misleading. In A brief history of relativity published in the above mentioned issue of Time, Stephen Hawking writes:
I still get two or three letters a week telling me Einstein was wrong. Nevertheless, the theory of relativity is now completely accepted by the scientific community, and its predictions have been verified in countless applications. [...] His idea was that mass and energy would warp spacetime in some manner ... Objects like apples or planets would try to move on straight lines through space-time, but their paths would appear bent by a gravitational field because space-time is curved.
Given such a statement by a respected physicists and with a great deal of experimental data to back up this claim is it reasonable to question this notion of gravity being a curvature in space-time? Is it reasonable to assume that the bent path an object takes when moving through a gravitational field is due to the of space-time curvature? Did Einstein actually hold the view that gravity is a curvature in space-time? At this point let us whet your appetite. No. Einstein never said nor implied in anyway that gravity is a curvature in space-time. This apparent disparity is a result of a change of interpretation. However it would be a great injustice to imply that there is no relationship between gravity and spacetime curvature. Curvature plays a very important role in general relativity and its importance should not be underestimated. However the aforementioned change in interpretation is most likely the source of various errors in the scientific literature.
The following is, in part, an historical journey through the origins of general relativity. Our purpose in doing so is to aid in our attempt to certain problematic areas. The following discussion will be address two different interpretations of general relativity. Therefore, for reasons of clarity, Einstein’s vision of general relativity, as defined in The Foundation of the General Theory of Relativity and other publications, will be referred to as EGR (Einstein’s General Relativity). General relativity as to it is widely understood at present will be referred to as MGR (Modern General Relativity).
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