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Originally Posted by jj_0001
So, a "radical departure" is what the original theorizer would have considered radical?
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That seems as reasonable a definition as any in this context.
The original quote you were responding to was this:
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Originally Posted by hhEb09'1
In a philosphical sense, as many have observed, subsequent theories are often radical departures.
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In the actual predictions of the theories, relativity produces almost exactly the same results as Newtonian mechanics, as long as we limit ourselves to fairly low relative velocities and small gravitational fields. However, in their views of the nature of space and time, relativity is completely different from Newtonian mechanics.
As for speaking for Newton, I try to make certain to only assume what someone might have been thinking when it's reasonably clear from writings. In Newton's case, he makes it quite evident in the introduction to the
Principia, as well as from documented arguments with many of his contemporaries (some of who thought that space and time were relative, although they probably would have also been shocked at just
how relative they are in Einstein's theory), that he felt strongly that absolute space and absolute time exist independently of any measurement thereof, and without reference to anything external. He further felt that it is perfectly meaningful to speak of the absolute motion of some object, and that this is quite distinct from motion relative to some other object. These are foundational principles of the
Principia, and Newton felt that they were important enough to devote a section at the beginning to discussing them in detail. Einstein's relativity says that any thought that there might be an absolute time or space that's independent of an observer is mistaken, and that there is simply no such thing as motion in any absolute sense. Relativity is derived from postulates which include the negation of Newton's underlying assumptions.