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The old Greek philosophy, which in Europe in the later middle ages was synonymous with the works of Aristotle, considered motion as a thing for which a cause must be found: a velocity required a force to produce and to maintain it. The great discovery of Galileo was that not velocity, but acceleration requires a force.

— Willem de Sitter The Astronomical Aspect of the Theory of Relativity (1933)

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More quotes by Willem de Sitter

All systems are receding, not from any particular centre, but from each other: the whole system of galactic systems is expanding.

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Is the density anywhere near that corresponding to the static universe, or is it so small that we can consider the empty universe as a good approximation?

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In Newton's system of mechanics... there is an absolute space and an absolute time. In Einstein's theory time and space are interwoven, and the way in which they are interwoven depends on the observer. Instead of three plus one we have four dimensions.

In Einstein's general theory of relativity the identity of these two coefficients, the gravitational and the inertial mass, is no longer a miracle, but a necessity, because gravitation and inertia are identical.

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How does it come about that we have been able to find satisfactory hypotheses to explain electricity and magnetism, light and heat, in short all other physical phenomena, but have been unsuccessful in the case of gravitation?

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