| Newton advanced his so-called "bucket argument" in support of the
belief that space is a substance existing independently of the matter it contains. He thought of space as a container - of which we can have no direct experience - holding
the objects of physics (matter and energy). The contrasting view, held notably by Leibniz, saw space simply as the system of relations between
those objects: no container, no unknowable metaphysical substance, just a
part of the experience we have of separated and extended objects.
The bucket argument is a thought experiment describing a
universe containing nothing but a pail of
water suspended from a rope. (We assume there is a
force comparable to gravity, but no mass below the bucket; the rope is likewise attached to a
tree that isn't there, keeping the bucket from falling.) Now suppose the bucket and the
water inside it are both at rest: the surface of the water will be level. If, however, the bucket and water are both spinning at
the same speed in the same direction, the water's surface will become concave. The problem for the relationalistic idea of space
arises from the fact that in both cases, the water and bucket are at rest with respect to one another. Since there is
nothing else in the universe (ignoring the rope, since we're good sports), there is nothing else with respect to which the water
is spinning; therefore, since the water is behaving as though it is spinning, there must be something absolute with respect to
which it is spinning: absolute space.
Einstein introduced the following version of this thought experiment. Two objects, consisting of liquid, are floating in space.
The liquid objects are contracted into spherical shape under under the influence of their gravity. The distance between them does
not decrease, indicating that the two are orbiting around a common center of mass. They are so far away from each other that
tidal effects are negligable. In this situation, interaction between just the
two objects suffices to explain the observation that the distance between them does not decrease. However, the two objects are
also observed to have a dissimilar angular velocity. Even when the situation is as symmetrical as possible, the two axes of
rotation of the two liquid objects, pointing straight to the other object's center of mass, an asymmetry remains: one is
perfectly spherical in shape, and the other is bulging at the equator (or they have dissimilar bulging at the equator).
This asymmetry cannot be explained as the result of an interaction between the two objects. There must be yet another
interaction involved. This other interaction then determines how much the liquid object will be distorted into a ellipsoidial
shape.
In general these thought experiments address the question of the origin of inertia. How does an object that is being accelerated
"know" that it is being accelerated? What is the messenger of that information? Presumably Newton decided that it would be absurd
to assume an action-at-a-distance between matter in one place, and distant matter in the universe, informing local matter what
the center of mass of the universe is. Presumably Newton decided that assuming absolute space was unavoidable. Newtonian absolute
space is telling matter in what way it is moving. Einstein did not follow Newton's decision to assume absolute space.
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