page_title: Absolute rotation
section_title: Newton's bucket argument
hierarchical_section_title: Absolute rotation / Classical mechanics / Newton's bucket argument
caption_attribution_description: English: Parabolic shape formed by a liquid surface under rotation. Two liquids are held between two vertical plexiglass sheets, with a slit in between. This narrow container is rotated around the center axis. The denser colorless liquid is being moved outwards by the centrifugal force while the lighter red liquid remains in the center. Taken at the Little Shop of Physics at the Lory Student Center, Colorado State University, February 26, 2005.
context_page_description: In physics, the concept of absolute rotation—rotation independent of any external reference—is a topic of debate about relativity, cosmology, and the nature of physical laws.
  For the concept of absolute rotation to be scientifically meaningful, it must be measurable. In other words, can an observer distinguish between the rotation of an observed object and their own rotation? Newton suggested two experiments to resolve this problem. One is the effects of centrifugal force upon the shape of the surface of water rotating in a bucket, equivalent to the phenomenon of rotational gravity used in proposals for human spaceflight.
  The second is the effect of centrifugal force upon the tension in a string joining two spheres rotating about their center of mass.
context_section_description: Newton suggested the shape of the surface of the water indicates the presence or absence of absolute rotation relative to absolute space: rotating water has a curved surface, still water has a flat surface. Because rotating water has a concave surface, if the surface you see is concave, and the water does not seem to you to be rotating, then you are rotating with the water.
  Centrifugal force is needed to explain the concavity of the water in a co-rotating frame of reference (one that rotates with the water) because the water appears stationary in this frame, and so should have a flat surface. Thus, observers looking at the stationary water need the centrifugal force to explain why the water surface is concave and not flat. The centrifugal force pushes the water toward the sides of the bucket, where it piles up deeper and deeper, Pile-up is arrested when any further climb costs as much work against gravity as is the energy gained from the centrifugal force, which is greater  at larger radius.
  If you need a centrifugal force to explain what you see, then you are rotating. Newton's conclusion was that rotation is absolute.
  Other thinkers suggest that pure logic implies only relative rotation makes sense. For example, Bishop Berkeley and Ernst Mach (among others) suggested that it is relative rotation with respect to the fixed stars that matters, and rotation of the fixed stars relative to an object has the same effect as rotation of the object with respect to the fixed stars.  Newton's arguments do not settle this issue; his arguments may be viewed, however, as establishing centrifugal force as a basis for an operational definition of what we actually mean by absolute rotation.
caption: Figure 1: The interface of two immiscible liquids of different density (a denser colorless liquid and a lighter orange-colored liquid) rotating around a vertical axis is an upward-opening circular paraboloid.
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