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Aether and general relativity
A spacetime that conforms to Mach's principle can be considered to display certain physical characteristics that might otherwise be considered to be properties of a "physical" light-medium, or "aether".
Einstein, 1920:
"Mach's idea finds its full development in the ether of the general theory of relativity. ..."
Aether and general relativity - The gravitational field considered as a medium
Under general relativity, the gravitational field parameters are supposed not just to 'modify' underlying pre-existing distances and times, but to define them, completely. Separation between objects is then not an independent mathematical property as it is with special relativity, the "quantity of space" between two objects is instead entirely defined by the properties of the region's gravitational field.
Einstein, 1952:
"... Physical objects are not in space, but these objects are spatially extended. In this way the concept "empty space" loses its meaning. ... On the basis of the general theory of relativity ... , space as opposed to "what fills space", which is dependent on the co-ordinates, has no separate existence. Thus a pure gravitational field might have been described in terms of the gik (as functions of the co-ordinates), by solution of the gravitational equations. If we imagine the gravitational field, i.e. the functions gik, to be removed, there does not remain a space of the type (1), but absolutely nothing, and also no "topological space". For the functions gik describe not only the field, but at the same time also the topological and metrical structural properties of the manifold. A space of the type (1), judged from the standpoint of the general theory of relativity, is not a space without field, but a special case of the gik field, for which -- for the co-ordinate system used, which in itself has no objective significance -- the functions gik have values that do not depend on the co-ordinates. There is no such thing as an empty space, i.e. a space without field. Space-time does not claim existence on its own, but only as a structural quality of the field."
Under general theories of relativity, the gravitational field typically "is" space and time, and technically counts as a "medium" ("that which is between") separating distant objects.
Aether and general relativity - Einstein on GR's aether
Einstein, 1920:
"... we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."
Many modern physicists would tend to consider Einstein's concept of a "non-particulate aether" to be stretching the meaning of the word "aether" past reasonable limits, but the idea does have historical precedent, for instance, in Optiks Query 21, Isaac Newton pondered whether the aether in his new model ought to be considered to be particulate or not, and did not reach a decision "...for I do not know what this Aether is".
Aether and general relativity - Physical properties of the GR aether
Einstein's "aether of general relativity" can be visualised as having the following properties:
- The medium is denser where the gravitational field is stronger ,
causing light-signals to take longer to traverse a region of higher gravity (Shapiro effect) and causing light and matter to be deflected towards the region of nominally-slowest lightspeed (conventional gravitation, gravitational lensing).
- Accelerated motion of a body through the medium
is resisted, and causes a disturbance that pulls along nearby matter and light (frame-dragging).
- Oscillatory motion of a body (repeated acceleration and deceleration)
throws wavelike disturbances into the medium that remove energy from the oscillating system and damp the motion (gravitational waves)
- "Smooth" rotational motion in the medium
creates a surrounding disturbance that pulls around nearby matter and light (frame-dragging)
- Rotation of an asymmetrical or irregular body (eg a double star system)
throws off wavelike disturbances into the medium, that remove energy from the rotating body and slow its rotation (gravitational waves)
- Constant-velocity motion through a perfectly-smooth (i.e. homogenous) gravitational environment
is not resisted (Newtonian relativity, special relativity).
- Constant-velocity motion through an irregular background
tends to "dampen" motion. Collision with a gravitational "feature" can involve momentum transfer (see: slingshot effect).
Although it may support an equivalent relative velocity between regions (eg terminal velocity due to a gravitational gradient, frame-dragging), it does not support absolute velocities.
Aether and general relativity - Utility of the GR aether concept
"GR aether" is a useful idea ...
... in that it allows a person with no mathematical skills to visualise most of the advanced phenomenology of general relativity, such as frame-dragging, in an intuitive way.
"GR aether" is not a useful idea ...
... in that it cannot be relied upon to safely generate quantitative predictions for general relativity. Physicists need to carry out exact calculations, and the particular form of GR's equations used is typically chosen according to very technical arguments. To a physicist working with GR, it is this mathematical and geometrical form that defines the theory (not considerations about how a hypohetical aether ought to behave), and the subsequent characterisation of general relativity as "an aether theory" is not likely to be considered to add anything new or useful to our knowledge of the theory.
Aether and general relativity - Validity of the GR aether description
Why GR looks like an aether theory.
Problems involving physical fluids or other physical media usually involve properties that are passed on or transmitted from particle to particle, producing smooth graduations in the value's property (e.g. heat transmission, velocity, momentum) and statistical modeling usually allows these to be dealt with using a field theory approach. Geometrical theories of gravitation and inertia are also normally constrained by field theory considerations, and consequently will often produce similar physical descriptions. For instance, in aether models, the rotational characteristics of a body are usually "smudged out" into the surrounding region by the transmission of forces between particles in the medium, while under GR, the rotation of the body influences the surrounding region because the properties of the rotating mass are imprinted on the body's external gravitational field. Both descriptions look superficially similar.
Why GR does not look like an aether theory.
Field behaviour usually discourages the appearance of abrupt "features" in a parameter's field, so aether models will, for instance, usually not have sharply-defined signal horizons (see: Visser,gr-qc/9712010, classical Hawking radiation). Although a general theory of relativity might be expected to be a "pure" field theory of inertia (i.e., a pure Machian theory), Einstein's general theory inherits some aspects of special relativity, which assumes that inertia is not a field effect. Consequently, general relativity's predictions about black hole event horizons are not what one would normally expect from an aether model.
Other related archivesIsaac Newton, Mach's principle, Machian theory, Shapiro effect, black hole, classical Hawking radiation, conventional gravitation, event horizons, field theory, frame-dragging, gravitational lensing, gravitational waves, homogenous, phenomenology, quantitative, slingshot effect, special relativity
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