Preferred frame

In theoretical physics, a preferred or privileged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different from those in other frames.

In theories that apply the principle of relativity to inertial motion, physics is the same in all inertial frames and no single inertial frame is privileged or preferred above any other.

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Preferred frame in aether theory

In theories that presume that light travels at a fixed speed relative to an unmodifiable aetherial medium, the "preferred frame" would be the frame in which this "aether" would be stationary. In 1887, Michelson and Morley tried to identify the state of motion of this hypothetical "preferred frame". To do so, they assumed Galilean Relativity to be satisfied by clocks and rulers; that is, that the length of rulers and periods of clocks are invariant under any Galilean frame change. Under such an hypothesis, a preferred frame should have been observed.

By comparing measurements made in different directions and looking for an effect due to the Earth's orbital speed, their experiment famously produced a null result. This experiment disproved the Galilean invariance hypothesis for ruler lengths and clock periods, and confirmed the universal validity of the principle of relativity of motion (the so-called Lorentz invariance). Thus, all inertial frames are physically equivalent. More precisely, provided that no phenomenon violates the principle of relativity of motion, there is no means to measure the velocity of an inertial observer with regard to a possible medium of propagation of quantum waves.

The Aether (ether) in 19th Century Physics

There arose a theory in 19th-century physics that there existed a hypothetical medium named the "Aether" (or ether). Some physicists suggested that the universe was filled with this substance known as "aether" which transmitted electromagnetic waves. It was also postulated that light arose from vibrations of the aether. Aether constituted an absolute reference frame against which speeds could be measured. In other words, the aether was the only fixed or motionless thing in the universe.

This theory arose out of what had seemed necessary to physics for over 2000 years since the Greek philosopher Aristotle (384-322 B.C.) said that the earth was immovable and a fixture from which all other points could be measured. This theory recorded later, by Ptolemy, remained intact in the earth-centered models of the universe until it was overthrown by the theory of Copernicus. Yet, in the Copernican system, though the Sun was now the center of the universe, the Sun itself took the place of the earth as the fixture upon which all measurements could be made. However, things changed in 1785 when William Herschel began to describe the position of the Sun within the Milky Way galaxy. Now the Sun was no longer a fixture in space and it appeared that there was no center to the universe. This was an unreconciliable idea to 19th century physicists who invented the concept of the aether to create a motionless fixture in the universe from which all measurements could be made. However, since electromagnetic waves passed through the aether and planets moved through the aether, the aether was postulated to have some wonderful properties i.e. it was sufficiently elastic that it could support electromagnetic waves, those waves could interact with matter, yet it offered no resistance to bodies passing through it. In fact, it was said that light could not pass from the Sun to the earth without the aether because it needed a medium to carry it just as sound waves travel in a medium. In trying to prove the existence of the Aether, 19th century physicists in their Michelson-Morley experiment accidentally disproved Galilean relativity assumption of rulers lengthes and clocks periods invariance and confirmed the universal validity of the principle of relativity of motion. This experiment stressed the impossibility of measuring the velocity of observers with regard to a possible aether thanks to any means complying with the principle of relativity of motion. It paved the way for Einstein's Special Theory of Relativity whose mathematical formulation relies on the principle of relativity of motion and doesn't require aether hypothesis.

Inertial frames preferred above noninertial frames

Although there is no preferred inertial frame under Newtonian mechanics or special relativity, the set of all inertial frames as a group may still be said to be "preferred" over noninertial frames in these theories, since the laws of physics derived for inertial motion only work exactly in this special category of frames.

No preferred frames

In theories that comply with Mach's principle, even the preference for inertial frames is removed: observations made by observers in nominally "inertial" and "noninertial" frames may seem superficially biased in favour of the inertial observer, but ultimately, a full description of the physics becomes similarly complex in either case.

References

• Einstein: Relativity, the special and the general theories (1954)