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Welcome to our Physics lesson on Symmetry of the Laws of Classical Physics. The Pseudo-Theory of Cosmic Ether, this is the fifth lesson of our suite of physics lessons covering the topic of Classical Principle of Relativity, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
As discussed in tutorial 16.7 "Faraday's Law of Induction", when we move a magnet towards to or away from a coil, a current is induced in the coil. The same phenomenon is observed even if the coil itself is moved towards to or away from a fixed magnet (any change in magnetic flux produces an induced emf and current in the coil, no matter how this change in flux is obtained). We can call the system connected to the coil at rest as S and that connected to the coil in motion as S'. If the speed of coil relative to the magnet is equal in these systems, both of them produce the same amount of induced current.
From this example, it is clear that we can extend the relativistic reasoning used in mechanical-related phenomena in electromagnetic-related ones as well. The same reasoning can be used in many physical phenomenon that are not directly related to classical mechanics or electromagnetism. The first scientist who understood this symmetry of inertial systems, was Henri Poincare, who extended this approach for all physical-related phenomena, not only for the mechanical and electromagnetic ones. For this reason, Poincare is often considered as the true founder of Relativity.
However, some questions which seemed insurmountable at the first glance, did appear by the development of this theory. They consisted on the light propagation in vacuum, which albeit at very high speed, is a process that occurs at a finite speed (about 300 000 km/s). In the classical physics, it was inconceivable a wave process (such as the light propagation) can occur in vacuum. Scientists believed at that time that a material medium is necessary for the propagation of light waves, just like occurs in other types of waves (mechanical waves). They tried to explain this drawback by "inventing" the concept of "cosmic ether" - a medium extending at infinity, odorless, massless, transparent and non-observable in normal conditions. In this way, they believed that all objects flow through this cosmic ether without any mechanical resistance. It was supposed to be there only for allowing the conduction of EM waves from one place into another. Furthermore, the fictional cosmic ether was supposed to penetrate all objects. The two constants ε0 and μ0 were thought to be the electric and magnetic constants that characterize the cosmic ether while ε and μ the corresponding electric and magnetic constants characterizing the space inside material objects.
In the Newtonian system, the supposed cosmic ether was though as the absolute system of reference and all systems moving at constant velocity in the universe, were thought as inertial systems of reference inside this infinite absolute system. Thus, the speed of light in vacuum (about 300 000 km/s) was considered as the speed of light relative to the cosmic ether. The explanation give was that light speed has the same value in all directions because the cosmic ether is homogenous everywhere, so the concept of isotropy [uniformity in all directions] of light propagation was a common belief in the scientific circles.
However, in the above reasoning there is a nonsense. Let's suppose an inertial system moving at velocity V relative to the absolute system at rest represented by the cosmic ether. If we emit a light ray in the direction of V, the relative speed of light would be
When light is emitted in the opposite direction of V, the relative velocity of light would be
In this case, we would obtain two different values of light speed in the same medium. This would violate the principle of isotropy of cosmic ether, i.e. there is a paradox in this reasoning. Therefore, it remains that there is no cosmic ether in the universe, so the above theory is wrong. The correct explanation of relativistic phenomena was first provided by Einstein during the first decade of the last century; a theory we will discuss in the next tutorial.
You have reached the end of Physics lesson 18.2.5 Symmetry of the Laws of Classical Physics. The Pseudo-Theory of Cosmic Ether. There are 5 lessons in this physics tutorial covering Classical Principle of Relativity, you can access all the lessons from this tutorial below.
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