A lot of people misinterpret that the Michelson Morley experiment represents strong evidence in favor of Einstein’s Theory of Relativity. It is true that the Michelson Morley experiment agrees with relativity theory, but it in no way represents strong evidence for the theory. There are several theories which attempt to interpret with respect to what light travels when it travels through empty space. The results of the Michelson Morley experiment only disagree with one of these theories-the stationary ether theory. Therefore, the Michelson Morley experiment has much more to do with disqualifying the stationary ether theory than it does with confirming Einstein’s.

The reason why the Michelson Morley experiment is so popularized probably has more to do with the fact that it was this experiment which caused Lorentz and Fitzgerald (who believed in the stationary ether theory) to propose their "hypothesis of contraction" (i.e. to rescue the stationary ether theory from the results of the MM experiment). This "hypothesis of contraction" is equivalent mathematically to Einstein’s transformations. It therefore serves, not only as an indication that the stationary ether theory was wrong, but also as an introduction to the origin of the mathematical transformation equations that Einstein uses in his theory of relativity. This is probably why the MM experiment receives so much attention in literature about relativity theory.

This paper is about the isotropic (same in all directions) velocity of light when it travels through empty space. With respect to what does light travel isotropic to, when it travels through empty space? Einstein’s theory of relativity assumes that it always travels isotropic to the observer. The ballistic theory assumes that it travels isotropic to some source of the light. The ether theory assumes that it travels isotropic to some ether.

There are different variations of the ballistic theory depending on when the "baton" is allowed to be passed from material to material. I will mention three kinds of ballistic theories in this paper. There are two primary variations of the ether theory. There is the stationary ether theory and there is the entrained ether theory. The entrained ether theory assumes that the ether is dragged by matter but the stationary ether theory assumes that the ether is not dragged by any matter.

These are the theories. But only experiment can decide which theories are wrong and which theory is right. There are two types of experiments. There are terrestrial experiments and there are stellar experiments. The terrestrial experiments attempt to measure the isotropy of light when it travels through empty space in the atmosphere here on earth. The stellar experiments attempt to measure the isotropy of light when it travels through the empty space in outer space before it reaches the atmosphere here on earth.

In this paper, we will consider the most famous experiments of each type. The most famous terrestrial experiment is the Michelson Morley. The Michelson Morley experiment showed that light, when it travels in our atmosphere, travels isotropic to the earth. The most famous stellar experiment is Bradley’s report of stellar aberration. This experiment showed that light, when it is emitted by a star, travels isotropic to the star.

But before we take a closer look at the theories and the experiments, let us take a quick look at which theories the experiments at least apparently discredit.

Stellar aberration apparently discredits Einstein’s Theory of Relativity. This is because stellar aberration suggests that light coming from stars travels isotropic to the stars and not to the observers (i.e. Bradley) on earth who are observing the phenomenon of stellar aberration. Einstein knows this. Einstein derives the aberration formula and the Doppler formula in section seven of his relativity paper of 1905. This author does not understand how it is that Einstein rescues his theory of relativity from this phenomenon. This derivation by Einstein will not be considered in this paper.

The Michelson Morley experiment apparently discredits the stationary ether theory. This is because the Michelson Morley experiment suggests that light traveling near earth travels isotropic to the earth. The stationary ether theory, however, thinks that the light, even when it travels near earth, should travel more isotropic to the sun and stars than the earth. Lorentz and Fitzgerald, who believed in the stationary ether theory, admit that the results of the MM experiment do indeed represent evidence against the stationary ether theory. However, they like the theory so much that they try to rescue it. To rescue their theory, they propose that materials moving through the ether get squeezed (this is called the "hypothesis of contraction"). We will consider this "hypothesis of contraction" in this paper.

The Doppler shift, double star phenomenon, and gravitational lensing (i.e. light bending around large masses) apparently discredit all ballistic theories. This is because, according to the ballistic theories, the light should always travel isotropic to the source. But in these cases, the light does not.

To my knowledge, there is no theory which, in and of itself, can explain all experiments regarding the isotropic velocity of light through empty space. This is because there seem to be several factors which dictate the motion of the light. The ballistic and ether theories are really just simple models, which may explain some experiments, but cannot be taken as strictly or universally correct.

It is the opinion of this author that the molecule to molecule change of hands model is the best model we have out there. But even this model cannot explain all phenomenon. However, the phenomenon which this model cannot explain (such as the Doppler effect, double star light, or gravitational lensing) can be explained by other reasonable means.

The ballistic theories assume that light travels isotropic to some source of the light. There are different variations of the ballistic theory depending on the extent to which the "source" is allowed to "change hands". By "change hands" or "pass the torch", I mean that the reference for isotropy has been transferred.

Let us consider visible light. All visible light can be eventually traced to an original light source. An original light source is "disturbed" in some way. "Disturbed" implies violent motion between the molecules and/or charges within the source. The sun, a light bulb, and lightning are all original sources of light. The original source is where the light is initially created. It is where the light originates.

During the journey of a light pulse, it may encounter macroscopic pieces of matter which reflect or refract the light. A mirror, prism, or the surface of water are such reflectors or refractors.

During the journey of the light pulse, it will encounter microscopic pieces of matter which absorb and emit the light. Molecules and/or charges are such microscopic pieces of matter.

An extreme ballistic theory might propose that the source can never "change hands". In other words, an extreme ballistic theory might say that light must always travel through the empty space throughout its journey with respect to the original source from which it was created. This ballistic theory is not very reasonable.

A less extreme ballistic theory might propose that the source can pass the torch, but only at the macroscopic pieces of matter which reflect or refract the light.

The least extreme ballistic theory (and the one which seems most reasonable to me) assumes that the torch is passed at every molecule which the light encounters. We now know that a finite time elapses for the process of "scattering" in molecules to occur. In other words, we now know that there is a finite time between when a molecule "absorbs" and then "emits" incident light. Therefore, it seems reasonable (to me, at least) that the molecule, itself, has some influence over the isotropic velocity of light which it propels.

The ether theories assume that light travels isotropic to some ether. "Ether" is a word used to describe some intangible or as yet undetected electromagnetic field resisting medium which permeates all so-called empty space. This ether could be a swarm of little bits of material floating around that we have not yet detected. This ether could be completely vacant of any material matter. This ether could be a sea of photons. This ether could be anything as long as it influences the motion of light while it is traveling through empty space.

There are two primary variations of the ether theory. There is the stationary ether theory and there is the entrained ether theory. The entrained ether theory presumes that the ether is dragged by materials which move through it, in much the same way that a physical gas or fluid is dragged by solids moving through it. This theory, therefore, presumes that the ether in the earth’s atmosphere is being dragged by the earth.

The stationary ether theory, by contrast, presumes no such dragging. Therefore, the stationary ether theory presumes that the earth and all other materials in the universe move through the ether without influencing the motion of the ether. This theory presumes that the ether is like one big immovable medium throughout the entire universe. It is homogeneously dense. It is still and rigid. But the question remains, what defines the motion of this absolute stationary ether?

It does not seem at all logical to assume that this ether, throughout the entire universe, is at rest to (i.e. moves with) the tides in the Atlantic Ocean. Why? Because the tides of the Atlantic Ocean are circumstantial. It seems more likely that the ether would be at rest to the earth. But the motion of the earth, too, is circumstantial. It seems more likely that the ether would be at rest to the sun. But the motion of the sun, also, is circumstantial. It seems more likely that ether would be at rest to the fixed stars in the sky. But why stop there? We can go on and on until we have scoped out to look at the entire universe (assume the universe is finite) as a whole. Let us say that we can do this and that the universe looks like a basketball. We would then say that a good estimation for the motion of this ether would be the motion of the basketball.

In theory, the above is what we would do. But in practice, we cannot do this. Fortunately, the validity of this theory can be tested without having to scope out of the entire universe. Rather, all we need to know is that the motion of the sun more closely coincides with the motion of this ether than does the motion of the earth. The Michelson Morley experiment tells us whether the isotropic velocity of light (here on earth) can be better referenced by the motion of the earth or by the motion of the sun. (It confirmed the former, in contrast with the stationary ether theory).

A common misconception about the ballistic and the ether theories, which began in the end of the 19^th century and continues to this day, is that linked with the ballistic and ether proposals are descriptions of or commitments to the character (i.e. wave or corpuscle) light. It is often misinterpreted that the ballistic theory presumes a corpuscular character of light and that the ether theory presumes a wave character of light.

On the contrary, neither theory makes any claims about what the character of light is. Rather, these theories are purely geometrical. They only offer proposals for what it is that light travels isotropic to when it travels through empty space.

The reason for this misconception stems from an analogy through which these theories are described. What should be remembered is that this is only an analogy. It is only a familiar example used to illustrate the content of these theories.

The analogy proceeds as follows: We are not familiar with the phenomenon of waves propagating through empty space. But we are familiar with the phenomenon of waves propagating through material mediums (like sound waves and water waves) and we are familiar with the phenomenon of materials moving through empty space (like a grenade exploding in outer space). The ether theory finds analogy with the former while the ballistic theory finds analogy with the latter.

As previously mentioned, the ether could be a sea of photons. In other words, the ether theory does not dismiss the possibility that light travels in a corpuscular form when it travels through empty space. Conversely, the ballistic theory does not dismiss the possibility that light, emitted spherically with respect to the last propelling molecule, travels in the form of a wave.

The experiments which are used to determine what it is that light travels isotropic to, when it travels through empty space, can be subdivided into two different types of experiments. There are terrestrial experiments, on the one hand, and there are stellar experiments, on the other hand.

Terrestrial experiments attempt to measure the isotropy of light when it travels through the empty space between molecules in mediums (usually gases) here on earth. Stellar experiments attempt to measure the isotropy of light when it travels through the empty space in outer space before it reaches the earth’s atmosphere.

Stellar experiments usually attempt to measure the light isotropy coming from stars. This is very practical because there is a lot of distance between the earth and stars. This allows us to treat the star as a point source and the earth as a point receiver (i.e. this treatment cannot be made in terrestrial experiments). Therefore, our ability to measure the isotropy of light (when it travels through outer space from stars to the earth) depends entirely on the accuracy with which we can measure the direction our telescopes are pointed in.

As mentioned, stellar experiments attempt to measure the isotropy of light when it travels through outer space before it reaches the earth’s atmosphere. We now have the Hubble telescope which is a telescope that is a satellite orbiting above the earth’s atmosphere. This telescope has the advantage of not having to account for the effects of the atmosphere on the light which enters it and then travels through it before it reaches the surface of the earth where most telescopes are.

However, we can still measure the isotropy of light when it travels through outer space before it reaches the earth’s atmosphere from our telescopic readings here on earth. Light from stars will refract slightly when it hits the earth’s atmosphere if it hits the earth’s atmosphere at an angle. But any refraction can be accounted for because we know what the index of refraction of the earth’s air is. If the light enters the earth’s atmosphere perpendicularly, there will be no refraction.

Bradley, as early as 1726, made such a measurement of light isotropy when it travels through outer space before it reaches the earth’s atmosphere. Essentially, what Bradley found was that the light traveling from stars travels isotropic to the stars. Bradley was led to this finding by noting that he could only explain his telescopic readings, throughout the course of a year, by assuming that the actual location of a star when it is seen to be directly above the surface of the earth (assuming it is still burning), is shifted 20 seconds of arc opposite to the direction of motion of the earth with respect to the star. The phenomenon which Bradley reported is called Stellar Aberration.

Let us assume that light from a star directly above the earth travels isotropic to the star. In such a case, the velocity of the light with respect to a telescope on earth can be described in terms of two components. One component is the velocity of light with respect to the star. This is c. The other component is the velocity of the star with respect to the telescope on earth. This is 30 km/sec. If the star is directly above the earth, then the two components are perpendicular to one another. If the light travels isotropic to the star, then the star can only be seen if we point our telescope in the direction of motion of the earth with respect to the star. The sine of the angle we would have to point our telescope in, in such a case, is c divided by 30 km/sec. This angle is 20 sec of arc.

The example is given of a man running in the rain. We assume that the rain falls at c with respect to the cloud directly above. We assume that the man is running at 30 km/sec. The man has a hollow cylindrical piece of wood that he is looking through. The rain will only travel through the hollow tube and hit him in the eye as long as the man points it in the direction he is running with respect to the cloud. The angle that he has to point his hollow tube in is 20 sec of arc.

A telescope on the earth travels at about 30 km/sec with respect to the sun (i.e. 29 km/sec from revolution plus or minus 1 km/sec from rotation). The sun travels at about 2 km/sec with respect to the fixed (i.e. single, not double) stars in the sky. So we assume that Bradley’s telescope on earth travels at about 30 km/sec with respect to the stars that he is observing in the sky (plus or minus, say, 4 km/sec). Bradley was able to measure the direction his telescope was pointed in to within about 1 second of arc. As mentioned, Bradley could not explain his telescopic readings without assuming that a star (which is viewed to be directly above the surface of the earth) is actually shifted 20 seconds of arc opposite to the direction of motion of the earth with respect to the star.

The best way to measure the isotropy of light through empty space when it travels here on earth would be to send it through a chamber which is completely evacuated of molecules. That is, it would be best to send the light through an artificially created vacuum.

However, we do not need to do this as long as we can assume that the medium, through which the light travels is homogeneously dense. This assumption can be made as long as the temperature of the medium can be controlled. What we have to do is make sure that the temperature, at all points in the medium, is the same. In other words, we have to make sure that the temperature is homogeneous throughout the medium.

A somewhat primitive method used to measure the isotropy (or speed) of light on earth was by sending the light through a toothed wheel and/or rotating mirror apparatus. But the isotropy of light can be much more accurately measured by sending it through an interferometer.

An interferometer usually consists of a monochromatic light source, one half silvered mirror, two or more fully reflecting mirrors, and a detector screen. The light originates at the monochromatic light source. It is sent towards the half silvered mirror. At the half silvered mirror, the single ray of light splits into two rays. These two rays proceed to travel along paths which are perpendicular to one another.

These two rays then hit a fully reflecting mirror (or set of fully reflecting mirrors) where they go out along a path and then come back along the same path. When the two rays hit the half silvered mirror on their way back, they recombine into one ray. This ray then proceeds toward a detector screen. By examining the interference at the detector, the experimenters can figure out by how much the two rays are out of phase.

The interferometer can credit its accuracy to its ability to capitalize on the wave nature of light. The key component in the interferometer is the half silvered mirror, or beam splitter. To my knowledge, the half silvered mirror was invented only a couple of years before Michelson made use of it by inventing the interferometer in 1880.

Let us say that the two perpendicular paths that the two rays travel in are equal in length. In such a case, if it is assumed that the light travels at the same speed in each direction, then it is expected that the two rays will recombine in phase. However, if it is assumed that the light does not travel at the same speed in each direction, then it is expected that the two rays will usually combine out of phase. More specifically, if it is assumed that the light does not travel at the same speed in each direction, then it is expected that the two rays will recombine in phase only when one ray returns an integral number of wavelengths ahead of the other ray.

Michelson performed this experiment first in 1881, and then, with the help of Morley, in 1887. The interferometer was placed at rest on the surface of the earth. Each time Michelson found that the two rays recombined in phase (or approximately in phase) no matter which direction the apparatus was pointed in. This is called the null result.

All theories that we have previously mentioned except for the stationary ether theory assumed that the light would travel at the same speed in each direction. Therefore, all of these theories expected that the light would return in phase. Einstein’s theory of relativity assumed this because the observers (Michelson and Morley) were at rest to the earth and the interferometer. Therefore, all of these theories expected a null result and the null result agrees with these theories.

However, the stationary ether theory did not assume that the light would travel at the same speed in each direction. Therefore, the stationary ether theory did not expect the two light rays to always recombine in phase. In other words, the stationary ether theory did not expect a null result. But a null result was found. Therefore, the null result of the Michelson Morley experiment stood as evidence against the stationary ether theory.

b. Lorentz and Fitzgerald’s Attempt to Rescue the Stationary Ether Theory

Recall if you will that the stationary ether theory assumes that the sun serves as a better reference for light isotropy than does the earth. Therefore, the stationary ether advocates assume that the light traveling through the interferometer travels isotropic to the sun. They then compare what would be expected under this assumption with what would be expected under the assumption that the light travels isotropic to the earth. If the stationary ether theory is true, then the former assumption should more closely agree to the experimental data than the latter assumption.

We can consider how the stationary ether theory assumes that light will travel through the interferometer, for the simple case where one of the two paths is directed in the direction of motion of the earth around the sun and the other path is (consequentially) directed perpendicular to the direction of motion of the earth around the sun. The stationary ether theorists assume that the light will travel isotropic to the sun when it travels through the interferometer. Identically, the stationary ether theorists assume that ether is at rest to the sun. In such a case, the interferometer is traveling through the ether at 30 km/sec (plus or minus 1 km/sec from rotation). And we have aligned one of the paths of the interferometer to point in this direction.

Instead of thinking of two rays of light traveling through an ether, we can think of two swimmers swimming through a river. The river is traveling downstream at the velocity of 30 km/sec. The swimmers can swim at the speed of c in still water. The river is a distance of, say, 100 yards across. The swimmers are going to race one another. One swimmer has to swim upstream to a tree which is 100 yards upstream, turn around and then come back. The other swimmer has to swim across the stream to the other side, turn around and then come back. It is calculated that the swimmer swimming across the stream and back should win.

Lorentz, Fitzgerald, and Michelson were all advocates of the stationary ether theory. All of them agreed that the null result of the Michelson Morley experiment stood as evidence against the stationary ether theory. All of them were discouraged with the result of the Michelson Morley experiment because they really liked the stationary ether theory. Lorentz and Fitzgerald, independently of one another, in an effort to rescue the stationary ether theory from the results of the Michelson Morley experiment, made a proposal. This proposal was the "hypothesis of contraction".

This "hypothesis of contraction" proposes that materials contract in their direction of motion through the ether. The interferometer and the earth would be such materials. If the interferometer was shorter in this direction, then stationary ether theory would expect the two light pulses to always return in phase, and therefore agree with the null result of the Michelson Morley experiment.

Let us consider this "hypothesis of contraction" for our two swimmers. The stationary ether theory (without the "hypothesis of contraction") expects that the swimmer swimming across the river and back should win. But the "hypothesis of contraction" says that the river will contract in the upstream/downstream direction. Therefore, the swimmer swimming upstream and then back downstream does not have to quite travel 100 yards up and 100 yards back. Rather, since the river is contracted, he only has to swim, say, 90 yards up and 90 yards back. It is because this swimmer does not have to swim quite as far that the swimmers can now tie the race. The "hypothesis of contraction" was calculated so that the swimmers would precisely tie.