Louisville Process Theology Network

A Short History of Relativity, Page 2

Mar 16, 2009

By the beginning of the 20th Century, we had a dichotomy in Physics. The Relativity principle held for the mechanical laws developed by Galileo and Newton. As Galileo explained it, ?“Any two observers moving at a constant speed and direction with respect to one another will obtain the same results for all mechanical experiments.?”

Theoretically, Galileo?’s Relativity means there is no such thing as absolute motion or absolute rest. Movement and position are relative to the observer?’s frame of reference.

And, all frames of reference are equivalent.

But, research into electromagnetic phenomena by Michael Faraday, James Clerk Maxwell and others from approximately 1750 to 1900 indicated the existence of an ?“absolute frame of reference?”. This idea clearly contradicted Relativity.

It was reasoned that electromagnetism would work only in frames at rest relative to the ?“medium?” through which electromagnetic waves circulate. The physicists of the time called this theoretical medium ?“ether?”, meaning it to be a substance that fills all the space of universe; sort of like air fills our atmosphere on Earth.

For other kinds of waves, the medium is easily recognized. The medium is water for water waves. It?’s the air for sound waves. It?’s the Earth for earthquake waves. The disturbance of that medium causes the waves.

The media of water waves, sound waves, and earthquake waves can be examined from many frames of references. If your frame of reference for observing any of these media is a ship in uniform motion, an airplane in uniform or the shore, the laws of physics are the same.

Yet, there was something different about electromagnetic waves.

In a ?“brilliant set of experiments," Faraday had discovered that the relationship between electricity and magnetism is completely reciprocal. A moving or changing electric field generates a magnetic field, and a moving and changing magnetic field generates an electric field. This discovery became the basis for the invention electric power generators.

Faraday?’s concept of a ?“field?” was only theoretical at this point, however. Maxwell would refine and extend this concept with mathematical equations around 1862.

Maxwell discovered a pattern right angle exchanges in transfers of energy and momentum among fields which flowed continuously. Light was already known to move in waves. This led Maxwell to consider whether light itself could be some arrangement of electromagnetic fields. The mathematical equations he developed out these studies predicted the existence of electromagnetic waves moving through the universe at the speed of light (speed c).

Maxwell could not tie down these movements to single set of equations. His equations were ?“not invariant?” in the words of one introductory textbook. Maxwell equations were different in different frames of reference.

So it appeared that all frames of reference are not equivalent for observing the speed of light. The obvious question for physicists was ?“with respect to what frame of reference, does light move at speed c??”

Maybe there was only one frame of reference for observing light waves and either; an absolute frame of reference. If so, there should be a relative uniform motion between frames of reference relative to the absolute. This would mean that Earth moves with respect to the either. Does it?

By 1887, Albert A. Michelson had constructed a highly sensitive ?“interferometer?” to measure shifts in light wave band spacing. This instrument was said to be so sensitive that it detected vibrations from the steps of a lab assistant stamping his feet outside the laboratory.

But, no shift was detected when the experiments were conducted. So apparently, Earth does not move with respect to the either.

Does this mean that Earth is the absolute frame of reference for observing light waves and either? If so, our planet would be the center of the universe. That idea had been ruled out by Copernicus hundreds of years ago.

Some physicists, most notably Henri Poincare, thought Michelson?’s experiment had probably ruled the existence of an absolute frame of reference. A new principle of relativity needed to be discovered.

Also, bear in mind that the existence of ?“either?” is only hypothetical.

Albert Einstein?’s insight was to see that motion of light waves was not the problem. The problem was the variables. This defied common sense because the variables Einstein wanted to re-examine were ?“space?” and ?“time.?” Our practical experiences tell us that space and time are absolute. Einstein imagined that both depend of the frame of reference of the observer.

Following Galileo?’s principle of Relativity, Einstein reasoned that ?“The velocity of light is same for all observers, in all directions, regardless of the motion of either the observer of the light source.?”

And, remember that all frames of reference are equivalent.

Einstein worked much of the theories based on his famous ?“thought experiments?”. One of his favorites was the thought experiment he named the Elastic Train.

Imagine train moving between two poles and there are three observers; a conductor at the front, another conductor at the rear, and signalman standing on the ground in between the two poles. A light flashes in the middle the train.

At that very moment, each of the conductors visually compares the length of the train to the distance between the two poles. It appears to be very long compared to the space between the poles from their perspective. But, the train appears shorter to signalman.

And, all three observers think they have seen the light at the same time. They haven?’t, because the light wave reached each of them at different times. The differences in these times were so small as to be imperceptible.

Einstein says all three frames of reference are valid because the laws of physics worked the same for each observer. Their observations are different because the variables for time and distance are different

Einstein also reasoned that observers separated by the vast distances of our universe cannot possibly observe what is happening to each other. An event that occurred light-years ago can appear to be happening now by the time the news of that event reaches you. Events that appear simultaneous to one observer may not actually be simultaneous for the other observer. We assume that everything we are seeing now is happening now. Not so.

The challenge is to estimate when a remote event happened by calculating how long it took the news to reach you at the speed of light.

A ?“transformation?” is needed to reconcile events that occur light-years apart; let?’s name it ?“y?”.

If a moving observer reports a time interval, then multiply by y to get your time interval in your time.

If a moving observer reports a length along the direction of motion, then multiply by y to get the length in your frame.

Time passes at the same rate for all observers. But, Einstein reasoned that there is a ?“time dilation?” effect between observers. One observer will see the time of another observer passing more slowly when measured by his own clock.

He was already familiar the Lorenz contraction of length and distances. This is named for Hendrick Lorenz who developed the trigonometric equation for the effect from his studies of light waves about 1892. Lorenz saw that distances in the direction of travel shrink for other frames of reference; even though length and distance do not change locally.

So, Einstein reasoned that space and time are relative to the speed the light. This reasoning brought time dilation and length contraction into Relativity.

What we thought was the variable turned out to be constant; what we thought were the constants turned out to be variables.

What happened to either? It turns out that sound waves and water waves need a medium, while light waves do not. Why? Light waves are made of electrical and magnetic fields. These kinds of waves are not supported by a particle medium such as air, water, or Earth.

And, light waves have the characteristics of both waves and particles.

Einstein?’s theory of 1907 is termed ?“Special Relativity?” because it relates to the special case ?– uniform motion. By 1915, he?’d worked out ?“General Relativity?”; which brought gravity and accelerating frames of reference into his theories.

Einstein?’s General Relativity was mostly famously verified by experiment in 1919 by astrophysicist Arthur Stanley Eddington. Observing a solar eclipse on May 29th, he saw that the light waves are curved by the effects of gravity. Einstein?’s theories have been verified by many more experiments since then.

As it was for Galileo, so it was for Einstein. The laws of physics are the same for all frames of references. This is what Relativity means in a nutshell.



Our reference -- We wrote this brief history of Relatively from the notes we jotted down while listening to the audio course, ?“Einstein?’s Relativity and the Quantum Revolution: Modern Physics for Non-Scientist,?” by Richard Wolfson. We checked it out of the public library. Did we get most of it right?