Louisville Process Theology Network

A Short History of Relativity, Page 1

Mar 06, 2009

The concept of Relativity has long history; much longer than many of us realize. We assume this idea began with Einstein?’s theory of Special Relativity in 1905, when actually, it goes all the way back to Galileo around 1630.

Galileo?’s Relatively is usually explained this way by the introductory physics textbooks ?– The laws of physics are the same for every object in uniform motion. Galileo, himself, is said to have expressed it like this. ?“Any two observers moving at a constant speed and direction with respect to one another will obtain the same results for all mechanical experiments.?”

By extension, this tells us that all frames of reference are equivalent. No mechanical experiment can tell you which frame of reference is moving and which is standing still. A frame of reference is a place that shares your motion.

For Galileo, these frames of reference were called ?“inertial frames?”; the word ?“inertia?” meaning ?“inactivity.?” Your personal frame of reference is the place where you are standing still.

So, this fundamental theory of Relatively is also known as ?“Galileo?’s Principle of Inertia.?” Sir Isaac Newton included it his famous ?“Laws of Motion?” about one-hundred and fifty years later. See Newton?’s first law.

I. Every object continues in its state of rest and its state of uniform motion until force is applied to change it.

II. The acceleration of an object is proportional to the force applied and goes in the same direction. Newton?’s famous mathematical equation for this is F=ma.

III. For every action there is an equal and opposite reaction.

Theoretically, Galileo?’s Relativity means there is no such thing as absolute motion or absolute rest. Both are relative to the observer?’s frame of reference. That idea bothered Newton a great deal even though he adopted it into his first law. He believed that absolute standards for motion and rest would be discovered eventually. (We are focusing mostly on the first law in this posting.)

Galileo?’s Relativity amounted to a rejection of Aristotle?’s theories on motion. Essentially, Aristotle believed in his personal observations which seemed to be common sense. Objects move when they are pushed and stop when the forces applied are removed or when they meet significant resistance. So, he reasoned that all objects on Earth naturally slow down and stop.

But, Aristotle lacked the means to subject his theories to scientific experiment, and he recognized that there were some exceptions that he could not explain. He saw for example that the acceleration of falling objects increases as they drop. Eventually, Aristotle decided that substances on Earth were different from the substances in the heavens and thereby subject to different laws of motion.

Galilean Relatively holds essentially that the laws of physics are the same everywhere; and the variables for motion (speed) and acceleration (velocity) are relative to the observer?’s frame of reference.

In introductory physics classes, the concept of ?“frames of reference?” is frequently demonstrated by the following hypothetical example.

Imagine you are standing inside a ship that is sailing at a constant speed in smooth water. You are deep inside the ship?’s hull and cannot see outside. And, you are performing mechanical experiments such as studying the paths of falling objects. Can you tell whether the ship is moving without going on deck or looking out a porthole? We know from our personal experiences that the answer is ?“no.?”

You cannot feel the motion of the ship because you are moving at its constant speed too. It would be the same if you were flying inside an airplane. You cannot tell you are moving when the plane is cruising at constant speed unless you look out the window.

This ship or airplane is your frame of reference. Galileo recognized that the mechanical experiments you conduct while the ship is moving at a constant speed would have the same results as similar experiments conducted on the shore. So, these frames of reference are equivalent.

On the other hand, you can indeed feel the movement of the ship when it speeds up or slows down. Your motions relative to each other are diverging.

Before we move to our next post, it might be a good idea to say something about what we mean by the expression ?“laws of physics.?” Broadly speaking, Physics is the study of the fundamental laws of nature. Some of these are obvious to us; however, physicists have also discovered over the last 400 years of so that nature is much more complex that we recognize. Many of their discoveries seem to be counter-intuitive.

The laws of physics they have developed relate to natural occurrences such as these: motion, gravity, conservation of mass and energy, conservation of momentum, thermodynamics, electrostatic charges, the characteristics of the Speed of Light, and quantum mechanics.