# Relativity and Light Speed

Relativity and light speed present a confusing concept in physics. My students always came into the physics class with prejudice based on what their family had told them. The relatives scared the students into thinking physics class was going to be hard. I always began the year by telling the students that physics was the easiest class they would ever take as long as they learn to speak algebra.

One year, a student enrolled in my physics class who had escaped Viet Nam and spoke virtually no English. The guidance counselor questioned how the student could handle my class with the language handicap. The young man smiled and said through his interpreter, “But I speak excellent algebra!” That was true, and he was my best student that year. Einstein’s theory of relativity is a physics unit that invokes fear in many minds, but it’s easy to understand if you know a little algebra.

The problem is not understanding relativity and light speed. The problem is believing it. Relativity begins with TWO BASIC POSTULATES:

THE FIRST is that the laws of physics are the same no matter where you are or what you are doing. If you are sitting in your chair reading this, all the laws of physics work very well. When you drop an object, it falls in accordance with the laws of motion. If you were in an airplane traveling near the speed of sound and you drop the same object, it would fall the same way as it did when you were sitting still.

THE SECOND postulate tells us that the speed of light is a universal constant. This one is easy to understand, but very hard to believe. Suppose I were in a rocket traveling toward you at half the speed of light. If I turn on my headlights, the light beam will travel at the speed of light. You are sitting still and measuring the speed of the light beam. What would your measurement be? You might be tempted to say, “The speed of the rocket, 0.5 times light speed, must be added to the speed of light. The answer would be 1.5 times light speed.” What Einstein’s postulate says is that you would measure it to be the speed of light–186,000 miles or 300,000,00 meters per second. That’s because the speed of light is a constant, independent of the motion of the light source or the observer.

Light speed is designed to be a universal constant according to Einstein relativity equations. You say, “How can that be?” According to Einstein, time is a created thing that depends upon the motion of the observer. As you go faster, time slows down. The algebraic equation is that the time you experience (t’) equals the time you would experience at rest (t), divided by the square root of 1 minus the velocity (v) squared divided by the speed of light (c) squared. Notice that the velocity cannot be higher than the speed of light. If it were, the denominator would be the square root of a negative number, which is not possible. If you don’t understand the equation, understand that time is not a fixed thing. It changes with velocity. The faster you go, the slower time passes. At light speed, time would stop.

Science fiction writers have suggested that this is a way to build a time machine. That won’t work, because time doesn’t reverse. Since the speed of light is always the same for all observers, time gets slower and slower but never stops. This is not wild speculation. Experiments at very high speeds in particle accelerators have verified what we have briefly sketched here.

Not only does this change our understanding of time, but it gives us a new understanding of space and mass. This knowledge helps us understand not only the creation but also the wisdom and design built into it. Everywhere we look, we see that a wonder-working hand has gone before us. Tomorrow we will look at the implications of relativity and light speed in Einstein’s Theory of Special Relativity.

— John N. Clayton © 2020