The early twentieth century saw the development of Special Relativity, General Relativity, and Quantum Mechanics. These important subjects will help us understand current theories about the origin of the universe. But before we jump into them, we’ll need to discuss the relativity of the seventeenth century.
Galileo and Newton pondered what moving objects would look like to a moving observer, or “relative” to that observer. Would they exhibit the same laws of motion? Galileo was the first to recognize that the motion of objects on a ship may look different when viewed from shore. Consider a ship moving at a constant speed over smooth water. An observer on the ship would see a rock dropped from the top of the ship’s mast fall straight down and land near the bottom of the mast. A stationary observer on the shore would see the rock moving with the ship horizontally as it fell vertically, following a curved path.
The basic principle of Galilean relativity is that a passenger cannot determine the difference between moving with a constant speed in a straight line and standing still without looking outside the ship. The surface of the Earth is bumpy and covered with air, so most of our modes of transportation, such as cars, ships, and airplanes, experience vibration and sound associated with their motion. Also, our earthbound modes of transportation need propulsion to move through air or water against the force of drag. This also provides vibration and noise, indicating that we are moving. A ship sailing smoothly across the water, or a spaceship coasting through the vacuum of space with engines off, does not experience these motion indicators.
Spaceship under different states of motion.
Imagine we are in a spaceship coasting at high speed through space outside the atmosphere, with the engine cut off and all the windows closed (left side of figure). We would not feel or measure any motion. We would not feel any gravity. We would be floating weightless through the interior of the ship. If we had a device that measured acceleration, such as a cell phone or a Fitbit with an accelerometer, it would say we are not moving because no acceleration is sensed.
After the window opens, we see other spaceships, planets, and asteroids floating by. We would know we are moving relative to them, even though we could not feel the motion. We can observe or measure our speed relative to other objects. On the other hand, we can experience and measure acceleration, changes in speed and direction, without looking out the window. If the spacecraft engine were activated, we would experience the force pulling our bodies toward the back of the spaceship.
Let’s go back to Earth and ride in an automobile. When a car makes a sudden stop, we can feel our body moving forward, as it strains against the seatbelt. The bag of groceries in the other seat might tip forward and dump its contents on the floor. We have just experienced an acceleration. Some people may refer to this as deceleration, but physicists generally use the term acceleration to describe any change in speed.
Acceleration can also mean a change in the direction of motion. Suppose we start the car moving again and make a sharp turn. We will experience ourselves being pulled to the opposite side. That is also an acceleration because our vehicle is changing direction. This is a consequence of the Law of the Conservation of Momentum. Everything in the car continues to move in the same direction it was already heading.
Let’s define some terms we will need later. A frame of reference is whatever an observer may be riding on, which could be an automobile, a spaceship, or an electron. A frame of reference moving at a constant velocity, with an unchanging speed and direction, is called an inertial frame.
According to the Galilean principle of relativity, we cannot determine how fast an inertial frame of reference is moving unless we measure it relative to another frame of reference. It also states that the laws of motion are most easily described in an inertial frame.
Motion is more complicated in accelerating reference frames because we must account for forces acting on the accelerating objects. Physicists add centripetal and centrifugal forces to describe motion inside rotating reference frames. These forces are also called fictitious because they don’t exist in inertial reference frames.
The Galilean relativity principle states that velocity can only be measured relative to a frame of reference. This implies that there is no absolute velocity. The universe lacks a preferred frame where the speed is zero. As I write this, I am moving around at high speed on the Earth’s surface as it rotates. The Earth also revolves around the sun at high speed. The sun is revolving around the galaxy at an even higher speed, and our Milky Way Galaxy is traveling rapidly through space relative to nearby galaxies. What reference frame should we use to determine our absolute velocity?
At the end of the nineteenth century, physicists thought they had things pretty well understood. However, there were still some mysteries, such as why atoms are stable and why the speed of light remains constant. It also seemed like the more we learned, the more we understood. In the twentieth century, the more we learned about space, time, and matter, the stranger it became. The next blog post will start to explore that strangeness.
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