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An introduction to Einstein's revolutionary ideas regarding inertial frames of reference and the constancy of the speed of light.
If you were traveling in a rocket at 99% the speed of light and turned on a flashlight, would the light beam crawl away from you at a snail's pace, or would it zip away at its usual speed? Einstein's answer to this question shattered our understanding of time and space forever.
Before we can understand relativity, we must define where the physics takes place. A frame of reference is essentially a coordinate system used to measure the position and motion of objects. An inertial frame of reference is one in which Newton's First Law holds true: an object at rest stays at rest, and an object in motion stays in motion at a constant velocity unless acted upon by a net force. Crucially, an inertial frame is not accelerating. Whether you are standing on a sidewalk or sitting in a plane flying at a perfectly steady in a straight line, you are in an inertial frame. In these frames, you cannot perform any mechanical experiment to 'feel' your motion.
1. Imagine you are on a train moving at a constant velocity of on perfectly smooth tracks. 2. You pour a cup of coffee. The liquid falls straight down into the cup, exactly as it would if the train were parked at the station. 3. Because the train is an inertial frame, the laws of mechanics (like gravity and inertia) behave exactly the same as they do on 'stationary' ground.
Quick Check
You are in a car driving around a circular track at a constant speed of 60 mph. Is this an inertial frame of reference?
Answer
No, because the car is changing direction, it is accelerating (centripetal acceleration), making it a non-inertial frame.
Einstein’s first postulate states: The laws of physics are the same in all inertial frames of reference. This might sound simple, but it is radical. It means there is no such thing as 'absolute rest.' If you and a friend are floating in deep space and you drift past each other, there is no experiment—mechanical, electrical, or magnetic—that can determine which one of you is 'actually' moving and which is 'actually' still. All motion is relative. If the laws of physics changed based on your constant velocity, you could use those changes to detect your 'absolute' speed, but Einstein asserted that nature doesn't work that way.
1. Consider two spaceships, A and B, in a void with no stars for reference. 2. Ship A sees Ship B moving toward it at . 3. Ship B sees Ship A moving toward it at . 4. According to Postulate 1, both viewpoints are equally valid. Neither ship can claim to be the one 'at rest' because the laws of physics (like ) work perfectly for both.
Quick Check
If the laws of physics were different in a moving car than in a stationary house, what would that imply about 'absolute rest'?
Answer
It would imply that there is a 'preferred' or 'absolute' frame of reference that is truly at rest, which contradicts Einstein's first postulate.
The second postulate is the real 'mind-bender': **The speed of light in a vacuum, , is the same for all observers, regardless of the motion of the light source or the observer.** In classical physics, if you throw a ball at from a car moving at , a person on the side of the road sees the ball moving at . However, light refuses to follow this rule. If you shine a laser from a rocket moving at , you measure the light at , and a stationary observer also measures that same light at exactly . This constancy is a fundamental property of the universe's geometry.
1. A spaceship travels toward Earth at . 2. The ship fires a laser beam directly at Earth. 3. Classical logic suggests Earth would see the light at . 4. However, Einstein's Second Postulate dictates that observers on Earth will measure the light speed as exactly . 5. To keep constant for everyone, other things we thought were constant—like time and length—must actually change (dilate or contract) as objects move faster.
Which of the following is an inertial frame of reference?
If a light source is moving away from you at , what speed will you measure for the light emitted by that source?
Einstein's first postulate implies that you can perform an experiment inside a windowless, non-accelerating plane to determine its speed.
Review Tomorrow
In 24 hours, try to explain the two postulates of special relativity to a friend or write them down from memory. Specifically, focus on why the second postulate is so counter-intuitive compared to throwing a baseball.
Practice Activity
Research the 'Michelson-Morley Experiment' to see the historical evidence that led scientists to realize the 'ether' didn't exist and that the speed of light was indeed constant.