Last November, Nova presented an hour-long program, Inside Einstein's Mind. Starting at the nine-minute mark, the program attempts to explain the thought experiment which led to "special relativity" in 1905.
Below, you see the transcript of that chunk of the program.
In that part of the program, Nova offers a brief account of the world of physics Einstein inherited. The program then describes a thought experiment which leads us to a series of "metaphysical statements."
The thought experiment involves a man on a railway platform; a woman moving past on a train; and a pair of lightning strikes. The visuals in the Nova program will help you picture the scene.
That thought experiment leads us to some sweeping statements. Three examples:
"The flow of time is different for an observer that is moving versus one that is standing still."
"Simultaneity, and the flow of time itself, depends on how you're moving."
"If there's no such thing as simultaneity, then there's no such as absolute time everywhere throughout the universe."
In theory, that thought experiment equips you to understand those statements. Also this:
"This concept, that time and space as well are relative, became known as special relativity."
Nova covers a lot of ground in a very short time. To watch the whole program, click here.
Starting around the nine-minute mark, this is the relevant transcript:
PROFESSOR SCHAFFER: Einstein's world in 1905 was dominated by two kinds of physics. One was about 200 years old, founded by Isaac Newton, British natural philosopher.That represents Nova's attempt to explain or elucidate "special relativity." Here's the question we'll be asking:
For Newton, all there is in the world is matter, moving.
NARRATOR: Newton showed that the motion of falling apples and orbiting planets are governed by the same force—gravity.
His equations are so effective we still use them today to send probes to the farthest reaches of the solar system.
The other important theory of Einstein's day covered electricity and magnetism. That branch of physics had been revolutionized in 1865 by the Scottish physicist James Clerk Maxwell.
Maxwell's theory described light as an electromagnetic wave that travels at a fixed speed.
In Newton's world, the speed of light is not fixed.
PROFESSOR SCHAFFER: Einstein could see that there's a contradiction between Newton and Maxwell. They just don't fit together. And one of the things Einstein, hated—hated!— was contradiction. If there's one kind of physics that says this, and another kind of physics that says that, and they're different, that's a sign that something's gone wrong, and it needs fixing.
NARRATOR: For months, Einstein wrestles with the problem. Eventually, to resolve this contradiction, he focuses on a key element of speed—time.
WALTER ISAACSON: He realized that any statement about time is simply a question about what is simultaneous. For example, if you say the train arrives at 7, that simply means that it gets to the platform simultaneous with the clock going to 7.
NARRATOR: In a brilliant thought experiment, he questions what "simultaneous" actually means, and sees that the flow of time is different for an observer that is moving versus one that is standing still.
He imagines a man standing on a railway platform. Two bolts of lightning strike on either side of him.
The man is standing exactly halfway between them, and the light from each strike reaches his eyes at exactly the same moment. For him, the two strikes are simultaneous.
Then, Einstein imagines a woman on a fast-moving train traveling at close to the speed of light. What would she see?
As the light travels out from the strikes, the train is moving towards one and away from the other. Light from the front strike reaches her eyes first.
For the woman on the train, time elapses between the two strikes. For the man on the platform, there is no time between the strikes.
This simple thought has mind-blowing significance. Simultaneity, and the flow of time itself, depends on how you're moving.
PROFESSOR CARROLL: If there's no such thing as simultaneity, then there's no such as absolute time everywhere throughout the universe, and Isaac Newton was wrong.
NARRATOR: This concept, that time and space as well are relative, became known as "special relativity." It led to remarkable results, such as the famous equation relating energy to mass.
Does that presentation make sense?