The wild frock of hair. The mischievous grin and sad, puppy dog eyes. And, of course, the famous equation E=mc2. The one scientist almost everyone knows not just by name but even by sight is, of course, Albert Einstein. He’s become a synonym for genius; so smart, so beyond our ordinary ways of thinking, that we don’t have any chance of understanding his ideas. And yet, that part of the legend is simply not true. You can understand some of Einstein’s most exciting discoveries, and you won’t need an advanced degree in physics to do it. All it takes is a little imagination. As Einstein said, “Imagination is more important than knowledge.” What better way to celebrate Einstein’s 134th birthday than by recreating in our own minds one of his most astonishing discoveries?
Albert Einstein was born 134 years ago, on March 14, 1879. He was slow to speak, but despite what is sometimes said he was not an unintelligent child. One day when he was five years old and sick in bed, Albert’s father brought him a magnetic compass. The boy was fascinated by the way the needle pointed stubbornly in the same direction, no matter how he turned the device about. "This experience made a deep and lasting impression on me," Einstein later wrote. "Something deeply hidden had to be behind things."
Einstein spent his life investigating those “deeply hidden” things, including the nature of light and time. As a teenager, Einstein wondered what it would be like to travel alongside a light beam. He realized that such a journey would lead to contradictions – contradictions he could only correct by changing the way we think of space and time. It is this new way of thinking about time that you are about to discover for yourself. Ready?
Imagine the world’s simplest and silliest clock. The clock is made of one moving part: a piece of light (called a photon) that bounces back and forth off two mirrors. One mirror is on the clock’s ceiling (just like the Hotel California), while the other is on the clock’s floor. The photon bounces back and forth between the two mirrors, and each time it hits a mirror, a “tick” is recorded. The photon moves at one well-defined speed – the speed of light. This is quite natural, since the photon is light. But one of the things that surprised and intrigued Einstein in his thoughts about light is that by its very nature light must always move at that single speed. It can’t slow down or speed up, but must always move at exactly the speed of light.
(That speed, by the way, is enormous. Light is so fast that it takes less than three seconds to travel from the Earth to the Moon and back again. The Apollo astronauts, traveling faster than any human in history, took six days to make the same trip, not counting time spent planting flags and hitting golf balls.)
So this photon, moving up and down in our mirror, must move at a single speed, the speed of light. Nothing we do can change that speed. This is a key point to remember. The photon must always move at one single speed, the speed of light.
Now watch the animation below. If we build two clocks, and give one a push, we’ll see that the photon in the moving clock bounces off the mirrors differently than the photon in the stationary clock. Why?
Because the photon in the moving clock takes a diagonal path between the moving mirrors, that path has to be longer than a straight up-and-down path. Prove it to yourself by measuring the lines, or by drawing your own pictures of moving and stationary clocks. The path of the photon in the moving clock has to be longer.
OK, we’re almost there. Here’s the key point. If both photons must move at the same speed (the speed of light) and the photon in the moving clock must go further, then the photon in the moving clock bounces off its mirrors less often. It makes less ticks.
But ticks are how the clock measures time! That means (here it comes) the moving clock must run slow!
This is astounding. By setting up this simple experiment in our imaginations, we’ve discovered a deep and profound truth about the universe. Moving clocks run slow.
But wait. Maybe this is just some weird quirk of this silly clock we’ve imagined. Maybe other clocks won’t show the same effect. Yet for the laws of physics to be consistent, all clocks must behave just like the light clock. All clocks must run slow when they are in motion. And in fact a multitude of experiments have again and again demonstrated this amazing effect.
Even your own internal clock, which tells you when to eat, when to sleep, and when to go to the bathroom, is affected by motion in just this way. Of course, you won’t notice this effect in any sort of everyday motion. Only when you travel near the speed of light, far faster than even those Apollo astronauts, does this time dilation become measurable.
But the fact that the effect is tiny in no way diminishes the astonishment. We think of time as so solid, so regular, so everyday. But time as revealed by Einstein is fluid, changeable, and different for every one of us. Just by taking an airplane trip, riding in a car, or even walking across a room, we each create our own unique time.
The light clock is just one step in understanding Einstein’s ideas. But it’s an important step. Once you own the light clock in your mind, it is yours forever. Just like Einstein, you will have grasped something profound about the universe; something amazing, fundamental, and yet “deeply hidden” beneath our ordinary experience. Think of the light clock as Albert Einstein’s birthday gift to you.
Happy birthday, Albert, and thanks for the present!
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