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Think Like Einstein

Adapted from an article by Rick Groleau

Ever since Einstein revealed his special theory of relativity, we've known that time travel—at least moving forward through time—is possible. Einstein didn't pull this theory, or even the notion that time travel is possible, out of thin air. Rather, he took the knowledge of the day, saw an inconsistency—a piece of a puzzle that didn't fit, so to speak—and thought about possible explanations.

In the following article you'll have to think like Einstein. You'll take a look at the same puzzle and see the problem, and you'll have to think about the same things Einstein had to think about to resolve the problem.

Maybe you've heard the recent reports about how physically unique Einstein's brain was. Don't panic—you won't need a superhuman brain to grasp the concepts presented. And when you finish, not only will you understand the special theory, you will have reasoned it out for yourself, just as Einstein did.

Part 1: Adding Velocities

Animation of train example

We begin with a basic concept—one that sets the scene...

You're on a train that's moving forward at 50 mph. You throw a ball in the direction that the train is moving. Relative to you and the train, the ball leaves your hand travelling at 20 mph.

Question: From the point of view of someone standing alongside the tracks, how fast is the ball moving?

Answer:  All you have to do is add the speed of the train (50 mph) and the speed of the ball (20 mph) which is a total of 70 mph.

Part 2: The Speed of Light

OK. Everything so far makes sense. Let's move on to the speed of light for a moment.

In 1887 two American scientists performed a now-famous experiment. The experiment seemed to show that the speed of light was independent of motion. In other words, that light always travelled at the same speed: 186,000 miles per second. It didn't matter if the source of the light was moving or if the observer was moving.

There was another indication that the speed of light was constant, too—one that Einstein found especially difficult to ignore. James Clerk Maxwell, the mind behind electromagnetic theory, had developed equations that described the nature of electricity, magnetism, and even light. These equations, the predictions of which were confirmed by experiment, by the way, implied that light always travelled at the same speed.

Which brings us to the next question...

flashlight & train animation

Again, you're on a train. This time, though, the train is moving much faster—at half the speed of light, or 93,000 mps (miles per second). And instead of throwing a ball, you turn on a flashlight.

Question: How fast is the light travelling relative to the observer standing alongside the tracks?

Answer:  While "common sense" might say the answer should be simply to add the speed of the train (93,000 mps) to the speed of light (186,000 mps), for a total of 279,000 mps, this doesn't work here.

Relative to the observer, the light is moving at 186,000 mps. Seems non-commonsensical, doesn't it? But this has proven to be true through many experiments over the years. 

Part 3: The Speed of Light

flashlight & train animation

Here's our last question. This one's like the previous one, but with a twist. Again, you're on a train moving at 93,000 mps, and again, you turn on your flashlight.

Question: How fast does the light travel relative to you?

Answer:  Relative to the man on the train, the light is moving ahead at 186,000 mps, just as it is for the observer outside the train. The speed of light remains constant for all observers.

Is there a solution to this paradox?

Part 4: The Solution

By now you probably understand the conflict: How is it possible that light always travels at the same speed, no matter how fast its source is moving? Einstein, when he was 16, thought about the same thing.

Are you familiar with the equation v=d/t? All it says is velocity (speed) equals distance travelled divided by time.

Here's an example of how it can be used...

And here are two more examples that show how speed can stay the same even though distance and time can change...

See the relationship between speed, distance and time?

If we use this equation in our first scenario—the one where you threw the ball—it works out fine. For you, within the train, as well as for someone standing by the tracks, we can calculate the speed of the ball by adding the distance the train travelled and the distance the ball travelled.

The equation does not work out so well in the second scenario, though, because we're dealing with the speed of light, so the "v" in the equation always has to be 186,000 miles per second.

Something has to give.

Question: What can we infer from what we've seen so far?

Answer: That time (and maybe even distance as well) is not the same for all observers.

Time Can Vary?

That's right! Contrary to what common sense tells us, time and distance are not fixed. This, too, is the assumption Einstein made.

In our second and third train examples, the speed of light turns out to be exactly the same for both you and the observer standing along the tracks because time, as measured by your watch, ticked along at a slower pace than time measured by the observer. Not only that, distance changed, too. For the observer, a one-foot ruler whizzing by on the train would have measured less than a foot.

The weird thing is that, for you on the train, time wouldn't seem to be moving slower and your ruler wouldn't be shorter—all would appear normal. However, time on the rest of the Earth would appear to be ticking along slower and its rulers would be shorter.

Now let's say you want to do some time travelling. You board a spaceship and take off for deep space.

The ship approaches the speed of light. Time for you seems to pass as it always has. It takes you about five seconds to tie your shoe. But to an observer on Earth (assuming he or she could watch you), you are moving at a snail's pace. It takes hours to tie your shoe.

Anyway, you continue on your journey. You slow down, stop, and accelerate back to Earth. You arrive home. You have aged two years during your flight. Two hundred years have passed on Earth. You have successfully travelled forward through time.

Now you want to go back? Sorry. According to relativity, you can only move through time in one direction.