Do you look around the natural world and wonder? Wonder why things work the way they do, why things are the way they are? If you do, then there’s a bit of physicist inside of you! Physics is all around us.

With the recent release of the movie Interstellar, lots of people have been wondering about time. Without giving too much away, characters in the movie age at different rates. While an hour or so passes for some, years go by for others. This might seem like the “fiction” part of science fiction, but time actually can and does run at different rates in different situations.

Albert Einstein realized that the speed with which light travels doesn’t depend on how fast the source of the light is moving. This is certainly not true for, say, a ball: it was well understood before Einstein came along that if you can throw a ball at 60 miles per hour and you throw a ball straight forward while riding a horse at 20 miles per hour, the ball will initially be moving at 60 plus 20, or 80 miles per hour. So said Isaac Newton.  

But not so with light! Light travels through empty space at about 186,000 miles per second. If you’re zooming through space at 100,000 miles per second and aim your laser pointer straight forward, the speed at which the light comes out is not 100,000 plus 186,000 miles per second. Rather it will be 186,000 miles per second, the same as if you weren’t moving. This is a fundamental law of the natural universe.

To see how the nature of light affects time, imagine a clock for which each tick is the time it takes for a light pulse shot from a laser to reflect off a mirror and return to the laser. When the clock is stationary, each light pulse travels twice the distance between the laser and the mirror during one tick of the clock.  

But imagine that our laser clock has been placed on a spaceship and you can see it as the spaceship zooms past Earth. As the light pulse travels from the laser to the mirror, the mirror (and the rest of the clock) moves, so the light has to travel farther to get there. The same is true as the light pulse heads back after reflection. For this reason, each light pulse must travel farther, perhaps much farther, than simply twice the distance between the laser and the mirror.  

Because the distance is longer, but the speed of the light is always the same (so said Albert Einstein!), you see the round trip for the light as taking a longer time. In other words, it takes longer for the clock to tick, and longer ticks mean that you see time as running slowly on the ship. The fabric of space and time is stretched for objects moving relative to you.

This does not address the affect that the massive black hole in the movie Interstellar has on time. That requires a bit of an extension of the discussion here.  We’ll leave it for another post.

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Questions to ponder:

When you’re driving, say up Highway 280, you know you’re moving by watching trees and houses moving backwards with respect to you. But suppose you are cruising along in space, with no nearby objects by which to judge your motion. Could you tell if you were moving?

Suppose you are cruising along in space, and you pass by another spacecraft pointed in the opposite direction. Could you tell which of the two spacecraft were moving – that is, could you tell if you were moving past the other craft or if it were moving past you?

Suppose you are out in space, and another spacecraft zooms past. We now understand that you will see timing running slowly on that other ship. But people on that ship think that you’re zooming past them – according to Einstein, they see your time as running slowly. Could that really be? [Hint:  yes!]