It's All Relative

At the end of the 1800s, the grain of sand in science's oyster was the conflict between what was known of motion and what was known of electricity and magnetism. Existing theories contradicted each other.

The resulting pearl was a revolution in science. The prophet of this revolution was a young Albert Einstein, whose theories of relativity neatly solved the contradiction, and made many predictions which were later proved correct.

The relativity of motion had been well understood for centuries. An object cannot be said to be moving or standing still by itself; it is moving or not moving only in relation to something else.

As an example, imagine you wake up tomorrow morning to find yourself on a spaceship. You look out the window, and see nothing but blackness all around. Is the spaceship moving, or is it sitting still? Without comparing to another object, it is impossible to say.

Now imagine that you look out the window, and see an asteroid float by. Is the asteroid standing still, and the spaceship moving past it? Is the asteroid moving past the motionless spaceship? Are they both moving? As far as the laws of nature are concerned, all of these views are equally valid. Anything you can say about a spaceship moving thirty miles per hour past an unmoving asteroid, equally applies if you consider the spaceship perfectly still and the asteroid moving thirty miles per hour in the opposite direction.

Another common-sense example is that if I am driving down the highway at sixty miles per hour, and someone drives past me at sixty-five miles per hour, they seem to me to be moving by at a slow five miles per hour, whereas they seem to be moving quickly to somebody standing still on the side of the road.

Einstein's discovery was this: no matter how fast you are going, light always appears to move at the same speed. If you are sitting still, light whizzes by at 300,000 kilometers per second. If you get in your trusty spaceship and fly at 299,999 kilometers per second, a beam of light still appears to whiz by you at 300,000 kilometers per second compared to you! Unlike the cars in our previous example, light's speed is always constant (in a vacuum), never relative.

How is this possible? Simple: if you go really fast relative to someone else, then time passes slower for you than for them.

Einstein's discovery was that time and space are not the same for everybody, but are different for two people depending on how fast the two people are going relative to each other. (The same, of course, applies to objects, but people can compare measurements.)

Omar and Abdul decide to test this theory. Omar stays comfortably on earth. Abdul hops in his spaceship, flies off near the speed of light for a while, then turns around and comes back to earth. Abdul's clock says that he was gone two years, and Abdul is indeed two years older than when he left. But Omar's clock says that Abdul has been gone for fourteen years, and Omar is indeed fourteen years older! Time passed slower for Abdul; for every second that passed in Abdul's time, more than one second passed in Omar's time.

Not only time, but space is relative to speed. By Abdul's measurements, he traveled a fairly small distance, while Omar's measurements say Abdul traveled a long distance.

There is nothing strange or contradictory in the idea of space and time being relative. It's how the universe appears to work. On the other hand, the idea that space and time are a fixed, universal backdrop, a stage on which the drama of life is played, is provably wrong.

Because time is relative to the observer, the concept of "now," of "simultaneous," also is relative. There is no single moment "now" that divides the entire universe into past, present and future.

Time is not a sequence of moments, which for any moment you could say what was happening at every spot in the universe. More accurately, you could say that, but it would only apply to your point of view. Abdul, moving on a spaceship near the speed of light, would slice up the events of the universe differently; just because you decided that some events happened "at the same time" doesn't mean he will come to the same conclusion. He will determine that the events happened at different times, and he will be right -- from his point of view.

The discovery of the relativity of time and space is a death-blow to naive realism. A naive realist might say that there is one "now" for everything, that everything before this now is past, and everything after is future. This is simply not true. It is impossible to say that things happen "at the same time," except from a particular point of view. Two points of view may measure two different durations of time and two different distances of space for the same interval. Two points of view may come to different conclusions about whether a particular event happened at the same time as another event, or before or after that event.

The line between past, present and future is not recognized by the universe. It is merely a point of view.

Next: The View From Outside