As an undergraduate scientist, amateur cyclist and chronic procrastinator, I concentrate a lot of my thoughts on the subject of time. I’m curious about many things related to time: the most pressing of these concerns is usually finding out where all of mine went and what to do with what little remains. President’s Day weekend is a particularly good example of this phenomenon in which I seem to blink, and then it’s Monday again. Otherwise, I usually consider things that are very impractical.
Stars are funny. They’re big, hot and for the most part, very far away. So far away that things like light take somewhere on the order of hundreds of thousands to billions of years to reach us. Just consider those distances, when it takes light less than a second to circumnavigate the globe.
It makes for something interesting: When we get sentimental, step outside and look at the night sky, we’re not just looking at a pretty arrangement of lights. We’re looking at a veritable intergalactic history lesson. We’re seeing, from one reference point, the state of affairs of things as they were in our neck of the universe before dinosaurs were a thing, before Earth was a thing, or even before our neck of the universe was a thing.
Some of these stars whose light we see today might not even exist anymore by the time their light reaches us.
This leads me to another bizarre quirk that’s widely accepted in some circles of physicists. The speed of light in a vacuum is constant. This is still the case even when we have a moving reference point. And now it gets weird.
While it’s easy to say, “Oh the speed of light is constant! That makes calculations on tests easier,” it’s still counterintuitive if you think about what speed is.
If we consider that we live in a universe defined by four dimensions, (x, y, z and time) then things can change their respective positions in the universe over a period of time and we get speed as a measure of this change. But let’s pretend we’re physicists and play around with some of these ideas.
Suppose you’re an observer. This shouldn’t be hard; it’s not wildly different from what you normally do. In this situation, you might be standing still, and a train goes by at 60 mph. A person on this train throws a ball at a speed of 10 mph in the same direction as the train’s motion. To you, the stationary observer, this ball moves at a stunning 70 mph. The situation is ordinary, but now I present another ordinary situation!
You are now hypothetically observing this ball from the train. If you had no knowledge that this train was moving, but observed the ball, you might logically perceive it as moving at 10 mph.
Okay, I’ve established that I can be really, really boring. Why does any of this matter?
If light behaved like this ball and moved with increased speed from a moving source, then some images from the same distance would reach the same stationary point faster or slower than others depending on if their source was moving.
Let’s pretend the speed of light is additive to a moving source’s speed.
If I were next to an intersection observing two objects about to collide, then things would appear different to everyone involved. If one of the objects was a car moving toward me and the other object was a motorcycle on a collision course with the car from a different road, but not any closer to me, then I’d see the car before I saw the motorcycle. The car would see the image of the motorcycle before I did, and to me, would maneuver to dodge for no apparent reason.
This is an oversimplification for demonstration’s sake, but it represents a qualitative example of why the behavior of light is at least worth some consideration.
Another example of why light’s behavior is hard to follow is as follows: suppose there’s a hypothetical aircraft traveling at half the speed of light. This craft is struck by lightning bolts simultaneously at both the front of the nose and the back at the tail. To an outside, stationary observer looking directly at the side of the plane, both bolts would hit the plane at the same time. However, to someone at the center of the plane, the light from the bolt that hit the nose would reach them quickly, while the light from the bolt that hit the tail would struggle to catch up. To the passenger, one bolt struck the plane before the other.
But if the speed of light isn’t changing as reference points move around, then what’s happening?
With the moving observer, we’re changing the distance that light must travel despite the fact that both lightning bolts started equidistant from the observer and radiated outwards at the same speed.
The idea is that for the speed of light to remain constant, either time or distance must change depending on the frame of reference of the observer.
The long and short of this is that in the universe, there shouldn’t be such a thing as absolute time or absolute distance because a moving clock will keep time at a rate different than a stationary clock. This keeps the speed of light constant for all frames of reference.
In all likelihood, for the amount that I still don’t understand, I spend far too much time thinking about time.
ALAN LIN is just profoundly confused himself and can be reached at email@example.com.