I am always digging through reams of material, both written and video, looking for interesting tidbits about my favorite topics, one of which is the philosophy of space and time and accompanying it, physics. Here, I found a very useful (I think) tidbit buried in a Youtube video put out by the World Science Festival. Called the World Science University (WSU), this is a compilation of lectures on a myriad of science topics by noted scientists, all available on Youtube, and all free! Somebody pinch me! They can be fairly long, so you may have to search within them for specific topics of interest. I will provide a link at the end to Greene’s video, but thought I’d explain what he is talking about, as this helps me understand it better, and if you don’t have the time or gumption to watch the video clip, you can get it here in a nutshell.
In this case, I was hunting for a description of time dilation that one could swallow without choking. If you watch science fiction movies, you have probably heard the phrase, “time dilation,” and know that when you hear it mentioned, the movie screen goes nuts and you see some intrepid adventurers, like the crew of Star Trek, moving crazily in slow motion with voices stretched out to near incomprehension. This type of movie magic is meant to visually display what happens when a star ship nears the speed of light because we now know that time slows down, way down, as the ship goes ever faster. This phenomenon, that clocks go slower the faster they move through space, is what is known as time dilation and it is a very real thing. In other words, time dilation is definitely NOT a Jedi mind trick!
While the movie effects and the idea itself are very cool to think about, it begs the question as to why? Why does a clock move slower through time when it is moving through space? Inquiring minds want to know!
Enter physicist Brian Greene. He offers us the answer and delivers it with what I think is the best description of time dilation that I have found to date. He calls his example a “mental mnemonic” that we can use to remember the answer to this important question.
Greene sets the stage by reminding us that what he is about to tell us is fully justified by the math. In other words, this isn’t theory, it’s real. In fact, it’s been proven thousands of times in experiments all over the world.
He tells us that for his example, we need to put time aside for a minute and think just about space.
Imagine, if you will, that there is a car on a highway traveling directly north at 100mph. Now, imagine the car suddenly veers off towards the east and starts traveling in a northeasterly direction at exactly the same speed. What happens? Well, we know that the car, once it changes direction to the northeast, is not as quick as the car when it was moving directly north. This is because of its diversion to the northeast. See the two graphics below.
In Graphic #1 we see 3 cars. The one going directly north and two others representing what happens when the car turns and begins heading northeast. (I’m not sure why he shows 3 cars, the third car is superfluous to his example.)
Time Dilation Graphic #1
In Graphic 2 below, we see what results from the car turning away from directly north and then going to the northeast. Notice that the car moving north is farther along than the other two cars moving to the northeast.
Time Dilation Graphic #2
So far, so good, right? I think if you drive a car, it’s easy to imagine this scenario; and well, it makes sense, doesn’t it? Because no driver would be surprised at this depiction.
Next, Greene tells us to now put space aside and apply this scenario to both space… and time.
Here, Greene says to imagine him just standing there on the stage. We would say that relative to us, Greene is not moving. He’s just standing there. “Aha!” he then tells us, (the “aha” is mine), he is in fact, moving! He explains that if he looks at his watch, he sees the seconds ticking off, one after the other. He is therefore moving… through time, a.k.a., the “time dimension,” although he is not moving through space, relative to us. See graphic #3 below, as he points to his watch.
Time Dilation Graphic #3
Now, he says, imagine he leaves his position on the stage and starts walking. See Graphic #4 below.
Time Dilation Graphic #4
Einstein, Greene explains, told us that as he walks, he is diverting some of his previous motion through time into motion through space, which means that now he is moving through time less quickly, similar to the example with the cars. Think about it. How cool is that? Once again, for clarity: When he starts to move, walking across the stage, he is diverting his motion through time into motion through space, so his motion through time slows down.
Greene says this is the simplest example he knows to describe time dilation. I can’t argue with that! So, if you want to think about why a clock slows down when it is in motion, just think to yourself that when a clock is sitting still, all of its motion is through time, but when it starts moving through space, it diverts some of its motion through time into motion through space, so it passes through time more slowly. And there you have the concept of time dilation!
This is the first time I have felt that I understand this concept! Thank you, Brian Greene!
The WSU video can be seen here in its entirety: WSU: Special Relativity with Brian Greene.
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