# Little help, please?

I have never in my life taken a physics course, so this may be obvious to other people who have. Parts of this article explaining relativity using only words with four or fewer words make sense to me and parts don’t. The stuff near the end is really hard for me to understand, but I figure it’s hard for most people to understand. What frustrates me though is the bit about Bert and Dana (when Bert is on the bus). I don’t understand the declaration that, since he’s moving, Bert would see the rocks come down at different times if they appeared to come down at the same time from Dana’s perspective. Maybe it’s true, but I have zero real-world experience of anything like that so I don’t get it. Does that really happen? Is there a way to experience it?

## 7 thoughts on “Little help, please?”

1. Deana says:

After a bit more study, I know that the distance between Dana and Bert means the light from one meteorite takes slightly longer to get to one of them than the light from the other. But why is movement important?

2. Deb says:

I fixed the link, no worries. I’m going to read the problem and see if I can help. I’ll bet Chris would be better though . . .

3. Deana says:

Thanks, Deb!

4. I’ve looked for a good way to make this plain, but it’s a strange one on all sides. Bert goes toward the light from one rock, so he meets it on the way there. He runs from one rock, so its light takes more time to catch him.

Bert looks at just the length between him and each rock, since he thinks he’s still. He does math to get how long the light should take to get to him, and finds that one rock hit first.

Dana looks at both the length and how fast Bert runs, since she sees him move. Her math doesn’t match Bert’s math, but says that they both came down at once.

The trick is that the math for the speed of light stays the same while the math for the speed of Bert does not. Since they don’t match, some part has to give to make them do so. Al said that the parts that give are space or time, thus the big deal.

(Dang, to say that with short words is hard but fun.)

5. (Okay, stepping away from the land of one-syllable words…)

In other words, the fact that Bert’s moving makes a difference in the equations he uses to determine how long each light pulse took to reach him from their starting points, but it does *not* make a difference in determining the speed of light. (This particular pickle came directly from experiments showing that the speed of light doesn’t change based on motion. The math would be tons easier if that wasn’t true, but oh well.)

So, since you can’t use a changing speed of light in order to make the equations match, something else has to change. Einstein’s brilliant idea was that distance and time could be changed instead, as long as they were changed together in a very specific way. The two together provide enough variation to match the difference in Bert’s speed and the speed of light, but the side effects are pretty crazy. Space shrinks in the direction of Bert’s motion, time becomes so skewed that events can’t be said to happen at the same time any more, and the effects get more pronounced the faster Bert goes.

Hope that helps. It’s one of those things that matches our everyday experience so poorly that it’s tough to actually picture at all. You end up going through the tunnel of equations to try to predict something you can actually picture (like everything going red-tinged or stars moving from their places in the sky) and then see if that actually happens.

6. deb says:

See, the big words make more sense to me. Thanks for the option. :)

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