I can’t seem to be able to phrase my question in any simpler way.
Basically, the question refers to Einstein’s theory of relativity, and to an example used to illustrate one of its principles in the text “[Short Words to Explain Relativity](https://www.muppetlabs.com/~breadbox/txt/al.html)”.
I tried to paste the relevant fragment in its entirety, but the bot flagged it as speculative. So here’s a trimmed version I hope will pass the tests:
>We have Bert and Dana. Take a bus, and put Bert on the bus. The bus goes down the road. Dana, she sits here, on the side of the road. He’s in the bus and she’s on her ass. And now take a rock off of the moon, and let it fall at them. It hits the air and cuts in two. The two bits burn, and then land just as Bert and Dana are side by side. One hits the dirt up the road a ways, and one hits down the road a ways. **Dana sees each rock at the same time, but Bert sees one rock and then sees the next rock**.
(continued on the site)
The basic idea is that depending on the point of reference (stationary Dana vs. mobile Bert), the two rocks hit the ground either at the same time or one after the other.
I cannot for the love of me imagine how that would work. Call me naive, but something touching the ground at the same time should look the same to all observers, whether they’re moving or not. So, although I feel stupid asking you to explain something written “in words of four letters or less”, can anybody dumb it down even further?
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In: 143
So the basic idea goes like this.
Bert and Dana are next to each other, Dana on her ass and Bert rolling along. The rocks also hit the ground at this same moment. The rocks miraculously hit the ground at the same distance from the two.
Bert and Dana have NOT seen the rocks hit the ground. Not yet anyway. At this exact moment the light from the event has not reached Bert and Dana yet.
Now, exactly one nanosecond later (because why bother with math when it’s all hypotheticals?) the light reaches Dana. Somehow incredibly it takes exactly one nanosecond to travel the required distance to Dana. She sees both rocks hit because she is exactly in the middle of the two rocks.
But what about Bert?
In that one nanosecond, Bert has moved. He’s only moved the teensiest amount along the road. He has seen one rock hit because the light of the event has reached him. But he DOES NOT SEE the other rock hit because the light has not yet reached him. He has moved just the tiniest amount away from the event such that in that nanosecond, the light has not got to him yet.
In this manner, Bert sees one event but will not see the other event occur till later.
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