How can two rocks hit the ground both at the same time and one after the other?

Question

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

Answer 1

We don’t actually need relativity to get this effect. You could even simulate it (with some effort) in real life, but with sound instead of light. The issue is that the information that something happened takes time to move, and someone closer gets the information first.

For sound you can even outrun the information (that is, going supersonic). Only when one adds that this is impossible with light (in vacuum) as it always seems to have the same speed for everyone, then we get relativity and all the weirdness it entails.

There is however one result from relativity that is not so obvious hidden behind this: there is no _objective_ time that says which of two things happened truly first; it _really_ depends on the observer. The only thing left is _causality_, which effect causes what others.