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

This is a bad example, while true in a technical sense it’d be imperceptible to a human observer I would think so it makes something which is already unintuitive harder to comprehend.

The important point to grasp is that there is no one ‘time’. It’s all relative based on the reference frame of the observer – for ‘human’ speeds in day to day life this doesn’t do anything but if you are moving very fast then it becomes a measurable difference.

For real world effects where it matters, a common example is GPS. Due to differences in relative speed and different gravity time dilation needing to be adjusted for for GPS navigation.