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
Imagine you’re driving a car at 80 mph and turn the headlights on. The light emitted by the headlights are going the speed of light relative to the car.
Imagine youre on the side of the road watching the same car go past. Is the light coming from the headlights going the speed of light plus 80 mph?
The answer is no. If something is moving in a direction relative to you, then it actually compresses slightly such that the distance it covers per unit time will be less. The compression is based on how fast the object is traveling. The math works out such that light always goes the speed of light, no matter how you are moving relative to the light source.
Equivalently, when an object moves relative to an observer, then the observer sees the past of the front of the object and the future of the back of the object. This is what causes the compression – the object is effectively time traveling along its length, and in the future, the back half is further along than the front so it appears shorter.
If I had a 15 foot garage and a 20 foot truck, and I drove the truck really fucking fast, an observer standing still would see my truck fit in the shed. Meanwhile, because from the drivers point of view the truck is stationary and the garage is approaching it, the garage would be even shorter and the truck would extra-not fit.
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