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
Relativity is VERY hard to ELI5 but I will give you my best shot. The first thing you need to know is that NO MATTER WHAT light always moves at the same speed. If you are on a plane going 500 mph and you throw a baseball forward at 100 mph (because you are a major league pitcher) you will measure the baseball going 100 mph (because you are also on the plane) but a person on the ground will measure it going 600 mph (the 100 form the throw + the 500 from the plane). Light does not work this way. If you turn on a flashlight on the plane you will measure the light going C (the speed of light) and someone on the ground will also measure it going exactly C, not C + 500 mph (going into exactly WHY this is is more complicated). For low speeds of real planes this is nearly impossible to notice but at high speeds you can see some very strange things happening.
Now we will imagine a very fast, very long, plane. The plane will travel at half the speed of light and be so long that standing in the middle of it our light travels an entire second before hitting the front or back of the plane from where we are. We will repeat our experiment only this time we aim two flashlights, one forward to the front of the plane and one backward to the back of the plane. From our perspective, we turn on the lights and then two seconds later we see both lights reflected back at us at the same time, exactly what we expect. An observer outside the plane would see something completely different. They would first see us turn on both lights. Then 2/3 of a second after the start they see the light strike the back of the plane (because it has moved closer relative to the start point so the light which must move at C had to travel a shorter distance). Then 2 seconds after the start they would see the light bounce off of the front of the plane (because it had moved away relative to the start point so the light, again forced to travel at C, had to move a longer distance). Finally they would see the two beams returning to us at the same time 2 and 2/3 seconds after the start.
The question we now need to ask is who is right? Both you and the observer saw the same events, but they took different amounts of time and even happened in different orders. The answer is, surprisingly, both of you are right! Because of this very strange property (light ALWAYS moves at C no matter how you measure it) we have just discovered that time (and for that matter distance) is relative, each observer has an equally valid but different perception of time, which also means that if I say two things are simultaneous that may be true for me even if you observe them at not being simultaneous (as long as they do not happen at the same place).
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