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

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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

20 Answers

Anonymous 0 Comments

Because “simultaneous” depends on your speed and velocity direction. This is hard to get your head around and it took the genius of Einstein to see it.

It is also not emphasized enough in most explanations and completely ruins space opera.

Anonymous 0 Comments

Does it have something to do with the way we “see”?
Just like how the suns light take 7 minutes to reach earth, so we always see the sun how it was 7 mins ago.

Anonymous 0 Comments

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.

Anonymous 0 Comments

Does it help to compare the situation with the speed of sound? If you have a bomb going off a mile to the south and another bomb a mile to the north, you could potentially hear them going off at the same time while someone two miles to the north of you will hear two separate bangs. Then consider for light, two stars exploding at the same time for an observer between them but at different times for someone on a different planet where the light from each travels different distances. This isn’t relativity, this is just speed of sound and speed of light. The relativity comes in for the case where movement is happening, which was Einstein’s big breakthrough, but you can have different events appearing to happen at different times even without Einstein’s relativity.

Anonymous 0 Comments

Essentially, it’s a matter of perspective. The Theory of Relativity tells us that how we see events is relative to our position at the time of the events. If your considering something at the speed of light, you need truly huge distances to have a noticeable difference.

Imagine thunder and lightning. If the lightning is several miles away, you’ll see a flash of light, then hear the thunder. To your perspectively, they did not happen at the same time. Whilst this is happening, I’m inside the storm cloud. I see the lightning, and hear the thunder at the same time. To me, it appears that they happened at the same time.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

Helpful question: when you look at the sun, do you see it as it is now, or how it was some time ago?

Anonymous 0 Comments

It all starts with knowing that the speed of light is always the same relative to you. This is different from all other speeds you experience in daily life.

Now imagine you are riding in a moving train car that has a lamp in the exact middle of the car. The lamp flashes on and off, and the light from the lamp hits the back and front of the train car at exactly the same time.

Now imagine your friend is sitting outside by the railroad tracks watching the train pass and he sees the lamp flash. To him, as he faces the train car. the light moves the same speed toward the right (where the front of the car is) as it does toward the left (where the back of the train car is. But of course the train is moving left to right. So your friend sees the left moving light collide with the back of the train care before the right moving light reaches the front of the train car. Thus you and your friend can’t agree on whether the light hit both front and back at exactly the same time or whether it hit the back first and then the front.

So you guys decide to do an experiment. You rig the train car’s front and back to drop a paint bucket on the train track when hit by the light. Then you can measure how far apart the buckets are. It turns out that relativity doesn’t just bend time, it also bends space. So your friend saw a much shorter train car passing by him than the train car you experienced while riding in it.

It’s really difficult to get an intuitive feel for it, but the math all works out.

One common objection is that if events happen in a different order depending on your perspective, then what if you drop a red paper and a blue paper? Which ends up on top if one person sees the red dropped first and the other person sees the blue dropped first? The key to that kind of problem is that how different the time perspectives of the events are depends a lot on how far apart in space the events are. And when you bring the objects back together so that one paper can be on top of the other, there is no longer any disagreement about which paper does what first. When two events happen in the exact same place, everyone agrees on which happened first.

Anonymous 0 Comments

To those talking about made up FTL travel in SciFi – I recently watched The Orville, and well- Spoilers – >!They travel back in time but their time device breaks. To get back to the future, they turn off the “quantum shield” of their FTL drive (which protects the ship from the effects of relativity) and travel at 99.9999% the speed of light to a star 200 lightyears away and back (for a total of 400). Because of time dilation, they experience less time on the ship and effectively travel to the future. In the show they say this only takes “a few minutes.” So as you can see, Sci Fi can just make up some words and devices to make all things possible!!<