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
I think the example in that passage isn’t great as other have said. So I’ll try to explain what is going on with the example that made it click for me!
The first thing we have to agree on is the equation that speed = distance divided by time. For example we measure speed as miles per hour, or meter per second etc.
We also have to agree that the speed of light is constant. No matter where you measure it, it’s always the same.
Let’s imagine a scenario where you have two mirrors facing each other on the roof and on the floor of the bus. We have some light bouncing up and down between them. Bert standing on the bus could measure how long it takes to bounce from the bottom mirror to the top mirror. Let’s say for example that it takes 1 second (it is much much faster, but let’s make the maths easy for ourselves!) and the mirrors are 1m apart.
Bert can see that the light then must be travelling at 1 meter per second!
Now let’s say Dana does the same measurement whilst the bus drives past her. For her the bus is moving 1 meter per second sideways. So when the light starts travelling from the bottom mirror, the whole thing will have moved by the time it reaches the top mirror. So from her perspective the light has actually travelled diagonally, 1m up and 1m left (a bit of Pythagoras gives the distance it travelled is the square root of 2, around 1.4m) so for Dana the light has moved 1.4m in 1 second.
However we agreed earlier that the speed of light is ALWAYS the same… So something in out equation speed = distance divided by time is wrong…
We know that the speed is fixed, we can also assume logically that the distance must be fixed between the mirrors. So this makes us thing that actually it’s the time that changed…
So how could this possibly work? If we say that actually time moves faster for Dana than for Bert because Bert is moving and we adjust how things are measured the maths works out.
So the very fact that Bert is moving compared to Dana makes times for him run slower compared to Dana. In day to day life the speed of light is much much faster so this effect is much much smaller and is practically unnoticeable. But when you start dealing with large scales and big speeds it becomes more that noticeable, for example early GPS systems didn’t account for this and very quickly went completely wrong as the satellites in space experienced time differently to us on earth. Every satellite since has to have clever programming to adjust their clocks to correct this.
ELI-Degree:
It actually gets slightly wilder than all of the above, because if we look at what speed (or really momentum is) it gets complicated, to make something speed up and move away from you, it requires energy. And it turns out it’s not so much the speed of something, but more the amount of energy (momentum) it has that affects how time changes. Einstein also gave us the equation E = MC^2 which tells us that energy and mass are actually the same thing and you can convert between the two. So that suggest that actually how much mass and energy something has determine how it experiences time.
So if you take light itself, which has no mass… It kind of doesn’t experience time at all? Everything that happens from the moment it’s created and then destroyed happens instantly…
Given that how much gravity something has is also directly affect by mass (and therefore energy) it turns out how things experience time is directly related to gravity. The nature of gravity is also quite complex and gravity is caused by mass essentially bending space like a ball on a stretched piece of rubber. So this means we end up with some very complicated maths that links together time, gravity and space and links of those and how they work and how they affect everything to mass (basically the more sweets I eat writing this the more I bend space, speed up time and and attract other large objects towards me!)
And let’s not get started on how magnets work…
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