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?
​
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).
It has been explained that because one person is moving and one is not, and because it takes time for light to travel a distance, therefore there is a difference in what the two observers see.
But it’s intended as a way to get you to think about the conundrum. It’s not an experiment that could be put into practice because we have no way of measuring the events with anything near the accuracy to prove it. Not with human observers involved. There is no way that Dana and Bert could perceive any difference on that small of a time scale.
Some kind of equivalent electronic setup could perform the experiment and produce the results. But that’s harder for people to relate to, which is where these kinds of thought experiments come in.
Maybe I’m misunderstanding your misunderstanding, but I used to have a similar issue with maths where I would get hung up on concepts being descriptions of stuff, rather than actually being stuff.
The comments here don’t seem to be doing a really great job at simplifying this. All this appears to be is a modified version of the “two lightning bolts on a train” thought experiment.
In short, Einstein realized that time is not a universal constant. Each person has their own clock and we do not observe the passage of time between each other the same way. A consequence of this is that we do not agree on the simultaneous nature of events.
If you’re standing motionless in the middle of a train platform and two bolts of lightning strike on either side of you then you might say that both strikes of lightning happened at the exact same time. To the person aboard a moving train they would say that one bolt of lightning – the bolt in the direction they’re moving towards – struck *before* the bolt on the other side of the platform – in the direction they’re moving away from. Even after you factor in the travel time of light both observers would still not agree whether each bolt of lightning struck at the same time or at different times. For one person two events happened at the same time whereas they happened at two completely different times for another.
If that seems odd think about relatively in a more familiar concept. If I’m standing on a train platform and there is a cat sitting next to me then from my perspective the cat is not moving – the distance between us never changes. For the person aboard the train the cat *is* moving – the distance between them *does* change. This misalignment between our spatial perspectives – is the cat moving or is it not moving – happens with time too. We don’t agree on whether the cat is moving or not, just like we don’t agree on whether the bolts of lightning struck at the same time or not. The only difference is that the latter doesn’t become noticeable until we’re moving at extreme speeds so it doesn’t seem intuitive for our every day experiences.
This video explains it decently I believe:
I can try to explain, but from the lady’s stationary view point, the light is hitting her eyes from both rocks at the exact same time
For the guy, he’s moving away from the light and so it takes longer to reach him, therefore even though they’re in the same physical 3D space, the information (light) of the second rock hasn’t reached him but has reached her
– one after the other
but since for her, who is not moving away from the light, she is stationary, she sees both rocks’ lights at the same time
– the same time
so there you go
Dana is right in the middle so the light from each rock impacting reaches her at the same time. Bert is moving towards one rock and away from the other, so as the light is moving towards him he is getting (a very tiny bit) closer to one rock than the other so he sees that one first.
It’s not a noticeable amount by any stretch. But it’s to illustrate a point. Another way to think about it is how when lightning strikes people near hear it before people farther away
Time moves measurably more ‘slowly’ in a gravity well than out of one. Say you drop a pocket watch into a black hole (very durable and stylish). From the watch’s perspective, it keeps ticking normally. You, having lost your watch, but outside that gravity well, will grow old and die and turn to dust before it ticks again from *your* frame of reference.
If you’re really interested in learning more about this topic, Bryan Greene has an incredible lecture on this very question. He also has a way of explaining complex topics in an easy to understand manner. I was confused, as you are for a long time, but watching his videos really helped my understanding
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…
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.
Latest Answers