Eli5: How does gravity affect time, dumb it down for me as much as possible
In: 286
TL;DR Gravity pulls EVERYTHING, and Time is a thing.
Sun’s gravity pulls Earth, Earth’s gravity pulls Moon, Moon’s gravity pulls ocean, you know this already.
Gravity also pulls the space BETWEEN stuff, and because space and time are connected just like anything else in the universe, that means it also pulls time. Let’s use an analogy (and yeah, this isn’t really how it actually works, but this is ELI5, not askScience):
Draw two Xs on a trampoline, we’ll call those points “start” and “end”. Now find the shortest path between them, we’ll call that the “race track”. Measure the length of the race track (which is a straight line), let’s call that the “race length”.
The speed of light is a constant, no matter what. It doesn’t matter what you do to light, light will ALWAYS go the same amount of fast if it’s traveling through the same stuff.
Speed = distance / time, you also know this. Let’s call that formula the “fast formula”.
Let’s make light run a race along our race track from Start to End. It’s going to take time – not a lot of time, but time.
Now put a bowling ball in the middle of the trampoline. Start and End didn’t move, because we didn’t erase them and the draw them again: they’re still exactly where they were, but the space they were on is getting pulled by the bowling ball now. Measure the race length along the trampoline’s surface again. It’s a little bit longer now, because even though we could draw a straight line between Start and End, that line wouldn’t be touching the trampoline anymore (it would be going through a different dimension). We can’t go that way (or if we can, we haven’t figured out how yet), we have to stay on the trampoline.
Let’s make light run the race again. It’s still going from Start to End, and it’s running along the same race track. How long will it take now?
Remember, light is ALWAYS the same amount of fast. That means that light’s speed without the bowling ball is the same as light’s speed with the bowling ball.
So, speed (no ball) = speed (yes ball).
Remember our fast formula: Speed (no ball) = distance (no ball) / time (no ball). Easy math, you know how to do this.
Let’s look at the fast formula for the second race.
Speed (yes ball) = distance (yes ball) / time (yes ball).
We know that the speed from each race is the same and that the race length got bigger when we added the ball, so let’s rewrite the fast formula.
Speed (no ball) = Speed (yes ball) = “distance (no ball) + gravity pull” / time (yes ball)
If the top of the fraction got bigger and the result stayed the same, that means the bottom of the fraction had to also get bigger, right? Let’s rewrite the fast formula again.
Speed = “distance + gravity pull” / “time + something”
We know that something had to make the bottom bigger, but the only “something” we changed between the two races was adding the bowling ball. We didn’t change the stuff light was running through to get from Start to Finish, we just pulled the race track a little and made it longer.
Let’s assume that gravity DOESN’T affect time. If the ball’s gravity didn’t pull on time, the speed of light would have changed, because it ran a longer race in the same time. But that can’t happen because light is always the same amount of fast, so the ball’s gravity must have also pulled on time.
Speed = “distance + gravity pull” : “time + gravity pull”
Latest Answers