# Curvature on the earth?

1. Feb 27, 2006

### varsha

curvature on the earth??????????

caused by the presence of a nearby massive object (like the Earth). but how does gravity act upon us? i got the idea of how it does in space. but earth? if it's a curvature, then what's keeping us on the ground. surely, there's no curvature on earth! can you explain this?
have a look at http://curious.astro.cornell.edu/relativity.php#links" [Broken].
can you explain the truck part in this? i got confused.

Last edited by a moderator: May 2, 2017
2. Feb 27, 2006

### HallsofIvy

Why do you say "there's no curvature on earth"? Surely the curvature of space is especially great around a massive body like the earth.

And please don't confuse "space" in this sense with "outerspace" in the astronomical sense. We are as much in space standing on the earth as if we were half way between Rigel and Antares!

As far as the truck in
The point is that light beams do NOT act like normal moving objects (such as a truck). The Michelson-Morley experiment showed that light behaves in the peculiar fashion the hypothetical truck is imagined behaving in.

Last edited by a moderator: May 2, 2017
3. Feb 27, 2006

### Staff: Mentor

I've never seen the truck analogy before - it looks like a good one. All it does is take the behavior of light and apply it to something that travels much, much slower. So if light traveled at only a few mph, that's how it would look.

And don't confuse that curvature with what your eyes see in normal 3d....

4. Feb 27, 2006

### Ich

I don´t like the analogy. The speed of light is the fastest possible speed, and this truck´s relative velocity seems to be a lot slower than the relative velocity of the two observers. This could be (and obviously is) confusing.
In the analogy, the truck could be slower than the third car, and this leads definitely to a wrong "mental image" of relativity.

5. Feb 27, 2006

### dicerandom

Have you ever jumped off of a swing? It's an interesting sensation, if you swing high enough and time it just right you travel in a parabola, yet you can feel no force acting on you.

How can an object travel along a curved path without any forces acting on it? Either the space is curved or the observer who wrote down the curved path is accelerating.