Understanding Gravity: Exploring Einstein's Theory of Curvature on Earth

[varsha]

curvature on the earth?

According to Einstein, gravity is not a force which pulls on things; rather, it is a curvature of space and time caused by the presence of a nearby massive object (like the Earth). When something comes along and moves past the massive object, it will appear to be pulled towards it, but in reality, it isn't being pulled at all. It is actually moving along the same straight line that it was moving along in empty space, but this straight line will now look like it is curved, due to gravity's warping of the underlying "space-time" continuum.

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" .
can you explain the truck part in this? i got confused.

 

[HallsofIvy]

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" .
can you explain the truck part in this? i got confused.

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

Imagine trying to measure the speed of a truck on a highway while driving in the lane next to it. The truck is driving a little faster than you are, so you see it creeping by - first it catches up with your rear wheel, then with your rear door. Suddenly, you decide to slam on the brakes. Instead of zooming by you, though, the truck continues to creep up - now it's in line with your front door. You hit the accelerator, and the truck doesn't fall behind - it continues to creep up past your front tire. Finally, you stop your car entirely and get out - still the truck creeps by.

It looks like the truck has been shadowing your every move, but then you compare notes with a friend who was driving in the third lane, on the other side of the truck. She thinks that the truck has been shadowing her, even though she was driving completely differently than you were - zooming along at the same time you were stopped, slowing down at the same time you were accelerating! Seems impossible? It might, but the Michelson-Morley experiment proved that if trucks behaved like light beams, this is exactly what they would do.

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.

 

[russ_watters]

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...

 

[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.

 

[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.