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Q & A: Some relativity questions

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Most recent answer: 05/29/2017
Q:
I have been reading a lot about special and general relativity recently, just the concepts obviously. I realize that Einstein's general theory of relativity about curved spacetime seem to be correct that gravity isn't a force, the pull of two objects is simply caused by the curvature of spacetime and they are both moving toward each other in the shortest distance possible. This curve in space is what causes planets to orbit and etc. Correct me if I'm wrong.Here goes my questions, I have four of them actually.First, since gravity isn't a force, then why do objects accelerate when getting closer to each other? For example, apple moving toward the earth. It doesn't quite make sense to me because there isn't an actual force acting on the objects.Another question is that if gravity isn't a force, then what's keeping us from floating away from the surface of earth? I know this sounds ridiculous, but what is the invisible "force" that stops up from doing that? Is it just that free moving objects will follow the curved space created by, in our case, earth? I tried to convince myself with the picture of a dent created by earth in a flat spacetime fabric, but it doesn't resonance with me because if there isn't a force, whats so hard about moving away from the dent?The last two are about black holes, I know nothing escapes a black hole beyond the event horizon, but in what way is it true? If I fall into a black hole while shooting a beam of laser away from the the singularity, why would it reach out of the even horizon? As I know, black holes don't suck, so the beam of light should eventually make its way out, right?Another confusion is about why objects experiencing stronger gravitational pull experience larger time dilation?Please be as specific as possible, I'm so stumped with these concepts.Thanks for all the help.
- Ian Lee (age 17)
San Diego, CA USA
A:

I'll try to answer those, not in order, but you may find some of the answers unsatisfactory.

"...why objects experiencing stronger gravitational pull experience larger time dilation?" Objects don't actually "experience" gravity. There's no feeling or other local change due to a uniform gravitational field, no matter how strong. The time dilation factor between two regions depends on which one is "down" in the gravitational field, and how far down.  You could have two planets so that somebody in between two would be at a zero-field point (in a conventional center-of-mass frame)  yet their clocks would run slower than those of somebody farther away who was uphill but still in a field.

For your first questions, yes, the geodesic (what passes for straight in this context) paths through spacetime of two objects, say you and the earth, can start off parallel and then approach each other at accelerating rates. That is, as you say, because their mass causes spacetime to be warped. Maybe this picture will help. Imagine a space that's the surface of a sphere, like the surface of the Earth but not imbedded in some bigger space. You and a friend start north from the equator, initially at a fixed distance, say 3 feet. Neither one of you bends or swerves but just continues on as straight a path as possible. You'll start getting closer and, when you reach the north pole, collide. Neither one of you will feel a sideways force at any time before the collision. Yet keeping track of your distance, it looks like one or both of you must have accelerated sideways. To keep from colliding, you have to push apart and experience sideways forces. I think that captures the basic idea behind what you're wondering about.

Mike W.


(published on 05/29/2017)

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