Gravity in relativity.

**ledzeppie** · Jul1-09, 08:43 PM

So I'm 16 and just starting to get into relativity. When I learned that bodies of mass bend space time, that got me thinking. So I was wondering whether space-time is sort of like rubber. Can it be stretched? Or is it more rigid and pulled from the outside, like a trampoline. I know this might be a bit of a stupid question but I'm reading A brief history of time right now and my brains kind of overloaded with a lot of new ideas.

If it is stretched, this raises another question. When it comes to time dilation, is time slower when its closer to a large mass because space-time is being stretched more?

And why is it that we get pulled towards the earth (in terms of space time being warped). I understand gravity in orbits, but not in things such as throwing an apple in the air. Could anyone clarify this to me?

**Naty1** · Jul2-09, 08:51 AM

Hi ledzeppie: You'll find a lot of people here who also like science.

that's a great book...I read it several times when I first came across it....it's a great way to think about many strange things, each of which could form the basis of an entire career in physics.

Can it be stretched? Or is it more rigid and pulled from the outside, like a trampoline.

Sure. Nobody even knows what spacetime is but we know it's NOT fixed and rigid and unchanging. In general, physics describes many systems via mathematics but does not explain WHY things happen, nor does it yet explain the most basic questions such as What is time? What is space? What is mass? How are they related?

Until Einstein refused to accept that space and time are fixed everybody thought they were "immutable", that is, unchanging. Einstein had a hunch that the speed of light WAS fixed and from there he eventually realized space and time were not! In fact he first began thinking about what would happen if he could catch up to light...what would he observe??: he was 16, like you, and thought about that for ten years until he published his special theory of relativity. He finally decided nobody can ever catch it!!

**keepitmoving** · Jul2-09, 08:42 PM

isn`t the center of gravity a moving target in this case?

**A.T.** · Jul3-09, 01:31 PM

Originally Posted by ledzeppie

And why is it that we get pulled towards the earth (in terms of space time being warped). I understand gravity in orbits, but not in things such as throwing an apple in the air. Could anyone clarify this to me?

Have a look at the visualizations linked in this post:
http://www.physicsforums.com/showpos...7&postcount=21

**atyy** · Jul3-09, 11:07 PM

Spacetime is rigid, but curved (somewhat like the earth's surface). The curvature of spacetime is determined by matter.

**shawn77** · Jul4-09, 11:59 AM

the gravity of falling bodies is also easy to understand with general relativity.

imagine the rubber sheet that so many books give as a mental image. put the bowling ball in the middle, and roll the smaller ball around it. thats orbit with general relativity. but this time, dont roll it, just place it down and watch how it moves towards the bowling ball without any angular momentum. thats how falling bodies work with general relativity. kinda watered down, but its the same general idea.

**A.T.** · Jul4-09, 12:55 PM

Originally Posted by shawn77

the gravity of falling bodies is also easy to understand with general relativity.

Indeed, but not this way:

Originally Posted by shawn77

imagine the rubber sheet that so many books give as a mental image. put the bowling ball in the middle, and roll the smaller ball around it. thats orbit with general relativity.

No, it's not.

Originally Posted by shawn77

but this time, dont roll it, just place it down and watch how it moves towards the bowling ball without any angular momentum.

Why does the ball start to move into the depression? Because gravity is pulling it down? You are using gravity to explain gravity.

Originally Posted by shawn77

thats how falling bodies work with general relativity. kinda watered down, but its the same general idea.

No, it is not even close. Gravity in GR is about moving along geodesics in curved space-time, not rolling into depressions in curved space. Where is the time dimension in your model?

Please, read this:
http://www.relativitet.se/spacetime1.html
and this thread:
http://www.physicsforums.com/showthread.php?t=286926

**ledzeppie** · Jul4-09, 05:38 PM

Exactly. I want to understand it more than using a rubber sheet. Not only that but I want to understand it, not parrot the answers that I've heard.

**A.T.** · Jul4-09, 06:45 PM

Originally Posted by ledzeppie

When it comes to time dilation, is time slower when its closer to a large mass because space-time is being stretched more?

Yes, you can interpret it geometrically like this. But you also have to add the idea that everything advances in space-time at a constant speed. So if a space-time dimension (here time) is stretched, it takes longer to pass coordinates (here clock ticks).

Originally Posted by ledzeppie

And why is it that we get pulled towards the earth (in terms of space time being warped).

For exactly the same reason, as above: Closer to the mass, the time dimension is stretched more. But again you have to add a second idea: Free falling objects advance on straight paths trough space time.

You can see both effects explained in terms of stretched time in this sketch:
http://www.physics.ucla.edu/demoweb/...urved_time.gif

**ledzeppie** · Jul4-09, 07:21 PM

Ohhh kayyy, Thanks! I get that part now

1 more quick question that I'd rather not open a whole new thread for.
For relativistic mass, could e=mc^2 be used (as a simple explanation obviously) to explain it. For example, as E(kinetic) increases, the relative mass would have to increase to keep both sides of the equation equal. It just seems like a simple way for my mind to wrap around it, I just want to make sure I'm understanding it somewhat correctly.