RIP Philo

### Something about curved space bugs me

So mass is supposed to distort space.  This is what causes objects to curve into them even though they're on a "straight" trajectory.

If gravity is not so much a force as it is a curvature of space, why do objects "at rest" still fall into masses?  If you lay motionless on a mountain and there's no force acting on you, you should stay in one spot, not slide down the slope.

Basically, if I moved an inch off the ground, why can't I stay there?  Is it because while I lift myself an inch off the ground on earth, I'm actually, from the POV of an observer on the moon, hurtling around the earth at ~24,000mph and this motion is what causes me to "fall" back into the planet?

If you were riding a shuttle in orbit around the earth, would you feel like you were making a sharp perpetual right turn, or would you feel that you were flying perfectly straight?
Michael B
March 9th, 2007 12:23am
Cool, is Michael our new academic dork.  We lost sharkfish; I guess you will do.
Bot Berlin
March 9th, 2007 12:25am
Gravity curved 4-dimensional space-time.  In 4-space, even if you're standing still in space, you're still moving "forward" on the t axis.

The nice picture of a rubber sheet w/ a grid on it being distorted by a ball only goes so far.
Ward
March 9th, 2007 12:56am
So in space-time, you're always moving, and you always tend to follow the shortest path - aka geodesic.  As far as you can tell, space is nice and rectalinear, but since it's really space-time, and it's curved, the shortest path isn't always where you'd think it is.
Ward
March 9th, 2007 1:06am
The rubber sheet distorted by ball analogy goes quite far, really. For example, if you place a small ball on the sheet it will roll down into the depression until it hits the big ball. Just like gravity. If you lay motionless on a mountain, you will of course tend to slide down it. Imagine if there was a slippery slide on the mountain side and you lied on that. Just try and stop yourself.

Gravity *is* a force acting on you. You can believe clever scientific pronouncements, or you can believe reality. Reality always wins.

On the other question: if you were riding a shuttle in orbit round the earth, you would be completely weightless. A shuttle in orbit is a zero gravity environment. (This one is not explained by the rubber sheet analogy though.)
bon vivant
March 9th, 2007 1:08am
Not by the rubber sheet, but the orbit is explained by the curvature of space-time - the circular (almost) orbit observed in space is because the shuttle is "falling" around the distorted space.  The feeling of weightlessness is because everything in the shuttle is falling at the same speed.
Ward
March 9th, 2007 1:21am
But you don't need curvature of space-time to explain gravity and orbits. Everything works just fine with classical Newtonian mechanics. Bringing relativity into it is just a way of frying your brain.
bon vivant
March 9th, 2007 1:27am
The precession of Mercury's orbit doesn't work w/ Newtonian gravity.
Ward
March 9th, 2007 1:44am
No, but space shuttle orbits, weightlessness and falling down mountains do. :-)

The discrepancy in Mercury's orbit is a very tiny effect. I doubt an average person could ever discover it with a telescope in their back yard.
bon vivant
March 9th, 2007 1:59am
The average person doesn't have a telescope.

The average person doesn't do science.

Science is done by obsessive compulsive detail oriented anal freaks.
Aaron F Stanton
March 9th, 2007 2:45am
Aaron: be sure to check out  http://www.bralessblog.com/

(assuming it's not a site you check every day...)
Ward
March 9th, 2007 3:01am
A fun way of looking at it is with a cup of java.

If you stir it with a spoon, you get bubbles and they attract eachother as if there was "gravity" affecting them.

What happens is that nature tries to reduce the tension on the surface to as few points as possible, and therefor it shuffles the bubbles into groups.

Some insects can walk on water because of this.

Nature doesn't like empty spaces or unbalance, and since the distribution of the tension is wacked, the bubbles develop "gravity" and re-organize until the tension is as low as can be, and as few points as can be.
Mikael Bergkvist
March 9th, 2007 4:30am
> A fun way of looking at it is with a cup of java.
> If you stir it with a spoon, you get bubbles and they
> attract eachother as if there was "gravity" affecting them.

That is the most elegant statement with the word 'java'
in it that I have ever heard.
Michael B
March 9th, 2007 9:03am
> So in space-time, you're always moving, and you
> always tend to follow the shortest path - aka geodesic.
> As far as you can tell, space is nice and rectalinear,
> but since it's really space-time, and it's curved,
> the shortest path isn't always where you'd think it is.

Time!  Of course!

(geodesic is one of those words that you go your whole life without hearing, then see it a thousand times when you ask a funny question.)
Michael B
March 9th, 2007 9:07am
>  We lost sharkfish; I guess you will do.

Yeah, sorry, I put out eventually.
Michael B
March 9th, 2007 9:11am
Maybe it's kinda like putting a bowling ball a foot away from a golf ball on a mattress. Unless it is one of those temperapedic deals, they are gonna go together.