Can you explain how space-time gravity works? I don't think it does.

[cytokinesis]

Originally posted by Cyberice
Ok, I have the 4d space-time dip down, but am still struggling with accelaration and how it "pulls" you to it's center.

Pretend you're accelerating in your car, and the road curves. You turn with the road, right? It's not really the accelerating that's pulling you, it's that while you're accelerating, you're accelerating through a curved portion of spacetime towards the centre of the earth. It is impossible to show a mechanical example because we cannot manipulate the 4th dimension at this point in time. Our brains are stuck on 3d mode and we cannot visualize the 4th dimension.

How can it pull you once you have left its surface?

Well since the Earth has a great enough mass to create a large disturbance in spacetime, even if we can't see it, were still accelerating towards it in the 4th dimension.

And if (when you are touching the ground) what you said is true about the equal and opposite reaction of us also pushing on the earth, wouldn't it cancel out the gravity since there is an equal upward and downward force?

In a sense it does cancel out. Because from your frame of reference, you don't feel like you're moving at all, right? You're just standing there. This was the puzzlement that led Einstein to his special theory. The question of whether or not there is such a thing as absolute motion, or absolute nonmotion.

...I think

 

[schwarzchildradius]

the Force is caused by a change in momentum - Newton's 2nd law : F = dP/dt
You are exchanging momentum with the Earth.
so let me rephrase your question, if I could:
"what property does mass have that causes space-time to bend around it?"

 

[meister]

Originally posted by Chi Meson
Where will we be flung if this Einstein idea hits the brakes?

Is this GR saying there is no gravitational force or is this classical physics?

 

[Cyberice]

"Pretend you're accelerating in your car, and the road curves. You turn with the road, right? It's not really the accelerating that's pulling you, it's that while you're accelerating, you're accelerating through a curved portion of spacetime towards the centre of the earth. It is impossible to show a mechanical example because we cannot manipulate the 4th dimension at this point in time. Our brains are stuck on 3d mode and we cannot visualize the 4th dimension."


Oh, so it's NOT accelerating in the 3rd dimention? If it's in the 4th dimention that would be a good reason why I didn't get it. You see, for some reason analogies stick w/ me and give me wrong impressions because they never give me the REAL repesentaions behind the analogy (like how the wave can be miss interpreted to only move up and down - when its only a representation). So did I ge it right THIS time?

 

[cytokinesis]

I think so. You just have to learn to visualize things in a way that might make sense. Like the rubber sheet and ball. The rubber sheet exists in 2 dimensions, so do the people affected by the ball, but the ball exists in 3. This is the same as the volume of stuff in 3 dimensions existing in the same 3 as we do, and the gravitational pull of the Earth existing in the 4th. You can't take the analogies so seriously BECAUSE all they are are analogies. They are used by theorists because it is impossible for our minds to comprehend the 4th dimension, and if we can relate the concept to soemthing we know already, it allows for a better understanding. The main point is, just try not to take the examples too seriously.

 

[cytokinesis]

Originally posted by meister
Is this GR saying there is no gravitational force or is this classical physics?

What do you mean by that? What is the 'this' to which you are referring?

 

[LURCH]

On my way here, I stopped by Cyto's thread over in "Theory Development". These two topics are very closely related, so each of you should probably watch the other's thread for possible insights into your own.

Regarding gravity as "acceleration"; perhaps the best way toward clear understanding is to take what you now know about the curvature being in the fourth dimension, and apply it to a principal which I'm sure you have heard of numerous times, that the fourth dimension is time. From this perspective, it can be said that your passage through time is what's being curved by gravity.

So, as you progress toward the future, the Earth's mass curves your path. Applying relativity to look at the situation from your perspective, your path appears straight and it is the Earth that is curving. The Earth's (from your perspective) curved trajectory across time causes it to maintain contact with your feet, continuously pushing you off of your path. This ongoing deflection from the path you would take is the acceleration that you feel as gravity.

 

[jcsd]

Graviataional fields curve all dimensions of space-time, spatial and temporal.

 

[meister]

Originally posted by cytokinesis
What do you mean by that? What is the 'this' to which you are referring?

I was referring to this whole post:

Originally posted by Chi Meson
You said:

"since you were inside of the elvator, when it accelarated the force pushed you against the floor to simulate gravity."

And there is your error in understanding. THere is no force at all pushing you against the floor. IF your book says that there is, throw it away.

THe force on you, is the floor pushing "up" on you.

Ohhhh, is he just talking about space where the force of the Earth's gravity can be neglected for the most part?

 

[LURCH]

Originally posted by meister


Ohhhh, is he just talking about space where the force of the Earth's gravity can be neglected for the most part?

Yes, the events in the elevator take place in some remote part of space where no noticable gravitational field exists. In such a location, you should float around in the elevator. But, if the elevator begins top move, you are held to the floor. As long as it continues to accelerate, you remain on the floor (in fact, the wall of the elevator you call "the floor" would be entirely defined by which way it accelerates). If there are no windows, no way of getting an outside refference with which to compare yourself, the difference between accelerating through empty space and sitting still on the ground on Earth is... NO difference at all!

 

[schwarzchildradius]

so the vector for the acceleration of gravity at the Earth's surface would be- straight up.

 

[pmb]

First a definition.

Inertial Force - The force on a given object due which results soley on observing the motion from a non-inertial frame of referance. Any force which is proportional to mass is regarded as an inertial force.

Originally posted by Cyberice

Anyway, Einstein supposedly proved that there was no force of gravity, that it could be subsituted with accelaration, and that gravity is made by bent space time. I have quite a few (semmingly) errors in which I may have just misinterpreted but desire to point out.

That's not quite correct. Einstein himself never said gravity was not a force. What he showed was that gravity and inertial forces are identical in nature. In Newtonian mechanics one might be tempted to think that inertial forces are not "real" forces. However Einstein saw things differently. To him, and thus his theory, inertial forces are "real."

The first is the gravity/accelaration annalogy of Einstien in a rotating elevator in space. The analogy is fine and makes sense. It states that if you were to be in a constantly accelarated (rotated) elevator in space it would pull you to the floor of the elevator and would simulate gravity, and if there were no windows you could not tell the difference if you were in an elevator on the earth.

Einstein's equivalence principle is stated as follows

A uniform gravitational field is equivalent to a uniformly accelerating frame of referance.

The rotating frame is not a uniformly accelerating frame of referance. You *can* tell you're in a rotating frame. Suppose you're standing on the surface of the Earth. If you stand up straight and drop a stone then the stone will fall straight down and land by your feet. If you're inside the rotating frame and are standing straight up and drop a stone thent the stone will *not* land by your feet. It will be deflected. This is the coriolis force.

And even if you're standing on the surface of the Earth you can, in principle, tell if you're in a gravitational field or an accelerating frame of referance in the absence of gravity.
The gravitational field of the Earth has tidal forces. No such forces are present in the absence of gravity.

And gravity is not a curvature in spacetime - Gravitational tidal forces are. I.e. gravitational tidal forces and spacetime curvature are one in the same thing.

But in general you don't have to have tidal forces to have a gravitational field. In fact the equivalence principle is based on this fact - that a uniform gravitational field has no tidal force an therefore there is no spacetime curvature. And there's no reason why there can't be uniform gravitational fields someplace.

Consider a spherical body which has a uniform mass density. If you hollow out a spherical cavity - the center of which is *off set* from the center of the body, then inside the cavity there will be a uniform gravitational field and thus no spacetime curvature.

Now suppose you're standing up in a uniform gravitational field. Then you are *at rest* in the field. That means that the total force acting on you is zero. The forces acting on you are the force of he floor pushing you up and the force of gravity pulling you down.

Pete

Pete