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

Cyberice
Ok I'm new here and I'm trying to get on my feet so I hope that this is the right forum to post this in.

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.

I will use two very popular annalogies used by Einstien (or rather by my physics book adapting them) on how grvity works.

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. Well easily explained, but if the floor were droped out we would fling out (obviosly). You are thinking "where is he going w/ this?" Well I'm getting there. See, since you were inside of the elvator, when it accelarated the force pushed you against the floor to simulate gravity. Now the point, We are ON TOP of the earth. How in the world, if this analogy is right, are we not flung off of the surface of the Earth at 1,000 something miles an hour off the earth? It would be if we were standing up-side-down (outside) on the elevator in our analogy. It would fling us off in a heart beat. It would not pull us toward its point of rotation, and we'd even have to use glue to put on our feet to stay on the outside (underside) of the elevator floor.

I'll continue my second analogy/arguments on my next post to save room.

Cyberice
Um... crap I have to go right now. I'll finish my posts in half an hour. In the mean time, if you can help please do.

Homework Helper
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. This is the same as when you acceleratin in a car. It feels like a force is pushing you into the seat, when in fact it is the seat that pushes against you. THese are called "inertial effects" or "pseudo forces." THe erroneous "centrifugal force" is another example.

So, this strange acceleration predicted by Einstein is in some unpointable dimension but we are left with the "inertial effect" of the Earth's surface pushing against us.

We don't get flung into space for the same reason that you do not get flung from your car while you are speeding up.

Where will we be flung if this Einstein idea hits the brakes?

Cyberice
Ok I'm back. Now to my second analogy.

The second analogy is of matter(mass) in space-time (4th dimention) that will create a dip or dent around it in space-time. Another well known analogy of why the planets revolve around the sun (since Einstien said there was no force of gravity) can be demonstrated by having a rubber sheet (that represents space-time) suspended off the ground (any amount so it is not touching the ground) and to lay a metal ball(sun) in the middle. The ruber sheet will bend around the ball and form sort of a funnel shape with the ball making a dip in the center. The planets (according to the analogy) rotate around this funnel (but don't move down it because of lack of friction). Pluto would be near the top of the funnel the bigger (thus longer) revolution around the sun, where mercury would be near the bottom making faster smaller revolutions. And suposedly the planets make their own little "sub-dips" in the big dip (sun's dip). Well that's fine too, but the sheet goes UNDER the object in this analogy, not in the center (where gravities "pull" is). It would work if we (say north america) were on the top of the ball(earth) because the dip would be directly below us. But for for the people in China (according to how the annalogy was illiustrated) would fall towards the sky because they would be on the under side of the ball (and the dip is under the ball). For a correct analogy the dip would somhow have to be in the center, and (again somehow)point in all 360 degrees to pull all things to the center. So perhaps 360 space time dips are facing in all 360 directions while in the center of the earth?! What?! NO WAY. That doesn't make sense either

Cyberice
THe force on you, is the floor pushing "up" on you. This is the same as when you acceleratin in a car. It feels like a force is pushing you into the seat, when in fact it is the seat that pushes against you. THese are called "inertial effects" or "pseudo forces." THe erroneous "centrifugal force" is another example.

Ok, I have to learn to be more "technical" with my words in physics when dealing with force.

Ok, now, I believe you that it pushes "up" on you, BUT if it pushes up on you what keeps you "down"? What keeps you from hitting the "top" of the elevator? Thats what I want to know, because the "down" force that keeps you from going "up" would be gravity. I'm trying to understand Einstiens point of veiw on gravity and not grasping it.

I'd much rather choose the "graviton's" veiw of gravity. But I don't know much about gravitons. How many are emitted by Earth (roughly) and do our gravitons (if we have any) attract the Earth gravitons to keep us on the ground? I read in some article that we emmit a graiton and Earth emmits one and when they intersect they "pull" (or push if you prefer) against each other which in turn forces you to the ground. Again i believe that I might have run into *another* bad analogy. So please help me figure out the "elevator" example, the "dip-funnel" in space-time example, and "graviton" example and put them all together (if possible).

It may be hard but please try, because I believe that I have fallen victim to bad teaching. Help if you can please. Thnx.

Originally posted by Cyberice
Ok I'm back. Now to my second analogy.

The second analogy is of matter(mass) in space-time (4th dimention) that will create a dip or dent around it in space-time. Another well known analogy of why the planets revolve around the sun (since Einstien said there was no force of gravity) can be demonstrated by having a rubber sheet (that represents space-time) suspended off the ground (any amount so it is not touching the ground) and to lay a metal ball(sun) in the middle. The ruber sheet will bend around the ball and form sort of a funnel shape with the ball making a dip in the center. The planets (according to the analogy) rotate around this funnel (but don't move down it because of lack of friction). Pluto would be near the top of the funnel the bigger (thus longer) revolution around the sun, where mercury would be near the bottom making faster smaller revolutions. And suposedly the planets make their own little "sub-dips" in the big dip (sun's dip). Well that's fine too, but the sheet goes UNDER the object in this analogy, not in the center (where gravities "pull" is). It would work if we (say north america) were on the top of the ball(earth) because the dip would be directly below us. But for for the people in China (according to how the annalogy was illiustrated) would fall towards the sky because they would be on the under side of the ball (and the dip is under the ball). For a correct analogy the dip would somhow have to be in the center, and (again somehow)point in all 360 degrees to pull all things to the center. So perhaps 360 space time dips are facing in all 360 directions while in the center of the earth?! What?! NO WAY. That doesn't make sense either

OK, learning to accept the fourth dimension is tough, no question. Steven Hawking once wrote," now if you can picture a four-dimensional curve...then you're not from this planet". But the important factor in grasping the rubber sheet analogy is to remember that it is only an analogy. Specifically, a 3-D illustration of a 4-D event.

This, I suspect, is where you run into the problem of people on the opposite side of the globe falling upward. Because their "upward" is the same direction in which the rubber sheet is curved. But the rubber sheet is only a two-dimensional representation of three-dimensional space. When left unperturbed, the sheet is flat. A marble rolled across the sheet may travel forward or back; or side-to-side. When a larger, heavier ball is introduced, the sheet stretches in a new direction, one which is 90o from the two directions already mentioned. A marble trying to roll near this ball will travel a path that is (to the left or right);or (in front of or behind) the ball. But the curvature of the sheet in this third direct (up-and-down) will cause the path of a marble to be altered.

General relativity basically claims that you can add one dimension to this rubber sheet and it will represent space. Place a heavy object in it, and spaced bends in a fourth direction which is at 19o to the three directions with which we are most familiar. Any object trying to pass nearby this "heavy object" may travel a path that is (to the left or right); (in front of war behind); or (above or below). But their trajectory in any of these three directions will be altered by the curvature of space in the fourth direction. So, just as the rubber sheet has only two dimensions while space has three, the dent in the sheet has only 3 dimensions while gravity has 4.

Last edited:
Cyberice
Ah, ok. I grasped most of what you said but if we tried to analagigize the 4th dimention veiw of the 3 dimention representation would it go around the ball or to through center? I mean which way is it pulling and how does it "balance"? Like you sain the 4th dimention is hard to grasp but try to make a picture/example. And what about gravitons (gravity waves as someone called it in another thread)? Do they play any role?

Einstien's Elevator

"Ok, now, I believe you that it pushes 'up' on you, BUT if it pushes up on you what keeps you 'down'? What keeps you from hitting the 'top' of the elevator? Thats what I want to know, because the "down" force that keeps you from going 'up' would be gravity. I'm trying to understand Einstiens point of veiw on gravity and not grasping it."
Ok so you have accepted that the elevator is moving upwards to meet you. I think the simplest way to explain to explain that you will not fly foward is Newton's Third law of motion. (For every action, there is an equal and opposite reaction.) Well the force of the elevator is upwards with the acceleration right so there must be an equal but opposite force pushing you down onto the elevator floor. This force would be the force felt when g-forces a produced on planes, elevators, and rollercoasters. Now someone might say, "well acceleration is not a force, so Newton's Law doesn't apply.". Well what they wouldn't know is that acceleration is also a force by the Equation F=MA. Any acceleration in essence is a force when the mass of the object is taken in account. Here is a diagram to explain:
eeeeeeeeeeee a f
e e a a a f
e e a f
e p e a f f f
e p e a f
eeeeeeeeeeee
Ok the e's is the elevator and the a's show the direction of the acceleration and the f's is the equal but opposite force that pushes the p's (person) down onto the elevator floor. This is my idea and I think this is right considering it applies to almost any object that accelerates on Earth. (including elevators, airplanes, rollercoasters, etc.)

Diagram

Ok well my diagram fell apart! Here it is fixed:
eeeeeeeeeeeeeee____a_______f
e_____________e__a_a_a_____f
e_____________e____a_______f
e______p______e____a_______f
e______p______e____a_______f
e______p______e____a_____f_f_f
eeeeeeeeeeeeeee____a_______f

Homework Helper
Cyberice said:
"Ok, now, I believe you that it pushes "up" on you, BUT if it pushes up on you what keeps you "down"? What keeps you from hitting the "top" of the elevator? Thats what I want to know, because the "down" force that keeps you from going "up" would be gravity. I'm trying to understand Einstiens point of veiw on gravity and not grasping it."

It's acceleration that you are not grasping. It's OK, you'd be surprised how many people think they know what it is yet have no idea. The push from the elevator floor is continuous. It is not a sudden "whap" (such as a bat hitting a ball) but a steady, uninterrupted push such as when you start at zero in your car and keep going faster and faster (and faster and faster [and faster and faster {and faster and faster...etc.} ] ).

We do not ever experience such continuous linear acceleration, so this is hard to imagine.

In real elevators there is only a short (half a second) acceleration followed by constant velocity for most of the trip and another short deceleration.

But during acceleration the inertia of your body (which doesn't "want" to accelerate) tries to hold you back while the elevator is pushing on you.

Again, there is NO force on you that pushes you into the elevator floor.

Cyberice
Ok I got the elevator analogy but, now, how does that apply to the surface of the earth, because we are not in a contained space (elevator) and we are *outside*(on the surface) of it. What is the accelaration of the earth, where is it and how do we counter it? I need a substance of some kind to envision it, not a "it just is" explanation. You said our body doesn't "want" to go up, so what pushes us down (on earth)? And do gravitons play any part?

Originally posted by Cyberice
Ah, ok. I grasped most of what you said but if we tried to analagigize the 4th dimention veiw of the 3 dimention representation would it go around the ball or to through center? I mean which way is it pulling and how does it "balance"? Like you sain the 4th dimention is hard to grasp but try to make a picture/example. And what about gravitons (gravity waves as someone called it in another thread)? Do they play any role?

Personally, I try to avoid thinking about gravitons within the "rubber sheet" analogy. I find that it tends to hurt just behind the forehead . Gravitons are quantum theory, the rubber sheet analogy is general relativity. So far, our greatest minds have not successfully combined the two. I'm not saying I couldn't do it, I just hate to show the rest of them up after they've worked so hard!

Perhaps this will help you to intuitively incorporate that all-important 4th dimension. I'm sure you have seen illustrations of the rubber sheet with grid lines drawn out on it. If you look straight down from above, these lines form perfect squares. But when the heavy object (which looks very much like a flat, two-dimensional circle from this vantage point) is rolled onto the sheet, all of the lines, whether (in front of or behind) or (to the left or the right) bend toward the heavy object. If you mark the point of maximum distortion for each of these lines with a dot, and then draw a line to connect these dots, the line you end up drawing will point directly at the middle of the heavy object.

Extending the rubber sheet to three dimensions, you'll find that the squares drawn on the rubber sheet become cubes that mark off areas of 3-D space. When a heavy object enters that space, all of the
lines whether (in front of or behind) or (to the left or the right) or (above or below) bend toward the object, causing the lines of distortion to point directly at the center.

Does that help?

Cyberice
Yeah, I'm slowly building. But then wouldn't there have to be another space time dip (like a mirror effect) to simulate gravity equaly on bothsides of the object. Because its like puting a half circle (space-time bend) in the center of a shpere. one side will have the dip end under it and the other the hill side. That wouldn't work.

I know you are trying to explain it to me but it is a very indepth and complicated thing to grasp.

Isotope
Warning: May not be the best example, nor 100% accurate.

Well, when someone says gravity, I think of the surface of water. Having a mass in the water causes a ripple, in which it does in space, too. Usually the ripples in water cast things away from you, but just pretend it attracted mass, creating a path in which to travel. Larger mass, more pulses. And as far as I know, gravity dilates time, like take the black hole for instance. This is a very obvious example; Someone looking in would likley see no change whatsoever across an infinite time of waiting, atleast until it went away, even though your image would be caught, let's not mind that for now. To you, it would seem as time was flowing normally, even though you'd be killed before you could notice. But that's some extreme time-dilation. All mass causes gravity, yet smaller objects are brought to the cetner of larger ones, or as far as they can. But I can't help you on your elevator problem.. Hope this helps at all.

Homework Helper
Cyberice said:

"You said our body doesn't "want" to go up, so what pushes us down (on earth)? And do gravitons play any part?"

With utmost respect I have to repeat that you are not grasping the concept of acceleration. I did not say that "the body does not want to go up." I said that the body does not want to accelerate. THis is the concept of inertia; this is Newton's first law which is "still valid after all these years" (Did paul Simon sing that song?)

And again: there are 2 competing theories; if Al Einstien's theory is right then gravity is not a force, but an inertial effect due to a constant state of acceleration near massive objects. If not, then there must be quantum particles called gravitons that cause the force interactions, and it would seem that these gravitons travel much faster than light or are not bound by our 3-D notion of travel.

I totally understand your frustration with trying to grasp "Big Al's" theory of general relativity. It is near the top of the list of difficult physical concepts Please re-read through all those previos replies to your post because I believe your questions have been answered.

Key point: you can't envision the direction into which we are all accelerating. Richard Feynman said that he could (actually his sister said that he said he could) but I really doubt that he had more than that 3-D "shadow" of 4-D going on in his head.

Originally posted by Cyberice
Yeah, I'm slowly building. But then wouldn't there have to be another space time dip (like a mirror effect) to simulate gravity equaly on bothsides of the object. Because its like puting a half circle (space-time bend) in the center of a shpere. one side will have the dip end under it and the other the hill side. That wouldn't work.

I know you are trying to explain it to me but it is a very indepth and complicated thing to grasp.

You're still thinking it dips in three dimensions; don't. Lurch is very correct in his anology. As soon as the sheet is given volume, the folds in space all point to the center of an object with mass given the fact that it is forth dimension space-time; radially. It means everything will be pulled toward the center of the object regardless of its orientation of 'top' and 'bottom' as you say.

Mentor
Despite knowing of the bowling ball on a trampoine analogy, I never really thought about it with people standing on earth.

Essentially, you put a marble next to that bowling ball and the curvature of the trampoline is what holds the two together. The marble is trying to accelerate down the curvature of the trampoline, but the bowling ball is in the way, creating the force between the two.

So basically the force you feel on your feet is the Earth stopping you from accelerating due to the curvature of space?

Cyberice
You're still thinking it dips in three dimensions; don't. Lurch is very correct in his anology. As soon as the sheet is given volume, the folds in space all point to the center of an object with mass given the fact that it is forth dimension space-time; radially.

Wait a minute! That's what I've been missing in my visualisation! I forgot to give the 2d "rubber sheet" volume to make it 3d. So, if the object/planet is placed in a 3d representation of space-time (is placed in the middle of it's "volume") then all points around it will bend toward the center of the displacement in space causing a curvature. Correct? Or am I off AGAIN? [?]

Eh
You're not supposed to visualize a volume at all, since human brains cannot see or imagine a curved volume. Just picture the 2D sheet as a universe where 2D stickmen live.

Originally posted by Cyberice
Wait a minute! That's what I've been missing in my visualisation! I forgot to give the 2d "rubber sheet" volume to make it 3d. So, if the object/planet is placed in a 3d representation of space-time (is placed in the middle of it's "volume") then all points around it will bend toward the center of the displacement in space causing a curvature. Correct? Or am I off AGAIN? [?]

YES! You have it exactly (or as exactly as any human can, at least)! Welcome to Mr Einstein's universe, You're going to like it here.

Last edited:
Cyberice
"YES! You have it exactly (or as exactly as any human can, at least)! Welcome to Mr Einstein's universe, You're going to like it here."

Sweetness!

kokain
I think I have something to add (but I am not smart, so I could be wrong). The reason that you stay on the Earth is not gravity (no such thing according to Einstein) and not because your body dosn't want to accelerate. Your body and everything for that matter is already accelerating. The dimples in time-space caused by the massive bodies (Earth) are like 4dim. hole that you are falling into. You want to reach the "middle" (hard to grasp in 4d) but the surface of the planet, the board you are standing on, the water you are floating on or the car that you are sitting in prevent you from falling any farther by pushing "up" on you eaqually as "hard" (hard HA HA) as you push on them. Am I close or not? Like I said, I have no idea what I am talking about.

Of all the forces of nature we understand gravity is odd because we don't seem to be able to do anything with it or manipulate and control it unlike atomic theory to make fission power or light theory to make lasers or electromagnetic theory to make electricity, generators, radios, televisions, etc...why?

See, since you were inside of the elvator, when it accelarated the force pushed you against the floor to simulate gravity. Now the point, We are ON TOP of the earth. How in the world, if this analogy is right, are we not flung off of the surface of the Earth at 1,000 something miles an hour off the earth?
because the force is directed toward the center of the earth, even at the north pole. Acceleration is absolute, not relative. Standing on Earth is equivalent to accelerating at 9.8 m/s^2 away from the surface of a massless earth. At the poles, where the centripital force approaches zero (as r->0) the force of gravity is stronger. At the equator, it is slightly weaker because of Earth's rotation. You can calculate this with just algebra, I think it's about a tenth of a m/s^2 difference.

cytokinesis
On a sort of related note, since gravity occurs as a constant acceleration in the 4th dimension, and the 4th dimension has come to be known as time, could time then be considered to be the constant acceleration which we all experience as gravity?

cytokinesis
hrm...

I think I may have put this in the wrong section, I'll make a new post in the theory development section.

kokain
As far as I understand (not saying much) the 4dim is not time. In our 3d world we use time as a fourth dimension only to identify points in space-time. The 4d universe that I was talking about is the 3 spatial dims we know and another dim perpendicular to those. Spacial 4d and 4d time-space is a completely different thing. I think.

cytokinesis
but it's a possibility?

kokain
Anything is possible! But the 4d curve as I see it is 3 dimensional space (the sheet of rubber) curved "upsilon" into the fourth spatial dimention.

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. I have yet to see an every-day mechanical example that can explain this. Especialy since it acts on you if you are in the air (which no mechanical device that I know of can do). I need a more detailed description of it. How can it pull you once you have left its surface? 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? And apparently this is not what happens.

If this is too hard for you to explain then could you please direct me to a link that can?

Last edited:
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

Last edited:
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.