# Gravity Analogy problem

Newton said Gravity is a FORCE. That is why two masses attract. Einstein said there is no gravitational force, just warped space time that causes the illusion of “attraction” between masses. The analogy used to explain space time is some version of a bowling ball on a trampoline. This causes a depression. Now roll a tennis ball onto the trampoline and it moves around the bowling ball just like it is being attracted to it, but there is no attractive force, just warped space time… says the analogy.
Here is my question : This analogy lets Einstein off the hook by ROLLING the tennis ball onto the trampoline. Putting the tennis ball in motion takes energy, given by the tried and true formula F=MA. Now just place the tennis ball on the trampoline but with ZERO motion relative to the trampoline. Since there is no Gravitational Force according to Einstein then there is nothing to make the tennis ball accelerate toward the bowling ball. It would just hang there on the sloped depression around the bowling ball, but not move toward it if there is no force. The law of force is always obeyed.. no force = no acceleration, no acceleration means no movement by the tennis ball. Same for holding a tennis ball and dropping it.. why does it hit the floor if I hold it perfectly still and then let it go ? Why did it accelerate ? Force of gravity is not an option, remember ?
Thanks for your comments,
James

September 2, 2008

### 12 Responses to Gravity Analogy problem

1. Anonymous June 30, 2009 at 3:57 am #

If there is no gravitational force, then when you hold a tennis ball above the floor and drop it, please TELL ME CLEARLY why the ball starts falling, called accelerating, toward earth.

I’ll give it a go:
Using General Relativity, the gravitational field is described by Einstein’s field equations, which although look simple, describe 4 dimensional spacetime using at least 16 variables.

These equations depend on both matter and energy (Newtons equations ‘just’ relied on teh distribution of matter) and remember that due to E=mc^2 that mass is just another form of energy.

The gravitational fields are described in general relativity as the curvature of spacetime. General relativity states that being in a region of curved space is equivalent to accelerating up the gradient of the field. By Newton’s second law (F=ma), this will cause an object to experience a fictitious force if it is held still with respect to the field.

I would also like to add that the trampoline or “sheet of rubber” example is an appalling way to describe the effects. It is fine for a conceptual view of how spacetime is curved, but it is only in 2-dimensions, when really it is curved in 4-dimensions, and our brains just aren’t good at knowing to to visualise this. It does nothing for explaining how things work at all.

Another way of thinking of it is, imagine that the bowling ball putting a dip in the trampoline. If your tennis ball was sitting at the bottom of the dip (presumably resting on the bowling ball) it would have a small amount of gravitational potential energy (GPE). Now suppose you move the tennis ball up the slope, you are increasing the GPE. Now ideally, it wants to have zero GPE, and the only way to do this is to move back down the slope. By placing the ball at some point on the curve, you are essentially giving it a certain amount of GPE that it ‘wants’ to get rid of.

This is a particularly crude way of putting it, but you are asking people to describe general relativity in Newtonian terminology, and it isn’t that simple. If you do that, you wouldn’t need GR.

Brian

2. MainFragger July 28, 2009 at 7:32 pm #

I think of it this way. An object at rest will stay at rest. If nothing had anything to support it, everything would just lie on the bottom of the universe. But the fact is, there are other forces that support planets, suns, moons, and galaxies. Electromagnetic, radiation, various forms of matter. The universe is not empty. And everything in the universe is like mixed color sand in a bottle. Leave it alone, and whatever pattern exists in the sand will stay as is forever. Move it, the pattern changes..all the sands now move up and down according to how you shake it, and may look different but all of the materials that cause that patterns are still there. So it is with gravity. The big bang created matter and motion in the universe. Any gravity we perceive is just an after effect of that motion interacting with all of the matter in the universe. Some celestial beings spin because of the big bang..others spin because varying sizes of matter interacted with other sources of matter, heat, radiation and magnetism and started spinning.

3. MainFragger July 28, 2009 at 7:18 pm #

Every atom between the magnet and the object alligns electro-magnetically in one direction. The atoms in the air air becomes a wire, which attaches itself to the magnet and the object. The electrons of the object then follows the magnetic inductance along the “air wire” to the electrons of the magnet.

4. Anonymous September 9, 2008 at 9:28 pm #

Thanks all who have responded. Some of you have deduced that I have a mechanical, Newtonian, older physics view of things. True. But I like to think I would accept something once I understood it. The Ah Ha experience is fun to have.
However I am not willing to allow objects to move without a source of energy. Does this not bother anyone ? If there is no gravitational force, then when you hold a tennis ball above the floor and drop it, please TELL ME CLEARLY why the ball starts falling, called accelerating, toward earth. Some of the most accepted and tested laws of physics are in the area of mechanics… force equals mass times acceleration is a bed rock of all physics and cannot be negated. So why does the ball rush to the floor ?
Discover magazine had an article awhile back where a technical reporter went around asking world famous physicists how a little magnet can make a piece of metal move at a distance. They all brushed him off.. some rudely.. but no one told him how a magnet can cause force at a distance ! I find that extrodinary and a dead certain indicator that all we can do is DESCRIBE the action with math but we have no UNDERSTANDING of the mechanism. I feel the same may be happening with space/time curvature being cited as the cause of what appears to be gravity. Just tell me what makes the ball accelerate,and be sure to include the energy source, since that cannot be violated. Thanks for your attention and interest, James.

5. Anonymous September 8, 2008 at 5:06 am #

David,
thanks for your comment. I think your several comments are helping me grasp the implications of curved space time paths. I have enough background to realize that there is no way for us to really visualize curved space time, four dimensions, that GR says a mass like earth causes. This curve would have to curve into itself from every where around the earth. The three dimensions we experience means we have no way of really “seeing” that. Math can describe it and verbally we can say it, but I think you have given me the clue that the problem is there is no good analogy because there are no four dimension visuals for us to use. Is that about what you are saying ? Side issue : I have been thinking of gravity in terms of what is called the Radiation Shadow Effect idea. You seem to mention that in answering someone else’s comment. The idea is that there is a flux of some sort…. and we keep finding more even in “empty” space.. and that mass absorbs some of this as it passes through. This makes an imbalance we feel as gravity. It works very well to the point that I can’t think of a way to disprove it, which I know is a necessary test of an idea : has to be disprovable. But every case I can think of conforms with known data. If you would like to chat more, I ran into some data on black hole minimum size theory that may be able to prove/disprove the radiation shadow model.
Well, thanks for your interest. Take care, James.

6. MainFragger September 5, 2008 at 10:59 am #

Technically, the planet doesn’t drop, it rolls. Tbe curve created by gravity is an analogy we can’t perceive to explain something we perceive as a visible force.

But keep in mind, that the universe is matter. If the universe movies or spins, or anything happens in it, matter is shifting.

When the univsers “curves” as in the droopinig example, whatever matter in the area is going to brush against whatever might be sitting on the event horizon of the curve. This is going to create “spin”..maybe not on a visual level, but perhaps at the subatomic or baryon level. The energy to move will seemingly generate from the object itself. This is why you can’t visibly “see” gravity.. because gravity is internal to everything. Think of one of those little ripcord toys they use to give away at fast food places. With the ripcord inserted and in place, the toy has potential energy, but not kinetic energy. Pull the ripcord, the internal workings of the toy start to spin, the toy takes off and moves along the table.

Thats my layman take on it, anyway..

7. Halliday September 3, 2008 at 4:54 pm #

Fizzizist:

By the way, the term “gravitational field” need not imply a “FORCE”. It need only imply a region of influence, or some other field-like “entity” intimately associated with gravity. For instance, within General Relativity (GR) one may refer to the curvature tensor as the “gravitational field”. (Though I don’t agree with the usage, I have even seen the metric tensor “field” referred to in such a way as to imply it is the “gravitational field”.)

(Note: The curvature tensor in GR is the closest thing analogous to the electromagnetic field tensor. The Christoffel symbols are the closest analogues to the vector “potentials” of electromagnetism and quantum field theories.)

David

8. Halliday September 3, 2008 at 4:38 pm #

“Particles, or something, rushing in” models of gravity (and even electromagnetism) have been tried, and failed, many times before.

Of course, don’t necessarily let that stop you from trying. Just recognize that it’s not going to be all that simple.

David

9. Anonymous September 3, 2008 at 2:37 pm #

Fizzizist:

The “bowling ball on trampoline” analogy for Einstein’s General Relativity (theory of gravity) has many problems (as do practically all analogies, actually). You have, perhaps, only hit on one such issue.

Part of the problem is that your concept of “acceleration” is rooted in an inertial, flat “reference frame”, while General Relativity (GR) deals with a curved spacetime (note: the curvature is not only in space, but time as well). The other aspect is that spacetime means that one cannot simply conceive of a stationary space in which all else resides and changes over time, but must deal with the temporal evolution, motion, of objects within the single spacetime.

Within spacetime, the question of “acceleration” becomes a question of what is the closest to “straight line travel”. (Think of constant velocity motion in the flat spacetime of special relativity. You can always draw a straight line in spacetime to represent such.) The closest thing to a “straight line” on a curved surface (like the surface of this Earth) is called, generally, a geodesic (note the “geo”, meaning Earth, part of the word). The same is used for the closest thing to “straight” in the curved spacetime of GR.

Since all objects that exist in spacetime exist for at least a finite time, their spacetime “paths” trace out curves, of some sort, within spacetime. This is true both for the ball you hold stationary above the Earth, and for the freely falling ball. The question is which ball is following a spacetime path that is most closely “straight”, meaning a geodesic.

I know it’s hard to visualize (though I have devised a couple of analogies that I believe better illustrate this than the trampoline or rubber sheet analogies), but the falling (apparently “accelerating”) ball is the one following a geodesic. (The mathematics of Riemannian geometry makes this quite clear, once you’re able to grasp the mathematics.) So the truth is that the ball you are holding stationary above the surface of the Earth is the one that is actually accelerating, and experiencing a force, while the freely falling ball is not.

It really does work, but, unfortunately, the mathematics is not so easy, and poor analogies like the rubber sheet or trampoline just confuse matters, in my opinion, for those that think deeply enough to see past them.

David

10. Anonymous September 3, 2008 at 2:05 pm #

A vacuum in space doesn’t really exert a force, does it? But particles rush to fill the empty space as if they were being pulled, yes?

Could gravitational attraction not be explained similarily?

11. Fizzizist September 3, 2008 at 8:09 am #

Thanks for your comments. Your reply uses the term
” gravitational field “. Use of this term implies there is a FORCE due to gravity. I restate that Einstein says there is no such thing as a force due to gravity. The problem I am pointing out is that no object can accelerate without force being applied. If that were not the case then objects could move for no reason.

So, think about this again if you like. I said the tennis ball could be placed on the slope of the trampoline and not move toward the bowling ball if there is no force to cause it to do so. Everything in the Universe may be in motion as you say.. or not… we have no way of knowing, that is Einstein’s point about relative motion, there is no absolute motion. We only know relative motion. Therefore I made the point that the tennis ball has zero motion relative to the trampoline and bowling ball….now I want to know why it would start moving. Remember, there is no gravitational force field according to today’s physics. Take care, james

12. Anonymous September 2, 2008 at 4:22 pm #

Dear James:

What you’ve left out of the equation is that the “ball” has to be within the gravitational field to be attracted. If you place the tennis ball on the far edge of the trampoline, of course it won’t roll toward the bowling ball. But if it is placed near the depression, it will. Rolling it is not required. The reason why that is included in some examples explaining gravitation in space is that in space everything is moving and things often move past large bodies that create gravitation fields strong enough to drawn objects moving past them, toward them.

As for holding a tennis ball still and letting it go, you are on Earth. Earth creates a significant gravitational field. Holding the ball and letting it go is called a gravity drop, and you can do measurements to calculate the rate of acceleration for a ball traveling from your hand to the floor. I know. I’ve done gravity drop measurements before.

No big mystery here. No problem.

MarshallBarnes