Someone explain gravity to me

Philosophy Forums Science, Technology, and Math
1

I know that under Einstein’s theory of gravity, space is compared to a trampoline. If you put a bowling ball on a trampoline it will weight itself at the center. Then if you put a tennis ball on this same trampoline, the tennis ball will roll to the bowling ball.

So space bends like the trampoline. And the bowling ball is like earth, or any cosmic body. And the tennis ball is like something that get pulled to the earth through gravity.

However the earth is a sphere. Space would have to bend in all directions for things to get pulled to earth in every direction. If you were to put a tennis ball underneath the trampoline it wouldn’t get pulled to the bowling ball.

I could have completely botched this explanation, but can anyone explain to me exactly how “space bends”?
Or if this is an outdated theory of gravity is there a better explanation?

I’m really shitty at Science.

2

It bends when there is a large amount of compacted matter, i.e. planets. I believe this “plane” that is our universe is very fragile, yet flexible. There is still much to be learned!

3

Yes, this makes sense.

But how can you compare space to a plane since it is 3 dimensional? This is my point. It would have to bend in all directions. Bodies are being attracted to earth in all directions, the north pole and the south pole. Does this mean space bends in all directions?

4

Heh, this is beyond me as well. Einstein’s theories have always been very confusing to me.

5

Yeah, it really bothers me because for a while I thought I understood gravity. Then when I really started thinking about it, I said “wait, that makes no sense at all!”

My hope is that some brilliant physics Ph.D. will pop on and explain it. But what would they be doing on ILP at 1:30 AM?

6

screw einstein ill explain it!

ok gravity basically is the attraction between 2 objects.

einstein or whoever observed that the closer the objects are to eachother, the stronger the attraction becomes. and that the strength of attraction is determined by the objects mass squared.

so if you want to be able to imagine what gravity does here is a quick way.

imagine space/time being like a big empty grid with xyz positions…

in this big empty grid matter is superimposed. (our 3 d universe)

the volume of any matter over this imaginary area holds a certain value, or strength.

and like electricity or magets this value determined by mass acts like an imaginary force like we see in starwars which subsequently pulls things closer together over time.

7

Um. No. First, it was Newton. Secondly, the mass isn’t squared, unless the mass of each object is the same. I think you meant distance. Newton’s approximation was this:

F = GmM/d^2

F - force of gravity
G - gravitational constant
m - mass of first body
M - mass of second body
d - distance between the two bodies

Dasein, you’re absolutely right: space stretches in every direction. It’s kind of hard to picture, but let’s use this that you’ve seen before:


And then do this:


And then just keep adding gravity wells around every point on the planet there. Hard to picture? Yeah. I can’t find a good picture online of what I’m thinking. I’ll try to draw it…


Ah, MS Paint. Well, it’s crappy, but the blue spheres show a region of space relatively unaffected by the gravity of the planet, so it’s uniform, i.e. the space between the blue spheres is the same. As you get closer to the planet, they spheres get further apart. The further apart the spheres are, the stronger gravity is…the easier it is for stuff to fall towards the planet.

Does that help?

8

Yes! Thank you for the post. It will take me some time to look these over and contemplate them, but I’m sure this will help.

Cheers.

9

ya sorry my memory of tenth grade physics is somewhat befuddled. (i know einstein didn;t make the equations but the base parts of an explanation are there)

yes, i got distance confused with mass

10

Ah, thanks for the pictures Anthem…makes sense to me now.

11

You can’t really picture how space-time curves because its 4D dimensions but you can make mathematical definations of curvature in any number of dimensions. Maybe the best way to understand why gravity must curve space and time is the equivalence principle. Basically this says that if your in a rocket which is accelerting at a constant rate call it g all the physics should be equilvalent to when you are standing on a planet where the acceleration due to gravity is g.

If you drop a ball in the rocket in space the the ball will apear to fall to the floor.( This is because the rocket is accelerating relative to the ball so the floor of the ball catches up with the ball.) If your standing on the planet the ball will drop beacuse of the gravity.

But now think about light. If we shine a light in the rocket from one wall to the other then in between it reaching the other wall the space rocket will have accelerted up wards so the light will appear to move down a little bit. We must then conclude from the equivalence principle that the light moves down in the gravitational field of the planet.

Now think about the idea that light trvels in “straight lines”. Obviously the light in the rocket and on the planet doesn’t travel in a straight line. But if you think about drawing a line on a curved surface like a ball you can’t. The closest thing you can do is draw a line that follows the curve, that is a curve that remains parrellel to the direction its moving or the shortest distance between 2 points (think about a mouse following its own noce on the surface of a ball) this is called a geodesic.

12

Gravity has nothing to do with “attraction”. It is a 4-dimensional negative pressure exerted upon universal Substance/ the lumineferous ether/ the spacetime fabric/ whatever you want to call it, and is directed radially inward towards a point called a singularity. Singularities have nothing to do with so-called black-holes. They exist in all gravitational fields.

Gravity is a cosmic-scale happening. As far as we can tell, there is nothing quantum about it, like the other forces of nature (electro, weak, strong).

It is 4-dimensional because it happens instantly upon vast expanses of of space. There is no propagation delay as with other forces.

It is negative pressure because it compresses these regions rather than expands them. (When you blow up a balloon, you are exerting positive pressure upon the rubber sheet. Gravity is like sucking the air out of a balloon.)

Gravity is the only force that gives us varying densities. In the atomic lattice structure of metals and crystals, the electro forces leave us with perfectly homogeneous arrangements. There is no natural “center” in such cases. A gravity field’s center is natural because its force field lines are always pointing directly into it.

Just imagine this…

All biomass is exerting downward pressure on the earth’s crust.

The crust and everything above it is exerting downward pressure on the earth’s mantle.

The mantle and everything above it is exerting downward pressure on the earth’s core.

As we descend deeper and deeper into the core, every new layer must support the cumulative weight of all layers above it.

As we continue to search for a bottom, we end up at a point of unimaginable density and smallness whose job it is to hold the entire weight of the world on its shoulders. This is a singularity.

Einstein was on the right track when he upheld that gravity has more to do with the large-scale structuring of the universe rather than a simple point-to-point attraction.

He didn’t go far enough, though.

Gravity fields are nothing short of the inhalations of God.

13

GRAVITY IS A UNIVERSAL LAW AND A GIRLS WORST NIGHTMARE AS IS WHAT EVENTUALLY CAUSES A GIRLS TITS ASS AND BELLY TO FLOP DROOP AND SAG

14

Oh boy ](*,)

Um…you know the drill. Reported.

15

what a load of crap. Go read a book!

16

Thanks, Fin. I didn’t want to be the first to shut-down the guy who thinks 4-dimensions means instantaneous travel.

17

The deep problem with typical quantum gravity theories concerns the question of quantum mass.

It is said that massive bodies cause gravitational fields. However, when forced to come up with a definition of what exactly it is that constitutes the quality of massiveness, physics is at a loss. The best we can do is to say that mass is the resistance to force. It other words, it is that which opposes all attempts to speed a thing up or slow a thing down. But this doesn’t tell us what it is, it just tells us what it does.

In our everyday experiences, we think we know what “massive bodies” are. But when it comes down to finding the very seat of massiveness, we are led down blind alleys. We can’t fully begin to ask the question of quantum gravity until we have settled the question of the fundamental unit of mass.

But if we assume that gravity is not a quantum force, but rather a cosmic one, then we no longer are faced with the “mass question”.

General relativists are fond of saying that mass-energy causes space to bend and that bent space causes mass-energy to alter its course.

However, if we take space to be everywhere filled with “mass-stuff”, then there is no difference between the two. We are left with the idea of gravity being an uncaused phenomenon. In this case, it turns out to be the spontaneous compression of universal substance into a central point.

Physicists don’t like this kind of arbitrarity, though. They like to be able to wrap their mits around the universe by way of formulas and laws. I guess whether you think that gravity is ultimately a caused quantum force or an uncaused cosmic force depends on whether you want to be able to wrap your mits around the universe.

Some people want the universe to be ultimately predictable so that there are no surprises, and there can be such a thing as perfect security.

Others, however, are profoundly attracted to an ultimately mysterious universe, always with a new treasure waiting to be discovered around the corner.

18

Welcome to existentialism. Watch out, Sartre is ranting in the corner. Take a flier and help yourself to refreshments.

19

Maybe string theory has it right. There’s a particle out there smaller than light that’s causing gravity. I wonder if light travel on gravitons? It seems weird that light can self-propagate.

20

Gravity is a structural force. No waves have been or will ever be found. There is no ‘pulling’, as this requires a reach, a grab and a pull. Gravity is a result of compression.