How does gravity actually work?

Factual Questions
1

I kind of don’t get it. You have one planet (very large, quite heavy), and another planet (larger, heaver), pop them in space near each other and without any other inputs, they move towards each other.

If there was no gravity, you simply couldn’t get them to move without a great deal of effort. But for some reason, in the absence of any other force, just having them within a couple of hundred million miles away is enough for them to move together. This is kind of mysterious isn’t it?

What’s going on here?

M

(Sorry if this question has been asked before - did a search and nothin much came up)

2

Well I’d love to hear others’ (better informed) explanations, but one interpretation I’ve heard requires you to understand “space” not as “nothing”, but rather something that can be manipulated.

Large masses (well, ok, any mass) actually bend, manipulate, curve, or whatever the space around them.

Here’s a picture visualization.

3

[QUOTE=BACI]
But for some reason, in the absence of any other force, just having them within a couple of hundred million miles away is enough for them to move together. This is kind of mysterious isn’t it?
[/QUOTE]

Without getting into the relativistic notions of spacetime…

Gravity is a force that works like this:

F = GM1M2/r^2

Where G = universal gravitational constant (essentially a fudge factor)
M1 = mass of first body
M2= mass of second body
r = distance between the two masses

Everything that has mass is attracted to everything else that has mass, not just planets. And the force operates over long distances, although it decreases like the square of the distance.

Did you have some specific question to ask, because “how does it work” doesn’t seem to be what you’re after. If you’re after “why is there gravity”, then the anwer is “because”. :slight_smile:

4

Despite what we know, gravity is still one of the biggest unsolved problems in physics. Nobody understands it at a fundamental level. Some scientists believe there is such a thing as “gravity waves” and they are in the process of building some insanely sensitive instruments in order to detect them.

5

Everything in the universe always moves at the speed of light through spacetime. If those two planets are at rest with respect to each other in space, then they are each moving at the speed of light 100% along the time axis in the same frame of reference. As Jayrot noted, mass distorts space; in fact, it distorts spacetime. So, similar to the image that he references, these two masses barrelling along with each other are slowly turned towards each other, which an observer sees as two planets that started out at rest with each other accelerate towards each other as time passes.

6

I think the OP is asking “where does the energy potential come from to get two huge masses moving towards each other?”

7

[QUOTE=Lumpy]
I think the OP is asking “where does the energy potential come from to get two huge masses moving towards each other?”
[/QUOTE]

In that case, it’s the energy that was required to separate them.

8

Couldn’t you say pretty much the same thing about any fundamental force?

You get this magnet over here and a refrigerator over there, and without any other inputs they move toward each other.

You put some neutrons near a proton, and somehow…

9

[QUOTE=Lumpy]
I think the OP is asking “where does the energy potential come from to get two huge masses moving towards each other?”
[/QUOTE]

I don’t think so. One might as well post the same OP about electromagetism. How does it work that two charged particles repel or attract each other? “In the absence of any other force” how do they do that?

10

[QUOTE=BACI]
I kind of don’t get it. You have one planet (very large, quite heavy), and another planet (larger, heaver), pop them in space near each other and without any other inputs, they move towards each other.

If there was no gravity, you simply couldn’t get them to move without a great deal of effort. But for some reason, in the absence of any other force, just having them within a couple of hundred million miles away is enough for them to move together. This is kind of mysterious isn’t it?

What’s going on here?

M

(Sorry if this question has been asked before - did a search and nothin much came up)
[/QUOTE]

I’m not sure anyone can tell you “why” any force exists. We know they exist because we observe it to be so. But you seem to be finding gravity particularly puzzling, apparently because of the fact that very massive objects are “hard” for everyone else to move, but “easy” for gravity to move.

Essentially, the reason why this is so is that the acceleration due to a force is inversely proportional to mass, but gravitational force is directly proportional to mass.

In other words, mass makes things harder to move, but mass also makes gravity pull on them more, and the two effects cancel out.

One could say that mass is a sort of “gravitational charge.” Just like the electric force between two objects is greater if they have more electric charge, the gravitational force is greater if they have more “gravitational charge”, meaning mass.

But why is the mass of an object equal to its “gravitational charge”? In the standard terminology of physics, the question would be “why is an object’s inertial mass equal to its gravitational mass?” So far as I know, this is an unanswered question. But all the experimental evidence indicates that they are equal.

11

[QUOTE=BACI]
If there was no gravity, you simply couldn’t get them to move without a great deal of effort. But for some reason, in the absence of any other force, just having them within a couple of hundred million miles away is enough for them to move together. This is kind of mysterious isn’t it?
[/QUOTE]
There are two “deep questions” here that are kind of woven together: action-at-a-distance, and the equivalence of gravitational & inertial mass. The idea of “action at a distance” was really first popularized by Newton when he came up with his original theory of gravity. To explain how two objects that were not obviously in contact could still exert a force on each other, he came up with the idea of a “gravitational field”: at every point in space, the field tells you how a mass will act under the influence of gravity. For example, if I’m near the surface of the earth, the gravitational field of the Earth is such that a mass will experience 9.801 Newtons of force for every kilogram of its mass, and that this force will be directed towards the center of the Earth.

You might protest that this doesn’t really explain anything; as I’ve described it, it just sounds like a convenient bookkeeping tool, but nothing more. As it turns out, though, the gravitational field turns out to have a life of its own; once you’ve figured out how to extend Newton’s theory of gravity to a more complete description of the Universe (Einstein’s General Relativity), you discover that the gravitational field has ripples and waves that can propagate through space on their own, without any need for mass nearby. So it certainly seems like the gravitational field should be considered an entity in its own right. (This is an analog of the link between electromagnetic fields and light, if you’re familiar with that.)

The other question is why the force of gravity seems to be so big; how is it that the gravitational field can act on a stupendously large mass to get it moving? This is where the equivalence between gravitational and inertial mass comes in. As John Mace noted, the gravitational force acting on an object is proportional to its mass; in other words, the more mass your object has, the stronger gravity acts on it. What’s more, Newton’s Second Law tells us that if a given force acts on an object, its acceleration (i.e. the amount it will move in response to the force) will be decrease as the mass increases. If you combine these two tendencies (with some not-too-complicated mathematics that I’ll spare you), you find that these two tendencies cancel out, and the acceleration that an object undergoes due to gravity is exactly the same, regardless of its mass.

This cancellation is due to the two notions of the mass — the “gravitational mass” which determines the force due to gravity, and the “inertial mass” which determines the acceleration due to a given force — being identical for all objects, at least to the accuracy we’ve been able to achieve with our experiments. This equivalence seemed somewhat fortuitous and mysterious until Einstein came along, with his aforementioned theory of General Relativity.

Einstein’s rather radical notion was to say that gravity isn’t really a force at all; rather, it’s a so-called “fictitious force”. A more easily comprehensible example of a fictitious force is the “force” that presses you backwards or forwards in your seat when you’re in a car that suddenly brakes or accelerates. You might think that there’s a mysterious force that suddenly grabs on to your body and pulls it forward when the car brakes; but really, it’s that the car is stopping, and the inertia of your body “wants” to keep moving forward. You experience this leftover inertia as a “force” that pulls you forward.

But gravity’s a real honest-to-Bob force, though, isn’t it? 'Fraid not. Einstein’s big idea was to say that spacetime is “curved”, and that an object left to its own devices will move in space and time in a certain natural way. What’s more, any massive object will cause spacetime to curve; and the resulting “natural” motion in space and time of another object due to this curvature is not to remain the same distance from the first object, but rather to approach it with a certain acceleration. (There are precise, elegant mathematical notions to go along with all this, but I’ll spare you those as well.) The reason we experience gravity as a “force” here on Earth is that we insist on staying the same distance from Earth, deviating from the “natural” path in space and time that our bodies “want” to take — just as when a car brakes, our bodies “want” to keep going, and we experience this as a force pulling us forward in the car.

This is probably much longer of an explanation than you wanted, but hey, what else am I going to do on a sweltering hot Tuesday night? :slight_smile: Hope this helps.

12

[QUOTE=Punoqllads]
Everything in the universe always moves at the speed of light through spacetime.
[/QUOTE]

How so?

Then everything since the big bang has been moving at C?

I can’t wrap my mind around that.

13

[QUOTE=Jayrot]
Large masses (well, ok, any mass) actually bend, manipulate, curve, or whatever the space around them.

Here’s a picture visualization.
[/QUOTE]

I’ve never liked that explaination. The idea that large objects bend space, and other objects slide down the slope doesn’t make sense to me- you have toassume that there is some gravity-like force acting “under” the universe causing things to roll down the bend in space, and where does that force come from? And so proceed ad infinitum.

14

A possible reason for BACI’s curiosity is that gravity is a naked force, frankly doing its thing uncloaked by an antisense. This makes it very obvious, despite being ten thousand trillion, trillion, trillion times weaker than electromagnetism - a force opposing itself everywhere because it comes in two senses.

15

[QUOTE=Rysdad]
How so?

Then everything since the big bang has been moving at C?

I can’t wrap my mind around that.
[/QUOTE]

In three dimensions you can represent the velocity of an object by a vector which has 3 components (x, y, z), and its magnitude can be anything at all less than c.

However in relativity Einstein tell us we have to include time. So now our vector must have four components (x, y, z, t). And when you calculate the magnitude of this vector it turns out it’s a constant and that constant is c.

So if an object is standing still it must be traveling through time at c and if it is moving through space at c then it must be standing still with respect to time.

16

[QUOTE=Peter Morris]
I’ve never liked that explaination. The idea that large objects bend space, and other objects slide down the slope doesn’t make sense to me- you have toassume that there is some gravity-like force acting “under” the universe causing things to roll down the bend in space, and where does that force come from? And so proceed ad infinitum.
[/QUOTE]

It’s not an explanation, it’s just a device to try to give people an intuitive feel for how curvature can effect massive bodies.

17

[QUOTE=Peter Morris]
I’ve never liked that explaination. The idea that large objects bend space, and other objects slide down the slope doesn’t make sense to me- you have toassume that there is some gravity-like force acting “under” the universe causing things to roll down the bend in space, and where does that force come from? And so proceed ad infinitum.
[/QUOTE]
This analogy is often stated poorly, but it is still a pretty good one. It’s not meant to explain gravitational attraction between objects, but more to help understand the effects of the curvature of space. When the rubber sheet elastically deforms around a heavy object, it actually gets what’s called intrinsic curvature–the same problem that mapmakers have in trying to draw flat maps of the Earth. Because of this curvature, the shortest paths on this surface will no longer look (to us, looking down at the surface) like straight lines. That’s what that orange light-ray on the left side of the gravity well is supposed to be showing. Even a massless object like a photon is deflected from what we might naively call a “straight” trajectory (the dotted white line), because the space in which it lives has become curved; the point is that this deflection is entirely a geometric property of the sheet and doesn’t depend on something pulling the test particles down into the dimples.

18

Oh, I don’t know about that. We have a perfectly good geometrodynamical theory of gravity, but we’re still struggling to produce such a theory for any of the other forces. From that point of view, gravity’s the only thing in the Universe we understand at a fundamental level ;).

19

[QUOTE=BACI]
But for some reason, in the absence of any other force, just having them within a couple of hundred million miles away is enough for them to move together.
[/QUOTE]
I think you’re a bit off in this.

Gravitational attraction between 2 objects is inversely proportional to the square of the distance between the objects, but only directly proportional to the product of their masses.

So I don’t think planetary masses would show a significant effect over “a couple of hundred million miles”.

20

[QUOTE=Punoqllads]
Everything in the universe always moves at the speed of light through spacetime. If those two planets are at rest with respect to each other in space, then they are each moving at the speed of light 100% along the time axis in the same frame of reference. As Jayrot noted, mass distorts space; in fact, it distorts spacetime. So, similar to the image that he references, these two masses barrelling along with each other are slowly turned towards each other, which an observer sees as two planets that started out at rest with each other accelerate towards each other as time passes.
[/QUOTE]

Is this true, that everything in the universe always moves at the speed of light through spacetime?

If so, this is the best explanation of gravity I’ve ever seen.