Boundaries and Surfaces
Imagine a drinking glass, half full of water. The boundary between the water and the air above it forms a disk shaped surface, which is neither water nor air.
If you tip the glass a little, the circular disk becomes elliptical, with a greater surface area. Tip it a little more, and the surface area increases even more.
If you set the glass on a table, and then thump the table, circular waves will form on the surface (think of the puddles of water when the dinosaurs approach in Jurassic Park). The circle remains the same size, but because of the up/down positioning of the waves, the air/water boundary increases in surface area.
But what is this boundary surface, and what is it that actually increases in size?
No one disputes that the surface is real, that its area can be measured. But it's not part of the water, nor part of the air. It exists only because the air and water exist.
This surface is a two-dimensional something with no volume or mass. It is created out of nothing and eventually returns to nothing. It's like it doesn't really exist, but nonetheless, we can easily measure it and create more of it.
Random Movement
Imagine you are visiting a farm, standing in a flat field. Now choose any random direction and take a step, north, south, east, west, or any direction between. There is no way anyone could predict where you would step to; every direction is just as likely.
But the field isn't really flat. It's a very small part of the spherical globe known as the Earth. If you could measure very small distances (or if you could take very large steps), you'd find that the situation isn't quite what your thought it was.
Consider the section of the USA/Canada border known as the 49th parallel.
Every point on the boundary is the same distance from the North Pole.
The border itself runs east/west. If you step to one side of the line, you move slightly north, and to the other side, slightly south.
Notice that this boundary is not a straight line; it curves toward the north. Draw a circle centered on the boundary and notice that the arc of the circle south of the line is a little longer than the arc of the circle north of the line.
Now suppose that the point you were standing on was exactly on the border. When you randomly stepped in any direction, you were actually slightly more likely to move south than north.
You of course don't have to move to Montana for this effect to happen. Close to the North Pole, the curvature of the latitude parallel will be much greater; at the Pole itself no matter which way you step will be south. Close to the Equator, the curvature will be much less; on the Equator itself the latitude has no curvature and you are equally likely to move north as south. In the southern hemisphere, the situation is reversed: stepping to the north is more likely.
If you continue taking random steps, after a very long while, it might even seem that something was mysteriously forcing you to move toward the equator.
But in fact it isn't. Compare a strip of land just north of the border with one of equal width just south of the border. The southern strip's southern border is longer than the northern strip's northern border and so the southern strip will have slightly more area than the northern one. If you are taking random steps, it's more likely you'll end up in the larger area than in the smaller area.
Space-time
General Relativity describes the Universe in terms of four dimensions, three for space and one for time.
The 49th parallel was a one-dimensional boundary in a two-dimensional world, and the air/water surface was a two-dimensional surface in a three-dimensional world. In the world of Physics, what we call space is a three-dimensional boundary in a four-dimensional world.
In the time dimension, the future exists on one side of us, and the past on the other.
What we call now
is simply the three-dimensional space that separates the past from the future.
(And just as the line on the map and the air/water boundary were artifacts that don't have any intrinsic existence of their own, perhaps our own now
is just as tenuous.)
Just as we could increase the length of a parallel of latitude by moving toward the Equator, and increase the area of the air/water surface by creating waves, it's also possible to increase the amount of space that we call now
.
According to General Relativity, mass distorts space, creating more space near itself. For small objects, the effect is too small to notice. But for massive objects (e.g. the Earth, the Sun, a black hole), the effect of this creation of space by mass is significant.
If we measured a distance in space using an incredibly long tape measure with no mass, and then placed a large mass near its centre, we'd see the measured distance increase. The two end points wouldn't have moved, but the space between them would have increased.
Near a large mass, what we think of as a thousand miles behaves as if it were actually a little more than that. The larger the mass, or the closer to it, the greater the difference. Where we would normally measure a thousand miles, we might see a thousand and one instead. Rulers in the extra space near the mass appear to be slightly shorter than our identical rulers.
Even stranger, when we observe light moving across that thousand miles, we realize that, in order to guarantee the constant velocity of light when measured with the apparently shorter rulers that exist near the mass, time itself must appear to run a little slower there too.
(In case you're thinking this is ridiculous, realize that the clocks in GPS satellites are set to run a little slower than normal in order to keep synchronized with the clocks on Earth, which are slowed down by being so much closer to the Earth's mass.)
Quantum fluctuation
Even if you didn't think that shrunken rulers and slow clocks were ridiculous, you'll have to agree that the Quantum world really is ridiculous.
Quantum physics describes extremely small objects and extremely small intervals of time. At this level, quantum particles are continually popping out of and into existence, splitting to form other types of particles, and almost immediately recombining to form the original. Particles disappear momentarily and then randomly and unpredictably reappear at a nearby location.
And quantum particles are of course what atoms, molecules, the Universe, and everything are made of.
You don't need to understand the details of how this happens (no one else does either), but accept that it is the only working theory that describes how the world actually behaves.
Gravity
Now, consider a quantum particle disappearing and reappearing nearby.
This is equivalent to the random step you took in the farmer's field. Just as the extra area created by the curved Earth caused you to be more likely to step to the south than to the north, the extra space created by a massive object makes a quantum particle more likely to reappear closer to the nearby mass than farther away.
And just as it seemed that you were mysteriously pulled toward the Equator, the quantum particle seems to be pulled toward the large mass.
But there isn't any actual force involved, nor is there any need to hypothesize graviton
particles.
The effect is very small, but there are an incredibly large number of quantum particles in even the smallest real world
object, and an incredible number of quantum interactions in even the smallest time interval.
The combined effect of quantum particles taking random jumps within space that is distorted by mass is what we observe, in the three-dimensional space we call now
, as gravity.
Objects aren't really attracted to each other; they just appear to be. But it's a whole lot easier for normal people to believe the myth.