Awesome question! The answer involves
understanding Einstein's theory of general
relativity, which is the best theory of gravity we
currently have (coincidentally, Einstein came up
with his theory in 1915, so this year is the 100th
birthday of general relativity!).
We're used to thinking of space as something we
move through - you can go up and down, forward and
back, left and right - but the space itself just
sits there. Similarly, we're used to thinking of
time as some universal thing that keeps track of
how everything changes from one moment to the next
- the steady ticking of all clocks. But here's
Einstein's mind-blowing insight: space and time
are actually part of the same thing, called
spacetime. Even crazier, spacetime doesn't
sit there - it can warp, bend, curve, and even
But what happens when an object (like a
a planet) tries to move through this warped
spacetime? Well, in ordinary flat (or unbent)
spacetime, stuff just moves in straight lines.
But when you bend the spacetime, that makes
objects move along curved paths. If that's
confusing, here's a nice analogy: imagine that
spacetime is like a trampoline. If you roll a
marble along the flat trampoline, the marble just
travels in a straight line. But what happens if
you bend the trampoline by putting a weight in the
middle of it? If you try to roll a marble along
it, the marble will move in a curved path, since
the surface of the trampoline (the "space" along
which it's moving) is curved.
General relativity says that this is what gravity
is: gravity is just the curvature of spacetime
caused by heavy objects. The reason we feel
pulled down towards the Earth isn't because the
Earth is really pulling on us; instead, the Earth
is bending the spacetime around it, and we feel
that curved spacetime as something pulling us
towards the center of the Earth (that is,
something pulling us "down"). Similarly, just
like the marble on the trampoline, if an object
tries to move by the Earth (say, like a satellite
or the moon), the curvature of spacetime causes
that object's path to change towards the Earth.
That's what creates orbits, and what keeps planets
in their orbits.
I hope this helped!
Gravity exerts force on objects in proportion
to their mass, i.e. the more massive the object,
the more force it exerts on other objects, and
the more force other objects exert on it. Planets
are already moving around fast enough that all
gravity is able to do is to keep them from flying
off in a straight line, which would lead into deep
space away from the sun. Things on Earth have are
typically not moving, so gravity is the only force
acting on them, so they fall downward until they
are on the ground.
Click Here to return to the search form.