what is gravity?
What really happens when something "falls"?
One of the greatest misconceptions we have about gravity is the idea that
objects are somehow "attracted" to the earth by the "force" of gravity. This is
sometimes thought of as a magnetic-like attraction, which is far from the truth.
If you hold an object, say a "bean bag", and let it go, it will appear, from
your perspective, to travel in a straight line directly to the floor. But
what has really happened here?
Einstein's remarkable thought was to imagine that
space itself is somehow warped by massive objects, and that objects moving
through space simply follow a "path of least resistance". This was an
incredibly bizarre concept for most people to comprehend and many physicists
could not bring themselves to believe it. But once Einstein imagined
curved space as being the source of gravitation, he knew he was right. It
was simply a matter of time before he could struggle through the complex
mathematics that describe the behavior.
Finally, in late 1915, with the help of his close
friend Marcel Grossman, he had it. Then, in 1916 he published, in the
German "Annals of Physics" (Annalen der Physik), the most momentous
scientific revelation of our time, "The Theory of General Relativity".
To understand gravity, there are a couple
fundamental concepts to understand:
1. WE ARE IN CONSTANT MOTION
- The earth is rotating and we are all
therefore constantly moving. Even if you happen to be at home sitting
at your computer, you are rotating along with the earth at a speed of about
800 miles per hour (depending on you location) to the east. Since we
rotate, we see the Sun rise in the east and set in the west.
2. THE SPACE Around us is "curved"
- Even though you cannot see it, the space
around the earth (and the space around you) is "curved". To
demonstrate this concept, imagine a large rubber sheet upon which you place
a heavy (or "massive") object. You can emulate this by
placing a heavy object, like a bowling ball, on your mattress and observe
the depression it makes.
- This effect is a "curvature" of the
fabric (mattress) due to the mass (bowling ball). As you might
imagine, the heavier the object you use, the more significant this curvature
will be. Similarly, any object with mass will have a similar effect on
the invisible space around it.
Einstein's visualizes gravitation
The idea above is exactly what Einstein was
thinking when he tried to imagine why things move they way they do.
He knew the earth was rotating and that he, along with the atmosphere and
everything around him, was in constant motion.
Now imagine that you are standing in your living
room holding a bean bag outstretched in your hand. Without doing
anything, remember that you and the bean bag are moving at a constant speed of
about 800 mph to the east. When you let go of the bean bag, it is still
moving with you at 800 mph to the east. You observe that the bean bag
travels directly to the floor. Why is this?
First, let's pretend it takes 1 second for the
bean bag to hit the ground. Let's also assume we travel 700 feet to the
east in that one second. If this sounds odd, just remember that you,
the bean bag, the furniture, and the house are all moving together at the same
speed. Nothing seems out of place because everything is rotating together
- including the atmosphere around your house.
So 1/10th of a second after letting the bean bag
go, both you and the bean bag have traveled 70 feet to the east. But
something else interesting has happened here. The bean bag is some small
distance below your hand. It has traveled away from your hand. Let's
think about the path that it has traveled.
From your perspective (or "relative to you"), the
bean bag has moved downward a couple inches. Imagine that you were
at "point A" (in space) when you released the bag. 1/10th of a
second later, you are now at "point B" (70 feet to the east). While the
bag has traveled with you, it has also begun to move downward (from your
perspective) to the floor.
Now let's think about the path the bean bag has
traveled from the perspective of some observer in space (an astronaut) with a
great set of binoculars and a bit of x-ray vision. :-). He looks
down and sees you, the bean bag, and your house rotating at about 800 mph to the
east. When you are at point A he sees you release the bean bag.
He continues watching and observes that the bean bag not only travels with you,
but also begins to move downward toward the floor. After 1/10th of a
second, he sees that you have moved 70 feet to the east, and the bag is a couple
inches closer to the earth. From his perspective (i.e., "relative to
him"), it appears as though this bean bag is following a curve - from point A to
point B. As he continues to watch, the bean bag follows a curved path (or
arc) from point A to point J (where there are 10 points from point A through
point J, each 70 feet apart). But from your perspective (again, "relative
to you"), this bean bag has simply appeared to travel straight down to the
So what is the "true" path traveled by the bean
bag? A curve, or a straight line? Well, it depends. It depends
on who is the observer. With this example, you have learned that there is
no so-called "absolute motion". No one can say what is the "true
path" of the bean bag. The motion or path an object takes must always be
described relative to some so-called "frame of reference". In the first
case, where the bag appeared to follow a straight line, the frame of reference
was with you, where you were standing. In the second case, where the bean
bag followed a parabolic curve, the frame of reference (or observation point)
was somewhere above the earths atmosphere.
a gravitational field on your mattress
Let's return to our mattress and bowling ball
example. You can try this at home and see the effect that curve space has
on objects that are passing nearby. Get your bowling ball, or any
dense, heavy object and place it on the middle of your mattress. It helps
to have a rather "flat" top on your mattress.
Now imagine that the bowling ball is the Sun, and
take a small marble (simulating the Earth, or an asteroid), and roll it past the
bowing ball. Depending on how close the marble is to the massive object,
the more it is effected by the warpage of the fabric of space. It's quite
interesting to actually try this with a marble and bowling ball on your
mattress. Both the speed of the marble and the distance from the
massive object will dictate whether it is "captured" by the gravitational
warpage and follow the contours of space in a catastrophic collision with your
[ If you don't have a bowling ball or other
heavy object, you can simply have someone take their fist and push downward on
the the mattress to cause the curvature you need. ]
As you observe the behavior of the marble with
various speeds and distances from the massive object, it becomes obvious
that the path traveled has nothing to do with a "magnetic" attraction or "force" of any sort. The objects are simply following the contours of space - the path of least
resistance. There is no "gravitational force", per se.
In truth, objects simply move through space (or
more accurately through "space-time").
Greg's comments on Curved Space
(from Yahoo! Answers)
What is "curved space"? -asked
Chosen as "Best
This is a bit of a
"layman's" answer but here goes:
Space, not just the world we all live in, but all
the space in the universe is "affected" by MASS
(massive objects) in a way that most people do not
realize. Most people think that gravity has
something to do with a "magnetic" force of some kind
that causes objects to be attracted to other
objects. The weird truth of it is that "gravity" is
a side-effect of the "geometry" of space-time.
What does that mean? What Einstein figured out is
that SPACE is actually "warped" or "curved" by mass
(or massive objects) like the Sun or planets, or
black holes - that sort of thing. Now while you
cannot really SEE this,
you can see the "effect" of it. When an asteroid
passes closely by a massive object, it may be
affected by the objects gravity and drawn toward it.
But this has nothing to do with magnetism - it is
actually the effect of the asteroid following the
"curvature of space" (since the massive object -
like the Sun - has warped space).
To think of a common, simple example: imagine a
BOWLING BALL sitting on a large rubber sheet (or
your mattress).... you will notice that the massive
ball makes an big depression in the sheet, or "dip"
in your mattress. Now imagine rolling a marble
across this sheet, or your mattress. If it is close
enough to the bowling ball, it will be "captured by
the gravity" (really warped space) and spiral inward
toward the bowling ball. So "Curved Space" is the
"warping of space" caused by "massive objects", and
what we see as gravity is really just objects
responding to the natural curvature of space.
Many people then ask why an apple falls straight to
the ground on earth. The general idea is that space
is "more steeply curved" when close to massive
objects (as we are standing on earth) and since we
are already ALWAYS in motion (you knew that - with
rotation and all), when you "let go" of a ball, it
follows the path of least resistance in curved
space, which is right DOWN to the ground (now that
is a "steep curve").
Mallon, La Plata, MD (submitted to Yahoo! Answers,