Why does gravity pull us down and not up?
Gravity is the reason that objects with mass or energy are attracted to each other. That’s why apples fall to the ground and planets orbit the stars.
Magnets attract some types of metals, but they can also repel other magnets. So why do you feel the pull of gravity?
In 1915, Albert Einstein understood the answer when he published his theory of general relativity. The reason gravity pulls you toward the earth is that all objects with mass, like our Earth, actually bend and curve the fabric of the universe, called spacetime. That curve is where you feel gravity.
What is Spacetime?
Before entering the complex world of gravity, you need to understand spacetime.
Space time is exactly what it sounds like: the three dimensions of space – length, width, and height – combined with a fourth dimension – time. Using some very brilliant mathematics, Einstein was the first person to realize that the laws of physics work in a universe where space and time are unified.
What this means is that space and time are connected – if you move fast in space, time slows down for you compared to someone moving slowly. This is why astronauts – who move so fast in space – age more slowly than people on Earth.
Matter Makes Gravity Wells, Not Gravity Hills
Remember, gravity is the idea that objects in the universe attract each other because spacetime is bent and curved. When Einstein developed general relativity, he showed that everything in the universe can be curved in spacetime – in physics terms that matter has mass and energy.
Because your brain usually thinks about the world in three dimensions, it is very difficult to think about the four dimensions of spacetime as an idea. So to make it easier to imagine, imagine the surface of a trampoline. If there’s nothing in it, it’s flat. But when you stand on the trampoline, it spreads your legs and creates a valley with you in the center. If there is a ball on the trampoline, it will roll around your feet.
This is a two-dimensional example of how spacetime works. Your mass stretches the trampoline, creating what is called a gravity well that the ball rolls. This is very similar to how the gravity of a heavy object – like the Earth – pulls objects like you and me towards it.
To make things even weirder, since space and time are connected, time is also stretched by heavy objects!
In the movie ‘Interstellar,’ the characters go to a planet near a black holeand while they are there, they age more slowly than others.
The heavier you are, the steeper the sides of the trampoline well. That’s why massive objects in the universe – like the Sun or black holes – have stronger gravity than Earth.
So why does gravity pull you and not away from you?
Imagine someone falls under a trampoline and is pushed up. The ball is rolling! This is a gravity hill, not a gravity well. As far as scientists know, the object – or objects – always create gravity wells and not gravity hills. Scientists can imagine things made of exotic matter or energy that cause gravity to push you into space, but so far, no one has found anything that could cause gravity to pull you away from Earth.
Written by Mario Borunda, Associate Professor of Physics, Oklahoma State University.
Adapted from an article originally published on The Conversation.
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