In 1907 a young Albert Einstein was looking for a way to extend his special theory of relativity to include acceleration.It took him eight years to come up with the mathematical solution that we know as the General theory of relativity.
It tells us that we all live in a 4-dimensional spacetime.We can move up and down,left and right,backwards and forwards,That's three physical dimensions,but we also move in time-in the direction of the future,That's dimension number four.
广义相对论认为 我们都生活在一个四维时空中,我们可以上下 左右 前后的运动,这是三维实体空间，但我们还能在时间维度上朝着未来前进。这就是第四个维度。
One consequence of Einstein's theory is that space-time can be stretched,a bit like a rubber sheet,He found out that mass,or energy,tells the 4 dimensional space-time how to deform,and vice versa,that deformation tells matter How to move.
It's way easier to think about if we consider less dimensions,Let's cut one out and limit ourselves to two spatial dimension,When we put in the Earth,its mass curves the space-time and our apple falls towards it,And this is what we call gravity.We can make it even simpler still,and remove another spatial dimension.
Here we are,you,me,the apple all falling towards Earth,While we can clearly see and feel the attraction from something really massive and nearby,Like the earth,the effect of things that are further away is harder to notice,
For example,the influence we feel from the moon is pretty weak,And the main effect we can observe is when parts of the ocean fall into the space-time curvature.That the moon causes,Most people like to call this effect tides.It's the gravity of the sun that holds our planet in its orbit,but we can barely feel this effect at all.This is because the gravitational force decreases with the square of the distance,So it becomes weak really quickly the further away you go.
In the end gravity may be a weak force,but while electric charges cancel each other out in large objects,that can't happen with gravity,Over large distances gravity always wins and will even allow us to perceive the universein a completely new way.While all modern astronomy sees the Universe,Einstein's general relativity predicts that: We can also hear the Universe with Gravitational waves.
These ripples in space-time are the sound of the universe and travel at the speed of light,They are emitted by the most extreme objects and are detectable from really really far away.This is pretty awesome,because soon the effects of gravity might be more to us than just the means by which stuff sticks to the ground.It will also provide us with a whole new way of observing our Universe and all the incredible objects that is contains.