Null hypersurface
Okay kiddo, a null hypersurface is like a big invisible wall that's really good at stopping light from passing through it. It's like when you put your hand up to block the sun from shining in your face.
Now, imagine that this wall is made up of a bunch of tiny points, kind of like the dots on your bed sheets. Each of these points is called an event, which is just a fancy word for a really specific point in spacetime.
These events can be connected by lines called light rays, which represent the path that light would take if it were passing through the wall.
When we say the wall is null, it means that the light rays followed by the line can never cross the wall at a right angle. Instead, they'll always be at a diagonal, kind of like when you try to draw a straight line across a sheet of paper with a ruler that's not perfectly straight.
This might sound like a strange thing to think about, but it's actually really important for understanding how gravity works in the universe. Scientists use null hypersurfaces to study things like black holes and the nature of spacetime itself.
Now, imagine that this wall is made up of a bunch of tiny points, kind of like the dots on your bed sheets. Each of these points is called an event, which is just a fancy word for a really specific point in spacetime.
These events can be connected by lines called light rays, which represent the path that light would take if it were passing through the wall.
When we say the wall is null, it means that the light rays followed by the line can never cross the wall at a right angle. Instead, they'll always be at a diagonal, kind of like when you try to draw a straight line across a sheet of paper with a ruler that's not perfectly straight.
This might sound like a strange thing to think about, but it's actually really important for understanding how gravity works in the universe. Scientists use null hypersurfaces to study things like black holes and the nature of spacetime itself.