To answer this question, we must first explain three things: 1) what is light, 2) what is gravity, and 3) what is a black hole?
1) What is light?
Light is just a type of energy, travelling through space. There are many different types of light we can’t physically see, but can detect and even use. For example, ultraviolet light that comes from our Sun is why you have to wear sunscreen – so the light doesn’t hurt your skin.
It’s important for us to remember that just because light doesn’t have mass, it still is very much a physical thing in our universe, following physical laws.
The neat thing is, no matter what type of light, it all follows the same physical laws in the universe. One rule is that light always wants to travel in a straight line through space.
This is where we now need to break down gravity, and what space is made of.
2) What is gravity?
Gravity is the force that keeps us safe here on Earth. It also keeps Earth circling around (orbiting) our Sun. But what causes gravity?
Many scientists in history pondered this question, and came up with all sorts of theories. But when Albert Einstein presented his theory on general relativity in 1915, we started to really understand exactly what gravity was, and how it affects our universe.
Einstein had mathematically worked out that we exist in something called “spacetime”. You can picture this as the fabric of our universe. Like a fabric, it can bend and stretch. I like to picture space like a trampoline. When you put something heavy (like a bowling ball) in the middle of a trampoline, the fabric underneath it bends and sinks down.
Now imagine a universe-sized trampoline, and we place our Sun on it. The dip in that trampoline represents the gravity of the Sun. And we can do this with every object that has mass.
When spacetime is bent by that mass, the lines that would normally be straight become slightly curved – you can see that in the picture below. This is most extreme around the really massive objects we call black holes.
