How can black holes attract light when light is not a physical object? – Will, age 8, Victoria
Very good question, Will! I wondered this too when I started studying the wonders of physics.
To answer that, we first have to explain three things: 1) what is light, 2) what is gravity, and 3) what is a black hole?
1) What is light?
Light is simply a type of energy that propagates in space. There are many types of light that we cannot physically see, but that we can detect and even use. For example, the ultraviolet rays that come from the Sun are the reason why we should wear sunscreen–so the light doesn’t hurt our skin.
The electromagnetic spectrum includes all types of electromagnetic radiation – that is, energy. The part in the middle with the rainbow and sun symbols on it indicates visible light. Shutterstock
It is important for us to remember that even though light has no mass, it is still a physical object in our universe, obeying the laws of physics.
What’s interesting is that, regardless of the type of light, it all follows the same laws of physics in the universe. One rule is that light always travels in a straight line through space.
This is where we need to decipher gravity, and what space is made of.
2) What is gravity?
Gravity is the force that keeps us safe on Earth. Gravity also keeps the Earth rotating around (orbiting) the Sun. So, what causes gravity?
Many scientists throughout history have pondered this question, and produced a variety of theories. However, when Albert Einstein presented his theory of general relativity in 1915, we began to truly understand what gravity really is, and how it affects our universe.
Einstein has mathematically proven that we exist in something called “space-time”. You can imagine it as the fabric that makes up our universe. Like fabric, it can bend and stretch.
I like to imagine space like a trampoline. When you place something heavy (like a bowling ball) in the middle of the trampoline, the fabric underneath will buckle and sink.
A trampoline bent by a bowling ball is like a spaceship bent by a heavy object. This is how gravity works. Sara Webb, CC BY
Now imagine a trampoline the size of the universe, and we put the Sun on it. The slope on the trampoline represents the Sun’s gravity. We can also do this for all objects that have mass.
When space is bent by that mass, normally straight lines become slightly curved. You can see it in the image below. This is most extreme in very massive objects that we call black holes.
The curvature of space around the Sun compared to a black hole. Notice the lines that were previously straight become bent and curved due to gravity.
Sara Webb/Wikimedia Commons, CC BY
3) What is a black hole?
In my opinion, black holes are one of the coolest things we have ever discovered in the universe. Black holes are areas in space that are so dense, nothing can escape.
Black holes usually form when very large stars become too heavy and collapse (explode). Astronomers suspect that all the mass inside a black hole is actually compressed into a single point in its center.
Black holes have a bad reputation for eating “everything nearby,” but that’s not true. Black holes do have a distance from their center, which we label as the point of no return. This is what is called the event horizon.
However, beyond this point, light and matter can orbit the black hole for a very long time.
So how do black holes attract light?
Once we have outlined these three important things, we can answer the big question posed by Will: how can black holes attract light?
When light moves closer to a black hole, it still tries to move in a straight line. As it gets closer to a black hole where space is curved, light will follow the curve.
When you are very close with a black hole, light is trapped in circles around it. That was because the space structure was bent to the extreme. As you remember, light is indeed a physical object and is influenced by space.
Perhaps my favorite part is, this fact doesn’t just apply to black holes.
Anything with enough mass can make light bend around itself, even the Sun. This is how scientists first confirmed Einstein’s theory of gravity most likely true in 1919.
Something very heavy, like a collection of galaxies clustered together, can bend space so that it acts like a magnifying glass and (shows a magnified image) the stars behind it.
Rahma Sekar Andini from Malang State University translated this article from English
2023-09-21 08:52:33
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