Does Light Have Mass? Why Does It Get Sucked Into Black Holes?
We've all been told that nothing can escape a black hole, not even light. Our teachers told us so, our books told us so, and now even documentaries are talking about black holes; always pointing out to us that even light will get sucked into black holes.
The basic premise of a black hole is fairly simple. A giant star builds up so much mass that it literally gets sucked into itself by the sheer amount of gravity it produces. We all know on an elementary level how gravity works. So it's easy to understand why objects passing by get sucked into black holes. On the other hand, we've always been taught that light is not matter and therefore not affected by gravity. Earth has gravity after all, and yet if you turn on a flashlight, the light doesn't eventually fall to the ground. So what makes black holes so special that their gravity can suck light in, never letting it go?
Black Holes and Spacetime
In order to understand why light gets sucked into black holes, it is first important to understand a few particular traits of the black hole.
As you may know, everything with mass has gravity. The more mass an object has, the more gravity it has. This is why the planets revolve around the sun, and not vice-versa. But contrary to what you may think, gravity is not the key component in a black hole's ability to trap light. The real culprit is the mass of a black hole, and its effects on spacetime. (Also referred to as spacetime or space-time)
Everything that has mass causes the spacetime around it to bend. More mass creates a larger bend in spacetime. To explain, imagine an empty trampoline sitting in your back yard. This is what space time would look like if there were no mass to distort it, except that space has three dimensions, not just two. Now put a bowling ball on top of the trampoline. That heavy ball creates a distortion in your trampoline. This distortion is exactly what happens in space wherever mass can be found. To make things far more complicated, black holes take this to the extreme. At the event horizon of a black hole, space time actually bends into itself!
The Shortest Distance Between Two Points
As a rule, light will always travel the shortest distance between two points. Here is a mind-bender for you: The shortest distance between two points is not always a straight line. Yes, your elementary teachers lied to you. Take that home, chew on it for a while.
The truth is, the straight line theory only works in two-dimensional space such as on a piece of paper. On a curved surface, this is not the case. Real life examples of this are actually in use on a daily basis. If you look at the figure to the right, this is the plot for an airline flight with no layovers from Seattle to London. One would normally assume that this flight would just cross over the US passing through Maine, then right over the Atlantic Ocean. Since Earth is spherical, however, taking that path would actually be far longer than the path depicted. (Check out other flight paths here) This is known in aviation as the great circle.
Black Holes and Light
Now that you are armed with the necessary information about how light travels, and how black holes bend spacetime, you can begin to understand why light will get sucked into black holes. Just like a plane using the curvature of the earth to travel between two points, light will follow the curvature of a warped spacetime, in order to get from origin to destination. This can be seen whenever light travels past a massive object. The light appears to bend. But on the contrary, it is spacetime itself that is bending, not the light.
When light travels into a black hole it will eventually hit the event horizon, and as spacetime continues to bend into itself; the light will follow. So really, light will never get sucked into black holes. Instead, light is simply following its normal behavior, and traveling straight into black holes on its own!