This week’s Cosmic Quiz question comes from James Sylvester, who asks,
“If the speed of light is the highest attainable speed, why can’t it escape a black hole?”
First, a few words about the speed of light, which is indeed the fastest speed attainable through space. How fast is it?
The speed of light is 299,792,458 meters per second. That works out to about 186,000 miles per second.
Trying to go at or faster than light through space requires inventing exotic new mathematics that permit real number answers to equations that involve division by zero and square roots of negative numbers. If you can figure out how to do this sort of math, a Nobel prize is yours for the asking.
The speed of light is more than just a zillion times faster than we’ve ever been able to achieve with our technology, it’s also a fundamental constraint on everything – both matter and energy – in the universe.
So if nothing is faster than light, than how can a black hole “trap” light?
Light is trapped in black holes because black holes bend space itself.
All objects with mass curve the space around them. Objects with little mass, such the Earth and Moon, only curve space a tiny amount, while objects with the mass of stars curve space a lot more. For a really massive object, like a black hole, the curvature of space they create in their vicinity is so severe that space is wrapped completely around itself.
Here’s a way to create a model of a black hole:
Take a sheet of paper. That’s the universe. To keep things simple, let’s declare that this is a one-dimensional universe, in that objects within this universe all exist along a single mathematical line and they can move in one direction only – left and right along that line. In this 1-d universe there is no such thing as moving up and down on the paper, nor can you be anywhere except on the paper.
To get from the left side of the paper (we’ll call that point “A”) to the right side of the paper (we’ll call that point “B”) you have to move in a straight line on the surface of the paper.
Without massive objects being present, the 1-d universe lies completely flat, and the shortest route (indeed, the only route) between A and B is along that flat straight line. So far so good. The shortest path between two points in flat universe is along a straight line.
But what if you introduce a massive object, like a star, into your 1-d universe?
The mass of the star bends space itself. You, living on the paper in this simplified universe, don’t see this curvature because your line of sight can only follow the line through space. Seen with the benefit of having extra dimensions (as you are when you hold the paper) you see a straight line traveling on a curved piece of paper. Is the line still straight? YES. It’s the space itself that’s curved.
In this 1-d universe imagining a jump from A to B without following the straight line is the equivalent of imagining a science-fiction jump through “hyperspace.”
What if the object on the line of your paper is so massive that it curves space completely around on top of itself? What if point B were inside the region where the curvature of space exceeds 360 degrees?
Then you’d have a black hole. Traveling along a straight line from A to B (as you must in this 1-d universe) you’d encounter a place where space had wrapped around itself and once you enter this region, no matter how fast you go, even at the speed of light, you can never leave.
That’s a 1-d black hole.
Now try imagining a point in space where space itself has been curved on top of itself in all dimensions – left-right, up-down, forward-backward, and time itself.
Black holes capture light (thus making them “black”) because light is trapped within a region of infinitely inward-curving space.
The term “mind-bending” seems appropriate, don’t you think?