by Ferdinando Catalano
A very special photo …
This picture went viral, and was shared around the world in a few days. This is a photograph of a BLACK HOLE.
What is it about the photo that proved, once again, that Albert Einstein’s theories were correct?
To see the importance of this photo, we must take a step back and think—in a simplified way—about the speed of light.
Imagine that you are in a train compartment. You want to go to the restroom, so you move forward along the train corridor, going in the same direction that the train is traveling. For anyone who might be watching from an outside train station along the route, the speed you are traveling would be combined with that of the train.
Does that make sense?
If not, here’s another example. Think about what happens when you walk up the steps of a moving escalator. You are moving faster than the normal speed of the escalator, right? In the first example, your speed is added to that of the train; in the second example, your speed is added to that of the escalator.
Now imagine that you are holding a flashlight in the train and moving toward the restroom, again, in the same direction that the train is moving. You might think that the speed of the light coming out of the flashlight will be added to the speed of the train, but it is not.
The speed of light has an upper limit, a value that cannot be exceeded, and is not dependent on any other reference system. This value is approximate (about 300,000 km/s), and is a fundamental part of Einstein’s Theory of Special Relativity (1905). To date, it has never been contradicted.
But what does this have to do with the black hole?
A little later in life (1916), Einstein presented the Theory of General Relativity, which states that it’s the presence of matter that causes the curvature of space. This concept can be difficult to understand, so here’s an experiment you can try at home . . .
Invite two friends to your house for coffee or tea. While you’re enjoying your drinks, explain that you need your friends’ help to understand the effects of a black hole. Get a ruler and a marker, and then tell your friends to take the opposite sides of the tablecloth and hold it open, taut in the air. Now draw two points, A and B, on the tablecloth and ask: What is the shortest line between A and B?
<< Of course your friends will tell you, it’s a straight line! >> And they would be right. So draw the line with your ruler and marker.
Now go to the kitchen and find something that weighs about 2 pounds (it could be canned food, or something else). Put it in the center of the pulled-tight tablecloth.
Next, draw a point C and a point D and then ask your friends (who are probably now wondering what’s wrong with you) what is the shortest distance between C and D. Is it still a straight line? No! That’s because the tablecloth, the physical space between points C and D, is now curved. And what caused that curve? It’s the extra weight that you placed onto the tablecloth.
What if, instead of 2 pounds, you put 10 pounds into the center of the tablecloth? The curvature of the space would have been even greater. What if the weight had been 100 pounds? You can see where this is going!
In the presence of a very high concentration of matter, cosmic space will “bend” and create a hole.
That means that in a cosmic space that’s very bent, Euclidean geometry does not work. That is, the geometry that says a straight line is the shortest distance between two points is no longer valid.
That’s all good so far. But why is this hole called “black”?
Imagine that you somehow find yourself in a hole in the ground. To get out, you’ll have to jump. But if you can’t jump fast enough, I’m sorry, you’ll stay in the pit.
It’s exactly the same for the light that’s near the black hole. The light is sucked into the hole like a ball falling into the dimple of the tablecloth. The curvature of the space is so deep that the velocity of escape required to get out of the black hole is even higher than the speed of light, which, as we saw at the beginning, is limited. In other words, the light, as fast as it is, is unable to “jump out” of the hole. Therefore, to us, the hole appears black. Just like in the photo.
P.S. The next time you happen to see or play on a trampoline, take a moment to think about this theory!