Can You See the Back of Your Own Head?
Have you ever wondered what the back of your head looks like? I know, I know; you can use two mirrors, reflecting the light in such a way that it bounces towards your eyes. But what if you had no mirrors available, and you desperately needed to check that your bald spot hasn’t increased in size? There must be a way!
Fear not; I can confirm that there is indeed a way to see the rear of your noggin!
For this, you will require:
- A ludicrously fast spaceship
- A spacesuit
Using this spaceship of yours, you’re going to travel quite a distance. You’re going to be travelling to a black hole.
The point of no return
A black hole is a point in space where gravity is so strong, that not even light can escape! This means that we can’t actually see a black hole; we can only measure how it affects objects near it. For example, we can indirectly observe a black hole by looking at how light is affected by the incredible pull of its gravity. This is called gravitational lensing – the bending of light due to gravity.
As the galaxy moves behind the black hole, the light it emits gets bent around the black hole // Animation via Wikimedia Commons.
Light-minutes and light-years
The closest black hole to us (which has the name V616 Monocerotis) isn’t just down the street; it’s 28,000,000,000,000,000 (28 quadrillion) kilometres away! For us, this seems like a huge number, but in terms of the universe, it’s actually tiny. So, instead of using kilometres to describe the distance between objects in space, we can use ‘light-years’. One light-year is the distance that light will travel through space in one year.
Light travels at just over 1 billion km/h. This makes our black hole a mere 3000 light-years away! In comparison, light from the Sun takes about 8 minutes to reach us here on Earth, which means that the distance between us and the Sun is 8 light-minutes. Maybe our black hole isn’t that close, after all! Still a bit bamboozled by light-years? This fantastic TED-Ed video explains it superbly.
Great! You’ve travelled 3000 light years in your spaceship, and you’ve arrived! Jump into that spacesuit and open up the airlock – it’s time to jump into the black hole!
While you’re on your way down into the black hole, try to picture a satellite in orbit around the Earth. Got it? This satellite is still feeling the Earth’s gravity; it is still falling towards the Earth. It stays in orbit, however, because it is travelling at just the right speed to fall off the side of the Earth, and so it keeps on going around and around.
A satellite in orbit around Earth // Photo by NASA via Unsplash.
The same concept applies to black holes – objects still orbit them, but at a much greater distance. When an object orbits closer to the black hole, it needs to be travelling heaps faster in order to “fall off the side” of the black hole. Remember how we said that light travels at just over a 1 billion km/h? Turns out that there is a point close to black holes that light is travelling just slow enough to be caught by the black hole’s gravity, but just fast enough that it doesn’t get sucked in – the point where light orbits the black hole.
This point is called the ‘photon sphere’!
Now, about that bald spot…
At this point, if you turned your head to the side so that you’re no longer facing the black hole, the light that is bounced off the back of your head would orbit the black hole and come all the way back around to you, allowing you to see the back of your head! How cool is that!
Unfortunately, your bald spot would become the least of your worries, as you would definitely die due to the immense gravitational forces ripping you apart. Still, you’d be the first human to be killed by a black hole, and that’s something!
Cover image by Ute Kraus via Wikimedia Commons.