Hello and welcome to the newest round of: random musings of an astrophysicist.
Invisibility is a popular ability in science fiction and fantasy. There is magical invisibility via spells, potions or cloaks. And there is technologically achieved invisibility in many forms. But if we were to ask the laws of physics, what would actually happen if we did turn invisible?
First, there are two ways to achieve true invisibility. Either light goes through you without any contact, or it goes around you.
And that's the problem. If light doesn't interact with you, then how do you see? No light falls into your eyes either, so you are blind. An interaction always works both ways. The world is getting no visual information that you are there, and in turn you get no information about the world.
But it is worse. If you were to turn perfectly invisible right now, you would pretty much explode and the particles that made up your body would fall into the Earth like little ghosts.
Why? Because the force that binds your molecules together and enables atoms to exist is the electromagnetic force (EM force). This is the force between two charged particles. How is this force transmitted? I mean, if you take two magnets and hold them close, how do they know they are there? How do they reach out across empty space and exert influence? Light!! Light is the carrier of the EM force. Your magnets fire particles of light at each other, and each such particle carries a bit of momentum and energy.
So, turning invisible in the way of having light go through you would mean to shut off the EM force. Only then would light from the outside no longer interact with your particles. But then, all your particles stop seeing each other as well. Shutting off the EM force = shutting off the glue that binds your particles together. BOOOM!! Hahaha.
But why would you fall into the Earth? Because gravity is currently pressing you against the ground, and the ground is pushing back via the EM force as well. This is true for all things you are touching. Touching simply means that your particles get close to other particles, which then interact via light. So this means .... shutting off the EM force = shutting off the ability to touch things. Thus your particles fall into the Earth as if it weren't there. Gravity still acts on those particles.
2. The end of Earth
Did you know that Earth as a living planet is now in the last 20% of its life time? Yes, only 1/5 of its life time is left.
There is a common bit of misinformation about this. Perhaps you have been told that the sun will become a red giant in about 4 billion years - it will expand and become so large that it may reach and swallow the Earth. But computer models can't entirely predict if the sun will become large enough for that or not. But no matter what happens, the Earth will be toast and life will end then. You heard about that, yes? It's WRONG.
The truth is, the sun will indeed become a red giant. This is part of the natural life cycle of our home star. But life will end 3 billion years earlier.
Right now, our sun is in a phase called the main sequence. All stars go through this phase, before becoming giants or other. During the main sequence, stars are not stationary - they become continuously more luminous, hotter and larger. In its youth, our sun was only 70% as luminous as it is today. In 1 billion years, it will be 10% more luminous than it is today. At that point, its light will blow away Earth's atmosphere and vaporize the oceans, then blow away the water vapor, leaving the Earth a dry rock without air. And thus all life will end.
Fun fact 1: The sun will actually turn into a red giant more than once. If becomes one, then shrinks again, then becomes one again. This cycle goes faster and faster and is quite irregular, while the sun is blowing away its layers and forming a planetary nebula that one day, a distant civilization will photograph and put as their desktop backgrounds.
Fun fact 2: the official name of our sun is Sol. Why don't we ever use it outside of sci-fi?!
Fun fact 3: the sun is entirely snow white, as seen from space as well as from Earth. Any picture of the sun showing it as orange is a false color image, allowing you to see details.
Random fun fact for those who read everything: A question mark ? shows the ass of a cat from behind, when the animal is curious. An exclamation mark ! shows the ass of a cat from behind when the animal is surprised. !??!?!?!?! lots of cat butts. Meow meow meow. Hahaha.
This concludes this round of: random musings of an astrophysicist (who also happens to be a science fantasy writer and can't help but think about this stuff)
Now, where did I put that calculator again...