You're reading this blog post, which means you're alive. Good.
I'll begin by assuming that you believe you're living on Earth as of now - a reasonable conclusion to draw, I suppose. But what if I told you that we were nothing but computer-generated simulations, living in an artificial world?
I doubt you'd be very willing to believe me, and I don't blame you - it's an impossible thing to prove. Well, nearly impossible. To get a little philosophical on you, since the beginning of philosophy itself many have speculated that our reality is nothing but an illusion - something fabricated by our own minds. Obviously, it seems ridiculous to assume that we are just simulants, artificial intelligence living in a digital world, but this is in fact a theory.
The modern form of this simulation theory was postulated by Nick Bostrom, philosopher at Oxford University, in the aptly named paper, "Are you living in a computer simulation?" He states that, due to the enormous computing power that humanity will likely develop in the future, it is more probable that we are simulations, living in a world generated by "posthuman" technology, than the belief that we are carbon based organisms inhabiting the "real" universe. This seems like an absurd claim to make, but he does use several probability calculations to back this claim up.
This theory corresponds with the "holographic principle" which implies that our three-dimensional Universe is a hologram/illusion, projected from information encoded on a two-dimensional chip.
But sane people have criticized this argument on the grounds that such a computer, with the power to artificially create our Universe, would have to be larger than the Universe itself, and would require more energy than the entire Universe has. Which is a valid claim - but as others quickly pointed out, it would take a heck of a lot less computing power to create an "imperfect" Universe, where the simulation is just good enough to fool us into thinking it's real. In other words, parts of the Universe would only be programmed as we observe them - like, for example, far-off galaxies only exist while we simulants are studying them with scientific equipment. And the second we look away, it's gone. This is reminiscent of solipsism, where things only exist as we observe them. Of course, this again delves more into philosophy.
Yeah, it's a creepy thought, and it lies just at the border of physics and philosophy. But don't let it worry you too much - I'm sure we're all real.