The originator of the so called Simulation Argument, Professor Nick Bostrom, assigns a rough 20% chance of us living inside a computer simulation.
Nick Bostroms argument, his revamping of the classical brain in a vat scenario, was presented in an article in the Philosophical Quarterly in 2003 and has since spurred a lot of attention.
Another prominent philosopher and author within the field Philosophy of Mind, Professor David Chalmers explains the argument in an interview with Scientific American journalist John Horgan:
…We should expect that in any universe that contains people as intelligent as us they are going to create great simulated worlds. I mean loook at our world. Simulation technology is already very good. We’ve got, you know, Sim Life, Sim City, Sim Earth, The Sims. Pretty soon, given a couple of decades, there’s going to be Sim Universe. We’re all going to run simulated universes in the background. There’s going to be simulated people within them. Then you start to think: Each of these simulated universes is going to have hundreds, millions, billions of simulated people. Maybe each of them will be running their own simulations and so on. Then you start to think: Count the numbers. The number of simulated people in the world is going to vastly outnumber the number of level zero people in the world. And now there’s a question of course about weather simulated people would in fact have their own consciousness and their own experiences… But some people, including me think there is good reason to believe that even simulated people, you know, silicon brains, so to speak, could have consciousness. Let’s just run with that for a second, and then say: What are the odds right now that in this world that probably has vastly more simulated people than real people that I’m one of the lucky ones at level zero?
…I mean I don’t know if this argument is right. There are various places where you might question it but I think it’s serious enough to think that it’s not out of the question…If you made me a bet. I’d give at least twenty percent odds that I’m in a simulation right now.
My own counterargument against the simulation argument is based upon what I believe to be the preconditions for an optimally detailed and precise simulation.
An efficient simulation on a microscopic level of nature’s trillions of processes every nano second is not optimal to run on a central single processor. An optimal simulation of nature can only be achieved where processing is maximally decentralized. A model for maximally decentralized processing is, as I see it, nature itself.
If you skip the idea of a central processor, you loose, as I see it, the essense of the simulation idea.
Nick Bostrom has a very good and clear Q & A web page about the Simulation Argument.
These are the questions he answers:
1. What is the simulation argument? 2. Do you really believe that we are in a computer simulation? 3. Is the simulation argument a variant of Descartes' daemon or the brain-in-a-vat argument? 4. If we are in a simulation, doesn’t that undermine the reasoning in the simulation argument? 5. “I can see glitches in the Matrix!” 6. What happens to the simulation argument if the universe is infinite? 7. Couldn't we simply be in the first generation, so that all the simulated creatures that will one day be created don't yet exist? 8. How has the Simulation argument affected how you live? 9. Isn’t the simulation-hypothesis untestable? 10. What is the relation between the simulation argument and religion? 11. What if we are simulated by aliens? 12. Why do you think there’s been so much interest in this? 13. If we are merely simulated, does that mean that the world isn't "really real"? 14. How did you come up with this? 15. "If the Universe is not simulated to a quantum degree of accuracy, the simulation can be immediately exposed via Bell's inequality or some similar test." 16. Can the simulation argument be generalized?
As mentioned above Nick answers the above questions here.