"Sooner or later disasters such as an asteroid collision or a nuclear war could wipe us all out, But once we spread out into space and establish independent colonies, our future should be safe."
Stephen HawkingTime travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a 'crank.'
Stephen HawkingOne might think this means that imaginary numbers are just a mathematical game having nothing to do with the real world. From the viewpoint of positivist philosophy, however, one cannot determine what is real. All one can do is find which mathematical models describe the universe we live in. It turns out that a mathematical model involving imaginary time predicts not only effects we have already observed but also effects we have not been able to measure yet nevertheless believe in for other reasons. So what is real and what is imaginary? Is the distinction just in our minds?
Stephen Hawking