Time Travel to the Past
We all wish we could go back in time to hang out with some dinosaurs or kick it with Hypatia at the Library of Alexandria. But sadly, this violates the physical law of causality, which is turning out to be a tough law to break. Time travel into the future is possible if a person travels close to the speed of light, but popping back to the past for a stroll around Xanadu is looking to be nigh impossible. With science, you can never say never, so let's cross our finger that we'll get a Flux Capacitor yet.