Well, going by Einstein's theory about time, time is a dimension, 4-d if you will. By this theory, using the right methods, one could go to any place in time, the relative past, present, and future. Using this ideology, one could theoretically go back in time using methods such as a worm hole or a rift in space-time. The only bad thing about that rather than using a theoretical time machine is you wouldn't be able to get back to the present, you'd be stuck where you ended up for the rest of your life, or until you found another rift that would lead you back to the current time. That in and of itself is a great challenge, for you would have no tools or guides on how to judge where a rift or worm hole will take you. If one could discern how to create these rifts in space-time and figure out how to know what time you'd be going to, you could theoretically make a time machine. Not one you'd carry in a pocket, or fit upon a ship, but one you would project into the planes of reality and travel. Not that a 15 year old high school student would know anything about that. >.>
You get an empty tonic can.
*Ic*
Edit: Also, you consider the notion that a wizard can survive and repair an injury so grievous as beheading. That is the concept that He Who Must Not Be Named held, that wizards and witches shouldn't be held to mere mortal death. Plain and simple, it didn't work that way. If it did, Sir Nicolas would have his head on, and not be nearly headless.