EXPERIMENTS WITH TIME
Some thoughts on accessing the past.

Is it possible?

Models of past time:

* Simple reverse of current events (like a reel of film)
* Negative direction of time only (new things happen while time runs in reverse direction)
* Doesn't exist at all (ie, 'dissolves' into the present)
* Only exists relative to another observer
* Exists independently for each observer
* Exists in another 'universe' (would have to be an infinite number of these?)

Possibilities for accessing past time (assuming it 'exists' in some fashion)

* Spinning superdense cylinders (or something spinning in a magnetic field that might distort into a cylinder) to drag spacetime (various suggestions made by W.J. Van Stockum, Frank Tipler, Robert L. Forward, among others)

* Circulating light beam creating a closed timelike curve (Ronald L. Mallett) - has horizon difficulties, that is, you can't go back further than when the effect was first created.

* Electromagnetic concentration of charges, based on a theory of matter as an electromagnetic distortion of time. This theory proposes a proton as a ring of negative time with a zero time shell (and yes, I did mean a proton, not a positron).

* High frequency gravitational waves, especially those using electromagnetic forces to generate (Robert M. Baker)

* Travelling faster than light (superluminal) - various suggestions for this.

* Wormholes (KipThorne) - also has horizon problems, can only travel back to the time the wormhole was formed.

* Entanglement (present particle entangled with a past one)

* Cosmic strings (J. Richard Gott III) - no evidence that strings exist?

Also, there are spinning universe theories (a là Godel), different models of space, VSR and symmetry theories, etc (but I'll keep my distance from the John Titor thing, at least for the moment)!

These are just my initial notes based on some quick preliminary reading, and I'll flesh them out as I do some more reading.

I haven't listed, or even taken much notice of, the paradoxes that people love to carry on about when they consider time travel. First of all, because paradoxes inspire even the most brilliant people to say some incredibly stupid things, like "this can't possibly happen so the universe will find some way to prevent it..." etc. Secondly, I can't see a problem with most of them. So someone travels back in time and kills their mother before they were born. It's a zig-zag - a different future now becomes a reality. The dice are re-rolled. Mother dies. Person is never born. End of story. What's so hard about that? (ah, but what happens to the original 'present', you might ask? Still working on that one...)

References to get started on (I am building myself a full annotated bibliography of time travel literature - it's a slow process)

Davies, Paul, How to Build a Time Machine, Penguin Books, NY, 2001
Gott, J. Richard, Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time, Mariner Books, NY, 2002
Mallett, Ronald L. and Bruce Henderson, Time Traveller: A Scientist's Personal Mission to Make Time Travel a Reality, Thunder's Mouth Press, NY, 2006

Time Machines and Physics: State of the Art - Examples of popular time machines. A summary of current time machine models (a little more comprehensive than my brief list)

Reversing Time at Sublight Speed, by David Anderson. An explanation of the rotating cylinder theory.

See also the discussions at the Physics Forums, especially those under Special and General Relativity.