A short Story:

John found a time machine.  He decided to see what would happen if he went back in time and retook a crucial test from his college days.  It was a test in Physics 201.  Would that have a positive impact on his grade point average?

His machine had two functions.  One for traveling through space and the other for traveling in time.  The machine implied that these are similar concepts.

To test this machine out, he decided to travel in space first.  The machine took him back to the space of his old college classroom.  The spot, on Earth, where he took that ill-fated exam, many years ago. 

Yes, he could go back to his old classroom, but he couldn't retake that test.  A new professor was teaching and the students were even learning a different subject.  That classroom was being used for English in 2003, rather than physics.  

He began to realize that one could travel in space to the old classroom, but there was no retaking that test and creating a paradox with his current grade point average.

Next, John decided to use the time travel function on his machine.  It brought him right back to the hour of that day, in 1978, when he took the test.   

Could he retake that exam, thus causing a paradox with his later years?

It turns out that he still couldn't.  John couldn't even find his test, or his old classroom.  He found himself in a dark vacuum instead.  Where is everything? he yelled.  He was in 1978, but it was a "nothing" place. 

Then Father Time appeared.  Father Time explained.  "Your classroom is no longer residing in 1978."  "It has moved on."  "Moved on into the future."  "That classroom is now somewhere in 2003; dispersed in such a way that you can not take that test."  

John realized, "yes, you can go back to a place in time called 1978, but this world is no longer there."  You can not alter something in this world, from a location in 1978, because this world is no longer in 1978. 

John found that the year 1978 was only an empty spot in time; empty except for, of course, Father Time.

Just then Father Time yelled, "Watch out."  "You had better go back to 2003." 

Like a train lumbering down the track, another world was rolling into 1978.  This "other world" was about to experience it's version of 1978. 

John didn't want to stick around because that world turned out to be nothing but a hot plasma.  It was another universe, about to enter 1978.  No way to retake that test.  Not even a classroom in that universe. 

John used his time machine to travel back to 2003 where his world resides.  He landed back in the space, on Earth, of his old classroom. 

As with going back to the old time of 1978, the old space of his classroom was still there.  It looked more familiar than what he found when he went back to 1978.  The classroom had been repainted, but at least it wasn't a hot plasma.    That English class, which was using the room in 2003, was about to dismiss, or one can also say, "disperse."

I don't know if this makes sense to people who understand physics.  It seems to resolve that causality paradox of going back to the past for me.  Then again, who am I to say.  Maybe I am full of it.  Let me know what you think. Write me.

Also see my blog piece on Retrocausality.