How Alex's time machine works?

Love this movie, one of my all time favorites but I always wondered how Alex's time machine worked?

I hypothesized that it got in a sort of interdimentional bubble that got out of the spacetime and sort of hang in there while spacetime go forward or backward, however that hypothesis is flawed with the scenes where an object (like Alex's hand lost Emma picture necklace on her first time trip to the future) gets out of the time bubble ... it would be impossible if my theory is true.

Therefore, I don't know how it works.