While the main plot of Outlander involves physician Claire travelling back in time to 18th century Scotland, there's one other act of apparent time travel that has generated a lot of fan theories. In the series' premiere, Claire’s first husband, Frank Randall, encounters a man dressed as a Highlander staring up at her from outside the window. Frank attempts to challenge him, but the Highlander turns to leave and vanishes. Later in the episode, after Claire travels back in time, Jamie Fraser is introduced, and there's a very strong resemblance.

Author Diana Gabaldon has confirmed the figure that Frank encounters is Jamie’s ghost, but just how remains a mystery. Gabaldon has said that the mystery surrounding Jamie's will be the last thing she writes in the series' final book. However, there are already five very good theories to explain just how Jamie may have moved forward in time.

RELATED: Time Travel in Outlander, Explained

Theory 1 - Astral Projection

The Astral Projection theory proposes that Jamie was able to send his consciousness through time and space. Gabaldon has confirmed that, as a ghost, Jamie wouldn't need the stones to travel through time. This theory might also explain Jamie’s “dreams” of events and people in the far future that he should have no knowledge of, such as the birthmark on Brianna’s neck.

This theory would fit with Jamie not needing to use the stones to travel through time and is based on his near-death experiences at Culloden and a snakebite. The thought of Claire is the thing that keeps him alive and returns him to his body. The projection of his soul or "ghost" could be explained through the traumatic experience of near-death or the power of deep love Jamie has for his unseen daughter.

Theory 2 - Purgatory State

Whether Jamie can pass through the stones to travel to the past or future doesn’t remove the fact that at some point Jamie will die. According to Catholic doctrine, Jamie would enter Purgatory to be purified of any wrongdoing in his life before entering Heaven. However, in Scottish lore, ghosts are freed to walk the Earth once more on the festival of Samhain. That means Jamie would be allowed to walk the Earth on the night before Claire traveled through time.

Some theorists feel that Jamie went to stop Claire from going to the stones. Others believe he may have been there to encourage her to go. Whatever the reasoning, this theory doesn’t answer the important question that lies at the center of it: Does this visit release Jamie to enter his eternal rest?

RELATED: Outlander: The Fraser Prophecy, Explained

Theory 3 - Time Loop

Jamie and Claire stand at the alter as they wed in Outlander

When considering time travel, the possibility of a time loop, or paradox, will pretty much always enter into fan theories. In the Time Loop theory, Jamie "meets" Claire for the first as a ghost walking about Inverness. He sees her through the window and it is then that he falls in love with her. When he meets her after she has traveled through the stones, their love for each other creates a never-ending time loop of love.

While it may seem a bit saccharine, this does bring the theme of fate into clear focus, as how they keep coming back to each other presents a "localized" time loop. They also seem to be, despite their best efforts, unable to change historical events. Perhaps these small loops of loss and re-connection are part of the grand loop of their lives, which are intertwined forever.

Theory 4 - Haunting Theory

The Haunting Theory is sort of the inverse of the Purgatory Theory in that it posits that Claire dies in the past with Jamie, perhaps before or after his own death. Jamie’s ghost then follows Claire throughout her entire life from birth until she is reunited with him in the past.

Paranormal ghost hunters claim that places or objects and even people can be the focal attachment for a ghost, and Jamie and Claire's bond through time is essential to the plot of the series. This theory then suggests that bond transcends life and death.

Theory 5 - Alternate Timeline

In the Alternate Timeline theory, Jamie dies at Culloden when he is 25 years old and then visits Claire in 1947. This "unlocks" Claire’s ability to go through the stones and into the past. Claire's presence then creates a new timeline where Jamie lives beyond the events of Culloden.

In opposition to the Time Loop Theory, the Alternate Timeline theory requires a linear movement of time where Jamie’s ghost in the 20th century is the catalyst to project Claire into a moment where her presence creates a diverging universe or timeline.

Developed by Ronald D. Moore, Outlander stars  Caitriona Balfe, Sam Heughan, Tobias Menzies, Duncan Lacroix and Graham McTavish.

Next: Outlander: Now Is the Perfect Time to Catch Up On the Time-Traveling Romance