Just what would time travel look like?  This question was posed to me by a movie director in L.A..  It turns out there are three parts to this question-- what physics suggests, what movies have done in the past, and what looks good.

The last is up to her and her special effects staff.  The middle one-- Hollywood traditions for time travel-- are worth examining to scope out possibilities.  I'll then conclude with what I think physics suggests is most likely.

Were I to invent categories for movie time travel effects, I'd create the following:

1.   techno with lots of lights and whooshing (ala 2001, though that wasn't time travel)



2.   high speed vehicle (similar to techno, but with speed lines)



3.   organic with the sun moving rapid arcs and fast-motion seasons passing by



4.   minimalistic where the screen goes white or black and then fades in on the traveller



5.   portal methods, doorways that open, pools you jump into, vortices, etc.



6.   flickering the images of the 2 times until is stabilizes on the new, overlays, etc



7.   surreal, where they just walk from one to the other in a quick pan, cut or overlay



8.   lazy: they go into a machine and then exit it, or go to sleep and wake up in the new time

It's all about the story the director wants to tell.  Is time travel easy or hard, disorienting or everyday, technological or magical, by people or by gear, on purpose or accidental, one time or repeatable, safe or dangerous, fun or scary, a blessing or a curse?

From a physics point of view, the problem simplifies greatly.  In fact, most time travel cases end up being self-explanatory, but the weird ones do take a bit more work.  I'll note that one set of time travels option assume "great speed leads to time travel".  In SF, generally they assume travel faster than light, or fast travel around the edge of an event horizon.  These adds some spatial movement to your method, but ultimately is the same as the static cases we will now cover.

First we look at 'Time Teleportation'.  If time travel involves a direct, instantaneous shortcut from "now" to "then", then time travel is exceedingly abrupt.  First you're here, then you're there.  You simply 'cut' from one scene to the next, with no intermediate view.  That's a bit of a no-brainer, since that's what 'instantaneous' implies.

Next is the 'Time Rift' method.  If time travel uses a door, portal, or wormhole, then you see whatever the door, portal, or wormhole passes through.  If it allows viewing, you see the scene.   On the other hand, if it blocks all light, you see nothing.  Again, very straightforward.

You do start to run into interesting violations of conservation if, indeed, the rift allows flow between the two locations.  If the portal also allows solid matter to pass, you can feel wind and weather. But if it happens to open into space, you'd get vacuum and can quickly remove all Earth's air.

You might wonder "why would there be vacuum on one side", and that leads to two interpretations of time travel.  One presumes an absolute reference frame for the universe, the other uses a local reference frame.

Local reference frames also work better in a narrative sense.  They presume you travel from geographic point A to geographic point A, the same relative location, just different in time.  Leave your house in 2009 to go to your house in 1809, for example.  Since there is no universal reference frame for the universe, this is as valid as any theory.

Another school of thought argues that, since the planets and stars and galaxies all move, when you travel in time you'll likely end up in deep space, in whatever happens to occupy that space in the universe where the Earth was when you left.  The Earth itself has moved on.

If you seek to find a 'universal reference frame, it can be hard to define.  There is no center of the universe, and the universe itself is expanding.  If time travel has a connection with gravity-- likely, since time flows at different rates in different gravity potentials-- then it's reasonable to assume time travel may be dragged around in your local inertial reference frame.  But if not, you can likely compute the motion of all the relevant bodies (earth rotation, earth orbit around sun, sun around galaxy center, galaxy center around local group) and get a good approximation for what your spatial shift will be.  In which case travel in time also yields space travel.

The third option is if it time travel has its own intrinsic nature.  This is identical to the case where time travel uses a shortcut through an intervening medium (timespace, hyperspace, hypertime, my closet).  The view is of whatever that intermediate space is.  This is really hard to imagine because it's dependent on which current laws of physics you are violating.  Perhaps the portal or intervening space emits its own radiation and kills you, rendering the problem moot.

Finally, there is the case where time travel simply moves you through time, able to view it as you go.  Your local elapsed time is different from outside time, and that difference yields the amount of time travel you can do.  We already time travel, just at 1 minute per minute.  If you could go, say, 1 hour per minute, then you're a bonafide time traveler.

In such a case, you would see the elapsed events around you-- seasons whirling, sun moving quickly, people in a blur.  They would see you as a nearly frozen statue.  Hopefully they won't poke at your or play tricks.

At fast time differentials, you could easily be blinded.  The total amount of photons per second arriving at your eye are increased.  If you travel 1 hour per minute, you would get an hour's worth of light in 1/60th the time.  The human eye has a range of about 10¹⁶ from 'dim' to 'glare', but enough of a time differential could increase even an ordinary light bulb to sun-level blinding.  Best choose a permanently dark chamber for this method of time travel.

And now comes the part of the article where, in theory, I wrap everything up and provide a twist or a 'kicker'.  But, alas, vacation looms and I have to go.  I ran out of ... time.

Alex, the daytime astronomer