You’re standing at The Crucifixion.

Dumbstruck, and open-mouthed, you can’t help but stare at the scene.  Perhaps the most famous in all of history.  It was an expensive package.  But your Time Travel Tour operator said it would be well worth the cash.  Just a few points to remember:  You must do nothing to disrupt history.  (Note to self: Don’t tread on any butterflies this time).  And when the crowd is asked who should be saved, you join in with the call, “Give us Barabbas!”

Suddenly, you realise something about the crowd.  Not a single soul from 33 AD is present.  The mob condemning Jesus to the cross is made up lock, stock and smoking barrel of tourists from the future.

Far-fetched?  Stephen Hawking thought so.  The story is Let’s go to Golgotha, a 1975 time travel tale by Garry Kilworth.  Hawking suggests that the apparent absence of such tourists from the future is a strong argument against the likelihood of time travel.  We may call it ‘Hawking’s Paradox’.

Of course, this doesn’t mean time travel is physically impossible.  It may merely mean that it is never developed.  And even if it is developed, there may be snags.

Assume we create a wormhole.  A wormhole is a region of warped spacetime.  It is essentially a ‘shortcut’ in space and time through which to travel.  Trouble is, time travellers would not be able to travel back in time to a date before the wormhole was created.  This may explain why we’ve not been overrun by tourists.  We’ve simply not created a wormhole yet.

It was science fiction writer John Campbell who invented such ‘space warps’.  In his 1931 story, Islands of Space, Campbell used the idea as a shortcut from one region of space to another.  In his The Mightiest Machine (1934), Campbell called this same shortcut ‘hyperspace’, another now-familiar coinage.

A year later, Einstein and Nathan Rosen penned a paper that nailed the notion of ‘bridges’ in spacetime. The imaginative American physicist John Wheeler re-dubbed these Einstein-Rosen bridges ‘wormholes’ in 1957.

A wormhole has at least two mouths, connected to a single throat. Such wormholes are valid solutions in relativity theory.  Matter may ‘travel’ from one mouth to the other by passing through the wormhole.  We haven’t observed one yet, but the Universe is still young.  And we haven’t been looking very long.

Of course, the idea of wormholes in the public imagination owes almost everything to fiction. The very premise of television shows such as Stargate SG-1 and Sliders owe their existence to wormholes.  Characters are either zipping from our Milky Way to the Pegasus galaxy, as in Stargate, or they are shifting between parallel universes, as in Sliders.

Indeed, Carl Sagan’s 1985 novel, and ensuing 1997 movie, Contact created a cottage industry among theoretical physicists. Jodie Foster’s character travels 24 light years through an Einstein-Rosen bridge to the star Vega.  Analysis of the story by Kip Thorne, as a favour to Sagan, is cited by Thorne as the initial impetus for his work on wormhole physics.

Unlike a black hole, a one-way journey to oblivion, a wormhole has two mouths.  An exit, and an entrance.  Thorne figured that some form of antigravity might control such wormholes so they can be used as the shortcut through hyperspace envisaged by John Campbell in 1931.

So, we need a dash of exotic matter.  Composed of hypothetical particles, this exotic material of negative mass and antigravity properties, would provide the pressure needed to keep the wormhole throat from imploding.  In this way, Thorne and many other physicists believe that a stable wormhole can be created, and readily turned into a time machine.

“This is a final passenger call for The Crucifixion”.

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