Fisher Drive (concept)

When humanity finally made First Contact with an alien species, they
gave us access to a FTL propulsion system that was used by species
across the galaxy (and presumably elsewhere too).

Humans named it the "Fisher Drive".  It was named, not after a person
named Fisher like you might think, but after the unique way it worked.
It "fished" for wormholes using entangled matter as "bait".

Thanks to quantum mechanics and the uncertainty principal, spacetime is
perpetually frothing with wormholes popping into existence for an almost
indescribably short period of time before vanishing again.

The problem is, 99.99999999999999% of those wormholes are utterly
useless for space travel (at least, the kind of space travel you could
ever hope to come back from) because they connected to parts of the
universe far outside human space.

In fact, given that the visible universe was only a microscopic fraction
of the entire universe, the vast majority of wormholes would take you on
a one-way trip "over the horizon", never to be seen again.  It was a
rare wormhole that would connect two points within the visible universe,
much less anywhere near your destination.  So you had to select your
wormhole very, *very* carefully...

Ships using the Fisher Drive were outfitted with libraries of "keys",
cylinders approximately the size of a soup can containing matter
entangled with matter at the destination.  Ships would take a key
corresponding to their chosen destination out of the library, load it
into the Fisher drive, and wait for a wormhole to "bite" their "bait".
And wait...  And wait...  Oftentimes, it would take years, on rare
occasions a few decades, before a wormhole would appear that had its
terminus close enough to the destination to trigger the drive.

Eventually, a wormhole that had its terminus near the destination would
briefly flicker into existence.  Quantum entanglement flux lines would
instantly bridge and trip sensors letting the drive know it had hit
paydirt.  The drive would energize, amplify the wormhole to traversable
size long enough for the ship to transit to the other end, and arrive at
the terminus, all much faster than the blink of an eye. The ship usually
popped up several thousand AU's from its destination, and has to spend
further months or years closing the distance to its destination on
fusion drive.

So while traversing the wormhole itself is instantaneous, or as close to
it as makes no difference, waiting for the correct wormhole to appear
can take years, occasionally decades.  So you have a FTL drive that can
travel from one side of the galaxy to another in 10 years, but can't
travel across the room in much less than 10 years either.

And the Fisher drive can't be used for exploring within the galaxy, as
it requires matter entangled with its destination to chose the proper
wormhole.  So most initial exploration is still conducted with good, old
fashioned fusion drives.  Fusion ships ply the cosmos for centuries
traveling to their destination, generate several million keys, bring
them back several centuries later and disseminate them, and *then* you
can start letting space tourists come visit.

It's therefore "expensive" (in terms of initial time investment) to
create a Fisher drive loop.  So it's not done willy-nilly to just travel
to any dead, boring star system.  That said, once the supply loop for
the Fisher drive is set up, it's easy to perpetuate, as the keys
themselves can be shipped via FTL.  The first time you go somewhere, it
may take thousands of years.  But the next time you go, it'll only take
5-20 years.

Most ships carry libraries with hundreds if not thousands of keys. In an
emergency situation, the ship could load *all* of its keys
simultaneously, significantly increasing the odds of it finding *some*
safe destination quickly, as long as they weren't too picky about where*
they went.

The Fisher drive requires matter entangled at the destination to choose
the correct wormhole.  Many alien races use that effect to limit any
external threat.  They will designate one world as their "port of
entry", and only exchange keys for that world with outside species.  Any
other incursion into their space would require fusion ships moving at
sub-light speed, which are relatively easy to detect.

As the Fisher drive is based on entanglement, it doesn't require the
distribution of coordinates.  It sort of functions like public key
cryptography, in that you can release the public part (the key) for
transportation without having to release the private part (the actual
physical location of the destination).  So in theory, aliens could allow
passage to their space, without actually admitting *where* their space
is.  Of course, any species capable of interstellar travel should be
able to astrogate their location anyway, but it does open intriguing
possibilities.

Keys don't last forever, though they can easily last several thousand
years before they lose entanglement as long as they are properly stored
in a "library" (a specialized container that helps preserve and maintain
the entanglement).  A library can be a cooler-size container that holds
one key, or a warehouse sized container that holds tens of thousands.

There are also instances of wormhole travel *without* using the Fisher
mechanism.  A young race somewhere experimenting with wormhole travel
that hasn't yet mastered the fundamentals, aliens sending "messages in a
bottle", or ships in dire straights engaging their drive blindly.  They
are all one-way, but they have allowed Milky Way civilizations some
knowledge of life beyond our galaxy.

In the event that a race possesses the Fisher drive, it can, in theory,
open the entire universe to them.  They can amplify a random wormhole,
blip to its destination even if it's on the other side of the universe,
and as long as they have a functioning Fisher drive, they can load their
key and blip back home.

In practice, the universe is undergoing eternal inflation, which means
that the vast majority of the universe someone might randomly land in
via this method is experiencing quasi-Big Bang conditions, and is
unlikely to survive more than a zillionth of a second.  Or that
particular patch of universe might have laws of physics substantially
different from the region you just left.  Or you might blip inside a
star, or too close to a supernova or black hole.  So as a rule,
non-suicidal races stick to the Fisher drive.

Humanity has been collaborating with other Milky Way species on ways to
mass produce cheap Fisher probes.  Send out a million, and if one
returns successfully, you now have a doorway to another part of the
universe. To date, the limitation has been in designing a probe AI that
is simultaneously competent enough and suicidal enough to pull it off.

Research continues, driven by the knowledge that such an incursion has
happened at least once before.  One Milky Way species speaks of their
beginnings, several thousand years earlier, aboard a desperate ship from
far over the cosmic horizon, as they blipped into our galaxy in dire
shape before a controlled crash landing on a marginally habitable world.
And if it's happened once successfully, it has likely happened thousands
of times under the radar.