The scale was so large, even travel time failed as a useful measure.
We’ll start with the smallest elements and work our way up from there.
The minds fighting this war were made from a series of reversible computing systems with low energy inputs and outputs. They were grown in and cut off the Dyson spheres wrapped around their parent stars. At the physical lower limit of muon-catalyzed fusion, they processed elemental hydrogen into helium and used the radiation at around one-third efficiency to alter computational patterns, manifest more muons and generate electrical charge.
In less technical terms, they were sentient computer-plants drinking the sunlight and feasting on rich hydrogen winds. They blossomed across worlds with more than 500 million times the surface area of Earth. They were creatures of the vacuum, evolved for space. In a pinch, they could even survive on the scant interstellar gas within a galaxy.
Two groups from distant Dyson spheres were at war—call them the Ferns and the Trees. They weren’t competing for resources, nor to expand or conquer. The reason they fought was colder, based on risk assessments developed across millions of years of recorded history.
Given enough time, two spacefaring species would inevitably find a reason to fight, if only because they feared the other side making this very calculation and arriving at the same logical conclusion. Peaceful civilizations were always vulnerable, and on the cosmic scale, one blow could be an extinction event.
Their galaxy was the Grand Barred Spiral in the constellation Eridanus. It languidly orbited the far side of the Eridanus Short wall, a dense filament made out of many superclusters of galaxies. It wasn’t born there, but migrated out from the filamentary core through gravitational slingshot, and in its galactic north, the Ferns and Trees could see the Eridanus Supervoid.
The Grand Barred Spiral was 110,000 light years long. At standard FTL velocities, it took 3 years to fly from one end of the galaxy to the other. The Tree-Fern War was scaled up to fight across the whole of this cosmic structure.
Emit, coalesce, scan and strike—there was little strategy and more brutal attrition. They leapt from star to star, murdered, refueled and rebuilt.
Their FTL drives worked by converting matter to tachyons and emitting energy in a controlled fashion. In the strange logic of tachyon physics, lower energies corresponded to higher velocities. Enough energy had to be maintained, however, or the ship’s drive couldn’t convert back.
They killed by exploiting the drives’ unstable nature. With the right pulse, one could trigger the exotic core of the enemy’s ship and cause an uncontrolled conversion. Energy would be naturally dumped as Cerenkov radiation, exponentially increasing the ship’s superluminal velocity and accelerating its loss of energy. Eventually, the average energy level of the core would drop below the vacuum energy of open space, and space would close up around the ship like a submarine in the deep ocean, crushed and imploded.
The Trees were winning the war.
The Ferns’ home was discovered first, and they lost their Dyson sphere to kinetic bombardment. Their only recourse was to pass through the galactic center—where they thought the Trees might’ve been born—and search frantically for the enemy sphere. Their entire plan was revenge.
The Trees met them near the galactic core in the only major ship battle of the entire war. Millions of vessels were converted to their destruction, slipping into tachyonic speeds, accelerating, and imploding somewhere around the supermassive black hole that presided over the slaughter.
The Fern leadership, if it could be called that, was forcefully converted by a Tree pulse during a sublight maneuver that left it facing the Eridanus Supervoid.
The Supervoid was the largest structure in the entire observable universe. It may have been the largest cosmic structure there ever was. From here, it looked like a cold spot in the microwave background radiation, as if somewhere beyond all the dense galactic warmth there was a deep emptiness in the sky.
Here was where scale truly began to fail us. 110,000 light years took 3 years for Ferns to cross under controlled travel. Out of control, the Fern leadership watched their ship drop down the quantum steps of the energy level stairs, acquiring such superluminal velocity that it would’ve crossed the galaxy in six hours. That was more than 160 million times faster than light.
They were able to stabilize at this velocity, just a few notches above implosion, and they rushed to repair their ship. They could hold it for about fifteen hours in this state while maintaining sufficient energy to switch back. Unfortunately, even at this speed the current ETA for crossing the Eridanus Supervoid was three years.
The Ferns despaired for good reason.
The Supervoid wasn’t empty like normal interstellar space. There weren’t scant stars or thin wisps of gas out there—there were no stars. There was no gas, because the filaments of galaxies captured it all at the beginning of time. These were the bubbles between the grand structures of being; they were the majority of the universe, the great emptiness that made even intergalactic space look rich and inviting.
The only thing out there was the vacuum and the threat of annihilation.
The Ferns planned. They would drop down to the last energy level above the vacuum. With luck, they would continuously feed the core exactly enough power to prevent a fluctuation below the fatal value, and in a single long streak of beautiful Cerenkov radiation, they would cross as much of the Supervoid as they could before converting back to normal matter.
160 million times became 1.6 billion times the speed of light—unfathomable numbers found through the powers of ten.
ETA at this rate to cross the Supervoid—four months.
1.6 billion became 160 billion times the speed of light. These values were inserted into their sentience via direct computer output, but it stood alone without association. No memory could connect with it. It might as well have been infinite speed.
ETA at this rate to cross the Supervoid—27 hours.
They couldn’t go faster. The next step down was into the grave. They repaired the ship and sailed on. After one more conversion, they wouldn’t have enough power to switch again. They’d have to coast at their original sublight velocity, barely relativistic, and hope that their reversible minds could survive the wait.
At the very last microsecond, the Ferns dumped their power reserves into the core, nudged over the threshold and coalesced out of tachyons into normal matter. They opened their artificial eyes and looked at the universe across the entire spectrum.
This far into the abyss, even the great X-ray sources from first eons were too dim to be seen with eyes this small. One of them caught a gamma sparkle somewhere ahead. It might’ve been the peak flash of a star going supernova 250 million years ago. Otherwise, there was nothing but the cool dim microwave background, the first light of a transparent universe soon after the Big Bang. Everything else was too distant—even their destination far ahead.
Twelve percent of the speed of light. 210 million light years of nothing gaped ahead. 1.6 billion years of travel time. The Ferns’ main, non-sentient computer was offline to conserve power.
Like all good reversible computers, the Ferns themselves could turn off their inputs and outputs but remain functional. Their last few operations would run forward, stop, and run backwards. The final minute before shutdown would be thought and unthought in ponderous succession with perfect efficiency.
In this state of eternal recurrence, the Ferns would relive the melancholy realization that traveling to the next star would take a vast period of time—almost more than what their minds could grasp. In 1.4 billion years, complex life had advanced from an ocean jelly to the post-biological Ferns of a Dyson sphere. They figured their journey would only take a little longer than that.
They shut their eyes and went to sleep, and they dreamed about their own anguish with the same fresh clarity they’d felt the first time around.
It’s very, very sad. They were so used to navigating with their computer’s aid, and they’d always thought about their journey in FTL terms. They’d forgotten something key. The universe was expanding, and the space between the grand filaments always grew. You had to fly excessively fast just to stand still.
The whole universe receded away from their ship at more than 30% of light speed.
There would always be more empty space to coast through, and nothing would ever appear to mercifully destroy them. Wrecked in the Eridanus Supervoid, immortal aliens slid forward into an ever expanding darkness. They dreamed of the conceivable scales when the truth was beyond conception.
Their destination at infinity. ETA at this rate—eternity.