Redshifted civilizations, galactic empires, and the Fermi paradox

This paper proposes that civilizations can overcome interstellar travel constraints and the Fermi paradox by utilizing time dilation near supermassive black holes to achieve galaxy-spanning expansion within a human lifetime using feasible Type II energy levels, while simultaneously offering a physical basis for the "dark forest" hypothesis through the vulnerability of such high-speed vessels.

The Gist

The Big Idea: Cheating Time Without Breaking Physics

Imagine you are a human with a lifespan of about 80 years. You want to explore the entire Milky Way galaxy, which is huge (about 100,000 light-years across). The problem? Even if you travel at the speed of light, it would take 100,000 years to cross. By the time you got there, your great-great-great-grandchildren would be long dead, and your home civilization would have changed beyond recognition.

This paper asks: Is there a way to travel across the galaxy within a human lifetime without using "magic" physics (like warp drives)?

The answer is yes, but it requires a very specific trick: Time Dilation.

Think of time not as a steady river, but as a rubber band. If you stretch the rubber band (travel fast or hang out near a heavy gravity source), time slows down for you relative to everyone else. To you, a trip might feel like 20 years. To everyone back home, that same trip might take 20,000 years.

The authors propose that advanced civilizations wouldn't just send out a few brave explorers; they would move their entire society into this "slow-motion" time bubble.

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Three Ways to Build a "Time-Slowing" Civilization

The paper outlines three ways a civilization could achieve this, ranging from "doable soon" to "super-advanced."

1. The "Black Hole Rollercoaster" (The Local Option)

• The Concept: Imagine a supermassive black hole at the center of our galaxy (Sagittarius A*). It has such strong gravity that time moves slower near it.
• The Analogy: Think of the black hole as a giant, spinning whirlpool. If you swim near the edge, the water drags you around, and time feels slower for you compared to someone standing on the shore.
• The Catch: You can't just sit still; you'd fall in. You have to orbit it at a breakneck speed.
• The Result: A civilization could orbit this black hole and experience time at a rate of 100 times slower than the rest of the galaxy. They could visit stars a few hundred light-years away and return in just a few years of their own time.
• The Cost: It requires a lot of energy to fight the drag of gas near the black hole, but a civilization that has mastered its home star (Type II) could handle it.

2. The "Galactic Subway" (The Travel Option)

• The Concept: Instead of orbiting a black hole, the civilization builds a fleet of spaceships that accelerate constantly at a comfortable 1g (like Earth's gravity) until they are almost at the speed of light, then turn around and slow down.
• The Analogy: Imagine a train that never stops accelerating. As it gets faster and faster, the passengers inside age very slowly compared to the people waiting at the stations.
• The Result: With a time-dilation factor of 10,000, a trip across the whole galaxy takes only 100 years for the people on the ship.
• The Network: The paper imagines a "Galactic Confederation." Ships travel between "stations" (nodes) in the galaxy. When they arrive, they slow down, meet other ships, swap passengers, and then speed up again. It's like a massive, synchronized dance of ships where everyone stays young while the universe ages around them.
• The Cost: This requires the energy output of a Type II civilization (harnessing the power of a whole star).

3. The "Black Hole Ring" (The Empire Option)

• The Concept: This is the most ambitious idea. Imagine building a giant ring of black holes around the galaxy.
• The Analogy: Think of a pinball machine. Instead of a ball bouncing off bumpers, a spaceship flies past black holes. The gravity of each black hole bends the ship's path, keeping it in a giant circle without the ship ever needing to use its engines to turn.
• The Result: The civilization lives on ships circling the galaxy forever, never aging much, powered by the black holes themselves (using a process called the Penrose process to steal energy from their spin).
• The Catch: Building this requires moving millions of black holes into place. It's a construction project that would take millions of years in "normal" time, but only a few hundred years for the people living in the time-dilated ring.

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Why Don't We See Them? (The Fermi Paradox)

The Fermi Paradox asks: "If the universe is so big and old, where are all the aliens?"

This paper offers a spooky answer: They are hiding because they are terrified.

The "Glass Cannon" Problem

If you are traveling at 99.9999% the speed of time dilation, you are incredibly fragile.

• The Analogy: Imagine you are driving a car at 1,000 mph. If a tiny pebble (a rock) hits your windshield, it hits with the force of a bomb.
• The Reality: For a civilization moving this fast, a single grain of dust or a small asteroid floating in space would hit their ship with the energy of a nuclear bomb.

The "Dark Forest"

Because they are so vulnerable, these civilizations have a huge problem: Anyone can destroy them.

• If a hostile civilization knows where your path is, they don't need a big laser. They just need to place a few heavy rocks in your path.
• Furthermore, time works against them. While they are traveling for 100 years (their time), thousands of years pass in the galaxy. In that time, a primitive hunter-gatherer tribe on a distant planet could evolve into a super-weapon civilization that decides to wipe them out.

Conclusion: The smartest move for an advanced civilization isn't to shout "We are here!" It is to stay silent, hide in the shadows, and move so fast that no one can see them coming. This supports the "Dark Forest" theory: the universe is a dark forest where everyone is a hunter, and the only way to survive is to stay quiet.

The "Sad Trombone" Signal

The paper also notes a cool side effect. If these civilizations did try to send a radio signal, the extreme speed and gravity would stretch the sound waves.

• The Analogy: It's like a "sad trombone" sound (wah-wah-wah) that gets lower and lower in pitch as the ship moves.
• Scientists might actually detect these civilizations by looking for radio signals that slowly drop in frequency, which looks a lot like the mysterious "Fast Radio Bursts" we see in the sky.

Summary

This paper suggests that advanced aliens might not be sitting on planets. They might be living on ships, orbiting black holes, or zooming through space at near-light speeds, living in a "slow-motion" bubble. They are likely hiding from us because in the cosmic game of survival, being too visible is a death sentence.

Technical Summary

1. Problem Statement

The paper addresses the fundamental constraints facing interstellar expansion: the finite speed of light ($c$) and the finite lifespan of biological organisms. Traditional solutions often rely on:

• Suspended animation or multi-generational ships: These create severe temporal separation between voyagers and their home civilization, leading to emotional and social disconnect.
• Faster-than-light (FTL) travel: This requires exotic matter or modifications to general relativity (GR), which are currently speculative and unproven.

The authors ask: Can a civilization explore a significant portion of the galaxy within a human lifetime (in the traveler's frame) without invoking new physics or sacrificing the possibility of reuniting with the home civilization?

2. Methodology

The authors analyze scenarios strictly within the framework of classical General Relativity (GR) and Special Relativity (SR), imposing three fundamental constraints:

1. Biological Tolerance: Acceleration must be tolerable for biological matter (approx. $1 \text{ m/s}^2$ to $10 \text{ m/s}^2$).
2. Validity of GR: Physics is assumed to hold on galactic scales without modification (e.g., no MOND effects at high accelerations).
3. Energy Conservation: Energy is non-negative and locally conserved.

The study evaluates three distinct migration strategies to achieve high time dilation ($\Gamma$ or $\gamma$):

• Orbital Migration: Utilizing unstable circular orbits near the photon radius of supermassive black holes (SMBHs).
• Linear Acceleration: Utilizing a network of vessels undergoing constant proper acceleration (Sagan's model).
• Gravitational Scattering: Constructing a ring of black holes to deflect vessels into closed galactic loops.

The authors calculate power requirements, tidal forces, and signal characteristics for each scenario, comparing them against the Kardashev scale (Type I, II, and III civilizations).

3. Key Contributions and Results

A. Time Dilation via Supermassive Black Holes (Sgr A*)

• Mechanism: Instead of static observers (who require infinite acceleration near the horizon), the authors propose unstable circular orbits near the photon radius ($r \approx 3M$) of a non-rotating Schwarzschild black hole.
• Time Dilation Factor ($\Gamma$): For a black hole like Sgr A* ($4.3 \times 10^6 M_\odot$), a time dilation factor of $\Gamma \approx 100$ is achievable.
• Biological Constraints: The limiting factor is tidal force. The authors derive that for a human-sized object ($\chi \approx 2$ m) to experience tidal acceleration $\le 1 \text{ m/s}^2$, the black hole must be supermassive. Stellar-mass black holes are unsuitable.
• Power Requirements: Counteracting drag from accreting plasma near Sgr A* requires power ranging from $10^{13}$ W to $10^{19}$ W. This is within reach of a Type I civilization (or slightly above), making this a viable near-term strategy for civilizations near the galactic center.
• Technosignatures: Signals from these orbits exhibit a downward frequency drift ("sad trombone" effect) due to the changing Doppler shift and gravitational redshift as the source moves relative to a distant observer.

B. Galactic Confederation via Linear Acceleration

• Mechanism: A civilization constructs a network of vessels that accelerate at $10 \text{ m/s}^2$ ($\approx 1g$) to ultra-relativistic speeds, decelerate at a midpoint, and accelerate again to the next node.
• Performance: To achieve a time dilation factor of $\gamma \approx 10^4$:
  • Proper travel time: $\approx 10$ years to reach peak speed.
  • Distance covered: $\approx 20,000$ light-years (1/4 to 1/5 of the Milky Way's diameter).
  • Total trip across the galaxy: $\approx 100$ years in the civilizational frame.
• Power Requirements: Accelerating a vessel with the mass of a km-sized asteroid ($10^{13}$ kg) to $\gamma \approx 10^4$ requires peak power $\approx 10^{22}$ W.
• Feasibility: A single Type II civilization (capable of harnessing $\sim 10^{26}$ W, the Sun's luminosity) could theoretically accelerate tens of thousands of such vessels, establishing a galaxy-spanning confederation.

C. Galactic Empire via Black Hole Rings

• Mechanism: A highly advanced civilization moves stellar-mass black holes to form a ring, allowing vessels to scatter off them. This creates a closed spatial trajectory without continuous engine thrust.
• Advantages:
  • Eliminates the need for constant acceleration/deceleration (reducing maintenance).
  • Allows extraction of rotational energy via the Penrose process. A single spinning black hole ($14 M_\odot$) contains enough rotational energy to power a Type II civilization for timescales exceeding the age of the universe.
• Construction Requirements:
  • To form a closed loop with $\gamma = 10^4$, a ring of $\approx 7.9 \times 10^5$ black holes is required.
  • Moving these black holes requires a power capacity of $\sim 10^{32}$ W (approaching Type III).
  • Time Scale: In the galactic frame, construction takes $\sim 2$ million years. However, due to the time dilation of the civilization ($\gamma = 10^4$), the construction time in the civilizational frame is reduced to $\sim 200$ years.

D. Implications for the Fermi Paradox (The "Dark Forest" Hypothesis)

The paper provides a novel physical mechanism supporting the "Dark Forest" hypothesis:

• Vulnerability: Ultrarelativistic vessels ($\gamma \approx 10^4$) are extremely vulnerable. A stationary object in the galactic frame appears to have kinetic energy $\approx 10^4$ times its rest mass in the vessel's frame. A small mass (e.g., 100 kg) impacting the vessel would release energy comparable to the Chicxulub asteroid impact.
• Predictability: Due to acceleration limits, these vessels have enormous turning radii ($\approx 10^8$ light-years), making their trajectories predictable.
• Rapid Emergence of Threats: From the perspective of a vessel with $\gamma = 10^4$, the entire history of a developing civilization (e.g., Earth's transition from hunter-gatherer to space-faring) occurs in less than a year.
• Conclusion: Civilizations in high-$\gamma$ frames must remain undetectable to avoid preemptive strikes. This creates a strong motivation for silence, explaining the Fermi paradox.

4. Significance

• Physics-Based Feasibility: The paper demonstrates that galaxy-spanning civilizations are theoretically possible using only known physics (GR/SR) and biological constraints, without requiring FTL or exotic matter.
• Reframing the Fermi Paradox: It shifts the explanation for the "Great Silence" from "civilizations are rare" to "civilizations are hiding due to extreme vulnerability." The time dilation effect creates a "blind spot" where civilizations cannot detect emerging threats until it is too late.
• Technosignature Prediction: The authors predict a specific observational signature for such civilizations: isotropic, monochromatic signals from near SMBHs that exhibit a characteristic downward frequency drift.
• Kardashev Evolution: The paper outlines a plausible evolutionary path for a civilization:
  1. Sub-Type II: Orbit Sgr A* ($\Gamma \sim 100$).
  2. Type II: Linear acceleration network ($\Gamma \sim 10^4$).
  3. Sub-Type III: Black hole ring construction ($\Gamma \sim 10^4$, closed loops).

5. Conclusion

Reiss and Feng conclude that while the engineering challenges are immense, there are no fundamental physical laws preventing a civilization from migrating to a "redshifted" frame. The primary barrier is not physics, but the power capacity required to move massive objects and the strategic imperative to remain hidden due to the existential threat posed by the rapid emergence of adversarial civilizations in a time-dilated context.