Burruss outlined a vast 1,000 miles wide disk-shaped Galaxyship, designed to accelerate to 0.4 c over a 50,000 year period, then coast to Andromeda’s M31 for 5 million years, then brake at destination over 10,000 years, before dispersing like a seed-pod to colonize a whole other Galaxy.
The disk-ship components – a billion hexagonal units with their own drive systems – can move around relative to each other, allowing avoidance of intergalactic hazards. One side, facing the direction of travel, is a radar array, while the other is a matter-antimatter drive. The drive produces 30 gigawatts of thermal energy. Some powers the mini-biosphere within and technological sub-systems, while the rest produces a thrust of gamma-rays.
Burruss didn’t specify the exact efficiency of the drive system, but imagined that 99% of the vehicle would be matter-antimatter, to annihilate on the way. A perfect photon-rocket with such a mass-ratio would cruise at 0.98 c, then brake to a halt at destination [thanks Ian Mallett – see comments]. Since the stated cruise speed is 0.4 c, meaning an effective specific impulse of ~0.19 c, the efficiency seems excessively low. Especially since the by-products are high-energy gamma-rays, thus bathing the vehicle and inhabitants. So what are the alternatives?
A perfect photon-rocket might be available, if Fred Winterberg is right about the ability to collimate an annihilating ambiplasma via relativistic self-focus and the Meissner effect. In that case, with a mass-ratio of 3, the Galaxyship could cruise at 0.5 c. Burruss imagined it would start with a mass of a trillion tons, but would be frittered away as gamma-rays to *just* 10 billion. The 1,000 mile disk, some 2 trillion square metres, would have an areal mass-density of just 5 kg by the end of its odyssey. Alternatively it’d shrink to 100 miles wide, sacrificing hexagons to the ravening gamma-ray engines. With a perfect photon-drive the final mass-density would be ~333 kg per square metre. That *might* be enough.
Could the Galaxyship be propelled in a different fashion?
In 50,000 years, accelerating at a constant rate to 0.5 c, a vehicle travels 25,000 light-years. The acceleration is 1E-4 m/s2 meaning a force of 1E+8 newtons is needed. If we push it with lasers, then each newton needs 150 megajoules of laser energy per second impinging on the ship. A total laser-power of 1.5 exawatts. Fortunately the Sun radiates some 400 million exawatts, so tapping a tiny fraction of that flow will be sufficient to propel a Galaxyship. Millions if we felt so inclined.
To direct the laser, keeping it focused on a reflective disk 1,600 km wide at a maximum range of ~25,000 light years, means we’ll need an immense optical system. The optical system – presumably some immense solar-collector/laser-array combination – would need to be 150,000 km wide.
Close passes to target stars with large solar sails on each hexagon component could be used to stop in the other galaxy. One star pass could be used to decelerate from about 4.6% of the speed of light. Slowing at one star and then more slowly coasting the actual target star could be used to stop from higher speeds. This would mean we would not need to carry the fuel for deceleration.
A hundred years or more to slowdown would not be that big a deal since it would take 2.6-8 million years to cross over to the Andromeda Galaxy.
Laser boosted speed and solar sail deceleration means that the photonic galaxy ship would be almost all payload.
Hypervelocity stars were observed in 2005. There are stars moving faster than the escape velocity of the Milky Way, and are traveling out into intergalactic space. There are several theories for their existence. One of the mechanisms would be that the supermassive black hole at the center of the Milky Way ejects stars from the galaxy at a rate of about one every hundred thousand years. Another theorized mechanism might be a supernova explosion in a binary system. These stars travel at speeds up to about 3,000 km/second. However, recently (November 2014) stars going up to a significant fraction of the speed of light have been postulated, based on numerical methods. Called Semi-Relativistic Hypervelocity Stars by the authors, these would be ejected by mergers of supermassive black holes in colliding galaxies. And, the authors think, will be detectable by forthcoming telescopes. These could be used by entering into an orbit around them and waiting.
Stellar engines could also be used by manipulating a star to generate propulsion.
The Kardashev scale is a method of measuring a civilization’s level of technological advancement, based on the amount of energy a civilization is able to use for communication. The scale has three designated categories:
- A Type I civilization – also called planetary civilization – can use and store energy which reaches its planet from the neighboring star.
- A Type II civilization can harness the total energy of its planet’s parent star (the most popular hypothetical concept being the Dyson sphere—a device which would encompass the entire star and transfer its energy to the planet(s)).
- A Type III civilization can control energy on the scale of its entire host galaxy
Once a civilization is at about Kardashev level 1.5 they would be able to build and power intergalactic worldship missions.