Donald Moffitt wrote big idea Science Fiction in the 1970s & 80s staying (more or less) within physics as we (sort of) know it. His “The Jupiter Theft” (1977) featured non-bilaterian aliens from Cygnus X-1 that steal Gas-Giants to travel at near lightspeed because they’re acrophobic. In his “Genesis” duology he features the ultimate CETI Message sent by humanity – our genetic code (in detail) and our collective knowledge beamed to the Pinwheel Galaxy (M101) some 37 million light-years away. The Nar, advanced and friendly aliens, receive the Message and recreate humanity. The Nar can communicate via sight and sound, but their main mode of communication is touch, something the freshly minted humans can never quite grasp. Additionally the Nar are much longer lived than the humans and a second-class society of humans develops living amongst a much richer culture of the Nar almost as treasured, yet pitied pets…
That’s where “The Genesis Quest” (1986) begins. The protagonist upends Nar/Human relations by the invention of physical immortality for humans, thanks to genetics breakthroughs. The humans, given an indefinite future existence, get restless and stage a revolt… leading to an exile on a starship back to the Milky Way for the restless.
“Second Genesis” is the tale of exiles as they escape the looming Galactic catastrophe brewing in M101’s Core – two super-massive black-holes are in-spiraling to collision, which will sterilize the galaxy in the equivalent of a Quasar out-burst. Sling-shotting in a close pass to the in-spiraling pair, the Space-Tree/Ramjet (which is more or less depicted by the above bookcover image) is flung out at almost light-speed.
How close to c? The distance is 37 million light-years and the flight-time is 528 years. That’s a gamma-factor (time-dilation factor) of about 70,075… but how do we turn that into proximity to lightspeed? Let c = 1 and the proximity we define as (1 – 1/e). Reversing the usual gamma-factor equation, when we’re really close to c we get 2(gamma)2 ~ e
In this case that’s e ~ 10 billion. Or about 3 centimetres per second shy of lightspeed (=299,792,458 metres per second.)
Only problem, which no one considered in the 1970s and 80s, is that space really isn’t empty. It’s full of leftover energy from the Big Bang, microwaves with a near perfect black-body spectrum of 2.726 kelvin. And that blackbody temperature is boosted by the gamma-factor to an effective temperature of 191,000 K, pushing the spectrum into the x-ray band. Additionally it’s pushed forward into a brilliant dot in the visible sky via aberration – the perceived direction of light-rays is changed due to the relative motion against the incoming photons.
Fortunately, for a 528 year trip from M101, the needed gamma-factor isn’t quite so high. The distance measured has shrunk since the 1980s from 37 million light-years to a bit over 21 million. A gamma-factor of only 40,000 is needed. Still results in a blazing x-ray sky, but be thankful for small mercies.
Once back in the Milky Way, the New Humans find what remains of Old Humanity after 74 million years of divergent evolution, as well as some surprisingly “alien” descendants of Old Earth’s biosphere. Definitely give the series a read if you like Imagination that spans across the billennia.
“The Jupiter Theft” features an early example of a gravity tractor. The Cygnans launch a high acceleration probe which circles Jupiter and sucks in its matter to power its annihilation drive. Eventually it’s spinning around Jupiter so fast that the Probe’s gravity matches Jupiter’s, so it can then drag it along. Gravity does funny things at high gamma-factors. The effective gravitational mass increases with the cube of the gamma-factor. If the Probe starts with a mass of a millionth of a Jupiter, then by gamma ~ 100 it’ll be tugging at Jupiter with about equal gravity. Of course that also means the thrust level required goes up as well – with the square of the gamma-factor to keep it in the same orbital radius. Additionally an annihilation drive needs to expel about 2(gamma) the mass of the object to get it to a gamma-factor. Thus the Cygnan Probe will reach roughly gamma ~ 100 by the time half the mass of Jupiter is expelled as annihilation beams. Once equality is achieved the probe will tug Jupiter forward with a slight backwards angling of the beam. The Cygnans use near 1 gee acceleration, climbing to about 0.98 c before flipping the drive