My goal is to generate a plausible means of interstellar travel without using FTL technology while supporting round trip travel time to nearby stars within a human lifespan (measured by the life of the humans left on earth).

At present, I'm contemplating a multi-stage process:

  1. Beginning with a probe of small mass, accelerated at a reasonable fraction of 1g, possibly using a solar sail and laser propulsion.
  2. Once around the target solar system (having slowed in transit sufficiently to orbit the target star), it has independent capabilities to locate and navigate to small asteroids, harvest resources, and manufacturer devices of modest size.
  3. Communicating with earth, the probe will employ designs arriving from earth to create tools/machines larger and more capable than itself, bootstrapping a manufacturing hub. Those machines will use the probe as an intermediary or communicate with earth directly.
  4. Once an environment/insulated biome/space station has been created that can support human life, the probe or another machine will accept patterns for human consciousness. The premise here is that, while most of the human body can be replicated without requiring the greatest accuracy, the component of the human brain that maintains consciousness must be replicated in exacting detail (to the extent that copying the pattern destroys the original) to maintain a semblance of identity.

This strategy avoids the challenge of attempting to corral and transmit the specification of an entire human (which in some estimates would require over 10^40 bits*). The fiction here is that we can efficiently extract and compress the pattern of an individual by looking at only a few ounces of brain matter distributed through the frontal lobe and cerebral cortex. The remaining brain can be copied at a much lower resolution while the body can be generated by off-the-rack plans. The other half of this fiction is that it is infeasible to create that spark of human consciousness using the same off-the-rack process used to create the body -- identity in the story would change radically without this stipulation.

In order to fulfill the 2nd half of this fiction, there would have to be some sort of no-cloning aspect to the technology (entanglement?). Otherwise, you'd be able to create an army of the same individual at the target site.

What sorts of challenges haven't I considered in this future world? Where are the plausibility issues?

The "Messenger Line" comes from the practice of using a light weight string from bows to reach between ships, dragging increasingly heavy rope to support the transfer of cargo.

I suspect that the heavy lifting of communication would be done from earth and that the transmission power of a probe that made it to Barnard's Star would be limited. Also, having a single, small probe capable of navigating around a solar system with a chance of sensing small asteroids sounds unrealistic. Perhaps a swarm of a thousand or a million probes whose paths are scatter shot around the system stands a more reasonable chance of sensing an asteroid of suitable composition.

Does this premise already exist in science fiction?

*[see Teleportation: will it ever be a possibility @ the Guardian]

  • $\begingroup$ I had this funny idea a while ago: we build a railgun out in deep space--hundreds or thousands of kilometers long--that accelerates a small "spacecraft" to near-light speeds, aimed in the general direction of a target exosolar body. En route, the spacecraft reassembles itself into a long, segmented chain thousands of kilometers long and perhaps less than a millimeter in width. Each partition in the chain acts as a powerful electromagnet that, at the proper time, will repel against the adjacent segment. (cont.) $\endgroup$
    – BMF
    Aug 4, 2020 at 20:37
  • $\begingroup$ When the spacecraft encounters the target body, it will do so explosively. The spacecraft will stream into a cloud of atomized material, however, the electromagnets, before contacting the body, decelerate trailing segments of the spacecraft at a rapid rate. The hope of it all is that out of the energetic explosion, small virus-sized nanomachines might survive with enough intact information and be propelled away from the epicenter. They might find some material and energy to work with to gradually build a human civilization from atom up. $\endgroup$
    – BMF
    Aug 4, 2020 at 20:37
  • $\begingroup$ Or perhaps build a laser facility to slow the real colonization effort still en route in interstellar space. $\endgroup$
    – BMF
    Aug 4, 2020 at 20:39
  • $\begingroup$ Is the sending of the consciousness FTL, or just by laser STL? $\endgroup$
    – NomadMaker
    Aug 5, 2020 at 0:33
  • $\begingroup$ @NomadMaker In my mind nothing in the conception is FTL. My expectation is that with the 'no cloning theorem' that I'm depending on to avoid multiple copies, extracting a consciousness from the original will have to occur on the order of seconds, if not faster. With that the bundle of information (+entangled particles?) will traverse the gap between Earth and points distant at no better than the speed of light. The way stations (see No Clones comment below) will have to be set up in advance in order to avoid delay of propagation. $\endgroup$ Sep 8, 2020 at 18:28

4 Answers 4


No Clones

There would have to be some sort of no-cloning aspect to the technology (entanglement?). Otherwise, you'd be able to create an army of the same individual at the target site.

Creating a clone army of 1,000 soldiers (for example) takes the same amount of resources (1,000 A.I. supercomputers + 1,000 robot chassis) that hiring or transporting an army of the same size would take.

Additionally, a "clone" army is demonstrably weaker (and thus less useful) than individuals : you'd only have one kind of specialist (heavy weapons, comm, research), for example.

Also, once exposed to varying roles, "clones" would decohere, as shown in the 1996 film Multiplicity. The once singular personality, exposed to different personality-shaping stimulus in each instance clone, would quickly become a unique personality. Like a very close sibling with common memories of growing up. It would be challenging in advance to know that the "team" of decoherent clones is effective together. It would be much more effective to take a team of individuals with proven competence as a team, and ship them.

Data rates, Power, and Attenuation

What sorts of challenges haven't I considered in this future world? Where are the plausibility issues?

Physics and safety concerns create a limiting factor to data rates.

Data rates are limited by the frequency of your carrier wave. A 1 GHz carrier, limits your data rate to $1 \times 10^9$ ${bits} \over {sec}$ The relationship between frequency and wavelength is fixed $\lambda = { c \over f}$.

Assuming the "message line" data is ${1 \over {100}}^{th}$ % of the whole body, estimated at $10^{40}$ bits, you'll need to transmit $10^{36}$ bits of data.

Time vs. Frequency to Xmit $10^{36}$ Bits :

  • Gamma rays ($10^{22}$ Hz) : 300 million years
  • Low-Frequency Cosmic rays ($10^{41}$ Hz) : 6 microseconds

There's a few problems here that might be hand-wavable. We can't create gamma ray transmitters yet (it's a hypothetical technology). Cosmic rays as tunable transmitters and receivers - even moreso.

The free space loss is the loss of transmitter power over distance. The equation for this is $L_{fs} = {20log({{4\pi d} \over {\lambda}})}$. The value is in decimals. As you can see, the smaller the wavelength ($\lambda$) over a distance (d), the higher the loss of power between transmitter and receiver.

Over an interstellar distance (10 ly), using low-frequency cosmic rays as ($2 \times 10^-33$ meter wavelength), the loss is -1,015 db ($10^{-101}$). That means for every Watt of transmitter power, $10^{-101}$ Watts of signal will be received at the remote site. A giga-Watt transmitter $10^9$ doesn't even begin to cut into that significant loss.

Solution : An Interstellar Cable

The solution takes another lesson from "message lines". As the ship travels, it could leave behind a Voyager-sized probe (400 kilograms) every 1 AU, creating a relay line in the probe's wake. The total mass cost for about 10 light years is around 5 tons (if I've done the math correctly).

Each of these lines is trying to reach only a distance to the next transmitter (1 AU = $150 \times 10^9$ meters). At this closer distance the free space loss is -899 dbm.

Let's Try This

Microseconds seems faster than I need to transmit a mind. Let's change that to 1 day (86,400 seconds) transmission time. That changes my carrier wave to $10^{-31}$ Hz. My carrier wavelength, then is $3 \times 10^{-23}$ meters. The free space loss at 1 AU is -667 dbM. Changing the timescale to week, month, or year doesn't make much extra improvement. Current transmitter and receiver technologies are in the range of ~100dbM; so you'd need to imagine several significant leaps forward in transmission and receiver technology.

May I Suggest Transmitting Synapses Instead?

The human brain only has $\approx 1 \times 10^{18}$ synapses. These might fully describe the mind beneath (or be in the right order of magnitude).

If you transmit synapses, you can transmit the entire data payload in 1 month using existing radio technology (gigahertz frequencies). The free space loss over 1 AU for this lower frequency transmission is -307 dbm; which is much more in the range of existing technologies.

  • $\begingroup$ Thanks for the comments about data rates and attenuation. I'm still working on absorbing it. The idea of repeaters sounds compelling given the distances and physical limitations of signal propagation. $\endgroup$ Aug 6, 2020 at 16:23

It's impossible to ship a human mind across a burst of data

Well, at least the way we understand the human mind to work, anyway. Consciousness is weird from a certain perspective. We know it exists, 'cogito ergo sum' and all that - but we don't know exactly how it exists and there's a lot of evidence to suggest that the conscious part of the human mind is not, in fact, entirely within the world that we can interact with - well, interact with using current scientific equipment. That is to say, human consciousness may very well have a quantum aspect which would pose certain issues in the attempt to duplicate said pattern and reinstate it into a cloned human mind, issues which cannot be solved with current technology.

The rest of the premise is fine - to use a bit of jargon, it's just a Von Neumann machine which is acting as a expeditionary scout. There are a few other problems with lightspeed transmissions, namely, its only as fast as light so jumping between systems even at light speed could take years so traveling 'there and back again' would mean that you don't age (at least not mentally) whereas everyone around you has. Not to mention that there's a chance said transmission could get deflected a bit from something in space and miss the target, thus resulting in a failed transference of conscious, which would presumably be bad.

That said, the quantum aspect of human consciousness isn't widely known and mind-swaps have been around in sci-fi for a while, so you're pretty good if you want to use this as a premise. Just be aware that this same technique can be used to keep putting humans in new bodies, so you've made everyone immortal.

  • 1
    $\begingroup$ I hadn't contemplated the immortality aspect of it. Something for me to think of. I'll read up on Von Neumann probes. $\endgroup$ Aug 4, 2020 at 5:26

Has this been done in fiction? Yes!

In Charlie Stross Accelerando, uploaded humans travel an interstellar distance in tin-can sized piece of computronium propulsed by a solar sail, on their way back they only transmit their consciousness. How do they guard angainst copying the same person? They don't.

Same author, different 'verse - in Neptunes Brood, people (more or less human like robots, humans went extinct way before the story is set) travel as a signal and buy or rent a new body upon arrival. The first probe to a new star system however is crewed by physical people, interstellar travel takes ages and is incredibly dangerous as is the first decade or two until people can be received proper. The financing of such an endavour is also quite complicated. Again, no real safeguard against multiple instances of one person running around, such is posthuman life.

Yet another take by the same author is in Glasshouse - wormholes exist, but also so called A-gates that disassemble people in one place and reassemble them in another, with drastic changes to the body possible. Again, no real safeguard against multiple instances of one person. The protagonist is an ex tank regiment.

Is it possible?
Brain uploading is a hotly contested topic among singularitans. I think proponents of the idea (as in: believe it's really feasible) vastly underestimate the complexity of a brain and the importance of bodies for cognition. I think it's not but more importantly you are about to write a story using established posthumanism/singularity tropes so maybe you want to look at those.

How to prevent multiple instances of the same person?
IMO impossible: You can always invent a destructive uploading process (brain is glassified, sliced and scanned?) but once an information is there, it can be copied or implemented physically as often as one has the means.

  • $\begingroup$ Altered carbon also uses this uploaded consciousness thing. And Avatar, right? There is a terrifying SF where the invading aliens use the transmission device to reconstitute escapees for questioning. The people who thought they were escaping instead wake up with the aliens and realize they are copies. They are merrily used up with the questioning because the aliens can always produce new copies for more questioning as the old ones are used up. Sometimes the new version and the old version overlap... $\endgroup$
    – Willk
    Aug 4, 2020 at 14:07
  • $\begingroup$ I didnt read AC, there's tons of examples in the tvtropes link. $\endgroup$
    – mart
    Aug 4, 2020 at 14:35

There are a number of issues with your scenario:

Assuming no 'breakthrough science' travel to other solar systems is, for the foreseeable future limited to speeds considerably less than the speed of light. Say .25C max with .1C to .2C being more likely based on limitations imposed by the velocity of the exhaust streams from various forms of nuclear propulsion. (You could get higher using various forms of sail propulsion but then slowing down at the other end becomes problematic.)

If your travel time is limited to the length of a human lifespan then solar systems within say 20 light years is the max you are likely to be able to achieve in a human lifetime. And normally even then any original crew will likely be too old contribute much on arrival (other than a psychological and emotional link to the 'mission' and Earth) when you talk about systems at the upper end of that range.

However there is hard science behind the idea of using hibernation (not suspended animation) to extend the duration of human extra solar travel. Basically we may be able to put humans into a torpor like state similar to what hibernating mammals undergo. So a human could, in theory spend say 6 months of every year during the journey asleep, potentially with some degree of anti-aging bonus. If used it could help enormously.

So, that said the process I would suggest is;

  1. Spaced based Very Long Baseline Interferometry or a similar technique is used to identify the best star system to send a mission to.

  2. A probe is sent consisting of a number of sub-units tasked with mapping the chosen system in detail;

  3. Your main mission is launched consisting of a large probe with the following sub units;

  • an advanced AI for mission control and communication;
  • mining sub units-and printer modules - to build a station;
  • a biological 'package' consisting of frozen, sperm, ova or embryos (animal and human) plus plant seeds and spoors and bacterial and viruses as required for a functioning ecosystem.

No humans are 'warmed up' until the station and a supporting ecosystem are up and running, artificial wombs being part of the production chain.


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