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In my current worldbuilding project, humans travel through space by putting their brains into advanced, miniature vehicles that supply the brain with the bare-minimum chemicals, electricity, etc, and then flying through space at very high speeds (likely through matter-antimatter interactions). At the end of their journey, their brain is transplanted into a different body (cyborg or otherwise), and they resume their activities as usual.

These brain vehicles travel distances of several light-days (usually about 0.02 lightyears or 0.03 lightyears), with the farthest planned destination being 0.2 lightyears away from the starting point.

My question is: How fast can these vehicles travel these distances, without killing the brain(s) within? What method of propulsion would be used, and how much would these vehicles accelerate? Would Expanse-style constant acceleration be involved?

I don't want to have mind uploading, by the way, because that would defeat the purpose of the story's themes. However, I will allow some modifications to the brain, as long as the brain continues to maintain the same "person" inside.

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    $\begingroup$ Pedantic note: the thing you care about is acceleration, not speed. Speed is only a problem when you hit something (or something hits you), but too much acceleration turns you to pâté. $\endgroup$ Jul 16, 2022 at 19:25
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    $\begingroup$ Also, now I look: you're asking multiple questions here, most unrelated to the title. That's a no-no on this site: ask one question at a time. You should edit your post to remove the excess. $\endgroup$ Jul 16, 2022 at 19:51
  • $\begingroup$ Hi, this was my first post on StackExchange! Thank you for replying to quickly, lol. I realized after I asked this question that I already knew how I wanted to do it lol. $\endgroup$
    – Zanly
    Jul 16, 2022 at 20:00
  • $\begingroup$ This is squishy Altered Carbon. Go with a cortical stack. $\endgroup$
    – Dave
    Jul 17, 2022 at 18:14
  • $\begingroup$ It's an even closer match to the Mi-Go -- see Fungi from Yuggoth, and "The Haunter in the Dark" $\endgroup$ Jul 17, 2022 at 18:28

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I'll leave aside the details of the rocket designs and trajectories, and just focus on the key thing: human tolerance to g-forces.

As you subject your brain-inna-jar to ever stronger accelerations, blood will tend to pool in the "bottom" part of the brain, and be depleted in the "top" part. Which parts those are depends on the orientation of the brain in the jar... there are probably some orientations better than others, but I've no idea what.

You can compensate for this by increasing the pressure of the blood supply. Problem is though, increase the pressure too much and eventually the human will suffer from a hemorrhagic stroke when some blood vessel pops and then they'll die in short order, or maybe just suffer some brain damage if the acceleration is quickly reduced and appropriate treatments administered.

Even without popping a blood vessel, there's a risk that bits of the brain at might suffer from inadequate blood supply, or maybe some of the smaller blood vessels are damaged by the high pressure.

There seems to be limited work on the effects of extended human exposure to high gravity. I've found at least one report of 24 hours at 2G, for example, but longer duration and higher acceleration studies may never have been done.

The effects of the elevated blood pressure will probably look like hypertensive encephalopathy, which is a real-world and moderately well studied thing. If you have good enough surgical techniques it might be possible to repair some of this damage, but there remains the possibility of permanent (and cumulative!) brain injury resulting in disability, memory loss, personality changes and all the rest. Mix and match to suit your plot needs.

If I had to make a wild guess, I'd say that more than 4G sustained for several days seems likely to be the limit. This is fine for sensible in-solar system journeys (a continuous acceleration 4G brachistochrone will take you 24 AU in a week, plenty for Uranus but not quite enough for Neptune) but not adequate for longer jaunts. Travelling .1ly will take nearly 120 days, and I bet your brain will be pretty scrambled by then.

Probably what you need is not just brain-removal, but also some kind of magical medi-nano-technological system for dramatically reinforcing small blood vessels. Such technology has quite a lot of potential knock-on effects, so be cautious about handwaving such things into your setting.

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  • $\begingroup$ Isn’t that the point of the Expanse’s “juice”? $\endgroup$
    – StephenS
    Jul 17, 2022 at 5:52
  • $\begingroup$ @StephenS pretty much. There's some reasonable fan theories on its composition and possible side effects, too. $\endgroup$ Jul 17, 2022 at 18:12
  • $\begingroup$ if all the blood "pools" at one end, why not just slowly rotate the brain around, so that the location the blood is pooling is keeps changing and so no part of the brain is deprived of blood for very long? $\endgroup$
    – Michael
    Jul 17, 2022 at 23:15
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    $\begingroup$ @Michael because brains are small, the accelerations are high, and hypoxic effects set in quick. It also presents and opportunity for the high-bloodpressure-induced "bruising", for want of a better term, to be nicely distributed across the whole brain. No, what you really want is to just not get rammed through space hard enough to get traumatic brain damage. $\endgroup$ Jul 18, 2022 at 15:49
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"How fast can these vehicles travel these distances?"

That is up to you, my friend. It depends on how much money (anti-money) you want to load into your engines. The bigger and badder you're willing to make your engine, the faster it can go.

I'll try to give you some equations and parameters you can use to model your antimatter engine. I'll be taking the figures for an antimatter beam core rocket at face value, found here on Atmotic Rockets:

  • Exhaust velocity: 100,000,000 m/s
  • Specific impulse: 10,200,000 s
  • Thrust: 10,000,000 N
  • mass flow: 0.10 kg/s

A beam core rocket converts small and equal amounts of matter and antimatter into gamma rays and charged particles. Charged particles are channeled by a magnetic nozzle and used directly for thrust. The dangerous levels of gamma rays are wasted and must be shielded against.

Say we want to travel distance $d$ [m] in time $t$ [s]. We can calculate our delta-v by: $$v_{d}=2\frac{d}{t}$$

$\frac{d}{t}$ gives us the velocity we'd need to cover distance $d$ in time $t$, and our delta-v is twice this amount because we have to speed up to that amount at the start of the trip, then slow down from that amount, coming to a relative stop again.

With $v_d$, and our given exhaust velocity, $v_e$, we can calculate the rocket's mass ratio, $R$:

$$R=e^{\left(\frac{v_{d}}{v_{e}}\right)}$$

The mass ratio is the ratio of fueled spacecraft to fuel-less spacecraft. And with that, we can calculate the mass of fuel required for the acceleration-deceleration, $m_p$ [kg] (based on the parameters for beam core we take at face value):

$$m_{p}=m_{1}R-m_{1}$$

$m_1$ [kg] is the mass of the spacecraft without matter-antimatter fuel, the "dry" mass. This includes the mass of the engine (which was given to us as 10,000 kg) and the mass of the cargo, or "everything else". This includes the brain support machinery and the shielding against the deadly gamma rays released by the beam core engine. As a rough guess, I used 20,000 kg for an estimate of $m_{engine}+m_{cargo}$.

Let's throw some numbers at it. Let's say we want to travel 0.02 ly in 2 years time. I get the following values:

  • $v_{d}$ = 6,000,000 m/s
  • $R$ = 1.062
  • $m_{1}$ = 20,000 kg
  • $m_{p}$ = ~1,240 kg

Half the mass of the propellent, $m_p$, is regular matter (protons), which means our antimatter mass is half that, ~620 kg.


Additionally, if you want to calculate acceleration, $a$ [m/s^2], divide thrust (given as 10,000,000 N) by the fully-loaded "wet" spacecraft mass, $m_0$ [kg]:

$$m_{0}=m_{p}+m_{1},$$

$$a=\frac{F}{m_{0}}.$$

For the 0.02 ly in 2 yr flight, I get an acceleration during the burn-phases of ~47.1 g. For a person, that kind of gee-force would really suck, but for a brain sitting in supporting armatures, pressurized fluids, etc., I would consider that near the limit of what's tolerable before damage starts occurring. You can reduce the acceleration by increasing the cargo mass (or engine mass, if you're feeling really conservative).
You can also reduce acceleration by reducing the efficiency of the rocket, but that would in turn result in using more antimatter propellent for a similar journey. If you're going to be reducing efficiency, you might want to consider using another rocket engine; beam core is already one of the most efficient ones out there.

To calculate the burn-time for the acceleration-deceleration phases, use:

$$b_{t}=\frac{\left(m_{p}\cdot v_{e}\right)}{F}$$

This gives you the total available burn time for the engine. Divide that amount by two for the burn time of each acceleration phase. For our example, I get a "half"-burn time of ~6,400 s, or about 3 1/2 hours. That's 3.5 hours of acceleration, followed by ~2 years of coasting, then 3.5 hours of deceleration. (Note: this is a hyperbolic trajectory, which, for a set delta-v budget, is technically faster than "brachistochronic" trajectories (constant acceleration), despite the name. You can plot the graph of velocity/time to see that for yourself.)


(Edit: Part of the problem with humans and gees is getting blood to the brain and different densities in the body (cavities, bones, etc.) leading to collapsing and tearing tissues. Assuming you have a mechanism to deliver blood to the brain adequately, the oxygenation problem isn't a problem. The brain is about as dense as water. Immersing the brain in a fluid as dense as it allows the acceleration pressure to be delivered across and throughout the brain, if all the nooks and crannies are filled. I'm lacking a source atm, but the last time I looked into this ~70 gees was right up against the threshold of survivability.)

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  • $\begingroup$ 2 years in a can ... they better have Netflix. $\endgroup$ Jul 17, 2022 at 18:40
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    $\begingroup$ @RossPresser They better have a dreamless, nothingness. If I had to watch Netflix with my mind for 2 years I'd go insane $\endgroup$
    – BMF
    Jul 17, 2022 at 18:54
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As fast as rocket equation allows them.

You choose one already - antimater based and there is nothing new reactive style

No, if you mean high accelerations, if you have not enough reactive mass for it, and for long distances you wont have it.

I guess main question here may be - on max acceleration - as tests show that brain can endure 80g, for some period of time, usually it tested as limits of a shock a human can endure at collision, so duration is not much and such, but there were tests by NACA and they were limited by their equipment at that time and half submerged human can hold at 30g and the tester wished more but limits of equipment. So a limit for normal operation mode can be higher when than 30g and it is not so much different from 80g.

So when body is out of question then it indeed may increase the rate of acceleration a brain can be okay with, and 80-100g can be a reasonable asumption especially if we consider is is helped specifically and some technological, medical means to help take care of this situation. And that number is for not modified brains. Add some reinforcment mesh, and technomagic and 500-1000g may be reasonable as well.

10k maybe not be possible because it will affect tissue at cellular sub cellular level - affecting things too much to function, I guess.

In general, if they can do all that, I do not see a sense to abandon a body for this purpose(if you do not do it for a reason to be a robocop type creature all the time, for robocop guys there is no difference then)

With 1g constat acceleration a distance of 0.2 ly it takes, about 320 days

And max speed at that time will be 0.1c (around that number)

So if there is the same proportion of reactive mass then - least time for such travel will be 35 days, sure a difference, but..

At 10g acceleration and all the same - it takes 100 days. Still sure a difference, may be important in some case, but a body lasts for decades, so idk.

30g - 58 days - 2x difference idk if one has some atachment to his body, I guess not big enugh reason to abandon it, and ship one needs is not much bigger than a brain ship.

Having technologies to preserve brains, it looks they can preserve body as well, not such a big deal, enough of cases where people are in comatose states for far longer. (One does not need to put one in such, but just play games, vr etc, but body is in a system like one of a vampire - in a box)

Over longer distances it also does not make much sense, acceleration part is not so long compared to travel time.

On short distances it does make sense - speed wise, but less so time wise.

So I would say it for those who need it to be a little bit cheaper, or preffer artificial bodies anyway - so geeks and poor and special forces while middle class keeps their bodies. Difference in resources is something like 10x so it may make sense for some, and for some reasons beside pricing, buut you in space, surrounded by spacescale of resources, energy, materials, soo...

But in general okayish setting, meat bags go home, borg collective for the win.

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