Okay, so our society has starting harnessing significant amounts of energy from its sun, and has started mass producing micro-black holes (although there have been experiments with gathering energy from the micro-black holes directly, because of engineering challenges, solar energy is cheaper. Many nations are trying to move away from solar power to a renewable energy source though, especially the outer colonies.) Now we can make a black-hole ship! The only problem is that we want to go far away really quickly, but living things die when you do that.

My question is, what ways can we get around that.

I am thinking something gravity based. If they are being pulled by gravity, it would pull on all parts of their body mostly equally, v.s. a spaceship transmitting all the acceleration into the feat/back.

  • They don't want to send cells that grow into babies or anything like that. That is like a weird sci-fi movie. Who would do such a thing!

    • Ideally, it should be comfortable. Who wants to be strapped in a spaceship for 6 months! However, if your method is really efficient, it would be okay be to uncomfortable since it wouldn't be really long.
    • Maximum comfort would be the crew experiencing 1 G of acceleration applied to their legs or whatever is touching the floor.
    • Extra good if their are other habitable regions of the ship with different accelerations for recreation or scientific purposes
    • Or having an adjustable knob to adjust subjective acceleation
  • Don't drain to much energy from the black hole!

  • Don't drain too much energy from the black hole engine. Thats more energy you have to carry! (That said, you do have a ton of energy by today's standards, since you can through rocks in and get hawking radiation. Just not infinite energy.)

  • The economy is doing pretty well now a days, so NASA has basically unlimited funding.

    • Although is it better if this technology could be created quickly, we can imagine that humanity wants this so much that they could gather resources for a couple of centuries in preparation.
  • I am looking for the most acceleration possible, that will be the main criteria for picking the answer.

  • 4
    $\begingroup$ Are you sure you want the hard-science tag? That requires equations and references from science journals for answers. Science-based is enough to restrict answers to known hard science knowledge. $\endgroup$ Commented Jul 21, 2015 at 20:13
  • $\begingroup$ @DanSmolinske Okay $\endgroup$ Commented Jul 21, 2015 at 20:48
  • $\begingroup$ @PyRulez Aww, but I have a hard-science answer! $\endgroup$
    – Samuel
    Commented Jul 21, 2015 at 21:12
  • $\begingroup$ @Samuel Hard Science is better. I would probably pick hard science over handwave science. $\endgroup$ Commented Jul 21, 2015 at 21:21
  • $\begingroup$ Also, in Macroscope, the people melted themselves to transport via a wormhole/hyperspace thing, as a liquid. $\endgroup$
    – JDługosz
    Commented Jul 22, 2015 at 6:33

6 Answers 6


Liquid breathing and black hole powered railguns

Full body fluid immersion with the air evacuated from the lungs will allow for the maximum acceleration. Without evacuation of the lungs, humans can withstand 24 g without any noticeable pain. This study found:

Animal studies with mice showed that, where the acceleration-time lethal threshold for water immersed mice is around 1300 Gx for 15 seconds, when their lungs are emptied from air, the maximum acceleration reaches 3800 Gx for more than 15 minutes without any physical impairment

Note: Gx is an acceleration in the positive x-axis, like you would have sitting in a vehicle.

The study goes on to mention that the mice in this case were not using liquid breathing, but extracorporeal circulation. This is where the blood is pumped through a separate system the oxygenates it. While complex and unimaginably terrifying this process would allow a person to be suspended in a fluid which they would otherwise drown in, without killing them. The maximum acceleration would be less for human suspended in any known breathable liquid, like perfluorocarbon, because that liquid is significantly more dense than a human.

They don't have any solid numbers for the actual maximum acceleration that a human could endure, but explain that:

It is difficult to estimate an ultimate acceleration limit possible with this set-up, but it presumably can be higher than hundreds of G.

Hundreds of g. That seems conservative considering the mice studies. In any case, that's exciting, as the paper goes on to say:

Completely new concepts, such as magnetic railguns, could also be considered for manned missions, should it be experimentally confirmed that the physiological stresses due to high acceleration loads vanish using this type of set-up

Some additions.

So there it is. Use the black holes to create black-hole-rail-guns that launch humans (and the reaction mass to slow down) for inner system transits. For leaving the solar system you can turn the black hole drive to eleven and accelerate at 400 g out to the stars. They will arrive at their destination far faster than those fools plodding along at one g.

  • 3
    $\begingroup$ I think a 1g constant acceleration would easily outrace a coasting craft that accelerated at a few hundred g for a few seconds or minutes. You can draw a graph and see where they cross for any specified initial boost, and at interstellar distances the parabola beats the straight line even when that line was pretty high up. $\endgroup$
    – JDługosz
    Commented Jul 21, 2015 at 22:57
  • $\begingroup$ @JDługosz You're neglecting the speed limit. If the maximum velocity for the craft is 0.99 c, getting there at 300g will beat a craft getting there at 1g. You can graph that as well :) $\endgroup$
    – Samuel
    Commented Jul 21, 2015 at 23:05
  • $\begingroup$ One line is (say) 300g for a few seconds via the sligshot or other external engine. Then it coasts. "The desired velocity is reached" you wrote. A constant acceleration will trump that beyond some distance d. $\endgroup$
    – JDługosz
    Commented Jul 21, 2015 at 23:14
  • $\begingroup$ @JDługosz You're describing a situation where you have intentionally set up the parameters so that constant acceleration beats an initial high acceleration (Why limit the high acceleration to minutes?). You then point to it and say "see, constant acceleration wins". I don't see why you're doing that. Constant acceleration does not inherently outrace a coasting craft if the coasting speed is as close as physically possible to the speed of light. In all cases, the rapid acceleration over finite time can be made to beat the 1g acceleration. $\endgroup$
    – Samuel
    Commented Jul 21, 2015 at 23:27
  • $\begingroup$ No, you specified that. The ultra high-g is a sligshot, not a constant engine source. Why limit it to a few minutes... implied, as that's what fly-past-massive-body does: it only works during that brief flypast. I see that if you reached ultrarelativistic speeds then it's the same limited speed to an outside observer. But a 100g acceleration won't reach that kind of speed during the brief duration that it's passing by a massive object. It would take a few days. $\endgroup$
    – JDługosz
    Commented Jul 21, 2015 at 23:40

Regarding high acceleration (or lack of), the late Robert L. Forward wrote about several interesting ideas both as a scientist/engineer and as a sf writer.

Within the solar system, it is wasteful to accelerate and de-accelerate a cargo, when in the end you just balance the momentum. Think of how a space elevator is different from a rocket, especially if counterweighted by incoming cargo.

Take a long pole and spin it in space, rapidly. The hub where it pivots is an easy place to dock, and then tractor rails pull it to one end where it is released, thrown out to the destination. Similarly, the arm can catch an incoming pod and carry it to the hub to be released.

The cargo pod will feel a large g force while it is being held to the wheel. The energy and spin-up of the tether can be balanced between incoming and outgoing, so it doesn't take new energy input to get from here to there.

For manned pods, a way of tolerating the acceleration would enable that use.

For black holes etc. A "slingshot" involves gravity and the ship does not feel any acceleration. When New Horizons passed behind Jupiter it gained angular momentum at the expense of Jupiter losing some, slowing its orbit around the sun. The ship gained 4 km/s, which isn't much on the scale of the solar system but did save 3 years, or in other uses can save fuel and expense.

If you collapsed Jupiter to a black hole using Clarke's monolith or somesuch, then you could pass much closer to the mass and get more attraction. But you are only closer for a brief time, so you have diminishing returns and it doesn't give as much as you would wish. In this case, the close encounter would give tidal forces and a ship would feel stress and the occupants high-g, as in Nivin's short story Neutron Star.

A chain of Saturn-mass black holes is absurd. Like normal planets they need to be spaced apart by billions of miles, and they only are useful when lined up just right.

Now back to Forward: imagine a super-dense material (not a black hole, but dense enough so gravity is useful) shaped like a torus. It's spinning around, such that a point on its surface is seen to go through the hole and circle around the limb (think of the motion of rolling down a sock while you're wearing it.

This would cause a gravito-magnetic effect and an object flying through the hole would be accelerated. Again, this acceleration is not felt by the ship since it affects every part of it. But, un-even-ness would be noted as g-forces.

If you had a set of rings so the ship passed through one after another it could build up acceleration. What do you make it out of, how do you keep it from collapsing into a sphere, how does it turn inside out like a smoke ring, and how do you replenish the spin after use? If you can build that, keeping biological bodies intact is not going to be an issue. The two topics should not meet, unless it's a found artifact or something like that.

Now consider a "railgun" of any technology. Not gravity but perhaps electric, or even pneumatic: whatever. Assume you can get a continuous acceleration, not just spots of high acceleration with gaps from one to the next. At 100g, how long would the barrel be in order to boost it up to ultra-relativistic speeds?

See this page for the math. Here is some GEL if someone who knows more how to use it wants to generate some graphs:

c = 1; # units used: c is 1 lyr/yr
g = 1.03; # 1g is 1.03 lyr/yr^2

function f_t (a,T) = (c/a) * sinh(a*T/c)
function f_d (a,T) = (c^2/a) * (cosh(a*T/c)-1)
function f_v (a,T) = c * tanh(a*T/c)
function f_T (a,t) = (c/a) * asinh(a*t/c);

day = 1/365.25
t = day
a = 100*g

T = f_T(a, t)   # proper time
d = f_d(a,T);  # distance traveled
v = f_v(a,T);  # velocity

display ("distance in miles", d*5.87849981e12)
display ("final velocity", v)

So, if your railgun could give a continuous 100g acceleration for one day, the projectile would have a final velocity of a mere 27% c, and the device would be 2¼ billion miles long.

After two days, you are up to 49% c and the barrel needs to be 8½ billion miles long.

What was that someone was saying about ultra-relativistic speeds, that a slingshot (or small number of them) could get up to 0.99c? Let's amp it up: 400g of continuous acceleration, applied for 8 days. And a railgun over 83 billion miles long. Sedna

The orbit of Sedna is not quite half of that. In this diagram, note the the purple orbit is Pluto.

why have high end-point acceleration if continuous 1g acceleration is available?

Someone earlier was thinking that high endpoint-only acceleration would give shorter transit time than 1g continuous acceleration. My own intuition is that any external mechanism (railgun) that is suitably compact will operate briefly, before the ship leaves the mechanism. Continuous acceleration builds up over time and you have the entire voyage to use it. So, there is no way that a gun will get a ship to its destination (or to the half way point, where both craft use the same on-board engine to show down) sooner than the 1g engine.

In terms of on-ship proper time, there is not the same speed limit. From the outside world, two ships traveling at near the speed of light will take the same time to transit. But on board, the one with higher dilation will experience less time during flight. So more is still better, from the passengers' point of view.

The advantage of something like a slingshot or external flinger of any kind is that you leave the engine behind and don't have to carry all that weight and fuel, and you can use conservation of round-trip counter momentum to reduce the actual energy needed. So even if you could build 1g craft, that would be the luxury passenger liner, while Walmart cargo would use the rotating tether for raw materials in one direction and finished goods in the other.

  • $\begingroup$ Do you have a link for the space pole idea? It sounds interesting. $\endgroup$ Commented Jul 23, 2015 at 0:26
  • $\begingroup$ No, I saw it in one of Forward's nonfiction essays, and used in various stories by him and others. A quick google search shows the skyhook variation. Maybe this ine: amazon.com/Indistinguishable-Magic-Robert-L-Forward/dp/… pick it up for a buck. $\endgroup$
    – JDługosz
    Commented Jul 23, 2015 at 5:50

The interesting thing about using a gravity slingshot is the ship is effectively in free fall during the manoeuvre, so the crew will not feel much acceleration either.

Depending on the size of the black hole, the crew could be in some danger as then approach the event horizon, since the massive gravitational gradients will induce a tide on the person, the ship and all the equipment. Too close and the entire structure can be pulled apart (scientists call this "spaghettification"). For micro black holes, this is probably not going to be an issue, but the small size provides two different problems:

  1. They are very small, by definition, so their gravitational influence will be minimal. You want to slingshot around massive objects the transfer momentum from them to you (i.e. Jupiter), not a black hole the mass of a small asteroid.

  2. Small black holes tend to evaporate, and as they do they release increasing amounts of energy. This exponential energy release will wreak havoc on your ship, unless you are prepared to harness it somehow. Solar sails deployed very close to the Sun are calculated to be able to generate large amounts of acceleration, enough to drive starships out of the solar system at 3G and reach Alpha Centauri in @ 1000 years. Lighter, unmanned probes are calculated to be able to a accelerate much harder, although harnessing it for human flight would be rather challenging.

Perhaps a compound system of micro black holes would work. The starship, equipped with a huge light sail, receives the energy of an evaporating black hole to accelerate to the gravitational engine. Rather than a single small black hole, the engineers have arranged for a large number to be orbiting around a common centre (think of a merry go round of black holes), and the ship uses the combined gravitational and kinetic energy of the "merry go round" to do the slingshot. Someone with better math sills can do the calculations on that.

  • $\begingroup$ Wouldn't large black holes work better? (You just need to feed it.) $\endgroup$ Commented Jul 21, 2015 at 23:45
  • $\begingroup$ Large black holes wold work much better, but would have some undesirable effects on the local real estate. As well, given the question suggested that we can create "mini" black holes, I tried to limit the answer to these constraints. The other issue is eventually you will be using most of the mass of the Solar System to get a "decent" sized black hole, and this would work even better with a black hole of several solar masses, which presents a bit of a problem right there... $\endgroup$
    – Thucydides
    Commented Jul 22, 2015 at 1:18
  • $\begingroup$ The gravitational engine is a ring of co-orbiting tiny black holes? How would it work? $\endgroup$
    – JDługosz
    Commented Jul 22, 2015 at 6:36
  • $\begingroup$ Tthre effects that can be harnessed with this gravitational engine: One, several mini black holes will create a deeper gravity well than an individual mini black hole. Two,the kinetic energy of the orbiting holes warping space will also be large, and if you slingshot in the same direction as the ring is turning, you will tap into some of the rotational energy too. The last effect you can tap is fire the rocket at perigee, to add all the effects together. $\endgroup$
    – Thucydides
    Commented Jul 23, 2015 at 0:31

Alcubierre drive

Use a bubble of opposing expanding and contracting spacetime to push/pull your travelers. Since the local reference frame is isolated from the gravity noise of the rest of the universe they are not subject to inertia and can be accelerated at virtually any rate.

If your civilization is already manufacturing black holes then I imagine it would be trivial for them to use the Woodward Effect to manufacture the negative mass required for an Alcubierre drive.


Well I am not somebody who can answer you question with variables or something but i can think logically pretty well. the thing with trying to go super fast without killing people or making any kind of injury or change to body that is a con for a living being is that you would most likely want to simulate a strong tornado-like magnetic field. It has to be strong so it can't be turned off or on without access to control panel of magnetic provider or conduit however you want to call it. Also it must be tornado-like. You ask why? The thing with tornados if you know them is that even though they have destructible power their middle is actually really peaceful. an example of not just tornado but wind itself has something to do with phrase peace before the storm or however it is said on english. You see when you experience a really peaceful weather and then suddenly there is storm it is because you are middle around which currents of wind caused rain with moving clouds through the same way and making them denser with pressure put on them. So I guess this answers your question.

  • 3
    $\begingroup$ I advise you take the tour and visit the help center to better understand what are good answers and question on this site. This is not answering the question in any way. $\endgroup$
    – L.Dutch
    Commented Sep 23, 2018 at 19:06

Some simple maths will tell you that with 1G of acceleration (~10 m/s2) will get you to the speed of light (299792458 m/s) within a year if you can sustain it. That's the quickest you could be going "far away".

1 year to get to the speed of light is a relatively small amount of time compared how long you'd have to be travelling at the speed of light to get anywhere useful.

  • $\begingroup$ 1) You wouldn't get to the speed of light even in thea hundred years 2) The speed is more for purposes of time/length dialation than going places. $\endgroup$ Commented Jul 21, 2015 at 21:25
  • $\begingroup$ Surely if you were getting 10m/s faster every second (1G of acceleration) you would get to 299792458m/s after 29979245.8 seconds? Divide this by 60 for minutes and 60 again for hours. Divide it by 24 for days and you get 346.98 which is less than 365...? For whatever purposes you choose the speed it's still how fast you're travelling through space to "go far away", although I know actually getting to the speed of light isn't very easy when you're lugging a black hole about... $\endgroup$
    – Varrick
    Commented Jul 21, 2015 at 22:10
  • $\begingroup$ @Webkanguru That's Newtonian acceleration - it's only accurate if you assume that you're not talking about reaching any significant fraction of light-speed. You need relativistic equations for this, and the results are quite different. $\endgroup$
    – Toby Y.
    Commented Jul 21, 2015 at 22:16
  • $\begingroup$ Yes I know because you get heavier, hence the last sentence in my comment. The initial problem was stated as humans having to withstand extreme G force, I'm merely pointing out there doesn't seem to be a need for them to when travelling such long distances. $\endgroup$
    – Varrick
    Commented Jul 21, 2015 at 22:36
  • $\begingroup$ As pointed out earlier, at relativistic velocities, you can't use simple vector addition; for the simple case of colinear velocities, you need the relativistic colinear velocity addition formula, which behaves quite differently as the velocities involved approach the speed of light. $\endgroup$
    – user
    Commented Sep 24, 2018 at 14:24

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