The Q-Drive and the difficulty of interstellar exploration

I’ve discussed the difficulties of interstellar exploration before.  To get a spacecraft to another star within a human lifetime requires accelerating it to an appreciable percentage of c (the speed of light), say 10-20%.  In general that requires titanic amounts of energy.  (Forget about the common sci-fi scenarios of going into warp drive or jumping through or into hyperspace.  Those are fantasy plot devices with either no science or highly speculative science behind them.)

The mass ratio of fuel-propellant to the rest of the craft, using the most plausible short term option, nuclear pulse propulsion, is something like 10,000 to 1 to reach 10% of c, that is, for every kilogram of spacecraft you want to reach the destination, you’ll need 10,000 kilograms of fuel.  Although multiple stages would help, when we consider everything that would be required to send humans, things start to look pretty bleak.  It’s a little bit more hopeful with uncrewed probes.

One solution being considered is Breakthrough Starshot.  Use tiny probes with light sails attached, which are accelerated by ground based lasers to 20% of c.  The biggest issues with this plan include the cost and logistics of the ground based lasers, the challenges in successfully miniaturizing the craft, and the fact that there’s no way to slow the probes at the destination, so they’d have to collect what data they could during the few hours they had when flying through the destination system.  And their small size limits their transmitting power, meaning sending back the resulting data would require decades.

Another old solution proposed in 1960 by Robert Bussard, is to collect fuel from the interstellar medium.  The Bussard Ramjet (BR) has a tremendous electromagnetic scoop in front of it, which brings in the diffuse hydrogen floating ahead of the craft, compresses it so that it undergoes nuclear fusion, and expels it as propellant.  The idea is that the faster the craft is moving, the more fuel available to it, and the faster it can accelerate.  The biggest issues with the BR is that the interstellar medium has been found since Bussard’s proposal to be far thinner than he believed, and the drag of the scoop limits its overall effectiveness.

Alex Tolley has a post up at Centauri Dreams discussing a new proposal: the Q-Drive, as put forward by Jeff Greason, chairman of the Tau Zero Foundation.  Like the BR, the Q-Drive uses the interstellar medium, but in a different manner.  Unlike the BR, this craft uses an inert stored propellant: water.  (The water is stored as a giant cone of ice in front of the craft, acting as a shield against interstellar particles.)  The water is ionized and accelerated out the back of the craft, propelling it forward.

What comes from the interstellar medium is the power to accelerate the water.  This involves two large magnets that create a couple of magsails (sails made of magnetic fields), but instead of using them as sails, they function sort of like a wind turbine, in that they collect energy by slowing down (relative to the craft) the passing ionic interstellar matter, transferring the difference in kinetic energy to the drive.  The faster the craft is moving, the more energy collected, the faster it can accelerate the propellant, and the higher the thrust.

Tolley’s diagram of the Q-Drive interstellar craft. (Click through for source.)

Notably, the craft has to be initially accelerated to some small percentage of c by other means, such as the nuclear pulse propulsion I mentioned above.  But once that is achieved, that first stage can be jettisoned, allowing the Q-Drive to supposedly then reach speeds around 20% of c.  It can decelerate by pointing the drive in the direction of travel.  (It’s less clear to me how the final stages of deceleration near the destination would work.)

If you’re interested in the details, I recommend reading Tolley’s post.  Tolley is clear that this is something that seems possible in principle, but the practicalities may be another matter.  In particular, there is a question of whether the energy conversion can be efficient enough to make the Q-Drive more effective than just using something like the nuclear pulse rocket for the whole trip.

However, if it does work, or it can be developed into something that works, it may make interstellar exploration far more practical than it currently looks.

If you’re interested in the hard core technical details, check out Jeff Greason’s paper, or his talk on the subject.

Greason’s primary message in the talk is to emphasize the key idea, that drag energy from the surrounding medium (the interstellar medium in interstellar flight, the solar wind in solar system flight) represents an untapped energy source.

It’s been a while since I saw a new idea in this space.  Interstellar travel is a very hard engineering problem.  Hard enough that some scientists think it’s effectively impossible.  It’s nice to see someone make a possible dent in that problem.

59 thoughts on “The Q-Drive and the difficulty of interstellar exploration

  1. Ii’s certainly the biggest engineering problem of all time, but I refuse to believe it is impossible, at least for unmanned probes. Human ingenuity is a formidable force, and if a miniscule probe could be built, perhaps it would be relatively easy to fire it off at speeds that are a moderate fraction of c. Given time – say, hundreds of years – I’m confident a solution will be found and it will become routine 🙂

    Liked by 1 person

  2. Wait, which kinetic energy are they capturing? Is it, I hope, the motion of interstellar medium perpendicular to the ship’s travel? Because otherwise it sounds like a phony perpetual motion machine. I suppose I could watch the dang video…
    Also, if you carry oxygen instead of water, and burn that, do you get a significant boost from chemical energy?

    Liked by 1 person

    1. Greason mentions in the video that just about nobody, including elite engineers, gets it on their first reading. I actually tried to stick to the most fundamental components here, but my first reaction too was, how are we getting net motion? You’re right, the stored propellant is what makes the difference.

      The special sauce of this drive is the speed of the propellant coming out the nozzle. I’m not sure adding the chemical energy of oxygen would help much, and I’m not sure it could be stored as easily.

      Liked by 1 person

      1. Sure. Well, you could certainly freeze oxygen in space. And if you invest in a burner, you can power the field generators that way instead of siphoning off your interstellar-matter kinetic energy supply. Dunno if it would be worth the weight of the burner.

        Liked by 1 person

  3. To get a spacecraft to another star within a human lifetime requires accelerating it [a lot]

    Ye of little faith. A consideration you’re not taking into account is a change to the extent of a “human lifetime”. The singularity is nigh.


    Liked by 1 person

    1. Yeah, I’m skeptical it’s right around the corner, but even if we extend the human life span, there are serious challenges with keeping a machine working that long away from any external power sources, like a sun. The Voyager spacecraft use plutonium as a power source, but it’s slowly ebbing as the plutonium decays. There are other longer sources, but with less energy. In other words, there are reasons other than just human mortality to keep the mission to just a few decades.


  4. Fascinating as this is I can’t find for myself a good reason to send humans. What if we spent money on AI, miniaturization, and automation. Huge payoffs for humans on Earth and an ability to send potentially hundreds of probes instead of a few with humans.

    Liked by 1 person

    1. Just to be clear, all of this is just to send an automated probe. Miniaturization has its own issues. In truth, we’ll probably have to do combinations of all these things (propulsion innovation, AI, miniaturization, etc) as far as we can take them.

      Sending biological humans is a whole other level of difficulty.


      1. Understood that.

        In Breakthrough Starshot, would it be possible to use the outbound solar plasma in the heliosphere of the destination star to slow the probe down. Sort like opening a parachute. The probe might need to be a little larger, of course, but if it could be slowed enough it might be able to enter an orbit if it had some slight capability of maneuvering.


        1. That might well be it. Greason even briefly alludes to the possibility of using the magsails as a break. And it sounds like the magsails, the electromagnetic field, would be tremendous, hundreds of thousands of miles, so the size might already be sufficient, although I imagine they might have to resize it for the different medium.


  5. The idea is certainly impressive, but I can’t help but see it as science fiction at this point. (But very cool SF!)

    I made a decision a few years ago to start ignoring “futurists”. We can’t even predict the weather that far in advance with any precision; guessing about the future is a lot like speculative philosophy or speculative science. It’s all a kind of hard SF with no characters or plot.

    These days I can’t help but think how, just a couple months ago, my short term future was mostly about the baseball season and plans to explore local brew pubs. The quote about the “best laid plans of mice and men” was never more apt.

    The thing about space exploration is that it’s most likely to be machines, and then what? Some form of NASA (or private company) will wait hundreds of years for anything to come back? What’s the point? What exactly are our machines going to do out there?

    Liked by 1 person

    1. I don’t really think of someone like Greason as a futurist. He’s not so much trying to predict the future as trying to figure out a way to make it happen. In my mind, that puts him closer to someone like Walter Hohmann (or at least a Hohmann wannabee).

      I think if it takes centuries, then it’s not going to happen. At least not until we all have much longer life spans. Decades is a little easier to imagine given how long current planetary missions last. As to what they’d be doing, exploration. Hopefully by the time we send a mission like this, AI and robot dexterity will have progressed that the machines can do most of what a human could.


      1. (Just to be clear, I didn’t say Greason was a futurist. I just said I’m not generally inclined towards guessing about how the future is going to turn out. Historically, it’s been hit and miss.)

        “I think if it takes centuries, then it’s not going to happen.”

        Exactly. We’re talking about a serious investment of time and money. What putative ROI makes it worth that investment?

        “As to what they’d be doing, exploration.”

        To what end? How long does it take for results to come back? What makes us think anything we find in some other star system will have any value here?


        1. Why explore? I would think for the same reason we explore the solar system. Just to learn, perhaps to find life, and who knows what other benefits. As our probes spread throughout the galaxy, we would build an ever increasing database.


          1. Sure, but it’s a vast expenditure of time and money for “who knows what other benefits.” It raises the same issues that new super-colliders raise, but in this case both the spend and the time-spans involved are even greater.

            Liked by 1 person

    2. What exactly are our machines going to do out there?

      I vote for:

      1. Exploring. Gathering info and sending it back.
      2. Gathering resources (and maybe sending them back).
      3. Preparing infrastructure.
      4. Building, staffing hotels.

      [I hope to visit Mars, but only after the robots have built nice hotels.]


      1. I can see visiting Mars as a Curiosity, but the round trip to other stars poses something of a problem.

        Frankly, I’d rather visit the Moon and play in low-grav. It’s closer and safer, and I was promised a vacation home (or at least a Hilton) there back in the 1960s.


        1. I expect the trip to stars would be more migration than visiting. Still, I’d want the robots to build the infrastructure, homes with a view, etc. Was it the show The Expanse that had the Mormons planning on doing something like that? It’ll be other planets/moons first, but stars eventually.



          1. Citing a science fiction TV show just emphasizes my point: It’s a great dream — I’ve been an SF fan all my reading life — but I’m dubious about humanity actually pulling it off. (Especially humanity these days.)

            I’m very dubious about migration. Humans living in space for any length of time has all sorts of issues to be solved. First, if it’s anything, it’ll obviously be robots. We might send our biology out (frozen and protected) to seed new planets, but I really doubt actual humans will ever cross the stars.


  6. I don’t get it, Mike, so am wondering if you can explain it? My immediate thought is you can’t strap a wind turbine on a car and drive into the wind and come out ahead, and then that was one of the analogies for this system on the link you gave, so that wasn’t reassuring. At least at face value. So if you understand it sufficiently to explain it succinctly, I’d be interested to know.

    A problem with the wind turbine on a car approach, I think, is that the drag force is related to the wind energy captured. The more energy you extract, the harder the wind is pushing against the sail area of your wind turbine and vehicle, right? And unless you have equipment that operates with 100% efficiency, the energy extracted will never equal the energy associated with the developed thrust. At very best, in a frictionless and entropyless world, it will equal it.

    From your conversation above I gather the difference is made up by expelling mass. But I don’t immediately see how that helps anything, because unless I’m missing something–which I probably am–the energy available to accelerate the mass that is ejected will be inherently less than the energy harvested, due to entropy and such. And the mass still has to be accelerated. To accelerate the ship AND the mass at the same time would require some of the extracted energy be diverted to each. So if 100% of the energy from drag were converted to accelerated a mass of propellant relative to the ship, what is left over to accelerate the ship?

    I’ll say as well that the equations on Tolley’s post are using a notation that is hard for me to follow, and the prose is really hard for me to follow.

    He writes, “The drag force is dependent on the velocity of the wind or the ship moving through the wind which affects the mass flow of the medium. However, it is the change in velocity of the medium as it passes through the energy harvesting mechanism rather than the wind velocity itself that completes this equation. Compare that to the thrust from the propellant where the mass flow is dependent on the square of the exhaust velocity. When the velocity of the ship and the exhaust are equal, the ratio of the mass flows is dependent on the ratio of the change in velocity (delta V) of the medium and the exhaust velocity. The lower the delta V of the medium as the energy is extracted from it, the lower the mass flow of the propellant. As long as the delta V of the medium is greater than zero, as the delta V approaches zero, the mass of the stream of medium is greater than the mass flow of the propellant. Conversely, as the delta V approaches the velocity of the medium, i.e. slowing it to a dead stop relative to the ship, the closer the medium and exhaust mass flows become.”

    Take the first sentence and try to make a mathematical statement out of that… I can’t do it. Two sentences later he writes that the mass flow of propellant is dependent on the square of the exhaust velocity. It’s very hard for me to imagine a system where velocity is the independent parameter, and mass flow the dependent one. I mean, mathematically you might be able to say the right side of the equation drives the left just as well as the left side drives the right, but that’s kind of like saying the force of an accelerating bullet was caused by its acceleration. It just doesn’t work that way…? Practically you cannot control velocity without first controlling mass flow, by imparting energy to the propellant and establishing the nozzle geometry.

    Anyway, sorry for the long note, Mike! I need to read the other post again, but I’m not following this for the present.


    Liked by 2 people

    1. Michael,
      On the car analogy, the key thing there was the propellant. In other words, the car is not only being propelled by the oncoming wind. You’re right, at best that would just cancel out. The car is being propelled by propellant expelled out the back, with the expelling being powered by the wind. Greason in his talk also provides some sailing analogies that might help.

      “So if 100% of the energy from drag were converted to accelerated a mass of propellant relative to the ship, what is left over to accelerate the ship?”

      Remember Newton’s third law of motion here: any action results in an equal and opposite reaction. Accelerating the propellant in one direction pushes the ship in the other direction. (At least provided we expel the propellant out the back.)

      Entropy is indeed an issue here. A key question is whether the design can be made efficient enough for the Q-Drive to improve on a standard rocket, and whether that efficiency scales as the amount of energy involved climbs with high speeds.

      On the mathematics, sorry, can’t help you there. You might be better off going to the source: Greason’s paper.

      Overall, remember this. The ship takes in energy from the drag. Yes, it does lose some portion of it in the translation, but the rest is used to accelerate and expel the propellant, which accelerates the ship. The idea is that as the speed increases, the amount of energy coming in also increases, which increases the velocity of the propellant, which in turn increases the overall ship acceleration, which increases the speed, increasing the energy more, etc.

      Unless of course we’re all overlooking some crucial flaw?

      Liked by 2 people

      1. So, if I stand on roller skates and throw rocks into the distance, I’ll roll the other way. I get that. But eventually I’ll get tired right? Because in theory I’m using energy in my body to accelerate the rocks.

        But, to keep with the car example, if the only place the energy to the throw the rocks is coming from is energy I obtain from pushing against the medium around me, then I won’t accelerate. Imagine you are stationary on flat ground with a wind turbine that is connected by a vertical post or stanchion to a cart on which you are resting, and the wind blows. You won’t move into the wind.

        Now maybe the sailboat analogy is more apropos, but that is a different scenario entirely, because there is a force vector acting on the sail in the direction of the ship’s travel, and that travel is MOSTLY sideways compared to the direction of the wind. If the idea is that interstellar plasma is blowing like a wind, and the craft is like a sail, then the sailing analogy works. It will be be able to move “into the wind” with a small percentage of its velocity with some clever sail rigging. But it wouldn’t accelerate indefinitely. A sailboat wouldn’t anyway. And the sailboat has the same thing going for it as this drive purports to have, the faster it goes the harder the wind pushes on the sail, but this doesn’t mean it accelerates indefinitely.

        Back to the wind turbine, stanchion, cart and motor scenario, if the premise is that I can take energy from decelerating myself in a medium (equal and opposite to decelerating the wind), to accelerate mass out the back, to thereby generate an even greater acceleration into that same medium, then I don’t think the equal and opposite reaction helps you. While the rock I throw out the back (using my wind powered pitching machine) is exactly counteracted by the forward movement of my cart, it is also the case that the wind blowing into the turbine is exactly counteracted by the force of the turbine stanchion against the cart, and that pushes the cart backwards. So in a perfectly efficient world without entropy or friction, the force of the wind on the turbine acts through the cantilever of the stanchion to push my cart backwards. The wind turbine doesn’t just fly downwind because it is connected to the cart, which is pushing the stanchion back with equal and opposite force. But the cart is going “backwards” at this point. Why? If the forces are equal and opposite?

        The same reason the boat accelerates. You can follow the force on the sail down the mast, where there is an equal and opposite force exerted by the boat against the mast, but until the boat is moving with some speed that creates a drag force equal and opposite to the wind force captured by the sail, it will accelerate. Because the water does not push back against a still boat. The boat must move through the water to generate this reaction. But it can only accelerate until the wind force on the sail is matched by the drag. And then the conversation ends. It is now in equilibrium, and all forces are balanced, and there is no acceleration. For a given wind speed there is very definitely a maximum sustainable velocity of the boat, assuming drag remains a force proportional to the velocity difference between the craft and the medium.

        Add an electric generator coupled to an electric motor into this system and it doesn’t change anything, because the two electrodynamically just push against each other. But for the purposes of this example, assume a 100% efficient wind turbine generator and motor and superconductance in the circuits and all that. Imagine the motor is operating a pitching machine throwing baseballs off the back, or turning a propellor to throw water out the back, or pumping out “propellant.” A thrust force equal and opposite to the baseballs will arise and tend to move our little cart “forward,” but is it enough to overcome the force of the wind pushing on the turbine, which is pushing me “backward?” Physics as we know it requires that it is not. If it was, this would be a free energy machine. For the car analogy, I can’t see any way this works.

        The only way it works for the space craft is if there is some additional dynamic related to electricity and magnetism that changes the analogy somehow. I would be surprised if there was, but if there is, I don’t think it’s explained by the video or the blog post.


        Liked by 2 people

        1. I have to admit, after thinking through how to reply on this, I’m somewhat less confident that it would work. Or to be more careful, I can’t say I understand how it would work. It does seem like the kinetic energy of the expelled propellant can’t be more than the kinetic energy harvested from the ISM.

          Greaser assures his audience in the video that if they run the numbers, they’ll see the result. I wish I knew how to do that.

          It seems like this part from the Centauri Dreams post may be a key one:

          Counterintuitively, such a propulsion system can work in principle. By ejecting the reaction mass, the ship’s kinetic energy energy is maintained by a smaller mass, and therefore increases its velocity. There is no change in the ship’s kinetic energy, just an adjustment of the ship’s mass and velocity to keep the energy constant.

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          1. Mike, this one’s a great brain teaser. I’m still mulling it over. I may have to eat some crow here having done some math, which is fine because it’s been an enjoyable meal. I’ve tried to explain this simply. I think the presentations mislead people just a bit, or don’t get to the salient points very efficiently. As you’ll see…

            So I haven’t studied the equations from Greason, but I wrote my own using basic dynamics (Newton’s Laws type of stuff), and they suggest there could be something to this. Assuming we ignore all the irreversibilities and real world inefficiencies that will want to get involved, let’s say you have 1 kg traveling at 100 m/s into the wind. Using the equation for kinetic energy of 1/2 * mass * velocity ^2 you would say that relative to a static frame of reference the ship has a kinetic energy of 5,000 Joules.

            Now, just to throw a number out there, say you have a sail device of some sort that recovers 50 Joules from the wind. And this is important: let’s say that the ONLY drag on the craft is your energy harnessing sail. The drag on everything else is negligible by comparison. Now, because that sail device is being pushed on by the wind with the same force as the ship is pushing back, the ship loses 50 Joules of kinetic energy in the process. The craft slows down to 99.5 m/sec. You could think of this energy recovery device as basically a break, no different from a regenerative braking system on a car. The question now is whether or not I can use that 50 Joules of kinetic energy to expel some mass and create thrust that will accelerate the ship.

            Let’s make the assumption that I shoot a pellet out the back that weighs 0.05 kg, and it has a velocity of 2,000 m/sec, for a kinetic energy of 50 Joules. So I just spent exactly the energy I harvested from the wind to launch that pellet.

            What happens to the other 0.95 kg? Well if it picks up 50 Joules of energy as we would expect it to, then it would get back to its original energy budget of 5,000 Joules. Because it has less mass than when it started, the velocity would be higher than before. It would be 102.6 m/sec.

            So this type of simple analysis suggests there is some merit to the concept.

            But this gets REALLY interesting (depending on what you enjoy doing with your spare time)… Using these basic equations I found that if we discharge 5% of our mass we can achieve a 2.6% increase in velocity. If we discharge 50% of our mass, the velocity gain would be 41%. And if we discharge 90% of our mass, the velocity gain would be 215%. I also tried this math for a sail energy recovery equal to 0.1%, 1% and 10% of the vessel’s initial kinetic energy. Turns out the velocity change was INDEPENDENT of how much energy is recovered by the sail, and is ONLY dependent on the quantity of mass exhausted. This kind of makes sense: we take some energy out as drag and we put it back where we found it, so it should really have no effect. But we reduce mass so the same energy means a higher speed.

            Basically this is a big drag-laundering machine that makes the effect of drag go away by assuming all the drag can be recovered and reconverted into thrust. But it only works while you can expel mass, and if you expel too much mass it won’t help as much, or at all.

            Imagine two box cars on a train rail. Pull the pin between them and push off of the back one from where you’re standing on the front one. The force of your push is negligible, relative to the masses. So the trailing box car really doesn’t fall behind. You’ve basically just lost half of your kinetic energy by halving your mass. You’ll still accelerate a bit though. Just not very much. So the trick here is to use the energy recovered from your regenerative breaking to accelerate a mass to a speed that is at least equal to your craft’s velocity relative to the static reference frame in which one is doing all the energy accounting. Then you can pull the pin, and transfer the energy back to the craft, only with less mass it will be moving faster.

            When we look at the real world, I think it would probably be tough to overcome all the inefficiencies and make this work. But I won’t say it’s theoretically impossible having looked at this simplified model I built. This can also tolerate some inefficiencies, I might add… You don’t actually need to convert 100% of the drag into thrust to accelerate, you’ll just accelerate more slowly or use up your mass faster.

            Can you carry enough mass to accelerate to 20% of the speed of light? No idea… Seems like you might need A LOT of mass.


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          2. Thanks Michael! I wondered if that kinetic energy balance might have something to do with it.

            The drag for everything other than the energy collecting mechanism should in fact be negligible. The proposal calls for the electromagnetic field to be thousands of kilometers in size, so the drag dynamics of the spacecraft itself should be a minor factor.

            The velocity change being independent of the energy is interesting, although I would think more energy equals more velocity change over a particular time period.

            On giving up mass, the proposal describes a craft initially massing 50,000 kg but with 48,000 kg of it being stored propellant (in the form of the ice shield). So the overall craft would be losing 96% of its mass throughout the transit.

            Crucially, it’s dependent on some other mechanism to get it up to 3% or so of c. Greaser discuses various other proposed ways to do that.

            It’s worth noting that this isn’t only an interstellar mission proposal. Greaser’s framework might enable a probe to use a magsail in the inner system to accelerate on the solar wind, but switch to q-drive mode for decelerating in the outer solar system, supposedly reaching Neptune in a year instead of the 12 years it took Voyager 2.

            But efficiency is the big question. Can the energy transfer be made efficient enough for this to outperform one of the more efficient rocket methods?

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          3. One question: Michael suggests a 215% velocity gain from throwing away 90% of your mass, and if another source is required to accelerate the ship to 3% or so, does that mean the top speed is about 7% of c?

            Another question: If a ship is coasting at a certain velocity, does merely losing mass make it speed up? (Imagine the mass is shed in a way that does not directly affect velocity.)

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          4. Hi Mike,

            As the drag increases, the energy “thrown off the back” must increase with it. The velocity change remains only the same percentage as I calculated above, at least using my rudimentary energy balance. So if the drag goes from 50 Joules in my example to 500 Joules, it doesn’t change the velocity increase, but because it’s a fixed percent, if you’re already going faster, then the absolute change in velocity would be higher. Presumably you have to be going faster to have a higher drag, so that percentage is being applied to a larger base, and the absolute change as I said would then be higher as the base velocity increases.

            Wyrd, I can’t respond to you directly, so hope you read this, but I also asked myself if just losing mass would help. If there was no drag, then yes, like a figure skater pulling her arms in, the ship would accelerate simply by throwing mass off the back, presuming it had some energy source on board to do so. The problem here is that you need the energy source to do so… The trick here is that you get the energy from the drag itself, so you don’t need the mass AND the energy.

            Just throwing mass out the back is basically back to just being a rocket, I guess.

            Also, it’s not just losing mass that matters, but losing it with sufficient velocity such that the kinetic energy from losing it overcomes the drag the ship is experiencing to start with.


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          5. (Michael: FWIW, if you use the WordPress Reader it’s possible to answer comments directly (as I’m doing here). The Reader has its flaws, but that’s one nice feature. I’ve gotten to the point of using it exclusively for commenting on other WP blogs.)


          6. Wyrd, I responded too fast. Merely losing mass is like the analogy I gave above with two train cars traveling at the same speed. You pull the pin to unhitch the two cars, but you don’t “push” from one to the other. This has no effect on the speed of either car. Simply dumping mass without accelerating it does nothing for you.

            As to the max speed, I think it depends on how you do this. My example was not for a continuous process, but a stepwise process. It may be a little different when done as a continuous process. But I did find that if you lose 2% of the mass at a time, and for each of those steps you gain 1% velocity, the max top end is only a 60% increase. That’s 1.01^48. Better to just heave all the mass at once (97%) according to my simple calculation. That gets you a lot more bang for the buck. Instead of landing at 160% of original speed, you end up at 5.8 times the original speed.


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          7. Sorry for the multiple replies here, Mike, but I want to clean up a conceptual error in what I sent you last night. There would be roughly 48 steps if each step involved losing 2% of the original mass. But the ship only needs to discharge 2% of the remaining mass at each step. That would take 175 steps, and 1.01^175 = 5.7. (A 2% loss in mass is a 1% gain in velocity.)

            So, until I uncover my next amateur mistake, it seems good on the whole that discharging all the mass at once, or discharging it in 175 small increments both get to the same end result. There is something about that which makes sense, and it sort of confirms that the main variable at play here is the mass.


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          8. No worries Michael. I’m grateful for your efforts and the information!

            I’m still a bit confused about the velocity ratio you described above. Along the lines of Wyrd’s question, it does seem like it doesn’t get us to 20% c.


  7. I read the Trolley post and Michal’s comments, and I’m a little dubious, too.

    My thinking starts with a car where the front wheels turn an electric generator that feeds an electric motor that turns the rear wheels. Assuming a 100% efficient system and a push to set the car in motion this perpetual motion machine should work.

    Except entropy, so it doesn’t.

    I’m not sure throwing mass off the back changes the equation. The energy gained by dragging an anchor seems necessarily balanced by the energy spent moving forward, regardless of the mechanism. I’m not even sure an ISM “headwind” helps because you’d have to accelerate against the headwind.

    As Michael suggests, maybe there is more to the math that generates gain — I get the impression that’s the case, but I’d have to read it a lot more carefully to make sense of it. (And I’d rather let someone else make the effort. 😀 😀 )

    I do think it’s a system that could keep a ship in motion once started with the initial push. Even if it operates at a loss, if that loss was small enough, it might be a viable drive. The way it gains energy as it speeds up is attractive, but I’m not sure where that gain comes from in the first place.


    1. I’m with you on letting someone else make the effort. I guess I could bone up on the relevant equations and then run the numbers, as Greason urges everyone to do, but it would be nice for a more clear English explanation.

      Liked by 1 person

  8. The difficulties of interstellar exploration seem to be a minor issue in most Fermi hypothesis discussions. In the Fermi hypothesis, we blithely assume all of the difficulties can be overcome and either aliens or replicating von Neumann probes could overcome all of these difficulties and multiply themselves around the universe.

    So I guess one of these things are true:

    1- Interstellar exploration is effectively impossible
    2- The aliens are already here.
    3- We are the only intelligent life.


    1. Another possibility, the one I currently think is the most likely, is that intelligent aliens are very far away, millions or billions of light years. That operationally is the same as 3, in that we may never get a chance to encounter them.

      The only shot we might have to distinguish those scenarios is if any of those civilization rise to a Kardashev Type III and we can detect them across cosmological distances. There are people trying to do this:

      Liked by 1 person

      1. I’m inclined to think there are gaps in stellar systems that are difficult to cross. The right stuff doesn’t exist for the next hop to support/sustain (emphasis on “sustain”) life for a long enough time or to replicate a probe. So there might be some civilizations that have explored a little ways and run into barriers that stopped greater expansion.

        On the other hand, I’m not ruling out that they are already here.


      1. In the absence of any actual pertinent information, I wouldn’t place odds on any of them. Or, I would say they are equally possible since all we have is little more than feeling or intuition about them.


        1. I think there are logical reasons to conclude that intelligent life is probably rare. It only evolved once on Earth, and seemed contingent on many low probability events. Of course, we could always be an outlier, but I weight 3 highest, or the 3b scenario I mentioned.

          1 seems like a lower probability scenario to me. If all else fails, we can lob a series of nuclear bombs out the back of the craft to propel it forward. The magsail mechanism, if nothing else, might at least keep a craft’s onboard equipment powered for however many centuries it takes to get to its destination. And, over the life of a star as it orbits the galaxy, it also has numerous “near pass” events, where it passes within 10-20% of a light year of another star, which should enable easier hops if the typical distances are hopelessly insurmountable.

          2 can’t be ruled out, but it seems like it’s in brain-in-a-vat territory. Maybe we’re all test subjects being monitored by the mice until we produce the ultimate question, but if so, it seems hopelessly outside of our ability to test.


          1. We can reasonably suspect intelligent life has evolved once on Earth so far. I think Adam Frank and somebody had a paper that suggested that intelligent life could have evolved previously and vanished and we might not see any evidence of it. It also could evolve again after we’re extinct.

            If you think #1 is unlikely, then I would think you would think #2 to be more likely. The wildcard with #1 and #2 is whether there are forms of transportation we can’t envision with current science. We only learned to fly a little over a 100 years ago.


          2. The evolutionary biologists I’ve seen comment on the Silurian hypothesis find it too discordant with what is known about evolutionary history. And Frank and Schmidt only meant it as a thought experiment anyway.

            Of course, we can speculate scenarios that can’t be disproven, but that doesn’t make those scenarios probable.


          3. All scenarios are speculation at this point. You can pick and choose your favorite statistics and versions of “facts” and come up with any conclusion you want.

            If interstellar travel is probable and feasible, then even if technological (probably better term than intelligent but I mean the same) civilization is rare, the aliens would more likely be here. You have to add in the fact of billions of years and the billions of stellar systems. Given enough coin flips eventually heads will come up a hundred times in a row.

            Actually we know very little about the dangers and difficulties of interstellar space travel. We have barely been outside Earth orbit. It isn’t just a matter of propelling something but a matter of sustaining life during the voyage and arriving at a hospitable destination with enough resources and viable population to get civilization going again.

            I like to speculate too but, if really pressed, I would stick with the equal odds because of the lack of knowledge.


          4. Crewed interstellar travel is orders of magnitude more difficult than for machines. I’m actually far less confident humans will ever cross interstellar distances. If we do, I doubt it will be in our naturally evolved form.

            Liked by 1 person

          5. After posting an idea for SciFi novel came to me. Of course, it has probably already been done.

            Colonization group of humans crosses dozens of light years to habitable planet. (We can make it a lot like Earth so we don’t need a lot of CGI when it becomes a B movie.) The colony lands and sets up. Gradually the technology fails, the ones who understand the technology die (in an epidemic, of course) and the remainder of the group revert back to hunting and gathering losing written language. Their landing craft and all their technology is swept away a flood. After which, the group sets out to settle their new world in the final scene.

            Liked by 1 person

          6. An interesting thing about sci-fi is the vast majority of interesting ideas were explored in the pulps before WWII. Although execution matters. Many of those early stories are unreadable by today’s standards, and there’s always room for ideas to be re-imagined.

            Did you ever watch the old BBC movie for The Hitchhiker’s Guide to the Galaxy? Or Battlestar Galatica (the reboot)? They both have related scenarios.

            Liked by 1 person

        2. I dunno; I think there is “pertinent information” we can use to form an opinion. We have a sense of the difficulties and dangers of space travel. We can play with the Drake Equation to see what results. We can apply logic.

          Even basic statistics can inform our opinion. Given all the galaxies and all the stars in those galaxies, assuming intelligent life arises sometimes, what are the odds of it being in this galaxy? Given the visible universe, very very low. I’ve long thought, statistically speaking, it’s likely we’re the only intelligent species in this galaxy at this time.

          Which puts a lot of weight on your #1 and #3 options. I maintain that #2 is just silly.


  9. I think maybe I know what the big deal is here. There isn’t any real magic, it’s just a rocket ship with the nice properties of using almost anything for fuel and not needing to carry an energy supply.

    I started thinking about this from the other end. I’ve gotten to some destination having used this drive, and my velocity is, let’s say, 15% c. I was given a push start that got me to, let’s say, 3% c. So my question is where did the energy come from that boosted me by 12% of c? It had to come from somewhere.

    In a conventional rocket, the energy is chemically bound in fuel and released during flight. Some level of thrust during the flight adds velocity until the fuel is exhausted. Given a steady impulse, as fuel mass decreases, that impulse is more effective.

    Alternatively, one might use a stored power source or burn fuel in a generator. That allows for an electric motor that can throw almost any reaction mass backwards for thrust. That allows a variety of things for reaction mass — in particular things you might find along the way and not have to process much. (Unlike trying to make rocket fuel.)

    But one still has to haul along fuel for the generator or some other significant power source.

    This Q drive claims to recover power from friction and to use that power to drive the motor that throws water (or whatever) out the back. If we assume a very efficient system, I can see that it might have very few losses given efficient motors. If most of the energy lost is drag is converted to thrust, you’d be getting around having to carry that power source.

    You just need a push start.

    The velocity gain, if I’m getting this right, comes from the loss of mass along the way while being able to keep the impulse constant. Merely being able to maintain thrust will increase velocity, and the loss of reaction mass will make the engine more efficient.

    I think. 🙂


  10. Hi Mike! Philosopher Eric’s plug brought me here! What an eclectic site you have! Here I was thinking (from the discussions at Ed’s blog) that you were just a neurological expert. Seems like you dabble in quite a large number of topics, many of which I am super interested in as well!

    I’ve been keeping tabs on the musings of Eric Weinstein recently, trying to discern if he is going off the deep end or if he is just extremely brave and fed up given the current unstable environment we find ourselves in (covid 19 pandemic time, for those reading this years from now). I don’t know if you follow him in any regard or know of him. But as I said in my last comment reply to you on Ed’s blog, he just released his theory of Geometric Unification (his attempt to finish what Einstein and others haven’t been able to do) and part of his motivation for doing so is that he feels that we are not likely able to handle the powerful memes we’ve developed in recent history (our mastery over the atom and the gene) and as such we are not capable of stewarding this planet. He thinks that if we are to have any long term chance we need to diversify our presence in this universe (a “don’t put all our eggs in one basket” mentality). I’m not sure I completely follow that, in the sense that if we can’t handle it here, how could we handle it out there. But maybe his assumption is that, here we can’t escape our history (maybe a kind of “old-world” meme inertia) enough to overcome our problems, but that if we can have fresh starts trying out different memeplexes (as Ed calls them) in different locations, we might be able to settle on a memeplex (or few) that allow(s) us to survive long term.

    Anyway, I think his thought is that if we can understand the underlying structure of the universe beyond the general relativity that Einstein brought us to, maybe we can master space travel as well and spread out. I felt like this post on the Q-Drive was a great place to bring this up to you.

    Liked by 1 person

    1. Hi Eric,
      Welcome! I aspire to be a science fiction writer in my retirement, so my interests often range over what you might expect from someone doing background research. On many subjects, that interest is fairly shallow. But the mind is a fascination that’s gripped me for the last several years, so that topic gets visited here more than any other.

      I’ve heard of Weinstein (in relation to “dark web” and Peter Thiel), but I’m not familiar with his theory. The issue I see with diversifying our presence is, to really increase our chances of survival, it will require establishing independent biospheres. We don’t currently know how to do that. Attempts to do it in isolated conditions here on Earth have failed. And doing it on places like Mars is orders of magnitude more difficult.

      Until we solve that issue, any colonies we might set up would be crucially dependent on a supply line from Earth. So if we ruin that biosphere, even a humanity spread throughout the solar system would be vulnerable. Even if we do solve the biosphere issue, the knowledge we gain might be more productively applied to fixing our own environment. If maximizing survival is our goal, underground cities are cheaper and safer.

      The biggest issue is that space is really not an inviting place for biological life. There’s a reason most of our progress has been with machines. Space really belongs to them. If humans are to spread throughout the universe, it will require that we change ourselves, or cede the universe to our AI progeny.

      Of course, if someone discovers a new physics, all that could change, but I don’t see that we can bank on that. And you’re right. Perfect post to bring this up. Thanks!


      1. I’m excited about reading any of your books! Based on the topics that you’ve been posting, I can imagine some great plots that must be brewing in your (cortex? haha, just trying to get some neuroscience in there for fun). I’m an aspiring indi computer game designer, with my main goal being a 4X space strategy game. I working on it now and so don’t plan on waiting until retirement to do it…but based on my progress and how easily I get distracted, finishing it may not happen until then! 😦

        Weinstein is an interesting guy. He details his theory here (in a lecture he gave at Oxford about 7 years ago).

        I thought the number of vocab words I’ve had to look up when you guys talk about philosophy and neuroscience on Ed’s blog was high. This talk is like listening to a complete foreign language! Apparently not much came of the Oxford presentation (I guess it’s tough for an outsider to be taken seriously by experts), and so I’m glad that he decided to finally post it in public so that it may get the scrutiny he has so long desired. From what I can gather, he is trying to unite quantum and general relativity in a manner more closely related to how Einstein was trying to do it, geometrically speaking, instead of trying to force gravity into the quantum world. I am very far from understanding any of it though, so I’m not going to attempt to summarize it any further than that, and even doubt that my summary is in any way accurate. He also posts his podcast interviews on this same YouTube channel and these are extremely interesting and varied in topic and scope.

        Yeah, I’m not sure I understand how Weinstein visualizes the timeline of his thought to diversify by spreading out into space. I mean, if he feels that we as a species are not capable of handling the power we have obtained in our recent advancements in understanding the atom and the gene, then I’m guessing that he feels we had better do something about it very soon. Yet to expand out into space (outside the solar system) is not something that I see could happen anywhere near soon enough to save us from ourselves. Even if his theory pans out in that regard, devising the real world applications from it would seem to be a lengthy process in itself. If we can avoid screwing things up irreversibly long enough to hold out until we have the technological capabilities to expand, then it seems as though we probably would be able to make it in the long run here as well at that point.

        Speaking of Mars, I wonder why Elon hasn’t been doing biosphere development iterations here to prep for his plans on Mars. Or maybe he has been and I just haven’t heard about it. I wonder if the past failed attempts here on Earth so far are more because of how difficult a design problem it is, or if it was more because no group working on such a project was able to get long term sustained funding to iterate and improve on what was learned from the previous mistakes. It seems to me that creating an isolated biosphere isn’t a insurmountable challenge that we are incapable of overcoming.

        I agree that we should never give up on fixing our environment here and then maintaining it in a sustainable manner as well. Ed recently mentioned in one of our discussions that he sees this period as a global meme selection process where all these cultures new and old are interacting and competing, and as in all evolutionary processes, eventually the most suitable traits are selected out. I wonder how this process will play out and what affect the fittest memes will have on the planet.

        I also found it interesting what you said about underground cities being cheaper. I wonder what our economical memes will look like at a point in time where we can do such things. Star Trek showed us a culture where economics seemed to be non-existent. And it seems that this pandemic is going to cause enough of an economic upheaval that I doubt we’ll come out on the other side with the same model in place that we’ve been running on for the last half century or so. Maybe by the time we are capable of building such underground cities, “cheaper” might not hold the same meaning as it does today.

        I agree that space is a hostile place for biological life. I hold optimism that given Star Trek-like technological prowess we could emulate that and not need to cede the universe to our robots/AI. Given our power over the gene, maybe we will have to end up changing ourselves first. That’s a deep topic for another time, haha! But getting our tech from that Star Trek level of science fiction to reality is not a guarantee in any sense either. So I agree that we shouldn’t bank on it. On a related note, I had a fun daydream recently (I’m not sure what inspired it, maybe the movie Prometheus or Star Trek or Battlestar Gallactica, or some other book I read in the past….or if it wasn’t inspired at all and I just flat out forgot where I heard the idea before) where life on Earth was actually seeded here by a species on another planet that attempted to exactly recreate their own biology yet didn’t completely succeed. Things like cancer, mental illnesses, etc. might be like bugs in their design and we are less perfect models of the biology that makes them up. We, in turn, are on our way towards trying to do the same thing and will eventually try to seed another planet with an imperfect model of our own biology. And the process would continue down the line with each successive iteration having more bugs than the previous iteration.

        Anyway! I’m glad I found your site! I’m going to give it a more thorough browse in the near future. Hope all is well and stay safe!


        1. I actually spent most of my career as a programmer who started out as a teenager wanting to write a game. (We’re talking Atari 400 8-bit technology here.) I actually wrote a couple, but never got them polished enough to actually do anything with. I did learn a lot from the efforts though. So I totally understand the impulse. Best of luck!

          On biospheres, there have been a few well publicized attempts:
          I agree that if Musk really wants to colonize Mars, this is an area he should be investing in.

          A global meme selection process? That’s an excellent point. I think about all the languages going extinct. Many bemoan this process, but I actually think about the fact that we’re able to communicate with each other much better than we ever have.

          Not sure what you’ll think about this, but your daydream could be considered an updated version of Plato’s theory of forms, the idea that there’s an idealized version of us and everything else, that we’re only an imperfect instantiation of. I tend to think we’re pattern matchers who create the idealized forms from those learned patterns, but I can understand the viewpoint from the other direction.

          Thanks Eric! Looking forward to our future conversations!


  11. Mike, your blog is a wonderfully inspiriting resource for a lot of people. You seem to be a genuinely up-beat person. One might search the internet for days on end without finding a blog as congenial to its Comments section as yours. Anyway, just wanted to remark that

    Liked by 1 person

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