A billion has been a thousand million in UK usage since the 1970s, with the Treasury adopting that definition in 1974. Not that too many non-scientists regularly needed to refer to numbers that large, of course.
Gamma radiation does more than heat the ship, it kills the occupants. You can’t get rid of that problem by using radiators. To reduce the threat from the gamma rays, the crew compartment needs to be a long way from the reaction, or the design must include a massive shield - this makes the spacecraft much heavier, also reducing the acceleration. Annihilation also produces lots of neutrinos, which are useless at creating thrust.
All in all, there doesn’t seem to be a practical and safe way to use antimatter efficiently. If you want to use it inefficiently, that’s fine; but bear in mind how expensive it is, and that you won’t get much more bang for your buck compared to fusion when all the other factors are accounted for.
Folgers crystals can be used in a pinch.
It is even worse than that. High energy gamma radiation is the exclusive yield of electon–positron reactionswith the momentum of the products is in random directions. There is no way of plausibly contain or direct gamma rays other than to have them be absorbed into some other thermal medium, and then you’re back to the limitations of a thermal rocket, albeit a very high temperature one. As noted, the yield of proton–antiproton reactions are neutral and charged pions; the neutral pions decay immediately into gamma photons and the charged pions decay to neutrinos (which as noted above are weakly interacting and provide no thrust) and leptons (electrons and muons, which themselves decay in short order), which could provide thrust via electromagnetic interactions but only at a small fraction of the total yield, notwithstanding all of the mass required for shielding against highly energetic reactions. For the energy and level of technology required there are any number of thermal rocket concepts that are just as efficient, albeit still marginal at best for achieving even tiny fractions of c.
It may be possible someday to produce antimatter more efficiently than we can now with partical accelerators or by impinging very hogh energy lasers on heavy nuclei to produce tiny numbers of antiparticles at ludicrously poor efficiencies, but using antimatter reactions to directly propel a spacecraft is not remotely plausible by any proposed concept.
Stranger
Stranger, roll back one. You’re saying that as the pions come off the antimatter reaction, it isn’t possible to redirect the fraction of them that are charged out the back? If it’s only 10% of them that can be redirected this way, that’s still an enormously efficient engine that would allow the ship to reach a fraction of the speed of light (proportional to mass fraction, of course)
Even 5% C dV for braking (you might use another method to accelerate) is enough to reach the stars if you’re moderately patient.
Where are you getting “thermal rocket efficiencies”. I do not see your basis for this. I am proposing only throwing away antiprotons + protons, the high energy gamma rays mostly either pass through your ship and are not absorbed, fly away into space, or are absorbed by the mass of the ship and the resulting heat is pumped into space through radiators that do not lose mass. (so they are sealed or they are tiny solid beads that fly free for a kilometer and are caught by a later part of the ship or something)
I am proposing making the antimatter by spontaneous pair production. I have read that free electron lasers are pretty efficient as the beam is being accelerated by superconducting magnets, and I’ve read that if the density of light in space reaches a critical threshold, you get spontaneous pairs of anti-particles. I’ve read of proposals that claim pretty high efficiency from this, but even 1% is plenty to fuel a starship with the hundreds of tons of antimatter you’d need.
But I’m not rocket scientist, I’m just reading about what sound like straightforward, plausible pieces. Aneutronic fusion, by contrast isn’t even proven to be net energy positive. (the method proposed for a fusion rocket). And you don’t get more than a tiny fraction of the mass energy when you do fusion.
Obviously the crew of a starship would not be biological beings resembling earth biology, that would be moronic, and I’m sure you are not proposing that. Only fully self repairing creatures with no lifespan limit and armoring against damage from all the gamma rays would be able to make a trip like this. (and a brain that is massively redundant, of course, so information is not lost as the circuits get disrupted)
I’m going to defer to your expertise but I’d like you to please elaborate further on why you think this concept isn’t feasible. Does spontaneous pair production not work that way? You don’t get that many charged pions or they lose their charge before you can gain momentum from them? I don’t know the answer and would like to learn.
It seems to me that the most plausible way to accelerate to a significant fraction of light speed is to go smaller. When we start talking about exotic antimatter propulsion and other technologies we don’t have the foggiest notion how to implement, it seems far more likely to me that our first interstellar ships will be tiny nano-probes. We are much closer to being able to build self-assembling nanobots than we are to building antimatter space drives.
I’m envisioning a tiny probe weighing a few hundred grams. The probe consists of nano assemblers and a simple program that causes them to seek out mass, attach themselves to it, and then begin to assemble a solar array to power a transmitter that can beam signals home. Eventually those assemblers might construct just about anything we need to have at that end.
The idea is that shipping mass is extremely hard to do over interstellar distances, so we want to limit the mass and send information - programs for manipulating mass at the other end of the trip.
These things may be extremely high risk on an individual basis, but we could mass produce them, and launch them using lasers by the thousands - or the millions. We would be like a dandelion blowing seeds into space with a laser. Any given one is sure to be lost, but you only need one to work.
Something with a mass that small will also be easier to brake at the other end. Perhaps when it gets near the star it extends many whiskers and creates a sort of web to interact with the star’s solar wind to slow it down. Or the star’s magnetic field, or gravity assist or something else we’ll have to figure out. But decelerating something that weighs a few grams is a whole lot easier than decelerating something that weighs tons.
So we get the thing there, it successfully attaches to some suitable mass in that system, and the assemblers swarm out and assemble a solar array and a transmission dish and our probe starts talking to us. Now we can send new firmware directing it to assemble whatever we need.
It’s far more likely, IMO, that we will figure out how to make nano-assemblers long before we figure out how to send massive probes to other star systems in a timely manner, so it seems likely to me that our first interstellar probes will be very small.
Sam, how are you going to accelerate your tiny spec? How does it brake at the other end? It’s not going to do anything for you if you burn through the target solar system at 1% or more of the speed of light.
Your only feasible option is the probe needs to carry the fuel and an engine able to shed at least 1% of the speed of light (any slower and why not spend the time to construct a faster ship, also competitors will beat you to the target star) once it arrives.
The reason big ‘probes’ make sense is that there are nonlinear scaling laws. Fusion engines, like what Stranger on a Train is proposing (I think he’s basically talking about this) get drastically more powerful and efficient if they are larger.
The reason is simply that you pay in containment field energy for the *surface *of the plasma volume where the fusion reaction is happening. So the bigger the plasma volume, the less surface area : enclosed volume and the greater the fusion gain you get. Obviously if you don’t get enough fusion gain you won’t even get enough energy to keep powering the various superconducting magnets and other active systems that your fusion engine uses to run.
Antimatter engines actually may work at smaller sizes, actually. Maybe. The particles coming off the reaction are near the speed of light, though, and you need an intense enough magnetic field to bend the charged ones to a direction that gives you thrust. And I think there’s similar scaling laws at work there.
Anyways, we’re talking about the engine/fuel of a starship here. I agree that the *payload *would basically be some sort of nanomachinery, as light as you can make it. Obviously.
Once you finish decelerating, the ship has shed almost all it’s mass (it probably can deconstruct empty fuel tanks and use the matter as propellant, etc, and near the very end of the journey it probably rebuilds the engine to be smaller, albeit less efficient, so it can use the excess mass as additional propellant) and is basically a minimum self replication unit of nano-machinery like you say.
It finds an appropriate small asteroid or other orbiting body with the right element composition, matches velocity and lands on it, and of course starts harvesting to exponentially rebuild the solar system full of infrastructure used to build the starship in the first place. I mean, obviously. This is the end-game of life, at least unless there are unknown physical principles out there that change the rules. The whole universe full of claimed solar systems by whoever was there first, all full of swarms of some kind of robotic system optimized to the limits allowed by physics, run by bricks of intelligent matter also similarly optimized.
Both seem to be on more or less equal footing to me.
We actually have “self-assembling nanobots” today; they’re called bacteria or archaea. Of course, they don’t exist in space, aren’t robust enough to withstand decades or centuries of bombardment by high energy radiation, and can’t assemble useful machines, but there is at least a basis to develop a workable future technology.
There is no reason expectation that we can produce or store antimatter in quantity with any practicable technology nor use it for propulsion due to the fundamental limitations described above.
Stranger
What limitations! I have asked you politely 3 or more times for what they are. As I understand it, you “merely” need to get enough photons into a small enough space and you’ll start getting matter and antimatter. The reverse reaction of E=MC^2. Obviously it’s theoretically capped at 50% efficiency and then the very focused, high quality laser beams you use are very low entropy, so they can’t be particularly efficient. But still, at this stage you’re building PV arrays in space with more surface area than the earth, you’ve got energy to burn.
What is the basis for your belief that this is not a feasible thing to do?
Once you have anti-hydrogen, obviously you need to get it to heavier anti-versions of the elements. Which has not been done experimentally, it’s possible that they are not stable but it’s a reasonable guess to assume that they are. Heavier bricks of anti-elements are going to have much higher melting points and densities, armoring them against cosmic rays that leak into your fuel tanks. Also would be easier to contain if you pick an element that is much more sensitive to magnetic fields, such as going to a Type-1 superconductor or even just fusing all the way to anti-iron.
And you need to do this with apparatus that never comes into physical contact, that part I think is reasonably straightforward. Just lots of lasers to do the manipulation, basically.
You accelerate them with big-ass lasers. That’s what Breakthrough Starshot is proposing.
As for deceleration - I don’t know. Like I said, maybe you could do it by having your probe assemble into a web or a series of long filaments that interact with the solar wind, or something else. I haven’t done any of the math on any of this so I don’t know if it’s feasible - it just seems like a more tractable problem to figure out a way to slow down a few grams of mass than a huge spaceship.
Or, you send thousands or millions of the probes out, and like Breakthrough Starshot they don’'t slow down, and they don’t have miraculous assemblers. We just figure out a way to allow very tiny bots to communicate across interstellar distances. Then they can just fly through the target system and keep on going - but if we send them by the thousands or millions and they arrive one after the other, we can make it look like one probe loitering the system for as long as we want - as one leaves, another arrives to take its place. maybe if we have a long chain of them between us and the target star, they can act as a communications relay. Who knows? We certainly can’t do any of this today - but if I had to bet on where our improvements in capability will come the fastest, I’d bet on going smaller rather than bigger. As Feynman said, there’s lots of room at the bottom.
There also scaling laws that go the other way. Which is why we had to go smaller to improve computer performance.
I am happy to stipulate that nano probes will not be fusion powered.
Perhaps. One answer to the rocket equation is always to build your payload as small as possible. But I question whether we can send a probe to another star system based on accelerating it with its own onboard fuel source. It seems iffy. Perhaps a hybrid system that is launched with laser power or some other method, and only uses onboard fuel to slow down. That doesn’t eliminate the issue, but it does drop your fuel requirement by an order of magnitude.
I hate to use the word ‘obvious’ when it comes to predicting anything about the future. We are on a random walk into an unknown future, and none of us have a clue what’s actually going to happen. Chances are, by the time we’re ready to explore other star systems things will have changed so much that conversations like this will be looked at as humorously as we look at people who thought maybe we could send people to the moon by shooting them there in giant artillery shells. Of course, people who thought that lived at a time when accurate artillery was just becoming a thing, so of course that would be the way to go…
Yes, but when people talk about self-assembling nanobots being used in space, they mean something that can land on an arbitrary rock, locate all of the elements necessary for the bot’s construction on that rock, mine those elements from solid rock, refine those elements and shape them into the parts necessary to build new copies of themselves, and then when enough copies are present expand that mining, refining, and assembling to a scale to build space colonies. Show me bacteria that you can sprinkle on a raw piece of dry carbonaceous chondrite in a vacuum that will proceed to turn it into a mass of bacteria and then I’ll begin to think that we are any closer to nanotech than we are to antimatter fuel.
Unknown physical principles. I covered that. I’m simply extrapolating based on the known ones. Also, optimized bricks of matter are the outcome of every possible future where life continues to exist and life continues to be able to intelligently improve itself. (that is, some disaster doesn’t happen that removes sentient life, kicking life back to using mere evolution as a design mechanism, which is not able to overcome local maxima)
Stranger is trying to say that the physical principles 100% mean that those bacteria, etc can work. We are completely certain they work. Some of the elements of my antimatter proposal, like making matter/antimatter from boiling the vacuum or making anti-elements so the stuff is stable enough to pack into a starship, etc, may hit hidden physics showstoppers that prevent them from working at all in that way.
Now you can try to say that bacteria need water, and thus may not work in a vacuum. That isn’t a showstopper - if you had to, you’d use a hybrid approach. You’d have an outer “container”/spaceship that carries the nanomachinery/bacteria. It would use tanks of water, etc. It would have digester systems that gather up bits of rock and sift out the elements needed and prepare them into a form the ‘nano-bacteria’ can use.
And then of course the ‘nano-bacteria’ can both self replicate and also make additional components for the outer “container-spaceship” that carried them.
Animal bodies do this very thing. They operate in an environment hostile to current life, the land. Using containers. They can digest large objects that bacteria cannot. Etc.
Though of course this is just to cover the “if that was the only way forwards” case. In reality, there is solid evidence that says chemistry works just fine in a vacuum and that nanorobotic systems are going to work in a vacuum.
Oh, there are no natural bacteria or archaea which can do this, and the degree of modification or novel biology to create some kind of self-organizing cooperative bacteria that could survive such a trip and do useful things is the realm of science fiction today. But we are in the infancy of synthetic biology and are already discovering just how flexible organic systems can be, and how surprisingly robust many natural lifeforms are in environments previously thought to be uninhabitable. It is not beyond conception to develop a kind of interstellar living probe (i.e. Freeman Dyson’s “Astrochicken”) that is capable of producing tailored nanoscale organic machines, e.g. bacteria-like workers which can reproduce and collect and process natural materials in situ to build more probe and construct larger purpose-specific devices for energy collection or exploration.
And rather than trying to break down “a raw piece of dry carbonaceous chondrite in a vacuum” the probe would likely seek out planets and moons with rich hydrocarbon deposits or atmospheres, or collect amino acids on water-rich comets or interstellar clouds. Again, this is not anything existing Earth lifeforms can or will be easily made to do, but is within the realm of plausibility given future advances in biological engineering, and doesn’t suffer any limitations of basic physics.
Stranger
And they don’t have to be universal assemblers that can transform anything into anything. They could be very purposefully designed. For example, one might simply land on a rock of a similar type that is common here, or on an icy body, and do something very specific like swarm over the ice and shape it into a large dish, then spin some fibers over it and pull the transmitter into the focus of the dish for communicating back to earth. The same dish could be used to focus light from the star onto solar collectors on the probe, which then store the power in a capacitor for use in ‘burst’ communications with Earth. Or something else hand-wavy.
The same dish might also be usable as a mirror, with the small probe containing the optical sensors. Then we can have a large reflecting telescope in orbit around another star system with a communications system capable of sending data home. All this could be tested in our own solar system until it’s foolproof.
Small probes have other advantages. For example Breakthrough Starshot’s proposal is to power the probe with a small amount of plutonium, and ‘thrusters’ can be as simple as a 1W laser diode. With unlimited power for the diodes and some navigation/sensing capability, these things could precisely control their entry into another star system. The first ones there could send data back as they find objects, and that data could be used to target those objects by sending direction change commands to the probes still on their way.
Again, even if we can’t solve the deceleration problem and each probe blasts through the system in an hour and never slows down, if you have a chain of thousands of them and they are arriving every hour, and we can precisely navigate them, we can make a ‘synthetic’ probe that looks like it’s staying in the system for thousands of hours. And each one can learn from the last one and reposition itself to go through the system at a different place to capture all the date we want.
And, if we can make these things en masse and energy is cheap enough, we are not limited to one star system. We could build a launch system that targets every star system within 50 light years and blasts 10,000 little nano probes at each one. Some get to their destination in a decade or two, some might take hundreds of years. But eventually we will get a fairly steady stream of data coming back from hundreds of star systems. In addition, a technique like this would allow us to learn a lot about the interstellar medium, whether it varies in different places, etc. It also maximizes our chance of a serendipitous discovery, and would be fault tolerant in that accidents or collisions won’t wipe out the entire effort. Interstellar space is not empty, and at the speeds we are talking about even hitting a grain of sand would be devastating. It’s estimated that a trip to Proxima Centauri would involve the collision of at least a thousand particles 0.1 μm in size or bigger per square centimeter of ship frontal area. So it seems to me that when you start talking about large spaceships, the odds of hitting something big enough to destroy you would be a serious risk. But with a swarm proposal, that risk is seriously diminished, as is the risk of loss of mission due to equipment failure.
And nano-assembly isn’t as far away as you think. In fact, it’s here now. How do you think CRISPR works? It basically takes control of the molecular machines our body uses to repair and copy DNA, and reprograms them to cut the DNA and splice it wherever we want - right down to an individual gene pair. We can even program it to insert a new DNA sequence of our choosing That’s some pretty serious nano engineering, and you can do it in a lab today for a few hundred bucks.
This is bad and only useful if you cannot figure out a way to decelerate. Learning about the nearby stars is cool, but if you can go there, whoever does it gets more real estate. They get a concrete return on their investment. Since entities able to travel the stars basically must be fully self repairing and thus unharmed from age, they will *personally *be able to enjoy whatever new star system they reach.
What would you rather do? Ride through a cake shop at 200 mph and get a faint whiff of the treats as you drive through or actually stop and eat some?
I’d rather do something that might actually be feasible within our lifetimes. And there is no way that we are going to be in a position to profit from extra-solar real estate. And then not until we ea,ed value out of every square meter of it in our own solar system.
I don’t think any of this is going to happen within our lifetimes unless
(a) the much publicized “singularity” pans out per the more hype filled models.
(b) we find some way of extending our lifetimes, either because (a) improved medical technology by the equivalent of thousands of years of progress in a single year or we get ourselves preserved so that a digital copy of us gets to see this happens.
Like you say, our own solar system has unfathomable (to our current resource utilization) depths of additional resources available. The Moon, which is literally right there, has as much solid matter as the entire earth’s crust and then some. Every element should be there in significant quantities, solar energy is abundant, and there is no biosphere to worry about polluting with dirty industry.
Maybe once we have expanded to the level of devouring the entire Moon we can start thinking about starships.