I would not fret too much about slight overheating of the payload; most of the launch mass is propulsion fuel anyway, and in worst-case the payload can rendezvous with the fuel in-flight, after the fuel has cooled down.
I would be very afraid of the launch mass, including solar sail / reflector loosing (1) reflectivity (you need a very good mirror that continues to be a good mirror when hot; imperfections will heat it) and (2) structural integrity.
I would guess that, even assuming technological maturity (can do anything that physics permits), you cannot keep structural integrity above, say, 3000K, for a launch mass that is mostly hydrogen. I think that this is still icy cold, compared to the power output you want.
So someone would need to come up with either
1. amazing schemes for radiative heat-dissipation and heat pumping (cannot use evaporative cooling, would cost mass),
2. something weird like a plasma mirror (very hot plasma contained by magnetic fields; this would be hit by the laser, which pushes it via radiation pressure and heats it; momentum is transferred from plasma to launch probe via magnetic field; must not loose too many particles, and might need to maintain a temperature gradient so that most radiation is emitted away from the probe; not sure whether you can use dynamo flow to extract energy from the plasma in order to run heat pumps, because the plasma will radiate a lot of energy in direction of the probe),
3. show that limiting the power so that the sail has relatively low equilibrium temperature allows for enough transmission of momentum.
No 3 would be the simplest and most convincing answer.
I am not sure whether a plasma mirror is even thermo-dynamically possible. I am not sure whether sufficient heat-pumps plus radiators are “speculative engineering”-possible, if you have a contraption where your laser pushes against a shiny surface (necessitating very good focus of the laser). If you have a large solar sail (high surface, low mass) connected by tethers, then you probably cannot use active cooling on the sail; therefore there is limited room for fancy future-tech engineering, and we should be able to compute some limits now.
---------------------
Since I already started raising objections to your paper, I’ll raise a second point: You compute the required launch mass from rocket-equation times final payload, with the final payload having very low weight. This assumes that you can actually build such a tiny rocket! While I am willing to suspend disbelieve and assume that a super efficient fusion-powered rocket of 500 tons might be built, I am more skeptical if your rocket, including fusion reactor but excluding fuel, is limited to 30 gram of weight.
Or did I miss something? While this would affect your argument, my heart is not really in it: Braking against the interstellar medium appears, to me, to circumvent a lot of problems.
---------------------
Because I forgot and you know your paper better than me: Do any implicit or explicit assumptions break if we lose access to most of the fuel mass for shielding during the long voyage?
If you could answer with a confident “no, our assumptions do not beak when cannot use the deceleration fuel as shielding”, then we can really trade-off acceleration delta-v against deceleration delta-v, and I stay much more convinced about your greater point about the Fermi paradox.
Thanks for these critiques! They are useful to hear and think about.
> I think that this is still icy cold, compared to the power output you want.
I think it’s not so much the power, but the range of the laser. If the target is large enough that a laser can hit it over distance of light years, for example, then we can get away with mild radiation pressure for a long time (eg a few years). But I haven’t run the numbers yet.
>I am more skeptical if your rocket, including fusion reactor but excluding fuel, is limited to 30 gram of weight.
I was imagining a sort of staged rocket, where you ejected the casing of the previous rockets as you slow, so that the mass of the rocket was always a small fraction of the mass of the fuel.
But Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the “ship”, then this doesn’t obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles—though the (tiny) acceleration of the laser/ship is). I’ll have to crunch the number on it.
>Do any implicit or explicit assumptions break if we lose access to most of the fuel mass for shielding during the long voyage?
We didn’t do the shielding very well, just arbitrarily assumed that impacts less energetic than a grenade could be repaired/ignored, and that anything larger would destroy the probe entirely.
As usual, Eric Drexler had a lot of fun shielding ideas (eg large masses ahead of the probe to inonise incoming matter and permanent electromagnetic fields to deflect them), but these were too “speculative” to include in our “conservative” paper.
>I was imagining a sort of staged rocket, where you ejected the casing of the previous rockets as you slow, so that the mass of the rocket was always a small fraction of the mass of the fuel.
Of course, but your very last stage is still a rocket with a reactor. And if you cannot build a rocket with 30g motor+reactor weight, then you cannot go to such small stages and your final mass on arrival includes the smallest efficient rocket motor / reactor you can build, zero fuel, and a velocity that is below escape velocity of your target solar system (once you are below escape velocity I’ll grant you maneuvers with zero mass cost, using solar sails; regardless, tiny solar-powered ion-drives appear reasonable, but generate not enough thrust to slow down from relativistic to below-escape in the time-frame before you have passed though your target system).
>But Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the “ship”, then this doesn’t obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles—though the (tiny) acceleration of the laser/ship is). I’ll have to crunch the number on it.
That does solve the “cannot build a small motor” argument, potentially at the cost of some inefficiency.
It still obeys the rocket equation. The rocket equation is like the 2nd law of thermodynamics: It is not something you can trick by clever calculations. It applies for all propulsion systems that work in a vacuum.
You can only evade the rocket equation by finding (non-vacuum) stuff to push against; whether it be the air in the atmosphere (airplane or ramjet is more efficient than rocket!), the solar system (gigantic launch contraption), various planets (gravitational slingshot), the cosmic microwave background, the solar wind, or the interstellar medium. Once you have found something, you have three choices: Either you want to increase relative velocity and expend energy (airplane, ramjet), or you want to decrease relative velocities (air-braking, use of drag/friction, solar sails when trying to go with the solar wind, braking against the interstellar medium, etc), or you want an elastic collision, e.g. keep absolute relative velocity the same but reverse direction (gravitational slingshot).
Slingshots are cool because you extract energy from the fact that the planets have different velocities: Having multiple planets is not thermodynamic ground state, so you steal from the potential energy / negative entropy left over from the formation of the solar system. Alas, slingshots can’t bring you too much above escape velocity, nor slow you down to below escape if you are significantly faster.
Edit: probably stupid idea, wasn’t thinking straight <strike> Someone should tell me whether you can reach relativistic speeds by slingshotting in a binary or trinary of black holes. That would be quite elegant (unbounded escape velocity, yay! But you have time dilation when close to the horizon, so unclear whether this takes too long from the viewpoint of outside observers; also, too large shear will pull you apart).</strike>
edit2: You can afaik also push against a curved background space-time, if you have one. Gravity waves technically count as vacuum, but not for the purpose of the rocket equation. Doesn’t help, though, because space-time is pretty flat out there, not just Ricci-flat (=vacuum).
That’s what I used to believe. But now, on closer analysis, it seems that it doesn’t. The rocket equation holds when you are continuously ejecting a thin stream of mass; it doesn’t hold when you are ejecting a large amount of mass all at once, or transferring energy to a large amount of mass.
The thought experiment that convinced me of this: assume you have a gun with two barrels; you start at rest, and use the gun to propel yourself (ignore issues of torque and tumble). If you shoot both barrels at once, that’s two bullets, each of mass m, and each of velocity v. But now assume that you shoot one bullet, then the other. The first is of mass m and velocity v, as before. But now the gun is moving at some velocity v’. The second bullet will have mass m, but will be shot with velocity v-v’. Thus the momentum of the two bullets is lower in the second case; thus the forward momentum of the gun is also lower in that case.
(The more bullets you shoot, and the smaller they are, the more the gun equations start to resemble the rocket equation).
But when you eject the payload and blast it with a laser beam, you’re essentially just doing one shot (though one extended over a long time, so that the payload doesn’t have huge acceleration). It’s not *exactly* the same as a one shot, because the laser itself will accelerate a bit, because of the beam. But it you assume that, say, the laser is a 100 times more massive than the payload, then the gain in velocity of the laser will be insignificant compared with the deceleration of the payload—it’s essentially a single shot, extended over a period of time. And a laser/payload ratio of 100 is way below what the rocket equation would imply.
If you disbelieve in 30g fusion reactors and set a minimum viable weight of 500t for an efficient propulsion system (plus negligible weight for replicators) then you get an additional factor of 1e7.
Combining both for fusion at 0.8c would give you a factor of 5e12, which is significantly larger than the factor between “single solar system” and “entire galaxy”. These are totally pessimistic assumptions, though: Deceleration probably can be done cheaper, and with lower minimal mass for efficient propulsion systems. And you almost surely can cut off quite a bit of rocket-delta-v on acceleration (Stuart assumed you can cut 100% on acceleration and 0% on deceleration; the above numbers assumed you can cut 0% on acceleration and 0% on deceleration).
Also, as Stuart noted, you don’t need to aim at every reachable galaxy, you can aim at every cluster and spread from there.
So, I’m not arguing with Stuart’s greater claim (which is a really nice point!), I’m just arguing about local validity of his arguments and assumptions.
Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the “ship”, then this doesn’t obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles—though the (tiny) acceleration of the laser/ship is). I’ll have to crunch the number on it.
(if the probe is very robust, we might be able to railgun it instead of using a laser—and railgunning a single mass, once is clearly not subject to the rocket equation).
I would not fret too much about slight overheating of the payload; most of the launch mass is propulsion fuel anyway, and in worst-case the payload can rendezvous with the fuel in-flight, after the fuel has cooled down.
I would be very afraid of the launch mass, including solar sail / reflector loosing (1) reflectivity (you need a very good mirror that continues to be a good mirror when hot; imperfections will heat it) and (2) structural integrity.
I would guess that, even assuming technological maturity (can do anything that physics permits), you cannot keep structural integrity above, say, 3000K, for a launch mass that is mostly hydrogen. I think that this is still icy cold, compared to the power output you want.
So someone would need to come up with either
1. amazing schemes for radiative heat-dissipation and heat pumping (cannot use evaporative cooling, would cost mass),
2. something weird like a plasma mirror (very hot plasma contained by magnetic fields; this would be hit by the laser, which pushes it via radiation pressure and heats it; momentum is transferred from plasma to launch probe via magnetic field; must not loose too many particles, and might need to maintain a temperature gradient so that most radiation is emitted away from the probe; not sure whether you can use dynamo flow to extract energy from the plasma in order to run heat pumps, because the plasma will radiate a lot of energy in direction of the probe),
3. show that limiting the power so that the sail has relatively low equilibrium temperature allows for enough transmission of momentum.
No 3 would be the simplest and most convincing answer.
I am not sure whether a plasma mirror is even thermo-dynamically possible. I am not sure whether sufficient heat-pumps plus radiators are “speculative engineering”-possible, if you have a contraption where your laser pushes against a shiny surface (necessitating very good focus of the laser). If you have a large solar sail (high surface, low mass) connected by tethers, then you probably cannot use active cooling on the sail; therefore there is limited room for fancy future-tech engineering, and we should be able to compute some limits now.
---------------------
Since I already started raising objections to your paper, I’ll raise a second point: You compute the required launch mass from rocket-equation times final payload, with the final payload having very low weight. This assumes that you can actually build such a tiny rocket! While I am willing to suspend disbelieve and assume that a super efficient fusion-powered rocket of 500 tons might be built, I am more skeptical if your rocket, including fusion reactor but excluding fuel, is limited to 30 gram of weight.
Or did I miss something? While this would affect your argument, my heart is not really in it: Braking against the interstellar medium appears, to me, to circumvent a lot of problems.
---------------------
Because I forgot and you know your paper better than me: Do any implicit or explicit assumptions break if we lose access to most of the fuel mass for shielding during the long voyage?
If you could answer with a confident “no, our assumptions do not beak when cannot use the deceleration fuel as shielding”, then we can really trade-off acceleration delta-v against deceleration delta-v, and I stay much more convinced about your greater point about the Fermi paradox.
Thanks for these critiques! They are useful to hear and think about.
> I think that this is still icy cold, compared to the power output you want.
I think it’s not so much the power, but the range of the laser. If the target is large enough that a laser can hit it over distance of light years, for example, then we can get away with mild radiation pressure for a long time (eg a few years). But I haven’t run the numbers yet.
>I am more skeptical if your rocket, including fusion reactor but excluding fuel, is limited to 30 gram of weight.
I was imagining a sort of staged rocket, where you ejected the casing of the previous rockets as you slow, so that the mass of the rocket was always a small fraction of the mass of the fuel.
But Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the “ship”, then this doesn’t obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles—though the (tiny) acceleration of the laser/ship is). I’ll have to crunch the number on it.
>Do any implicit or explicit assumptions break if we lose access to most of the fuel mass for shielding during the long voyage?
We didn’t do the shielding very well, just arbitrarily assumed that impacts less energetic than a grenade could be repaired/ignored, and that anything larger would destroy the probe entirely.
As usual, Eric Drexler had a lot of fun shielding ideas (eg large masses ahead of the probe to inonise incoming matter and permanent electromagnetic fields to deflect them), but these were too “speculative” to include in our “conservative” paper.
>I was imagining a sort of staged rocket, where you ejected the casing of the previous rockets as you slow, so that the mass of the rocket was always a small fraction of the mass of the fuel.
Of course, but your very last stage is still a rocket with a reactor. And if you cannot build a rocket with 30g motor+reactor weight, then you cannot go to such small stages and your final mass on arrival includes the smallest efficient rocket motor / reactor you can build, zero fuel, and a velocity that is below escape velocity of your target solar system (once you are below escape velocity I’ll grant you maneuvers with zero mass cost, using solar sails; regardless, tiny solar-powered ion-drives appear reasonable, but generate not enough thrust to slow down from relativistic to below-escape in the time-frame before you have passed though your target system).
>But Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the “ship”, then this doesn’t obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles—though the (tiny) acceleration of the laser/ship is). I’ll have to crunch the number on it.
That does solve the “cannot build a small motor” argument, potentially at the cost of some inefficiency.
It still obeys the rocket equation. The rocket equation is like the 2nd law of thermodynamics: It is not something you can trick by clever calculations. It applies for all propulsion systems that work in a vacuum.
You can only evade the rocket equation by finding (non-vacuum) stuff to push against; whether it be the air in the atmosphere (airplane or ramjet is more efficient than rocket!), the solar system (gigantic launch contraption), various planets (gravitational slingshot), the cosmic microwave background, the solar wind, or the interstellar medium. Once you have found something, you have three choices: Either you want to increase relative velocity and expend energy (airplane, ramjet), or you want to decrease relative velocities (air-braking, use of drag/friction, solar sails when trying to go with the solar wind, braking against the interstellar medium, etc), or you want an elastic collision, e.g. keep absolute relative velocity the same but reverse direction (gravitational slingshot).
Slingshots are cool because you extract energy from the fact that the planets have different velocities: Having multiple planets is not thermodynamic ground state, so you steal from the potential energy / negative entropy left over from the formation of the solar system. Alas, slingshots can’t bring you too much above escape velocity, nor slow you down to below escape if you are significantly faster.
Edit: probably stupid idea, wasn’t thinking straight <strike> Someone should tell me whether you can reach relativistic speeds by slingshotting in a binary or trinary of black holes. That would be quite elegant (unbounded escape velocity, yay! But you have time dilation when close to the horizon, so unclear whether this takes too long from the viewpoint of outside observers; also, too large shear will pull you apart).</strike>
edit2: You can afaik also push against a curved background space-time, if you have one. Gravity waves technically count as vacuum, but not for the purpose of the rocket equation. Doesn’t help, though, because space-time is pretty flat out there, not just Ricci-flat (=vacuum).
>It still obeys the rocket equation.
That’s what I used to believe. But now, on closer analysis, it seems that it doesn’t. The rocket equation holds when you are continuously ejecting a thin stream of mass; it doesn’t hold when you are ejecting a large amount of mass all at once, or transferring energy to a large amount of mass.
The thought experiment that convinced me of this: assume you have a gun with two barrels; you start at rest, and use the gun to propel yourself (ignore issues of torque and tumble). If you shoot both barrels at once, that’s two bullets, each of mass m, and each of velocity v. But now assume that you shoot one bullet, then the other. The first is of mass m and velocity v, as before. But now the gun is moving at some velocity v’. The second bullet will have mass m, but will be shot with velocity v-v’. Thus the momentum of the two bullets is lower in the second case; thus the forward momentum of the gun is also lower in that case.
(The more bullets you shoot, and the smaller they are, the more the gun equations start to resemble the rocket equation).
But when you eject the payload and blast it with a laser beam, you’re essentially just doing one shot (though one extended over a long time, so that the payload doesn’t have huge acceleration). It’s not *exactly* the same as a one shot, because the laser itself will accelerate a bit, because of the beam. But it you assume that, say, the laser is a 100 times more massive than the payload, then the gain in velocity of the laser will be insignificant compared with the deceleration of the payload—it’s essentially a single shot, extended over a period of time. And a laser/payload ratio of 100 is way below what the rocket equation would imply.
How much does the argument break down if we use the rocket equation? I apologize for being a lazy reader.
I assume that if you are using a galaxy’s power for colonization, then it doesn’t matter at all.
In that case contacting us would still be mostly-useless.
If you have to use the rocket equation twice, then you effectively double delta-v requirements and square the launch-mass / payload-mass factor.
Using Stuart’s numbers, this makes colonization more expensive by the following factors:
0.5 c: Antimatter 2.6 / fusion 660 / fission 1e6
0.8 c: Antimatter 7 / fusion 4.5e5 / fission 1e12
0.99c Antimatter 100 / fusion 4.3e12 / fission 1e29
If you disbelieve in 30g fusion reactors and set a minimum viable weight of 500t for an efficient propulsion system (plus negligible weight for replicators) then you get an additional factor of 1e7.
Combining both for fusion at 0.8c would give you a factor of 5e12, which is significantly larger than the factor between “single solar system” and “entire galaxy”. These are totally pessimistic assumptions, though: Deceleration probably can be done cheaper, and with lower minimal mass for efficient propulsion systems. And you almost surely can cut off quite a bit of rocket-delta-v on acceleration (Stuart assumed you can cut 100% on acceleration and 0% on deceleration; the above numbers assumed you can cut 0% on acceleration and 0% on deceleration).
Also, as Stuart noted, you don’t need to aim at every reachable galaxy, you can aim at every cluster and spread from there.
So, I’m not arguing with Stuart’s greater claim (which is a really nice point!), I’m just arguing about local validity of his arguments and assumptions.
Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the “ship”, then this doesn’t obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles—though the (tiny) acceleration of the laser/ship is). I’ll have to crunch the number on it.
(if the probe is very robust, we might be able to railgun it instead of using a laser—and railgunning a single mass, once is clearly not subject to the rocket equation).