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The Cereal Box

“No matter how these issues are ultimately resolved, Centauri Dreams opts for the notion that even the back of a cereal box may contain its share of mysteries.” I wrote that line in 2005, and if it sounds cryptic, read on to discover its origins, ably described by Christopher Phoenix. I first encountered Christopher in an online discussion group made up of physicists and science fiction writers, where his knack for taking a topic apart always impressed me. A writer whose interest in interstellar flight is lifelong, he is currently turning his love of science fiction into a novel that, he tells me “incorporates some of the ideas we talk about on Centauri Dreams as a background setting.” Today’s essay examines the ideas of a physicist who dismissed the idea of interstellar flight entirely, while using a set of assumptions Christopher has come to challenge.

by Christopher Phoenix


“All this stuff about traveling around the universe in space suits — except for local exploration which I have not discussed — belongs back where it came from, on the cereal box.”

Over fifty years ago, physicist Edward Purcell penned the boldest dismissal of interstellar flight on record in his paper “Radioastronomy and Communication Through Space”. In that paper, Purcell uses the elementary laws of mechanics to refute the possibility of starflight in total. There are many people, of course, who share his belief that we will never reach the stars.

Keeping a firm grounding in the laws of physics is absolutely necessary when researching interstellar travel. A healthy skeptical attitude can help keep researchers honest with themselves. Certainly, not everything we imagine is possible. Nor can we hope to ever reach for the stars if we do not keep our feet firmly planted in reality.

However, sometimes such extreme skepticism deserves some healthy skepticism itself. Even though Purcell’s equations aren’t wrong, he didn’t prove that starflight belongs back on the cereal box. Instead, he defines the problem of interstellar travel in such a way that it seems to be insurmountable.

Radioastronomy and Communication Through Space


Before we begin, I want to quickly introduce Purcell and this paper. Edward M. Purcell made important contributions to physics and radioastronomy. He shared the 1952 Nobel Prize in Physics for discovering nuclear magnetic resonance (NMR) in liquids and solids. Later, Purcell was the first to detect radio emissions from neutral galactic hydrogen, the famous “21cm line”. Many important developments in radioastronomy resulted from his work.

“Radioastronomy and Communication Through Space” was the first paper in the Brookhaven Lecture Series. These lectures were meant to provide a meeting ground for all the scientists at Brookhaven National Laboratory. In this paper, Purcell argued that traditional radio SETI, not interstellar travel, is our only way of learning about other planets in the galaxy.

Image: Edward Mills Purcell (1912–1997). Credit: Wikimedia Commons.

Purcell builds to his conclusion in three sections. The first section discusses then-recent discoveries in radioastronomy. Purcell tells how astronomers mapped the galaxy by observing radio emissions from neutral galactic hydrogen (the 21cm line). He notes in particular that we gathered all this information by capturing an astonishingly tiny amount of radio energy from space. Over nine years, the total amount of radio energy captured by all 21cm observatories added up to less than one erg (10-7 J).

The paper then jumps from radioastronomy to more speculative topics. In the second section, Purcell takes on the idea of interstellar travel and runs some calculations on relativistic rockets. He concludes that interstellar flight is “preposterous”. In the final section of his paper, Purcell argues that radio messages can be sent between the stars for relatively little energy cost, while the energy required for interstellar travel is unobtainable.

I shall primarily discuss the second part of this paper, where Purcell argued against the possibility of interstellar travel.

“This is preposterous!”

From the start, Purcell considered fast interstellar travel as our only option. Purcell noted that relativity is not the obstacle to reaching another star within a single human lifetime. We cannot travel faster than light. However, if a we travel at speeds close to that of light, time dilation becomes an important factor, reducing the amount of time that passes for us on our trip. You will age much less than your friends back home if you travel to the stars at relativistic speeds.

This is perfectly correct, in my view, so far as it goes. Special relativity is reliable. The trouble is not, as we say, with the kinematics but with the energetics… Personally, I believe in special relativity. If it were not reliable, some very expensive machines around here would be in deep trouble.

The problem, Purcell says, is building a rocket capable of carrying out this mission. He develops this argument by examining a particular example flight.

Let us consider a trip to a place 12 light years away, and back. Because we don’t want to take many generations to do it, let us arbitrarily say we will go and come back in 28 years earth time. We will reach a top speed of 99% speed of light in the middle, and slow down and come back. The relativistic transformations show that we will come back in 28 years, only ten years older. This I really believe… Now let us look at the problem of designing a rocket to perform this mission.

So, Purcell has defined the problem in a certain way. The starship must fly to another star and return to Earth within a human lifetime. To do so, it will reach a top speed of 99% the speed of light (C) in the middle of the voyage. The craft is a rocket, and it must carry all its propellant from the beginning of the trip. It cannot refuel anywhere. To reach 99% C within a short amount of time, the rocket must maintain an acceleration of one g for most of the trip.

Having laid out the starting assumptions for our trip, Purcell uses the relativistic rocket equation to calculate the amount of propellant the rocket will require to complete the trip. Remember that rockets are momentum machines. They throw a certain mass of propellant out the back, and the reaction force pushes the rocket. When that propellant is all gone, only the payload remains and the rocket has reached its final speed.

A rocket engine’s performance is determined by its exhaust velocity (Vex). This is the velocity at which propellant leaves the engine as measured by the rocket. The higher the Vex, the more efficiently the rocket engine uses propellant. Engineers refer to rocket efficiency as specific impulse (Isp). A rocket’s specific impulse is determined by its exhaust velocity.

If you have a rocket of a certain Vex, and you want to accelerate it to a certain maximum velocity (Vmax), physics imposes a certain relationship between the initial and final mass of the rocket. Engineers call this ratio a rocket’s mass ratio. This relationship is shown by the rocket equation. Unfortunately, if our Vmax is much larger than our Vex, mass ratios increase exponentially. This is because the rocket must not only accelerate the payload, but also all the as-yet unused propellant. To go faster, you need more propellant, but you need more propellant to carry that propellant- and so on.

So, our next problem is choosing an engine. We want to travel close to the speed of light, so we need an engine with the highest exhaust velocity (and thus highest Isp) possible. Chemical rockets have much to low a Vex to do this- they would require an unimaginably large amount of reaction mass to approach the speed of light. We need a far more powerful engine.

One type of engine that could perform far better than chemical rockets is the nuclear fusion rocket. So, Purcell first proposes using idealized nuclear fusion propellant. In this case, the rocket’s initial mass must be a little over a billion times its final mass to reach 99% C. A ten ton payload will require a ten billion ton rocket at the start of the journey. This is simply too much mass!

We need something far more potent. Purcell turns to idealized matter-antimatter (M/AM) propellant. Again, we assume the fuel is utilized with perfect efficiency. Matter annihilates with antimatter, and the resulting energy is exhausted as massless electromagnetic radiation (gamma rays), giving us a Vex of C. We can’t beat that.


Image: VARIES (Vacuum to Antimatter Rocket Interstellar Explorer System) is a concept developed by Richard Obousy that would create its own antimatter enroute through the use of powerful lasers. Credit: Adrian Mann.

The situation is vastly improved by M/AM propellant. To reach 99% C, the rocket’s initial mass must be only 14 times its final mass. But we must also slow down at the destination, and slowing down requires just as much effort as accelerating in the first place. After that, we must turn the ship around and return to Earth.

So, during the course of our flight, the rocket shall undergo four accelerations. On the trip away from Earth, the rocket will accelerate to 99% C, and then decelerate back down to rest at the destination star. After turning around, it will accelerate back to 99% C on the trip home and then decelerate back down to rest at Earth. To do this, the rocket must start with an initial mass 40,000 times its final mass. To send a ten ton payload on this round trip will require a 400,000 ton rocket, consisting half of matter and half of antimatter.

The starship must accelerate at one g for most of the trip. At the outset of its journey, this rocket must radiate 1018 watts of radiant energy to accelerate its 400,000 tons of mass at one g. This is a little over the total power that the Earth receives from the sun. Only this energy is in gamma rays, which presents a shielding problem for any planet near the ship. In addition, once the rocket achieves relativistic velocities, cosmic dust and gas present a shielding problem for the ship itself. At these speeds, even tiny specks of matter will behave like pinpoint nuclear explosions, and individual protons will be transformed into deadly cosmic rays.

Purcell concludes that these calculations prove that interstellar flight is “preposterous”, in this solar system or any other.

Rigging the game

There isn’t anything wrong with Purcell’s calculations. The problem is that Purcell wants to take this one set of calculations and prove that any form of interstellar travel is impossible. This isn’t very fair, since the starting conditions he picked in his example lead to his pessimistic conclusions. Let’s examine these assumptions.

Purcell’s first assumption is we must travel at 99% C. Why must we travel so fast? Even to complete a trip to a nearby star within a human lifetime, you can travel slower than that. Purcell is committed to these extreme relativistic speeds in order to take advantage of time dilation and complete the round trip in a decade.

If we are willing to travel much slower, perhaps 10% C, or even 1% C, and let multiple generations of crew make the trip, the difficulties are greatly reduced. At slower speeds, propulsion requirements are far more reasonable, and deadly collisions with cosmic dust would be easier to defend against.

Of course, there are many very difficult challenges to solve before we can launch such a ship. The travelers must recycle all their air and water, grow their own food, and build a stable society able to last for centuries. Some form of artificial gravity must be provided to prevent muscle and bone loss in microgravity. The habitable sections of the ship must be shielded from cosmic rays. But none of these represent hard physical limits arising from the laws of mechanics and nothing else.

This is all assuming humans are making the trip. Slow travel is made even easier if humans do not make the trip, just as we have done with our current robotic exploration of the solar system.

The second assumption is the starship must return to Earth. Particularly if we must carry all the propellant we use from the outset, a round trip mission is far more difficult than a one-way trip. But why must the starship return to Earth? There are many interesting missions that do not require the spacecraft to return to Earth. A colonizing expedition does not have or even want to return. Neither does a robotic probe. A fly-by probe like Daedalus doesn’t even need to carry propellant to slow down at the destination.

Purcell’s third questionable assumption is an interstellar vehicle must carry all its energy and reaction mass on board from the start of the trip. Is this really true? Think about in-situ resource utilization. An interstellar expedition could mine propellant from planetoids encountered at the destination. We can use propulsion systems that use the resources present in space, like gravitational assists, solar sails, or even interstellar ramjets. Granted, gravitational assists and solar sails could not get you anywhere near relativistic speeds, but they could work for slower travel.


Image: A Bussard ramjet in flight, as imagined for ESA’s Innovative Technologies from Science Fiction project. Credit: ESA/Manchu.

If the natural resources of space are not sufficient, there are other options. Rockets carry all their energy and reaction mass from the start. Beam-rider propulsion systems are an alternative that leave heavy engines, energy sources, and propellant back home. One such craft is a photon sail pushed by a laser. Another is a spacecraft propelled by a stream of relativistic pellets, each transferring momentum to the craft. As a cursory read of Mallove and Matloff’s excellent book The Starflight Handbook shows, we are not limited to rockets only.

Ultimately, Purcell’s conclusion that all speculation about interstellar travel belongs back “on the cereal box” simply doesn’t hold air in the space vacuum.

SETI vs interstellar travel?

Purcell’s paper underscores an unfortunate split in the ranks of scientists. Many scientists interested in SETI maintain that interstellar flight is simply not feasible for any civilization. They argue that we don’t need to physically travel to other planetary systems in order to learn about the rest of the universe. We need only turn our radio telescopes to the sky and search for broadcasts from more advanced civilizations. If we find them, these advanced civilizations will hopefully tell us everything we want to know. We might even find that mature civilizations in space have formed a galactic community of communicating societies. Perhaps they might allow us to join the conversation once we demonstrate enough maturity to engage in interstellar radio communications. This an exciting possibility, if a bit idealistic, and SETI deserves our support.

However, it is important to realize it is not an either-or question. We can research interstellar travel and carry out SETI searches at the same time. Even if SETI searches find communicative aliens to talk to, that will not negate the usefulness of interstellar travel. We will still need interstellar flight to investigate the countless solar systems where such civilizations are not present, and starflight is absolutely necessary for interstellar migration. But it seems like some SETI supporters don’t see it that way.

Denying starflight has become a fundamental tenant of the SETI worldview. It speaks directly to the question of whether it might be dangerous to contact alien civilizations. Many SETI supporters claim that we don’t have to worry about this question. If we assume interstellar travel is impossible, no civilization in space can physically threaten another. As Purcell claims in his paper:

It [communicating with ETI] is a conversation which is, in the deepest sense, utterly benign. No one can threaten anyone else with objects. We have seen what it takes to send objects around, but one can send information for practically nothing. Here one has the ultimate in philosophical discourse – all you can do is exchange ideas, but you do that to your heart’s content.

In my opinion, this is the real reason why Purcell argues so vehemently against the possibility of interstellar flight. In order for communication with ETI to be completely safe, interstellar travel must be impossible for any civilization anywhere in the universe. Contact with ETI becomes more complicated if there is a possibility of encountering them or their technology physically. Of course, we can’t be entirely sure messages from ETI will be entirely harmless either, if they contain instructions or information that might pose a danger.

I suspect that Purcell’s pessimistic arguments against starflight were driven more by his desire to believe that discourse with aliens comes without risks than a genuine interest in the future of space travel. Whatever the disposition of aliens, we can’t allow our personal hopes and dislikes to bias our conclusions. While interstellar travel is very difficult, we can already conceive of ways that a sufficiently motivated civilization could reach the stars.


Comments on this entry are closed.

  • Michael November 20, 2015, 11:34

    There is no requirement to carry the fuel, if the fuel is held by say sails that are accelerated by laser or microwave beams the craft need only catch up to the next fuel cargo and then fuse the fuel to get to the next fuel cargo and so on, in effect the exponential rocket equation issue is by-past. All the energy for moving the fuel is in the solar system perhaps using large solar-to-laser arrays near the sun. Now if we send fuel/sail packages decades later as the craft approaches the target star system they can be collected by the craft (it turns around to collect the fuel packages) as they encounter the other star system to slow down, so in effect a buzzard ram jet hybrid.

    Now we only need to accelerate within the solar system out to Saturn at 1g to get to a significant fraction of light speed. Another advantage of the sail/fuel concept is that they can be accelerated to very high values, so in effect we would have a stream of fuel packages going outwards. Yes there will be some mechanisms required to collect the packages perhaps a laser/microwave dish on the front of the craft will aid in their collection. The impact hazard could be mitigated by these same accelerated sails going ahead as stand off shields guided again by the laser/microwave dish in front of the craft.

  • JoeP November 20, 2015, 11:39

    Chris, I enjoyed this article. The schism around whether communication with alien civilizations could be dangerous or not reminds me of the climate change debate. Both sides trying to claim that the “science is settled” and debating is is preposterous!

  • David Cummings November 20, 2015, 11:49

    Excellent post. Very well written.

    I just have a question: Does the SETI community still resist talk of interstellar travel?

  • Hop David November 20, 2015, 12:00

    The impossibility of interstellar space flight woundn’t confer safety.

    There must be dozens of science fiction stories where ETI has sent us malevolent memes. Sending us the technology to destroy ourselves is a common device. Or if ETI minds are software, these can be transmitted as well. Some Vinge and Brin stories come to mind.

    As you point out, Purcell doesn’t consider generation ships.

    KSR’s argument against generation ships is the difficulty of establishing a CELSS (Closed Ecological Life Support System) that would last generations.

    Early ecological life support systems would not be closed. Main belt colonies would be able to trade with their neighbors as well as Earth and Mars. They would also have access to in situ resources.Pointing to the Oracle, AZ Biosphere2 fiasco does nothing to debunk the notion of space colonies.

    Past incentive for colonization breaking the logistic growth ceiling. Given limited resources, growth is limited. The Main Belt and Sun Jupiter L4 and L5 Trojans would give us room to grow for many generations (not to mention Mars, moon and moons of Jupiter).

    When this frontier is settled the KBOs and Oort cloud would likely be the next frontier. As well as the moons of Saturn, Uranus and Neptune. This would give humanity room to grow for 1000s of years.

    by the time the outer Oort cloud is settled, we will have had millennia of experience in building ecological support systems. In the outer Oort trade with neighbors is more difficult so there is incentive to develop indepdendent, closed ecologies.

    Eventually we’ll fill the sun’s Hill sphere. By the time we reach that logistic growth ceiling, closed ecologies lasting many generations will be routine.

  • Mark Zambelli November 20, 2015, 12:44

    Yes, Purcell seems to have used the reductio absurdum trick to steer his agenda and when disected as you’ve done, it does come across as unfair. You are right to point out it’s not an ‘either-or’ situation.
    (I was a little surprised to see mention of Bussard’s ramjet though)

  • Black SciFi November 20, 2015, 12:56

    I firmly believe that any manned interstellar journey will only become practical when in-situ resources are the primary energy source for propulsion.

    This, of course, is dependent on a far more focused R&D which at the present time is still within the realm of the (off the record) military budgets of the space-faring nations of this planet.

    If an “in-situ” use of space resources is developed it will be because we are looking for a better way to deliver a bomb or a better bomb. I have zero faith in the funding mechanism for advanced physics to have any interest to further research into, for example, field propulsion.

    Within that context I foresee research funding to originate from space entrepreneurs like Musk/Space X rather than NASA, et al.

    Long range deep space projects that entail anything approaching generation ships is a non-starter. Given the current political climate, funding is only going to be available for “in our lifetime” results.

    The good news is that the cost of the imagination of Hard SF writers is the most cost effective means of generating new deep space propulsion ideas based on current and future physics. I’d bet all the tea in the Vega system that a really revolutionary in-situ fueled deep space propulsion system will originate from the fertile imagination of a lone writer in his study.

  • Jim Strom November 20, 2015, 13:09

    Even if relativity keeps a traveller’s trip time under a lifetime, everyone on Earth they knew will be long gone upon return.

    So what is the point of hurrying, or even being awake? A slow vessel with passengers in suspended animation would be a simpler solution, assuming that true suspended animation is possible.

  • spaceman November 20, 2015, 13:39

    What are some projected velocities for the VARIES concept?

  • James Salsman November 20, 2015, 14:40

    Why is cryonic preservation still virtually ignored, especially compared to esoteric technology speculations which haven’t had even a tiny fraction of the empirical verification? Whole-body supercooling vitrification and resuscitation by controlled warming have had huge advances recently, including in the organ banking literature where transplant surgeons have started using CASfresh brand industrial Cells Alive System supercooling vitrification freezers. As far back as 1954, the success rate from rewarming supercooled rodents has been 100% and now that we can do the same to hundreds of kilograms instead of just kilograms, serious proposals for interstellar travel need to catch up with science fiction.

  • Paul Gilster November 20, 2015, 16:23

    spaceman asks:

    What are some projected velocities for the VARIES concept?

    Good question. I have the issue somewhere around here but haven’t yet reviewed the paper; it’s Obousy, R.K., “Vacuum to Antimatter Rocket Interstellar Explorer System“, JBIS 64 No.11/12 pp 378-386 (2011). I don’t off the top of my head know what sort of velocities Dr. Obousy has projected. Can any of the readers help? I’ll drop him a note about this.

  • Andrew Palfreyman November 20, 2015, 16:44

    The black hole space drive of Crane & Westmoreland, already discussed on these pages, is easily the most efficient route to interstellar travel – far superior to even matter-antimatter drives. Add to this some optimisations that I’ve discovered of late involving staggered operation of BH lifetimes, and it really rocks :).

    And don’t let the fact that it’s currently technically impossible put you off; the same can be said for sustained fusion at the moment. The physics is sound, and we will eventually master the engineering challenges.

  • Tom Baty November 20, 2015, 16:49
  • Ron S November 20, 2015, 16:55

    Why are we flogging long-dead scientists?

  • Christopher Phoenix November 20, 2015, 17:25

    Thanks for all the great comments!

    @Michael What you described is an externally fueled fusion rocket. I first encountered this propulsion proposal here. I have not run any numbers yet to see what advantage this gives us, but any way we can avoid accelerating all the fuel from the start of the trip is helpful. And this shows again that Purcell’s assumptions were unfair.

    @David Cummings Some of this talk is probably still with us, but not every scientist agreed with Purcell. Carl Sagan, for instance, was interested in interstellar travel. A chapter of Intelligent Life in the Universe discusses cyronic suspension and relativistic ramjets as possible means of direct contact between civilizations.

    @ Mark Zambelli What surprised you about seeing ramjets mentioned? In this case, I was not attempting to debate the feasibility of any particular propulsion system, but to point out that there are other options. Also, I didn’t mean only the traditional Bussard ramjet, but all the ramscoop derivatives, including RAIR, externally fueled ramjet, stellar ramjet, and perhaps mostly importantly Zubrin’s magnetic sail.

    @James Salsman Yes, a slow trip with a cryonically suspended crew would be a much easier way for humans to reach the stars. I’m reminded of Robert Forward’s comments about photon rockets, to the effect that attempting to accelerate the vehicle to high relativistic velocities results in large energy cost with little increase in speed (more and more energy goes into increasing the starship’s mass, instead). We eventually hit the point of diminishing returns. This suggests that it would be better to turn off the engine and coast for a while.

    Unfortunately, we don’t yet know how to freeze someone’s entire body and thaw them out later without causing lethal damage.

  • Christopher Phoenix November 20, 2015, 17:43

    @James Salsman

    Why is cryonic preservation still virtually ignored, especially compared to esoteric technology speculations which haven’t had even a tiny fraction of the empirical verification?

    I didn’t mention cryopreservation in this post because I consider it a questionable technology. We don’t yet know how to freeze entire astronauts and thaw them out without damage. Most of our research goes toward preserving biological samples, not entire living beings for centuries. If there are new developments on this front, though, I will be excited to read about them!

    Even if we do find a way to freeze people without damage (currently, only smaller masses of relatively homogeneous cells can be preserved), don’t forget that the human body is full of naturally occurring radioactive atoms. Normally, cellular damage caused by this constant low level of radioactivity is repaired by the body, but while you are frozen it won’t be. After centuries, enough damage might have occurred that the automatic warming systems will thaw out a corpse.

    Multigenerational crews present less difficulties. Our main problem is developing a CELSS. I think it will be easier to develop a self-sustaining space habitat than to untangle all the problems with cyropreservation.

    If by “esoteric technology speculations” you mean things like mind uploading, I’ve been puzzled too by how often people mention transhumanism while virtually ignoring cryonics. They both need comparable levels of magictech (nanotech etc.) to get working!

  • Charlie November 20, 2015, 18:08

    A question here. This protest against the types of flights that are involved. I wonder if it is in fact A HIGHLY IMPROBABLE that there will be interstellar flight for many, many, many years. If ever. I’m not being absolute in this, but the question that people can be either suspended in a hibernation (with no associated aging I might add), as well as being a generational ship is at all practical add any juncture. The first might not even be possible. And secondly, the second one has the implication that people won’t go crazy being locked up inside basically a huge tin can far what amounts to generation after generation of people. Are people really that motivated to go ahead and make such EXTREME voyages ? I mean, we know that people can go for a long times on journeys, but they had the expectation that they would eventually return to their home situation. This is not something we would see on interstellar voyages which would be many, many light years away with the idea that you never come back.

  • John Freeman November 20, 2015, 18:42

    A person like Purcell, clearly of intelligence and knowledge, must have been able to see the flaws in his oen argument:a flyby probe does not need to slow down or come bavk, and the same logic applies to missiles. would a desire for SETI to be ‘safe’ really have led such a smart guy to ignore such simple and obvious flaws in his argument?

  • James Salsman November 20, 2015, 21:27

    Christopher, thank you for your replies on cryogenic preservation for suspended animation.

    On the question of capability, I urge you to review the literature of 1954, I want to say Audrey Smith of England, but maybe one of her coauthors. Once they figured out how to supercool rodents, the rewarding survival rate went to 100%. Only in the past decade do we have means to supercool hundreds of kilogram tissue masses, including whole bodies. If you raise the funds to supercool and rewarm human-sized livestock, and get sufficiently similar results, I volunteer to test it myself.

    As for incorporated radionuclides, for the beta and gamma radiation, bringing the temperature to around 25 Kelvin, as a sleeper ship with a reasonable energy budget might cruise at, the amount of ionization incurred per equivalent energy is reduced because the chemical bonds can absorb additional energy without disturbing molecular structure. Alpha radiation presents a somewhat but not entirely different scenario, where the ionization per radiation particle and energy is still reduced, but helium ion damage may not be.

  • Steven Tupper November 20, 2015, 21:41

    Right around 1980-ish it was noted by at least a few people that electronic communications devices like the telephone, radio and television would inevitably converge with that shiny new Apple II on your desktop. That happened of course, and the result is in your pocket right now. Similarly, work on AI, while slow and difficult now, will very likely at some point in the future converge with the human brain; the only real question is when. Will it be before or after our civilization gets it together to launch a sizable interstellar spacecraft? If before, then the “we” that is assumed in the article and discussion will be whatever we have become by that time, and all those assumptions about human lifetimes may no longer be valid.

  • Joe G November 20, 2015, 21:47

    I always liked the idea of a massive rail gun to accelerate a robotic probe… Look ma, no engine… I wonder if anyone has ever done a feasibility study on this idea. Mostly I wonder if there is some upper limit to speeds that can be achieved. True enough, a robotic probe can make a one way high speed flyby… However its always made me wonder if creating a Volkswagen sized chunk of metal traveling at relativistic speeds might be an irresponsible thing to do… Perhaps it will never ever encounter anything, on the other hand you’d never see it coming… I’d really hate to be on the planet or ship that accidentally moved into its path.

  • Larry November 21, 2015, 1:15

    Fast relativistic star flight is an interesting challenge. We would need, first of all, to develop different methods of controlling energy than we now know. Antimatter is no more useful than coal if all you know how to do is generate heat, boil water to make steam and turn a turbine to make electricity. And the shielding required for a reactor putting out 10^18 watts omni-directionally would be massive if all we know is to pile up matter in front of it. There is much we need to learn before we can consider going to the stars.

    On the other hand, I find the idea of a generation ship to be preposterous. Imagine a journey of 250 years – 10 generations. The people embarking on the voyage would be intrepid explorers and volunteers! The next nine generations would be slaves. They would have very little freedom of any kind, and much involuntary work to do – a life they never signed up for, and were never given a choice – or the mission would fail and they would all die. They would spend their entire lives in a relatively tiny vehicle, knowing only a few people around them, no scenery but the unmoving stars, with no opportunity to grow or explore, and never living to see the world their parents or great grandparents had condemned them to hurtle toward. They would be required to mate and have children, whether they wanted to or not. I can imagine the social situation getting pretty bad, yet there is no escape save through suicide, which could be frequent. What kind of psychopaths do you imagine would arrive at the destination after ten generations of forced labor, inbreeding, and isolation? Not intrepid explorers, I’m pretty sure.

  • Michael November 21, 2015, 4:37

    @Christopher Phoenix November 20, 2015 at 17:25

    ‘@Michael What you described is an externally fueled fusion rocket. I first encountered this propulsion proposal here. I have not run any numbers yet to see what advantage this gives us, but any way we can avoid accelerating all the fuel from the start of the trip is helpful. And this shows again that Purcell’s assumptions were unfair.’

    I agree about Purcell’s one-sidedness and therefore unfair justification, he just looked at the hard equations which he was correct on but just failed to use his imagination on them. As for the pellet concept I ran some numbers awhile back and found that the accelerator becomes very large and the pellets do not scale well magnetically or electrostatically, so it was not a good design.

    However when I looked into sails with fuels attached they offered some great advantages with their large surface areas for not only energy capture and therefore huge accelerations which would lower beam divergence issues but also they can be manipulated from the spacecraft when collecting the packages more easily. These sails could also be smart and alter their shape to keep on beam and the sail materials after capture could also be used as an erodible protection of the reaction chamber walls.

    So in effect we have star system power laser or microwave beams which make use of the free solar energy near the Star and fuel packages been sent out which greatly reduce the fuel requirements of the space craft.

    We can also use this concept on chemical propellants and perhaps we could apply this concept to incorporate anti-matter and then we will truly be laughing.

    Great post by the way.

  • Antonio November 21, 2015, 5:47

    @Christopher Phoenix:

    About cryonics’ state of the art, see the references at the end of this letter: http://www.evidencebasedcryonics.org/scientists-open-letter-on-cryonics/

  • Mark Zambelli November 21, 2015, 7:19

    @ Charlie… there are a few reasons that groups of people may jump at the chance to get away from the rest of us. A few commentors on CD over the years have suggested such reasons as political or religious freedom as an incentive for an isolationist mentality (a driving force behind small collections of my countrymen and women setting sail to the New World 400yrs ago btw). Maybe ‘communes’ or cultist sects want to set up their own edens out in the big beyond and view themselves as having an ‘ark’ to do it in? Personally, I’m claustrophobic so, just as in the TV show ‘Ascension’, the tin can would have to be huge for my tastes ;)

    @Christopher… sorry for casting dispersions on the good ol’ Ramjet. I appreciate their inclusion in your list of alternatives (certainly the hybrids you went on to mention) but is there a design that can be shown to accelerate as intended, rather than act like a magnetic brake as has been shown?… the braking aspects would be good for decelerating your ramjet at destination. I hope so as Bussard really struck a chord with such a brilliant design.

    Thanks again for the article.

  • Michael Spencer November 21, 2015, 8:15

    Oh, dear. Here we go again.

    Everybody knows that heavier than air flight is impossible, and that computers will never populate home desktops. The major science problems were resolved before the twentieth century, after all. Heck, even the illustrious Dr. Friedman recently predicted that we will never populate our own solar system beyond Mars.

    Our knowledge of the universe we find ourselves inhabiting is–well, pitiful. Dark matter? Dark energy? Quantum whatever? We are so ignorant that comparing the 21st century to Leonardo pales.

    Let us continue research. And let us plan for our future. Our future in the stars.

  • Brian November 21, 2015, 11:18

    I have a bit of an ethical issue with long manned interstellar missions. Unmanned sure, but putting people on a ship for decades and centuries is problematic. I think we are simply better off working on the fast solution. I’ve also wondered has anyone ever looked into the Krasnikov tube as a solution to the twin paradox? combined with relativistic travel you basically are left with a poor man’s warp drive. Has anyone ever looked into its plausibility?

  • Robert November 21, 2015, 14:01

    I think that the theoretical breakthrough in recent years involving solutions of General Relativity involving variants of the Alcubierre solutions changes the dynamics of the discussion.

    Oh sure, the required parameters are fantastic yet getting far less fantastic over the years since the discovery. The theoretical requirements have gone from a Jupiter sized mass of exotic matter down to a mass the size of a Jupiter probe, literally a few thousand kilograms. Even at this site we recently discussed the possibility of sending a few photons through a sort of mini wormhole in order to send messages effectively superluminal.

    So, I believe a reasonable working assumption is that a truly advanced technological civilization would have long surpassed our understanding and perhaps arrived at a workable solution if one is possible.

  • Egad November 21, 2015, 16:03

    Slightly echoing Joe G above, are there any analyses of robotic fly-by probes moving at, say, 10% c? Think New Horizons at Pluto but a lot faster and a lot farther away. The longevity of current spacecraft (decades) suggests that probes capable of lasting a century or more wouldn’t be out of the question, and there is a fair number of stars that could be flown by in that time frame.


  • Paul Gilster November 21, 2015, 16:57

    Project Daedalus assumed 12 percent lightspeed. For more on it, check the archives here. The current Icarus effort is an attempt at a more modern redesign.

  • Christopher Phoenix November 21, 2015, 18:02


    On the other hand, I find the idea of a generation ship to be preposterous. Imagine a journey of 250 years – 10 generations. The people embarking on the voyage would be intrepid explorers and volunteers! The next nine generations would be slaves.

    I’ve noticed that a lot of people react like you did to the idea of multigenerational space travel, which is a pity since it is a liberating concept for interstellar travel. People who insist on reaching the stars within a single human lifetime struggle to arrive at a viable design to reach at least 30% C, and they are then limited to trips to the nearest stars.

    Personally, I think that we need to set our priorities straight. People who consider generation ships a form of slavery are all focusing on the destination. Thus, all the generations of people in between just miss out, right? Nowhere to go or explore.

    But there is another way to do it. We can plan to make the voyage as pleasant as possible, and build the ship to be a tiny, viable world in an of itself. This will require careful planning.

    Crew size must be large enough to create a viable society. If the crew size is large enough, you won’t know everyone on board. The relevant limit is Dunbar’s number. The habitat interior could be sculpted to provide an Earth-like environment, if that is necessary for psychological health. Humans generally desire a purpose. I don’t think they would consider the jobs created to do the work of maintaining the starship to be “slave labor” any more than the engineers who maintain modern cities do. There would be plenty of leisure activities as well. Some of the crew will contribute to the arts, because this is a vital activity for a healthy society.

    And there will be other opportunities as well. Scientists aboard would do valuable scientific research on the way. The ship would be an excellent platform from which to study the interstellar medium. As it travels further from Earth, it will provide a longer baseline for parallax measurements of stars.

    Is this so different from life in an isolated village on the Earth? Nowadays we take rapid travel with some assurance of safety for granted, but in earlier times, not many people left their home village. We have casual connections with a multitude over the internet. But is this really so superior to the stronger relationships that will grow between the crew on a generation starship?

    Some of the issues you raise aren’t any different than life on Earth. Our parents all made choices that affected us in ways beyond our control. Being born on a starship isn’t any different. If the starship is a healthy environment for the crew, there is no “ethical dilemma” here.

    They would be required to mate and have children, whether they wanted to or not. I can imagine the social situation getting pretty bad, yet there is no escape save through suicide, which could be frequent. What kind of psychopaths do you imagine would arrive at the destination after ten generations of forced labor, inbreeding, and isolation? Not intrepid explorers, I’m pretty sure.

    Most people want to have children at some point. I think we will have to work harder to keep the population from growing too much than to get people to have children at all. If this were not the case, we would not be approaching 7 billion humans on Starship Earth. On Earth, people delay having children due to financial reasons or lack of a stable environment, but it will be different on a generation ship. Starship society will be structured to support the children who will become the next generation of crew.

    As for inbreeding, the initial crew size will have been large enough to avoid this. Studies have already been done to determine the necessary crew size. No one will be launching a starship so badly planned that the crew arrives “inbred after generations of forced labor.” This is all assuming we don’t routinely use genetic engineering to repair damaged genomes by the time we have large interstellar spacecraft, in which crew size is a less pressing issue.

    In conclusion, I don’t think life on a generation ship will be anywhere near as terrible as you describe. Life on a generation ship will be like life on Earth, only in miniature. The crew will work to build a stable, healthy society, because that is the only way to succeed. The lurid language you use, such as “forced labor, inbreeding, isolation, psychopaths” brings images to mind that seem entirely inappropriate for describing life on a generation ship. It sounds like you think the crew is expected to spend ten generations rowing the ship, like ancient Roman galley slaves!

  • NS November 21, 2015, 18:03

    Re various sects etc wanting to get away from their fellow humans, wouldn’t it be orders of magnitude easier to setup even very large habitats within the solar system than to attempt interstellar flight? Unless you’re bound and determined to live on another Earth-like planet someplace it seems like you could have just about anything you wanted right around here, without the difficulty of traveling to another star.

    SETI proponents were/are very anxious to separate themselves from the UFO crowd. The alleged impossibility of interstellar flight lets them discuss intelligent life in other stellar systems without contending with all the people who think they’ve MET intelligent life from other stellar systems (or wherever*). If interstellar flight is theoretically possible then UFOs etc can’t be so readily dismissed.

    * I should note that nowadays many in the UFO “field” don’t think UFOs are from other stellar systems anyway, but are rather the product of other intelligences on or near the Earth.

  • Christopher Phoenix November 21, 2015, 18:14

    @Mark Zambelli

    sorry for casting dispersions on the good ol’ Ramjet. I appreciate their inclusion in your list of alternatives (certainly the hybrids you went on to mention) but is there a design that can be shown to accelerate as intended, rather than act like a magnetic brake as has been shown?… the braking aspects would be good for decelerating your ramjet at destination. I hope so as Bussard really struck a chord with such a brilliant design.

    Sadly, it seems like the original Bussard Ramjet concept can’t accelerate past the point where its magnetic field produces more drag than the engine does thrust. Unless we can tune up the concept to inject the energy lost as the particles spiral into the engine back into the exhaust stream, or something like that. And proton-proton fusion is not a good reaction for the ramjet, even though you will mostly be scooping up protons.

    There have been some posts on CD about the ramjet, though. The search terms “catalytic ramjet” should turn up some articles on the idea of catalyzing fusion with the CNO cycle in a ramjet. Researchers are continuing to review the idea. We should not give up the possibility of unlimited fuel and reaction mass without a fight!

  • Alex Tolley November 21, 2015, 18:41

    Perhaps a good example of Clarke’s First Law. However I think Christopher Phoenix makes a good case that this was a case of a straw man argument.

    We shouldn’t forget that we all tend to make such cases, especially where it confirms a bias. Mind uploading for the journey? – impossible say some, even though it seems to not violate any physics. AI’s instead of humans? – many hate the idea that humans will not be the Earth’s emissaries to the stars, but that our machine descendants may be.

    @Michael – I see the fuel pellets working for the outbound acceleration, but I have difficulty with seeing how the pellet stream can be used to decelerate the ship. (It isn’t a matter of just reversing the thrust direction).

    In many respects, we are like Victorians speculating about interplanetary flight, with very limited knowledge of physics, and no great ability to foresee trends in technological development. A century from now, star flight might look very different from teh proposed approaches favored today.

  • Charlie November 21, 2015, 20:05

    What has happened to all this business concerning faster than light travel via general relativity? I haven’t heard anything at all in quite a while concerning all that

  • PW November 21, 2015, 20:45

    I sort of agree with Purcelle.

    Without some sort of FTL, interstellar travel is mostly a non-starter.
    To be undertaken only when there is no choice. Earth is doomed and we must escape and there are no options.

    The incredibly huge galaxy was not not built for humans to escape. One of a kind, desperate, one way missions are most likely to fail.

    If we are to be confined to Einstein’s brilliant physics, we are fated to sit here and intelligently record our planet side demise.

  • Christopher Phoenix November 22, 2015, 1:13

    @ PW

    Without some sort of FTL, interstellar travel is mostly a non-starter.
    To be undertaken only when there is no choice. Earth is doomed and we must escape and there are no options.

    Well, Purcell’s argument doesn’t prove these statements… so you’ll need to construct one to support it yourself. Deliberately picking the parameters of the mission to make it all-but-insurmountable doesn’t bolster the argument that interstellar flight is a non-starter.

    As beloved the “flee dying Earth” scenario is to SF, it seems pretty unlikely this will be the motivation behind building our first starship. It would take a long period of stable development to support building starships. There are also few (likely?) disasters that would make it easier to flee to another star than to stay here and cope with it as best we can.

    That said, colonizing other solar systems will make the human species impervious to any local disasters, gradual or sudden.

    Let’s not forget that robotic missions are a possibility for interstellar exploration. We don’t have to send humans to find out how things are in the rest of the universe!

    “If we are to be confined to Einstein’s brilliant physics, we are fated to sit here and intelligently record our planet side demise.”

    I don’t think of Einstein’s brilliant physics as being a trap. Relativity gave us the equivalence of mass and energy and time dilation, two potentially liberating concepts for interstellar travel. And brilliant as he was, Einstein didn’t have all the pieces of the puzzle. I don’t know what, if any, breakthroughs might come out of new developments in physics, but nobody can say for certain that there aren’t any.

    All our current propulsion concepts came out of a development in physics at some point. Maxwell’s theory of electromagnetic radiation predicted that light had pressure, which led to the light sail. The unexpected discovery of radioactivity eventually led to the discovery of nuclear energy and the nuclear rocket. Dirac predicted the existence of antimatter, and was confirmed when the positron was discovered. Without this, the antimatter rocket wouldn’t be a concept. Einstein theoretically predicted stimulated emission, which eventually led to the invention of the laser. A number of propulsion proposals depend upon lasers. Pair lasers with lightsails and you have Robert Forward’s laser-pushed lightsail. The list could go on.

    We’ve already have breakthroughs in propulsion physics, and quite a number of them!

  • Adam November 22, 2015, 2:00

    The unstated assumption is that all the action worth visiting is around stars. However microlensing data suggests thousands of planet-sized objects between the stars, putting them at 1/10 the distance of the stars. Or less. With a bit of effort, humanity (and any +H up-grades) could colonize thousands of new worlds before we have to worry about Alpha Centauri.

  • Christopher Phoenix November 22, 2015, 2:07

    @Alex Tolley

    We shouldn’t forget that we all tend to make such cases, especially where it confirms a bias. Mind uploading for the journey? – impossible say some, even though it seems to not violate any physics. AI’s instead of humans? – many hate the idea that humans will not be the Earth’s emissaries to the stars, but that our machine descendants may be.

    Hey, nice to see you round Alex! Yes, it is easy to allow personal bias to decide the issue for you and then build an argument with the aim of supporting your views. Feeling has a tendency to trump intelligence.

    Re mind uploading, I still haven’t seen much to convince me it is a near future technology. We still don’t know exactly how consciousness arises from the brain. How can we hope to reconstruct a brain if we don’t even understand it? A great many transhumanist arguments are built on the comparison between a brain and a computer, but that analogy is deeply flawed.

    A brain doesn’t start as blank chassis that passively accepts programs. We don’t program children in school with lines of code. Instead, humans learn by experience and repetition. We have intelligence and self-awareness. A computer never manages to do more but passively follow instructions we give it. And so forth.

    Also, the human brain isn’t separate from the body, but exists as part of the entire system. Without regular feedback from the rest of the body, a human brain would probably go mad from a whole body form of phantom limb pain. So if we emulate an entire mind/brain, we better also emulate the experience of having a body!

    Let’s say that we someday do figure out how to create “brain emulations”. We upload your brain. But here there is a snag for people pursuing personal immortality. Your “upload” cannot be a continuation of your sense of self. From the moment it is created, it is a separate individual growing steadily away from your original mind.

    Perhaps it will be easier to teach an AI to copy the mannerisms and personality of a person. However, even if this emulation manages to convince us it is the original person, it certainly isn’t the original. Does it even have awareness and understanding of what it is doing? Remember the Chinese room thought experiment.

    Getting down to the nuts and bolts, no one has explained exactly how we are going to hack into the workings of the brain to recreate a human mind and personality in the first place. Hopefully such a process won’t involve destroying the original brain.

    Mind uploading is an interesting idea. However, I want to see more coherent ideas about how exactly to do it before I’ll say I’m convinced. Just dropping the word “Singularity” (Rapture of the nerds?) isn’t enough.

    Mind uploading will certainly raise all sorts of interesting questions for society if/when this technology is developed. If grandpa has an emulation made before he dies, how do we explain the meaning of death to his grandchildren? What about the original mind? Grandpa’s brain isn’t in the computer box. It’s in the ground. Do we accept the emulation as being grandpa? Or is it simply a locked Chinese room? Can children understand the difference? What if we now have a society where people die but their thoughts and personalities can go on? You died, but everyone else can talk to your emulation. We may find that we discovered immortality for everyone but the original person.

    Perhaps, however, all this is irrelevant to the issue of space travel. Maybe mind uploading won’t be way to secure immortality, but a way to create an AI patterned on a human mind. An AI with prior human knowledge and experience could be useful for a starprobe.

    Re AI, AI is fascinating, and whatever direction artificial intelligence takes in the future, I am sure that it will have important implications for space travel. Adaptable AI is a must for future robotic interstellar missions. I still hope humans will make the trip too, though!

  • Michael November 22, 2015, 4:40

    @Alex Tolley November 21, 2015 at 18:41

    ‘@Michael – I see the fuel pellets working for the outbound acceleration, but I have difficulty with seeing how the pellet stream can be used to decelerate the ship. (It isn’t a matter of just reversing the thrust direction). ‘

    Unfortunately pellets don’t work but fuel pellets attached to a light accelerated sail will.

    When the craft approaches the target Star the craft turns around to face where the stream of sails is coming from. Now earlier another stream of sails was sent out after the craft has attained its maximum speed from the first stream. Each later sail with fuel has less velocity than the one in front but more than the craft after it gets fused in the sequence so the craft decelerates to a lower velocity and now the next sail arrives as it is now faster than the craft so on and so on. There is no need to store the fuel and recreate the rocket equation issue by gather mass/fuel to slow it down.

    The ability to control the timing of the stream of sails is crucial, no mean feat. Headway in controlling the stabilisation of the sail has already been made, https://www.centauri-dreams.org/?p=33450 Now by manipulating the surface of this cone which would be a lot easier than a flat disc we could have a greater control to keep the sail on beam.

    @PW November 21, 2015 at 20:45

    ‘If we are to be confined to Einstein’s brilliant physics, we are fated to sit here and intelligently record our planet side demise.’

    Einstein physics describes matter and energy in space-time, it does not explain what space-time is. Everything that has mass and energy is limited by the speed of light c, but space-time itself may move a lot quicker. We don’t have viable equations that can describe both the quantum world AND the relative one at the moment. Perhaps by the melding of the two it would describe space-time itself and the effect it has on the material universe would allow us greater freedom of actions.

    Don’t give up yet!

  • BRS November 22, 2015, 8:38

    I think the ethical issues of the “generation ship” will never be relevant because solving human aging is a much simpler problem than traveling between stars.

    The ageless people who undertake such journeys may or may not avail themselves of cryonic suspension. I suspect not, because virtual reality environments can provide endless amusement, stimulation and novelty. Not many people would choose a long sleep over absolute creative freedom and every kind of pleasant pastime imaginable.

    Having offspring probably won’t be high on their priority list. That’s something people do when the years start catching up to them.

  • Al Jackson November 22, 2015, 12:36

    As a bit of trivia, I can’t remember from the 1940’s or 1950’s an American Cereal Box that has space flight or Rockets on it*. Could be, even in the 40’s and 50’s there was a lot of cereal boxes, tho, lord knows not as many as are on the shelves these days.
    Maybe in England?

    I mean I can’t even remember Buck Rogers or Flash Gordon on cereal boxes.
    Though I did have a 1947 Lone Ranger Atomic Bomb Ring with I was 7 years old! Lost it. Dang!

  • Alex Tolley November 22, 2015, 13:00

    @Michael – Thank you. I see how the combination or fuel and sail works. Are there any published papers on this that I can study?

  • Alex Tolley November 22, 2015, 13:05

    @BRS – Poul Anderson wrote about exactly this in “The Boat of a Million Years” (1989). Last section is relevant.

  • Alex Tolley November 22, 2015, 13:23


    If grandpa has an emulation made before he dies, how do we explain the meaning of death to his grandchildren?

    McDevitt’s “Alex Benedict” novels have simulations of live and dead people. In the immediate term, there are proposals to use online entries as the source for simulating social media posts. Can you hear Grandpa now?

  • Eniac November 22, 2015, 15:22

    Alex Tolley:

    We shouldn’t forget that we all tend to make such cases, especially where it confirms a bias. Mind uploading for the journey? – impossible say some, even though it seems to not violate any physics. AI’s instead of humans? – many hate the idea that humans will not be the Earth’s emissaries to the stars, but that our machine descendants may be.

    Well said, Alex!

    I hope Christopher won’t mind when I point out that his objections to cryogenics and mind uploading seem to include some of the same logic as that which Purcell is being criticized for.

    People commonly postulate that “the mind is not just a computer”, and then derive from that certainty that personality or consciousness or intelligence will never be ported to machinery. They do not, usually, consider that the original postulate is unprovable and, borrowing the words of Pauli, “not even wrong”. Most often the postulate is supported by statements such as “A computer never manages to do more but passively follow instructions we give it”, which are both trivially true and irrelevant.

    Another common fallacy, which is very close to Purcells straw-man argument, is to note how incredibly complicated the human brain appears to be and that a full and accurate simulation of it can be ruled out of the question. This argument implicitly assumes that a full simulation at the neuronal or even atomic level is required, and that there are not higher order abstractions that can yield the desired result with far less massive computations. Similar to Purcell’s “requirement” of 99% of c and a return voyage.

    We still don’t know exactly how consciousness arises from the brain. How can we hope to reconstruct a brain if we don’t even understand it?

    Very good point. This is the crux of the issue, and it should be obvious that the route to progress in AI will not be in simulating neurons, but in understanding the mind. However, that we currently don’t understand enough about it does not mean that we never will. Or even, that we might not do so in as little as a decade.

    The mind is composed of perceptions, feelings, thoughts, and so on, not neurons. It is this domain that we have to think about to try to understand the mind. Psychology is the relevant biological science here, more so than neuroscience. I would absolutely expect that once we understand better how the mind works on the level of thoughts and feelings, applications will be forthcoming that result in artificial minds that are comparable to ours in all the relevant aspects: reasoning, intelligence, self-awareness, and so on. Perhaps including minds that are so closely modeled on existing ones that they are, for all intents and purposes, “uploaded”.

    As an aside, I also expect that the amount of computing power required to this will be laughable compared to what some postulate, and that such minds will be able to run on future cell-phones.

    Just to list a few more often heard arguments which can easily be refuted by the most rudimentary understanding of psychiatry: The “going crazy without a body” argument is quickly refuted by pointing to sane quadriplegics, the one about “going crazy without integrated sensory perception” will be objected to by the many sane people who are blind or deaf, the one about the “absolute accuracy in copying” or “failure to retain identity” is quickly refuted by stroke or accident victims insisting on being the same person after losing a substantial part of their brains. And so on.

  • Eniac November 22, 2015, 15:26

    Let me be clear that I find Christophers post to be exceedingly well-reasoned and interesting, in other words: Awesome. I do not wish the quibbles in my previous reply to detract from this…

  • Eniac November 22, 2015, 15:31


    Each later sail with fuel has less velocity than the one in front but more than the craft after it gets fused in the sequence so the craft decelerates to a lower velocity and now the next sail arrives as it is now faster than the craft so on and so on.

    How slow do the decelaration sails/pellets get? It seems to me that you could easily get into a situation where you need to have sent them millenia ago, which kind of defeats the purpose of a fast journey.

  • Ben November 22, 2015, 15:52

    I think the reason Purcell made these arguments is the same reason people fight over space research and development now: the finite pool of funding. When you’re desperate for research funds, the easy thing to do is to tear down the ideas of the competition. The trouble is, this doesn’t make it any more likely that you will win. The funding agencies could decide “Great point. Let’s fund something else,” and send the money back to more established programs that aren’t pushing the boundaries. Just because you don’t find the ideas of the competition compelling, doesn’t mean they don’t have value. I see this kind of thing in the advanced propulsion field, but the truth is the different ideas are complementary and help us go farther, and establish a more lasting human presence in the solar system and beyond. For example, missions to Mars or the gas giants could benefit from a combination of solar sails, ion engines, aerocapture, electrodynamic tethers, and nuclear propulsion by operating at different phases of the mission where each works best. Of course, where an idea has real and specific problem, by all means we should be openly skeptical. But, asserting that an idea is bad based on strawman arguments doesn’t help any of us.

    Rather, we should keep our competition for existing resources civil, and work together to grow human activities and presence in space, whether by proxy or in person. That way, we will grow the opportunities and resources for all of our ideas. For example, if there were multiple commercial space stations operating in Earth orbit, it would be far easier for any R&D project to get time in space to try out their ideas, from interstellar propulsion technology to SETI listening devices. By the time we launch the first interstellar voyages, I expect the SETI community to have more access to space for building and operating listening (and transmitting) station than they know what to do with.

  • Eniac November 22, 2015, 15:57

    Christopher Phoenix:

    As for inbreeding, the initial crew size will have been large enough to avoid this. Studies have already been done to determine the necessary crew size.

    As is pointed out often, but apparently not often enough: The problem of inbreeding is most easily addressed by taking frozen gametes (e.g. sperm bank and/or egg repository). It therefore need not be a constraint on the ideal population size.

    In general, I agree with those here pointing to cryogenics as a much more sensible solution than generation ships, and with those who point to isolated communities here on Earth as examples that there is no evidence that confinement to a spatially limited society has to drive people crazy. The human psyche is much more robust than some give it credit for.

    Most importantly, though, I think the initial wave of exploration will certainly be robotic. Whether or not human bodies and/or minds participate in that expansion, later, has little bearing on what it means for our knowledge about the universe and our place in it. Whether or not you believe our machines will replace us, they will certainly precede us to the stars. Within our own solar system, they have already done so.

  • Charlie November 22, 2015, 16:02

    It just dawned on me that if you can get a ship up to 99% the speed of light. There would be absolutely no need for a generational ships whatsoever (barring the possibility that you would be going to galaxies fourteen billion light years away, that is at the edge of the universe).
    All ships that are moving at that high-speed will experience time dilation and even the most reasonably close stars will be able to be accessible in the lifetime of the astronauts so generational ships will be in no starter.

    I do take issue however with anyone who would say that traveling in a generational ships could be a rather pleasant little experience. I seriously doubt that there would be people on board who would be joyously happy to be in this situation. It just seems inevitable that somebody would be unhappy and that would fester in their souls. As was said, there are people who live in small tiny villages on earth and can handle that, very, very well. Of course they can handle it very, very well because they know for a fact that whenever they wish they can leave that situation for a while – and later on, go back to their village refreshed and ready to go. NOT SO ON A STARSHIP. There you are, and you are stuck with it no matter what may be your personal feelings. In the book “Aurora” there was the also added factor that various important materials such as elements could become dispersed among the structural elements and failed to be recycled. Keeping material and energy balances would probably be far, far harder than we can easily imagine.