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Deep Time: Targeting Another Galaxy

Interstellar flight isn’t about possibility as much as it is about time. We know we can launch a payload to another star if we’re willing to burn up enough millennia — about seventy — to get there in the form of a Voyager-style flyby. That’s with today’s technology, and we can extrapolate how the time frame can be shortened with improved materials and propulsion techniques. So as Robert Forward always pointed out, it’s not that interstellar flight is impossible — it’s that it’s very difficult, and our expectations of the kinds of missions possible have to adapt to that fact.

Intergalactic flight, though, is such an immense undertaking that I’ve rarely discussed it in these pages. Is there any conceivable technology that might make such a thing possible? Well, Carl Sagan and Iosif S. Shklovskii considered the situation in Intelligent Life in the Universe (Holden-Day, 1966), working with the opportunity for time dilation opened up by special relativity. Accelerate at 1 g continuously and time dilation helps you to reach nearby stars in just a few years as time is measured aboard your spacecraft.

The numbers get more and more mind-boggling as you continue to work the equations. With that same 1 g acceleration (and just how you achieve that is of course the grand question), you can make it all the way to the center of the Milky Way in about 21 years — tens of thousands of years would have passed on Earth by the time you arrived at the galactic core. Or go for the ultimate journey: A voyage to the Andromeda galaxy. To reach M31 in a ship of this sort would take 28 years ship-time as you nudged ever closer, but never reached, the speed of light. As always, I point to Poul Anderson’s Tau Zero as the novelistic embodiment of Sagan and Shklovskii’s musings, as fresh today as when it ran in Galaxy in 1967.


Image: M31, the closest major galaxy to our own. Continuous acceleration at 1 g would allow a human crew to reach it, but what conceivable technology would allow such a craft to be built? Image credit & copyright: Lorenzo Comolli.

Yesterday Adam Crowl mentioned Martyn Fogg’s paper “The Feasibility of Intergalactic Colonisation and Its Relevance to SETI,” in the context of Robin Spivey’s ideas on galactic migration. Fogg runs through the possibilities for intergalactic journeys including constant 1 g acceleration of the kind Sagan and Shklovskii talk about. Robert Bussard had proposed in 1960 that this kind of acceleration could be achieved through a fusion-powered ramjet design that fed off the interstellar medium. It would be a tough engine to light but once you got the ship moving fast enough, a self-sustaining reaction seemed to be possible, and it’s more or less the concept that Anderson used in Tau Zero for his starship.

But Fogg nails the problem with the Bussard ramjet, or I should say, the problems, there being several intractable issues to be faced. Robert Zubrin and Dana Andrews have shown that as the ramjet accelerates, the vast electromagnetic ‘scoop’ being used to collect fusion fuel actually begins to act as a brake. See Starships: The Problem of Arrival for more. And as early as 1972, A. R. Martin had discussed structural limitations that would prevent such a ship from sustaining accelerations this close to the speed of light.

Surmount even these difficulties and you run into perhaps the biggest showstopper. Gas density between the galaxies is, in Fogg’s estimate, 10-5 lower than what the ramjet would encounter within a galaxy. Gaining enough fuel, then, would be the challenge, and as we’ve learned more about the interstellar medium, it seems clear that a ramjet would have problems even within the Milky Way depending on the local density of interstellar material. That leaves only David Froning’s suggestion of a ‘quantum ramjet’ using quantum fluctuations of the vacuum, an idea that would assumedly not be constrained by the Bussard ramjet’s fuel problems.

Is there any other way to save the idea of intergalactic travel close to c? Fogg doesn’t think so. From his paper:

…irrespective of the propulsion system used, hyper-relativistic intergalactic travel would be fundamentally limited. The velocity of the ship at midpoint would be so close to c that no interaction with intergalactic matter would be permitted. Dust grains would impact like cannon shells and hydrogen atoms would take on the characteristics of a lethal and penetrating form of cosmic radiation.Thus, leaving aside the feasibility of a “quantum drive”, the use of time dilation to significantly reduce elapsed intergalactic voyage times would not be practical unless a way could be found of preventing impacts from oncoming particles.

That’s a pretty good list of seemingly insurmountable problems, all generated by the fact that we’re trying to get the crew that set out from Earth all the way to its destination. Give up on that constraint and a number of other possibilities open up that make us weigh interstellar and intergalactic flight against our concepts of deep time and the lifetime of civilizations. Sagan and Shklovskii didn’t pin their entire argument on Bussard-style craft. They also explored the potential of human hibernation on journeys lasting thousands of years. But there are other possibilities, some of them discussed by Adam Crowl yesterday, that I want to explore tomorrow.

The Fogg paper is “The Feasibility of Intergalactic Colonisation and its Relevance to SETI,” Journal of the British Interplanetary Society Vol. 41 (1988), pp. 491-496 (available online). Robert Bussard’s ramjet paper is “Galactic Matter and Interstellar Spaceflight,” Astronautica Acta 6 (1960), pp. 179-194. Andrews and Zubrin’s paper on the Bussard ramjet and drag is “Magnetic Sails and Interstellar Travel,” International Astronautical Federation Paper IAF-88-5533 (Bangalore, India, October 1988), although I’ll also point you to Zubrin’s Entering Space: Creating a Spacefaring Civilization (New York: Tarcher/Putnam, 1999). The Froning paper is “Propulsion requirements for a quantum interstelllar ramjet,”JBIS, 33, 265-270(1980).


Comments on this entry are closed.

  • James D. Stilwell June 24, 2014, 12:42

    Deep Time: Targeting Another Galaxy
    by Paul Gilster on June 24, 2014

    An excellent post…
    The Bell Theorem…questions…
    It might be just information sent…
    Yet isn’t this the nature of “Q”…
    And the transporter…

  • Michael June 24, 2014, 13:27

    We could wait 4 billion years or so for Andromeda to collide with us or hitch a ride on a hypervelocity star, right mass/age, going in the right direction. The stars magnetic field and solar wind should give ample protection against cosmic radiation as well as heat and light. The thing we don’t need to go 1 g all the time, when the crew sleep or laying down, they could handle higher g forces, 4-5 g for 8 hours during sleep, might need forced breathing though (iron lung).

  • RobFlores June 24, 2014, 13:59

    Once you set your mind to cross to another galaxy.
    Would it not be more compelling to travel to one outside the Local Group
    What about a target such as M74, about 31 million miles. It is a well behaved
    galaxy with a recognizable habitable zone if you want to colonize After all there is no extra propulsion cost, once you get up to speed on your vessel.

    What would be a prudent speed to send such a vessel. I don’t think you want
    to mess with speeds of more than .4C due to the need to scan ahead for
    debris than can do damage. It also saves fuel to go slower and gives you
    more options for vector changes once closer to your initial nominal destination. 20% C seems technologically feasible to me. This will make
    the trip last 135 MYa. The reason you need to wait 1,000 Yrs, is to perfect
    your automation to maintain the ship and biologicals. None of your crew will be awake during period of travel in intergalactic space Even having a rotating crew of 6 would consume too much of the ships endurance capacity over 135 million years. Apprehensiveness about getting into a coffin and
    not knowing if there is a end to the sleep (in a good way) is understandable.
    Once a target planet is located Automated units will make the surface
    fertile and the infrastructure for 50,000 colonists is ready , then the
    adventure would begin. Back in your home galaxy, Maybe galactic
    empires will risen and fallen, by the time you arrive to paradise.

    The main reason this might be attempted is a blood feud within
    humanity, and the need for one party to depart to parts unknown before
    they are totally captured, conquered or wiped out. (see TOS Space Seed)

  • DavidJ June 24, 2014, 14:38

    Why always so much focus on c? If there are problems ‘near c‘ then aim for e.g. half c, and we can still colonize star systems within e.g. 50 light years of earth.

  • DavidJ June 24, 2014, 14:46

    Sorry, I realize my question seems a bit stupid in some sense. Obviously the possibilities for travel close to light speed are enticing, and confers benefits of slowed aging. I guess I just meant it more as a counter to what seems to sometimes be a sort of pessimistic attitude of ‘if we can’t get close to ‘c’ interstellar travel isn’t feasible’ …. I think in the near term it may be helpful to focus on what we can achieve, and by the time we’ve reached the nearest stars, we’ll then also have a better idea of whether speeds close to ‘c’ are attainable or not, as our knowledge will have advanced in the meantime, and it will be less ‘guesswork’.

  • swage June 24, 2014, 15:32

    ‘hitchhiking’ high velocity stars, as suggested from the last entry seems an intriguing idea to me, if you can preserve life long enough. There are basically two options: spread so much that you get every ride available and bet on probabilities and chance or selective, thats determining which star to use to get somewhere else. But the important thing to realize is that ‘velocity’ and ‘time’ are interchangeable values. I think ‘time’ is the easier road, but i am all up for a big surprise from Mr. Harold White and alike.

    There are organisms on earth which can hibernate for millions of years – maybe one or multiple orders of magnitude more than that. Technically this is totally impossible: there is radiative decay of certain key elements in the organisms structure… but still… testing is producing the most phenomenal data in that regard. We are just beginning to decipher how that works… molecular acid which encloses genetic information, hardening it versus double strand breaks and so on… and these things come back to life in the most unlikely of circumstances. Here is something already working which we can try to understand.

  • WILLIAM June 24, 2014, 20:31

    These questions that are stated here bring up in my mind certain ideas that have been turned over and over by myself but for me they are without answer. Perhaps someone here might wish to take a swing at it and give it a try.

    Although I am a firm believer in physical science I’ve often wondered if there’s any way to lower the energy bill that we know arises from classical physics. Specifically what I’m asking is does the kinetic energy of an object ALWAYS need to be one half of the mass times the square of the velocity? Does anyone think that there may be perhaps a revolutionary physics that would permit the energy needed to be less than that defined quantity?

    I also wish to say that the story tau zero is a truly wonderful story I’d love to see it made into a big screen movie under the direction of a competent movie director.

  • Heath Rezabek June 24, 2014, 20:34

    Wanted to note a related resource: “Eternity in six hours: intergalactic spreading of intelligent life and sharpening the Fermi paradox”


    “Abstract: The Fermi paradox is the discrepancy between the strong likelihood of alien intelligent life emerging (under a wide variety of assumptions), and the absence of any visible evidence for such emergence. In this paper, we extend the Fermi paradox to not only life in this galaxy, but to other galaxies as well. We do this by demonstrating that traveling between galaxies { indeed even launching a colonisation project for the entire reachable universe { is a relatively simple task for a star-spanning civilization, requiring modest amounts of energy and resources. We start by demonstrating that humanity itself could likely accomplish such a colonisation project in the foreseeable future, should we want to, and then demonstrate that there are millions of galaxies that could have reached us by now, using similar methods. This results in a considerable sharpening of the Fermi paradox.”

  • xcalibur June 24, 2014, 22:42

    It seems to me that theoretical technologies of manipulating space-time (foldspace etc.) are the most feasible method of intergalactic travel. Of course, the energy requirements and engineering challenges of this are massive.

    I’m more optimistic about interstellar travel within our galaxy by accelerating to significant fractions of c.

  • Brian Swiderski June 25, 2014, 1:51

    If we don’t have warp drive by the time the Milky Way starts feeling crowded, or haven’t transcended into immortal beings who sneeze at million-year transit times, we must not have been trying very hard. We are talking basically about 100,000 to 1 million years from now. A million years ago “we” weren’t even us yet – we were more or less chimps. And that was unguided biological evolution without conscious STEM functionality. The us of Galactic Civilization will resemble our present nature about as closely as we resemble mollusks.

  • Michael June 25, 2014, 3:09

    ‘Is there any other way to save the idea of intergalactic travel close to c? Fogg doesn’t think so. From his paper:

    The velocity of the ship at midpoint would be so close to c that no interaction with intergalactic matter would be permitted. Dust grains would impact like cannon shells and hydrogen atoms would take on the characteristics of a lethal and penetrating form of cosmic radiation. Thus, leaving aside the feasibility of a “quantum drive”, the use of time dilation to significantly reduce elapsed intergalactic voyage times would not be practical unless a way could be found of preventing impacts from oncoming particles.’

    Dust and gas can be encouraged to move out of the way by the use of a particle beam emanating from the front of the craft. As you increase the velocity of the craft the energy is also added to the beam from the front. The nice thing is that we can use many types of atoms, heavy proton rich atoms would be helpful in creating a tunnel or shock front ahead of the craft when the atoms breakup through collisions with the on coming material. Also due to the very high velocity of the charged particles they would have the effect of ionising the neutral gas and dust as they move past them.

    A while back I was thinging of the idea of using the exhaust gases from a fusion or fission drive engine been directed towards the front through the use of a narrow radiation channel at the appex of the exhaust chamber. The uncharged particles, light and neutrons would be internally reflected along this tube and out the front to act as the active shield. The channel could also be charged and a magnetic pinch control system that would allow some of the charged contents down the tube as well could aid the active shield. A bonus would be that the channel would be mass neutral, structurally, and could be connected easily through a multi-staged craft as it working principle is based on internal reflection.

    I can’t see dust/gas been a show stopper at high velocities.

  • Michael Spencer June 25, 2014, 6:38

    Say you are standing on a planet in Andromeda. Aside from knowing where you are, there’s no sensibility that you are actually in a different galaxy. You could be standing upon one of the billions of planets inside our own galaxy.

    We find ourselves in a truly huge place. I wonder if we will ever grasp the immensity of it? I’ve just finished reading (listening to) Greene’s “The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos”. He managed to convey the true spatial dimensions in a new and interesting and terrifying way.

  • John Hartley June 25, 2014, 9:14

    One way to stimulate new thinking about a problem is to exaggerate it into a much larger problem that can’t be solved through variations of known solutions. Going to the moon? Build a rocket. Mars? A bigger rocket! Nearby stars? A really big rocket!! Andromeda? How about visiting the Hubble deep field? Whatever the answer is (if there is one…), it’s probably not a rocket (Tau Zero and the runaway ramjet excepted). Sometimes blowing a problem up into the realm of the “impossible” opens up the imagination to explore ideas otherwise overlooked. How would you visit that faint smudge in the Hubble deep field?
    @StarshipBuilder on twitter

  • Al Jackson June 25, 2014, 10:41

    Looking through the few histories of modern prose science fiction I have does not turned up a history of intergalactic flight in SF… so I don’t know the earliest story.
    I do know that when I started reading SF in the early 50’s I was immediately educated about the difference between interplanetary and interstellar flight.
    A fact to still seems to be scrambled in the minds of Hollywood producers and writers. Star Trek was an excitement for SF fans since there were , only two , I think films, This Island Earth and Forbidden Planet that used interstellar flight and made the distinction, tho it very clunky in This Island Earth , but quite sophisticated in Forbidden Planet, why it was 10 years between Forbidden Planet and Star Trek always puzzled tru blu SF fans!

    I do remember me and my fellow SF fans in the early 1960’s being pleased with A for Andromeda Hoyle and Elliot 1962 , tho not spaceship intergalactic , ol clever Hoyle came up with an idea , not unlike ‘matter transmission’ that involved our closest ‘major’ galaxy. Oddly , this was originally a BBC TV show, it never played on any TV station I was around, so me and my friends thought it was just an original novel! (The Wikipedia article about is all about the original BBC production and two TV movie remakes , which I have also never seen.)
    A for Andromeda , like The Black Cloud, was the kind of ideas that one might have thought A.C. Clarke would have written, but was no surprise that Hoyle thought them up.
    To this day , as mind blowing as the plot ideas were, for someone used to reading Heinlein, Clarke, Asimov , Pohl, Sturgeon, …. so on… (at that time) Black Cloud and A for Andromeda came off a little bit like ‘Popular Mechanics’ SF , nowhere up to the standards of characterization and plot sophistication of Heinlein , say, to be read for the ideas and not the narrative.

    One notes that the Sagittarius Dwarf galaxy is only about 50,000 light-years from the Milky Way, Omega Centauri about 15,800 light-years. So far we have counted 11 dwarf galaxies orbiting the Milky Way. ~16 K light years sounds a little better than 2.5 million , tho not by much!

  • andy June 25, 2014, 12:39

    As for visiting another galaxy, maybe as a more feasible (though still extremely challenging) alternative, how about targetting Kapteyn’s star instead? It seems more within the realms of possibility…

  • kzb June 25, 2014, 13:26

    You could never visit at least some of the galaxies in the Hubble deep field: they’re receding from us at apparently superluminal velocities. So no matter how good you make your ramjet, there are still some places you could never go.

  • Michael June 25, 2014, 13:29

    @Michael Spencer June 25, 2014 at 6:38

    ‘We find ourselves in a truly huge place. I wonder if we will ever grasp the immensity of it?’

    That may be a problem, we are intimidated by the immenseness of Space, it can be paralysing.

  • Ross June 25, 2014, 15:06

    Regarding the comment by Fogg from his paper:

    …irrespective of the propulsion system used, hyper-relativistic intergalactic travel would be fundamentally limited. The velocity of the ship at midpoint would be so close to c that no interaction with intergalactic matter would be permitted. Dust grains would impact like cannon shells and hydrogen atoms would take on the characteristics of a lethal and penetrating form of cosmic radiation.Thus, leaving aside the feasibility of a “quantum drive”, the use of time dilation to significantly reduce elapsed intergalactic voyage times would not be practical unless a way could be found of preventing impacts from oncoming particles.

    Another paper was produced in 2006, by Dr. Franklin Felber with the comment:

    Felber’s research shows that any mass moving faster than 57.7 percent of the speed of light will gravitationally repel other masses lying within a narrow ‘antigravity beam’ in front of it. The closer a mass gets to the speed of light, the stronger its ‘antigravity beam’ becomes.

    From the link:


    If this is true, and we can somehow reach those high speeds, we might not have to worry about the impacts mentioned by Fogg. Was Felber’s research ever discounted? Why haven’t we heard more about it or him?

  • Paul Gilster June 25, 2014, 16:55

    Ross, my thoughts on Felber are here:


    and may explain why nothing much has come of this. In short, testing these ideas will require time and money, and to get the latter they have to be considered credible. Read the extended comment on the post I link to above by Ron S for a thorough look at the problems inherent in Felber’s work.

  • Rob Henry June 26, 2014, 1:31

    Al Jackson, I feel your being a bit unfair on Fred Hoyle’s science fiction writing ability. I feel that, not just his science, but his whole recreation and description of the scientific ‘atmosphere’ pertaining to the processes of discovery was superior to any other SF writer I have read.

  • Rob Henry June 26, 2014, 2:45

    Above, I should have also mentioned that Hoyle gave a reason for intergalactic travel in ‘The inferno’. Crucial to the plot is that a quasar has more potential to disrupt the galaxies magnetic field, and so has greater potential to shower the whole galaxy with a burst of charged particles than a much closer by supernova would. In the story Earth was streaming off atmosphere by what I later recognized as hydrodynamic flow. Anyhow, it made me wonder if the eventual merger of Andromeda’s and the Milky Way’s black holes might provide impetus to move.

  • kzb June 26, 2014, 7:32

    @Ross: another point is, does relativistic mass have gravity? In other words, as the ship gains mass as it approaches c, does its gravitational field increase? This seems to be a taxing question for the best of physicists when you try and research the question !

  • drs June 28, 2014, 2:40

    …seems to me the most feasible way of getting a crew to an arbitrary distance to have them be immortal or suspended. Engineering humans to be immortal or suspendable is hard but seems a lot easier to me than massive antimatter rockets, and doesn’t require wishful-thinking physics like warp drives.

    (‘immortal’ = indefinitely long lifespan given energy for repair and maintenance.)

  • Michael June 28, 2014, 13:10

    @drs June 28, 2014 at 2:40

    ‘(‘immortal’ = indefinitely long lifespan given energy for repair and maintenance.)’

    A person travelling at light speed still has the same life span as any other, maybe a little less due to radiation damage.

    Time just slows down for them.

  • drs June 28, 2014, 23:30

    What made you think I was talking about light speed? I was talking about biological longevity. Thought that was clear from “engineering humans”. (As I suspect you’ll have to mess with the genes to get really long lifespans, quite possibly as well as having outside medical interventions; engineering people to be easier to intervene with could also be part of it.)

    (Of course, with strong AI/intelligent robots, the longevity or suspension needn’t be biological.)

  • Michael June 29, 2014, 12:37

    @kzb June 26, 2014 at 7:32

    ‘another point is, does relativistic mass have gravity? In other words, as the ship gains mass as it approaches c, does its gravitational field increase? This seems to be a taxing question for the best of physicists when you try and research the question !

    I tried to answer this myself with my limited knowledge of relativity, I should have paid attention in class really. I thought it did as well but it looks as if it does not. I imagined a neutron star been loaded with mass and then collapsing to form a black hole. Now the falling mass is accelerated to great relativistic velocities and so you would think it would gain ‘mass’ from falling in but the black hole will still remain the same mass as what formed it.

    That is my thoughts on the matter. Anyone else have an idea how to explain it?

  • James M. Essig July 2, 2014, 15:43

    Perhaps another method of stellar engine propulsion would rely on efficient stellar light conversion to electrical power which might then power electrodynamic-hydrodynamic-plasma-drive systems where a solar or stellar lampshade type capture mechanism would do the power conversion.

    Electrodynamic methods as such may include one or more of the following: 1) electro-hydrodynamic-plasma-drives; 2) magneto-hydrodynamic-plasma-drives; 3) electro-magneto-hydrodynamic-plasma-drives; and 4) electromagnetic-hydrodynamic-plasma-drives.

    The reaction mass can include a fusion jet plasma originating from the sun and/or background plasma within the interstellar and intergalactic medium.

    The solar or stellar lamp-shade may also employ linear induction current generation which could then power electromagnets for field effect propulsion off of interstellar and intergalactic magnetic fields. Such a magnetic field effect mechanism is not ordinarily stable but can be made stable by numeric fine-scale adjustments.

    Interstellar ramjet options using the lamp-shade are also plausible. The very large diameter shade would provide an excellent mechanism for scooping up interstellar hydrogen and helium, perhaps also using the CNO bicycle fusion processes.

    Provided suitably relativistic velocities could be attained, then anisotropic CMBR which would be highly blue-shifted in front of the craft could be directly absorbed and converted into propulsive electrical power to operate electrical propulsion systems. The gamma factor of the starship would need to be rather high in order to obtain meaningful power input.

    While all of these propulsion modes and energy sources are being engaged, antimatter may be generated in the form of positrons perhaps embodied in positronium, or anti-hydrogen and anti-helium could be produced and used to heat concentrated portions of collected nuclear fusion fuel. The heat generated may be used to induce nuclear fusion throughout the concentrated fusion fuel species collected by the interstellar ramjet mode.

    The effective specific impulse of the system can be enhanced beyond that commonly contemplated by stellar engine theorists by extracting potential energy resources from the interstellar and intergalactic medium.

    There are likely other options for enhancing the effective specific impulse of stellar engines which have not yet been contemplated.

    One of my favorites is the use of suitably strong magnetic-fields to induce non-zero dipole moments in background electrically neutral atoms and molecules, and perhaps even atomic nuclei. The magnetic field intensities would need to be very high, and when distributed over a large volume, may manifest an impractical additional stellar sail inertial. However, the additional inertia may be reduced to negligible levels using creative field configurations.