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Fritz Zwicky On Moving Stars

The great Ukrainian mathematician Israil Moiseyevich Gelfand was famous for his weekly seminars in Moscow, where sudden switches in topics and impromptu presentations were the norm. Although his listeners had heard it many times, Gelfand liked to tell this story: In the early 20th Century, a man approaches a physicist at a party and says he can’t understand how the new wireless telegraphy works. How is it possible to send a signal without using wires?

The physicist tells him it is simple. “To understand wireless telegraphy, you must first understand how the wired telegraph system works. Imagine a dog with its head in London and its tail in Paris. You yank the tail in Paris and the head in London barks. That is wired telegraphy. Wireless telegraphy is the same thing except without the dog.”

It always got Gelfand a laugh, but he liked to use the story for a deeper purpose. According to Edward Frenkel, who in his youth attended and presented at some of Gelfand’s seminars, Gelfand would use the tale whenever he thought a problem needed a more radical solution than anyone had proposed. “What I am trying to say,” he invariably added, “is that we need to do it without the dog.” Read Frenkel’s charming Love & Math: The Heart of Hidden Reality (Basic Books, 2013) for a look at Soviet-era mathematicians and the world they worked in.

I’m thinking that if there was ever a man who worked without the dog, it was Fritz Zwicky (1898-1974). The name of the Bulgarian-born physicist who spent his career in the United States inevitably came up in the light of our discussions on moving entire stars. This was a man with a deeply independent mind whose six-volume catalog of 30,000 galaxies, based on the Palomar Observatory Sky Survey, remains a touchstone in the study of galaxy clusters. But he was also a thorn in the side of many astronomers, routinely disparaging their work and their characters.

Zwicky 2

If a combative colleague — he coined the term ‘spherical bastards’ to describe his fellow astronomers, who he said were bastards from any angle you chose to observe them — Zwicky was kind to students, university administrators and people outside his profession. He was a man who liked to think big. Zwicky broached the subject of what we might call ‘stellar propulsion’ in a May, 1948 lecture at Oxford University, where he said there was a possibility of:

“…accelerating…[the Sun] to higher speeds, for instance 1000 km/s directed toward Alpha Centauri A in whose neighborhood our descendants then might arrive a thousand years hence. [This one-way trip] could be realized through the action of nuclear fusion jets, using the matter constituting the Sun and the planets as nuclear propellants.”

Zwicky’s lecture was published later that year in The Observatory (68:121-143). In a June, 1961 article in Engineering and Science called “The March Into Inner and Outer Space,” he followed up on the idea, although as before only in broad terms shorn of detail. In Zwicky’s view, a journey to the stars should not necessarily demand leaving the Earth behind. Instead, accelerate the Sun, letting it pull the planets along with it, and you maintain your own environment on the most comfortable of all generation ships. As to how to do it:

In order to exert the necessary thrust on the sun, nuclear fusion reactions could be ignited locally in the sun’s material, causing the ejection of enormously high-speed jets. The necessary nuclear fusion can probably best be ignited through the use of ultrafast particles being shot at the sun. To date there are at least two promising prospects for producing particles of colloidal size with velocities of a thousand kilometers per second or more. Such particles, when impinging on solids, liquids, or dense gases, will generate temperatures of one hundred million degrees Kelvin or higher-quite sufficient to ignite nuclear fusion. The two possibilities for nuclear fusion ignition which I have in mind do not make use of any ideas related to plasmas, and to their constriction and acceleration in electric and magnetic fields.

Zwicky would amplify his stellar propulsion ideas in his book Discovery, Invention, Research through the Morphological Approach (Macmillan, 1969), where he described how these directed exhaust jets would accelerate the Sun to a velocity sufficient to reach Alpha Centauri in about fifty human generations. Left for the reader’s imagination is the question of how a moving Solar System would decelerate once it arrived in the vicinity of the Alpha Centauri stars, presumably to join them to form a new triple (or quadruple, counting Proxima Centauri) star system.

We’ve seen how Leonid Shkadov conceived of wrapping a thruster around a star in such a way as to create propulsive forces, an idea now explored in the Greg Benford and Larry Niven novels Bowl of Heaven and Shipstar. But it’s clear that a star journey via a propulsive Sun is yet another idea that Zwicky had early, although he never went ahead to work out all the ramifications. In addition to manipulating the Sun’s own fusion, Zwicky’s Engineering and Science article described other broad concepts: The benefits of taming fusion for power and rocket propulsion, the need for a human presence in nearby space, particularly the Moon, and the possibility of using what he called ‘terrajet engines’ to burrow into the interior of the Earth.

In tribute to Zwicky, it’s necessary to mention several of his insights, including the notion that cosmic rays are produced in the explosion of massive stars which he began, in 1931 lectures, to describe as ‘supernovae,’ as opposed to the more common and less powerful novae. Working with Mount Wilson Observatory astronomer Walter Baade, he went on to extrapolate the creation of neutron stars that were dense as atomic nuclei but only a few kilometers in diameter, an idea that was met with skepticism.

The Palomar Observatory Sky Survey grew out of Zwicky’s work, and Zwicky himself discovered 122 supernovae. But his work hardly ended with exploding stars. Investigating the Coma cluster of galaxies, he worked out that the mass of the cluster was far too little to produce the gravitational forces needed to keep the cluster together. Unseen matter — and he coined the term ‘dark matter’ — must be making up the difference in mass. How to investigate the idea? Zwicky suggested gravitational lensing, now a common technique but a novel solution in his day.

In his dark matter work in particular, Zwicky can be said to have done what Israil Moiseyevich Gelfand urged. He learned how to do it without the dog, to take an enormous conceptual leap into an answer that later observation would prove suggestive and worthy of intense follow-up. He was a stormy genius, an irritated, irritating treasure of a man.


Comments on this entry are closed.

  • James M. Essig July 2, 2014, 10:39

    Using Red Dwarfs as stellar engines would be awesome. With a life-time of up to 10 trillion years, a red dwarf solar system, planets, moons, and all upon reaching 1,000 km second could (0.00333)(10 EXP 13) light-years = 33.3 billion light-years which is over twice our current cosmic light-cone. Add space-time expansion into the mix and the final distance from the Milky Way is several additional orders of magnitude greater.

    Stellar engines is a topic I know little about but an intriguing one for me now that Paul has been doing lots of post as late on TZ-CD.

  • Tom Mazanec July 2, 2014, 13:05

    1000 km/sec reaches Alpa Centauri in one thousand years? It does not compute!

  • Andrew Palfreyman July 2, 2014, 16:08

    A directed particle beam using the sun as source is another possibility. The pointing problem can be traded off against profligacy because there’s so much stuff available to chuck.

  • Michael July 2, 2014, 16:15

    @James M. Essig July 2, 2014 at 10:39

    ‘Using Red Dwarfs as stellar engines would be awesome…’

    If particle beams are used to create fusion hot spots then any gas giant or brown dwarf will do.

  • djlactin July 3, 2014, 3:42

    @Tom: I calculate about 1300 y.

  • James M. Essig July 3, 2014, 7:09

    Michael, interesting prospect indeed. Brown dwarfs and gas giants would alleviate the solar flare and end of stellar life cycle red giant bloating associated with at least some stars.

  • Michael July 3, 2014, 9:48

    @djlactin July 3, 2014 at 3:42

    ‘I calculate about 1300 y.’

    It is a little less as the Star systems are coming towards each other at around 20km/s.

  • Frank Smith July 3, 2014, 13:50

    Wasn’t Zwicky the astronomer who, while having lunch in the prime focus cage of the Mt Wilson 100-inch telescope, dropped a sandwich on the mirror?

  • ljk July 7, 2014, 9:56

    Any red dwarfs detected that seem to be moving much faster than they normally should through the Milky Way galaxy?

    Just because red dwarfs may not be as friendly to developing native life as we hope does not mean they would not attract starfaring civilizations, especially due to their very long life spans.

    And red dwarfs may one day become more attractive places to live – in about one trillion years. Quoting from this page:


    “As they get older, red dwarfs increase in luminosity but never become red giants or become white dwarfs. If left unmodified, many red dwarfs will eventually increase in brightness until they become as bright as a sun-like star; for smaller dwarfs, this era will not occur for a trillion years or so. Eventually the oldest dwarfs will cease to shine and become large, helium enriched black dwarfs, slowly radiating their heat and contracting.”

  • Michael July 7, 2014, 10:27

    All stars eventually get faster as they encounter each other in their orbits around the Milky way. As for Red dwarfs I like the fact that they turn blue as they get much older!


    ‘Another interesting feature in Figure 2 is the track of the star with M = 0:16M . Near the end of its life, such a star experiences a long period of nearly constant luminosity, about one third of the solar value. This epoch of constant power lasts for nearly 5 Gyr, roughly the current age of the solar
    system and hence the time required for life to develop on Earth.’

    @ljk July 7, 2014 at 9:56

    ‘Just because red dwarfs may not be as friendly to developing native life as we hope does not mean they would not attract starfaring civilizations, especially due to their very long life spans. ‘

    They can also be used as powerful gravitational lenses with much shorter focal distances than our Sun.

  • Michael July 7, 2014, 13:12


    The URL address did not copy across as planned, it should be,


  • Michael July 7, 2014, 13:14

    It did it again, not sure what the problem is there, but they just need to copy the whole address into the search field.

  • Paul Gilster July 7, 2014, 13:33

    Not sure what the problem is there, Michael, and I can’t seem to reproduce it. But yes, the thing to do is to copy the entire link and then paste into a browser. Sorry about this and if I find the answer, I’ll post it.

  • Walter R. Johnson July 8, 2014, 10:52

    TO: Tom Mazanec

    RE: “1000 km/sec reaches Alp[h]a Centauri in one thousand years? It does not compute!”

    It actually comes out to about 1,400 years, assuming that Alpha Centauri is about 4.4 light-years from Earth.

  • ljk July 9, 2014, 12:04

    Moving the Earth

    by Robert Zubrin

    Monday, July 7, 2014

    The Sun is becoming about 10 percent hotter every billion years. If nothing is done to deal with this problem, our planet could become uninhabitable in just a few billion years!

    In just 500 years we will have enough power available to be able to undertake this project, with only one percent of our total power generating capacity needed to support the job.

    But what can we do? One suggestion might be to modify the Sun, to keep it from heating up. But no one has any idea of how to do that. Fortunately, there is an alternative plan which should be much more practical to implement; move the Earth.

    Our home planet, is after all, only about 1 millionth the mass of the Sun, much cooler, much closer, and thus, overall, much more readily available for manipulation. Furthermore, since solar heating falls as the square of the distance, to cope with a ten percent solar flux increase, we only have to increase the distance of the Earth by five percent. This will make things much easier.

    So let’s see what it would take to move the Earth outward from the Sun by five percent over the next billion years, thereby compensating for increased solar heating. A little bit of fancy math shows that to do this, a velocity change of 1,200 meters per second will need to be imparted to our home planet. That works out to an acceleration rate of 1.2 microns per second per year, or 3.8 x 10-14 m/s2.

    Now the mass of the Earth is 5.97 x1024 kilograms. So, force equals mass times acceleration, to get the thrust required to accelerate the Earth at the required rate, we just multiply the above two figures together and obtain a thrust of 2.27 x 1011 N, or 227 billion newtons. That’s really not that much, when you think about it: it’s the weight of a cube of water 284 meters on a side.

    So what kind of rocket could be used to generate that amount of thrust? A Saturn V had a first stage thrust of 33.4 million newtons, so thrusting together, 6,796 of them could do the job. Making that many rockets should not be a problem: the Germans produced more than 4,500 V-2’s during 1944 alone. Unfortunately, however, it’s not so simple. Because the average exhaust velocity of a Saturn V first stage is only about 3,000 meters per second, to generate a velocity change of 1,200 meters per second would require using about a third of the mass of the Earth as propellant—and that’s just for the first billion years of operation! Clearly we need to use a rocket with a higher exhaust velocity.

    Full article here:


    To quote:

    Now, as original as this discussion may seem, it has undoubtedly been entertained before. There are hundreds of millions of habitable planets in our galaxy alone, and the residents of nearly all of them are facing this very same problem. The laws of the universe are the same everywhere. As above, so below. If we are going to need to do this someday, many others elsewhere are probably already doing it now.

    Might it be possible for us to spot them? What would a 68-million-terawatt rocket exhaust look like, if pointed directly at us, by people trying to save their planet located in a star system many light years away?

    The power output of our Sun is about 3.85 x 1014 terawatts, or about 5.7 million times the power of our planet-moving photon rocket. Of course, the rocket will be focused to point just in one direction, so if we assume a gain of 1,000 in apparent power by such focusing, the photon rocket would be 1/5,700 times as bright as the Sun. That’s about a difference of nine stellar magnitudes. Now, if seen from 10 light years away, the Sun would be about a 2nd magnitude star, so our planet rocket would be 11th magnitude, and readily visible using a good amateur telescope. But there are only a few stellar systems within 10 light years, so we would have to be really lucky to spot one so close.

    However, there are over 12,000 stellar systems within 100 light years. That would drop the apparent magnitude of the planet-moving rocket to 16th, about the brightness of Pluto’s moon Charon as seen from the Earth. While beyond the capability of all but the most dedicated and well-equipped amateurs, there are many professional-grade telescopes that could spot such an object. Of course, this rocket flare would be positioned close to a star, which would make it harder to spot, but it would still be thousands of times brighter than an Earth-like planet or even a Jupiter-like planet as seen from interstellar distances, so if we can spot one of those, we should be able to spot one of these.

    The trick, however, will be to catch it when it is pointing at us, which will only be for a brief period of time during each orbit, after which we will have to wait a whole planetary year to catch it again and prove reproducibility of the event. But with enough time and effort, it should be possible.