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Cepheid Variables: A Galactic Internet?

Making contact with an extraterrestrial civilization, whether by microwave, laser or neutrino, highlights the problem of time. Suppose you are looking for a newly emerging technological culture around another star. When do you transmit? After all, even the most powerful signal sent to Earth a million years ago would have no listeners, which is why some have suggested putting actual artifacts in promising solar systems. Rather than transmitting over time-scales measured in eons, you leave an object that can be decoded and activated for communications. All kinds of interesting science and science fictional scenarios flow from that idea.

But what if you want to contact not just a few highly targeted systems, but instead send a signal intended for everyone in the galaxy with the means to receive it? As John Learned (University of Hawaii) and team speculate in a new paper, one way to do that would be to select highly visible and important stars to carry your message. Cepheid variables are a natural fit. Because of the clear relation they show between luminosity and variability, they’ve helped us figure out distances on a galactic scale, and have been crucial in our studies of the Hubble constant. As well as being important to science, they’re bright and easily observed, like the ‘water-hole frequencies’ an obvious place to search for signals.

Image: This NASA Hubble Space Telescope image of a region of the galaxy M100 shows a class of pulsating star called a Cepheid Variable. Though rare, these stars are reliable distance indicators to galaxies. Could they also be put to work as galactic communications beacons? Credit: Dr. Wendy L. Freedman, Observatories of the Carnegie Institution of Washington, and NASA.

Exactly how might a super-civilization modulate an entire star? The authors speculate about using neutrinos for the purpose:

Neutrinos would seem to be the ideal delivery means for transporting a pulse of energy to the stellar core, both due to penetration ability and to speed. If one wanted to employ infalling material as the trigger, this disturbance would propagate at near the local speed of sound, and might well evaporate prior to reaching the core in any event. Neutrinos of about 1 TeV would have an attenuation length of about 106 km at solar densities. We have not modeled Cepheids in order to determine the optimum neutrino energy, but it is irrelevant for the present discussion: we leave it as an engineering problem for the star tickling civilizations out there. The initial flavor mix of the neutrino beam makes little difference since oscillations will mix the flavors and in any event the cross-sections are flavor independent.

The Star Ticklers — it sounds like a 1950’s title, maybe from one of the Ace science fiction double paperbacks. And it’s a curiously provocative concept. Deeply interesting here is the supposition that, if a modulated signal were being broadcast through the use of Cepheid variables, we might already have evidence for it in the abundant data collected about these stars. Normally, Cepheids are studied through a small number of randomly spaced observations conducted over a long period of time. The authors run through the methods of data collection and analysis, and point to places where those methods might actually mask the presence of an interesting signal. And they suggest a way to pull a signal, assuming one is there, out of the noise. This is also worth quoting:

Thereafter the conclusive finding of an ETI signal would come about by identification of regularities which are hard to understand from any natural oscillator, such as involving repeated complex sequences, prime numbers, a limited “alphabet” (e.g. something akin to the familiar ASCII code), or even an apparent raster-like arrangement. At this point we cannot but fall back upon the supposition that we will recognize unnatural stellar fluctuations when we see them. In sum, we think the normal method of employing Fourier transforms (with windowing, gap filling, long term averaging, etc.) and other methods in common use to analyze the periodicity of Cepheids may cloud the discovery of intentional modulation, and that cycle-by-cycle periods need to be examined.

The prospect that existing records of Cepheids and possibly other variables might contain the signature of a galactic internet is an exciting one indeed. It’s also interesting to look at the mechanism involved in generating the signal. The neutrino trigger generator might consist of a power station perhaps 100 AU out, one that draws energy from the radiation of the Cepheid itself, accumulating it over the course of a complete cycle and then setting off the trigger pulse. An unstable stellar system becomes the universe’s biggest signal amplifier.

Anything that helps us with what the authors call the ‘needle in a haystack’ problem is all to the good. We want to find ways to narrow the SETI search in productive directions. Here Cepheids are useful because the entire galaxy contains only about 500 known examples. Giant yellow stars massing between five and ten times the mass of the Sun, with 103 to 104 times solar luminosity, Cepheids would be not only visible but highly studied by any civilizations trying to learn about the cosmos. That makes them a productive medium for the kind of culture that can muster the resources to impose a signal upon them.

The paper is Learned et al., “The Cepheid Galactic Internet” (abstract).

Comments on this entry are closed.

  • Dave September 8, 2008, 11:59

    I just wanted to say I love this blog! I started reading a few weeks ago. Keep up the great work!

  • Usman Saeed September 8, 2008, 12:38

    You know what, I like this. These schemes of communication seem to be in vogue lately.

  • Mark Phelps September 8, 2008, 13:49

    Hi Paul

    Great article and it reminded me of this book,
    ‘Talk of the Galaxy” Paul laViolette ,which has a similar view
    using pulsars….there is as yet poorly understood science behind
    a lot of the pulsar ‘signal ‘ and he jumps ahead and suggests that not
    only are pulsars good navigational beacons over galactic and intergalactic
    distances but are a ‘cheap’ way to broadcast a ‘message’. Cheap is a relative
    term but in a sense is a good place to look beside the water hole.Methinks too
    it might be a bit less difficult to manipulate,piggyback the output of a pulsar
    than that of a Cepheid Variable.(that is getting close put something in the beam
    path or more interestingly do something on the surface of the pulsar where the
    beam is generated. Stage 2 civilization work indeed.


    ps there is some fluff in the book but read past…ie ‘electrogravic what?..pass on..

  • Phil September 8, 2008, 14:45

    Cool… Very Unlikely.. and How do you get an internet from one way signaling?

  • Mark Phelps September 8, 2008, 15:28

    Galactic Internet…lots of time for the handshake and just think about lost packets…

  • tacitus September 8, 2008, 15:36

    Well, it’s nice to see the professional astronomers catching up :-). I posted this comment on the “Bad Astronomy” forum way back in November 2005 (still Googleable):

    Another way could be to use one of the other “standard candles” for measuring inter-galactic distances — Cepheid variables. If you could modulate the output of a Cepheid in such a way as to piggy-back a signal on top of it – perhaps by playing with it’s chemical composition – then you could be seen as far away as 50 million light years.

    One advantage of using Cepheids would be that astronomers in other civilizations are likely to be looking at them since they are a valuable tool in determining distances between galaxies.

    Far fetched, yes, of course, but not outside the realms of possibility. If it turns out that the physics of our universe means it will be always impractical to travel between galaxies then I don’t think it’s beyond reason to believe that advanced civilizations, isolated by physics, would want to reach out and touch someone in this manner.

    Yeah, talking about “chemical composition” isn’t exactly right, but the concept of modulation or tickling is certainly the same!

    Two other super-advanced civilization communications tricks could include modulating the rotation of rapidly spinning pulsars — not as long range as Cepheids, but could have a higher bandwidth and more easily detected (given the precision of the base spin rate) — and triggering supernovae.

    If intelligent life is extremely rare and a super-intelligent civilization finds itself alone in a galaxy or even galactic cluster, then the only way (we know of) to send an obvious “we are here” signal that will reach across millions of light years is via supernova (another standard candle).

    One could imagine blowing up a pattern of supernovae (a directional signal since if the explosions are see at an angle and thus 100s of years apart, their significance will probably be missed). One could also imagine exploding supernovae at regular intervals over a long period of time.

    With supernovae signals, the trick would not be to convey a message other than “we are here”. But once you have someone’s attention — i.e. another advanced civilization’s best probes and telescopes — you can use a second, less powerful form of transmission knowing you there is a better chance that someone is paying attention.

  • andy September 8, 2008, 16:07

    Regarding pulsars, the easiest way to use a pulsar for signalling is to drop stuff on it. The energy yield is fantastically good: for a 1.4 solar mass neutron star with a radius of 10 km, the maximum yield (for matter initially at infinity) is about 20% of the rest mass energy.

    The maximum yield from dropping stuff onto a Schwarzschild black hole would be 50% of the rest mass energy, but since there is no solid surface to stop it at the event horizon, I’m not so sure things would be so simple.

  • Hans Bausewein September 8, 2008, 16:35

    Each ISP a Cepheid.

    Choose from a few hundred in your galaxy.

    Latency measured in millions of years instead of milliseconds.

    You have to scale up everything a bit. Even your lifetime, I guess.

  • John Hunt September 8, 2008, 16:50

    I would think that the purpose of affecting cepheids or pulsars would not be to use as a galactic internet so much as a beacon to direct others to your location where you could find a more efficient broadcast signal. Initially it would be a one-way internet. But if they were broadcasting plans for a FTL modem then it could become two-way. The signal could also indicate where local Bracewell probes were so as to conduct essentially two-way communications.

  • tacitus September 8, 2008, 17:13

    I know this is getting into the realm of science fiction, but there is a chance that there is a way to circumvent the limits of light-speed communications that we just don’t know about yet, and perhaps the physics concepts behind it is so alien that it could be hundreds of years before we would find it on our own.

    Now, if that’s the case, and our putative super-advanced civilization is out there kicking its heels waiting for all us non-super-advanced civilizations to catch up so they can talk to us, why wouldn’t they want to give us all a helping hand?

    So, rather than wait around, they could simply engineer a simple, repeating “bootstrap” message via Cepheids or pulsars (perhaps with a brief “Cooee! Over here!” message via supernova first), containing the basic concepts behind the thoery of superluminal communications.

    Then all they would have to do is wait for us to login to the real broadband of the connected Universe.

  • tacitus September 8, 2008, 18:31

    Great minds, John :)

  • Mark Phelps September 9, 2008, 11:07

    And of course…”ITAR”….I wonder when State will decide SETI and METI are objects
    of interest…..or even gedanken about such macro projects….

  • ljk September 9, 2008, 13:24

    Pulsars have been considered as good candidates for
    ETI signalling as well. Their discovery in 1967 was even
    considered to be an actual alien signal at first, so who
    knows, maybe the initial guess will ultimately prove to
    be right some day in certain cases:


    Supernovae have also been considered as sources for
    getting someone’s attention on a galactic scale – though
    I would be a bit concerned about beings who can detonate
    stars just for that purpose.


  • tacitus September 9, 2008, 17:56

    …though I would be a bit concerned about beings who can detonate stars just for that purpose.

    ljk, there is absolutely no cause for concern. A civilization that turns to detonating stars for communications purposes is mostly likely one that has failed to discover other intelligent life in its own galaxy already since a civilization that can explode suns is likely to have had the longevity and technology to fully explore their home galaxy.

    Therefore the closest supernova-signal we have any reasonable chance of receive is from Andromeda, 2 million light years away. A civilization capable of superluminal travel is hardly likely to want to use light-speed communications at that distance so even if we exploded a few of our suns in reply, it would be another 4 million years before they even had a chance of dropping by.

    Even then, the odds are that such a signal would come from much further away than our nearest neighbor (which is, after all, only one of billions of galaxies).

    So, it’s highly unlikely that, if we ever see a supernova message, it is snare set by an aggressive civilization as a prelude to an invasion millions of years into the future. Much more likely, it would be an attempt by some lonely, aging super-advanced civilization that is reaching out and perhaps seeking to impart their knowledge to others in a sparsely populated Universe.

  • Stevo Darkly September 9, 2008, 20:24

    This is a really, REALLY cool idea!

    Of course, if Cepheid variables really are being uses as a “Galactic Internet,” when the signals are detected and deciphered, they will probably be something like:






  • Benjamin September 11, 2008, 2:40

    As regards the hostile intent of any civilisations who modulate cepheids, think of it this way. Yes, they must be enormously more advanced than us to be able to do that. But suppose we respond, and they send an invasion fleet. Even moving arbitrarily close to the speed of light, they get here x million years later, and have 3 main possibilities:

    1) We no longer exist. Bit of a waste to send that fleet!

    2) We have collapsed into a dark age, and have nothing they could want to kill us for except our planet, and there’s plenty of them going around anyway.

    3) We have spent the x million years advancing, and we have technology as nasty as, or nastier than, anything they had when they launched, and we defeat them or offer enough resistance to make their venture unprofitable.

    Not too scary, eh?

    I think detailed observations of cepheids might be a very interesting project for SETI after reading this.

  • forrest noble September 11, 2008, 16:08

    The value of this possibility could be utilized in the present by being aware of the possibility that an advanced civilization might use such a system for distant communications. We could “listen” from time to time.

  • ljk September 12, 2008, 19:38

    I am not worried about an invasion, but if they decide to
    restructure the galaxy to their needs, will we be important
    enough to consult on the Galactic Renovation Project, or
    will we be like ants at a construction site?

    Plus I see the ability to move and detonate stars as a
    way of saying We’re really powerful – don’t mess with us.

    And they leave us alone unless we start irritating them a lot.

    Or come up with our own construction plans.

  • ljk September 20, 2008, 1:51

    Pinning down the Milky Way’s spin (ESO 30/08)

    New, very precise measurements have shown that the rotation of the
    Milky Way is simpler than previously thought. A remarkable result
    from the most successful ESO instrument, HARPS, shows that a much
    debated, apparent ‘fall’ of neighbourhood Cepheid stars towards our
    Sun stems from an intrinsic property of the Cepheids themselves.

    Read more in ESO 30/08 at


  • ljk September 30, 2008, 14:02
  • PsychicSEO October 26, 2008, 17:30

    Hi there Ijk – our Prof.I. thaught about another interstellar Version of the Internet himself – have a look: http://www.psychic-seo.de/283-interstellares-internet-pr12-backlinks/