aa r X i v : . [ phy s i c s . pop - ph ] N ov Galactic Neutrino Communication
John G. Learned , ∗ Sandip Pakvasa , † and A. Zee ‡ Department of Physics and Astronomy,University of Hawaii, 2505 Correa Road,Honolulu, Hawaii 96822 U.S.A. Kavli Institute for Theoretical Physics,University of California, Santa Barbara,California 93106 U.S.A. (Dated: October 29, 2018)We examine the possibility to employ neutrinos to communicate within the galaxy. We discussvarious issues associated with transmission and reception, and suggest that the resonant neutrinoenergy near 6.3 PeV may be most appropriate. In one scheme we propose to make Z o particles inan overtaking e + − e − collider such that the resulting decay neutrinos are near the W − resonanceon electrons in the laboratory. Information is encoded via time structure of the beam. In anotherscheme we propose to use a 30 P eV pion accelerator to create neutrino or anti-neutrino beams. Thelatter encodes information via the beam CP state as well as timing. Moreover the latter beamrequires far less power, and can be accomplished with presently foreseeable technology. Such signalsfrom an advanced civilization, should they exist, will be eminently detectable in existing neutrinodetectors.
PACS numbers: 95.85.Ry, 98.70.Sa, 84.40.Ua
I. INTRODUCTION: NEUTRINOS FORGALACTIC COMMUNICATION
The search for extraterrestrial intelligence (SETI) hasnow gone on for decades without detecting a signal. Thusfar, the search has presumed that the transmission wouldbe in photons within radio or optical bands. In this note,we suggest that it may be more sensible to search for apossible signal in neutrinos. We will discuss some of thephysics issues related to both transmission and reception.Why would one want to employ the notoriouslydifficult-to-observe neutrino to communicate? Some rea-sons are obvious.i) The obscuration/opaqueness of material between thesource and detector makes photons less useful within thegalactic plane and particularly for lines-of-sight anywherenear the galactic center. Neutrinos arrive almost with-out attenuation from any source direction (though at theenergies we suggest below, they are attenuated throughthe earth).ii) Neutrinos are rare, and from a given direction are allbut negligible (particularly at high energies as we discussbelow). For photons the signal/noise (S/N) problem is ∗ Electronic address: [email protected] † Electronic address: [email protected] ‡ Electronic address: [email protected] not at all negligible in any band and is worse for all direc-tions where galactic civilizations may reside (presumablyin the galactic plane). For neutrinos the band can beessentially noise free.iii) Even when photons are not completely blocked,their scattering introduces jitter in arrival time as well asdirection. As we discuss below, the reasonable encodingfor maximally energy-efficient information transfer mayemploy the time interval between quanta. Moreover, inone scheme, data may be encoded via the use of neutrinosversus anti-neutrinos.There have been early general proposals for the useof neutrinos for interstellar communication and for SETI[1]. There were also more recent and more specific pro-posals about the use of neutrinos in SETI [2], [3]. In thisnote we propose a new way of using neutrinos for commu-nication by ETI and point out that existing and futurefacilities will be able to look for these signals without anyneed for new construction or expense.
II. WHAT NEUTRINO ENERGIES ARE MOSTSUITABLE?
What neutrino energies are best suited for galacticcommunication?First, let us consider relatively low energy neutrinos,those typical of nuclear, solar and supernovae (SN) pro-cesses. One strike against this energy region, roughly upTypeset by REVTEXto around 10 MeV (40 MeV for SN) is that such neu-trinos are produced in abundance naturally, so there issome signal-to-noise barrier to overcome. If one thinks ofthe natural emission from radioactive decays, from starsand from supernovae across the universe, this region isnot so attractive since these sources are enormously pow-erful and one must compete against them (The “noise”is small, but the inherent signal-to-noise ratio S/N is alsosmall for any imaginable source.) Moreover, directional-ity at the lower energies is at best difficult. Also, S/Naside, at low energies the neutrino-nucleon interactioncross-section is dauntingly small, of order 10 − cm .(For the same reason of vanishingly small cross-sections,we dismiss further discussion of even lower neutrino ener-gies.) We have also considered the possibility of employ-ing resonant nuclear energies, but we have not identifiedany viable mechanism which will beat the problems ofinherent S/N and low cross sections.For these reasons we are driven to consider higher en-ergies. The neutrino cross section grows with energylinearly until the 100 TeV energy range, and then log-arithmically. The (dominant) terrestrial neutrino back-grounds fall rapidly, one power in energy more steeplythan the cosmic ray spectrum. For a beam, there arefurther gains of order E due to the solid angle of thebeam, and typically one would gain altogether by a fac-tor of > E in signal, and > E in S/N on a per particlebasis.The energy chosen should be such that it would beclear at once that it is an artificial source such as ETIand not some random background. One choice is to make Z o ’s at rest as was proposed earlier [2]. Then the neu-trinos from such a source have energies of exactly m Z /2,about 45 GeV, and are easily identifiable as due to Z o decay: there are no natural sources of ν s of this preciseenergy. In this case the neutrinos are emitted in a spheri-cally symmetrical manner, and because of that the powerrequirements for galactic distances, reach the scale of to-tal solar power (as estimated there) to obtain a signifi-cant counting rate. Of course one might argue that thisis not “our” problem, but one to be solved by the pos-tulated advanced civilization with technology we cannotyet imagine. But resorting to harnessing (Dyson) starscertainly moves the potentiality of such communicationto the distant future, if indeed such is ever practical for acivilization. It was further proposed there that the detec-tion process employed electrons accelerated such that theincoming electron-anti-neutrino interact at the Glashowresonance[4] i.e. producing a W boson on-shell (an elec-tron energy E e = m W / m e , about 35.5 GeV) with alarge cross-section.We presume that the ETIs, though in our galaxy, areremote. Even if an ETI has been observing us, it maybe a long while (timescale of thousands of years) beforethey would send us an introductory message. So if theywant to send a message in advance, saying hello and wel-come to the galactic network, they are going to have tospeculate about when to bother to transmit. From the jittering of advances in speciation, with the great die offs,it seems clear that evolution is a stochastic process, withfluctuations on a timescale of many millions of years. Theevolution of technology may fluctuate over a timescale ofthousands of years, as exemplified by the long periodsof lack of technological progress in post-Roman Europe,China and India. One must reason that no useful predic-tion could be made as to when the industrial revolutionwould take off and high technology would arise. Thus theETI would have to be transmitting speculatively over along period.Communication may be initiated for many hypotheti-cal reasons. For example, to simply welcome a new soci-ety to the galactic club, or to warn of dangers from withinor without, or for reasons we cannot now guess. Indeed,it seems best not to speculate on motives for transmis-sion at this point but to focus on physics issues. In anyevent, it seems that there might be two stages, the firstbeing to simply get the attention of the recipient andthe second to send information. One might also imagineETI sending information to military outposts via secureneutrino beam, on a known schedule and in a small solidangle, and we happen to be in the transmission path,but the probability of our intercepting such is evidentlynegligible.Given light transmission time over galactic dimensions,compared to evolution time in our world, we cannot fore-see much of a dialogue, but that is of course also true forelectromagnetic communication. Note, for example, thatsignals leaving the center of our galaxy at the time of thefirst human settlements would not arrive for another 14millenia. Monologues are what we can anticipate at best.That is the case unless there are means to beat the speedof light via wormholes or extra dimensions, but such donot now appear to be viable within established physics.It has been argued persuasively[5] that transmission oflarge amounts of data via radio or light are very ineffi-cient compared to the deposition of artifacts, snail mailas it were. Artifacts can hold huge amounts of data, andneed only be sent once or very seldom to promising starsystems and left to be discovered later. We thus imaginethat there is not a great need for a high data rate channelin the galaxy, but perhaps only as stated above, for someminimal information of overwhelming importance, whichmight include instructions on where to find the artifact. III. DIRECTIONAL TRANSMISSION ANDDETECTION VIA RESONANT NEUTRINOS
Instead of omnidirectional broadcast, let us next con-sider the possibility of sending out focused neutrinobeams in a specified direction, aimed at promising starsystems.Sending out a focused beam has the advantage of notbeing seen by all, perhaps a worthy security measure.Many have speculated on the danger of attracting un-wanted attention by potentially aggressive species (whichconceivably could wish to take over a nice proven hab-itable planet, or even enslave the inhabitants of such aplanet — the subject of much science fiction but not ob-viously a wrong presumption.) Indeed, some have sug-gested that our civilization should consider measures tokeep our galactic visibility to a minimum for just this rea-son. Hence transmitting to an unknown new society hasrisks, particularly since with a beam one may reveal thelocation of the transmitting entities. If this is indeed areal danger, perhaps an advanced civilization would em-ploy a transmitting station, a lighthouse, at some removefrom their home. In view of this, perhaps an advancedcivilization may be more inclined to transmit to a newlytechnically emergent society (TES) such as ours. Suchaction might offer the rewards of heading off more unde-sirable consequences possible in the longer term, when aTES is not in a position to be too territorially acquisitive.Or perhaps the ETI would simply like to initiate trade aswhen Europeans first visited China and India. However,our own history gives no precedent in terms of communi-cation prior to contact, and indeed that history is a bitfrightening, since first contacts soon led to exploitation.Let us assume, moreover that the ETI will guess that acivilization ready to hear their messages will have devel-oped to the point of constructing large neutrino detectorsin the process of studying neutrinos and attempting tobegin neutrino astronomy. It is hard to justify from somefuture viewpoint, but we see now that a high energy neu-trino detector of the scale of 1 km is reasonable (IceCubeunder construction at the South Pole, and NESTOR,ANTARES and NEMO and an expanded Lake Baikaldetector are all proposed or under development[6]). So,for discussion purposes we shall assume the ETI will aimat communicating with a detector of cross-section on theorder of 1 km .Suppose we make a beam of electron-anti-neutrinos,and get them to a very high energy, to be precise E G = M W / m e = 6 . P eV . Then these can be sent in somechosen directions to be received by observers who canemploy a detector seeking the reaction ¯ ν e + e − → W − atresonance, the so-called Glashow resonance, as illustratedin Figure 1. The production and decay of a W − into ashower provides a unique signature; given more than onesuch event from a given direction the source would beimmediately known to be due to an ETI as there areno natural sources of 6.3 PeV ¯ ν e ’s. To contrast withthe proposal made in 1994[2], here we boost the initialbeam rather than the electrons in the detector; in bothone employs the Glashow resonance and its high cross-section. The range of such a resonant neutrino in water isabout 100 km, so that these neutrinos would penetrate tothe deepest detectors on earth, but would be attenuatedin arrival directions below the horizon. This also impliesthat the detection fraction in a 1 km detector would beabout 1% of the traversing neutrinos.An efficient mechanism for such a beam generationwould be to collide electron-positron beams at a center-of-mass energy at the Z o mass, but in a fast moving ref- erence frame, so that the decay neutrinos would be at 6.3PeV. In this instance the electrons might be overtakingthe positrons in the laboratory frame, so that the Z o isfast forward moving and the beam direction determinedthereby.The size of the region illuminated by such a beam froma distance of 1 kiloparsec (about 3,000 light years, 3 × km) is about 3,000 AU across. If we require a beam suchthat there would be at least 100 neutrinos per km area,then the individual pulse would have to have around 10 neutrinos(here we use an opening angle in the Z decay ofabout M Z /E ν ).Needless to say, the numbers we cite are illustrativeonly as we clearly could not anticipate all possible sce-narios. For instance, the ETI may be relatively nearby,either on a extrasolar planetary base or in a space station,waiting for us to build a suitable neutrino telescope. FIG. 1: Electron and muon (and tau same as muon) neutrinoand anti-neutrino interaction lengths on target electrons ver-sus neutrino energy, including Glashow resonance. From [7].
IV. ENERGY COSTS
Accelerators are marvelously good at transformingelectrical energy into beam energy, typically putting tensof percent of the wall-plug power into the beam. One canimagine a total energy transfer from the delivered powerto neutrinos of perhaps 0.1%. In contrast, radio is better,by perhaps one or two orders of magnitude. Lasers arecurrently not very efficient, but their technology is stillevolving rapidly. One may thus argue that the cost ofmaking a neutrino beam is not prohibitive in comparisonto photon beams. Also, the radio beams have the poten-tial disadvantage of sidelobes, which could be a stronglynegative factor if the security issue is real or perceived tobe so.At an energy cost of order 10 Joules per pulse (al-lowing for accelerator efficiency and the fact that the Zdecays into neutrinos only 20% of the time, and of thatonly 1/6 will be electron anti-neutrinos), this is a hugeenergy output. If the pulses were fired once per second,the accelerator power would be about 3% of the solar lu-minosity. In fact, taking into account the flat spectrumof the neutrinos from Z decay after the boost, there is afurther factor of m Z / Γ Z , which makes the required powerabout equal to the solar luminosity. This is clearly nota task we can imagine carrying out on any projection ofour present technology.However, we do not know the methods that may beavailable to advanced civilizations to make a neutrino (orany other) beam. We have direct evidence in the 10 eV cosmic rays, the gamma ray bursts (GRBs), the micro-Quasars, and the amazingly collimated jets from activegalactic nuclei (AGN), so that we might suspect thatwe do not yet understand some fundamental issues onparticle acceleration. For example, how does one get anearth mass accelerated to a gamma of 1000 in a distanceof a few light seconds, as has been inferred for gamma rayburst jets or “cannonballs”? There is also the possibilitymentioned earlier of employing “Dyson” stars. So, forpresent purposes, we shall assume that an ETI would findit affordable and worthwhile to expend such resources tocommunicate with our TES. V. INFORMATION ENCODING VIA PULSETIMING
What about encoding? Since we are talking about rel-atively rare events, it seems evident that the encodinginformation by relative timing of the neutrino pulses pro-vides the only mechanism, much as the use of a simpleMorse code in the early days of electromagnetic commu-nication. Also, neutrinos are fermions and presumablyencoding could only involve classical physics, rather thansay some hypothetical analog of the laser.Neutrino oscillations might permit some further en-coding, but given the distances, oscillations are averagedout. For neutrinos coming from a distance near that tothe galactic center, the solar oscillations will have madeabout a million cycles. If the beam energy were sharp toparts per million, then in fact one would have to worryabout the phase of the cycle (earth could be in a null),but this seems not a problem. In any event if only elec-tron anti-neutrinos are detected (as we are consideringhere), then no information can be encoded in neutrinoflavor.Thus there would be some interval between pulses,which we interpret as some number of time incrementslong, and that is the message. What would be the nat-ural time increment? One possibility would be the lep-ton associated 0.3 picosecond lifetime of the tauon. This timing over a large detector is not presently practical,and would seem to be not possible in the foreseeable fu-ture. The muon lifetime of 2.2 microseconds would bemuch easier and more practical. The minimum time in-terval detectable with present detectors is of the orderof 1 nanosecond, and may reach 100 picoseconds in afew decades. With maximum resolution, if events werespaced apart by, say (arbitrarily) 2200 sec on average,then this is 10 intervals per pulse, or equivalent to 30bits, or an equivalent data rate of about 0.014 baud, notso bad for interstellar communications! If we assumetransmission with, say, three repetitions then this wouldbe 143,000 bits per year, quite a respectable amount ofdata.Some thought should be given to how the ETI mightencode data in a way which would be most simple todecode. For example, coarse level timing might encodefor the simplest messages helping to establish the linkand achieve synchronization, with finer and finer detailencoding more and more complex data.Finally, at the risk of speculating beyond what wouldbe warranted at present, we might try to guess the con-tent of a message from an advanced civilization. Wecould ask what we might say if we were in a positionto start transmitting. As suggested in another context[9], in light of the physicist’s well documented and al-most irresistible urge to publish, an advanced civiliza-tion might just want to announce that it has figured outhow the universe ticks. A concise summary would be thegauge algebras of the three non-gravitating interactionssuitably coded. protonprotonacceleratoracceleratortargettarget pionpionhornhorn10 PeV linear10 PeV linearacceleratoraccelerator muonmuonkillerkiller EarthEarth10 PeV 10 PeV pionspions
Pion Accelerator Neutrino Beam Concept all neutrino all neutrino ororantineutrino typesantineutrino typesmuonsmuons andandmuon neutrinosmuon neutrinos
FIG. 2: Cartoon of muon neutrino production from a pionaccelerator. Neutrinos mix and all flavors are detected, butonly anti-neutrinos will have Glashow resonant events.
VI. AN ALTERNATIVE WITH NEUTRINOSUPERBEAMS
As this paper was being drafted, a preprint appeared[3] which proposed that communication from nearby starsmight be conducted with neutrino beams such as willbe inherent in a muon collider. In present discussionsof such, one would rapidly accelerate muons made frompion decays in the 1 GeV energy range (the pions madein collisions of a few megawatt proton beam of a fewtens of GeV with a fixed target). These muons wouldthen circulate in order to study high energy muon paircollisions. The muons decay and create a powerful (andeven dangerous) neutrino beam. And, there has beenmuch discussion of neutrino factories, using muons asdiscussed, but only keeping them in a race track (withlong straight sides) shaped ring or a nearly triangularring, with sides pointed at distant detector. Indeed suchinstruments seem to provide the path towards detailedmeasurements of neutrino mixing, and we think will bebuilt in the foreseeable future.We propose an alternative to this which seems to beeven more interesting, as illustrated in the cartoon in Fig-ure 2. Our idea is to accelerate the charged pions beforethey decay to muons, and to take them to energies inthe range discussed previously, ≃ P eV , so that the de-cays would make neutrinos in the 6.3 PeV energy range.The decay distance for pions of this energy will be about0.5 million km, or about the distance to the moon. Thepositively charged pions will decay into positive muonsand muon neutrinos, and negative pions vice versa, withresulting anti-neutrinos. If one aims the beam at a rel-atively thin shield (rock) one will kill the muons (whichradiate very copiously at this energy), but leave the neu-trinos. Hence in selecting whether positive or negativepions are accelerated, one may choose neutrinos or anti-neutrinos as the beam.In case of π + , only ν µ ’s are produced and from π − only¯ ν µ ’s are produces. As is well-known[10] the averaged outoscillations, after a distance of about a few light days,convert these into a mixture of all three flavors in theproportion given by ν e : ν µ : ν τ = 4:7:7, but keeping the par-ticle/antiparticle nature as is. Hence, since it is a matterof sign selection in the accelerator one can switch the neu-trino beam between particles and antiparticles, switchingon and off the Glashow resonance. There will be a con-stant signal from either beam due to the other chargedand neutrino current reactions. Thus there will thus bedifferent signatures for neutrinos and anti-neutrinos, andone may encode information in the nature of each pulse,without regard to timing. In addition, timing can beemployed as well, as discussed earlier for the resonant Z o scheme, and we can substantially boost the data trans-mission rate.A strong further attraction of the pion scheme is thatthe maximum neutrino angle relative to the pion is( m π − m µ ) / m π /E ν so the beam is much narrower thanthe Z o beam and the target area is smaller by about afactor of about 10 . All transmitter powers involved aredramatically reduced so that we would be consideringa total beam energy requirement per pulse of order onegigajoule. This is on the order of the energy per pulsewhich is being discussed for near future controlled fusionreactions on Earth. Given such “modest” power levels,one may imagine transmitting at rates higher than hy-pothesized for the resonant Z o method, perhaps one per second, as is easily foreseeable with present technology (agigawatt of power, less than many present nuclear powerstations).The penalty for the tight beam however is that the ETImust know the precise planet they are targeting and knowits ephemeris, since the 10 million km beam spread (from1 kpc) is less than 0.01 AU. If they have surveyed thestellar systems and singled out earth, then this would notseem a barrier as they would of necessity have determinedthe orbital parameters, and extrapolation of the earth’sorbit over a few millenia should pose no problem.The only thing we do not yet readily foresee in thisscheme is how to accelerate the pions to this high energy(30 P eV ). Yet, a linear accelerator (in space) with a gra-dient of 100 GeV/m (already achieved for short distancesin the laboratory) would require a length of 1,000 km,which seems not wildly implausible for a future civiliza-tion. Also, while aiming with sufficient precision wouldcertainly pose problems for us at present (at the level of0.07 milliarcseconds), such a scale would be needed bythe ETI to optically resolve the earth in any event.
VII. CONCLUDING REMARKS
We have outlined a method for intragalactic commu-nication via directed 6.3 PeV beams of electron anti-neutrinos, and other neutrinos. Such beams can be cre-ated with reasonable energy efficiency by a civilizationwithout a long stretch from the technology we now pos-sess. Detection of such a beam, possible given a de-tecting civilization with our present level of neutrino de-tectors (cubic kilometer scale), would be evidently dueto ETI with only a few events detected since there isno known mechanism for making neutrinos at only thisenergy range. Plus, having a few as two neutrinos ar-rive from precisely the same direction would be very un-likely unless accompanied by a huge burst of radiation inother bands. Data would accumulate at a rate plausiblyabout 1 Hz equivalent bandwidth and decoding the pat-tern would take perhaps one year. This would amount totransmission of 1000 pages of material per year, a tremen-dous amount of information. Given that the transmittingentity would have to know about the earth’s (and othertargets’) ephemeris and possibly even day cycle, theirtransmission might be set to repeat several times dailyand again on a longer cycle. Given that the ETI haveno a priori knowledge of when observations will begin,and cannot get feedback for millenia afterward, multipletransmissions would be necessary, but perhaps only oncein a few years, as perhaps they would illuminate othersystems alternately and if our picture of the device isat all accurate, redirecting the accelerator would requiresome time.No special action is required on our part, since if thisspeculative hypothesis should be correct, we will soondiscover such signatures, but perhaps such will not ar-rive for some time. This adds motivation to keep allneutrino telescopes operating for long timescales, suchas the watch for supernovae in our galaxy and unpre-dictable burst events of all types. We humans should cer-tainly think about and continue to explore other meansfor such communications, but to us neutrinos seem toprovide some special opportunities.
Acknowledgments
This work was supported in part by the U.S.D.O.E. un-der grant DE-FG02-04ER41291 at University of Hawaii, and by the N.S.F. under grant 04-56556 at University ofCalifornia at Santa Barbara. Two of us (S.P. and A.Z.)would also like to thank Xiao-Gang He, Pauchy Hwangand their colleagues at the NTU for their hospitality andthe stimulating atmosphere where this work was begun.We also thank Xerxes Tata for useful discussions. [1] J. M. Pasachoff, and M. L. Kutner,” Neutrinos for Inter-stellar Communication”, Cosmic Search 1, 2(1979), M.Subotowicz, ”Interstellar Communication by NeutrinoBeams”, Acta Astronautica , 213 (1979), A. W. Saenz,A. W. Ueberall, H. Kelly, D. W. Padgett, and N. See-man, ”Telecommunications with Neutrino Beams”, Sci-ence, , 295(1977)[2] J.G. Learned, S. Pakvasa, W. A. Simmons, and X. Tata,“Timing data communications with neutrinos:A new ap-proach to SETI”, Q.J.Roy.Astron.Soc. , 321 (1994).[3] Z.K. Silagadze, “SETI and the muon collider”,arXiv:0803.0409 (2008).[4] S.L. Glashow, “Resonant scattering of anti-neutrinos”, Phys. Rev. , 316 (1960).[5] C. Rose and G. Wright, “Inscribed matter as an en-ergy efficient means of communications with an extra- terrestrial civilization”,
Nature ,431 (2004).[6] J.G. Learned and K. Mannheim, “High Energy and Neu-trino Astrophysics”, Ann. Rev. Nucl. Sci. , 679 (2000).[7] R. Gandhi, C. Quigg, M.H. Reno, I. Sarcevic, “Neu-trino interactions at ultrahigh energies”, Phys. Rev. D ,093009 (1998).[8] P. Zucchelli, “A novel concept for a ¯ ν e / ν e neutrino fac-tory: The Beta Beam”, Phys. Lett. B , 166 (2002).[9] S. Hsu and A. Zee, “Message in the Sky”,
Mod. Phys.Lett.
A21 , 1 (2006), arXiv:physics/0510102; “The Never-Ending Days of Being Dead” by Marcus Chown,