Tag Archives: Space

Galaxy Calc Shows Aliens

What makes a planet a good host for life? That is, what does a planet need for life to originate there and then evolve to something at the human level? Astronomers today say a planet at least needs a star that 1) lasts long enough, 2) has enough heavy elements, and 3) is not too often hit by nearby supernovae or gamma ray bursts. Using such criteria, several astronomers (mentioned below) have tried to calculate “galactic habitable zones,” i.e., galactic distributions of good-for-life planets, in both space and time. Such calculations are far more important than I had realized – they can help say how common are aliens! Let me explain.

Imagine that over the entire past and future history of our galaxy, human-level life would be expected to arise spontaneously on about one hundred planets. At least it would if those planets were not disturbed by outsiders. Imagine also that, once life on a planet reaches a human level, it is likely to quickly (e.g., within a million years) expand to permanently colonize the galaxy. And imagine life rarely crosses between galaxies.

In this case we should expect Earth to be one of the first few habitable planets created, since otherwise Earth would likely have already been colonized by outsiders. In fact, we should expect Earth to sit near the one percentile rank in the galactic time distribution of habitable planets – only ~1% of such planets would form earlier. If instead advanced life would arise on about a thousand planets, Earth should sit at the 0.1 percentile rank. And if life would arise on a thousand planets, but only one in ten such life-full planets would rapidly expand to colonize the galaxy, Earth should again sit near the one percentile rank.

Turning this argument around, if we can calculate the actual time distribution of habitable planets in our galaxy, we can then use Earth’s percentile rank in that time distribution to estimate the number of would-produce-human-level-life planets in our galaxy! Or at least the number of such planets times the chance that such a planet quickly expands to colonize the galaxy. If Earth has a low percentile rank, that suggests a good chance that our galaxy will eventually become colonized, even if Earth destroys itself or chooses not to expand. (An extremely low rank might even suggest we’ll encounter other aliens as we expand across the galaxy.) In contrast, if Earth has a middling rank, that suggests a low chance that anyone else would ever colonize the galaxy – it may be all up to us.

At the moment published estimates for Earth’s time percentile rank vary widely. An ’04 Science paper (built on an ’01 Icarus paper) says: Continue reading "Galaxy Calc Shows Aliens" »

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A Galaxy On Earth

Our galaxy has about three hundred billion stars, and Earth today has about seven billion people. Assuming only half as many useable planets as stars, we could combine these two numbers into an initial crude guess for the size of a galactic civilization, and define a “galaxy of people” to be a thousand billion billion (or 1021) people. Now consider some famous galactic civilizations in science fiction.

One of the most popular science fiction stories ever was Issac Azimov’s Foundation series. It tells of the fall and rise of a galaxy-wide civilization, whose capital, Trantor, was a planet-wide city a kilometer deep into the ground. Trantor’s population was said to be forty billion, in a galaxy with millions of populated planets and a total population of a million billion (or one millionth of a “galaxy” as defined above).

Star War‘s Coruscant is also a planet-wide city and capital of a galaxy wide civilization, with planetary population of a thousand billion, in a galaxy also of millions of planets and a total population of a million billion. Some say Coruscant’s buildings averaged two kilometers tall. In Star Trek‘s Federation of 150 planets a few centuries hence, which controlled a few percent of the galaxy, each planet had no more than about our Earth’s seven billion, though some say the Federation held ten thousand billion people.

These all seem like dramatic underestimates to me. If Earth were paved over with a city the density of Manhattan today (1.6 million in 59 square kilometers), Earth would have a population of 14 thousand billion. Since Manhattan now has an average building height of 25 meters, a two kilometer deep version could hold a million billion people, and a two thousand kilometer deep version (Earth’s radius is 6400km) could hold a billion billion people.

There is roughly another thousand times as much useable material nearby, in other planets, comets, and the sun itself, allowing a solar-system population perhaps a thousand times larger. This brings us to a thousand billion billion, or a “galaxy” of people, the same as my initial crude population estimate for an entire galaxy above, and vastly larger than most science fiction galaxy estimates.

Furthermore, android ems (whole brain emulations in simulated bodies) could take up a lot less space than humans. I once somewhat conservatively estimated that an em might stand at 1% of human height (and run one hundred times faster). Since such an em would take up only one millionth of a human’s volume, a two kilometer deep Earth city could hold a “galaxy” (or thousand billion billion) of ems. And a solar system civilization might fit a billion billion billion ems, or a million “galaxies.”

Of course we have a long long way to go, not only to generate such huge populations, but also to develop the energy, manufacturing, heat-dumping, etc techs to allow us to support them. And yes, eventually we would run out of energy and material near our Sun, and need to go elsewhere to grow.

But we have strong economic reasons to stay close to one another as long as there is enough energy and material nearby, and especially as long as we continue to innovate. So most of our descendants’ economy should stay close to our sun until congestion here gets severe. We may well have a solar system population of a billion billion billion before the time comes when most of our descendants are closer to other stars.

Most science fiction seems to vastly underestimate the population that a single planet or star can hold, and the strength of the economic pressures to keep an economy close together, rather than spread across vast distances. Someday we will learn to tell stories that treat planets and stars as the vast spaces of possibilities that they really are.

Added 11a: Even an unmodified sun radiates enough energy to cover the calorie consumption of over a hundred “galaxies” of humans, and far more ems.

The timescale to grow from today’s population to a “galaxy” of descendants would be 600 years at an industry-style 15 year doubling time, 40 millennia at a farming-style thousand year doubling time, and four years at at next-singularity-style monthly doubling time.

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Space vs. Time Genocide

Consider two possible “genocide” scenarios:

  • Space Genocide – We expect the galaxy to have many diverse civilizations, with diverse behaviors and values, though we don’t know much about them. Their expansion tendencies would naturally lead to a stalemate, with different civilizations controlling different parts of the galaxy. Imagine, however, that it turns out we luckily have a chance to suddenly destroy all other civilizations in the galaxy, so that our civilization can expand to take it all over. (Other galaxies remain unchanged.) Let this destruction process be mild, such as sudden unanticipated death or a sterility allowing one last generation to live out its life. There is a modest (~5%) chance we will fail and if we fail all civilizations in the galaxy are destroyed. Should we try this option?
  • Time Genocide – As their tech and environments changed, our distant ancestors evolved differing basic behaviors and values to match. We expect that our distant descendants will also naturally evolve different basic behaviors and values to match their changing tech and environments. Imagine, however, that it turns out we luckily have a chance to suddenly prevent any change in basic behaviors and values of our descendants from this day forward. If we succeed, we prevent the existence of descendants with differing basic behaviors and values, replacing them with creatures much like us. There is a modest (~5%) chance we will fail and if we fail all our descendants will be destroyed or exist in a mostly worthless state. Should we try this option?

Probably, more people can accept or recommend time genocide than space genocide, even if success in both scenarios prevents the existence of a similar number of relatively alien creatures, to be replaced by a similar number of creatures more like us. This seems related to our tending to admire time-stretched civilizations (e.g., Rivendale) more than space-stretched civilizations (e.g., Trantor), even though space-stretched ones seem objectively more prosperous. But what exactly is the relation?

The common thread, I suspect, is that the far future seems more far, in near/far concrete/abstract terms, than situations far away in space, or in the far past. The near/far distinction was first noticed in how people treated the future differently, and our knowing especially little detail about the future makes it especially easy to slip into abstract thought about the future.

As we are less practical, more idealistic, and more uncompromising in far mode, we see civilizations time-stretched into the future as more ideal, and we are more willing to commit genocide to achieve our ideals regarding such a civilization, even at a substantial risk.

Of course the future isn’t actually any less detailed than the past or places far away in space. And there isn’t any good reason to hold the far future to higher ideals now than we’d be inclined to want when the future actually arrives. If so, time-genocide should be no more morally acceptable than space-genocide. Beware the siren song of shiny far future thought.

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Space vs. Time Allies

Consider two possible civilizations, stretched either across time or space:

  • Time: A mere hundred thousand people live sustainably for a billion generations before finally going extinct.
  • Space: A trillion people spread across a thousand planets live for only a hundred generations, then go extinct.

Even though both civilizations support the same total number of lives, most observers probably find the time-stretched civilization more admirable and morally worthy. (more)

Our distant ancestors struggled against nature and other species, but competed most directly for mates and resources with others in their species, especially others in the same generation. More distant generations, like grandparents or grandkids, tended more to be allies in their efforts to promote their genes and culture. Because of this, Katja and I suggested, humans evolved intuitions that see time-stretched civilizations as more full of comforting allies, and hence more worthy, than space-stretched civilizations.

Modern economies, however, differ in many important ways from the forager bands where these intuitions evolved. So let us compare the relative promise of time-stretched versus space-stretched modern economies with similar total numbers of people.

  • Scale Economies – Spatially large civilizations can specialize more in the production of goods and services, and take advantages of economies of scale, to get more of everything. Temporally large civilizations, in contrast, can only take advantage of scale economies for extremely durable goods like music. This issue favors spatial stretching.
  • Dependence Fragility – The more that the parts of a civilization depend on one another, the more that damage to one part can put the whole at risk. In a time stretched civilization a very bad outcome for any one generation risks the destruction of all future generations. It is a long chain of dependence that is only as strong as its weakest link. In contrast, a space stretched civilization allows for more redundant and parallel dependence paths. It can be more like a net that holds even when many of its strands are broken. This issue favors spatial stretching.
  • Innovation – A finite speed of light imposes delays on how fast innovations developed in one part of a spatially separated civilization can be used elsewhere.  [Added 8a: parallel innovation attempts also make info delays.] The more that a civilization is time-stretched, as opposed to space-stretched, the smaller are such delays. Our civilization is now compact enough that such delays are only a minor issue. This will also cease to be an issue when innovation has ended, i.e., when we have basically discovered all that is worth knowing. This issue favors time-stretching, but only during a (perhaps short) innovation era and only for very spatially stretched civilizations.

Overall, for similar numbers of total people, modestly spatially-stretched civilizations seem more promising. Thus in contrast to our evolved intuition that temporal associates are our allies while spatial associates are our rivals, spatial associates seem to actually be more useful, and hence are more naturally our allies. Beware relying on ancient evolved intuitions.

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Is Time Us, Space Them?

(This post co-authored by Robin Hanson and Katja Grace.)

In the Battlestar Galactica TV series, religious rituals often repeated the phrase, “All this has happened before, and all this will happen again.” It was apparently comforting to imagine being part of a grand cycle of time. It seems less comforting to say “Similar conflicts happen out there now in distant galaxies.” Why?

Consider two possible civilizations, stretched either across time or space:

  • Time: A mere hundred thousand people live sustainably for a billion generations before finally going extinct.
  • Space: A trillion people spread across a thousand planets live for only a hundred generations, then go extinct.

Even though both civilizations support the same total number of lives, most observers probably find the time-stretched civilization more admirable and morally worthy. It is “sustainable,” and in “harmony” with its environment. The space-stretched civilization, in contrast, seems “aggressively” expanding and risks being an obese “repugnant conclusion” scenario. Why?

Finally, consider that people who think they are smart are often jealous to hear a contemporary described as “very smart,” but are much happier to praise the genius of a Newton, Einstein, etc. We are far less jealous of richer descendants than of richer contemporaries. And there is far more sibling rivalry than rivalry with grandparents or grandkids. Why?

There seems an obvious evolutionary reason – sibling rivalry makes a lot more evolutionary sense. We compete genetically with siblings and contemporaries far more than with grandparents or grandkids. It seems that humans naturally evolved to see their distant descendants and ancestors as allies, while seeing their contemporaries more as competitors. So a time-stretched world seems choc-full of allies, while a space-stretched one seems instead full of potential rivals, making the first world seem far more comforting.

Having identified a common human instinct about what to admire, and a plausible evolutionary origin for it, we now face the hard question: do we embrace this instinct as revealing a deep moral truth, or do we reject it as a morally irrelevant accident of our origins? The two of us (Robin and Katja) are inclined more to reject it, but your mileage may vary.

(This is cross-posted at Meteuphoric.)

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The Mysterious Desert

How bright is our future? That depends greatly on how feasible is interstellar travel. And we don’t really know that, because we are still pretty ignorant about what lies between the stars. Oh it all looks pretty empty, but looks could be deceiving. If you look at a logarithmic map of the universe, the scales on which we seem the most ignorant (below 10Bly) are the three orders of magnitude between the furthest planets and the nearest stars. Now we see clues that unexpected and powerful things happen there:

Between May 2009 and May 2010, IceCube detected 32 billion cosmic-ray muons, with a median energy of about 20 TeV. These muons revealed, with extremely high statistical significance, a southern sky with some regions of excess cosmic rays (“hotspots”) and others with a deficit of cosmic rays (“cold” spots).

Over the past two years, a similar pattern has been seen over the northern skies by the Milagro observatory in Los Alamos, New Mexico, and the Tibet Air Shower array in Yangbajain. … It’s a mystery because the hotspots must be produced within about 0.03 light years of Earth. Further out, galactic magnetic fields should deflect the particles so much that the hotspots would be smeared out across the sky. But no such sources are known to exist.

One of the hotspots seen by IceCube points in the direction of the Vela supernova remnant … almost 1000 light years away. Cosmic rays coming from such large distances should be constantly buffeted and deflected by galactic magnetic fields on route, and should thus have lost all directionality by the time they reach Earth. …

There could be a “tube” of magnetic field lines extending between the source and our solar system, funnelling the cosmic rays towards us. … [This] theory is highly speculative. … Others have proposed that … solar magnetic field lines cross and rearrange, converting magnetic energy to kinetic energy – could be accelerating local cosmic rays … creating the observed hotspots. … “That’s also crazy, but it is at least less crazy than other explanations.” (more)

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The SETI Game

When listening for signals from aliens, it isn’t enough to just point an antenna at the sky. One must also choose details like directions, angles, frequencies, bandwidths, pulse widths, and pulse intervals. Apparently most SETI searches assume that for a given signal power density, aliens would pick details to make it as easy as possible for us to detect their signals. So standard SETI searches are optimized for such easily-seen signals. Two excellent papers, published back in July, instead consider what sort of signals would be sent by “beacon” building aliens, who seek to create the maximum possible power density at any given distance away from them.  (One of the authors is SF author Greg Benford.) Such signals are quite different, and most of today’s SETI searches are not very good at seeing them:

Minimizing the cost of producing a desired power density at long range … determines the maximum range of detectability of a transmitted signal. We derive general relations for cost-optimal aperture and power. … Galactic-scale beacons can be built for a few billion dollars with our present technology. Such beacons have narrow “searchlight” beams and short “dwell times” when the beacon would be seen by an alien observer in their sky. … Cost scales only linearly with range R, not as R2. … They will likely transmit at higher microwave frequencies, 10 GHz. The natural corridor to broadcast is along the galactic radius or along the local spiral galactic arm we are in. …

Cost, spectral lines near 1 GHz, and interstellar scintillation favor radiating frequencies substantially above the classic “water hole.” Therefore, the transmission strategy for a distant, cost-conscious beacon would be a rapid scan of the galactic plane with the intent to cover the angular space. Such pulses would be infrequent events for the receiver. Such beacons built by distant, advanced, wealthy societies would have very different characteristics from what SETI researchers seek. … We will need to wait for recurring events that may arrive in intermittent bursts. …

A concept of frugality, economy. … directly contradicts the Altruistic Alien Argument that the beacon builders will be vastly wealthy and make everything easy for us. An omnidirectional beacon, radiating at the entire galactic plane, for example, would have to be enormously powerful and expensive, and so not be parsimonious. … For transmitting time t, receiver detectability scales as t1/2. But at constant power, transmitter cost increases as t, so short pulses are economically smart (cheaper) for the transmitting society. A 1-second pulse sent every 10 minutes to 600 targets would be 1/600 as expensive per target, yet only *1/25 times harder to detect. Interstellar scintillation limits the pulse time to >10-6 s, which is within the range of all existing high-power microwave devices. Such pings would have small information content, which would attract attention to weaker, high-content messages. …

Cost-optimized beacons … can be found by steady searches that watch the galactic plane for times on the scale of years. Of course, SETI literature abounds with consideration of the trade-offs of search strategy (range vs. EIRP vs. pulse vs. continuous (continuous wave, CW) vs. polarization vs. frequency vs. beamwidth vs. integration time vs. modulation types vs. targeted vs. all-sky vs. Milky Way). But, in practice, search dwell times are a few seconds in surveys and 100–200 seconds for targeted searches. Optical searches usually run to minutes. And integration times are long, of order 100 s, so short pulses will be integrated out. …

Behind conventional SETI methods lies the assumption that altruistic beaming societies will send persistent signals. In searches to date, confirmation attempts, when the observer looks back at a target, in practice usually occur days later. Such surveys have little chance of seeing cost-optimized beacons. … Distant, cost-optimized beacons will appear for much less time than as assumed in conventional SETI. Earlier searches have seen pulsed intermittent signals resembling what we (in this paper) think beacons may be like, and may provide useful clues. We should observe the spots in the sky seen in previous work for hints of such activity but over year-long periods. (more)

Of course both the usual assumption that aliens will pay any cost to make a given power density signal easy for us to see, and the new assumption that aliens ignore our costs and merely seek to maximize power density, are both somewhat unsatisfactory. It would be better to model this interaction as a game, where each side has a limited budget and seeks to maximize the probability of at least one successful communication, holding constant the behavior it expects from the other side. Each side would of course also have to integrate over possible locations and budgets for the other side.

I’m very interested in working with (sim, math, or physics) competent folks to more formally model this SETI communication game.

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On Berserkers

Adrian Kent is getting a little publicity for posting his ’05 paper on the berserker hypothesis, “that evolution has very significantly suppressed cosmic conspicuity”, i.e., that many aliens are out there, but hiding from each other. He advocates taking the hypothesis seriously, but doesn’t actually argue for the coherence of any particular imagined scenario. Kent’s excuse:

It would be very difficult to produce a model that convincingly predicts the likelihoods and spatial distributions of the various strategies, since the answer surely depends on many unknowns.

He instead just claims:

The hypothesis is certainly not logically inconsistent and it seems to me not entirely implausible.

So what then is Kent’s contribution? Apparently it is a bunch of strategy fragments, i.e., strategy issues that aliens might consider in various related situations. It is not clear that these are much of a contribution, at least relative to the many contained in related science fiction novels. But, well, here they are: Continue reading "On Berserkers" »

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Earth Is Not Random

The great filter is whatever obstacles stand in the way of simple dead matter eventually giving rise to a visibly expanding interstellar civilization. It is now confirmed that a non-trivial chuck of that filter is in planets having special orbits that let climates be stable over time:

Planetary anthropic selection, the idea that Earth has unusual properties since, otherwise, we would not be here to observe it, is a controversial idea. This paper … [compares] Earth to synthetic populations of Earth-like planets … [for] high (or low) rates of Milankovitch-driven climate change. Three separate tests are investigated: (1) Earth-Moon properties and their effect on obliquity; (2) Individual planet locations and their effect on eccentricity variation; (3) The overall structure of the Solar System and its effect on eccentricity variation. In all three cases, the actual Earth/Solar System has unusually low Milankovitch frequencies compared to similar alternative systems. All three results are statistically significant at the 5% or better level, and the probability of all three occurring by chance is less than 10^-5. It therefore appears that there has been anthropic selection for slow Milankovitch cycles. This implies possible selection for a stable climate, which, if true, undermines the Gaia hypothesis and also suggests that planets with Earth-like levels of biodiversity are likely to be very rare. Continue reading "Earth Is Not Random" »

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Are Gardens Fertile?

Cosmologists tend to think that the physics we see around us is not universal. There is instead a vast “landscape” of possible ways a local physics could be, and different (large far away) places in the universe embody or express these different physics.

When adjacent space-time places have different local physics, there must be a common “meta” physics that describes their border. This meta-physics will say how often places of one type lead to places of other types nearby, including “ends” where nothing is nearby.

Let us distinguish two special kinds of places:

  • Gardens support life and possibly civilization.
  • Fertile places tend to lead to more fertile places nearby.

The existence of any fertile place implies an expected infinity of connected fertile places. Thus when meta-physics maintains a one-to-one state map across a time dimension, there should be no finite upper bound to the entropy of a fertile place. Thus the entropy at a fertile place is always vastly lower than is possible, and entropy would increase in some local time direction. Since this low entropy should infect adjacent places, non-fertile places “close enough” to fertile ones should also have entropy increasing away from the fertile side. Thus we can explain our local “arrow of time” by assuming that our place is connected to a fertile place in our distant past.

Is our garden fertile? If both gardens and fertile places are rare, and these properties are not very correlated, then fertile gardens would be especially rare – it would be quite unlikely that our garden is fertile. In this case, while our universe is infinite, our future is finite, and will see and influence only a finite amount before our space and entropy run out.

Cosmologists today, however, tend to think that fertile places are not very rare. They expect places with a “positive vacuum energy” and a “low vacuum decay rate” to generate many “baby universes”, and that many of these baby universes also satisfy this description. In fact, they guess that our place here satisfies this description, and so is fertile. (This is, basically, Sean Carroll’s account of our arrow of time.)

But a whole lot of guess work goes into all this. For example, it could be that vacuum decay rates are much higher, and that baby-universe-generating rates are much lower, than they’ve guessed. My guess is that this property of being fertile is rarer than cosmologists now guess, which lowers the chance of our garden being fertile.

A correlation between being a garden and being fertile might result if civilizations tended to work to increase the rate at which their places lead to more places nearby. But it might be that for most gardens there isn’s much civilizations can do.  In which case if fertile places are rare, then most gardens are not fertile, our future is finite.

Finally, even if our place is fertile, it might be that the border between our place and other different places has no “hair” letting us send specific influences from here to there. In this case, our future influence would still be finite.

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