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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  • nazgulnarsil

    this doesnt sound like useful speculation.

  • uchicagoman

    All fun and games to think about, but doesn’t mean sh**t without an once of experiment. As much as I love Sean and co, cosmology is getting a bit stale. Entropy this, entropy that, I have less and less time for this speculation and blather.. you know I can watch Carl Sagan videos from here to eternity if I please..

    A hungry scientist should focus on biology for now, THAT is the new physics. So many unanswered questions right in front of us. Yet, so fundamentally mysterious. We have so much to learn about the universe right here, right now.

  • Buck Farmer

    Might gardens be a type of fertility? I.e. life, intelligence, and civilization are manifestations of one mechanism for fertile low entropy spaces to propagate themselves (like the ruthless imperialists they are)?

    Robin, I’ve encountered these ideas before, and I think there’s something quite valuable here, but I don’t feel I’ve a firm enough grasp on the definitions and their implications to reach a conclusion.

    Do you know any good primers for non-cosmologists with a strong general background in physics? Any reccommended texts? I don’t think I saw cosmology in LW’s textbook list.

  • Charlie

    I’ll try and give a more complete picture of the physics, since that seems necessary.

    Our current best guess is that the various parameters of a universe (speed of light, particle types, etc.) are initialized near-randomly. These parameters determine the energy of the vacuum via things like virtual particles, and there can be wildly different vacuum energies for different parameters.

    So the vacuum is like a quantum particle in an energy landscape – picture a soccer ball in a land of hills and valleys. Like a quantum particle, and unlike a soccer ball, it has a chance to tunnel through the surrounding hills to some other valley, releasing energy in the process. So suddenly the vacuum could change properties, effectively spawning a new universe! As these new parameters spread into the old universe at near the speed of light there’s a drop in the energy stored in the vacuum, which is released inside the new universe to form matter. This new matter creates entropy and makes the process only go one way – towards lower vacuum energy.

    How does this interact with life? Well, life only develops in universes that are fairly stable. You probably can’t develop life if your universe collapses in a million years, or doubles in size (and halves in density) every million years. So life-bearing vacuums can only have a very small range of vacuum energies near 0 (to about one part in 10^100). Thus when a “garden” decays, the result is not another garden. The result is a universe with lower vacuum energy, and lower than 0 vacuum energy means it crunches in on itself.

    So those are the gardens covered. What is this idea of “fertility?” Well, a fertile universe would be one that spawns many baby universes – a universe on top of a high hill with only a small barrier around it. Since even a tiny piece of a high-energy universe can expand fast enough (like stretching space, not expanding “into” anything) to replenish itself, fertile universes can be extremely fertile. These ridiculously fast-expanding universes would be quite inhospitable to life. In string theory the potential landscape for parameters is near-random, and in the standard model I think it’s unknown, so a fertile vacuum should probably decay into a wide variety of babies (given the large number of dimensions in the space there are a large number of neighbors). At different places in the energy landscape, fertile universes will spawn different kinds of babies, but because there are so many possibilities the chances of us having a “neighbor” universe with life in it are slim (and 10^-100 is a hell of a prior to overcome).

    I keep calling them “universes,” so does that mean you can’t communicate in or out? Well if you lived 13.7 billion years ago you might have sent a gravitational signal out, but if you were around then you were probably an elementary particle with nothing interesting to say. And then there are the gravitational signals sent in, which would show up as ripples in the matter of the early universe (visible today as the cosmic microwave background). If you had the power to throw black holes around when our universe was born, you might encode a few bits of information and send it to us that way. But if you were in a highly fertile universe you were probably an elementary particle with nothing interesting to say. Then, inside an expanding “garden” like ours, eventually the space in between you and the edge of the universe will be expanding faster than light, cutting off any information from outside.

    On the whole, it’s cool physics but not great for sci-fi hypotheticals.

    • Robin Hanson

      Thanks for the elaboration Charlie. Sean Carroll seems to think that our place is fertile, i.e., that it will via tunnelling birth baby universes out in its infinite future expanse of exponential expansion. Do you think otherwise, or just consider ours to be much less fertile by comparison?

      • Charlie

        We’re not the lowest-energy possible universe, so if the multiverse does exist we will eventually have baby universes with lower vacuum energy than ours. But since the stability we have is 10^100 special, a baby universe will, at the same time as its edges spread through our universe at near the speed of light, be shrinking how much space is within those edges, and generally not being a nice place to live.

        On the bright side, the fact that we’re not dead yet seems to indicate that our universe expands faster than it spawns babies, so we should be safe at least until the LHC gets up to full power (joke).