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.
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).
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?
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.
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.
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.
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).
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?
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.
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.
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.
this doesnt sound like useful speculation.