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Tuesday
Oct122010

Fish in a Pond

Things are so because things couldn't have been any different… In recent years, cosmological observations have painted a remarkably detailed picture of the history of our universe. The bad or good news (depending on your perspective) is that some of the conclusions are astounding: results suggest an extremely delicate balance all around us, so delicate that if things were to be a tiny bit different at the beginning of time, perhaps we would not be around to ask any questions…

There are several such "coincidence" and "fine tuning" issues. In the beginning, the universe was filled with dark energy (see previous post) and underwent a dramatic explosive expansion. This expansion was exponential - physicists call it the Inflationary Epoch - where the fabric of space stretched faster than the speed of light. As the universe expanded, some normal matter was generated during a period cryptically called a "graceful exit"… The expansion was so violent, that it was highly highly (and I mean highly) sensitive to the initial condition of the dark energy pervading the universe. If things were a little different, this crucial epoch of expansion of the universe may not have been realized. And this inflationary epoch is crucially needed to explain why our universe is around… Here's another perverse coincidence. As the universe continued to expand, and is now known to undergo an accelerated expansion, space stretches away from us faster than light can catch up with it… so, our horizon - farthest extent we can see into the universe - is shrinking fast… As it happens, we live around the right period that allows us to just be able to see the whole universe… Several hundred million years later, the edge of the universe would have receded away from our visual horizon… On cosmological timescales, this coincidence is quite shocking and highly unlikely.

In the context of String Theory, these issues get addressed rather explicitly. String Theory is described by a set of equations whose solution is presumably our universe. The problem is that we sort of have a situation of an embarrassment of riches: the equations admit many many solutions - amongst them potential candidates for our universe - but these realizations are often very disparate in their conclusions on how the world should look like. Too many of these solutions do look like the world we live in - but the devil is in the details. And we don't know all the solutions… You may then say that String Theory allows many universes as possibilities: call it the Multiverse picture. How can we predict anything in such a situation! Which universe are we in? Are there other ones around? Where the heck are they? Why are we in this one? There comes the anthropic principle, an old idea that has received new life in the context of these modern questions. The idea is simple: of all possible universes, only a few select ones have the right conditions to have our kind of miserable life evolve in it… we are in this universe because if things were different, we wouldn't have been around… This does have a flavor of a mentality from the Middle Ages, doesn't it? it is absurd to ask some fundamental questions because there is no answer to them beyond: "it is so, because we need it to be so to be able to ask the question"...

Here's my revised version of an argument that goes back to the Cosmologist Linde in support of the anthropic principle. Imagine a species of sophisticated fish living in a pond whose temperature is 15C. After living their lives as high quality Sushi for a while, these fish develop intelligence and become sentient. Some of the fish adopt unglamorous careers of hard work with little benefits as physicists, and start measuring the temperature of the water. Fish with even lower self esteem become theoretical physicists, and start asking: "Why is the temperature so?". Can we derive some equations that predict the temperature? You are standing outside the pond looking into it and wandering: "These must be the stupidest fish in the world: the temperature is so because if it wasn't, this species of fish wouldn't survive and be around to ask the question"…

Personally, this scenario is very troubling to me. Does this mean there are some fundamental questions in physics we can never unravel beyond a lame anthropic argument? is this the end of fundamental physics then? I don't think so. There are some recent suggestions that one may be able to get a statistical handle on such questions: we may not be able to predict which universe of the many possible ones we live in, but perhaps we can say which ones are the most likely without considering a biological factor… And that may be good enough, whether we like it or not… after all, if the fish are to evolve, they need to start looking outside the pond...

The accompanying video is slightly on the edge… but still interesting enough to lead you to ponder over some of the implications of this subject that straddles physics and philosophy.

A reference article: Leonard Susskind (2007). The Census Taker's Hat http://arxiv.org/abs/0710.1129v1 arXiv: 0710.1129v1

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Reader Comments (2)

Is it not better to tell the theoretical fishicists "yes, this universe has the biologically ideal temperature of 15C, but in other ponds where the temperature is different there are no fish and so the question is not asked?" In that case, though the overall meaning is the same, the stress is on the fact that all the variations occur, and that in the case where it is conducive to questions, asking takes place, and where it is not, asking does not take place (rather than stressing the "luck" of being in the right universe at the right only time? From that standpoint, I think the anthropic principle can actually do a lot of epistomological work.

October 12, 2010 | Unregistered CommenterMichael Maindi

Yes, but the troubling aspect in all this is the loss of scientific predictive power. It is one thing to have a philosophical understanding of the grand scheme of things, another to actually be able to compute predictions and verify with the world around us without resorting to some ill-defined notions of biological evolution and mental development... Physics is an experimental science; and we want to eventually understand and compute everything about Nature. Otherwise, there will always be these "numbers" around us that we just have to accept without being able to derive them from first physical principles. And that sort of leaves a void deep in my heart...

October 12, 2010 | Registered CommenterVatche Sahakian

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