Wednesday, August 12, 2009

The String Landscape And Multiverse Thought Experiments

I enjoyed reading a thought experiment proposed by SteveP: Would it matter if you switched to an identical universe? He wonders if it would matter if you were transported to an identical universe where the only difference is the size of some fish. You should all read his post for more details as it is an interesting question.

This reminds me of the highly speculative, but nevertheless mainstream, theoretical physics known as the string landscape. See here for peer reviewed articles. (Again this stuff is very speculative.)

For those who don't know, string theory started out in the late seventies as an attempt to explain the strong interactions. It soon became apparent string theory may finally be the theory that explains the quantum nature of gravity. A major early problem was there were several consistent string theories. Which one describes our universe?

Then came M-Theory that unified all the string theories into a single theory that that predicts, by one popular interpretation, there are 10^500 types of universes all connected together in a giant multiverse. (For those who don't know, 10^500 is a really, really big number.) Each of these "types" of universes can happen any given number of times.

Anyways, as I read SteveP's post I couldn't help but think of the 10^500 different universes an alien might take me too if the string landscape is a correct idea. Some would support life and some wouldn't. Some would have Obama's socialist death squads for senior citizens and others would live in the paradise of Glenn Beck's imagination. (Okay, I made that one up. :) ) But some would be just like us, and if Nobel Prize winner Gell-Mann is correct, just the right accident may happen so that the only difference between us and them is the size of a fish.

I'll tell you how it goes when I get there SteveP. But in the meantime it was a great thought experiment.

PS: Just an FYI, string theory is not the only theory that predicts a multiverse. Almost all quantum cosmology theories predict a multiverse of some sort. The string landscape just predicts a really big multiverse.


  1. One should note there is starting to be quite a backlash against the multiverse. Even Lee Smolin has moved away from some of his multiverse views if not quite the whole idea.

  2. Thanks for the comment Clark. It is true the multiverse has become a polarizing idea leading many people to turn elseware. I won't deny that.

    However a few things.

    1. There is no "better" quantum gravity theory than string theory and it's accompanying landscape. That doesn't mean it is correct by any means! But whatever bad things someone has to say about it, all other "theories of everything" are even in worse shape.

    2. Lee Smolin is popular with the general public because he is a great popular science writer. However, most researchers don't take him very seriously. He does a good job criticizing string theory, but can't come up with anything even close to having the potential string theory has. (He thinks he has, just has a hard time convincing other physicists his ideas have much merit.)

    3. But in fairness who am I to make judgments? I'm just a graduate student right? True, but I am sure if you follow the biggest names in physics, (biggest in the eyes of researchers), you will find they are saying these same things.

    4. All these theories are so speculative that the odds are they are all wrong, but as I have said in other places, at least they are well motivated by physical principles we know are true.

    5. I see you are a Python programmer. I have a lot of respect for you. Python is awesome!

  3. I'm not sure that's entirely fair about LQG. Certainly String Theory has the mindshare over LQG but it's just not fair to portray him as out there alone. There are a lot of people who favor LQG. To say it's not even close to having the potential M-Theory has is a bit of an exaggeration. It has problems (which Smolin is pretty upfront about) but then so does M-Theory.

    Of course just because one hasn't found a better alternative doesn't mean M-Theory ought be embraced too much - as you note.

    The problem with applying principles we know are right is that we know they are right in certain circumstances. However where we are applying it are well beyond where we know they are right. And there are good reasons to think our principles are wrong. I think everyone remembers the obvious principles of the 19th century.

  4. You raise some good points Clark. I personally don't have a problem with loop quantum gravity. (I've even tried to read some review articles on it.) Nevertheless, as you noted, the mind share among physicists seems to favor something like string theory over LQG.

    Full disclosure: I am the type of person who has a hard time thinking the minority is more likely to be right. If it was 45/55 that would be one thing. But it seems it is more like 10/90 in favor of LQG.

    But, as you said, I may be being unfair. Lee Smolin is not the only person who favors LQG and sometimes the minority is correct. However, the odds are both theories need some serious work before they are ultimately correct.

    The really important thing is we need some way to test them.

    Again, your comments are really appreciated, especially because they keep me honest.

  5. My feeling is that the whole movement is in crisis and folks are looking for something new. I admit a preference to LQG just because of the issue of background independence (philosophically I think GR is more likely right there than QM) and because I love E&M and LQG has the obvious similarities to classical E&M. Of course I also love group theory and String Theory obviously built off of the classical supersymmetry work in the early 70's. (Which is about as far as I can get before the math overwhelms me - especially since I've been doing philosophy and chocolate much more than physics the past decade!)


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