Here is the link to the final, fourth episode of Brian Greene's new PBS/NOVA documentary:
But only when I watch it with an extra Blogger.com tab opened at the same time. Let me say in advance that this is obviously the episode dedicated to the most speculative topic and it's not right to pretend that these questions are more settled than they are.
New York and masterpieces fail to be unique. Duplicates are inevitable if a multiverse theory is right. Not too deep comments about other possible universes with an Earth boasting Saturn rings and Brian Greene who is a conductor. Guth, Weinberg, and Susskind add a few words. Susskind says that physicists don't like to ask "whether it is science, religion, or philosophy: they just follow the logic". Well, true, except for the final conclusion that "logic seems to lead to the multiverse". It doesn't.
It's just a possibility but a strictly "single" Universe separated from others, making all "other" components totally unphysical, is also a possibility. Thank to the documentary, this viewpoint is instantly presented, too. Andreas Albrecht says he isn't feeling nice about the multiverse. David Gross says that the multiverse may "exist" in the same sense as angels. ;-) Weinberg and Guth bet that it could be real and Guth adds that it could be established in a century.
Alan Guth is given some space to present cosmic inflation which gave the bang to the Big Bang. They review the cute paradoxical origins of inflation in Guth's and Henry Tye's calculations motivated by other expectations. The vacuum energy is presented as negative or repulsive gravity. This leads most people to wrong interpretations but it's also right when treated properly. Still, it's an oversimplification stimulated by pop-science interests because people like to speak about antigravity. (The masses never repel because of their very existence.)
Guth says he didn't care about mysteries of the Universe; he just wanted tenure. Ouch. ;-) But he's honest. I am sure this is true for hundreds of others who wouldn't openly admit such a thing. CMB, COBE, WMAP testing inflation. Fingerprints matched. The pictures of the fingerprint give a somewhat misleading picture of the data that was actually matched but OK. ;-)
Andrei Linde and Alex Vilenkin are described as "two Russians who showed that inflation was hiding a [multiverse] secret". Vilenkin "got it" during his shower. Unnaturally long pictures showing a naked man. Bangs happen at many places, inflation stops somewhere, continues elsewhere: eternal inflation. Growing Swiss cheese with holes. Nicely visualized: the holes don't grow much, the remaining cheese in between them does.
Vilenkin tried to impress Guth with the eternal inflation but Guth did the same thing as during your humble correspondent's string theory classes: fell asleep. ;-) He still asks the best questions afterwards! Of course, I would never leave the Harvard classroom just because my student Alan Guth fell asleep. Alex Vilenkin has different priorities and he left the MIT office. To close this triangle, when we visited Alex Vilenkin and Delia Perlov at Tufts, I ultimately had to leave because Vilenkin suggested it was natural for some silly reason I forgot. I remember my Harvard friends were offended by Vilenkin's behavior and would be bringing me some apologies from Vilenkin later but I think they made them up. ;-)
Back to the documentary. Physicists weren't interested in Vilenkin's comments about marvelous things behind the cosmic horizon. This attitude is instinctive and may even be very reasonable and quantitatively justified (cosmic horizon complementarity etc.). For a while, the PBS/NOVA program doesn't even admit it could be the case and prefers to paint the physicists as idiots. But they quickly show Albrecht, Weinberg, and Steinhardt who try to explain why it may be scientifically reasonable to consider the regions behind the horizons to be "unreal" or at least "outside the reach of science".
Andrei Linde came up with his own version of eternal inflation. Andrei was more resilient and, using his words, more arrogant than Vilenkin (I strongly doubt it!). Cool reception continued. Up to the discovery of the cosmological constant, which is said to support the anthropic/multiverse reasoning and which Raphael Bousso counts as one of the major experimental discoveries in the history of science (not sure about that, given the fact that except for this single number, it has led to zero convincing scientific insights and ramifications), as well as insights in string theory.
The case for the anthropic explanation of the smallness of the C.C. is presented persuasively.
Brian Greene gets to a hotel. He receives the room number 10,000,001. It's surprising but if the hotel is big, it's not surprising. I disagree. Even if the hotel had 20 million rooms, the probability of having such a special number is still low, so it is surprising. Of course, when one proves that almost all the other rooms are incompatible with life, the surprise goes away (or at least goes down). But it still doesn't imply that there isn't a better explanation of our "room number" and it isn't really new evidence for the anthropic principle because the prior probability that we would find a value compatible with life was 100% – even before we measured the C.C. and calculated which values of the C.C. are compatible with life! Exactly because the hospitability was guaranteed to start with, the theory hasn't passed any nontrivial test so its probability shouldn't have increased.
A short introduction to string theory was taken from The Elegant Universe on PBS, Brian's first documentary of this sort. Susskind atypically mentions "elegance" of string theory and Joe Polchinski says that strings vibrate in all the dimensions. An ant on the wire seems to be completely copied from The Elegant Universe TV show. The extra dimensions are the DNA of the Universe, Susskind says. The shapes resemble subspecies of cabbage. Shamit Kachru of the KKLT says that there are many possible compactifications, 10 to the 500.
String theorists were depressed by this pile of solution but Susskind claims to have been happy from the beginning. That's exactly what he wanted – and cosmology needs. Obviously, I don't have to explain how philosophical this attitude is and how non-robust any hypothetical argument in favor of this picture is. Delia Perlov finally appears on the screen, too, offering some female charms.
Eternal inflation, dark energy, and string energy become 3 legs supporting the multiverse chair. A picture of one of those panels I remember with a disgusted David Gross who has to listen to the multiverse babbling by Vilenkin et al. says a lot but they correctly mention that the multiverse advocates are no longer alone. Linde says that the genie is out of the bottle: you can't put it back. Well, I can't put it back but I may wait when it gets dispersed, diluted, and its density drops to zero. ;-)
The diverse properties of the multiverse are a bit confusing – the very different "levels" of the multiverse are mixed with each other (the stringy landscape that still obeys some universal laws, e.g. GR coupled to matter at low energies, is mixed with totally differently-physical or unphysical universes, universes just like ours with people with 3 hands, etc.) so this is really just an inspiring video, not a presentation of any particular scientific research.
Three Brian Greenes explain that with many copies, possibilities start to get repeated, a not-excessively-deep theme given lots of space in The Hidden Reality. Clifford Johnson, Shamit Kachru, and others say random things that are different in other worlds out there. Alex Vilenkin actually says that the huge size made him depressed. David Gross correctly says that such a reasoning may be applied whenever you don't have a better explanation.
Why the Earth-Sun distance is right for life? Kepler tried to calculate it. But it can't be calculated. Of course, in the case of many planets, the anthropic explanation is right. But that doesn't mean that the same reasoning is right for totally different questions in which we don't observe the "other worlds". Susskind calls the anthropic reasoning "the leading hypothesis" because there's no better one. That's a fair description but rationally, it also says that if this is the main argument for its validity, it's almost certainly wrong. Hypotheses that are right almost always have much more robust arguments behind them – they pass some a priori non-automattical tests.
It was a nice and balanced show about a very speculative layer of research. This topic is of course mostly about personal philosophies, not convincing scientific evidence (relevant for the existence of a multiverse) in one way or another, so a popular program about it inevitably focuses on the sociology, just like this episode. That's not Brian's fault; it's guaranteed by the uncertain status of these hypotheses. Although this multiverse business became an emotionally controversial topic in recent years not just because of scientists themselves but also because of various cranks and sourballs, and despite my personal differences with Brian's attitude, I didn't find any portion of this episode unbalanced or downright incorrect in any way, and I guess that most physicists on both sides of the anthropic divide could agree that it was a fair program about this topic.