Multiverse mania started seriously among string theorists around 2003, with a defining event Susskind’s February 2003 The Anthropic Landscape of String Theory. At the time I was finishing up writing what became the book “Not Even Wrong”, and my reaction to Susskind’s paper was pretty much “This is great! Susskind’s argument implies that string theory can’t ever be used to predict anything. If people accept that, they’ll have to give up on string theory since it has come to the end of the line.” Over the next year or two it became clear that devotion to multiverse mania wasn’t just localized at Stanford (where Andrei Linde had always been pushing this, even before the string theorists climbed aboard). Other proponents of the string theory landscape were up and down the California coast, including Raphael Bousso at Berkeley and Joe Polchinski at UCSB. One West Coast holdout was David Gross, who that summer at Strings 2003 quoted Churchill’s words to his country during the Nazi bombardment of London: “Never, never, never, never, never give up”. On the East Coast, the center of the resistance was at the IAS in Princeton, where several people told me that Witten was privately strongly making the case that this was not physics.
I ended up adding an additional chapter to the book about this, and covering developments closely here on the blog. For many years I found it impossible to believe that this pseudo-scientific point of view would get any traction among most leaders of the particle theory community. How could some of the smartest scientists in the world decide that this was anything other than an obviously empty idea? After a while though, it became clear that this was getting traction and that there was a very real danger that particle theory would come to an end as a science, with most influential theorists giving up, justifying doing so by claiming they now had a solid argument for why there was no point in trying to go further. String theory is the answer, but the answer is inherently unpredictive and untestable.
It has become clear recently that we’ve now reached that end-point. From the new video of his discussion with Rovelli, it’s clear that David Gross has given up. No more complaints about the multiverse from him, and his vision of the future has string theory solving QCD 80 years from now, nothing about it ever telling us anything about where the Standard Model comes from. Today brought an extremely depressing piece of news in the form of a CERN Courier interview with Witten. Witten has also given up, dropping his complaints about the string theory landscape:
Reluctantly, I think we have to take seriously the anthropic alternative, according to which we live in a universe that has a “landscape”of possibilities, which are realised in different regions of space or maybe in different portions of the quantum mechanical wavefunction, and we inevitably live where we can. I have no idea if this interpretation is correct, but it provides a yardstick against which to measure other proposals. Twenty years ago, I used to find the anthropic interpretation of the universe upsetting, in part because of the difficulty it might present in understanding physics. Over the years I have mellowed. I suppose I reluctantly came to accept that the universe was not created for our convenience in understanding it.
I’ve never really understood the kind of argument he is making here, that the problem with the string theory multiverse is that it’s upsetting, but we just have to get control of our feelings. Feelings have nothing to do with it: the problem is not that the idea is upsetting, but that it’s vacuous.
The rest of the interview is also pretty depressing. At the high energy physics experimental frontier, Witten promotes “split supersymmetry”, something which does little more than try to keep on life support failed ideas about supersymmetry and “naturalness”:
There is also an intermediate possibility that I find fascinating. This is that the electroweak scale is not natural in the customary sense, but additional particles and forces that would help us understand what is going on exist at an energy not too much above LHC energies. A fascinating theory of this type is the “split supersymmetry” that has been proposed by Nima Arkani-Hamed and others.
On string theory, he follows Gross in referring to not “string theory” but “the string theory framework” and describes the situation as
We do not understand today in detail how to unify the forces and obtain the particles and interactions that we see in the real world. But we certainly do have a general idea of how it can work, and this is quite a change from where we were in 1973.
The situation with string theory unification is that it’s a failed idea, not that it’s a successful general idea just missing some details.
Finally, Merry Christmas and best wishes for the New Year. Fundamental physical theory may now be over, replaced with a pseudo-science, but at least that means that things in this subject can’t get any worse.