| String theory, and all its many modified versions, claim we exist in more than the 4 dimensions of spacetime — time plus 3 dimensions of space — described by Einstein’s theories of relativity. Some theories claim the dimensionality (“D”) of our spacetime is 5 (1 extra), while others claim D=11 (7 extra). Most such theories posit the extra dimensions are compactified — curled up in loops trillions of times smaller than an atomic nucleus — as this image suggests. Others propose immensely large extra dimensions. These expansive notions can now be tested with Gravitational Waves (GW), first observed by LIGO and announced in 2016 (see my Oct. 2016 newsletter). A recent paper by Du, Tahura, Vaman, and Yagi of the University of Virginia compares actual GW events observed by LIGO with the predictions of a version of string theory in which GWs propagate through D dimensions while normal matter and radiation are restricted to the usual 4. They calculate that for D>4 GWs are phase shifted (delayed) by 5(D–4)/(7D–16), which equals 0.26 for D=5 and increases slowly to 0.57 for D=11, as shown in red in this graph. Based on all events from LIGO’s first two observing campaigns, there is no evidence of an anomalous phase shift. But, should such a phase shift actually exist, LIGO says it must be less than 0.1, the black line in the graph. It seems this version of string theory has been convincingly disproven. However, Leo Susskind, a leading string theorist, claims there are at least 10500 possible versions of string theory, with each version corresponding to a different folding of the extra dimensions and/or a different string wrapping. Susskind says we will never know which of those 10500 versions describes our universe. He says each version leads to different answers to every question. For example, the different versions would predict (if we could do the math, which we can’t) 10500 different values for the electron mass — since that effectively spans all possible values, the theory is completely useless. So it’s 1 version of string theory down, and (10500–1) to go. String theorists may be crazy, but they’re very clever at ensuring their own job security. Best Wishes, Robert February 2022 Note: Previous newsletters can be found on my website. | 