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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.
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