String Theory: Now Circling the Drain
The largest physics experiment ever built is now testing the nature of reality. String theory, supersymmetry and other theories beyond the Standard Model are under scrutiny. More than 10,000 people have been involved. Total cost is nearing $10 billion. This, of course, is the Large Hadron Collider (LHC), which helped discover the Higgs Boson.
Simultaneously, the ACME experiment, run by a team of less than 50, built for a few million dollars (and much, much smaller), has created a more precise test of these advanced theoeries. This experiment hinges on an extremely painstaking and precise method to picture the shape and size of electrons.
Electron electric dipole moment (EDM) is brutal physics jargon. It's a direct measurement of the interal structure of an electron. If an electron is an absolutely perfect sphere of negative charge with an infinitely small radius, the EDM is 0. If it's actually an extremely tiny speck made of even tinier specks of electric charge sitting next to one another, the EDM is some tiny value greater than 0.
String theory and similar theories require many unknown physics measurements. One of these is the magnitude of the electron electric dipole moment that this experiment probes. Each type of theory requires an EDM within a certain range of values to work.
What we know so far is that every experiment yet built has measured EDM = 0. (Not precisely zero, but zero within a margin of error, effectively establishing a tiny maximum experimental error value as the largest possible size.) Is the electron really like a tiny black hole with infinite density filling infinitely small space? Answering this question sheds light on the bigger one as well: Just how plausible are theories beyond the Standard Model?
Measurements taken for several decades have gradually reduced the possible size of the EDM. This year, the ACME collaboration published the best measurement ever made of the EDM: smaller than 8.7 x 10^-29 cm. (Free copy here.) It's an unimaginably small number. (Imagine an ant's size compared to that of the universe. This number is 100 times smaller than the ant.)
What this tiny value does is severely limit the likelihood of several particular varieties of theory stretching beyond the Standard Model.
de is the EDM, and the blue, red, green and purple areas are various theories beyond the Standard Model. Every theory to the left of the red line is now impossible! Large portions of the imaginary world of String Theory (e.g., versions of SUSY or "supersymmetry" shown above) are being ruled impossible by this machine 1/1000th the size and cost of the LHC. Previously, String Theory was on life support. Now, it might be circling the drain.
A more powerful upgrade of the ACME experiment coming on-line in the next few years may be able to push the elimination line about two more powers of ten to the right, refuting more theories.
Most importantly, finding funding for future physics projects as big as the LHC is going to be difficult. Managably small experiments like ACME relying on clever and precise methods may displace those relying on pure size and power.