## June 12, 2018

### AND THAT'S HOW THE LEOPARD GOT HIS SPOTS:

There Are No Laws of Physics. There's Only the Landscape. (Robbert Dijkgraaf, June 4, 2018, Quanta)

Suppose Alice and Bob are both asked to prepare a meal. Alice likes Chinese, Bob likes Italian. They each pick their favorite recipe, shop at the local specialty store, and carefully follow the instructions. But when they take their dishes out of the oven, they are in for a big surprise. The two meals turn out to be identical. We can imagine the existential questions Alice and Bob must ask themselves. How can different ingredients produce the same dish? What does it even mean to cook Chinese or Italian? And is their approach to preparing food totally flawed?This is exactly the perplexity experienced by quantum physicists. They have found many examples of two completely different descriptions of the same physical system. In the case of physics, instead of meats and sauces, the ingredients are particles and forces; the recipes are mathematical formulas encoding the interactions; and the cooking process is the quantization procedure that turns equations into the probabilities of physical phenomena. Just like Alice and Bob, quantum physicists wonder how different recipes lead to the same outcomes.Did nature have any choice in picking its fundamental laws? Albert Einstein famously believed that, given some general principles, there is essentially a unique way to construct a consistent, functioning universe. In Einstein's view, if we probed the essence of physics deeply enough, there would be one and only one way in which all the components -- matter, radiation, forces, space and time -- would fit together to make reality work, just as the gears, springs, dials and wheels of a mechanical clock uniquely combine to keep time.The current Standard Model of particle physics is indeed a tightly constructed mechanism with only a handful of ingredients. Yet instead of being unique, the universe seems to be one of an infinitude of possible worlds. We have no clue why this particular combination of particles and forces underlies nature's structure. Why are there six "flavors" of quarks, three "generations" of neutrinos, and one Higgs particle? Furthermore, the Standard Model comes with 19 constants of nature -- numbers like the mass and charge of the electron -- that have to be measured in experiments. The values of these "free parameters" seem to be without any deeper meaning. On the one hand, particle physics is a wonder of elegance; on the other hand, it is a just-so story.

Posted by Orrin Judd at June 12, 2018 4:27 AM