Seven Brief Lessons on Physics Read Online Free Page A

is given the technical term “renormalization.” It works in practice but leaves a bitter taste in the mouth of anyone desiring simplicity of nature. In the last years of his life, the greatest scientist of the twentieth century after Einstein, Paul Dirac, the great architect of quantum mechanics and author of the first and principal equations of the Standard Model, repeatedly expressed his dissatisfaction at this state of things, concluding that “we have not yet solved the problem.”
    In addition, a striking limitation of the Standard Model has appeared in recent years. Around every galaxy, astronomers observe a large cloud of material that reveals its existence via the gravitational pull that it exerts upon stars and by the way it deflects light. But this great cloud, of which we observe the gravitational effects, cannot be seen directly and we do not know what it is made of. Numerous hypotheses have been proposed, none of which seem to work. It’s clear that there is
something
there, but we don’t know what. Nowadays it is called “dark matter.” Evidence indicates that it issomething
not
described by the Standard Model; otherwise we would see it. Something other than atoms, neutrinos, or photons . . .
    It is hardly surprising that there are more things in heaven and earth, dear reader, than have been dreamed of in our philosophy—or in our physics. We did not even suspect the existence of radio waves and neutrinos, which fill the universe, until recently. The Standard Model remains the best that we have when speaking today about the world of things; its predictions have all been confirmed, and, apart from dark matter—and gravity as described in the general theory of relativity as the curvature of space-time—it describes well every aspect of the perceived world.
    Alternative theories have been proposed, only to be demolished by experiments. A fine theory proposed in the 1970s and given the technical name SU(5), for example, replaced the disordered equations of the Standard Model with a much simpler and more elegant structure. The theory predicted that a proton could disintegrate, with a certain probability, transforming into electrons and quarks. Large machines were constructed to observe protons disintegrating. Physicists dedicated their lives to the search for an observable proton disintegration. (Youdo not look at one proton at a time, because it takes too long to disintegrate. You take tons of water and surround it with sensitive detectors to observe the effects of disintegration.) But, alas, no proton was ever seen disintegrating. The beautiful theory, SU(5), despite its considerable elegance, was not to the good Lord’s liking.
    The story is probably repeating itself now with a group of theories known as “supersymmetric,” which predicts the existence of a new class of particles. Throughout my career I have listened to colleagues awaiting with complete confidence the imminent appearance of these particles. Days, months, years, and decades have passed—but the supersymmetric particles have not yet manifested themselves. Physics is not only a history of successes.
    So, for the moment we have to stay with the Standard Model. It may not be very elegant, but it works remarkably well at describing the world around us. And who knows? Perhaps on closer inspection it is not the model that lacks elegance. Perhaps it is we who have not yet learned to look at it from just the right point of view, one that would reveal its hidden simplicity. For now, this is what we know of matter:
    A handful of types of elementary particles, whichvibrate and fluctuate constantly between existence and nonexistence and swarm in space, even when it seems that there is nothing there, combine together to infinity like the letters of a cosmic alphabet to tell the immense history of galaxies; of the innumerable stars; of sunlight; of mountains, woods, and fields of grain; of the smiling faces of the young at parties; and of the