“There are no laws of physics,” observed Robbert Dijkgraaf, mathematical physicist and director of the Institute for Advanced Study, where Einstein spent his last 22 years. Instead, he observes, there is a terrifying complex “landscape” of possibilities, a nearly infinite, subtly connected network of complementary versions of reality each with its own set of fundamental particles, forces, laws and dimensions.”
The current Standard Model of particle physics,” writes Dijkgraaf in Quanta, “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.”
“This landscape, also known as the multiverse,” writes Dennis Overbye in the New York Times, “is the vision of string theorists who have vaulted past Einstein in the current scientific imagination.” String theory unites gravity, which curves the cosmos, with quantum mechanics, which describes the randomness that lives inside it, by envisaging the fundamental constituents of nature as tiny strings of energy vibrating in 11 dimensions.
By contrast, wrote Dijkgraaf in Quanta, Albert Einstein famously believed that, given some general principles, there is essentially a unique way to construct a consistent, functioning universe. In Einstein’s quasi religious 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 theory has been described as a piece of 21st-century physics that fell into the 20th century by accident — and which might require 22nd-century mathematics to understand, continues Overbye. “The result is a mathematical labyrinth with 10⁵⁰⁰ solutions, each one a different potential universe. In principle, one of those universes is ours — but nobody knows, because the math and physics are so horrendously complex.”
If our world is but one of many, how do we deal with the alternatives? asks Dijkgraaf. “The current point of view can be seen as the polar opposite of Einstein’s dream of a unique cosmos.”
Scientists were drawn to Dijkgraaf’s conjecture by the discovery, two decades ago, that a mysterious unseen yet to be proved force — dark energy — is accelerating the expansion of the universe, making the galaxies retreat from each other faster and faster in spacetime.
This dark energy, observes Overbye, “bears all the earmarks of a fudge factor, called the cosmological constant, that Einstein inserted into his equations a century ago, and later rejected as a blunder. But the amount of this dark energy is smaller than the predicted value of the cosmological constant by a factor of 10⁶⁰.”
Leading physicists to assume that the value of Einstein’s constant is random across all potential universes; we live in one with the right amount of dark energy to allow stars and galaxies to form.
The game changer that led to this switch of perspective has been string theory, the only viable candidate for a theory of nature able to describe all particles and forces, including gravity, while obeying the strict logical rules of quantum mechanics and relativity.
In string theory, summarizes Dijkgraaf, certain features of physics that we usually would consider laws of nature — such as specific particles and forces — are in fact solutions: “They are determined by the shape and size of hidden extra dimensions. The space of all of these solutions is often referred to as “the landscape,” but that is a wild understatement. Even the most awe-inspiring mountain vistas pale in comparison with the immensity of this space. Although its geography is only marginally understood, we know it has continents of huge dimensions. One of the most tantalizing features is that possibly everything is connected — that is, every two models are connected by an unbroken path. By shaking the universe hard enough, we would be able to move from one possible world to another, changing what we consider the immutable laws of nature and the special combination of elementary particles that make up reality.”
If scientists want any gift for the holidays, it’s some new physics that would break the stalemate of these “standard models” and provide new clues to our existence.
Maybe that breakthrough will come from finally figuring out what dark matter is, or from the Large Hadron Collider, which will continue banging together subatomic particles for the next 20 years in search of new forces and phenomena. Every collision recorded is another step into the unknown.
Cosmologists, a notably fractious group, have agreed on their own standard model of our particular universe, which posits that baryonic matter, atoms — the stuff of you, me and the stars — account for only 5 percent of the cosmos by weight. Dark matter, of which we know nothing, writes Overbye, “except that its collective gravity sculpts and holds the galaxies together, amounts to 25 percent. The remaining 70 percent is dark energy, pushing everything apart; we don’t know anything about that, either. We only know that this ‘dark sector’ exists because of the effect of its gravity on the luminous universe, the motions of stars and galaxies.”
The upside of this unproven theory that leaves 95 percent of the universe unidentified is that it’s an exciting sign that science is hardly over –or perhaps only that the human species has arrived at the end of the beginning.
Read more of Dijkgraaf’s Quanta article here