Scientists seek a single description of reality. But modern physics allows for many different descriptions, many equivalent to one another, connected through a vast landscape of mathematical possibility.
Quanta Magazine said: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.
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The game changer that led to this switch of perspective has been string theory. At this moment it is 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. The good news is that string theory has no free parameters. It has no dials that can be turned. It doesn’t make sense to ask which string theory describes our universe, because there is only one. The absence of any additional features leads to a radical consequence. All numbers in nature should be determined by physics itself. They are no “constants of nature,” only variables that are fixed by equations (perhaps intractably complicated ones).
Which brings us to the bad news. String theory’s space of solutions is vast and complex. This is not unusual in physics. We traditionally distinguish between fundamental laws given by mathematical equations, and the solutions of these equations. Typically, there are only a few laws, but an infinite number of solutions. Take Newton’s laws. They are crisp and elegant but describe an incredibly wide range of phenomena, from a falling apple to the orbit of the moon. If you know the initial conditions of a specific system, the power of these laws allows you to solve the equations and predict what is going to happen next. We do not expect, nor demand, an a priori unique solution that describes everything.
In string theory, 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.
But how do we explore the vast landscape of physical models of the universe that might easily have hundreds of dimensions? It’s helpful to visualize the landscape as a largely undeveloped wilderness, most of it hidden under thick layers of intractable complexity. Only at the very edges do we find habitable places. In these outposts, life is simple and good. Here we find the basic models that we fully understand. They are of little value in describing the real world, but serve as convenient starting points to explore the local neighborhood.
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