Most of the quantum fields that fill our universe have one, and only one, preferred state, in which they’ll remain for eternity. Most, but not all.
True and False Vacuums
In the 1970s, physicists came to appreciate the significance of a different class of quantum fields whose values prefer not to be zero, even on average. Such a “scalar field” is like a collection of pendulums all hovering at, say, a 10-degree angle. This configuration can be the ground state: The pendulums prefer that angle and are stable.
In 2012, experimentalists at the Large Hadron Collider proved that a scalar field known as the Higgs field permeates the universe. At first, in the hot, early universe, its pendulums pointed down. But as the cosmos cooled, the Higgs field changed state, much as water can freeze into ice, and its pendulums all rose at the same angle. (This nonzero Higgs value is what gives many elementary particles the property known as mass.)
With scalar fields around, the stability of the vacuum is not necessarily absolute. A field’s pendulums might have multiple semi-stable angles and a proclivity for switching from one configuration to another. Theorists aren’t certain whether the Higgs field, for instance, has found its absolute favorite configuration—the true vacuum. Some have argued that the field’s current state, despite having persisted for 13.8 billion years, is only temporarily stable, or “metastable.”
If so, the good times won’t last forever. In the 1980s, the physicists Sidney Coleman and Frank De Luccia described how a false vacuum of a scalar field could “decay.” At any moment, if enough pendulums in some location jitter their way into a more favorable angle, they’ll drag their neighbors to meet them, and a bubble of true vacuum will fly outward at nearly light speed. It will rewrite physics as it goes, busting up the atoms and molecules in its path. (Don’t panic. Even if our vacuum is only metastable, given its staying power so far, it will probably last for billions of years more.)
In the potential mutability of the Higgs field, physicists identified the first of a practically infinite number of ways that nothingness could kill us all.
More Problems, More Vacuums
As physicists have attempted to fit nature’s confirmed laws into a larger set (filling in giant gaps in our understanding in the process), they have cooked up candidate theories of nature with additional fields and other ingredients.
When fields pile up, they interact, influencing each other’s pendulums and establishing new mutual configurations in which they like to get stuck. Physicists visualize these vacuums as valleys in a rolling “energy landscape.” Different pendulum angles correspond to different amounts of energy, or altitudes in the energy landscape, and a field seeks to lower its energy just as a stone seeks to roll downhill. The deepest valley is the ground state, but the stone could come to rest—for a time, anyway—in a higher valley.
A couple of decades ago, the landscape exploded in scale. The physicists Joseph Polchinski and Raphael Bousso were studying certain aspects of string theory, the leading mathematical framework for describing gravity’s quantum side. String theory works only if the universe has some 10 dimensions, with the extra ones curled up into shapes too tiny to detect. Polchinski and Bousso calculated in 2000 that such extra dimensions could fold up in a tremendous number of ways. Each way of folding would form a distinct vacuum with its own physical laws.