Most of the quantum fields that fill our universe have one, and only one, preferred state in which they remain for eternity. Most but not all.
True and false vacuums
In the 1970s, physicists learned the importance of another class of quantum fields, whose values prefer not to be zero, even on average. Such a “scalar field” is like a collection of pendulums, each levitating at, say, a 10-degree angle. This configuration can be the ground state: the pendulums prefer this angle and are stable.
In 2012, experimenters 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 states the way water can freeze into ice, and its pendulums all rose to the same angle. (This non-zero Higgs value gives many elementary particles the property known as mass.)
For scalar fields, the stability of the vacuum is not necessarily absolute. The pendulums of a field can have several semi-stable angles and a propensity to change from one configuration to another. Theorists aren’t sure if the Higgs field, for example, has found its all-time favorite configuration — the true vacuum. Some have argued that the current state of the field, although 13.8 billion years old, is only temporarily stable, or “metastable.”
If so, the good times won’t last forever. In the 1980s, physicists Sidney Coleman and Frank De Luccia described how a false vacuum of a scalar field can “decay”. Any moment enough pendulums move to a more favorable angle in one place, they pull their neighbors toward them, and a bubble of true vacuum flies outward at nearly the speed of light. It will rewrite the physics on its way and destroy the atoms and molecules on its way. (Don’t panic. Even if our vacuum is only metastable, given its staying power so far, it’s likely to last for billions of years to come.)
In the potential variability of the Higgs field, physicists identified the first of virtually infinite ways that nothingness could kill us all.
More problems, more vacuums
As physicists attempted to fit the established laws of nature into a larger set (and thereby fill in huge gaps in our understanding), they concocted possible theories of nature with additional fields and other ingredients.
As fields accumulate, they interact, affecting each other’s pendulums and forming new mutual configurations in which they tend to get stuck. Physicists visualize these vacuums as valleys in a hilly “energy landscape.” Different pendulum angles correspond to different amounts of energy or heights in the energy landscape, and a field tries to decrease its energy just as a rock tries to roll down a hill. The deepest valley is the ground state, but the rock might – at least for a time – come to rest in a higher valley.
A few decades ago, the landscape exploded in scale. Physicists Joseph Polchinski and Raphael Bousso investigated certain aspects of string theory, the leading mathematical framework for describing the quantum side of gravity. String theory only works if the universe has about 10 dimensions, with the extra ones curled up into tiny shapes to discern. Polchinski and Bousso calculated in 2000 that such extra dimensions could fold up in myriad ways. Each kind of folding would form its own vacuum with its own physical laws.