An empty room is still full of fields. Remove the furniture, the air and every particle you can find, and physics does not leave you with a featureless nothing. There is still a state of those fields. That is what makes the question of vacuum stability so peculiar: even “empty” can have more than one possible arrangement.
A useful picture is a ball resting in a valley. It will stay there through small disturbances. But there might be a deeper valley on the other side of a ridge. The first valley is stable enough for everyday purposes without being the lowest place the ball could reach.
Physicists call such a state metastable. The possibility that our vacuum is in one of these valleys is a serious theoretical question. The discovery of the Higgs boson did not prove that it is.
What the Higgs changed
The Higgs field has a nonzero value in the vacuum. Interactions with it give mass to the W and Z bosons and, through Yukawa couplings, to elementary fermions. This is not an explanation of every kilogram around us: most of the mass of ordinary matter comes from the strong-interaction energy inside protons and neutrons.
Finding the Higgs boson in 2012 gave physicists a new measurement with which to test this account. Its mass, together with the top quark's properties and other inputs, also bears on how the Higgs potential behaves at much higher energies. CERN's account of the discovery and subsequent measurements explains why those questions remain open.
Here the calculation becomes conditional. Extrapolate the Standard Model far beyond the energies directly tested, and measured parameters can put the electroweak vacuum in a metastable region. The result depends on those parameters and on what physics, if any, appears beyond the Standard Model. It cannot be read off a simple ratio of two particle masses.
- MeasurementsHiggs, top quark, couplings
- CalculationChoose a theory; extrapolate
- Stability estimateConditional on those choices
Getting through the ridge
The ball picture eventually stops helping. A classical ball needs enough energy to cross the ridge. A quantum field can make a transition through tunnelling. Sidney Coleman's 1977 work on the fate of the false vacuum gave a way to calculate this kind of decay.
In the picture he studied, a bubble of the lower-energy phase forms and grows. Its wall can approach the speed of light. Inside it, the vacuum has changed. What matter can exist there depends on the theory describing that vacuum. “Everything disappears” is a memorable line; it is less informative than asking what properties of matter would change.
This is also different from the technical idea called a bubble of nothing. A new vacuum phase is not simply a hole where space has been erased.
- Metastable phaseLocally long-lived
- TunnellingA new phase nucleates
- Expanding bubbleThe vacuum changes inside
A lifetime with assumptions attached
Standard Model calculations can give our vacuum a lifetime enormously longer than the present age of the universe. Buttazzo and colleagues' analysis studies the stability boundary and its dependence on the measured parameters. Those calculations are a reason to investigate fundamental physics, not a countdown.
There is an important limitation to carry with the number. A precise answer obtained by extrapolating a theory is still conditional on that theory continuing to apply. Additional fields or interactions at higher energies could change the potential. Even more precise measurements at the LHC would not remove every assumption about unobserved physics. Branchina and colleagues examine how new physics can alter the stability analysis.
What catches me is how much depends on a state we usually mistake for absence. The vacuum has properties. Those properties permit the matter we know. We can measure some of them and calculate further than we can observe, but the calculation eventually reaches questions the measurements have not answered.
The universe has lasted long enough for us to ask whether its valley is the deepest one. So far, that question remains open.