False vacuum decay

If empty space is not at its lowest energy, a bubble of true vacuum could grow at light speed: what that means, how it could start, and why it is no danger

A what-if: physics allows it on paper, but no one has ever seen one. Everything shown follows the theory as it stands.

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Empty space is not empty

Particles get their mass from how strongly they feel that value. The electron feels it weakly and is light; the top quark feels it most of all and weighs nearly as much as a gold atom. Change the value and every mass changes with it, and with the masses go the sizes of atoms and the whole of chemistry.

The Higgs boson, found at CERN in 2012, is a ripple in this field. The violet shimmer is drawn: the field itself cannot be seen and would look the same everywhere.

A ball in a dip

Picture the field's energy as a landscape and our vacuum as a ball resting in a dip. If a deeper valley lies beyond the hill, ours is a false vacuum: steady, but not the lowest.

A ball in a dip is stable against small nudges: push it a little and it rolls back. But if the land beyond the hill goes lower still, the dip is only metastable, like supercooled water, which stays liquid below freezing until something starts the ice. Physicists call such a state a false vacuum, and the lowest valley the true vacuum.

Two measured masses

Degrassi and colleagues showed this in 2012, the year the Higgs boson was found, and later work agrees. The masses are the Particle Data Group's averages of the ATLAS, CMS and Tevatron measurements. They sit close to the line, which some physicists take as a clue in itself; a much heavier top quark would have made the vacuum decay long before now.

It is not certain. The top quark's mass is hard to tie to the number the theory needs, and the strength of the strong force adds doubt. Andreassen, Frost and Schwartz worked out in 2018 that ruling out a stable vacuum for sure would need the top quark's mass known to within 250 MeV.

How it could start

Quantum physics lets things cross barriers they could not climb, the way an alpha particle escapes a nucleus. For the vacuum it means that a small region could jump to the lower valley with no cause at all. A bubble too small shrinks away again; one just above a critical size gains more energy from its inside than its wall costs, and grows.

That figure holds only if the Standard Model is the whole story up to enormous energies. Unknown particles could make the vacuum stable, or shorten its life. Some theorists have asked whether tiny black holes could seed bubbles (Burda, Gregory and Moss, 2015), an idea with no evidence either way.

No warning

Once a bubble is big enough to grow, the energy freed as the vacuum drops lower pushes its wall ever faster, and within a tiny time it moves at practically the speed of light. Seen from outside, the bubble grows by a light-year every year: across the Milky Way's 100,000 light-years in about 100,000 years.

Light from the bubble's birth travels only just ahead of its wall, so there is nothing to see until it is here. The picture shows the bubble from outside as no eye could: in reality the stars ahead of it would shine on undisturbed until the moment the wall arrived. It would cross a person in a few billionths of a second, far faster than any nerve can signal, so no one would feel it.

Inside the bubble

Inside, the Higgs field would hold a vastly larger value, and the masses of particles and the forces between them would change with it. Atoms, and so chemistry and life, could not exist there.

Every particle that gets its mass from the Higgs field would weigh enormously more inside the bubble, and the balance of forces that holds atoms together would be lost. What the laws there would look like in detail is not known: the Standard Model cannot be trusted at such field values. What is clear is that the matter we are made of would not survive the wall.

Gravity makes it worse. The true vacuum lies lower than ours, whose energy is almost exactly zero, so its energy would be below zero, and Coleman and De Luccia showed in 1980 that space inside such a bubble would then collapse in on itself. The dark violet inside is drawn: no one knows what, if anything, it would look like.

Can a collider start one?

No. Cosmic rays strike the Moon, the planets and the stars with far more energy than any collider can reach, and have done so for billions of years. The vacuum is still here.

Piet Hut and Martin Rees made this argument in 1983: if collisions at such energies could set off a bubble, one would have formed long ago somewhere in our past, and we would not be here. The LHC's safety review of 2008 came to the same answer. Colliders recreate in the lab what nature does far more violently every day.

Is it real?

No one knows. Our vacuum may be the lowest after all: the answer hangs on the top quark's exact mass and on physics beyond the Standard Model, which we know must exist.

The Standard Model leaves out dark matter, gravity at the highest energies and much else. New particles at high energies could change the shape of the Higgs field's energy, making our vacuum stable, or less so. Sharper measurements of the top quark, at the LHC and at future colliders, will narrow the answer down.