First Experimental Test of Process that Might Have Created Universe (Patreon)

[This is a transcript with links to references.]

One of the most disturbing ideas in physics, or maybe *the most disturbing idea, is that space can fall apart. That’s because it could be what’s called “false” vacuum. A false vacuum can remain in this innocent reliable looking form for billions of years, but eventually a quantum fluctuation could be enough to cause it to decay. This would release enormous amounts of energy and kill all of us. Yes, cheerful thought.

So far, this has all been theory. But a new experiment has now for the first time observed that quantum fluctuations can actually trigger a transition like that, luckily not for the entire universe, but for a small and safe test-setting in the laboratory with ultracold gases. Let’s have a look.

You’d think that empty space is about as boring as it gets. But the question of just what it means for space to be empty is more difficult than you might think. You see, physicists believe that our entire universe was in some sense created from empty space. And if the empty space which we find around us now is not a true but a “false vacuum”, then it will eventually decay, too.

These vacuum decays are similar to phase transitions that we can observe in nature around us.
A good example of such a phase transition is chemical handwarmers like this one that I dug out of my children’s room. These handwarmers contain a liquid which is supercooled with a thin metal plate in it. That it’s supercooled means that it’s at a temperature below its freezing point, yet it isn’t a solid.

Such a supercooled liquid contains energy that it wants to release by switching into another more stable solid state. You can trigger this transition by a large enough perturbation,  for example pressure or a small electric spark by twisting this metal plate. The liquid then suddenly makes a transition to a solid, and the energy is released. This makes it warm up, which is why the thing is used as a handwarmer.

In physics, a state that isn’t totally stable, but kinda stable until you disturb it too much is called “meta-stable”. It’s nothing to do with metaphysis, I swear.  Maybe they should just have called it kinda-stable. Metastable states are like teenagers basically. Just fine if you leave them alone, but if you disturb them, weird things can happen.

What happens with meta-stable states is often summarized in an energy diagram like this. The vertical axis is the energy of the system and the horizontal axis some parameter that describes the phase transition you could think of fluid stuff to the left and solid to the right. This upper valley is the metastable state. You see if the system is perturbed a bit, it will roll back into the same valley. Disturb it too much and it’ll roll over into this other valley.

Normally such phase transitions are caused by external forces.  But theoretically they can also happen by quantum fluctuations in which case the system tunnels from the metastable to the stable state. These quantum fluctuations need no external cause. They just happen all by themselves. It’s god rolling dice, as --Einstein put it.

And this is one of the theories for how our universe might have been created with all that stuff in it.  Once upon a time there was a false vacuum. It decayed and released a lot of energy that energy turned into matter. Some billions of years passed and now there’s us, the leftovers of that energy from the false vacuum.

Now you might think that if our universe was created by vacuum decay then it can’t decay again. But it isn’t that easy. You can easily imagine there being a sequence of these metastable states. So maybe our universe will decay again. Physicists believe that this would happen by a process called “bubble nucleation” in which a quantum fluctuation triggers the decay in one place, and then it spreads to neighbouring places.

All of this is theory, mathematics equations. So far no one has actually seen a quantum fluctuation trigger such a phase transition or a bubble nucleation.  And this is now what the new paper is about.

They didn’t look at vacuum decay, but a phase transition triggered by quantum fluctuations and the bubble nucleation. For this they used a cloud of sodium atoms cooled to near absolute zero and put it into a magnetic field. The sodium atoms have spins, and they want to align themselves along the magnetic field.  They also want to align themselves towards each other though.  This creates a metastable state in which the atoms are all aligned in the same direction but not ideally aligned with the external field. Increase the strength of the external field and they’ll start flipping. They don’t flip on the borders because the magnetic field is weaker there.

This is what they observed, and what you see in this video which shows the transition from the metastable phase that is blue to the stable phase that is red. You can see that there’s actually some oscillation going on and a kind of antibubble in the bubble forming.

What does that mean? It means they have confirmed that these theories of quantum triggered phase transitions are correct and they also measured the dependence of the probability of the decay as a function of the energy. They’re now planning to do more experiments but don’t worry that they’ll accidentally destroy the universe, I think it’s very unlikely.