What exactly is vacuum decay?
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A rule of the universe is that **”things” like to have as little energy as possible**, because that means they are more stable. To use a real-life, hands-on example: a ball at the top of a hill generally doesn’t want to stay there – it wants to roll down the hill. A **decay (coming from the latin for “to fall off”)** is the general term for a process by which the thing goes from the less stable to the more stable state, so in this case the ball “decays” by rolling down the hill.
By rolling down the hill it ends up more stable as a result. Once its at the bottom of the hill, there’s nowhere else for it to roll, and balls generally don’t roll uphill, and if they do, they will roll straight back down. So at the bottom of the hill, we say the ball is in **a stable state**. It will stay there indefinitely, unless acted upon. You could put your own energy into returning it to the top, but otherwise it will stay put.
However, there might be circumstances where the ball **WANTS** to lose its energy, but can’t. For example, if there’s a big crater at the top of the hill. The ball won’t just spontaneously roll down the hill, as to do so it would need to get over the small hump of the crater first. But equally it’s unlikely to just roll up over the crater lip by itself. So the ball might appear to be “stable” but its not **AS** stable as truly being at the bottom of the hill. This is what we call a **metastable state,** or if you prefer, a **false** **stability**.
However if it gets some energy from somewhere (say, someone kicking it), it might be able to get out of this metastable state and then plummet down the hill to the **true stable state**.
Now, on the scale of the universe, **”vacuum”** (i.e., space and time with absolutely nothing in it, is generally treated as the universal “bottom” of the hill. There’s no particles to lose energy, nothing moving, no way to get more “stable”.
Or is there?
What if **spacetime itself** was in its own metaphorical “crater” at the top of another metaphorical “hill”, so to speak? What if the vacuum that we see, and assume IS stable, was in fact, just in a metastable state **(a “false vacuum”)** and could in fact get more stable, if it was given the right kick? That’s **vacuum decay.**
And if it happens, we have problems, because just as a big ball rolling down the hill can break things on the way down, the vacuum decaying will break things. And by things, I mean, **all the laws of physics.** And all matter in the universe, which depends on these laws functioning exactly how they do, suddenly breaks as well. **And we go “poof”.**
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