The universe is going to end.

But of all the  possible ends of the universe vacuum decay   would have to be the most thorough - because  it could totally rewrite the laws of physics.

Today I hope to help you understand  exactly how terrified you should be.

It seems pretty lucky that the universe  is how it is.

Its just the right size,   has just the right expansion rate, and has just the right particle properties to allow stars and   planets and people to exist.

The habitability of our universe is largely defined by the properties   of the quantum fields that pervade all space.

The quantum fields give rise to the particles   that make up all matter and all forces - and  if those quantum fields were a bit different,   none of the familiar structures from atoms  to galaxies - would be possible.

In fact,   for most possible configurations of the quantum fields, no structures would exist at all.

Fortunately things are they way they are … although don’t get too cosy - there’s   at least one mechanism that could rewrite  the laws of physics across the universe.

That mechanism is vacuum decay, and  some physicists think it’s inevitable.

First to give you a crude picture - vacuum  decay looks like a bubble of annihilation   that expands at the speed of light, rewriting  the nature of the quantum fields as it passes.

To understand whether and when this might happen,   we first need to understand the  quantum fields that it threatens.

For that let’s try an analogy.

Think of all space as being sort of springy at every point.

As a   simplistic example, imagine a rubber ring at each point.

If you compress the ring in one direction   it bounces back and begins to oscillate around its equilibrium shape.

But each ring is also connected   to its neighboring rings, and so it can transfer its oscillations, causing a wave to propagate   through space.

And there are other ways for the rings to oscillate - it has different vibrational   modes.

For example it could twist back and forth on different axes, or deform or more complex ways.

We can think of each quantum field as a set of these modes of oscillation.

And each quantum field   - each type of oscillation - has a corresponding particle - that’s the oscillation itself.

Just like the deformed ring, a quantum field  wants to return to its equilibrium position.

That position is the field value where the energy is minimized.

Physicists like to represent this   by plotting the energy in the  quantum field versus field value.

It takes more energy to get further away  from the minimum - the equilibrium point,   just like it takes more energy if you want to deform the rubber ring.

Another nice physical analogy is a ball rolling in a dip.

The ball will roll up and down,   oscillating around the equilibrium, and the height it reaches depends on how much energy it has.

For most quantum fields, the minimum  energy is where the field value is zero.

For example, for an electromagnetic wave  - a photon - the electric and magnetic   fields rise and fall between positive and  negative values, but the average field value   is zero.

And after the photon passes the  electromagnetic field goes back to zero.

But there’s one quantum field that breaks these rules.

That’s the Higgs field.

The minimum energy   state of the Higgs field is not where the field strength is zero.

Instead, the Higgs field likes   to settle into an equilibrium value where it  has a real, positive value.

This means that   the entire universe is filled with this soup of  Higgsiness.

Most elementary particles that have   mass gain their mass due to interactions with this ubiquitous field.

I go into all the detail of the   mass-granting power of the Higgs mechanism in a previous video.

For today, all you need to know is   that there's this one quantum field that has a minimum energy value where the field strength is non-zero - and   that this also means that the energy carried by the Higgs field is non-zero, even at this minimum.

And that's where the trouble starts.

The Higgs field may have yet another weird property.

It may have more than one possible minimum value.

This would look like multiple dips in   our graph of energy versus field strength.

A quantum field with multiple minima   like this will tend to find its way into one  of these dips and get stuck there.

Just like   a ball rolling on an undulating surface.

In order to move between dips, the ball,   or the quantum field, needs to gain enough  energy to overcome this barrier between them.

That could happen, say, in an extreme energy environment like the big bang or near a black   hole or in a sufficiently large particle accelerator.

But there’s another to make this jump.

The Heisenberg uncertainty principle tells  us that there’s a fundamental uncertainty   in - well, pretty much everything, but certainly in the value of a quantum field.

This results in   fluctuations in the field strength that  can cause it to spontaneously shift,   and perhaps find itself stuck in an adjacent  dip.

We call this quantum tunneling.

So what does this mean for the Higgs field?

Well some theorists do believe that the Higgs   field has at least two minima, and those minima should have different energy values.

There’s a   true minimum - the true lowest energy, or what we call the true vacuum.

This is where the field would   prefer to spend its time given the choice.

And then there’s another local minimum with a higher   energy - what we call that a false vacuum.

The false vacuum is a so-called metastable state - it seems   stable as long as the field doesn’t learn about the more stable, lower energy state.

Somewhat   worryingly, we don’t know which of these minima our universe’s Higgs field is in right now.

Let’s think about the potentially  catastrophic consequences of   this.

Firstly, if the Higgs field is in the  true minimum then no big deal.

Even if the   field at one location of space tunneled  or was excited into the false minimum,   it would quickly find its way back to the true minimum for reasons I’ll get to.

But what if the   entire universe is in the false vacuum?

Then a tunneling event would be a little more dramatic.

Imagine a universe filled with the Higgs field in a false vacuum.

At a single point in space,   a quantum tunneling event drops the field into the true vacuum.

This creates a tiny bubble   that is at an energetically favorable  state compared to the surroundings.

The interior of the bubble immediately  tries to drag the surrounding Higgs field   down with it.

This happens because quantum fields are connected and tug at their   adjacent points across space, which is why  their oscillations propagate as particles.

Now this isn’t necessarily an instant disaster.

The interior of the bubble wants to expand,   but the shell of the bubble is actually in  an energetically unfavorable state - it’s in   this unpleasant transitional state between  the two minima of the Higgs field.

In fact,   the bubble wall experiences a sort of surface tension that tries to collapse the bubble again.

Things get awkward if the initial bubble  exceeds a certain size.

The bigger the bubble,   the more its interior wants to expand into its surroundings.

But you also have more bubble surface,   so more surface tension to resist that  expansion.

But the interior of the bubble   increases with the cube of the radius, while  the surface increases only with radius squared.

That means that above a certain size, the  hunger of the bubble interior dominates.

The bubble grows, and that growth very quickly approaches the speed of light.

Once it gets going,   the bubble is unstoppable, and will drag  the Higgs field through the entire universe   down into the true vacuum.

This is vacuum decay.

It’s a phase transition of the quantum fields.

In fact it has a lot of similarities with the sort of phase transitions that you get in matter.

For example, when water boils - it undergoes a phase transition to water vapor - that   phase transition also starts in small bubbles that grow into its surroundings.

The formation of   these phase-transition bubbles in water or in a quantum field is called bubble nucleation.

OK, let’s review the horrors that  would result from vacuum decay.

First up, everything gets fried.

The energy  released in the decay of the Higgs field fills   the expanding bubble with a hot soup of energetic  particles.

It’s similar to how the energy held in   the latent heat of boiling water is released  into the kinetic energy of gas molecules.

But that’s not the worst of it.

As I mentioned, the Higgs field gives elementary particles their   masses.

Those masses depend on the energy in the field - the so-called vacuum expectation   value.

Drop the energy in the field and you  reduce the masses of the elementary particles.

The ability for stars to form and undergo  nuclear fusion, and the ability for chemistry   to work as it does, depend sensitively on  the properties of the elementary particles.

Life and structure could not exist as we know it, and may not exist at all.

I should also   add that there are other possible types  of vacuum decay beyond Higgs field decay.

For example, there may be weird fields within string theory that exist in false vacuum states.

The decay of those could rewrite the  laws of physics in far more drastic ways.

This all sounds moderately bad.

But can this actually happen?

First let’s ask whether we really   are living in one of these nasty false  vacuums - then we’ll talk about whether   that vacuum might decay.

Assuming the Higgs field really does have multiple minima, the question becomes   which minimum are we in?

The true one or a false one?

It turns out that the theorists aren’t sure.

We can determine the shape of the Higgs field with   precise measurements of the particles  that gain their mass from the Higgs.

The most important are the Higgs particle  itself as well as the top quark, which is   the most massive elementary particle.

And our measurements of these masses tell us that … we are probably in the false vacuum - but we’re actually very close to the boundary.

We   MIGHT be in the true vacuum - which would be good, but it’s a bit more likely that we’re in the bad,   false one.

Further study of the Higgs particle and top quark should help us figure this out.

OK, so the next question is could the vacuum really decay?

As long as decay is possible,   it’s inevitable.

At any instant in time there’s a tiny but real probability that a patch of   space will tunnel between the false and true vacuums.

Wait long enough and it WILL happen.

Happily, there’s only a very small probability  that a bubble of true vacuum will randomly form   that’s large enough to actually continue  growing.

Physicists have not converged on   a single timescale - they vary from roughly the current age of the universe to 10^hundreds times   the age of the universe for a single such bubble to be likely to appear somewhere in our observable universe.

So, somewhere between vaguely  unlikely and staggeringly unlikely.

Before you get too comfortable, remember  there’s another way to hop between   energy minima.

If enough energy can be pumped into a patch of space, the Higgs field can hop   between minima without tunneling.

This would be like dropping specks of dust into superheated   water - dust provides sites for bubble nucleation, and the entire body of water rapidly evaporating.

Now,   there was some fear that our giant particle  colliders like the LHC might nucleate a vacuum   decay bubble.

But that was unfounded.

After all, Earth is constantly being bombarded by cosmic   rays of much, much higher energy than the LHC can produce, and no annihilation just yet.

Overall, it’s very unlikely  that a vacuum decay bubble   will reach us in the lifespan of our species,  let alone your own lifetime.

Of course, in an   infinitely or sufficiently large universe then vacuum decay has definitely started somewhere.

But as long as we’re far enough away we’re safe.

If the vacuum decay starts beyond several billion   light years, the accelerating expansion of the universe will throw us away from it faster than the   bubble of annihilation can grow, even though that growth is at nearly the speed of light.

And this leads to one more comforting fact.

If the vacuum does decay inside our cosmic horizon   we’ll never see it coming.

Because no light  can outpace the expanding bubble to warn us   of its approach.

One moment you might  be watching your favorite YouTube show,   the next … well, there would be  no next.

Who knows - perhaps the   bubble edge has already swallowed your  computer screen a nanosecond ago.

…   I guess not.

Good.

So, just for now let’s  enjoy whatever time we have left - perhaps   mere billions of years - before vacuum decay swallows this fragile and metastable space time.