How to pronounce "hanging"

So last year, on the Fourth of July,

experiments at the Large Hadron Collider

discovered the Higgs boson.

It was a historical day.

There's no doubt that from now on,

the Fourth of July will be remembered

not as the day of the Declaration of Independence,

but as the day of the discovery of the Higgs boson.

Well, at least, here at CERN.

But for me, the biggest surprise of that day

was that there was no big surprise.

In the eye of a theoretical physicist,

the Higgs boson is a clever explanation

of how some elementary particles gain mass,

but it seems a fairly unsatisfactory

and incomplete solution.

Too many questions are left unanswered.

The Higgs boson does not share the beauty,

the symmetry, the elegance,

of the rest of the elementary particle world.

For this reason, the majority of theoretical physicists

believe that the Higgs boson could not

be the full story.

We were expecting new particles and new phenomena

accompanying the Higgs boson.

Instead, so far, the measurements

coming from the LHC show no signs of new particles

or unexpected phenomena.

Of course, the verdict is not definitive.

In 2015, the LHC will almost double

the energy of the colliding protons,

and these more powerful collisions

will allow us to explore further the particle world,

and we will certainly learn much more.

But for the moment, since we have found

no evidence for new phenomena, let us suppose

that the particles that we know today,

including the Higgs boson,

are the only elementary particles in nature,

even at energies much larger

than what we have explored so far.

Let's see where this hypothesis is going to lead us.

We will find a surprising and intriguing result

about our universe, and to explain my point,

let me first tell you what the Higgs is about,

and to do so, we have to go back

to one tenth of a billionth of a second

after the Big Bang.

And according to the Higgs theory,

at that instant, a dramatic event took place

in the universe.

Space-time underwent a phase transition.

It was something very similar to the phase transition

that occurs when water turns into ice

below zero degrees.

But in our case, the phase transition

is not a change in the way the molecules

are arranged inside the material,

but is about a change

of the very fabric of space-time.

During this phase transition, empty space

became filled with a substance

that we now call Higgs field.

And this substance may seem invisible to us,

but it has a physical reality.

It surrounds us all the time,

just like the air we breathe in this room.

And some elementary particles interact

with this substance, gaining energy in the process.

And this intrinsic energy is what we call

the mass of a particle,

and by discovering the Higgs boson, the LHC

has conclusively proved that this substance is real,

because it is the stuff the Higgs bosons are made of.

And this, in a nutshell, is the essence of the Higgs story.

But this story is far more interesting than that.

By studying the Higgs theory,

theoretical physicists discovered,

not through an experiment

but with the power of mathematics,

that the Higgs field does not necessarily exist

only in the form that we observe today.

Just like matter can exist as liquid or solid,

so the Higgs field, the substance that fills all space-time,

could exist in two states.

Besides the known Higgs state,

there could be a second state in which the Higgs field

is billions and billions times denser

than what we observe today,

and the mere existence of another state

of the Higgs field poses a potential problem.

This is because, according to the laws

of quantum mechanics, it is possible

to have transitions between two states,

even in the presence of an energy barrier

separating the two states,

and the phenomenon is called,

quite appropriately, quantum tunneling.

Because of quantum tunneling,

I could disappear from this room

and reappear in the next room,

practically penetrating the wall.

But don't expect me to actually perform the trick

in front of your eyes, because the probability

for me to penetrate the wall is ridiculously small.

You would have to wait a really long time

before it happens, but believe me,

quantum tunneling is a real phenomenon,

and it has been observed in many systems.

For instance, the tunnel diode,

a component used in electronics,

works thanks to the wonders

of quantum tunneling.

But let's go back to the Higgs field.

If the ultra-dense Higgs state existed,

then, because of quantum tunneling,

a bubble of this state could suddenly appear

in a certain place of the universe at a certain time,

and it is analogous to what happens when you boil water.

Bubbles of vapor form inside the water,

then they expand, turning liquid into gas.

In the same way, a bubble of the ultra-dense Higgs state

could come into existence because of quantum tunneling.

The bubble would then expand at the speed of light,

invading all space, and turning the Higgs field

from the familiar state into a new state.

Is this a problem? Yes, it's a big a problem.

We may not realize it in ordinary life,

but the intensity of the Higgs field is critical

for the structure of matter.

If the Higgs field were only a few times more intense,

we would see atoms shrinking, neutrons decaying

inside atomic nuclei, nuclei disintegrating,

and hydrogen would be

the only possible chemical element in the universe.

And the Higgs field, in the ultra-dense Higgs state,

is not just a few times more intense than today,

but billions of times,

and if space-time were filled by this Higgs state,

all atomic matter would collapse.

No molecular structures would be possible, no life.

So, I wonder, is it possible

that in the future, the Higgs field

will undergo a phase transition and,

through quantum tunneling, will be transformed

into this nasty, ultra-dense state?

In other words, I ask myself, what is the fate

of the Higgs field in our universe?

And the crucial ingredient necessary

to answer this question is the Higgs boson mass.

And experiments at the LHC found that the mass

of the Higgs boson is about 126 GeV.

This is tiny when expressed in familiar units,

because it's equal to something like

10 to the minus 22 grams,

but it is large in particle physics units,

because it is equal to the weight

of an entire molecule

of a DNA constituent.

So armed with this information from the LHC,

together with some colleagues here at CERN,

we computed the probability

that our universe could quantum tunnel

into the ultra-dense Higgs state,

and we found a very intriguing result.

Our calculations showed

that the measured value of the Higgs boson mass

is very special.

It has just the right value

to keep the universe hanging

in an unstable situation.

The Higgs field is in a wobbly configuration

that has lasted so far

but that will eventually collapse.

So according to these calculations,

we are like campers

who accidentally set their tent

at the edge of a cliff.

And eventually, the Higgs field

will undergo a phase transition

and matter will collapse into itself.

So is this how humanity is going to disappear?

I don't think so.

Our calculation shows that quantum tunneling

of the Higgs field is not likely to occur

in the next 10 to the 100 years,

and this is a very long time.

It's even longer than

the time it takes for Italy to form a stable government.

(Laughter)

Even so, we will be long gone by then.

In about five billion years,

our sun will become a red giant,

as large as the Earth's orbit,

and our Earth will be kaput,

and in a thousand billion years,

if dark energy keeps on fueling

space expansion at the present rate,

you will not even be able to see as far as your toes,

because everything around you

expands at a rate faster than the speed of light.

So it is really unlikely

that we will be around to see the Higgs field collapse.

But the reason why I am interested

in the transition of the Higgs field

is because I want to address the question,

why is the Higgs boson mass so special?

Why is it just right to keep the universe

at the edge of a phase transition?

Theoretical physicists always ask "why" questions.

More than how a phenomenon works,

theoretical physicists are always interested in

why a phenomenon works in the way it works.

We think that this these "why" questions

can give us clues

about the fundamental principles of nature.

And indeed, a possible answer to my question

opens up new universes, literally.

It has been speculated that our universe

is only a bubble in a soapy multiverse

made out of a multitude of bubbles,

and each bubble is a different universe

with different fundamental constants

and different physical laws.

And in this context, you can only talk about

the probability of finding a certain value of the Higgs mass.

Then the key to the mystery

could lie in the statistical properties

of the multiverse.

It would be something like what happens

with sand dunes on a beach.

In principle, you could imagine to find sand dunes

of any slope angle in a beach,

and yet, the slope angles of sand dunes

are typically around 30, 35 degrees.

And the reason is simple:

because wind builds up the sand, gravity makes it fall.

As a result, the vast majority of sand dunes

have slope angles around the critical value,

near to collapse.

And something similar could happen

for the Higgs boson mass in the multiverse.

In the majority of bubble universes,

the Higgs mass could be around the critical value,

near to a cosmic collapse of the Higgs field,

because of two competing effects,

just as in the case of sand.

My story does not have an end,

because we still don't know the end of the story.

This is science in progress,

and to solve the mystery, we need more data,

and hopefully, the LHC will soon add new clues

to this story.

Just one number, the Higgs boson mass,

and yet, out of this number we learn so much.

I started from a hypothesis, that the known particles

are all there is in the universe,

even beyond the domain explored so far.

From this, we discovered that the Higgs field

that permeates space-time may be standing

on a knife edge, ready for cosmic collapse,

and we discovered that this may be a hint

that our universe is only a grain of sand

in a giant beach, the multiverse.

But I don't know if my hypothesis is right.

That's how physics works: A single measurement

can put us on the road to a new understanding

of the universe

or it can send us down a blind alley.

But whichever it turns out to be,

there is one thing I'm sure of:

The journey will be full of surprises.

Thank you.

(Applause)