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)