Have we reached the end of physics Harry Cliff
A hundred years ago this month,
a 36-year-old Albert Einstein
stood up in front of the Prussian
Academy of Sciences in Berlin
to present a radical new theory
of space, time and gravity:
the general theory of relativity.
General relativity is unquestionably
Einstein’s masterpiece,
a theory which reveals the workings
of the universe at the grandest scales,
capturing in one beautiful line of algebra
everything from why apples fall from trees
to the beginning of time and space.
1915 must have been an exciting year
to be a physicist.
Two new ideas were turning
the subject on its head.
One was Einstein’s theory of relativity,
the other was arguably
even more revolutionary:
quantum mechanics,
a mind-meltingly strange
yet stunningly successful new way
of understanding the microworld,
the world of atoms and particles.
Over the last century,
these two ideas have utterly transformed
our understanding of the universe.
It’s thanks to relativity
and quantum mechanics
that we’ve learned
what the universe is made from,
how it began
and how it continues to evolve.
A hundred years on, we now find ourselves
at another turning point in physics,
but what’s at stake now
is rather different.
The next few years may tell us
whether we’ll be able
to continue to increase
our understanding of nature,
or whether maybe for the first time
in the history of science,
we could be facing questions
that we cannot answer,
not because we don’t have
the brains or technology,
but because the laws of physics
themselves forbid it.
This is the essential problem:
the universe is far, far too interesting.
Relativity and quantum mechanics
appear to suggest
that the universe
should be a boring place.
It should be dark, lethal and lifeless.
But when we look around us, we see we live
in a universe full of interesting stuff,
full of stars, planets, trees, squirrels.
The question is, ultimately,
why does all this interesting stuff exist?
Why is there something
rather than nothing?
This contradiction is the most pressing
problem in fundamental physics,
and in the next few years, we may find out
whether we’ll ever be able to solve it.
At the heart of this problem
are two numbers,
two extremely dangerous numbers.
These are properties of the universe
that we can measure,
and they’re extremely dangerous
because if they were different,
even by a tiny bit,
then the universe as we know it
would not exist.
The first of these numbers is associated
with the discovery that was made
a few kilometers from this hall,
at CERN, home of this machine,
the largest scientific device
ever built by the human race,
the Large Hadron Collider.
The LHC whizzes subatomic particles
around a 27-kilometer ring,
getting them closer and closer
to the speed of light
before smashing them into each other
inside gigantic particle detectors.
On July 4, 2012, physicists
at CERN announced to the world
that they’d spotted
a new fundamental particle
being created at the violent collisions
at the LHC: the Higgs boson.
Now, if you followed the news at the time,
you’ll have seen a lot of physicists
getting very excited indeed,
and you’d be forgiven for thinking
we get that way every time
we discover a new particle.
Well, that is kind of true,
but the Higgs boson
is particularly special.
We all got so excited
because finding the Higgs
proves the existence
of a cosmic energy field.
Now, you may have trouble
imagining an energy field,
but we’ve all experienced one.
If you’ve ever held a magnet
close to a piece of metal
and felt a force pulling across that gap,
then you’ve felt the effect of a field.
And the Higgs field
is a little bit like a magnetic field,
except it has a constant value everywhere.
It’s all around us right now.
We can’t see it or touch it,
but if it wasn’t there,
we would not exist.
The Higgs field gives mass
to the fundamental particles
that we’re made from.
If it wasn’t there, those particles
would have no mass,
and no atoms could form
and there would be no us.
But there is something deeply mysterious
about the Higgs field.
Relativity and quantum mechanics tell us
that it has two natural settings,
a bit like a light switch.
It should either be off,
so that it has a zero value
everywhere in space,
or it should be on so it has
an absolutely enormous value.
In both of these scenarios,
atoms could not exist,
and therefore all the other
interesting stuff
that we see around us
in the universe would not exist.
In reality, the Higgs field
is just slightly on,
not zero but 10,000 trillion times weaker
than its fully on value,
a bit like a light switch that’s got stuck
just before the off position.
And this value is crucial.
If it were a tiny bit different,
then there would be
no physical structure in the universe.
So this is the first
of our dangerous numbers,
the strength of the Higgs field.
Theorists have spent decades
trying to understand
why it has this very peculiarly
fine-tuned number,
and they’ve come up
with a number of possible explanations.
They have sexy-sounding names
like “supersymmetry”
or “large extra dimensions.”
I’m not going to go
into the details of these ideas now,
but the key point is this:
if any of them explained this weirdly
fine-tuned value of the Higgs field,
then we should see new particles
being created at the LHC
along with the Higgs boson.
So far, though, we’ve not seen
any sign of them.
But there’s actually an even worse example
of this kind of fine-tuning
of a dangerous number,
and this time it comes
from the other end of the scale,
from studying the universe
at vast distances.
One of the most important consequences
of Einstein’s general theory of relativity
was the discovery that the universe began
as a rapid expansion of space and time
13.8 billion years ago, the Big Bang.
Now, according to early versions
of the Big Bang theory,
the universe has been expanding ever since
with gravity gradually putting
the brakes on that expansion.
But in 1998, astronomers made
the stunning discovery
that the expansion of the universe
is actually speeding up.
The universe is getting
bigger and bigger faster and faster
driven by a mysterious repulsive force
called dark energy.
Now, whenever you hear
the word “dark” in physics,
you should get very suspicious
because it probably means
we don’t know what we’re talking about.
(Laughter)
We don’t know what dark energy is,
but the best idea is that it’s the energy
of empty space itself,
the energy of the vacuum.
Now, if you use good old
quantum mechanics to work out
how strong dark energy should be,
you get an absolutely astonishing result.
You find that dark energy
should be 10 to the power
of 120 times stronger
than the value we observe from astronomy.
That’s one with 120 zeroes after it.
This is a number so mind-bogglingly huge
that it’s impossible
to get your head around.
We often use the word “astronomical”
when we’re talking about big numbers.
Well, even that one won’t do here.
This number is bigger
than any number in astronomy.
It’s a thousand trillion
trillion trillion times bigger
than the number of atoms
in the entire universe.
So that’s a pretty bad prediction.
In fact, it’s been called
the worst prediction in physics,
and this is more than just
a theoretical curiosity.
If dark energy were
anywhere near this strong,
then the universe
would have been torn apart,
stars and galaxies could not form,
and we would not be here.
So this is the second
of those dangerous numbers,
the strength of dark energy,
and explaining it requires an even more
fantastic level of fine-tuning
than we saw for the Higgs field.
But unlike the Higgs field,
this number has no known explanation.
The hope was that a complete combination
of Einstein’s general
theory of relativity,
which is the theory
of the universe at grand scales,
with quantum mechanics, the theory
of the universe at small scales,
might provide a solution.
Einstein himself
spent most of his later years
on a futile search
for a unified theory of physics,
and physicists have kept at it ever since.
One of the most promising candidates
for a unified theory is string theory,
and the essential idea is,
if you could zoom in on the fundamental
particles that make up our world,
you’d see actually
that they’re not particles at all,
but tiny vibrating strings of energy,
with each frequency of vibration
corresponding to a different particle,
a bit like musical notes
on a guitar string.
So it’s a rather elegant, almost poetic
way of looking at the world,
but it has one catastrophic problem.
It turns out that string theory
isn’t one theory at all,
but a whole collection of theories.
It’s been estimated, in fact,
that there are 10 to the 500
different versions of string theory.
Each one would describe
a different universe
with different laws of physics.
Now, critics say this makes
string theory unscientific.
You can’t disprove the theory.
But others actually
turned this on its head
and said, well,
maybe this apparent failure
is string theory’s greatest triumph.
What if all of these 10 to the 500
different possible universes
actually exist out there somewhere
in some grand multiverse?
Suddenly we can understand
the weirdly fine-tuned values
of these two dangerous numbers.
In most of the multiverse,
dark energy is so strong
that the universe gets torn apart,
or the Higgs field is so weak
that no atoms can form.
We live in one of the places
in the multiverse
where the two numbers are just right.
We live in a Goldilocks universe.
Now, this idea is extremely controversial,
and it’s easy to see why.
If we follow this line of thinking,
then we will never be able
to answer the question,
“Why is there something
rather than nothing?”
In most of the multiverse,
there is nothing,
and we live in one of the few places
where the laws of physics
allow there to be something.
Even worse, we can’t test
the idea of the multiverse.
We can’t access these other universes,
so there’s no way of knowing
whether they’re there or not.
So we’re in an extremely
frustrating position.
That doesn’t mean
the multiverse doesn’t exist.
There are other planets,
other stars, other galaxies,
so why not other universes?
The problem is, it’s unlikely
we’ll ever know for sure.
Now, the idea of the multiverse
has been around for a while,
but in the last few years,
we’ve started to get the first solid hints
that this line of reasoning
may get born out.
Despite high hopes
for the first run of the LHC,
what we were looking for there –
we were looking
for new theories of physics:
supersymmetry or large extra dimensions
that could explain this weirdly
fine-tuned value of the Higgs field.
But despite high hopes, the LHC
revealed a barren subatomic wilderness
populated only by a lonely Higgs boson.
My experiment published paper after paper
where we glumly had to conclude
that we saw no signs of new physics.
The stakes now could not be higher.
This summer, the LHC began
its second phase of operation
with an energy almost double
what we achieved in the first run.
What particle physicists
are all desperately hoping for
are signs of new particles,
micro black holes,
or maybe something totally unexpected
emerging from the violent collisions
at the Large Hadron Collider.
If so, then we can continue
this long journey
that began 100 years ago
with Albert Einstein
towards an ever deeper understanding
of the laws of nature.
But if, in two or three years' time,
when the LHC switches off again
for a second long shutdown,
we’ve found nothing but the Higgs boson,
then we may be entering
a new era in physics:
an era where there are weird features
of the universe that we cannot explain;
an era where we have hints
that we live in a multiverse
that lies frustratingly
forever beyond our reach;
an era where we will never be able
to answer the question,
“Why is there something
rather than nothing?”
Thank you.
(Applause)
Bruno Giussani: Harry,
even if you just said
the science may not have some answers,
I would like to ask you a couple
of questions, and the first is:
building something like the LHC
is a generational project.
I just mentioned, introducing you,
that we live in a short-term world.
How do you think so long term,
projecting yourself out a generation
when building something like this?
Harry Cliff: I was very lucky
that I joined the experiment
I work on at the LHC in 2008,
just as we were switching on,
and there are people in my research group
who have been working on it
for three decades,
their entire careers on one machine.
So I think the first conversations
about the LHC were in 1976,
and you start planning the machine
without the technology
that you know you’re going to need
to be able to build it.
So the computing power
did not exist in the early ’90s
when design work began in earnest.
One of the big detectors
which record these collisions,
they didn’t think there was technology
that could withstand the radiation
that would be created in the LHC,
so there was basically a lump of lead
in the middle of this object
with some detectors around the outside,
but subsequently
we have developed technology.
So you have to rely on people’s ingenuity,
that they will solve the problems,
but it may be a decade
or more down the line.
BG: China just announced
two or three weeks ago
that they intend to build
a supercollider twice the size of the LHC.
I was wondering how you
and your colleagues welcome the news.
HC: Size isn’t everything, Bruno.
BG: I’m sure. I’m sure.
(Laughter)
It sounds funny for a particle
physicist to say that.
But I mean, seriously, it’s great news.
So building a machine like the LHC
requires countries from all over the world
to pool their resources.
No one nation can afford
to build a machine this large,
apart from maybe China,
because they can mobilize
huge amounts of resources,
manpower and money
to build machines like this.
So it’s only a good thing.
What they’re really planning to do
is to build a machine
that will study the Higgs boson in detail
and could give us some clues
as to whether these new ideas,
like supersymmetry, are really out there,
so it’s great news for physics, I think.
BG: Harry, thank you.
HC: Thank you very much.
(Applause)