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)