Faster Than the Speed of Light?

These are the Appennine mountains in Central Italy.

Buried underneath them

is one of the most sophisticated science labs in the world.

Last month, an international group of scientists

working here on a particle physics experiment

called OPERA made an astonishing claim.

They said they had detected particles that seemed to travel

faster than the speed of light.

It was a claim that contradicted

more than 100 years of scientific orthodoxy.

It has created a furore.

If it's true the implications are amazing.

They're mind-blowing. They really will turn things on their heads.

This is earth-shaking if true.

You would be able to travel back in time.

We have to tear up all the textbooks and start again.

My name's Marcus du Sautoy.

I'm a mathematician and as a mathematician,

I'm used to dealing with ideas that seem impossible in the real world.

For me, it's moments like this

when data clashes with theory that are always rather thrilling.

You can almost feel the shudder that passes through the entire

scientific community when a result as strange as this comes out.

Everybody's talking about it.

Is this the moment for a grand new theory to emerge that makes

sense of all the mysteries that still pervade physics?

Or has there just been a mistake in the measurements?

I'm going to explore one of the most dramatic

scientific announcements for a generation.

What does it mean and why does it matter?

Our story starts with light.

For centuries, light has fascinated us.

Our ancestors built monuments to capture light

from the sun at particular times of the year.

Light gives us colour. It's how we see the world.

Light floods the cosmos.

But it has always been mysterious.

One of the biggest mysteries about light is how fast does it travel?

Unravelling this question, would lead to one of the greatest

and most surprising leaps in the history of science.

Until 350 years ago,

many scientists argued that light didn't really travel at all.

It was transmitted instantaneously from source to eye.

But then an astronomer, making careful observations

of the moons of Jupiter showed it took a finite period

of time for light waves to reach Earth.

That meant light travel couldn't be instantaneous.

It had to have a finite speed.

Another puzzle remained.

If light was a wave, then scientists concluded it must be travelling

through some medium, in the same way as sound travels through air.

This medium was given a name - the ether.

It was thought that the ether was able to flow like the wind.

Therefore, light waves that were travelling in the same direction

as the ether should travel faster than those fighting against it.

In the 1880s, scientists tried to measure variations

in the speed of light travelling in different directions.

But to their surprise, they found no difference.

However you measured it, light always went at the same speed.

As the 20th century dawned,

scientists were still wrestling with the strange behaviour of light

and in particular, what speed it travelled at.

The stage was set for the arrival of a genius who would unravel

the mysteries of light

and in the process, transform our understanding of the universe.

In 1902, a young physicist arrived in the Swiss town of Berne.

He trained as a physics and maths teacher in Zurich,

but had been unable to find a teaching job.

Eventually, he found work in the Swiss patent office.

It was far from a lofty, academic institution,

but it turned out to be just the environment he needed.

His name was Albert Einstein.

An amateur scientist, someone who didn't have an academic position.

This patent clerk who worked on physics

when he wasn't doing his day job

was quite an unusual person to be the one

who revolutionised our ideas of space and time.

I don't know what the workload was in the patent office.

Maybe there weren't so much patents coming in

in Switzerland in those days and he had a lot of time to think.

Anyway, somehow or other, he was able to think

very long, very hard and very deep.

The clerk's work gave Einstein time

to ponder thought experiments, deceptively simple scenarios

that enabled him to explore the most complex of concepts.

Einstein was very much an individual, lone scientist

thinking his deep thoughts and, perhaps precisely because

he was working by himself, he got insights other people hadn't seen.

Einstein was fascinated by the mysterious behaviour of light.

It was a wave, yet it also had the properties of a particle,

what came to be known as a photon.

How fast did it travel, he wondered? And did it have a speed limit?

From the age of 16, Einstein had been pondering a thought experiment.

If I look into this shaving mirror and I accelerate faster and faster

towards the speed of light, then does my image suddenly disappear?

If you think about it, the photons from my face have got to travel

the distance from my face to the mirror and if I am going

at the speed of light, then those photons have to go travelling faster

than me. Namely, the light is travelling faster

than the speed of light.

Now, Einstein believed his image wouldn't disappear,

so he started to think about how to resolve this paradox.

In the spring of 1905, Einstein was ready

to launch his ideas on the world.

In that one year, Einstein published four papers, any one of which

would have been enough to create a sensation in their own right.

It was, arguably, one of the most sustained and extraordinary bursts

of scientific creativity the world has ever seen.

One of those papers transformed our understanding of light.

And here it is, all 31 pages of it.

It's an astonishing paper in many different ways, not least

because if I look at one of the papers I have published,

then, at the end, I reference 39 other papers that I rely on.

In Einstein's paper there are no references at all.

It contains a set of scientific laws that define not just our world,

but also our entire universe.

At the centre of these is the statement that the speed of light,

when it travels through a vacuum, is absolute.

Nothing can travel faster.

It was an incredibly audacious piece of reasoning.

Einstein realised that the way we looked at the universe was wrong,

particularly our intuitive sense of how time and space worked.

We can see how, by doing a thought experiment of our own

with the help of the 12.12 to Ashford.

If I shine this torch while standing still on the platform,

then the beam of light from the torch

is going to be going at the speed of light.

That's straightforward.

But what happens to the same beam of light went I'm on a moving train?

Now, I've just asked the conductor how fast the train is going.

He says it's going at 140 miles an hour.

This is the same torch I had on the platform.

If I switch it on, the question is,

for somebody standing outside in the field,

how fast do they think the light is travelling?

Because logic would suggest that the light is travelling

at the speed of light from the torch, but then I need to add on

the 140 miles an hour that the train is going.

But Einstein said no. The speed of light is a constant.

It doesn't matter where you are in the universe, how you measure it,

on a train to Ashford or on a spaceship

travelling across the universe or standing still outside,

the speed of light is the same.

Einstein's brilliance was to realise

that if the speed of light was the same regardless

of where you measured it from, then something else had to give.

He concluded that it was time that was changing.

Time was not a constant.

Instead it changed depending on how quickly you were moving.

The faster you travel, the slower time passes.

Einstein's view of the universe was seen as radical at the time

and it's still hard to grasp.

But over the years, countless experiments have proved him right.

These theories have a practical impact in the real world,

an example being the GPS or Global Positioning System.

'US Naval Observatory Master Clock.

'At the tone, Mountain Daylight Time

'18 hours, 48 minutes, 5 seconds.'

BEEP

GPS uses a network of satellites orbiting

at speeds of 14,000 kilometres per hour

to accurately pinpoint locations all over the globe.

To ensure precision, it's vital that the time kept by the satellites

is the same as the time kept by the receivers on the ground.

But the satellites travel so fast that, compared to the receivers

on earth, time runs slower by seven microseconds a day.

If we didn't use Einstein's theories and take this into account,

the accuracy of our GPS systems would drift

by more than two kilometres a day.

Einstein didn't stop there.

He theorised that not only did light travel at a constant speed,

but that speed was also the speed limit of the universe.

Nothing can travel faster.

That's because of the relationship between mass and energy.

Einstein said that mass and energy were two sides of the same coin.

That means that if the amount of energy an object has increases,

then so does its mass.

Crucially, increasing an object's speed increases its energy.

The faster I travel on this train, the more mass I gain.

For example, if I was travelling at 90% of the speed of light,

then my mass would be twice that as if I was stationary.

The more I accelerate, the more my mass increases

and the more energy I'm going to need to make me accelerate.

Until, when I reach the speed of light,

the equations force my mass to be infinite.

I'm going to need an infinite amount of energy to get there.

But no-one can possess infinite energy however hard they tried.

That's why, according to Einstein, it's impossible

to cross the speed of light barrier.

From a thought experiment,

Einstein was able to radically alter our view of the world.

He concluded that the speed of light is constant

and that nothing with mass can travel faster than the speed.

These concepts are at the heart of our modern understanding

of the universe.

The results picked up by the OPERA team in Italy were so shocking

because they raise serious questions not just about Einstein's theory,

but all the evidence that's been gathered to support it.

That said, in some ways, we shouldn't be so shocked

by the results because the OPERA scientists were studying

one of the strangest and least understood particles there is -

the neutrino.

And if there was one particle that was going to break the rules,

it was this one.

The neutrino's been the bad boy of physics,

basically, putting physicists out of their comfort zone.

I think that's the best way to put it.

A lot of unusual things have been revealed by the neutrino,

so maybe we shouldn't be surprised by this novelty.

There are 16 types of fundamental particles that are the smallest

and simplest building blocks in the universe.

Together, they explain the world and what holds it together.

Three of those elementary particles are neutrinos.

Their assistance was first predicted in 1930

by Austrian physicist Wolfgang Pauli.

But Pauli didn't think it would ever be possible to find one,

because their properties make them incredibly difficult to spot.

It's a very anti-social particle. It doesn't like to talk to the world.

Right now, you are being crossed by billions of neutrinos per second,

and you don't feel them because they go through you.

They go through the earth, through everything without interacting.

And still, the universe is pervaded by them. It's full of them.

There is a swarm of neutrinos going around,

many more neutrinos than particles of light and atoms

and anything you are used to.

It order to understand how neutrinos are able to travel straight

through matter without being noticed,

we need to think about what matter is made of.

Every physical thing in the world around us,

from mountains and buildings to you and me, is made of atoms.

And atoms are made up of a nucleus at the centre,

surrounded by orbiting electrons,

a bit like a solar system with a sun and orbiting planets.

The mind-boggling thing about matter is that, although it looks

and feels solid, it's actually mostly empty space.

There are vast swathes of nothingness between the tiny nucleus

and the orbiting electrons.

And the neutrino is so small without any charge,

that it can pass through this space very easily.

In fact, the neutrino's so tiny that if the atom

is the size of the solar system, the neutrino is the size of a golf ball.

These tiny particles existed in theory for a quarter of a century

without anyone being able to see them.

But then something happened to change that.

The nuclear bomb.

The power of a nuclear bomb comes from a chain reaction

of spitting atomic nuclei.

In the 1950s, a young researcher called Fred Reines

realised that this chain reaction would produce

an intense burst of neutrinos,

and so be the perfect place to hunt for the elusive particle.

But detecting neutrinos from a nuclear explosion wasn't practical.

So Reines turned his attention to the much more controlled

chain reaction in a nuclear reactor.

Although most neutrinos produced by the reactor passed through

the gaps inside atoms, so many neutrinos were produced

that every now and then, one would collide with an atom's nucleus.

When it did, a charged particle would be ejected.

He set up his experiment, which he called Project Poltergeist,

and waited for the characteristic signal of this interaction -

a distinctive double pulse of energy.

In June 1956, Reines announced that he had detected the neutrino.

Since that discovery, we've become a bit more adept at creating

and observing this most elusive of particles.

We've created neutrinos in man-made particle accelerators

like the ones in CERN in Geneva,

as well as detecting them naturally in cosmic rays and from the sun.

We now know they are essential to our existence.

All of the elements are made by nuclear reactions

that would be impossible without neutrinos.

We also know that despite their tiny size,

they do still have a small mass, which means, according to Einstein,

they can't travel faster than the speed of light.

But that theory has now been challenged by a small group

of scientists working in one of the most unusual science labs

in the world.

Assergi is a sleepy town nestled beneath Gran Sasso,

a 3,000 metre peak in the Apennines of Central Italy.

In the early 1980s, a new road was planned here

that would cut right through the mountain.

Italian scientists had a brilliant idea.

They realised that the road would give them

a unique opportunity to create a physics lab like no other.

It would give them easy access to the heart of the mountain,

the perfect place to build a neutrino detector.

Here we have 15 different experiments,

and there are roughly, 100 physicists per day working here.

Neutrinos so rarely interact with matter that it is easy

for an experiment to be swamped by false readings,

readings triggered by naturally occurring radiation

and charged particles such as cosmic rays hitting the experiment.

The only way to study neutrinos is to find some way to weed out

as many of these interfering particles as possible.

Now we are in the middle of the gallery.

And near here, we have the experiments.

On top of us, we have 1,400 metres of rock,

the top of Gran Sasso mountain.

Here, the cosmic rays are very few, because outside,

there are 200 per square metre per second.

Here, just one per square metre per hour. This is a very huge shielding.

Thanks to the mountain above it,

this vast chamber is a natural laboratory for neutrino research.

It was here, in 2008, that scientists began work

on a sophisticated experiment designed to study

the nature of neutrinos.

It was called the Oscillation Project with Emulsion Tracking Apparatus,

or OPERA for short.

At this stage, they had no idea of the impact that OPERA would have.

The OPERA experiment is an experiment designed to study

the properties of neutrinos.

It consists of a huge detector which is designed

to try and find as many of them as it can.

Once it's found them and counted them,

it wants to test their properties and enable us to know more about

what they're doing, what their nature is

and in fact, anything we can find out about them.

To begin with, measuring the speed of neutrinos

was not at the forefront of the scientists' minds.

They were trying to understand how the three different types

of neutrinos were formed and how they behaved.

The first step of the experiment was to create some neutrinos.

For this, they turned to another underground lab,

CERN in Switzerland.

CERN is most famous for the Large Hadron Collider.

But it was two much less-heralded particle accelerators

that began the OPERA experiment.

The scientists started by generating a beam of protons

which they accelerated around CERN's Proton Synchrotron.

The proton beam was then passed into the Super Proton Synchrotron

to accelerate them even further.

The resulting high-energy beam of protons

was slammed into a graphite target.

This produced a cocktail of exotic sub-atomic particles,

including neutrinos, which then flew off through the Earth

in the direction of Gran Sasso.

The 730 kilometre journey took them 2.4 milliseconds.

They came from that direction. Geneva is in that direction.

Several billions of neutrinos are produced every day

at the CERN accelerators.

They go through the Earth's crust and they reach the OPERA detector.

Even with billions of neutrinos streaming into the laboratory,

detecting them still wasn't easy.

The key was the huge detector at the heart of the Gran Sasso lab.

It's made from 150,000 bricks of lead, and weighs 4,500 tonnes.

Lead is particularly dense,

which increases the chances of a neutrino encountering a nucleus.

As the neutrinos smashed into the lead nucleus,

they created charged particles,

which are detected as tiny flashes of light.

You can see that with OPERA, it's a waiting game.

You fire a neutrino beam and you wait,

and you count as many of these interactions as you can.

The process generated about 30 flashes of light a day,

and provided a chance to test more

than just the type of neutrino arriving.

The nice thing about this experiment is, although it was set up

to study the behaviour of neutrinos in a very fundamental sense

and the types of neutrinos and how they might change into each other,

is that you can also study more basic properties of them.

And what OPERA decided they could measure was the speed

at which neutrinos travel.

That's quite an easy thing to measure because you know a distance,

you know where neutrinos were produced,

you know where you're finding them and how long they took to get there

if you have a clock where you produced it

and a clock in your experiment where you've made the measurement.

That's speed. Speed is just the distance covered

in a certain amount of time.

Nobody had anticipated what happened

when they started measuring how long it took the neutrinos to arrive.

They seemed to arrive early. Earlier than the laws of physics allow.

60 billionths of a second, or 60 nanoseconds sooner

than a beam of light would, if it were to cover the same distance.

That meant that the neutrinos had travelled at just over

two thousands of 1% faster than the speed of light.

If I was on a motorway, I wouldn't expect to get into trouble

for exceeding the speed limit by that small amount,

but not in physics.

The thing about an absolute speed limit is that it is absolute -

it can't be exceeded in any circumstances,

by however small an amount.

Under our current understanding of the universe,

this just isn't possible.

The researchers themselves were pretty shocked by the results.

They spent many months looking for mistakes.

They brought in outside experts.

They pored over the figures hundreds of times,

searching for an error.

They even made sure they'd factored the movement

of the continents that changes the distance

between Italy and Switzerland by small amounts.

But they couldn't find any mistakes, so they decided to publish.

When the news broke, it caused a sensation.

The theory that nothing travels faster than the speed of light is challenged.

The measurements could be wrong or there's some unknown...

Scientists have discovered that some tiny particles

seem to break that rule.

They seem to be travelling faster than the speed of light.

For physicists, this is earth-shaking if true.

It has created a huge furore, basically because if it was true,

then it would be so astonishing and important.

If the velocity of light turned out not to be absolute,

we just have to tear up all the textbooks and start all over again.

For me, it would mean the direction

of my own research was wrong.

So...it WOULD be a revolution, but to me,

it would also mean that nature's just playing tricks with us.

On the other hand, it would be nice if it were true.

Ever since the paper was published,

the internet has been buzzing with debate.

There are over 100 papers that have been uploaded in the last few weeks.

For me, this is a great example of science in action.

The OPERA team found some data that they couldn't explain.

For months, they'd been questioning it, doubting it, repeating it,

and only after intense scrutiny did they eventually publish it,

not in some triumphalist way, but asking the scientific community

to see where they might have made a mistake.

Not surprisingly, many of the responses have been sceptical.

And there are good reasons for doubting the figures,

based on both theory and experimental data.

The first problem is that the finding calls into question

one of the fundamental principles

that underpins our understanding of the universe -

cause...

and effect.

Cause and effect is a simple, yet powerful idea.

One thing follows another in a logically-ordered sequence.

The important thing is that events stay in the same order.

If I drink my coffee, I drink the coffee before I put the cup down.

A happened before B.

That's important cos A might have caused B.

Einstein's theory respects the relationship

between cause and effect, because with an absolute speed limit,

the speed of light, time can only flow in one direction.

If that isn't the case,

then the world can quickly become a very strange place indeed.

Here's an example of what might happen.

I'm going to send a text to my friend

with the winning lottery ticket numbers which were just announced.

The lottery numbers were...

2, 3, 5, 7, 11, and 13.

Press "send".

Now, let's suppose my friend and I have both got phones

that can send messages faster than the speed of light.

For this to work my friend has got to be moving relative to me,

so let's suppose that she's on a spaceship.

It's a spaceship that travels close to the speed of light.

This means that if I send a message that can travel faster

than the speed of light,

then, as far as she's concerned, it would arrive

before it had been sent.

Then, it's possible for me to send her a text

and for her to reply so that I get the reply before

I've even sent the original text, which is pretty weird.

Things get even weirder

if you start to think whether I can actually act on my friend's text.

I could now change my lottery numbers to the winning numbers

and become a millionaire.

I can change my past, which just doesn't make sense.

With the order of events all scrambled up, we find ourselves

in a universe more traditionally inhabited by science fiction.

If something can travel faster than the speed of light,

then, in principle, time travel is possible.

You'd venture into that forbidden region where you are influencing

things that you shouldn't, according to Einstein.

This causes paradoxes because you can go back in time

and kill your grandmother before you were born, all this nonsense.

For physicists, a consistent theory of the universe in which

we can travel back in time to win the lottery or kill

our grandmother is almost impossible to imagine.

It makes you wonder,

are the speeding neutrinos playing some sort of joke on us?

A barman says, "Sorry, we don't serve neutrinos."

A neutrino walks into a bar.

In other words, neutrinos that travel faster

than the speed of light imply all sorts of ideas

that don't tally with our everyday experience of the universe.

Another reason why many scientists are sceptical

that neutrinos really can break the light barrier

is because it contradicts previous results.

This is not the first time that the speed of neutrinos

has been measured.

In fact, there's one particularly famous observation

that was made back in the 1980s.

The reason you probably haven't heard about it

is because the results were in perfect accord

with Einstein's theories, so no news headlines and no TV programmes.

The action began on February 23rd, 1987.

Astronomers realised that a star on the fringes

of the Tarantula Nebula in the Large Magellanic Cloud had exploded.

It's called a supernova, one of the most violent

and destructive events in the universe.

We observed it in 1987. It actually happened over 100,000 years ago

and it took the light from that supernova,

the energy from that supernova, over 100,000 years to reach us.

This star exploding threw out enormous amounts of energy.

Most of it was in neutrinos, some of it was in light.

The light from the supernova and the neutrinos from the supernova

reached us almost at exactly the same time.

Scientists calculated that the neutrinos travelled

just a tiny bit slower than the speed of light,

just as you'd expect if Einstein was right.

Had the neutrinos from the supernova

travelled at the speed that the OPERA scientists recorded,

in other words, a little bit faster than the speed of light,

then they would have arrived here

four years before the light from the supernova.

That didn't happen.

Given this rock-solid verification of Einstein's theory,

it's not surprising that when the OPERA results were published

this year, suggesting that neutrinos travelled faster than light,

most people thought that, somewhere along the line,

they must have made a mistake.

When I first heard the result, I was...sceptical

and I think that most of my colleagues were very sceptical also.

I heard about this result in the coffee bar at CERN

about two weeks before it came out and I laughed.

I was like "Ah, well, they've got something wrong, haven't they?!"

Data error seems plausible when you consider the details

of what they were measuring.

Remember, those neutrinos arrived 60 billionths of a second,

that's 60 nanoseconds, early.

It's not the sort of measurement where a standard stopwatch

would be much use.

It's worth considering the astonishing nature

of the measurements we're talking about.

The world of athletics provides a good comparison,

a high precision sport relying on super accurate measurements.

In a 100 metre sprint the race is often so close

that it results in a photo-finish.

The winning athletes may be separated from the rest by just

100th of a second. A gap of 100th of a second in time

translates into roughly ten centimetres in distance.

Now, compare that to the neutrinos' journey from Switzerland to Italy.

The neutrinos that arrived in Gran Sasso

did so just 60 billionths of a second ahead of schedule.

If a 100 metre sprint were to be won by 60 billionths of a second,

then that would mean the winner would have been just under

1,000th of a millimetre ahead of the field.

So the OPERA team were attempting to measure time

over almost inconceivably small periods.

Even the tiniest error could have huge implications.

The scientists themselves have admitted that there are inaccuracies

with their measurement.

Firstly, they could have got the distance between CERN

and Gran Sasso wrong, but only by about 20 centimetres.

It is also difficult to pin down the exact moment the neutrinos hit

the target at Gran Sasso.

But by far the biggest uncertainty comes from recording exactly

when the neutrinos left CERN.

Yet, even adding together all the potential errors identified so far,

it only gives you around ten nanoseconds.

That still doesn't come close to explaining why the neutrinos

arrived 60 nanoseconds early.

But some of the physicists who have been poring over the results

reckon that a much larger inaccuracy could be lurking

deep in the detail of when exactly the neutrinos started their journey.

So what they do is measure this kind of pulse of the protons at CERN

and these things leave with some kind of shape.

Then, in OPERA, they sit there waiting.

There are billions of protons at CERN producing lots of neutrinos.

Very few of those neutrinos actually interact in the OPERA detector,

so you sit there and wait and you get a bang. There's one, bang.

There's another one.

Over time you build up a shape of the arrival time of the neutrinos

and you fit the two together. You fit the proton pulse shape

and you fit the neutrino arrival shape.

But the neutrino arrival shape is made up of many fewer events

than the proton one.

John Butterworth is concerned that the OPERA scientists

have assumed that these two shapes are the same

when there are good reasons why they might not be.

As far as I can see, they assume that the underlying shape

of the neutrino arrival is identical to the underlying shape

that they know very well, of the protons leaving.

It's not obvious to me that that's true

because the OPERA experiment, you see a very small fraction of the beam.

The beam is much bigger than the detector.

It's a kilometre across and the detector's much smaller than that.

Also, these protons, a lot happens to them before they become neutrinos.

There are various ways in which that shape

could be slightly different. You don't need much of a difference

to undermine the precision of the measurement.

I'm not saying this is definitely a mistake,

but I'm surprised that they didn't treat that more seriously

and I think I'd have gone, "That needs to be checked."

So far, dozens of suggestions have been made about potential errors

in the experiment, but none of them

have yet been proven to explain the faster than light measurement.

What strikes me about the paper the team have prepared

is just how meticulous it is.

This must be one of the most accurate measurements ever made.

So, at this stage, I think it's right to keep a sceptical,

but open mind.

There's one intriguing additional piece of evidence

that offers some support for the OPERA team.

In 2007, scientists from Fermilab,

the high energy physics laboratory just outside Chicago,

made a similar, but less precise, neutrino measurement

using an experiment called MINOS.

MINOS fired neutrino beams similar to those detected at OPERA

to a detector in a mine 800 kilometres away in Minnesota.

They measured the time between Chicago where the particles

are produced and the this mine in Minnesota and they get an effect

which goes in the same direction as what OPERA has seen,

so that the neutrinos are a bit faster than you'd expect.

The MINOS neutrinos did seem to be moving faster

than the speed of light.

However, because their equipment was less precise,

the MINOS scientists had to allow for a larger uncertainty

than the Italians.

And when this lack of precision was accounted for,

the results didn't appear to be statistically significant.

So nobody got really very excited about this at the time.

Now, this will mean, with this new result coming out,

that MINOS and another experiment in Japan, which is called T2K,

will both work very hard to get a similar measurement

with a similar position in the next few years.

But it will take a few years, I think.

So until we've got evidence there really is an error

in the OPERA results, it only seems fair to explore other options.

This is where it becomes particularly interesting,

especially if you're a mathematician.

Because there's a whole range of other theories

that could explain this.

At stake is one of the greatest prizes of science,

a theory of everything.

The first issue is to consider whether the speed of light

is really the absolute barrier that Einstein described.

There are at least two arguments that suggest it might be possible,

in certain circumstances, to travel faster than the speed of light.

The intriguing thing is that, mathematically speaking,

travelling faster than the speed of light isn't quite as difficult

as the popular interpretation of Einstein's series suggest.

In fact, from a mathematical point of view, it isn't impossible at all.

To understand why, you need to explore the relationship

between physics and maths.

There are many examples in the history of physics

where maths predicts something that, at first sight,

seems counter-intuitive only for the maths to then to be proved right.

Back in the 1920s,

a scientist called Paul Dirac came up with equations to describe

what happened to electrons when they travel close to the speed of light.

But his equations led to a peculiar conclusion.

They predicted that every particle had an equivalent antiparticle

with an opposite electric charge.

These antiparticles would combine to form antimatter.

At the time, the idea of antimatter seemed mad,

but eventually, incontrovertible evidence

for its existence was found.

And we've seen something similar happen with the prediction

that neutrinos would exist before they'd been observed.

So maths can sometimes suggest solutions that appear impossible

in the real world, but then turn out to be feasible after all.

Surprisingly enough, there are mathematical solutions

to Einstein's equations which do allow particles to go faster

than the speed of light.

We even have a name to describe these theoretical particles

that can do this. They're called tachyons.

Now, I have to admit that, on the surface,

tachyons are pretty strange.

Most notably, their mass is an imaginary number,

but however strange that sounds, it doesn't mean they couldn't exist.

A surprisingly large part of the universe

is built on imaginary numbers.

So what's special about tachyons?

How could they travel faster than the speed of light?

The key is this...

Einstein's formula forbids any particle to travel THROUGH the speed

of light, because as it accelerates, its mass get greater and greater.

But if a particle is formed when it's already travelling

BEYOND the speed of light, then it gets past this problem.

Even before these results, a few scientists have suggested

that neutrinos might have a tachyonic behaviour.

In other words, there might be a link between tachyons and neutrinos.

At this stage, it's too early to say whether this theory has any legs,

but it's still good to know from a mathematical perspective

that it IS possible to travel faster than the speed of light.

There's another reason for doubting that Einstein's speed limit

is quite as absolute as it appears.

In fact, there are certain circumstances where the idea

of an ultimate speed limit doesn't make any sense.

The exciting thing for me about controversial results like these

are that they shake things up.

They provoke lots of questions, demand new ideas.

In doing so, they shine a light on theoretical problems that tend

to get swept under the carpet.

Unless you study science, you could be forgiven

for thinking that the theories used by academics to describe

the universe all join up nicely, but that's not always the case.

Obviously, this result contradicts what you find in textbooks,

but if you're actually working in the frontier of physics

and trying to find new theories, this is not as tragic as you might think.

It's a crisis, but we need a crisis because there are lots of things

in physics, in those textbooks, which don't really make any sense.

Einstein's theories describe with astonishing accuracy

the universe we can see.

The planets, the stars, even the distant galaxies.

And here, the speed of light is indeed the ultimate speed limit.

But even within this familiar universe,

there are places Einstein's theories don't work.

In extreme conditions, the rules break down.

Physics hasn't yet developed the language to understand

what happens inside a black hole, for example.

Einstein's ultimate speed limit also causes problems

in trying to explain how the universe evolved

from the birth of everything...

EXPLOSION

..the Big Bang.

Physicists think that at the moment of the Big Bang,

everything in the universe was crammed into one tiny point,

smaller than an atom.

At the Big Bang, the universe expanded at astonishing speed.

As it expanded, it cooled, allowing fundamental particles,

then protons and neutrons, to condense out of the energetic soup.

All of this happened in less than a second.

Over the next 400,000 years,

the universe cooled enough to allow the first hydrogen atoms to form,

creating vast clouds of gas that finally began to collapse

into the familiar stars and galaxies

that make up the universe as we see it today.

But here's the big problem.

Accepted science only seems to account for what happened

just after the Big Bang.

If you want to understand what happened to our universe

in its very first moments, Einstein can't help you.

And there's a particular problem with Einstein's idea

of a constant cosmic speed limit, the speed of light,

when you apply it to the Big Bang.

Some physicists believe that for the universe around us

to be as we see it today, then that speed limit must have been

broken in these instants immediately after the Big Bang.

In cosmology, it's very difficult to explain why the Big Bang universe

is what it is if you have the speed limit which is very constraining

in the early universe.

You don't have enough time to produce the universe if you have this

speed limit which limits your range of action and ties your hands.

So raising the speed limit could be exactly the missing ingredient

for explaining the Big Bang.

This is a controversial theory.

But it does support the idea that there are extreme circumstances

in which the speed of light

is not the ultimate speed limit of the universe.

However, the most exciting attempt to explain

how neutrinos could travel faster than light

comes from the very frontier of theoretical physics.

Scientists are attempting to create a unified theory of everything.

At the moment, there are two sets of theories that explain the universe.

Einstein's theories which explain the world of the large,

the things we can see in the universe.

And a second theory, called quantum mechanics, describes the world

of the small, like subatomic particles.

And they just don't join up.

The dilemma we faced at the beginning of this century is that the two main

pillars of the last century's physics seem to be mutually incompatible.

So if something big has to give...

and this August, perhaps, a new scientific revolution.

There are, however, a number of candidates

for this grand unifying theory. The main one is string theory.

And the exciting thing that's beginning to form

in some scientists' minds is that perhaps the OPERA results

are the first experimental proof of it.

String theory is based on the idea that we only have

a very partial view of the universe.

It suggests that the fundamental particles we see in the universe

are all related to each other through a string.

In string theory, the particles are still there,

but they no longer occupy centre stage.

The fundamental object is a one-dimensional string.

One can think in an analogy of a violin string.

The string can vibrate and each mode of vibration, each note,

if you like, represents a different elementary particle.

So this note is an electron, that note a quark,

and yet another note could be a Higgs boson.

So it's a much more economical way of describing dozens

of elementary particles by a single string.

There are plenty of mathematical equations

that describe string theory,

but they lead to a rather uncomfortable conclusion.

The universe needs a lot more dimensions

than we are used to dealing with.

We are used to the idea of living in a three-dimensional world.

Forwards, backwards, up, down, left, right.

And time is the fourth dimension.

But string theory says there have to be an extra six.

But they'd have to be curled up to one unobservably small size,

or else rendered invisible in some other way,

if they're to describe the universe we find ourselves in.

Scientists have come up with wonderful language

to describe this multi-dimensional world.

The 3D universe we are familiar with is known as a membrane,

or "brane" for short.

But this is just part of something much larger,

which includes all the other membranes or dimensions.

And this all-encompassing entity is known as the bulk.

A one possibility is that our universe, you, me

and everything in it, is a three dimensional brane...

which lives itself in a higher dimensional bulk space time

which may have 10 or 11 dimensions.

And there can be other universes parallel to ours.

The analogy would be slices of bread in a loaf.

So the bulk is the loaf, the brane is the slice of bread.

And we live on the brane, and light is confined just to the brane.

It doesn't travel in the bulk.

So here, then, is one possible explanation.

The neutrinos left CERN travelling at just below the speed of light

on our brane.

They then took a short cut through the bulk and popped back

into our universe or membrane in time to be picked up at Gran Sasso.

If a particle were to leave the brane,

travel in the bulk and reappear on the brane,

it would create the impression to someone living on the brane

that it had travelled faster than light.

There are a couple of rather satisfying elements to this theory.

First, Einstein's theories still hold.

Light still forms the ultimate speed limit in our membrane,

just as Einstein said.

But if particles like neutrinos can travel in the bulk,

they can do so at a faster speed.

Second, it might explain why the supernova neutrinos

that were detected in 1987 travelled slower than the speed of light.

Think of it this way.

Most of the time, when ocean waves form, they behave

in a predictable way, because the energy that forms them

is fairly consistent.

But every now and then, there's a freak wave

formed from a particularly violent collision.

In the same way, neutrinos created at CERN are the products

of incredibly violent collisions, and this could be enough to throw

some of them briefly out of our membrane and into the bulk.

It all sounds rather elegant.

If this explanation is right,

then these faster than light neutrinos offer tantalising evidence

that string theory could indeed be a theory of everything.

But it's only fair to say that many string theorists

are far from convinced.

I've been working on the idea of extra dimensions for over 30 years.

So no-one would be happier than I if the experimentalists were

to find evidence for it.

However, to be frank, although I like the idea of extra dimensions,

this is not the way they are going to show up, in my opinion.

So I am not offering extra dimensions as an explanation

for the phenomenon that the Italian physicists are reporting.

So for the time being, there is no theory that convincingly explains

how the neutrinos appeared to break the speed of light barrier

travelling between Geneva and Gran Sasso.

All scientists have is some idea of the right place

to look for a theoretical explanation.

This could be one of those moments that turns our understanding

on its head yet again, lets us see further into the universe,

lets us understand more about how it ticks, how it sticks together,

how things are related inside it.

If it does that, if we understand more, then it is one of those

magical moments that you get in the history of physics

that just twists your understanding and brings the universe into focus.

If we are seeing the start of that now and we're documenting it,

then we're really, really privileged to be doing so.

At this stage, the argument is nicely poised.

Measurement error, or the beginnings of a seismic breakthrough

in our understanding of the universe?

Nobody knows. What's needed, of course,

is the thing that underpins all of science.

The scientific method demands replication of the results.

If other scientists can't repeat the findings coming from Italy,

we have to begin to doubt the accuracy of those measurements.

However, if they do repeat them, the stage is set for a major challenge

to Einstein and the creation of a grand unifying theory of everything.

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