Science: Inside the mysteries of the light fantastic: Scientists in Germany are using a giant particle accelerator to glimpse the secret world of the photon. Brian Foster reports on their first discoveries

THE NEXT time you go into a dark room and casually press the light switch, spare a thought for the hundreds of physicists working at a laboratory just outside Hamburg in Germany. For them, light is something of a puzzle.

At the Desy laboratory, the scientists' pride and joy is a machine called Hera, which accelerates subatomic particles to very high energies. The first data garnered from Hera has shown that light, when looked at under this most powerful of 'microscopes', is not as simple as it appears.

Hera accelerates two kinds of subatomic particles: protons, the positively charged particles that form the nucleus of the hydrogen atom, the simplest of all the chemical elements; and electrons, the negatively charged particles that carry electricity along the cables into our homes. The electrons circulate inside a huge vacuum chamber 6.3km (3.9 miles) in circumference and the protons circulate in the opposite direction, before they are brought together to collide head-on.

In this giant electron microscope, the violence of the collisions between the electrons and protons means that physicists can probe the interior of the proton with 10 times the precision previously attainable. Although the main purpose of the experiments is to examine the proton's innermost structure, Hera is also starting to uncover the mysterious properties of light.

Light is familiar to everyone. Yet, to physicists brought up on quantum mechanics, any beam of light is actually a stream of elementary particles, called photons. Via the wonders of quantum mechanics, the myriads of individual particles - photons - behave in many situations as if they were waves of light. The photon is a sort of intermediary particle that 'carries' the electromagnetic force, familiar from the electric current in a light bulb or the force between a piece of iron and a magnet.

The Hera accelerator produces a very intense source of photons. Physicists believe that, in the violent collisions between the electrons and protons, the electron emits a photon which a 'quark', one of the proton's inner constituents, then absorbs. The photons in the most violent collisions appear to be very simple, point-like particles, conveying the electromagnetic force between the electron and quark. These very violent collisions happen relatively infrequently, and therefore only a handful have been seen in the experiments. It is the properties of photons involved in less violent but more probable collisions that have proved so interesting.

In the less violent collisions, photons no longer behave as if they were simple particles with zero size. Instead, they seem to behave as if they were a very complex, spatially extended, jumble of quarks, antiquarks, electrons and other particles.

For a photon to turn itself into a quark-antiquark pair violates a fundamental principle of physics: that energy is always conserved. But in quantum mechanics, Heisenberg's Uncertainty Principle allows the photon to do just that, provided it does so for an extremely short time. Heisenberg's principle puts a limit on how precisely we can measure energy and time, so there can be a large 'uncertainty' in the photon's energy (as it transforms itself into a quark-antiquark pair) but only if its duration keeps within the limit set by Heisenberg's principle.

Thus, if we can examine a photon over such short times, it may appear not as a simple intermediary of the electromagnetic force, but as subject to other forces including the 'strong' and 'weak' nuclear forces - responsible respectively for holding the nuclei of atoms together and for radioactive decay. The once simple photon now appears as a complicated object. This complexity implies a richness of possible phenomena and a unique chance to examine the interplay of fundamental forces.

In previous experiments carried out at lower energy on other machines, the photon often interacted with protons as if it felt the strong force. But the evidence for this could be inferred only indirectly. The maximum energy of photons in previous experiments was about 20 times lower than can be achieved at Hera. Now the two major experiments at Hera, known as H1 and Zeus (each involving complex arrays of electronic detectors weighing several thousand tons and operated by several hundred physicists), have been able to utilise the much higher photon energy to observe unambiguously the transitory quark constituents of the photon.

Quarks have never been isolated in experiments. The strong force acts rather like an elastic band tying two quarks together. If a photon strikes a quark in a weak collision, the elastic band merely stretches and restores, returning the quark to its neighbours and causing the parent particle to deviate slightly. More energetic collisions cause the elastic band to break. There the analogy with elastic bands fails, because a new quark-antiquark pair appears at the break, and then the shorter elastic band is stretched further as the original quark continues to recede.

Further breaks occur, until the final result of the original collision is not an isolated quark but a number of particles of similar type to the proton, each containing quarks and antiquarks. At low energies, the pieces of the broken elastic band originating from the photon cannot travel very far and get inextricably jumbled up with similar particles produced by the quarks within the proton. At the high energies of Hera, however, the struck quark travels a large distance, producing a swarm of particles known as a 'jet'. The jet follows the original direction of the struck quark quite closely. By measuring the energy of the jets observed as the particles travel through electronic detectors, the experimenters can reconstruct the properties of the original quarks inside the photon.

Having firmly established the existence of quarks inside the photon, scientists at Hera have gone on to make detailed measurements of the energy and properties of these fundamental constituents of our material world.

The observations reported here were made with data collected in the first few weeks of Hera's operation, and with intensities of protons and electrons several hundred times smaller than will eventually be achieved. A great increase in the quantity and quality of data is therefore expected over the coming months. This should allow a fascinating glimpse inside the mysterious world of the photon, and how light flashes into matter and then back into light again.


The photon is important in particle physics because of its central role in the 'Standard Model'. This seeks to explain the behaviour and evolution of the universe in terms of four forces acting on 12 'elementary' particles. (Each particle has its antimatter equivalent, known as an antiparticle, with equal mass but otherwise opposite properties.)

The four forces are electromagnetism, gravity, the weak force and the strong force. The electromagnetic force and gravity are familiar in everyday life. The strong and weak forces are unfamiliar since they govern the behaviour of matter inside the unimaginably small atomic nucleus.

Six of the 12 elementary particles, the ones known as quarks, feel the effects of all four forces, while the other six, the leptons, do not feel the strong force. The electron, for example, is a lepton.

All the forces act on the elementary particles via intermediary particles. The photon is the intermediary of the electromagnetic force, and thus conveys this force between electrically charged quarks and leptons.

Dr Brian Foster is a reader in physics at Bristol University.

(Photograph and graphic omitted)

Suggested Topics
John Travolta is a qualified airline captain and employed the pilot with his company, Alto
people'That was the lowest I’d ever felt'
Life and Style
healthIt isn’t greasy. It doesn’t smell. And moreover, it costs nothing
Jonas Gutierrez (r) competes with Yaya Toure (l)

Newcastle winger is in Argentina having chemotherapy

Arts and Entertainment
Blossoming love: Colin Firth as Stanley and Emma Stone as Sophie, in 'Magic in the Moonlight'

Actors star in Woody Allen's 'Magic in the Moonlight'

peopleThe Times of India said actress should treat it as a 'compliment'

Watch this commuter wage a one-man war against the Circle Line
We are phenomenally good at recognising faces; the study showed that humans have been selected to be unique and easily recognisable

Human faces unique 'because we don't recognise each other by smell'

Arts and Entertainment
You've been framed: Henri Matisse's colourful cut-outs at Tate Modern
artWhat makes a smash-hit art show?
Home body: Badger stays safe indoors
lifeShould we feel guilty about keeping cats inside?
A male driver reverses his Vauxhall Astra from a tow truck

Man's attempt to avoid being impounded heavily criticised

Arts and Entertainment
US pop diva Jennifer Lopez sang “Happy Birthday” to Gurbanguly Berdimuhamedow, president of Turkmenistan
musicCorporate gigs become key source of musicians' income
ebooksAn unforgettable anthology of contemporary reportage
Arts and Entertainment
While many films were released, few managed to match the success of James Bond blockbuster 'Skyfall'
filmsDaniel Craig believed to be donning skis as 007 for first time
The Guildhall School of Music and Drama is to offer a BA degree in Performance and Creative Enterprise

Arts and Entertainment
Emma Thompson and Bryn Terfel are bringing Sweeney Todd: The Demon Barber of Fleet Street to the London Coliseum

Returning to the stage after 20 years makes actress feel 'nauseous'

Arts and Entertainment
Pulp-fiction lover: Jarvis Cocker
booksJarvis Cocker on Richard Brautigan
Arts and Entertainment
Robin Thicke and Pharell Williams in the video of the song, which has been accused of justifying rape
music...and he had 'almost no part' in writing it
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

March On Cancer™ - Local Marketing and Promotions Volunteer

This is an unpaid voluntary role.: Cancer Research UK: We need motivational vo...

SEN Coordinator + Teacher

£1 per day: Randstad Education Leeds: Job Purpose To work closely with the he...

Maths Teacher - Evening session

Negotiable: Randstad Education Birmingham: I am looking for a qualified experi...

Teaching Assistants

£50 - £85 per day: Randstad Education Preston: Rapidly developing and growing ...

Day In a Page

Mystery of the Ground Zero wedding photo

A shot in the dark

Mystery of the wedding photo from Ground Zero
His life, the universe and everything

His life, the universe and everything

New biography sheds light on comic genius of Douglas Adams
Save us from small screen superheroes

Save us from small screen superheroes

Shows like Agents of S.H.I.E.L.D are little more than marketing tools
Reach for the skies

Reach for the skies

From pools to football pitches, rooftop living is looking up
These are the 12 best hotel spas in the UK

12 best hotel spas in the UK

Some hotels go all out on facilities; others stand out for the sheer quality of treatments
These Iranian-controlled Shia militias used to specialise in killing American soldiers. Now they are fighting Isis, backed up by US airstrikes

Widespread fear of Isis is producing strange bedfellows

Iranian-controlled Shia militias that used to kill American soldiers are now fighting Isis, helped by US airstrikes
Topshop goes part Athena poster, part last spring Prada

Topshop goes part Athena poster, part last spring Prada

Shoppers don't come to Topshop for the unique
How to make a Lego masterpiece

How to make a Lego masterpiece

Toy breaks out of the nursery and heads for the gallery
Meet the ‘Endies’ – city dwellers who are too poor to have fun

Meet the ‘Endies’ – city dwellers who are too poor to have fun

Urbanites are cursed with an acronym pointing to Employed but No Disposable Income or Savings
Paisley’s decision to make peace with IRA enemies might remind the Arabs of Sadat

Ian Paisley’s decision to make peace with his IRA enemies

His Save Ulster from Sodomy campaign would surely have been supported by many a Sunni imam
'She was a singer, a superstar, an addict, but to me, her mother, she is simply Amy'

'She was a singer, a superstar, an addict, but to me, her mother, she is simply Amy'

Exclusive extract from Janis Winehouse's poignant new memoir
Is this the role to win Cumberbatch an Oscar?

Is this the role to win Cumberbatch an Oscar?

The Imitation Game, film review
England and Roy Hodgson take a joint step towards redemption in Basel

England and Hodgson take a joint step towards redemption

Welbeck double puts England on the road to Euro 2016
Relatives fight over Vivian Maier’s rare photos

Relatives fight over Vivian Maier’s rare photos

Pictures removed from public view as courts decide ownership
‘Fashion has to be fun. It’s a big business, not a cure for cancer’

‘Fashion has to be fun. It’s a big business, not a cure for cancer’

Donatella Versace at New York Fashion Week