EinsteinNews from the National Physical Laboratory

Winter 2005 | Issue 18

FUNDED BY THE DTI

National Physical Laboratory | Hampton Road | Teddington | Middlesex | United Kingdom | TW11 0LW

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Einstein’s discoveries included:

time can stretch or shrinkmatter and energy can change intoeach othertime flows at different rates in differentplaceslight behaves as streams of particlesa way of mathematical modelling theuniversethe nature of gravitymany ways of measuring atomic size

After an eventful upbringing in Germany, Italyand Switzerland, Einstein settled for a whilein Switzerland, working in the patent officeand producing some of his most importantwork in 1905. A number of universityappointments followed as his prestige grew.During the First World War he campaignedfor peace and developed his theory of gravity(general relativity).

A demonstration of the distortion of starlightduring a solar eclipse in 1919 led to world-widefame. When the Nazis came to power in1933, Einstein campaigned against them -and they against him - and he left Germany,

settling finally in the United States. After thewar, though plagued by illness, he workedtirelessly for peace, campaigning against thehydrogen bomb, and McCarthyism.Scientifically, his major contributions endedin the 1920s but for the rest of his life heworked on a new theory that would unify allforces together with time, space and matter:a theory he never completed but which setthe direction for today’s embryonic ‘Theoriesof Everything’.

To measure directly the effects at thevelocities that vehicles can achieve on earth,or those caused by the Earth’s gravity,extremely accurate clocks - atomic clocks -are required. The first operational atomicclock was developed at NPL, and NPL wasinvolved in a recent verification of relativityusing an atomic clock (see page 2).

In 1905, Einstein showed that light can beviewed as a stream of many tiny particle-likepackets of energy called photons. Most lightsources (such as lasers, light bulbs or stars)generate photons at enormous rates so that acontinuous beam is all we can see, but NPL ismaking photons one by one (see page 4).

Albert Einstein was born in Ulm, Germany in1879. When he died in 1955 he hadtransformed our understanding of the worldin the most profound and fundamental ways.Some of his most important breakthroughswere published in 1905, and the NationalPhysical Laboratory (NPL) is supporting thecentenary of that year.

Demonstrating Relativity by Flying Atomic Clocks

At speeds encountered in everyday life,relativistic effects are very small. WhenEinstein published his work on relativity theclocks then available were not sufficientlyaccurate to observe the predicted effects ontime. Instead, indirect evidence obtained fromastronomy and particle physics was used tosupport the relativity theories.

Highly accurate atomic clocks, thefirst of which was built andoperated at NPL in 1955, openedup the possibility of directly

measuring relativistic effects.Atomic clocks enable time to

be measured far moreaccurately than

any otherphysical quantity. The

original NPL clock, likemany that followed it,

maintained time bycounting cycles of an

atomic frequencytransition present

within thecaesium atom.

Atomic clockshave been

developedthat are

sufficiently portable to be flown in an aircraft,and possess sufficiently good timekeepingproperties that the relativistic effects maybe observed.

In 1971 Keating and Hafele operating fromthe United States Naval Observatory inWashington DC, performed the most famousflying clock experiment. Four commercialcaesium atomic clocks were flown around theworld on commercial aircraft, firstlytravelling from east to west,and then travelling

from west toeast. The results of the

experiment did indeedconfirm the relativisticpredictions.

To commemorate the 25th anniversary of theKeating and Hafele experiment, NPL featuredin a BBC Horizon programme that involvedflying a single caesium atomic clock fromLondon to Washington and then back again.The timekeeping properties of atomic clockshad improved very significantly over the 25years since the original experiment, and as aresult relativistic effects would be observablefollowing much shorter clock trips.

Several relativistic effects need to beconsidered when performing a flying clocktrip, including:

The speed of the aircraft relative to anobserver on the ground. This is aresult of the special theory of relativity.

The height and hence gravitationalpotential of the clock on the aircraftrelative to an observer on the ground.This is a result of the general theoryof relativity.

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Before the start of the experiment, thetravelling clock was compared directlyagainst the UK’s national timescale at NPL,to establish both its offset and rate. Duringthe flights the height, speed and estimatedposition of the aircraft were regularlymonitored. From these measurements thefollowing predictions were made regardingthe expected time gain of the travelling clockrelative to NPL’s reference timescale.

Results included:

The combined flight times of 14 hoursand mean height in excess of 10 kmresulted in a predicted clock gain of53 ns. This followed the principle thata clock in a weaker gravitational field(higher altitude) will run faster.

The effect of the aircraft’s speedrelative to the Earth’s surface resultedin a predicted clock loss of 16.1 ns.This followed the principle that amoving clock runs slow.

On return to NPL the travelling clock waspredicted to have gained 39.8 ns, includingan additional geometric factor. Thiscompared remarkably well with a measuredgain of 39.0 ns. We estimated the uncertaintydue to clock instabilities and noise to bearound ±2 ns. This short flying clockexperiment therefore provided a cleardemonstration of relativistic effects.

Research is continuing to develop newatomic clocks that are either more stable,such as NPL’s caesium fountain standard,or are very significantly smaller than thoseused in these flying clock experiments.There is considerable potential for futureimprovements. Atomic clocks now operatecontinuously in space on the US GlobalPositioning System satellites. Theobservation of relativistic effects and theircorrection has become an important part ofboth the operation of satellite navigationsystems and of international timekeeping.

Essen (right) and Parry with the original NPLatomic clock

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© Crown Copyright 2005. Reproduced with the permission of the Controller of HMSO and Queen’s Printer for Scotland.

For additional copies of this newsletter, or for more information on any aspect of NPL’swork and the range of services available from the Laboratory, call the NPL Helpline:Tel: 020 8943 6880 | Fax: 020 8943 6458 | Switchboard: 020 8977 3222E-mail: enquiry@npl.co.uk | Website: www.npl.co.ukNational Physical Laboratory | Teddington | Middlesex | TW11 0LW

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NPL scientists are developing single-photonsources and the diagnostic instrumentationrequired to quantify their relevant parameters.

Einstein’s proposal concerning photons wasthe key to a satisfactory explanation ofexperiments on the photoelectric effect. Eachphoton of visible or infra-red radiation has anextremely small energy. Moreover, it ispossible to generate and detect individualphotons. Technologies making use of thespecial properties of single photons includequantum cryptography, for the securedistribution of cryptographic keys. In thefuture, it may be possible to processquantum information using single photonsand linear optics. For either of these quantumtechnologies, a source capable of generatingsingle photons is a desirable resource.

Examples of physical systems that may beused to generate single photons include asingle atom, and a single quantum dot in asemiconductor. In both instances, the energy

levels of the system are discrete, or quantised,and the system may be determined to be inone state only at any given time. A solitaryatom or dot in an excited energy level,decaying radiatively to a lower energy level,will emit one and only one photon at a time;it will never emit two photons simultaneously.It is this property of an individual quantumemitter that is fundamental to the generationof “single photons”.

A single-photon source possessesfundamentally different properties whencompared to a laser, a light bulb, or a star.Precise knowledge of these properties isrequired to quantify the performance of theselight sources for scientific applications. Inquantum cryptography for example, a lessthan perfect source opens the door to anundetectable eavesdropper, rendering thecommunication channel insecure. Futuristictechnologies such as quantum computationwill require similar characterisation of sources.

Einstein and Single Photons

The NationalPhysical Laboratory

(NPL) is proud to besupporting Einstein

Year 2005 the UK & Ireland’scontribution to World Year of Physics(WYP) and marks the centenary of thepublication in 1905 of Einstein’s threeground-breaking papers on specialrelativity, the photoelectric effect andBrownian motion. These papers providedthe foundation of modern physics, andactivities throughout Einstein Year willexplore ideas in contemporary physics aswell as showing how our everyday livesare influenced by Einstein’s legacy.

Einstein Year is a whole year ofactivities that will get you fired up aboutphysics, more information can be found atwww.einsteinyear.org

NPL’s The Learning Roomwww.npl.co.uk/thelearningroomWorld Year of Physics www.wyp2005.orgInstitute of Physics www.iop.org

NPL produces a range of free colourful,fun and educational posters aimed atGCSE/A-level students and are a veryuseful teaching resource. For a free setplease fill in the on-line form atwww.npl.co.uk/thelearningroom/posters.htmlor contact the NPL Helpline.

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