Presentation Speech by Professor Carl Nordling of the Royal Swedish Academy of Sciences

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

This year’s Nobel Prize in Physics has been awarded to Russell Hulse and Joseph Taylor for the discovery of a new type of pulsar, a discovery that has had a great impact on gravitational physics.

When a star dies and its light fades away, it may be transformed into a pulsar. It then vanishes from the visible firmament. We no longer see it. But it is there, sending out radio signals instead of light, and in its new guise it has taken on remarkable characteristics.

As an astronomical object it is now tiny, only about ten kilometers in diameter. It consists entirely of nuclear matter, chiefly neutrons. Its density is extremely high. One pinhead of matter in such a pulsar would weigh hundreds of thousands of tons. It rotates at enormous speed, perhaps approaching a thousand revolutions per second. It continuously emits a radio signal in two beams that sweep across space, resembling the light beam from a lighthouse.

A terrestrial radio receiving antenna aimed at this transmitter picks up a signal that pulsates at the same frequency as the extinguished star rotates. Its frequency is very stable, fully comparable with that of terrestrial atomic clocks. This pulsating radio signal was the origin of the name “pulsar.” During a series of observations employing refined techniques to study the occurrence of pulsars, Joseph Taylor and his doctoral student Russell Hulse made the discovery that is being rewarded with this year’s Nobel Prize. At a position in the sky with the celestial coordinates 1913 + 16, they found a new pulsar. In itself, this was not a remarkable discovery, since many new pulsars had been identified in the course of their work.

But this object behaved differently from previously known pulsars. The time between its radio pulses – 59 milliseconds – was not constant, but showed periodic changes. The pulsar was being subjected to some kind of disturbance. Hulse and Taylor supplied an explanation for this phenomenon that was simple, yet stirred the imagination: Their pulsar had an invisible companion!

These two objects orbit each other, with the pulsar sometimes moving toward the earth and the radio antenna, sometimes away from them. When the pulsar moves toward the earth, the antenna picks up a higher frequency signal. When the pulsar moves away from earth, the antenna receives a lower frequency signal. This phenomenon, called the Doppler effect, occurs in many contexts. We experience it every time we hear an ambulance siren changing its audible frequency as the ambulance first approaches, then moves away from us.

This effect was what revealed the presence of the invisible companion – invisible both to the eye and to the radio receiver with its 300 meter diameter antenna dish. This companion is probably also a neutron star. Perhaps it is a pulsar that emits two beams of radio frequency radiation into space, although neither of them sweeps over our little planet. But perhaps they sweep over some other planet. Perhaps a radio astronomer in a distant civilization is sitting right now, recording these pulses and pondering why “his” pulsar is demonstrating certain irregularities in its radio signals.

Hulse and Taylor found that the pulsar moved at a speed of up to 300 km per second in its whirlwind dance around its companion. This is a high speed, ten times higher than the speed at which the earth travels in its orbit around the sun. Hulse and Taylor realized that their binary pulsar thereby provided a unique opportunity to observe effects that Einstein had predicted sixty or seventy years earlier in his theories of relativity. New hope was awakened among relativity theorists throughout the world. Now they had an object tens of thousands of times more favourable than the planet Mercury, the classic object for testing the general theory of relativity and competing theories.

One of the most fascinating predictions of relativity theory is that massive objects in vehement motion emit a new kind of radiation, known as gravitational radiation. This phenomenon is also described as a wave motion, as ripples in the curvature of space-time, and we speak of “gravitational waves.”

No one has yet succeeded in recording a gravitational wave in a terrestrial or extraterrestrial receiver, but the Hulse-Taylor pulsar has convinced us that this type of radiation actually exists. This is because the orbiting period of the pulsar around its companion gradually diminishes with time – extremely little, but in exactly the way the general theory of relativity predicts, as a result of the emission of gravitational waves. This triumph puts the Hulse-Taylor pulsar in a class by itself as a laboratory for gravitational physics.

Dr. Hulse, Professor Taylor,

You have been awarded the 1993 Nobel Prize in Physics for your discovery of the first binary pulsar, PSR 1913 + 16, a discovery which has had a great impact on gravitational physics. It is my privilege to convey to you the heartiest congratulations of the Royal Swedish Academy of Sciences, and I now ask you to receive the Prize from the hands of His Majesty the King.