Question

The discovery of pulsars –rotating neutron stars that generate regular pulses of radiation at their spin rate- was
fortuitous. A group of astrophysicists from Cambridge University lead by Anthony Hewish was looking for
quasars (these are extremely bright, compact and remote objects that emit up to a thousand times as much light
as our entire galaxy). For that purpose the research group designed a radio-telescope that was built on the flat
fields surrounding Cambridge in central England. This telescope did not look like the visible light refractors,
which the average person identifies with the word "telescope", but instead consisted of over a thousand posts
strung with more than 2000 dipoles between them, and up to 120 miles of wire to connect the whole. The
researchers with the help of university students built the apparatus themselves, and it took them two years to
finish the work.
The astrophysicists were in search of scintillation sources of electromagnetic radiation in the radio frequency
range. Scintillation is the apparent fluctuation in intensity of electromagnetic emissions, that is, the apparent
'twinkling' of electromagnetic sources. Most of the sources of radio emissions of the universe are large, such as
galaxies or extended regions of gases and dust where new stars are forming. They knew that twinkling of small
compact sources is much stronger than scintillation of extended sources. Hence, quasars being smaller than
galaxies should scintillate more. Radio sources that scintillated a lot were very good candidates for quasars.
Jocelyn Bell, a physics graduate student working on her Ph. D., was responsible for operating the radio telescope
and for analyzing the data, which consisted of nearly 30 meters of chart that was generated daily, all of which
had to be analyzed by hand. It was better to inspect the data visually to become familiar with the behavior of
the telescope and its receivers before designing a computer program. In addition the researchers took into
account that people can easily recognize signals of different character whereas it is difficult to program a
computer to do so.
Two months after the beginning of the experiment, Bell found something strange on the records, a signal that
did not resemble other scintillating sources. After more careful analysis of the observations, Bell could
determine that the odd source emitted short pulses with a perfect period of 11/3 seconds. Such period is too brief for something as big as a star. Hewish believed the pulses to be man-made interference, as with a gap so
regular they seemed too precise to be natural.
The researchers considered a variety of possible explanations for the curious phenomenon, such as being a
signal reflect from the Moon or emitted by an artificial satellite in an unusual orbit. They ruled out potential
sources of man-made interference one-by-one. It could be that the mysterious signal was created by the
telescope itself. However another group of researchers working with a different telescope managed to pick-up
the same signals, removing instrument malfunction as the possible source of the surprising emission.
The enigma deepened. Another astrophysicist, Pilkington, measured the dispersion of the signal and found that
the source was outside the solar system but inside our galaxy. It could be that the rapid signal originated with
an extraterrestrial intelligence. If the signal was coming from an inhabited planet, and since planets orbit
around a star, then a Doppler Effect shift had to be observable on the radio pulses: the frequency of the radio
emission would be higher when the planet was moving towards us and lower when it was moving away.
However the researchers measured only changes in the radio frequency due to the motion of our own planet
around our Sun. The signal continued to be a puzzle.
Bell continued with the analysis of the data produced by the telescope and found three other similar pulsating
radio sources at very distant points in the sky. These findings excluded the explanation of alien civilizations as it
was very unlikely that different groups of extraterrestrial intelligent creatures extremely far from each other
were choosing a similar frequency to send signals to the same planet, the Earth. It seemed highly improbable
that the signals were generated by intelligent beings; however nobody in the Cavendish Laboratory at
Cambridge University had any other explanation.
Hewish and Bell wrote a paper describing the first pulsating source and submitted it to the journal Nature,
where it was published on February, 1968. A few months later after the publication of the findings, Thomas
Gold, a professor at Cornell University, come out with a good explanation for the pulsed signals. Gold suggested
that the radio signals originated on the surface of very compact, exceedingly fast spinning neutron stars.
Neutrons starts continuously emit radio waves from two opposite directions that usually are not aligned with
the star’s rotation axis. As the star rotates these radio signals are swept around the sky in a circle. When the
emitting zones of the neutron stats point toward the Earth, we detect the radio signals. This is the 'lighthouse'
explanation of the pulsars which Gold proposed.
Up to now Dr. Gold’s explanation has not been disproved, and is unanimously accepted by the scientific
community because of additional evidence and coherence with current explanations of stellar evolution.
Anthony Hewish was awarded the Nobel Prize for Physics in 1974 for the discovery of pulsars, along with Martin
Ryle (the head of the Cavendish laboratory) for his work with radiotelescopes. Jocelyn Bell, despite her
instrumental role in the discovery of pulsars, did not share the prize.
Adapted from “Little Green Men, White Dwarfs or Pulsars?” By S. Jocelyn Bell Burnell. Annals of the New York Academy
of Science, vol. 302, pages 685-689, Dec., 1977.
Now reread the passage and answer the following questions.
1. Initially what type of experiment (observation, testing or application) was conducted by Hewish and Bell? What was
their goal?
2. How did the original experiment become an observation experiment?
3. List all of the explanations proposed.
4. What experiments were designed to test each one of the explanations? What did the explanations predict for the
outcome of the experiments?
5. How is an explanation different from a prediction?

6. Why did Bell and Hewish need to conduct testing experiments?
7. Why did they rule out most of the explanations?
8. Why did scientists accept Gold’s explanation?

Answer 1

Ans 1) Initially the astrophysicists were looking for quasars (these are extremely bright, compact and remote objects that emit up to a thousand times as much light as our entire galaxy). The experiment was of testing type as they were making various tests to look for quasers.

Ans 2) The goal of astrophysicists were in search of scintillation sources of electromagnetic radiation in the radio frequency range. They knew that twinkling of small compact sources is much stronger than scintillation of extended sources. Hence, quasars being smaller than galaxies should scintillate more. But Bell found something strange on the records, a signal that
did not resemble other scintillating sources. And various other observations were made to correctly identify the signal was of unknown or unidentified sources. Therefore, it was observation the variation and fluctuations in the waves that could give the end goal of the experiment. Hence, it became an observation experiment.

Ans 3)

The astrophysicists believed the pulses could be

1) man-made interference

2) created by the telescope itself.

3) the rapid signal originated with an extraterrestrial intelligence.

4) the radio signals originated on the surface of very compact, exceedingly fast spinning neutron stars.

Ans 4)

1) man-made interference :

such as being a signal reflect from the Moon or emitted by an artificial satellite in an unusual orbit. They removed it one by one.

2) created by the telescope itself :

It was removed by another group of researchers working with a different telescope managed to pick-up the same signals, removing instrument malfunction as the possible source of the surprising emission.

3) the rapid signal originated with an extraterrestrial intelligence :

Doppler Effect shift had to be observable on the radio pulses as the frequency of the radio emission would be higher when the planet was moving towards us and lower when it was moving away. However the researchers measured only changes in the radio frequency due to the motion of our own planet
around our Sun. Continued analysis of the data produced by the telescope found three other similar pulsating
radio sources at very distant points in the sky.

4) the radio signals originated on the surface of very compact, exceedingly fast spinning neutron stars :

the radio signals originated on the surface of very compact, exceedingly fast spinning neutron stars. Neutrons starts continuously emit radio waves from two opposite directions that usually are not aligned with the star’s rotation axis. As the star rotates these radio signals are swept around the sky in a circle. When the emitting zones of the neutron stats point toward the Earth, we detect the radio signals. This is the 'lighthouse' explanation of the pulsars.

Ans 5)

Explanation is quantifying or understanding a substance when it has already occured or dicovered.

Prediction is forecasting events or occurances with given data by careful repeated analysis.

Ans 6)

Bell and Hewish needed to conduct testing experiments to rule out potential sources that interfered in the data that was found.

Ans 7)

They ruled out most of the explanations to get the best possible case for the fluctuation in the signal.

Ans 8)

Gold's explanation had the best answer to the problem and could not be refuted with various experiments like the others predicted by Bell and Hewish.

If any doubt feel free to comment.

(P.S : This is a subjective question. If you do not agree with any answer feel free to share and let me know)