2.4 Investigations into Radio Pulsars
55
However, Hulse and Taylor noticed that one of these pulsars was always unpredictable during several days. After timing the radio pulses for some time, there was
a systematic variation in the arrival-time of the pulses. Sometimes, the pulses were
received a little sooner than expected; sometimes, later than expected. These variations changed in a smooth and repetitive manner, with a period of 7.75 h. Initially,
they thought that the phenomenon possibly resulted from using an inappropriate data
analyzing method. And thereby, Hulse shortened the sampling interval from 10 to
1 ms and made observations once again, but it was shown from the observational
results that the periodic change rate of the pulsar was uncertain yet. By repeating
observations and comparing researches, Hulse and Taylor eventually realized that
the phenomenon could well be interpreted if the pulsar was in a binary orbit with
another star. For the binary system, the pulsar is more and more away from the Earth
in a half time of the orbital period, and thus its observed period will be longer and
longer; during another half time of the orbital period, the pulsar is more and more
close to the Earth, and thus its observed period will also be shorter and shorter. So,
the periodic change rate of the pulsar in the binary system is far greater than that of
other pulsars.
The radio pulsar in the binary system is named PSR B1913+16, and also called
Hulse−Taylor binary, the first binary pulsar system to be discovered. Later, it was
confirmed that the companion of the pulsar is a neutron star. Nevertheless, the pulses
from the companion neutron star have not been detected, but it might only be the
result of an unfavorable viewing angle. There is not material exchange between two
sub-stars in the binary system, and the semi-major axis of the orbit of two sub-stars
is 1.95 × 10
6 km, its period 7.75 h and its eccentricity 0.617. Consequently, this
pulsar has extremely high orbital velocity, about 10
4 km/s, equivalent to one-tenth
of the light velocity.
According to the general relativity, any object with mass will produce the gravitational waves as it is accelerating. The masses of objects on the Earth are so small
that the gravitational waves resulted from them are extremely weak and thus cannot
be detected by current technical means. For an ordinary star, although its mass is
very huge, it is generally unable to be accelerated, and thus the gravitational wave
radiated from it is also very weak. Based on the observed characteristics of the binary
pulsar system, Hulse and Taylor speculated that this was possibly what physicists
had been trying hard to find with extremely huge mass and extremely high velocity.
It was shown from the investigation that the gravitational radiation from the binary
system is so strong that the orbital period change is very obvious and gets to −2.6 ×
10
–12 s/s. To observe so small periodic change rate, it is required to highly accurately
measure pulse arrival-time. Hulse and Taylor had continually observed and investigated PSR B1913+16 for more than 20 years by using the Areciboradio telescope.
And finally, they got a result that there is only 0.4% difference between the actual
observational values and the theoretic ones from the general relativity. In other words,
they indirectly confirmed existence of the gravitational waves. In 1993, Hulse and
Taylor won together the Nobel Prize in physics “for the discovery of a new type of
pulsar, a discovery that has opened up new possibilities for the study of gravitation”
while at this time they both were being employed at Princeton University.
55
However, Hulse and Taylor noticed that one of these pulsars was always unpredictable during several days. After timing the radio pulses for some time, there was
a systematic variation in the arrival-time of the pulses. Sometimes, the pulses were
received a little sooner than expected; sometimes, later than expected. These variations changed in a smooth and repetitive manner, with a period of 7.75 h. Initially,
they thought that the phenomenon possibly resulted from using an inappropriate data
analyzing method. And thereby, Hulse shortened the sampling interval from 10 to
1 ms and made observations once again, but it was shown from the observational
results that the periodic change rate of the pulsar was uncertain yet. By repeating
observations and comparing researches, Hulse and Taylor eventually realized that
the phenomenon could well be interpreted if the pulsar was in a binary orbit with
another star. For the binary system, the pulsar is more and more away from the Earth
in a half time of the orbital period, and thus its observed period will be longer and
longer; during another half time of the orbital period, the pulsar is more and more
close to the Earth, and thus its observed period will also be shorter and shorter. So,
the periodic change rate of the pulsar in the binary system is far greater than that of
other pulsars.
The radio pulsar in the binary system is named PSR B1913+16, and also called
Hulse−Taylor binary, the first binary pulsar system to be discovered. Later, it was
confirmed that the companion of the pulsar is a neutron star. Nevertheless, the pulses
from the companion neutron star have not been detected, but it might only be the
result of an unfavorable viewing angle. There is not material exchange between two
sub-stars in the binary system, and the semi-major axis of the orbit of two sub-stars
is 1.95 × 10
6 km, its period 7.75 h and its eccentricity 0.617. Consequently, this
pulsar has extremely high orbital velocity, about 10
4 km/s, equivalent to one-tenth
of the light velocity.
According to the general relativity, any object with mass will produce the gravitational waves as it is accelerating. The masses of objects on the Earth are so small
that the gravitational waves resulted from them are extremely weak and thus cannot
be detected by current technical means. For an ordinary star, although its mass is
very huge, it is generally unable to be accelerated, and thus the gravitational wave
radiated from it is also very weak. Based on the observed characteristics of the binary
pulsar system, Hulse and Taylor speculated that this was possibly what physicists
had been trying hard to find with extremely huge mass and extremely high velocity.
It was shown from the investigation that the gravitational radiation from the binary
system is so strong that the orbital period change is very obvious and gets to −2.6 ×
10
–12 s/s. To observe so small periodic change rate, it is required to highly accurately
measure pulse arrival-time. Hulse and Taylor had continually observed and investigated PSR B1913+16 for more than 20 years by using the Areciboradio telescope.
And finally, they got a result that there is only 0.4% difference between the actual
observational values and the theoretic ones from the general relativity. In other words,
they indirectly confirmed existence of the gravitational waves. In 1993, Hulse and
Taylor won together the Nobel Prize in physics “for the discovery of a new type of
pulsar, a discovery that has opened up new possibilities for the study of gravitation”
while at this time they both were being employed at Princeton University.