282
Appendices
numbers, but with integer spin? These supersymmetric particles (called sparticles) would comprise sleptons, squarks, neuralinos and charginos with spins
differing by ½ from the normal fermions. Unfortunately, once again these
new particles are so heavy that they are beyond the energy range of today’s
particle colliders.
A.10.1 The Measurement of Distance
in Astronomy
The measurement of distances in astronomy is fraught with difficulties.
Nevertheless, it is common to see in newspapers, popular science magazines,
and books such as this one, that objects have now been discovered at a
distance of 10 billion light years. How are such measurements even possible?
We have already explained in Chap. 10 how nearby stars observed from
earth appear to move against the background of distant stars as the earth
circles the sun, and how the observed parallax leads to the measurement of
distance. The standard unit of distance in astronomy is the parsec; a star
with one arc-second of observed parallax, measured when the earth is on
opposite sides of the sun, is said to be at a distance of 1 parsec. (An arcsecond is 1/3600 degrees.) The distance to any star in parsecs is the reciprocal
of its measured parallax in arc-seconds. To convert from parsecs to units
more familiar to non-astronomers (kilometres or light-years), we need to
know the distance of the earth from the sun. This distance is defined as the
Astronomical Unit (AU) and must be measured independently.
It is generally recognised that the first successful measurement of the
Astronomical Unit was carried out in 1672 by Jean Richer and Giovanni
Domenico Cassini, who measured the parallax to the planet Mars from two
different locations on earth. Knowing the separation of these terrestrial locations provides a baseline length for the parallax measurement, and enables
the distance from earth to Mars to be calculated. Kepler’s Laws of planetary
motion give the interplanetary distances in terms of the Astronomical Unit,
so working backwards, a knowledge of the earth-Mars separation can be used
to estimate the value of the Astronomical Unit. The currently accepted value
is: 149,597,870,691 ± 30 m. From this value for the AU, 1 parsec is found
to be about 3.3 light-years.
In the early ‘90s, the Hipparcos satellite was used to take parallax measurements of nearby stars to an accuracy much greater than possible with
earth-bound telescopes, thereby extending the distance measurements of
Appendices
numbers, but with integer spin? These supersymmetric particles (called sparticles) would comprise sleptons, squarks, neuralinos and charginos with spins
differing by ½ from the normal fermions. Unfortunately, once again these
new particles are so heavy that they are beyond the energy range of today’s
particle colliders.
A.10.1 The Measurement of Distance
in Astronomy
The measurement of distances in astronomy is fraught with difficulties.
Nevertheless, it is common to see in newspapers, popular science magazines,
and books such as this one, that objects have now been discovered at a
distance of 10 billion light years. How are such measurements even possible?
We have already explained in Chap. 10 how nearby stars observed from
earth appear to move against the background of distant stars as the earth
circles the sun, and how the observed parallax leads to the measurement of
distance. The standard unit of distance in astronomy is the parsec; a star
with one arc-second of observed parallax, measured when the earth is on
opposite sides of the sun, is said to be at a distance of 1 parsec. (An arcsecond is 1/3600 degrees.) The distance to any star in parsecs is the reciprocal
of its measured parallax in arc-seconds. To convert from parsecs to units
more familiar to non-astronomers (kilometres or light-years), we need to
know the distance of the earth from the sun. This distance is defined as the
Astronomical Unit (AU) and must be measured independently.
It is generally recognised that the first successful measurement of the
Astronomical Unit was carried out in 1672 by Jean Richer and Giovanni
Domenico Cassini, who measured the parallax to the planet Mars from two
different locations on earth. Knowing the separation of these terrestrial locations provides a baseline length for the parallax measurement, and enables
the distance from earth to Mars to be calculated. Kepler’s Laws of planetary
motion give the interplanetary distances in terms of the Astronomical Unit,
so working backwards, a knowledge of the earth-Mars separation can be used
to estimate the value of the Astronomical Unit. The currently accepted value
is: 149,597,870,691 ± 30 m. From this value for the AU, 1 parsec is found
to be about 3.3 light-years.
In the early ‘90s, the Hipparcos satellite was used to take parallax measurements of nearby stars to an accuracy much greater than possible with
earth-bound telescopes, thereby extending the distance measurements of