Astronomy makes use of more than 20 decades of the electromagnetic spectrum, from radio to gamma rays. The observing techniques vary so much over this enormous range that there are distinct disciplines of gamma-ray, X-ray, ultraviolet, optical, infrared, millimetre and radio astronomy, often concentrated in individual observatories. Modern astrophysics depends on a synthesis of observations from the whole wavelength range, and the concentration on radio in this text needs some rationale. Apart from the history of the subject, which developed from radio communications rather than as a deliberate extension of conventional astronomy, there are two outstanding characteristics that call for a special exposition. First, the astrophysics: long-wavelength radio waves are most often observed as a continuum in which the interaction with matter follows classical electrodynamics. High-energy electrons are involved; they are created in a variety of circumstances, and their radiation as they circulate in magnetic fields gives evidence of new phenomena, often showing a close link to the phenomena observed in X-rays and gamma-rays. At the shorter wavelengths the low quantum energy gives access to spectral lines from atomic and molecular species at comparatively low temperatures. Second, the techniques: radio astronomy takes account of the phase as well as the intensity of incoming radio waves, allowing the development of interferometers of astonishingly high angular resolution and sensitivity

The electromagnetic signals that give information about the Universe have the characteristics of random noise. More specifically, in the radio part of the spectrum, the signals are composed of Rayleigh, or Gaussian, noise, the result of an assemblage of many random oscillators with random frequency and phase. As one moves to shorter wavelengths, through the infrared and into the optical, ultraviolet and X-ray bands, the discrete character of photons becomes increasingly dominant, and the random noise obeys Poisson statistics, sometimes called shot noise. Throughout the spectrum, the process of detecting and measuring the signals gathered by a telescope is almost always electronic; for the optical astronomer the eye and the photographic plate are not sensitive enough, and at both the radio and X-ray ends of the spectrum electronic means have always been essential. The device that measures the power of the incoming signal is a radiometer; when it measures power as a function of frequency, it is a spectrometer.

At wavelengths shorter than about 100 μm, immediate detection of the received power is almost always forced on the observer because the laws of quantum mechanics require any amplifier to add extraneous noise. For the radio astronomer, the incoming signal is amplified before its power is measured in a detector, and the construction of low-noise amplifiers has become an art.

Cygnus A (Figure 13.1) is one of the strongest radio sources in the sky. It was discovered by Hey in his early survey of the radio sky (Hey et al. 1946a, 1946b); its trace can even be discerned on Reber's 1944 map (see Appendix 3 for a brief history). It is, however, an inconspicuous object optically, and it was not identified until its position was known to an accuracy of 1 arcmin (Smith 1951; Baade and Minkowski 1954). Its optical counterpart was found to be an eighteenth-magnitude galaxy with a recession velocity of 17 000 km s-1, that is, with redshift z= 0.06, indicating a distance of almost 1000 million light years. The source was shown to be double, with an overall size of more than a minute of arc, through the pioneering interferometry observations of Jennison and das Gupta (1953). Furthermore, the large angular size and double-lobed shape of Cygnus A were such distinctive features that similar radio sources might well be recognizable at very much greater distances; this was confirmed in 1960 when the radio source 3C 295 was identified by Minkowski with another similar galaxy, this time with a redshift of 0.45. Another galaxy, NGC 5128, had already been identified as a radio source known as Centaurus A (Bolton et al.1949); this again showed the characteristic double-lobed shape, but with an angular diameter of 4° it was obviously much closer.

In astrophysical contexts, the propagation of radio waves is governed, as for other parts of the electromagnetic spectrum, by the laws of radiative transfer and refraction. In radio astronomy, however, there is an emphasis on classical (non-quantized) radiative and refractive processes. Synchrotron radiation is the dominant radiation process at the longer wavelengths; spectral-line emission is observed mainly at shorter wavelengths. Maser action, the microwave equivalent of lasers, is encountered in several astrophysical contexts: this is due to the low energy of radio photons which can be significantly amplified by small population inversions in rotational and vibrational energy levels. Refraction is important in astrophysical plasmas; even though these are usually electrically neutral, protons have a negligible effect and the electron gas can have a significant effect on the velocity of radio waves. In the presence of a magnetic field, birefringence can lead to Faraday rotation of the plane of polarization.

In this chapter we set out the basic theories of radiative transfer, and outline the processes of radiation that are of particular importance in radio astronomy: free-free emission, line emission (and particularly maser emission) in dilute gas and synchrotron radiation. Free- free emission, or bremsstrahlung, is the main source in ionized hydrogen clouds, whereas synchrotron radiation is responsible for the background radiation in our Galaxy (Chapter 8) and is also practically universal in discrete radio sources from supernova remnants to quasars.

Introduction

In 2003 the American Association for the Advancement of Science, Program of Dialogue on Science, Ethics, and Religion, invited over twenty scholars from diverse fields, scientists active in astrobiology, as well as philosophers, historians, ethicists, and theologians, to explore together the philosophical, ethical, and theological implications of research and discoveries in astrobiology. A major motivation for this effort was the recognition that the very questions that define astrobiology as a discipline – Where do we come from? Are we alone? Where are we going? – are multidisciplinary in nature and have broad appeal to the public-at-large.

It is unavoidable that the science of astrobiology will intersect with, and inevitably challenge, many deeply held beliefs. Exploration possibilities, particularly those that may include the discovery of extraterrestrial life, will continue to challenge us to reconsider our views of nature and our connection to the rest of the universe. Much work has already been done in this area. What is unique about our present circumstance is that past theoretical musings may soon benefit from a renewed urgency that is awakened both by new discoveries and by technological advances. Many of the astrobiologists assembled for this workshop have in common another interest, working proactively to provide more opportunities for non-scientists to both share in the excitement of this field, and to be informed participants in a public dialogue that considers the opportunities and challenges associated with astrobiology in the near future.

Astrobiology, encompassing the search for life on other planets, laboratory studies of the origin of early life forms from precursor materials, and prospects for the discovery of microbial life on other planets reflects outcomes at the cutting edge of science and technology. Yet the issues that such investigations raise are profound, for not only do they bear on our own sense of self in relation to the cosmos, but they also raise deep philosophical and religious issues concerned with purpose, meaning, and human identity. Given these profound challenges, the ethical and moral frameworks within which such developments take place need to be carefully considered, for outcomes of such deliberations have public and social importance alongside a potential scientific gain. The intention of this chapter is to:

1. analyze those philosophical and theological themes that arise in the context of the origin of life, focusing particularly on the intersection of physical and evolutionary parameters in the interplay of chance and necessity, alongside debates around purpose and design;

2. consider some of the ethical issues associated with both the origin and future of life from a Christian ethical perspective, including responsibility for future generations;

3. argue for the recovery of a sense of wisdom, from both a theological perspective and through phronesis or practical wisdom.

It would be impossible to do justice to the full range of possible positions that a theologian might take in relation to theological and ethical issues raised by astrobiology.

Introduction

The search for ET life is encompassed within a broad spectrum of research efforts in the field of astrobiology. In general, this multidisciplinary field seeks to understand the origin, evolution, and fate of life in the universe. While searches for ET life represent just a subset of the overall astrobiology goals, they command a disproportionate share of the public interest. To scientists it may be obvious that finding an ET microbe on Mars would be quite different than getting a message from an intelligent civilization somewhere in the Milky Way. However, to the public, the nature and implications of different types of discoveries are probably less clear. Without a systematic analysis of the varied research efforts and a consideration of the science and issues associated with them, it is impossible to get an overview of what it would mean to discover ET life. This chapter attempts such an analysis and focuses additionally on the kinds of societal issues and concerns that need to be communicated to public audiences when discussing astrobiological research and exploration efforts, particularly those centered on searching for ET life.

When searching for ET life, astrobiology uses diverse methods to identify and study potentially habitable locations, understand their environmental conditions, analyze processes that may be associated with life, and, finally, seek evidence for ET life (which may or may not be the same as finding life itself).