The Davey Dialogues - An Exploration of the Scientific Foundations of Human Culture. John C. Madden

      Figure 4.2 – Measurement of the distance to Proxima Centauri by the parallax method.

      – Davey, I wonder if you ever studied trigonometry in your equivalent of our high schools? If so, you may recall that the sine of the angle a is r/d. Hence d, the distance to Proxima Centauri is r/sin a, which works out to 4.2 light years.

      – Peter, you continue to underestimate my intelligence. I suppose that humankind has been the most intelligent species on Earth for so long that it cannot admit to itself that there may be other beings that are more intelligent than humans. I’ll let you know if you ever tell me something I cannot understand!

      – Thanks a lot! Perhaps I should call you Mr. Smartypants! I suppose I should have known that you would think you are a lot cleverer than I am, especially after all you have told me about yourself!

      Davey noticed my sarcasm. He at once apologised and asked me to continue. His apology did not at all address his claim to be much smarter than me, a claim I was not yet ready to concede.

      – Distance measurements are obviously fundamental to gaining an understanding of our universe. Some method of measuring our separation from more distant objects was badly needed. Solutions to the problem came from unexpected directions.

      In 1895 Henrietta Leavitt was given a job at the Harvard University observatory. In accordance with the practices of the day, because she was a woman, she was not given full status as an astronomer but was instead given leadership of the department that studied the photographic images of stars to determine their magnitude. During her career there, she discovered over 2400 stars whose brightness varied periodically in magnitude, usually with a period ranging from several hours to several tens of days. These variable stars are called cepheids after the name of the first variable star discovered (in 1784), Delta cephei. Leavitt was able to determine that there is a direct correlation between the brightness of these stars and the period of magnitude variations. This was a very significant discovery, since it permitted observers to determine both the actual brightness of a star and to measure its apparent brightness when seen from Earth. Since brightness is known to decrease as the square of the distance from the light source, it became possible to determine the distance to the star, having first calibrated cepheids, which were close enough to measure using the previously described geometric (or parallax) method to provide the baseline distances.[11]

      However, it is the third link in the chain of astronomical distance measuring techniques that is the most spectacular.

      In the 1920s, the vast majority of astronomers believed that the universe and the Milky Way galaxy were one and the same thing. Nebulas were thought to be stars (in the Milky Way) in the process of formation. The universe was thought to be a static entity, though the theory included the idea that the birth and death of stars was part of a continual regeneration process.

      The first real clue that this might not be so came from an American astronomer, Vesto Melvin Slipher who started in 1909 to study nebulas in the hopes of learning more about how our sun came about.[12] Slipher looked first at the Andromeda nebula and observed that the spectral lines of light from the nebula, including the lines from excited hydrogen gas, were displaced from their proper position on the frequency scale.

      As you may already be aware, Davey, both light and sound from a source that is moving relative to the observer will undergo an apparent frequency shift audible or visible to the observer, a shift that is referred to as the Doppler shift (after the effect’s discoverer, Christian Doppler [1803–53], an Austrian mathematician and physicist). As it happens, the Andromeda nebula is moving toward us, so the frequency of the spectral lines were shifted toward the blue, or high-frequency, end of the visible spectrum. However, by 1914 Slipher had discovered that some other nebulas were apparently receding from Earth at very high velocities, since the spectral lines were shifted down in frequency, i.e. toward the colour red. Calculations showed that one nebula was apparently receding from us at an astounding 1100 km/sec, or about 4 million km/hr! How could something moving this fast be in our galaxy?

      It was Edwin P. Hubble who provided the evidence to resolve this dilemma. Hubble was an American Rhodes Scholar who studied law at Oxford (to please his father), was briefly a school teacher, served in the US Army during the First World War, and, finally, in 1919 returned to his first and enduring passion, astronomy, when he accepted an offer of employment at Mount Wilson Observatory in California. In 1923 Hubble launched into a study of very bright stars (novae) in spiral nebulas. To his surprise, he came across a cepheid in the Andromeda nebula, and, by applying the by then standard calculations, discovered that this nebula was a million light years from us, putting it well outside the Milky Way. In rapid order, he found other cepheids, and found that many of them, too, were well removed from our own galaxy. The implication was clear. The Milky Way was not the only galaxy in town! There were many others. In fact the current estimate is that there are over one billion galaxies in our universe, each with, on average, about 100 billion stars! There are thus about 10²⁰ (i.e. 100,000,000,000,000,000,000) stars in the universe.[13]

      But there was yet another major surprise still in store.

      Slipher’s data had shown that some nebulas were moving away from us at high speeds. There were two physicists who dared to wonder aloud whether this phenomenon might be due to the expansion of the universe. The first was Alexander Friedmann, a Russian World War One fighter pilot and professor in St. Petersburg, and the second was Georges Lemaître, a young Belgian abbé and professor. To begin with, the evidence was spotty. Hubble changed all that. By 1929 he had collected enough evidence to show that the rate at which a galaxy was apparently receding from Earth was directly proportional to its distance from us. The constant of proportionality came to be called “the Hubble constant”[14] Alexander Friedmann and Georges Lemaître were right.

      Humanity thus took another great leap toward insignificance, or at least to lack of centrality. Not only was our planet not at the centre of the universe, neither was the sun, nor even our galaxy. Our sun is only one of a hundred billion billion stars, and who knows how many habitable planets there are in the universe? Furthermore, like all the other planets and stars, we resulted from a cosmic Big Bang since calculated to have taken place about 13.8[15] billion years ago!

      The idea of the Big Bang rose directly from Hubble’s observation that the universe seemed to be exploding outward. By running the events in reverse, everything seemed to come back together to a time of origin of one, huge, “universal” explosion over 13 billion years ago.

      All this took some time to digest, even amongst the scientists. Arthur Eddington, the leading astronomer of the day, wrote about these findings.

      I paused to find my quote from Eddington.

      – “. . . it seems to require a sudden and peculiar beginning of things. . . . As a scientist I simply do not believe that the present order of things started out with a bang; unscientifically I feel equally unwilling to accept the implied discontinuity of the divine nature.”[16] Others have since found the concept that everything started with one Big Bang equally uncomfortable. Was there really nothing at all prior to the Big Bang? We all have some freedom to speculate here, since, as of now, there is really not much hope of making any observations outside of our universe.

      At this point I waited deferentially for a comment from