Women, priests, and astronomy

ps200306

Last week Google did a Doodle for Georges Lemaître's 124th birthday.



I think Lemaître is sometimes viewed a bit suspiciously as the Catholic priest who came up with a theory of creation. In fact he was a hard-nosed scientist who warned the Pope against any over-enthusiastic philosophical speculation. A few newspapers did blurbs in his honour last week ...

https://www.express.co.uk/news/world/989888/Google-Doodle-Georges-Lema-tre-124th-birthday-Big-Bang-theory-universe-Albert-Einstein

Lemaître's story reminded me of what seemed like one of the great milestone achievements of my youth -- figuring out how to balance a sweeping brush upside down on the palm of your hand, and to prevent it from toppling by constant small adjustments of hand position. Imagine if instead of a sweeping brush you were trying to hold up the entire contents of the universe. That was the situation in the static universe proposed by Einstein in 1917, two years after his publication of the General Theory of Relativity. He found his equations permitted the addition of a cosmological constant, representing a kind of anti-gravity that would support a changeless universe and keep it from collapsing under its own weight.

The "sweeping brush problem", as one might call it, was pointed out by Georges Lemaître in 1927. While the static universe was possible, it required very special conditions and was hopelessly unstable under the most general assumptions. Rather than try to rescue Einstein's universe, Lemaître pointed out that an expanding universe avoided the problem altogether. The rest, as they say, is history. Well, except it isn't ... Hubble's discovery of the cosmic expansion in 1929 should have been Lemaître's crowning glory, but in fact practically nobody had heard of him. His 1927 paper was published in an obscure journal that was little read outside Belgium.

In 1930 Sir Arthur Eddington, the giant of British astrophysics, wrote an English language article about Lemaître's ideas in the Monthly Notices of the Royal Astronomical Society. I haven't read Lemaître's paper, but Eddington's is available here and I was motivated to read it since Google stuck up their Lemaître doodle last week. It's a fascinating read even if you have to skip a lot of the maths. A primary insight is that the clumping together of galaxies would cause any initially stable configuration to destabilise. It's also interesting to note that Hubble's measurement of the cosmic expansion the previous year was too high by a factor of seven compared to modern accepted values. This gave -- to use Eddington's words -- an uncomfortably recent beginning of the universe, no more than one or two billion years ago.



Albert Einstein, Georges Lemaître, Agnes Mary Clerke, William and Caroline Herschel.


Much of Eddington's paper would not be out of place in a modern textbook but it is also interesting from a historical perspective. Sometimes science can be presented as too much of a fait accompli when in fact its development is invariably a mishmash of often mutually incompatible ideas. These grow up side by side, only for some to flourish while others wither. So recently I listened to the audio book version of Agnes Mary Clerke's Popular History of Astronomy in the 19th Century. I can't recommend it highly enough, although it's not a beginner's book and you'll need a passing knowledge of the subject material in order to appreciate which ideas have been subsequently refined or superseded. The first edition of the book came out in 1885, but the audio book version is the 4th edition from 1902. In the short span of time between those editions, physics had made considerable leaps. The preface mentions that "Clerk Maxwell's medium no longer figures as an indispensable factotum", a reference to null results of experiments to detect the aether which was supposed to fill all of space.

Other major problems in astronomy were nowhere near a resolution in 1902. As an example, the processes that powered the Sun were completely unknown. Conversion of mass into energy as a result of Einstein's Special Relativity was still some years away, but even then it was treated as a curiosity with Einstein doubting that it was a physical possibility. The detailed reactions were not worked out by Hans Bethe until 1939 and he didn't get his Nobel prize for it until 1967!

Meanwhile, in the 19th century there were some interesting theories doing the rounds. The possibility that the Sun was actually burning like a lump of coal was considered, although calculations showed it would have difficulty persisting over geological time which was already known to be vast. Compressional heating by friction -- which we now understand is how stars get started -- was similarly known to have problems explaining sustained energy output. An ingenious extension of the nebular theory of star formation was conceived. The idea was that the Sun continued to accrete material to the present day, in the form of comets falling into it. Nowadays we know the rate of such accretion is much too low to sustain the Sun, but it was a reasonable guess for 19th century astronomy.

And let's face it, there were much wackier ideas floating around. For a 19th century writer, the preeminent astronomer had to be William Herschel, the discoverer of the planet Uranus. He dominated British astronomy for half a century, including the first quarter of the 19th, and his son John took up the standard thereafter. William was born only a decade or so after the death of Isaac Newton (who was born the year Galileo died). Even in Herschel's time, you got hints of pre-scientific ideas still floating around. So Herschel believed the Sun was a planet like our own, with people living on it. He reckoned that if the solar inhabitants looked at our own planet, they would perceive it as a fiery ball, like we do the Sun. The roots of this idea, of course, are in the classical Greek elements. Even though Earth had been displaced from the centre of the universe, fire still had a tendency to rise heavenward, and this was where the great Herschel got his unscientific idea.

There is lots more fascinating stuff in Clerke's book which I recommend you read or listen to yourself. The author is an interesting person in her own right. She wrote several other books on astrophysics and cosmology, and is one of the foremost science popularisers of the 19th century in spite of having no university degree or even any formal schooling. But then, orthodox routes to learning were very limited for women of the time. She was the daughter of a bank manager in Skibbereen and got interested in astronomy as a child, while the elder Herschel was still alive. She was home schooled by her parents, her father being a graduate of TCD, and an amateur astronomer. He owned a transit telescope which would have provided a time service to the townsfolk of Skibbereen in the days long before standardised time zones. The younger Clerke got to view the planets through this instrument and studied astronomy at home in addition to Latin, Greek, and maths. In her twenties she lived in Italy where she was also able to study science (though her first published article was on the Sicilian mafia!).

Later in London Clerke became a successful popular science writer. This gained her many connections with scientists of the time, and her circle grew to include astronomers from across Europe as well as South Africa, and major universities and observatories in the USA. She got to spend time at observatories and became something of an expert on spectroscopy.She was a founding member of the British Astronomical Association, but later became a member of the Royal Astronomical Society and the Royal Institution. She was only the third female member of the RAS, after astronomer Caroline Herschel (brother of William and aunt of John) and Mary Somerville, another self-motivated student and friend of the Herschels. And that reminds me, my next reading or listening project has to be the Memoir and Correspondence of Caroline Herschel, written by John's wife Margaret Herschel in 1879.

mickmackey1

I find it strange that they are still using the phrase 'cosmological constant' in today's attempts to grapple with the accelerating universe. They had no knowledge of dark matter or dark energy in 1917, so if the phrase is still relevant it can hardly be used in the manner which they intended back then.

ps200306

But they didn't intend any manner, either then or now. Einstein's cosmological constant (denoted Λ, lambda) was a giant fudge to prop up a static universe. The only reason he put it in was because philosophically he fancied a static universe and General Relativity didn't require Λ to be zero. Once the cosmic expansion was discovered by Hubble, Λ became surplus to requirements. Today's dark energy is just Λ dusted off and recycled to match the accelerating expansion rate of the universe observed by Perlmutter and Riess. Nobody, then or now, has the slightest idea what the nature of Λ is so there's no point pretending our modern one is different.

We do know what units Λ must be in to fit into Einstein's field equations and produce the required effect. Dimensionally it's an energy density: an amount of energy per unit volume of space, sometimes referred to as the vacuum energy. If we rewrite energy as force times distance, then divide through by distance, we end up with a force per unit area which, of course, has dimensions of pressure. So Λ can be thought of as a negative pressure, inflating the universe.

The big difference between Einstein's static universe and an expanding universe is that, if Λ is associated with the vacuum energy, then the more space there is the more energy there is. That's why the expansion accelerates, leading to a possible "Big Rip" as the universe tears itself apart right down to the level of subatomic particles.

mickmackey1

Hmmm well the whole subject seems embroiled in a ridiculous level of complexity and to paraphrase Clint Eastwood: 'There are two types of people in the world, those who haven't got a clue and those who understand that they haven't got a clue.'

ps200306

It's quite simple really: nature abhors a vacuum. Therefore when we're stuck for an explanation, someone always steps in to make something up :pac:

mickmackey1

ps200306 wrote: »

It's quite simple really: nature abhors a vacuum. Therefore when we're stuck for an explanation, someone always steps in to make something up :pac:

That's just macro nature. Quantum nature loves and abhors a vacuum all at the same time

Fourier

Bit old, but only read this now.

Just to mention, Lemaître wasn't the first person to discover that General Relativity implies an expanding universe, that was actually Alexander Friedmann.

General Relativity basically reduces to an equation of the form:

(Matter looks like/is arranged like this) means (Space and Time are shaped like this)

Friedmann assumed (and he is basically correct) that on the largest scales the universe looks roughly like a smooth soup. General Relativity then tells you that if the matter looks like a smooth soup, spacetime will take the form where space is growing/expanding with time, hence an expanding universe. In his paper Friedmann actually found several different types of expanding universes were possible and made no real guess as to which one was right.

Friedmann's paper was even known to Einstein, as Einstein was the one who had to allow it into the Zeitschrift für Physik journal as he acted as a referee for the paper.

Note that this is of major importance in regard to General Relativity. The theory was originally only supposed to be a theory of gravity, not cosmological history. However it was Einstein himself who first realised it would also be capable of describing the history of the universe. His first attempt being the 1917 paper on a static universe that ps200306 mentioned (although he knew about the potential for cosmology in 1916).

When Friedmann wrote his paper in 1922, Einstein accepted it into the journal as good work, but he disagreed that the universe was expanding. He thought it was an artefact of the matter being so simple (a perfectly smooth soup).

Lemaître in 1927 then repeated Friedmann's work, but he got out something extra. He found that if space was expanding this way, then light would be redshifted. So in essence Friedmann discovered the expansion of the universe was implied by General Relativity, but Lemaître found a consequence of that expansion that you could actually observe, i.e. he provided something you could actually check observationally. As mentioned above Lemaître also discovered that Static Universe were unstable. So where Friedmann found expanding ones were possible Lemaître found that they were necessary, i.e. only expanding were possible.

As mentioned by ps200306 above, Hubble observed this redshift. However this isn't what convinced Einstein. Rather between 1929 and 1931 he studied Friedmann's paper and from it came up with a stronger proof that the universe couldn't be static, a better one than Lemaître's demonstration. He also took Friedmann's guess at the total mass of the observable universe (Lemaître didn't make a guess at this) and quoted Friedmann's age for the universe, which was ~10 billion years old (close to the modern 13.7 billion).

In 1932, while staying in Los Angeles after a bit of a tour of the Americas, he coauthored a paper with Willem deSitter which was a more realistic model of the expansion of the universe. As I mentioned above Friedmann's paper had covered several possible expanding universes. Einstein and deSitter selected the simplest one and covered it in more detail.

The Einstein-deSitter model of the universe was then valid until 1998, when it was found that the expansion is accelerating. However an accelerating universe was one of the possibilities in Friedmann's original paper and Einstein himself thought that the Friedmann model he and deSitter chose would eventually prove incorrect and one of Friedmann's other models would be right.

ps200306

Fascinating stuff, Fourier. The history is much twistier than I realised. It seems also that there was a flurry of relevant contacts and influences around 1930-31 between Friedmann, de Sitter, Eddington, Lemaître, Jeans, Tolman, Einstein, Zwicky and others. You're right, Friedmann had come up with the expanding universe solution before Lemaître. And also Lemaître had anticipated the possibility of verification via galactic redshifts in 1927, before Hubble's later observations. But this also wasn't the stroke of sheer inspiration that I imagined. Lemaître already knew about the discovery of redshifts by Vesto Slipher, which went all the way back to 1912, albeit that the discovery of the redshift-distance relation would have to wait for Hubble.

Lemaître also didn't come up with his primeval atom idea at the same time -- that didn't happen until 1931. And that apocryphal story about Einstein declaring the cosmological constant to be his "greatest blunder" upon Hubble's discovery -- that turns out to be a second hand story put about by George Gamow and others, and almost certainly isn't true. In fact, Einstein tried to recycle his cosmological constant into something useful again in 1930-31, by making it responsible for matter creation in a steady state theory that predated the one by Hoyle, Bondi and Gold by nearly two decades. He just plain didn't like evolutionary theories of the cosmos, although he abandoned his steady state idea very quickly (and didn't like Hoyle's one either by the time the idea came round again).

Two fascinating papers about the goings on in cosmology around 1930, looking at an unpublished paper of Einstein's:

Einstein's aborted attempt at a dynamic steady-state universe -- https://arxiv.org/abs/1402.4099

Einstein's steady-state theory: an abandoned model of the cosmos -- https://arxiv.org/abs/1402.0132

Fourier

Thanks for the arxiv links ps200306.

I know Einstein continued in cosmology for a bit after this, exploring more consequences of General Relativity, until he eventually discovered wormholes with Nathan Rosen. After that he seemed to get absorbed in the meaning/interpretation of Quantum Mechanics, a follow up to the debates he had with Bohr in 1927 and never really returned to cosmology in a major way.