Freeman Dyson, RIP

Freeman Dyson died today, Friday, 28 February 2020, age 96, at a hospital in his longtime home of Princeton, NJ. A thoughtful obituary by George Johnson just appeared (Friday night, 28 Feb 2020) on the New York Times website.

Dyson was a mathematical physicist who knew the greats -- Feynman, Oppenheimer, Bethe, Teller, and the rest -- and became great himself by contributing the theory of quantum electrodynamics. He was one of the final living members of the World War II generation of scientists who invented cryptography, radar, and, of course, nuclear weapons and who later wrote about and lobbied vigorously for their safe and controlled use. These scientists saw themselves also as humans, citizens, and small-d democrats who knew the essential need for public discussion of issues raised by the technology they created.

Dyson reminds me of Jacob Bronowski. Both were Englishmen, both mathematicians, and both authors of books overlapping science and the humanities. Bronowski is best known for his public television series The Ascent of Man (1973). Dyson, likewise, wrote many popular books, though in an unpopular way. He dared to bring in Johann Wolfgang von Goethe, Bertold Brecht, Ingmar Bergman, T.S. Eliot, and other nonscientist people of culture.

One of those books, his 1979 memoir Disturbing the Universe, I read over a cross-country Greyhound bus trip in January 1983. What I discovered in it was not science, nor memoir, but humanity. Dyson, for example, compared Richard Feynman to Jof the Juggler in Ingmar Bergman's film, The Seventh Seal (1956). That film, central now to my understanding of life, I did yet know, but Dyson's book sent me out to see it when it finally caught up to me. Thank you, Professor Dyson. One does not expect to learn such things in a scientist's memoir! (Feynman, of course, I have long known. Like so many of my generation, I learned physics partly by perusing all three volumes of The Feynman Lectures on Physics.)

Dyson was technological optimist. Problems on earth -- be they overpopulation or climate change -- either were not as serious as claimed or could be overcome by even more technology, such as space colonies somewhere else in the universe. One would travel there by spaceships powered by nuclear explosions. It is here I lose touch with Dyson. Perhaps I lack his imagination. Certainly I lack his faith in technological solutions to problems of human nature.

Disturbing the Universe introduced me as well to Goethe's Faust, particularly this favorite line from its Part I. It is a fitting quotation to a remembrance of Freeman Dyson: "All theory is gray, / But the tree of life, my friend, is green."

posted 2020-02-28, edited 2020-02-29

How old is old?

What is the oldest thing found on earth?

Rocks, right? Some of the oldest rock formations are found in northern and eastern Canada and date back about four billion years. Immensely, unimaginably old they are, and yet the earth itself is older, dated to about 4.54 billion years. On earth, rocks are continuously recycled, broken down by weathering and erosion, and moved by water, wind, and gravity. And rocks are also built back up, reformed by sedimentation and remelting, pushed up again by our earth's ever-moving crust. Original rocks, present when our newly-created, molten planet had cooled and solidified, are likely long since recycled and remade by our dynamic living planet.

So how about meteorites, the rocky remains of "shooting star" reaching the earth's surface still intact? They are older still. At the local New Mexico Museum of Natural History and Science is found an iron-nickel meteorite collected from South America and dated to 4.55 billion years ago. "PLEASE TOUCH" says the sign by the meteor. Awed, I place my hand on its rusty metallic surface and try to feel the vastness of time that my rational mind cannot comprehend. "This may be the oldest rock you will ever touch" suggests the sign. Indeed.

Iron-nickel meteorite at the New Mexico Museum of Natural History and Science

Earth, sun, and the seven planets [1] of our solar system all originated together about 4.57 billion years ago. Originally part of rocky objects smaller than Earth but still part of the solar system, rocks that would become meteorites cooled faster and solidified sooner than Earth. Thereby they are older.

But can we go even farther back in time? Can we find even more ancient mineral-based substances on earth? Until recently I thought not. But now, I stand corrected.

Older matter ought to exist somewhere. Old as Earth and the solar system are, we are but recent arrivals. As the Big Bang creation is dated to about 13.7 billion years ago, some two-thirds of the universe's history had already passed before our solar system was created. Generations upon generations of stars and galaxies predate us and our sun. The vastness of earth's history therefore contacts to something termed "recent". Absurd! My mind rebels at this absurdity and this monstrosity. I am a trained experimental scientist. I do believe these ages. Still, I need something solid, something tangible, something to touch and feel, if only with scientific instruments, so that I can also believe in my heart and marrow as I believe in my brain. I need more evidence.

Timeline of the Universe

Late last month a scientific paper [2] appeared providing some of that evidence. The paper's title is instructive: "Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide". The words "presolar" and "interstellar" suggest matter older than the solar system and originating outside of it. "Dust" suggests solid matter, and "silicon carbide" is a particular type of mineral, one of the first minerals to form in the history of the universe [3]. Presolar grains are not new. What this paper contributes is a new method of dating them. The grains they dated are derived from the Murchison meteorite that landed in Australia in 1969. While most of the meteorite's matter dates from just after the formation of the solar system, embedded within it are trace grains of far older matter, matter containing the mineral silicon carbide (SiC). The oldest grains date to 1 to 5 billion years prior to the solar system's formation, or 5.6 to 9.6 billion years ago. As I touched the meteorite at the local museum, might my hand have come into contact with similar ancient presolar grains [4]?

Should we believe this paper? Key to answering that question is considering carefully the methods used. The authors face two challenges: (1) convincing us that the SiC grains came from outside the solar system and (2) establishing a date for their formation. For (1) the authors relied on a well-established method: demonstrating by mass spectroscopy isotopic anomalies characteristic of matter originating outside of the solar system. For (2) the authors developed a new dating method based on cosmic ray exposure. They had to. As they note in their paper, "Dating of interstellar dust directly with astronomical methods is not possible. Neither is dating based on the decay of long-lived radioactive nuclides, due to current analytical limitations and unknown initial isotopic compositions."

But can we go back even further in time?

Yes, we can go almost all of the way back, back more than 13.6 billion years, if we relax one key assumption. Up to this point, this blog post has been written from a geologist's point of view. What counts to the geologist are minerals, those underlying chemical components of terrestrial rocks, meteorites, and presolar grains. Key to the definition of mineral is the requirement that atoms continuously exist in an intact, regular, and solid structure. Recycling of rocks on earth's surface, for example, reshuffles this structure, thereby resetting a mineral's age to the most recent episode of (re)mineralization.

Chemists, in contrast, see things differently. They are more willing to let go of the requirement that dating apply to elements in their mineral form. What matters is only the elements themselves, and not their chemical neighborhood and bonding partners. All hydrogen, some helium, and traces of lithium were created in the Big Bang and predate even the first stars. Cosmologists estimate that by 379,000 years after the Big Bang, the expanding universe had cooled enough for the nuclei of the hydrogen, helium, and lithium to combine with electrons forming the first atoms.

I sip some tea, and in the water composing it, I taste and consume hydrogen dating back nearly to the Big Bang 13.7 billion years ago. Oh the places that hydrogen has been!


NOTES

1. Missing from the list of eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) is Pluto. Based on its small mass and the aberrant characteristics of its orbit (specifically, its eccentricity and large deviation from the ecliptic), Pluto was reclassified in 2006 from a "planet" to a "dwarf planet", the same status shared by Ceres, one of the larger known asteroids.

2. The article is by Philipp R. Heck et al.  and was published on 28 January 2020 in the Proceedings of the National Academy of Science, 117(4): 1884-1889. The original article is available here at no cost.

3. See Professor Robert Hazen's course, "The Origin and Evolution of Earth" (The Great Courses, 2013), especially lectures four and five. See also the Wikipedia entry on "presolar grains".

4. I am doubtful. The Murchison meteorite, a carbonaceous chondrite, has a composition distinctly different from iron-nickel meteorite displayed at the NM Museum of Natural History and Science.


ACKNOWLEDGMENT

Professor Carol Hill, her course "Planetary Geology" (now being taught at the University of New Mexico), and my fellow classmates motivated and informed this blog post.

posted: 2020-02-20, last modified: 2020-02-20