Neutrino Puzzles (Part 1)

24 December, 2020

Merry Xmas, Ymas, and Zmas—and a variable New Year!

For a long time I’ve been meaning to update this list of open questions on the Physics FAQ:

Open questions in physics, Physics FAQ.

Here’s what it said about neutrinos as of 2012:

• What is the correct theory of neutrinos?  Why are they almost but not quite massless?  Do all three known neutrinos—electron, muon, and tau—all have a mass?  Could any neutrinos be Majorana spinors?  Is there a fourth kind of neutrino, such as a “sterile” neutrino?

Starting in the 1990s, our understanding of neutrinos has dramatically improved, and the puzzle of why we see about 1/3 as many electron neutrinos coming from the sun as naively expected has pretty much been answered: the different neutrinos can turn into each other via a process called “oscillation”. But, there are still lots of loose ends.

It’s held up fairly well: all of those questions are still things people wonder about. But I should add a question like this, because it’s nice and concrete, and physicists are fascinated by it:

• Is the tau neutrino heavier than the mu and electron neutrinos, or lighter?

This is a bit sloppy because the neutrinos of definite mass are linear combinations of the neutrinos of definite flavor (the electron, mu and tau neutrinos). The neutrinos of definite mass are called mass eigenstates and the neutrinos of definite flavor are called flavor eigenstates. This picture by Xavier Sarazin makes the two competing scenarios clearer:

In the normal hierarchy the mass eigenstate that’s mainly made of tau neutrino is the heaviest. In the inverted hierarchy it’s the lightest.

We don’t know which of these scenarios is correct. The problem is that we can’t easily measure neutrino masses! The rate at which neutrinos oscillate from flavor to flavor gives us information about absolute values of differences of squared masses! Currently we’re pretty sure the three masses obey

|m_1^2 - m_2^2| \approx 0.00008\; \mathrm{eV}^2


|m_2^2 - m_3^2| \approx 0.003 \;\mathrm{eV}^2

So, m_1 and m_2 are close and m_3 is farther, but we don’t know if m_3 is bigger than the other two (normal hierarchy) or smaller (inverted hierarchy).

We also don’t know which is bigger, m_1 or m_2. And as the FAQ points out, we’re not even sure all three masses are nonzero!

By the way, I will bet that we’ve got the normal hierarchy, with m_1 < m_2 < m_3. My reason is just that this seems to match the behavior of the other leptons. The electron is lighter than the muon which is lighter than the tau. So it seems to vaguely make sense that the electron neutrino should be lighter than the mu neutrino which in turn is lighter than the tau neutrino. But this ‘seems to vaguely make sense’ is not based on any theoretical reason! We haven’t the foggiest clue why any of these masses are what they are—and that’s another question on the list.

I also want to change this question to something less technical, so people realize what a big deal it is:

Could any neutrinos be Majorana spinors?

A less technical formulation would be:

• Is any kind of neutrino its own antiparticle?

On the one hand it’s amazing that we don’t know if neutrinos are their own antiparticles! But on the other hand, it’s really hard to tell if a particle is its own antiparticle if its very hard to detect and when you make them they’re almost always whizzing along near the speed of light.

We’d know at least some neutrinos are their own antiparticles if we saw neutrinoless double beta decay. That’s a not-yet-seen form of radioactive decay where two neutrons turn into two protons and two electrons without emitting two antineutrinos, basically because the antineutrinos annihilate each other:

Physicists have looked for neutrinoless double beta decay. If it happens, it’s quite rare.

Why in the world should we suspect that neutrinos are their own antiparticles? The main reason is that this would provide another mechanism for them to have a mass—a so-called ‘Majorana mass’, as opposed to the more conventional ‘Dirac mass’ that explains the mass of the electron (for example) in the Standard Model.

I will bet against the observed neutrinos being their own antiparticles, because this would violate conservation of lepton number and an even more sacred conservation law: conservation of baryon number minus lepton number. On the other hand, if some so-far-unobserved right-handed neutrinos are very heavy and have a Majorana mass, we could explain the very light masses of the observed neutrinos using a trick called the seesaw mechanism. And by the way: even the more conventional ‘Dirac mass’ requires that the observed left-handed neutrinos have right-handed partners, which have so far not been seen! So here’s another interesting open question:

• Are there right-handed neutrinos: that is, neutrinos that spin counterclockwise along their direction of motion when moving at high speeds?

So many unanswered questions about neutrinos!

My list of references hasn’t held up as well:

For details, try:

The Neutrino Oscillation Industry.

• John Baez, Neutrinos and the nysterious Maki-Nakagawa-Sakata Matrix.

• Paul Langacker, Implications of neutrino mass.

• Boris Kayser, Neutrino mass: where do we stand, and where are we going?.

The first of these has lots of links to the web pages of research groups doing experiments on neutrinos. It’s indeed a big industry!

In fact the first page is now full of silly random posts, but oddly still titled Paul Langacker’s page is missing. Boris Kayser’s review uses an old link to the arXiv, back when it was at His review is still on the arXiv, and it’s nice—but it dates to 1998, so I should find something newer!

What are the best places to read a lot of clearly explained information about neutrino puzzles? Are there other big neutrino puzzles I should include?

Theoretical Physics in the 21st Century

22 December, 2020

In 2021, March 8–13 will be “Sustainability Week” in Switzerland. During this week, students at all Swiss universities will come together to present their current work, promote a sustainable lifestyle and draw extra attention to changes that must be made at the institutional level. Anna Knörr, a third year Physics Bachelor student at ETH Zürich, is president of the Student Sustainability Commission. She and Professor Niklas Beisert invited me to give the Zurich Theoretical Physics Colloquium on Monday the 8th of March.

She proposed the modest title “Theoretical Physics in the 21st Century”. I like this idea because it would give me a chance to think about the ways in which theoretical physics is stuck, the ways it’s not, and the ways theoretical physics can help us adapt to the Anthropocene. So, I could blend ideas from these two talks:

Unsolved mysteries in fundamental physics, Cambridge University Physics Society, October 3, 2018.

Energy and the environment—what physicists can do, Perimeter Institute, April 17, 2013.

but update and improve the second one. I think it’ll be pretty easy for me to explain that the Anthropocene is about much more than global warming. The hard part is giving suggestions for “what physicists can do”.

Of course we can all resolve to fly less, etc.—but none of those suggestions take advantage of special skills that physicists have. Anna Knörr correctly noted that many theoretical physicists have trouble seeing what they can do to help our civilization adapt to the Anthropocene, since many of them are not good at designing better batteries, solar cells, fission or fusion reactors comes easily. To the extent that I’m a theoretical physicist I fit into this unhappy class. But I think there are more theoretical activities that can still be helpful! And I have more to say about this now than in 2013.

One lesson I may offer is this:

If something is not working, try something different.

This applies to the Anthropocene as a whole, all the social problems that afflict us, and also fundamental physics. I just ran into a talk that the famous particle physicist Sheldon Glashow gave 40 years ago, called “The New Frontier”. He said:

Important discoveries await the next generation of accelerators. QCD and the electroweak theory need further confirmation. We need to know how b quarks decay. The weak interaction intermediaries must be seen to be believed. The top quark (or the perversions needed by topless theories) lurks just out of range. Higgs may wait to be found. There could well be a fourth family of quarks and leptons. There may even be unanticipated surprises. We need the new machines.

That was in 1980. The ‘weak interaction intermediaries’—the W and Z—were found three years later, in 1982. The top quark was found in 1995. The Higgs boson was found in 2012. No fourth generation of quarks and leptons, and we now have good evidence that none exists. To the great sorrow of all physcists, particle accelerators have found no unanticipated surprises!

On the other hand, we have for the first time an apparently correct theory of elementary particle physics. It may be, in a sense, phenomenologically complete. It suggests the possibility that there are no more surprises at higher energies, at least at energies that are remotely accessible.

He’s proved right on this, so far.

Proton decay, if it is found, will reinforce belief in the great desert extending from 100 GeV to the unification mass of 1014 GeV. Perhaps the desert is a blessing in disguise. Ever larger and more costly machines conflict with dwindling finances and energy reserves. All frontiers come to an end.

You may like this scenario or not; it may be true or false. But, it is neither impossible, implausible, nor unlikely. And, do not despair nor prematurely lament the death of particle physics. We have a ways to go to reach the desert, with exotic fauna along the way, and even the desolation of a desert can be interesting.

Proton decay has not been found despite a huge amount of effort. So, that piece of evidence for grand unified theories is missing, and with it a strong piece of evidence that there should be a “desert” of new phenomena between the electroweak unification energy scale and the GUT energy scale.

But, we’re not seeing anything beyond the Standard Model: no “exotic fauna”.

Glashow’s “new frontier” was the “passive frontier”: non-accelerator experiments like neutrino measurements, and this is indeed where the progress came since 1980: we now know neutrinos are massive and oscillate, and there is still some mystery here and room for surprises—though frankly I suspect that neutrino masses will work very much like quark masses, via coupling to the Higgs. (This is in a sense the most conservative, least truly exciting scenario.)

So, very little dramatic progress has happened in particle physics since 1980—except for a profusion of new theories that haven’t made any verified predictions. I’ll argue that physicists should turn elsewhere! There are other things for them to do, that are much more exciting.

Consolidated Appropriations Act, 2021

22 December, 2020

You may not have noticed, but the US Congress just passed the biggest climate-related bill in long time, with measures to help save the ozone layer and speed progress on solar, wind and nuclear energy, battery storage and carbon capture. It’s big news, though it’s been overshadowed by the pandemic and resulting economic disaster. Everyone is focused on another portion of the 5593-page Consolidated Appropriations Act: namely, Division M, the Coronavirus Response and Relief Supplemental Appropriations Act.

This is important. We’ll get through this pandemic, though the US at least has been doing a bad job so far. Global warming will be a much tougher test of our resolve. So it’s good to see this step toward recognizing its gravity.

• Sarah Kaplan and Dino Grandoni, Stimulus deal includes raft of provisions to fight climate change, Washington Post, 21 December 2020.

In one of the biggest victories for U.S. climate action in a decade, Congress has moved to phase out a class of potent planet-warming chemicals and provide billions of dollars for renewable energy and efforts to suck carbon from the atmosphere as part of the $900 billion coronavirus relief package.

The legislation […] wraps together several bills with bipartisan backing and support from an unusual coalition of environmentalists and industry groups.

It will cut the use of hydrofluorocarbons (HFCs), chemicals used in air conditioners and refrigerators that are hundreds of times worse for the climate than carbon dioxide. It authorizes a sweeping set of new renewable energy measures, including tax credit extensions and new research and development programs for solar, wind and energy storage; funding for energy efficiency projects; upgrades to the electric grid and a new commitment to research on removing carbon from the atmosphere. And it reauthorizes an Environmental Protection Agency program to curb emissions from diesel engines.

The legislation also includes key language on the “sense of Congress” that the Energy Department must prioritize funding for research to power the United States with 100 percent “clean, renewable, or zero-emission energy sources” — a rare declaration that the nation should be striving toward net-zero carbon emissions.

“This is perhaps the most significant climate legislation Congress has ever passed,” said Grant Carlisle, a senior policy adviser at the Natural Resources Defense Council.

The HFC measure, which empowers the EPA to cut the production and use of HFCs by 85 percent over the next 15 years, is expected to save as much as half a degree Celsius of warming by the end of the century. Scientists say the world needs to constrain the increase in the average global temperature to less than 2 degrees Celsius compared with preindustrial times to avoid catastrophic, irreversible damage to the planet. Some places around the globe are already experiencing an average temperature rise beyond that threshold.

Advocates say the $35 billion of new funding for renewable technology and energy efficiency in the legislation will also help reduce pollution that is driving global warming and provide a much-needed boost to federal energy programs that haven’t been updated since 2007.

“It doesn’t have regulations or mandates in it,” Sasha Mackler, director of the energy project at the Bipartisan Policy Center, said of the energy package. “But from the bottom up it’s advancing the technology that’s needed. … This is definitely a bill that creates the enabling conditions for decarbonization.”

Support among lawmakers for the package suggests that tax incentives and research funding may be a rare area of common ground between two parties that have been divided on climate change.

Despite President Trump’s numerous efforts to roll back climate regulations, leading Republicans backed the package, which has been a top priority for Sen. Lisa Murkowski (R-Alaska) for years. Senators John Barrasso (R-Wyo.) and John Neely Kennedy (R-La.) helped craft the bipartisan agreement to scale down polluting refrigerants.

“These measures will protect our air while keeping costs down for the American people,” Barrasso, chair of the Senate Environment and Public Works Committee, said in a statement Monday.

Sen. Thomas R. Carper (D-Del.), an ally of President-elect Joe Biden and co-sponsor of the HFC provision, called it “a watershed moment” that bodes well for lawmakers interested in working with the incoming administration on climate change.

“The debate on whether climate change is real is over. It is real. It’s not getting better,” Carper said in a recent interview. “Our Republican colleagues, they get it, for the most part.”

The agreement comes on the heels of a major United Nations climate report, which found that nations’ current plans to reduce greenhouse gasses are just one-fifth of what’s needed to avoid catastrophic warming.

If leaders invest heavily in green infrastructure and renewable energy as part of coronavirus stimulus spending, the world could trim as much as 25 percent from its expected 2030 emissions, the U.N. report said.

Democrats and environmental groups say the legislation is not quite the sweeping “green stimulus” that’s needed. Though it meets Biden’s call to extend tax incentives for solar and wind generation and provide more money for clean energy research, it falls short of his requests for aggressive subsidies for electric vehicles and new requirements that utilities eliminate their contributions to global warming by 2035.

It also excludes a provision from earlier versions of the bill that would have set voluntary standards for energy efficiency in buildings — something that could significantly curb emissions from cities.

“Let’s be clear: Are these provisions enough to meet the demands of the science? No,” said Senate Minority Leader Charles E. Schumer (D-N.Y). “But are they a significant step in the right direction? Yes.”

The HFC rule lays the groundwork for the United States to sign onto the Kigali Amendment, an international agreement in which more than 100 nations committed to replacing the chemicals with refrigerants that have a smaller climate impact. Signed in the final days of the Obama administration, the treaty was never submitted by Trump for ratification by the Senate. By voting to curb the climate pollutant now, Congress has eased the path for approval once Biden takes office.

Included in the energy package are roughly $4 billion for solar, wind, hydropower and geothermal research and development; $1.7 billion to help low-income families install renewable energy sources in their homes; $2.6 billion for the Energy Department’s sustainable transportation program; and $500 million for research on reducing industrial emissions.

It also authorizes $2.9 billion for the Advanced Research Projects Agency-Energy, a program that funds high-risk, high-reward research and that Trump has sought to eliminate multiple times.

The increased funding is expected to make emerging clean-energy technology cheaper and more widespread. This is especially significant for ideas that have proved effective but are struggling to make the jump to commercial viability.

“This is an opportunity to not only make significant advances in climate action and reducing HFCs, but to help maintain leadership of U.S. technology and our competitiveness in that global market,” said Marty Durbin, an energy lobbyist at the U.S. Chamber of Commerce, the largest corporate lobbying group in Washington.

In a boon for renewable energy companies, Congress extended tax credits for wind and solar and introduced a new credit for offshore wind projects, which Heather Zichal, chief executive of the American Clean Power Association, called “America’s largest untapped clean energy source.” One Department of Energy analysis suggested that developing just 4 percent of the total U.S. offshore wind capacity could power some 25 million homes and reduce the nation’s greenhouse gas emissions by almost 2 percent.

But many green groups were critical of provisions dedicating more than $6 billion to efforts to remove carbon from the air and store it, as well as funding for enhanced oil recovery projects, which reuse carbon dioxide to flush residual oil from existing wells.

“It just perpetuates the fossil fuel system,” said Jean Su, an attorney and director of the energy justice program at the Center for Biological Diversity. “If you pass something like this, you’re not doing the best we can do in terms of transforming our energy system.”

Others see carbon capture as a necessary tool for mitigating emissions from sources that aren’t easily decarbonized, such as air travel. The bill directs the energy secretary to estimate “the magnitude of excess carbon dioxide” that needs to be removed from the air to stabilize the climate.

The legislation includes more than $11 billion for nuclear energy [….]

Theories of Aether and Electricity (Part 1)

19 December, 2020

I’ve been reading an amazing book, a little bit every night in bed:

• Edmund Whittaker, A History of the Theories of Aether and Electricity, Two Volumes Bound As One. Volume I: The Classical Theories. Vol. II: The Modern Theories, 1900-1926. Dover, 1989, 753 pages.

How in the world did our species figure out the laws governing the electric field, magnetic field, and charged particles? A lot started with pure luck. Two unusual stones played a key role: amber and lodestone.

The first, really fossilized tree sap, easily acquires an electric charge if you rub it against wool or silk. This was one of human’s introductions to the electric field, and electrons. Indeed, the ancient Greek word for amber was ēlektron. The second, called magnetite, is naturally magnetic.

How odd that of all the minerals in nature, there were two with peculiar abilities to attract and repel! This duality foreshadowed the duality between electric and magnetic fields, now understood mathematically using the Hodge star operator. Who could have guessed that a pair of stones would eventually lead to such deep discoveries?

Isaac Newton caught a glimpse of it. In the early 1700s he commented about both amber and lodestones in the third book of his Opticks, called simply The Queries. He was imagining challenging someone skeptical of the existence of aether:

Let him also tell me, how an electrick Body can by Friction emit an Exhalation so rare and subtile, and yet so potent, as by its Emission to cause no sensible Diminution of the weight of the electrick Body, and to be expanded through a Sphere, whose Diameter is above two Feet, and yet to be able to agitate and carry up Leaf Copper, or Leaf Gold, at the distance of above a Foot from the electrick Body? And how the Effluvia of a Magnet can be so rare and subtile, as to pass through a Plate of Glass without any Resistance or Diminution of their Force, and yet so potent as to turn a magnetick Needle beyond the Glass?

While these are brilliant questions, he and some later thinkers had to struggle for a long time to sort out the relation between what we’d later call electrons and the electric field. It’s easy to see why, since they’re so intimately related.

As it turns out, electrons are not emitted but absorbed by amber when it rubs against wool. Later there were long arguments about whether there were two kinds of ‘electrical fluid’, positively and negatively charged, or just one. But maybe the ‘exhalation’ he mentions is really the electric field, just as the ‘effluvia’ of a magnet are the magnetic field.

There is a lot more to say about all this, but I think I’ll do it in short bits, to avoid writing a 753-page tome like Whittaker’s.


18 December, 2020

On 11 December 2020, Ceres, a sustainability nonprofit that works with investors on climate change, announced that a consortium of 30 investors managing $9 trillion in assets have committed to investing to support the goal of net zero carbon emissions by 2050.

This is what the 30 investors signed:

The Net Zero Asset Managers Commitment

In line with the best available science on the impacts of climate change, we acknowledge that there is an urgent need to accelerate the transition towards global net zero emissions and for asset managers to play our part to help deliver the goals of the Paris Agreement and ensure a just transition.

In this context, my organisation commits to support the goal of net zero greenhouse gas (‘GHG’) emissions by 2050, in line with global efforts to limit warming to 1.5°C (‘net zero emissions by 2050 or sooner’). It also commits to support investing aligned with net zero emissions by 2050 or sooner.

Specifically, my organisation commits to:

a) Work in partnership with asset owner clients on decarbonisation goals, consistent with an ambition to reach net zero emissions by 2050 or sooner across all assets under management (‘AUM’).
b) Set an interim target for the proportion of assets to be managed in line with the attainment of net zero emissions by 2050 or sooner.
c) Review our interim target at least every five years, with a view to ratcheting up the proportion of AUM covered until 100% of assets are included.

In order to fulfil these commitments my organisation will:

For assets committed to be managed in line with the attainment of net zero emissions by 2050 or sooner (under commitment b)

1) Set interim targets for 2030, consistent with a fair share of the 50% global reduction in CO2 identified as a requirement in the IPCC special report on global warming of 1.5°C.
2) Take account of portfolio Scope 1 & 2 emissions and, to the extent possible, material portfolio Scope 3 emissions.
3) Prioritise the achievement of real economy emissions reductions within the sectors and companies in which we invest.
4) If using offsets, invest in long-term carbon removal, where there are no technologically and/or financially viable alternatives to eliminate emissions.
5) As required, create investment products aligned with net zero emissions by 2050 and facilitate increased investment in climate solutions.

Across all assets under management

6) Provide asset owner clients with information and analytics on net zero investing and climate risk and opportunity.
7) Implement a stewardship and engagement strategy, with a clear escalation and voting policy, that is consistent with our ambition for all assets under management to achieve net zero emissions by 2050 or sooner.
8) Engage with actors key to the investment system including credit rating agencies, auditors, stock exchanges, proxy advisers, investment consultants, and data and service providers to ensure that products and services available to investors are consistent with the aim of achieving global net zero emissions by 2050 or sooner.
9) Ensure any relevant direct and indirect policy advocacy we undertake is supportive of achieving global net zero emissions by 2050 or sooner.


10) Publish TCFD disclosures, including a climate action plan, annually, and submit them to the Investor Agenda via its partner organisations for review to ensure the approach applied is based on a robust methodology, consistent with the UN Race to Zero criteria, and action is being taken in line with the commitments made here.

We recognise collaborative investor initiatives including the Investor Agenda and its partner organisations (AIGCC, CDP, Ceres, IGCC, IIGCC, PRI, UNEPFI), Climate Action 100+, Climate League 2030, Paris Aligned Investment Initiative, Science Based Targets Initiative for Financial Institutions, UN-convened Net-Zero Asset Owner Alliance, among others, which are developing methodologies and supporting investors to take action towards net zero emissions. We will collaborate with each other and other investors via such initiatives so that investors have access to best practice, robust and science based approaches and standardised methodologies, and improved data, through which to deliver these commitments.

We also acknowledge that the scope for asset managers to invest for net zero and to meet the commitments set forth above depends on the mandates agreed with clients and clients’ and managers’ regulatory environments. These commitments are made in the expectation that governments will follow through on their own commitments to ensure the objectives of the Paris Agreement are met, including increasing the ambition of their Nationally Determined Contributions, and in the context of our legal duties to clients and unless otherwise prohibited by applicable law. In some asset classes or for some investment strategies, agreed net zero methodologies do not yet exist. Where our ability to align our approach to investment with the goal of net zero emissions by 2050 is, today, constrained, we commit to embark with determination and ambition on a journey, and to challenge and seek to overcome the constraints we face.

The Dome Fire

13 December, 2020

This August a fire swept through the Mojave National Preserve in southern California and killed about a million Joshua trees. Let us take a moment to mourn them, along with the ancient giant sequouias that we also lost this year.

(The article has a subheading that mistakenly says “countless ancient redwoods” also died, but the article itself does not claim this, though it has a section on coastal redwoods and the fires affecting them.)

• John Branch, They’re among the world’s oldest living things. The climate crisis is killing them, New York Times, December 2020.

This lavishly illustrated article talks about all three species. I’ll just quote the part on Joshua trees, since they live pretty close to here.

Mojave National Preserve, Calif. — On the August day when fire broke out on Cima Dome in the Mojave National Preserve, the California desert already was making international headlines. The thermometer at nearby Death Valley had reached 130 degrees, the highest temperature reliably recorded on Earth.

As photos of tourists smiling at the thermometer ricocheted around the world — a paradoxical bit of gee-whiz glee on a day portending a dire future — a million Joshua trees were on fire.

Cima Dome is a broad mound, a gentle and symmetrical arc on the vast desert horizon. It is visible from the interstate connecting Los Angeles and Las Vegas. Scientists considered it home to the world’s densest concentration of Joshua trees.

“To the untrained eye or the person not familiar with this region, most wouldn’t even notice it as they go by at 90 m.p.h.,” said Todd Esque, a desert ecologist for the United States Geological Survey. “But for those who do know, this is a huge loss.”

Joshua trees—a yucca, not a tree, named by Mormon settlers—already teeter toward trouble. Their range is shrinking, and they are not well-suited to outrun the quickening pace of climate change. Scientists worry that future visitors will find no Joshua trees in Joshua Tree National Park, the way some worry that Glacier National Park will be devoid of year-round ice.

“It’s a possibility,” Dr. Esque said.

Now wildfires, scarcely a threat historically, are taking out huge swaths at once, aided by climate change and invasive grasses.

The Dome Fire consumed 43,273 acres and killed most of the estimated 1.3 million Joshua trees it burned, according to Mr. Kaiser, the vegetation program manager for Mojave National Preserve.

“Cima Dome was a model for where the Joshua tree could persist for the next 100 years,” Mr. Kaiser said. “It was a beautiful, lush, decadent Joshua tree forest. But they’re wiped out.”

While there are plans to replant the millions of dead with thousands of young Joshua trees, “It’ll never come back like it was,” Mr. Kaiser said. “Not with climate change.”

Joshua trees can grow more than 40 feet tall with spiky, Seussian eccentricity. They typically live about 150 years.

But their range is shrinking faster than the trees can spread to more livable climes—higher in elevation and latitude, generally. The species is thwarted by slow migration (their large seeds, once transported by ground sloths that are now extinct, do not travel far from where they fall) and the overall population appears to be aging. Even at Cima Dome, there were relatively few young Joshua trees.

Those are persistent threats, too. Humans chop down Joshua trees to make room for neighborhoods, roads, even solar farms. And with Joshua trees often sharing the landscape with ranching, invasive grasses are fueling more fires than ever.

While the Dome Fire was shocking in its scope and ferocity, it was not surprising to the scientists who know the area best. “This was just a fire waiting to happen,” said Debra Hughson, chief of science and resource stewardship at Mojave National Preserve.

For more than a century, until 2002, cattle grazed on Cima Dome. Among the legacy of livestock is invasive perennial grasses like red brome. Weirdly, though, those same grasses may have helped the Joshua tree flourish.

Young Joshua trees need a nurse plant to hide under, and the prickly, woody blackbrush—unappetizing to livestock—is a perfect partner. As cattle chomped on grass, leaving vegetation sparse enough to prevent potential fires from spreading, Joshua trees took hold on Cima Dome more than in other places.

“A lot of what we were calling a year ago ‘the largest and densest Joshua Tree forest in the world’ probably didn’t exist in the early part of the 20th century,” Dr. Hughson said.

And after cattle were banned, and the invasive grasses grew uninterrupted, “It was just waiting for a spark,” she said.

The spark came in August, with a lightning strike. With resources stretched because of so many other California fires, the Dome Fire spread uncontrolled. It jumped from Joshua tree to Joshua tree and across park roads and fire lines, fueled by winds that became swirling firenados.

In two days, the blaze had done almost unimaginable damage.

“I was preparing myself for the worst,” Mr. Kaiser said as he toured the burn area. “And it pretty much was the worst.”

US Fusion Reactor Plan

10 December, 2020

A while back I mentioned the SPARC fusion reactor, a relatively new design. What about more traditional approaches to fusion? Here’s a good article about the state of funding for these in the US:

• Adrian Cho, U.S. physicists rally around ambitious plan to build fusion power plant, Science, 8 December 2020.

Here’s the start:

U.S. fusion scientists, notorious for squabbling over which projects to fund with their field’s limited budget, have coalesced around an audacious goal. A 10-year plan presented last week to the federal Fusion Energy Sciences Advisory Committee is the first since the community tried to formulate such a road map in 2014 and failed spectacularly. It calls for the Department of Energy (DOE), the main sponsor of U.S. fusion research, to prepare to build a prototype power plant in the 2040s that would produce carbon-free electricity by harnessing the nuclear process that powers the Sun.

The plan formalizes a goal set out 2 years ago by the National Academies of Sciences, Engineering, and Medicine and embraced in a March report from a 15-month-long fusion community planning process. It also represents a subtle but crucial shift from the basic research that officials in DOE’s Office of Science have favored. “The community urgently wants to move forward with fusion on a time scale that can impact climate change,” says Troy Carter, a fusion physicist at the University of California, Los Angeles, who chaired the planning committee. “We have to get started.”

Impact climate change… with a prototype in the 2040s? Don’t hurry yourselves too much, folks! But you might want to read this:

• Coral Davenport, Major climate report describes a strong risk of crisis as early as 2040, New York Times, 2018 October 7.

INCHEON, South Korea — A landmark report from the United Nations’ scientific panel on climate change paints a far more dire picture of the immediate consequences of climate change than previously thought and says that avoiding the damage requires transforming the world economy at a speed and scale that has “no documented historic precedent.”

The report, issued on Monday by the Intergovernmental Panel on Climate Change, a group of scientists convened by the United Nations to guide world leaders, describes a world of worsening food shortages and wildfires, and a mass die-off of coral reefs as soon as 2040—a period well within the lifetime of much of the global population.

Thanks to Keith Harbaugh for pointing out the article in Science.

Applied Compositional Thinking for Engineers

7 December, 2020

Hey! There’s a new online course coming up!

Applied Compositional Thinking for Engineers. January 7, 8, 11-15, 20-22, and 25-29. Taught by Andrea Censi, Jonathan Lorand and Gioele Zardini.

It’s not a coincidence that the acronym for “Compositional Thinking” is “CT”, just like “Category Theory”.

The course is free but registration is required. During the sign-up process you will be asked for the most convenient times for you; the instructors will choose what works best for a majority of participants. Each session will consist of short explanatory videos (to be watched in advance), and a 50 minute online lecture (allowing for questions and discussion).

Andrea Censi is known for his work on co-design. He and the others are at ETH Zürich, but this is not an official ETH Zürich course.

Here’s a video explaining the course:

Great Conjunction

5 December, 2020

I’ve been seeing Saturn and Jupiter coming closer to each other in the sky lately. Jupiter passes by Saturn every 19.6 years, and it’s called a great conjunction. But I just learned that on December 21st they’ll look closer than they have since March 1226! They’ll be just 0.1 degrees apart: 6.1 arcminutes, to be precise. That’s less than a fifth of the Moon’s apparent width.

Here’s the expected view from New York on December 16th, 45 minutes after sunset, when there will also be a crescent Moon:

Jupiter and Saturn were even closer on July 17, 1623—just 5.2 arcminutes apart—but the glare from the the Sun made them invisible from Earth. There will be another close great conjunction on March 15, 2080. Jupiter and Saturn will be just 6.0 arcminutes apart then! If you’re young, maybe you can see that one. Not me.

On February 16, 7541, Jupiter will actually pass in front of part of Saturn! This called a transit. But if you can wait that long, you might as well wait for June 17, 7541, when Jupiter will completely block the view of Saturn. This is called an occultation.

So yes, Jupiter passes by Saturn more than once that year! In fact it’ll do it three times: this is called a triple conjunction. Because the Earth moves around the Sun much faster than Jupiter or Saturn, these planets sometimes seem to move backwards in the sky, and thanks to this, there are some great conjunctions where Jupiter and Saturn come close to each other in the sky three times in rapid succession, like in 1682–1683:

I got this picture from here:

• Patrick Hartigan, Jupiter-Saturn conjunction series from 0 CE to 3000 CE.

You can have a lot of fun reading this. Since Jupiter and Saturn are in a 5:2 orbital resonance—that is, Jupiter orbits the Sun 5 times in the time it takes Saturn to go around twice—the great conjunctions are not random. Instead, they follow interesting patterns!

Puzzle. Why are triple conjunctions more common than double conjunctions?

Graph Transformation Theory and Applications

4 December, 2020

I love graph rewriting—the study of ways to change one graph into another by changing one small part at a time. My student Daniel Cicala did his thesis on this! So I’m happy to hear about the new virtual seminar series GReTA: Graph TRansformation Theory and Applications.

It aims to serve as a platform for the international graph rewriting community, promote recent developments and trends in the field, and encourage regular networking and interaction between members of this community.

Seminars are held twice a month in the form of Zoom sessions (some of which will be live-streamed to YouTube). Go to the link if you want to join on Zoom.

You can get regular updates on the GReTA seminars in several ways:

• Subscribe to the GReTA YouTube channel.

• Subscribe to the GReTA Google Calendar (or alternatively import it in iCal format).

• Subscribe to the GReTA mailing list.

Here are the two talks so far. Any subject that can promote talks on both logic and chemistry must be good! Thinking of chemistry and logic as two aspects of the same thing is bound to trigger new ideas. (Just as a sequence of chemical reactions converts reactants into products, a proof converts assumptions into conclusions.)

Speaker: Barbara König
Title: Graph transformation meets logic

Abstract. We review the integration of (first-order) logic respectively nested conditions into graph transformation. Conditions can serve various purposes: they can constrain graph rewriting, symbolically specify sets of graphs, be used in query languages and in verification (for instance in Hoare logic and for behavioural equivalence checking). In the graph transformation community the formalism of nested graph conditions has emerged, that is, conditions which are equivalent to first-order logic, but directly integrate graphs and graph morphisms, in order to express constraints more succinctly. In this talk we also explain how the notion of nested conditions can be lifted from graph transformation systems to the setting of reactive systems as defined by Leifer and Milner. It turns out that some constructions for graph transformation systems (such as computing weakest preconditions and strongest postconditions and showing local confluence by means of critical pair analysis) can be done quite elegantly in the more general setting.

Speakers: Daniel Merkle and Jakob Lykke Andersen
Title: Chemical graph transformation and applications

Abstract: Any computational method in chemistry must choose some level of precision in the modeling. One choice is made in the methods of quantum chemistry based on quantum field theory. While highly accurate, the methods are computationally very demanding, which restricts their practical use to single reactions of molecules of moderate size even when run on supercomputers. At the same time, most existing computational methods for systems chemistry and biology are formulated at the other abstraction extreme, in which the structure of molecules is represented either not at all or in a very rudimentary fashion that does not permit the tracking of individual atoms across a series of reactions.

In this talk, we present our on-going work on creating a practical modelling framework for chemistry based on Double Pushout graph transformation, and how it can be applied to analyse chemical systems. We will address important technical design decisions as well as the importance of methods inspired from Algorithm Engineering in order to reach the required efficiency of our implementation. We will present chemically relevant features that our framework provides (e.g. automatic atom tracing) as well as a set of chemical systems we investigated are currently investigating. If time allows we will discuss variations of graph transformation rule compositions and their chemical validity.