Andrew H. Jaffe

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Last post: Sep 11, 2026
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The Talk I Gave on September 12
Sep 11, 2026 · original
More 25th anniversary thoughts and recollections. I had moved to Oxford less than two weeks before, still settling into my new life in the UK, working at Imperial College in London. That day, I was heading off to Durham, in the north of England, to a conference called “ A New Era in Cosmology ” — my first big talk now that I had started my permanent academic job. I was going to be a bit late, only able to arrive toward the end of the first day of the conference: September 11, 2001. My train left in the late morning. A few hours in, passengers were starting to talk about an attack on New York City. This was in the days before smartphones and constant communication — I didn’t even have a mobile phone. The discussions around me were getting more and more frantic, and I was doing my best to piece together the story. I was born in New York City, and much of my family still lived in the area.
Second test post
Sep 9, 2026 · original
With some more text…
Test post
Sep 9, 2026 · original
Checking some infrastructure…
25 & 60
Sep 8, 2026 · original
Twenty-five years ago, I moved from San Francisco to the UK, from a fellowship at Berkeley to a permanent job at Imperial College, London. A lot has changed since then. I was 35; now I am 60. It was two weeks before 9/11; the world hasn’t seemed as open and free since. I arrived from the Bay Area just after the first dot-com bubble burst. London, adjusting to New Labour after almost two decades of Thatcher and Thatcherism, felt exciting and vibrant. But just weeks after I arrived came the horror of 9/11, and its years-long aftermath, especially the Iraq war which eventually doomed Blair’s premiership and probably led the way to the 2010 election, the disaster of “austerity” as a wrong-headed attempt to deal with the 2008 recession, and eventually to the more-disastrous Brexit on this side of the Atlantic. Similar politics, though with very different timing, led back in the USA to Obama,
Around the world in 383 days
Aug 2, 2026 · original
It’s been exactly two years since the start of our sabbatical year away from England, and almost a year since we returned. I’m only now understanding the shape of that year and the effect it had on me, my science, and my family. Imperial College has a competitive process for requesting sabbatical leave: you have to choose between (paid) “intellectual refreshment” and (unpaid) “personal refreshment”. Having made careful arrangements with colleagues around the world, I was granted a coveted year’s leave for intellectual refreshment, allowing my family and me to travel to Asia, North America and Europe over the course of about 13 months. I would visit and collaborate with those colleagues, start new projects, and use the time to finish my book, The Random Universe . By the time we left, I had submitted the draft manuscript but, as I discovered, there was still a lot of work to do. We travel
What My Thirty-Year-Old Algorithm Taught an AI
Jul 9, 2026 · original
As a scientist in the later stages of my career , the managerial and mentorship load has increased, leaving less time for math and programming. These technical activities were also why I wanted to be a scientist in the first place, and I regretted losing the time for this hands-on research. Formerly the core of my scientific work, these activities require sustained attention, hours at a time, a resource now in short supply. Also like many scientists, I’ve watched the coming of artificial intelligence over the last few years with growing interest. The technical predecessors and underlying substructure of these large language models ( LLMs ) are neural networks , a computer technology that has already begun to revolutionise many scientific fields, including cosmology and astrophysics. I wrote about LLMs in my recent book, The Random Universe , but hadn’t really used them for my own work. S
The Only Gaijin in the Onsen
Jan 15, 2025 · original
After living in Japan for about four months, we left in mid-December. We miss it already. One of the pleasures we discovered is the onsen , or hot spring. Originally referring to the natural volcanic springs themselves, and the villages around them, there are now onsens all over Japan. Many hotels have an onsen, and most towns will have several. Some people still use them as their primary bath and shower for keeping clean. (Outside of actual volcanic locations, these are technically sento rather than onsen .) You don’t actually wash yourself in the hot baths themselves; they are just for soaking, and there are often several, at different temperatures, mineral content, indoor and outdoor locations, whirlpools and even “electric baths” with muscle-stimulating currents . For actual cleaning, there is a bank of hand showers, usually with soap and shampoo. Some can be very basic, some much mo
Discovering Japan
Dec 17, 2024 · original
My old friend Marc Weidenbaum, curator and writer of disquiet.com , reminded me, in his latest post , of the value of blogging. So, here I am (again). Since September, I have been on sabbatical in Japan, working mostly at QUP (International Center for Quantum-field Measurement Systems for Studies of the Universe and Particles) at the KEK accelerator lab in Tsukuba, Japan, and spending time as well at the Kavli IPMU , about halfway into Tokyo from here. Tsukuba is a “science city” about 30 miles northeast of Tokyo, home to multiple Japanese scientific establishments (such as a University and a major lab for JAXA, the Japanese space agency). Scientifically, I’ve spent a lot of time thinking and talking about the topology of the Universe, future experiments to measure the cosmic microwave background, and statistical tools for cosmology experiments. And I was honoured to be asked to deliver
It’s been a while
May 20, 2024 · original
If you’re reading this, then you might realise that I haven’t posted anything substantive here since 2018, commemorating the near-end of the Planck collaboration . In fact it took us well into the covid pandemic before the last of the official Planck papers were published, and further improved analyses of our data continues , alongside the use of the results as the closest thing we have to a standard cosmological model, despite ongoing worries about tensions between data from Planck and other measurements of the cosmological parameters . As the years have passed, it has felt more and more difficult to add to this blog, but I recently decided to move andrewjaffe.net to a new host and blogging software (cheaper and better than my previous setup, which nonetheless served me well for almost two decades until I received a message from my old hosting company that the site was being used as par
The Milky Way
Mar 30, 2024 · original
Planck: Demographics and Diversity
Aug 13, 2018 · original
Another aspect of Planck s legacy bears examining. A couple of months ago, the 2018 Gruber Prize in Cosmology was awarded to the Planck Satellite . This was (I think) a well-deserved honour for all of us who have worked on Planck during the more than 20 years since its conception, for a mission which confirmed a standard model of cosmology and measured the parameters which describe it to accuracies of a few percent. Planck is the latest in a series of telescopes and satellites dating back to the COBE Satellite in the early 90s, through the MAXIMA and Boomerang balloons (among many others) around the turn of the 21st century, and the WMAP Satellite (The Gruber Foundation seems to like CMB satellites: COBE won the Prize in 2006 and WMAP in 2012 ). Well, it wasn t really awarded to the Planck Satellite itself, of course: 50% of the half-million-dollar award went to the Principal Investigato
(Almost) The end of Planck
Jul 19, 2018 · original
This week, we released (most of) the final set of papers from the Planck collaboration — the long-awaited Planck 2018 results (which were originally meant to be the Planck 2016 results , but everything takes longer than you hope …), available on the ESA website as well as the arXiv . More importantly for many astrophysicists and cosmologists, the final public release of Planck data is also available. Anyway, we aren t quite finished: those of you up on your roman numerals will notice that there are only 9 papers but the last one is XII — the rest of the papers will come out over the coming months. So it s not the end, but at least it s the beginning of the end. And it s been a long time coming. I attended my first Planck -related meeting in 2000 or so (and plenty of people had been working on the projects that would become Planck for a half-decade by that point). For the last year or mor
Leon Lucy, R.I.P.
May 18, 2018 · original
I have the unfortunate duty of using this blog to announce the death a couple of weeks ago of Professor Leon B Lucy , who had been a Visiting Professor working here at Imperial College from 1998. Leon got his PhD in the early 1960s at the University of Manchester, and after postdoctoral positions in Europe and the US, worked at Columbia University and the European Southern Observatory over the years, before coming to Imperial. He made significant contributions to the study of the evolution of stars, understanding in particular how they lose mass over the course of their evolution, and how very close binary stars interact and evolve inside their common envelope of hot gas. Perhaps most importantly, early in his career Leon realised how useful computers could be in astrophysics. He made two major methodological contributions to astrophysical simulations. First, he realised that by simulati
WMAP Breaks Through
Dec 4, 2017 · original
It was announced this morning that the WMAP team has won the $3 million Breakthrough Prize . Unlike the Nobel Prize, which infamously is only awarded to three people each year, the Breakthrough Prize was awarded to the whole 27-member WMAP team, led by Chuck Bennett, Gary Hinshaw, Norm Jarosik, Lyman Page, and David Spergel, but including everyone through postdocs and grad students who worked on the project. This is great, and I am happy to send my hearty congratulations to all of them (many of whom I know well and am lucky to count as friends). I actually knew about the prize last week as I was interviewed by Nature for an article about it. Luckily I didn’t have to keep the secret for long. Although I admit to a little envy, it’s hard to argue that the prize wasn’t deserved. WMAP was ideally placed to solidify the current standard model of cosmology , a Universe dominated by dark matter
The Chandrasekhar Mass and the Hubble Constant
Oct 24, 2017 · original
The first direct detection of gravitational waves was announced in February of 2015 by the LIGO team, after decades of planning, building and refining their beautiful experiment. Since that time, the US-based LIGO has been joined by the European Virgo gravitational wave telescope (and more are planned around the globe). The first four events that the teams announced were from the spiralling in and eventual mergers of pairs of black holes , with masses ranging from about seven to about forty times the mass of the sun. These masses are perhaps a bit higher than we expect to by typical, which might raise intriguing questions about how such black holes were formed and evolved, although even comparing the results to the predictions is a hard problem depending on the details of the statistical properties of the detectors and the astrophysical models for the evolution of black holes and the sta
JSONfeed
May 19, 2017 · original
More technical stuff, but I m trying to re-train myself to actually write on this blog, so here goes… For no good reason other than it was easy, I have added a JSONfeed to this blog. It can be found at http://andrewjaffe.net/blog/feed.json , and accessed from the bottom of the right-hand sidebar if you’re actually reading this at andrewjaffe.net . What does this mean? JSONfeed is an idea for a sort-of successor to something called RSS , which may stand for really simple syndication , a format for encapsulating the contents of a blog like this one so it can be indexed, consumed, and read in a variety of ways without explicitly going to my web page. RSS was created by developer, writer, and all around web-and-software guru Dave Winer , who also arguably invented — and was certainly part of the creation of — blogs and podcasting. Five or ten years ago, so-called RSS readers were starting to
Python Bug Hunting
May 19, 2017 · original
This is a technical, nerdy post, mostly so I can find the information if I need it later, but possibly of interest to others using a Mac with the Python programming language , and also since I am looking for excuses to write more here. (See also updates below .) It seems that there is a bug in the latest (mid-May 2017) release of Apple’s macOS Sierra 10.12.5 (ok, there are plenty of bugs , as there in any sufficiently complex piece of software). It first manifested itself (to me) as an error when I tried to load the jupyter notebook , a web-based graphical front end to Python (and other languages). When the command is run, it opens up a browser window. However, after updating macOS from 10.12.4 to 10.12.5, the browser didn’t open. Instead, I saw an error message: 0:97: execution error: "http://localhost:8888/tree?token=removed" doesn't understand the "open location" message. (-1708) A li
Knightian Uncertainty
May 3, 2017 · original
[ Update : I have fixed some broken links, and modified the discussion of QBism and the recent paper by Chris Fuchs — thanks to Chris himself for taking the time to read and find my mistakes!] For some reason, I’ve come across an idea called “ Knightian Uncertainty ” quite a bit lately. Frank Knight was an economist of the free-market conservative “Chicago School”, who considered various concepts related to probability in a book called Risk, Uncertainty, and Profit . He distinguished between “risk”, which he defined as applying to events to which we can assign a numerical probability, and “uncertainty”, to those events about which we know so little that we don’t even have a probability to assign, or indeed those events whose possibility we didn’t even contemplate until they occurred. In Rumsfeldian language, “risk” applies to “known unknowns”, and “uncertainty” to “unknown unknowns”. Or,
SOLE Survivor
Jan 24, 2017 · original
I recently finished my last term lecturing our second-year Quantum Mechanics course, which I taught for five years. It’s a required class, a mathematical introduction to one of the most important set of ideas in all of physics, and really the basis for much of what we do, whether that’s astrophysics or particle physics or almost anything else. It’s a slightly “old-fashioned” course, although it covers the important basic ideas: the Schr dinger Equation , the postulates of quantum mechanics , angular momentum , and spin , leading almost up to what is needed to understand the crowning achievement of early quantum theory: the structure of the hydrogen atom (and other atoms). A more modern approach might start with qubits : the simplest systems that show quantum mechanical behaviour, and the study of which has led to the revolution in quantum information and quantum computing . Moreover, the
Electoral woes and votes
Nov 22, 2016 · original
Like everyone else in my bubble, I’ve been angrily obsessing about the outcome of the US Presidential election for the last two weeks. I’d like to say that I’ve been channelling that obsession into action, but so far I’ve mostly been reading and hoping (and being disappointed). And trying to parse all the “explanations” for Trump’s election. Mostly, it’s been about what the Democrats did wrong (imperfect Hillary, ignoring the white working class, not visiting Wisconsin, too much identity politics ), and what the Republicans did right (imperfect Trump, dog whistles , focusing on economics and security). But there has been an ongoing strain of purely procedural complaint: that the system is rigged, but (ironically?) in favour of Republicans. In fact, this is manifestly true: liberals (Democrats) are more concentrated — mostly in cities — than conservatives (Republicans) who are spread more

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