The Big Picture: What It’s All About

Many years ago I had the pleasure of attending a public lecture on cosmology by Martin Rees, one of the leading theoretical astrophysicists of our time and a wonderful speaker. For the most part his choice of material was unimpeachably conventional, but somewhere in the middle of explaining the evolution of the universe he suddenly started talking about the possibility of life on other planets. You could sense the few scientists in the room squirming uncomfortably — this wasn’t cosmology at all! But the actual public, to whom the talk was addressed, loved it. They didn’t care about fussy academic boundary enforcement; they thought these questions were interesting, and were curious about how it all fit together.

Since then, following Martin’s example, I have occasionally slipped into my own talks some discussion of how the particular bit of science I was discussing fit into a larger context. What are the implications of quantum mechanics for free will, or entropy for aging and death, or the multiverse for morality? To (what should be) nobody’s surprise, those are often what people want to follow up on in questions after the talk. Professional scientists will feel an urge to correct them, arguing that those aren’t the questions they should be asking about. But I think it’s okay. Science isn’t just about solving this or that puzzle; it’s about understanding how the world works. The whole world, from the vastness of the cosmos to the particularity of an individual human life. It’s worth thinking about how all the different ways we have to talk about the world manage to fit together.

That idea is one of the motivating considerations behind my new book, The Big Picture: On the Origins of Life, Meaning, and the Universe Itself, which is being released next week (Tuesday May 10 — I may be visiting your area). It’s a big book, covering a lot of things, so over the next few days I’m going to put up some posts containing small excerpts that give a flavor what what’s inside. You can get a general feeling from glancing at the table of contents. I also can’t resist pointing you to check out the amazing blurbs that so many generous people were thoughtful enough to contribute — Elizabeth Kolbert, Neil Shubin, Deborah Blum, Alan Lightman, Sabine Hossenfelder, Michael Gazzaniga, Carlo Rovelli, and Neil deGrasse Tyson.

This book is a culmination of things I’ve been thinking about for a long time. I’ve loved physics from a young age, but I’ve also been interested in all sorts of “big” questions, from philosophy to evolution and neuroscience. And what these separate fields have in common is that they all aim to capture certain aspects of the same underlying universe. Therefore, while they are indisputably separate fields of endeavor — you don’t need to understand particle physics to be a world-class biologist — they must nevertheless be compatible with each other — if your theory of biology relies on forces that are not part of the Standard Model, it’s probably a non-starter. That’s more of a constraint than you might imagine. For example, it implies that there is no such thing as life after death. Your memories and other pieces of mental information are encoded in the arrangement of atoms in your brain, and there’s no way for that information to escape your body when you die.

More generally, ontology matters. What we believe about the fundamental nature of reality affects how we look at the world and how we choose to live our lives. We should work to get it right.

The viewpoint I advocate in TBP is poetic naturalism. “Naturalism” being the idea that there is only one world, the natural world, that follows the laws of nature and can be investigated using the methods of science. “Poetic” emphasizes the fact that there are many ways of talking about that world, and that different stories we tell can be simultaneously valid in their own domains of applicability. It is therefore distinguished from a hardcore, eliminativist naturalism that says the only things that really exist are the fundamental particles and forces, and also from various varieties of augmented naturalism that, unsatisfied with the physical world by itself, add extra categories such as mental properties or objective moral values into the mix.

Along the way, we meet a lot of fun ideas — conservation of momentum and information, emergent purpose and causality, Bayesian inference, skepticism, planets of belief, effective field theory, the Core Theory, the origin of the universe, the relationship between entropy and complexity, free energy and the purpose of life, metabolism-first and replication-first theories of abiogenesis, the fine-tuning argument, consciousness and philosophical zombies, panpsychism, Humean constructivism, and the basic finitude of our lives.

Not everyone agrees with my point of view on these matters, of course. (It’s possible that literally nobody agrees with me about every single stance I take.) That’s good! Bring it on, I say. Maybe I will learn something and change my mind. There are plenty of things I talk about in the book on which respectable good-faith disagreement is quite possible — finer points of epistemology and metaphysics, the connections between different manifestations of the arrow of time, how to confront radically skeptical scenarios, the robustness of the Core Theory, approaches to the mind-body problem, interpretations of quantum mechanics, the best way to think about complexity and evolution, the role of purposes and causes in our ontology, free will, moral realism vs, anti-realism, and so on.

The point of the book is not to stride confidently into multiple ongoing debates and proclaim that I have it All Figured Out. Quite the opposite: while the subtitle correctly implies that I talk about the origins of life, meaning, and the universe itself, the truth is that I don’t know how life began, what the meaning of it all is, or why the universe exists. What I try to advocate is a particular framework in which these kinds of questions can be addressed. Not everyone will agree even with that framework, but it is very explicitly just a starting point for thinking about some of these grand issues, not the final answers to them.

There will inevitably be complaints that I’m writing about things — biology, neuroscience, philosophy — on which I am not an academic expert. Very true! I’m pretty sure nobody in the world is an expert on absolutely everything I talk about here. But I’m just as sure that different kinds of experts need to occasionally wander outside of their intellectual comfort zones to discuss how all of these pieces fit together. My primary hope in TBP is not to put forward some dramatically original view of the universe, but to work toward a synthesis of a wide variety of ideas that have been developed by smart people of the course of centuries. It is at best a small step, but if it helps spark ongoing conversation, I’ll consider the book a great success.

So from people who don’t read the book very carefully, I’ll no doubt get it from both sides: “Knowing physics doesn’t make you an expert on the meaning of life, how dare he presume?” and “I read the whole book and he didn’t tell me what the meaning of life is, what a cheat!” So be it.

One thing that I meant to include in the Acknowledgements section of the book, but unfortunately it slipped my mind at the last minute: I should have mentioned how much my ideas about many of these topics have been shaped and sharpened by interacting with commenters on this very blog. Longtime readers will recognize many of the themes, even if the book presents them in a different way. I’ve definitely learned a lot from questions and arguments in the comment sections here (even if I’m usually too busy to participate very much myself). So — thank you, and I hope you enjoy the book!


Posted in Big Picture | 46 Comments

Youthful Brilliance

A couple of weeks ago I visited the Texas A&M Physics and Engineering Festival. It was a busy trip — I gave a physics colloquium and a philosophy colloquium as well as a public talk — but the highlight for me was an hourlong chat with the Davidson Young Scholars, who had traveled from across the country to attend the festival.

The Davidson Young Scholars program is an innovative effort to help nurture kids who are at the very top of intellectual achievement in their age group. Every age and ability group poses special challenges to educators, and deserves attention and curricula that are adjusted for their individual needs. That includes the most high-achieving ones, who easily become bored and distracted when plopped down in an average classroom. Many of them end up being home-schooled, simply because school systems aren’t equipped to handle them. So the DYS program offers special services, including most importantly a chance to meet other students like themselves, and occasionally go out into the world and get the kind of stimulation that is otherwise hard to find.


These kids were awesome. I chatted just very briefly, telling them a little about what I do and what it means to be a theoretical physicist, and then we had a free-flowing discussion. At some point I mentioned “wormholes” and it was all over. These folks love wormholes and time travel, and many of them had theories of their own, which they were eager to come to the board and explain to all of us. It was a rollicking, stimulating, delightful experience.

You can see from the board that I ended up talking about Einstein’s equation. Not that I was going to go through all of the mathematical details or provide a careful derivation, but I figured that was something they wouldn’t typically be exposed to by either their schoolwork or popular science, and it would be fun to give them a glimpse of what lies ahead if they study physics. Everyone’s life is improved by a bit of exposure to Einstein’s equation.

The kids are all right. If we old people don’t ruin them, the world will be in good hands.

Posted in Academia, Humanity | 14 Comments

Being: Human

cover6-150Anticipation is growing — in my own mind, if nowhere else — for the release of The Big Picture, which will be out on May 10. I’ve finally been able to hold the physical book in my hand, which is always a highlight of the book-writing process. And yes, there will be an audio book, which should come out the same time. I spent several days in March recording it, which taught me a valuable lesson: write shorter books.

There will also be something of a short book tour, hitting NYC, DC, Boston, Seattle, and the Bay Area, likely with a few other venues to be added later this summer. For details see my calendar page.

In many ways, the book is a celebration of naturalism in general, and poetic naturalism in particular. So to get you in the mood, here is a lovely short video from the Mothlight Creative, which celebrates naturalism in a more visual and visceral way. “I want to shiver with awe and wonder at the universe.”

Being: Human from Mothlight Creative on Vimeo.

Posted in Big Picture, Humanity | 19 Comments

Science Career Stories

The Story Collider is a wonderful institution with a simple mission: getting scientists to share stories with a broad audience. Literal, old-fashioned storytelling: standing up in front of a group of people and spinning a tale, typically with a scientific slant but always about real human life. It was founded in 2010 by Ben Lillie and Brian Wecht; I got to know Ben way back when he was a postdoc at Argonne and the University of Chicago, before he switched from academia to the less well-trodden paths of communication and the wrangling of non-profit organizations.

By now the Story Collider has accumulated quite a large number of great tales from scientists young and old, and I encourage you to catch a live show or crawl through their archives. I was able to participate in one about a year ago, where I shared the stage with a number of fascinating scientific storytellers. One of them was one of my mentors and favorite physicists, Alan Guth. Of course he has an advantage at this game in comparison to most other scientists, as he gets to tell the story of how he came up with one of the most influential ideas in modern cosmology: the inflationary universe.

It’s a great story, both for the science and for the personal aspect: Alan was near the end of his third postdoc at the time, and his academic prospects were far from clear. You just need that one brilliant idea to pop up at the right time.

But everyone’s path is different. Here, from a different event, is my young Caltech colleague Chiara Mingarelli, who explains how she ended up studying gravitational waves at the center of the universe.

Finally, it is my blog, so here is the story I told. I basically talked about myself, but I used my (occasionally humorous) interactions with Stephen Hawking as a hook. Never be afraid to hitch a ride on the coattails of someone immensely more successful, I always say.

Posted in Academia, Personal | 8 Comments

Did LIGO Detect Dark Matter?

It has often been said, including by me, that one of the most intriguing aspects of dark matter is that provides us with the best current evidence for physics beyond the Core Theory (general relativity plus the Standard Model of particle physics). The basis of that claim is that we have good evidence from at least two fronts — Big Bang nucleosynthesis, and perturbations in the cosmic microwave background — that the total density of matter in the universe is much greater than the density of “ordinary” matter like we find in the Standard Model.

There is one important loophole to this idea. The Core Theory includes not only the Standard Model, but also gravity. Gravitons themselves can’t be the dark matter — they’re massless particles, moving at the speed of light, while we know from its effects on galaxies that dark matter is “cold” (moving slowly compared to light). But there are massive, slowly-moving objects that are made of “pure gravity,” namely black holes. Could black holes be the dark matter?

It depends. The constraints from nucleosynthesis, for example, imply that the dark matter was not made of ordinary particles by the time the universe was a minute old. So you can’t have a universe with just regular matter and then form black-hole-dark-matter in the conventional ways (like collapsing stars) at late times. What you can do is imagine that the black holes were there from almost the start — that they’re primordial. Having primordial black holes isn’t the most natural thing in the world, but there are ways to make it happen, such as having very strong density perturbations at relatively small length scales (as opposed to the very weak density perturbations we see at universe-sized scales).

Recently, of course, black holes were in the news, when LIGO detected gravitational waves from the inspiral of  two black holes of approximately 30 solar masses each. This raises an interesting question, at least if you’re clever enough to put the pieces together: could the dark matter be made of primordial black holes of around 30 solar masses, and could two of them have come together to produce the LIGO signal? (So the question is not, “Are the black holes made of dark matter?”, it’s “Is the dark matter made of black holes?”)

LIGO black hole (artist's conception)

This idea has just been examined in a new paper by Bird et al.:

Did LIGO detect dark matter?

Simeon Bird, Ilias Cholis, Julian B. Muñoz, Yacine Ali-Haïmoud, Marc Kamionkowski, Ely D. Kovetz, Alvise Raccanelli, Adam G. Riess

We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M≲Mbh≲100M where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they can radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will then rapidly spiral inward due to emission of gravitational radiation and ultimately merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2−53 Gpc−3 yr−1 rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next generation experiments will be invaluable in performing these tests.

Given this intriguing idea, there are a couple of things you can do. First, of course, you’d like to check that it’s not ruled out by some other data. This turns out to be a very interesting question, as there are good limits on what masses are allowed for primordial-black-hole dark matter, from things like gravitational microlensing and the fact that sufficiently massive objects would disrupt the orbits of wide binary stars. The authors claim (and quote papers to the effect) that 30 solar masses fits snugly inside the range of values that are not ruled out by the data.

The other thing you’d like to do is figure out how many mergers like the one LIGO saw should be expected under such a scenario. Remember, LIGO seemed to get lucky by seeing such a big beautiful event right out of the gate — the thought was that most detectable signals would be from relatively puny neutron-star/neutron-star mergers, not ones from such gloriously massive black holes.

The expected rate of such mergers, under the assumption that the dark matter is made of such big black holes, isn’t easy to estimate, but the authors do their best and come up with a figure of about 5 mergers per cubic gigaparsec per year. You can then ask what the rate should be if LIGO didn’t actually get lucky, but simply observed something that is happening all the time; the answer, remarkably, is between about 2 and 50 per cubic gigaparsec per year. The numbers kind of make sense!

The scenario would be quite remarkable and significant, if it turns out to be right. Good news: we’ve found that dark matter! Bad news: hopes would dim considerably for finding new particles at energies accessible to particle accelerators. The Core Theory would turn out to be even more triumphant than we had believed.

Happily, there are ways to test the idea. If events like the ones LIGO saw came from dark-matter black holes, there would be no reason for them to be closely associated with stars. They would be distributed through space like dark matter is rather than like ordinary matter is, and we wouldn’t expect to see many visible electromagnetic counterpart events (as we might if the black holes were surrounded by gas and dust).

We shall see. It’s a popular truism, especially among gravitational-wave enthusiasts, that every time we look at the universe in a new kind of way we end up seeing something we hadn’t anticipated. If the LIGO black holes are the dark matter of the universe, that would be an understatement indeed.

Posted in arxiv, Science | 62 Comments

Einstein’s Girl

One of the joys of living in LA is being surrounded by talented and creative people of all stripes, both inside and outside the world of science. Recently, for example, my friend Gia Mora helped me redesign my main website. This is a long-overdue upgrade; the site has existed in one form or another since 1996, and has been hacked together in html by me. Remarkably, things have changed over the last twenty years, and what sufficed in the waning years of the twentieth century had become unwieldy and unable to cope with the challenges of modern webbery. Now I have a working site that should be ready, for example, for the publication of The Big Picture.

Einsteins_logo But all of that is just an excuse for mentioning that web design is not Gia’s primary calling — it’s acting and singing, often with a bit of scientific flair. For anyone in the LA area, I encourage you to check out her show Einstein’s Girl, appearing at the Malibu Playhouse on Saturday Feb. 27. I’ve seen the show, and it’s great, plus I’m told there will be highly topical gravitational-wave jokes added in for just this occasion.

Gia’s singing about physics recently took center stage at Caltech, for the Institute of Quantum Information and Matter’s One Entangled Evening event. Besides the famous Rudd/Hawking quantum chess video, and maybe a few science talks, the highlight of the program was John Preskill, resplendent in black tie, joining Gia to sining a duet about quantum entanglement. John is not technically a professional singer like Gia is, but he did write the lyrics, and I can testify that he’s very good at quantum mechanics. Hats off to both the performers.

Posted in Entertainment, Personal | 10 Comments

Gravitational Waves at Last

ONCE upon a time, there lived a man who was fascinated by the phenomenon of gravity. In his mind he imagined experiments in rocket ships and elevators, eventually concluding that gravity isn’t a conventional “force” at all — it’s a manifestation of the curvature of spacetime. He threw himself into the study of differential geometry, the abstruse mathematics of arbitrarily curved manifolds. At the end of his investigations he had a new way of thinking about space and time, culminating in a marvelous equation that quantified how gravity responds to matter and energy in the universe.

Not being one to rest on his laurels, this man worked out a number of consequences of his new theory. One was that changes in gravity didn’t spread instantly throughout the universe; they traveled at the speed of light, in the form of gravitational waves. In later years he would change his mind about this prediction, only to later change it back. Eventually more and more scientists became convinced that this prediction was valid, and worth testing. They launched a spectacularly ambitious program to build a technological marvel of an observatory that would be sensitive to the faint traces left by a passing gravitational wave. Eventually, a century after the prediction was made — a press conference was called.

Chances are that everyone reading this blog post has heard that LIGO, the Laser Interferometric Gravitational-Wave Observatory, officially announced the first direct detection of gravitational waves. Two black holes, caught in a close orbit, gradually lost energy and spiraled toward each other as they emitted gravitational waves, which zipped through space at the speed of light before eventually being detected by our observatories here on Earth. Plenty of other places will give you details on this specific discovery, or tutorials on the nature of gravitational waves, including in user-friendly comic/video form.

What I want to do here is to make sure, in case there was any danger, that nobody loses sight of the extraordinary magnitude of what has been accomplished here. We’ve become a bit blasé about such things: physics makes a prediction, it comes true, yay. But we shouldn’t take it for granted; successes like this reveal something profound about the core nature of reality.

Some guy scribbles down some symbols in an esoteric mixture of Latin, Greek, and mathematical notation. Scribbles originating in his tiny, squishy human brain. (Here are what some of those those scribbles look like, in my own incredibly sloppy handwriting.) Other people (notably Rainer Weiss, Ronald Drever, and Kip Thorne), on the basis of taking those scribbles extremely seriously, launch a plan to spend hundreds of millions of dollars over the course of decades. They concoct an audacious scheme to shoot laser beams at mirrors to look for modulated displacements of less than a millionth of a billionth of a centimeter — smaller than the diameter of an atomic nucleus. Meanwhile other people looked at the sky and tried to figure out what kind of signals they might be able to see, for example from the death spiral of black holes a billion light-years away. You know, black holes: universal regions of death where, again according to elaborate theoretical calculations, the curvature of spacetime has become so pronounced that anything entering can never possibly escape. And still other people built the lasers and the mirrors and the kilometers-long evacuated tubes and the interferometers and the electronics and the hydraulic actuators and so much more, all because they believed in those equations. And then they ran LIGO (and other related observatories) for several years, then took it apart and upgraded to Advanced LIGO, finally reaching a sensitivity where you would expect to see real gravitational waves if all that fancy theorizing was on the right track.  Continue reading

Posted in Science | 91 Comments

Guest Post: Grant Remmen on Entropic Gravity

Grant Remmen“Understanding quantum gravity” is on every physicist’s short list of Big Issues we would all like to know more about. If there’s been any lesson from last half-century of serious work on this problem, it’s that the answer is likely to be something more subtle than just “take classical general relativity and quantize it.” Quantum gravity doesn’t seem to be an ordinary quantum field theory.

In that context, it makes sense to take many different approaches and see what shakes out. Alongside old stand-bys such as string theory and loop quantum gravity, there are less head-on approaches that try to understand how quantum gravity can really be so weird, without proposing a specific and complete model of what it might be.

Grant Remmen, a graduate student here at Caltech, has been working with me recently on one such approach, dubbed entropic gravity. We just submitted a paper entitled “What Is the Entropy in Entropic Gravity?” Grant was kind enough to write up this guest blog post to explain what we’re talking about.

Meanwhile, if you’re near Pasadena, Grant and his brother Cole have written a musical, Boldly Go!, which will be performed at Caltech in a few weeks. You won’t want to miss it!

One of the most exciting developments in theoretical physics in the past few years is the growing understanding of the connections between gravity, thermodynamics, and quantum entanglement. Famously, a complete quantum mechanical theory of gravitation is difficult to construct. However, one of the aspects that we are now coming to understand about quantum gravity is that in the final theory, gravitation and even spacetime itself will be closely related to, and maybe even emergent from, the mysterious quantum mechanical property known as entanglement.

This all started several decades ago, when Hawking and others realized that black holes behave with many of the same aspects as garden-variety thermodynamic systems, including temperature, entropy, etc. Most importantly, the black hole’s entropy is equal to its area [divided by (4 times Newton’s constant)]. Attempts to understand the origin of black hole entropy, along with key developments in string theory, led to the formulation of the holographic principle – see, for example, the celebrated AdS/CFT correspondence – in which quantum gravitational physics in some spacetime is found to be completely described by some special non-gravitational physics on the boundary of the spacetime. In a nutshell, one gets a gravitational universe as a “hologram” of a non-gravitational universe.

If gravity can emerge from, or be equivalent to, a set of physical laws without gravity, then something special about that non-gravitational physics has to make it happen. Physicists have now found that that special something is quantum entanglement: the special correlations among quantum mechanical particles that defies classical description. As a result, physicists are very interested in how to get the dynamics describing how spacetime is shaped and moves – Einstein’s equation of general relativity – from various properties of entanglement. In particular, it’s been suggested that the equations of gravity can be shown to come from some notion of entropy. As our universe is quantum mechanical, we should think about the entanglement entropy, a measure of the degree of correlation of quantum subsystems, which for thermal states matches the familiar thermodynamic notion of entropy.

The general idea is as follows: Inspired by black hole thermodynamics, suppose that there’s some more general notion, in which you choose some region of spacetime, compute its area, and find that when its area changes this is associated with a change in entropy. (I’ve been vague here as to what is meant by a “change” in the area and what system we’re computing the area of – this will be clarified soon!) Next, you somehow relate the entropy to an energy (e.g., using thermodynamic relations). Finally, you write the change in area in terms of a change in the spacetime curvature, using differential geometry. Putting all the pieces together, you get a relation between an energy and the curvature of spacetime, which if everything goes well, gives you nothing more or less than Einstein’s equation! This program can be broadly described as entropic gravity and the idea has appeared in numerous forms. With the plethora of entropic gravity theories out there, we realized that there was a need to investigate what categories they fall into and whether their assumptions are justified – this is what we’ve done in our recent work.

In particular, there are two types of theories in which gravity is related to (entanglement) entropy, which we’ve called holographic gravity and thermodynamic gravity in our paper. The difference between the two is in what system you’re considering, how you define the area, and what you mean by a change in that area.

In holographic gravity, you consider a region and define the area as that of its boundary, then consider various alternate configurations and histories of the matter in that region to see how the area would be different. Recent work in AdS/CFT, in which Einstein’s equation at linear order is equivalent to something called the “entanglement first law”, falls into the holographic gravity category. This idea has been extended to apply outside of AdS/CFT by Jacobson (2015). Crucially, Jacobson’s idea is to apply holographic mathematical technology to arbitrary quantum field theories in the bulk of spacetime (rather than specializing to conformal field theories – special physical models – on the boundary as in AdS/CFT) and thereby derive Einstein’s equation. However, in this work, Jacobson needed to make various assumptions about the entanglement structure of quantum field theories. In our paper, we showed how to justify many of those assumptions, applying recent results derived in quantum field theory (for experts, the form of the modular Hamiltonian and vacuum-subtracted entanglement entropy on null surfaces for general quantum field theories). Thus, we are able to show that the holographic gravity approach actually seems to work!

On the other hand, thermodynamic gravity is of a different character. Though it appears in various forms in the literature, we focus on the famous work of Jacobson (1995). In thermodynamic gravity, you don’t consider changing the entire spacetime configuration. Instead, you imagine a bundle of light rays – a lightsheet – in a particular dynamical spacetime background. As the light rays travel along – as you move down the lightsheet – the rays can be focused by curvature of the spacetime. Now, if the bundle of light rays started with a particular cross-sectional area, you’ll find a different area later on. In thermodynamic gravity, this is the change in area that goes into the derivation of Einstein’s equation. Next, one assumes that this change in area is equivalent to an entropy – in the usual black hole way with a factor of 1/(4 times Newton’s constant) – and that this entropy can be interpreted thermodynamically in terms of an energy flow through the lightsheet. The entropy vanishes from the derivation and the Einstein equation almost immediately appears as a thermodynamic equation of state. What we realized, however, is that what the entropy is actually the entropy of was ambiguous in thermodynamic gravity. Surprisingly, we found that there doesn’t seem to be a consistent definition of the entropy in thermodynamic gravity – applying quantum field theory results for the energy and entanglement entropy, we found that thermodynamic gravity could not simultaneously reproduce the correct constant in the Einstein equation and in the entropy/area relation for black holes.

So when all is said and done, we’ve found that holographic gravity, but not thermodynamic gravity, is on the right track. To answer our own question in the title of the paper, we found – in admittedly somewhat technical language – that the vacuum-subtracted von Neumann entropy evaluated on the null boundary of small causal diamonds gives a consistent formulation of holographic gravity. The future looks exciting for finding the connections between gravity and entanglement!

Posted in Guest Post, Science | 10 Comments

Quantum Fluctuations

Posted in Science | 41 Comments

We Suck (But We Can Be Better)

One day in grad school, a couple of friends and I were sitting at a table in a hallway in the astronomy building, working on a problem set. The professor who had assigned the problems walked by and noticed what we were doing — which was fine, working together was encouraged. But then he commented, “Hey, I’m confused — you’re all smart guys, so how come the girls have been scoring better than you on the problem sets?” Out loud we mumbled something noncommittal, but I remember thinking, “Maybe they are … also smart?”

This professor was a good-hearted guy, who would have been appalled and defensive at the suggestion that his wry remark perhaps reflected a degree of unconscious bias. Multiply this example by a million, and you get an idea of what it’s like to be a woman trying to succeed in science in a modern university. Not necessarily blatant abuse or discrimination, of the sort faced by Marie Curie or Emmy Noether, but a constant stream of reminders that many of your colleagues think you might not be good enough, that what counts as “confident” for someone else qualifies as “aggressive” or “bitchy” when it comes from you, that your successes are unexpected surprises rather than natural consequences of your talent.

But even today, as we’ve recently been reminded, the obstacles faced by women scientists can still be of the old-fashioned, blatant, every-sensible-person-agrees-it’s-terrible variety. A few months ago we learned that Geoff Marcy, the respected exoplanet researcher at Berkeley, had a long history of sexually harassing students. Yesterday a couple of other cases came to light. U.S. Representative Jackie Speier gave a speech before Congress highlighting the case of Timothy Slater, another astronomer (formerly at the University of Arizona, now at the University of Wyoming) with a track record of harassment. And my own institution, Caltech, has suspended Christian Ott, a professor of theoretical astrophysics, for at least a year, after an investigation concluded that he had harassed students. A full discussion can be found in this article by Azeen Ghorayshi at BuzzFeed, and there are also stories at Science, Nature, and Gizmodo. Caltech president Thomas Rosenbaum and provost Edward Stolper published a memo that (without mentioning names) talked about Caltech’s response to the findings. Enormous credit goes to the students involved, Io Kleiser and Sarah Gossan, who showed great courage and determination in coming forward. (I’m sure they would both much rather be doing science, as would we all.)

No doubt the specifics of these situations will be debated to death. There is a wider context, however. These incidents aren’t isolated; they’re just the ones that happened to come to light recently. And there are issues here that aren’t just about men and women; they’re about what kind of culture we have in academia generally, science in particular, and physics/astronomy especially. Not only did these things happen, but they happened over an extended period of time. They were allowed to happen. Part of that is simply because shit happens; but part is that we don’t place enough value, as working academic scientists, professors, and students, in caring about each other as human beings.

Academic science — and physics is arguably the worst, though perhaps parts of engineering and computer science are just as bad — engenders a macho, cutthroat, sink-or-swim culture. We valorize scoring well on tests, talking loudly, being cocky and fast, tearing others down, “technical” proficiency, overwork, speaking in jargon, focusing on research to the exclusion of all else. In that kind of environment, when someone who is supposed to be a mentor is actually terrorizing their students and postdocs, there is nowhere for the victims to turn, and heavy penalties when they do. “You think your advisor is asking inappropriate things of you? I guess you’re not cut out for this after all.”

In 1998, Jason Altom, a graduate student in chemistry at Harvard, took his own life. Renowned among his contemporaries as both an extraordinarily talented scientist and a meticulous personality, he left behind a pointed note:

“This event could have been avoided,” the note began. “Professors here have too much power over the lives of their grad students.” The letter recommended adoption of a three-member faculty committee to monitor each graduate student’s progress and “provide protection for graduate students from abusive research advisers. If I had such a committee now I know things would be different.” It was the first time, a columnist for The Crimson observed later, that a suicide note took the form of a policy memo.

Academia will always necessarily be, in some sense, competitive: there are more people who want to be researchers and professors than there will ever be jobs for everyone. Not every student will find an eventual research or teaching position. But none of that implies that it has to be a terrifying, tortuous slog — and indeed there are exceptions. My own memories of graduate school are that it was very hard, pulling a substantial number of all-nighters and struggling with difficult material, but that at the same time it was fun. Fulfilling childhood dreams, learning about the universe! That should be the primary feeling everyone has about their education as a scientist, but too often it’s not.

A big problem is that, when problems like this arise, the natural reaction of people in positions of power is to get defensive. We deny that there is bias, or that it’s a problem, or that we haven’t been treating our students like human beings. We worry too much about the reputations of our institutions and our fields, and not enough about the lives of the people for whom we are responsible. I do it myself — nobody likes having their mistakes pointed out to them, and I’m certainly not an exception. It’s a constant struggle to balance legitimate justifications for your own views and actions against a knee-jerk tendency to defend everything you do (or don’t).

Maybe these recent events will be a wake-up call that provokes departments to take real steps to prevent harassment and improve the lives of students more generally. It’s unfortunate that we need to be shown a particularly egregious example of abuse before being stirred to action, but that’s often what it takes. In philosophy, the case of Colin McGinn has prompted a new dialogue about this kind of problem. In astronomy, President of the AAS Meg Urry has been very outspoken about the need to do better. Let’s see if physics will step up, recognize the problems we have, and take concrete steps to do better.

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