Wednesday, October 9, 2019

This Year's Nobel Prize in Physics

Jim Peebles was awarded half of this year's prize for his foundational contributions to cosmology.  I've known him for 40 years -- he was my senior thesis advisor at Princeton and had already been the most influential figure in the field long before I met him.

Cosmology was actually a bit of a backwater from the 1930s until the discovery of the cosmic microwave background in 1965.  Peebles had a tangential role in that discovery (look it up -- it's a very famous story), and he played a central role in the development of the field for decades afterwards.  His most influential work has to do with the way that large-scale structure forms in the universe -- how small fluctuations in the density grow with time to give us galaxies and everything that goes with them.  (Peebles was also the best physics teacher at Princeton, at least when I was there).  So this is a well-deserved prize.  Jim is the nicest of all of the geniuses I've met -- and I've met quite a few.

Thursday, September 12, 2019

David Brin speaking at Vanderbilt Sept. 23

David Brin (author of, among others, The Postman, Kiln People, and the Uplift series), will be giving a special colloquium in the Department of Physics and Astronomy at Vanderbilt University on Monday, Sept. 23, at 4:00 in room 4327 Stevenson Center.  The talk is free and open to the public:


Our place in the Cosmos and Is Anyone Out There? 

In both science and literature, the question of ‘others’ can be a mirror illuminating our own origins and plausible destinies. Are we a fluke? Might we be the first to navigate the minefield of existence? Astrophysicist and novelist David Brin will (briefly) survey both what we know and can speculate about life in the universe.


Tuesday, July 30, 2019

More on "Portle"

I have a guest blog post about the origins of "Portle" out at the Analog website. You can read it here.

Thursday, June 20, 2019

Guest Commentary on "Portle"

My short story, "Portle", has just appeared in the July/August issue of AnalogI'll have a blog post about the story at the Analog website soon, but in the meantime I have this comment about the story from Adrian Melott, a fellow cosmologist (and collaborator) at the University of Kansas, to share:


In the spirit of literary criticism, this is the meaning of your story, which is a parable about physicists. It is true because I say it is, whether you intended it or not. By writing about it, I will make sure that everyone thinks of it this way.

Physicists couldn't tolerate the grandiose vision implied by the many-worlds understanding of quantum mechanics. Retreating from this, they narrowed their consciousness. This is called "collapse of the wave function". What happened to the little girl is a metaphor for the Copenhagen Interpretation.

Tuesday, April 2, 2019

Does Science Fiction Predict the Future?

Well, does it? I address this question in a brief article over at the online Observations section of Scientific American.

Thursday, February 14, 2019

The Most Famous Person You Would Never Recognize in a Photo

Think of the most famous people of the past 100 years:  Einstein, Churchill, Gandhi.  All of them, and hundreds more, are instantly recognizable from their photos:






So here is a question to ponder: who is the most famous person of the past century whom most people would never recognize from a photo? I have a nominee -- it's this guy:


Do you recognize him? Who is it? I'm being a little unfair here, as he became famous at a much younger age. Try this photo instead:


Time to guess: who are we talking about?

Friday, February 1, 2019

New Things in Analog

I've just had a couple of items accepted by AnalogThe first is a nonfiction article that discusses the similarities and differences between "doing" theoretical physics and coming up with new ideas for science fiction. (I need to specify here that my cosmology research does not fall under the category of "science fiction"). And the second is the short story to which I alluded here. I can't tell you what it's about -- you'll just have to wait and see.

Friday, September 7, 2018

An Astronomy Math Puzzle

It's pretty easy to measure the radial velocities of distant stars or galaxies. (The "radial" velocity is the motion of an object directly toward or directly away from us). Astronomers use the Doppler shift, the stretching or compression of light from the moving object. Light from stars or galaxies moving toward us gets "bunched up," and the wavelength shrinks, making the light bluer, while light from objects moving away from us gets stretched out and becomes redder. The Doppler effect works so well that we can pin down the radial velocities of the most distant galaxies in the universe -- these observations provided the first evidence for the expanding universe.

But it's a lot harder to measure the sideways, or "transverse" velocity. The only way to measure a transverse velocity is to keep careful watch on an object and wait... and wait... and wait... We've been able to measure the transverse motions of stars for a long time -- the star with the fastest transverse motion is Barnard's Star, named after Vanderbilt's E.E. Barnard. But even Barnard's star is moving at a glacial rate across the sky -- 10 arcseconds a year.  At that rate, it will take almost 200 years to move the width of a full moon.

Galaxies are much farther away than stars, so their transverse motions are minuscule in comparison and have never been observed. But recently the Gaia satellite has allowed astronomers to pinpoint the locations of distant objects with unprecedented accuracy. So there's speculation that if we could monitor distant galaxies for a long enough time (10 years? 20 years?) we might be able to measure their transverse motions.

All of which leads to a puzzle. We don't want to sit around for 10 years, only to discover that we've been watching a slowpoke galaxy that's hardly moving sideways at all. If we could monitor only a handful of distant galaxies, which ones are likely to have the biggest transverse velocities? Are they the galaxies with the largest radial velocities? That makes sense -- a galaxy with a large radial velocity is more likely to have a large total velocity, so the component of its velocity in the transverse direction is also likely to be large. But here's an argument in the opposite direction: if all the galaxies are moving at about the same speed, then a large radial velocity means that the galaxy is likely to be moving almost directly toward or away from us, so it will have a small transverse velocity, while a galaxy with almost no radial motion is likely to be moving perpendicular to our viewing direction and will have a very large transverse velocity. In that case, we should monitor the galaxies with the smallest radial velocities.

So what's the answer?

Friday, July 6, 2018

Time Travel Stinks

Time travel is one of the most enduring themes in science fiction, as well as one of the most implausible -- I've discussed the science of time travel here. But there's one piece missing from almost all fictional discussions of time travel: the smell.

Monday, June 25, 2018

Dark Energy: The Final Exam

Where do theoretical physicists get their ideas? That's a hard question to answer. But in the case of my most recent paper, which just appeared in Physical Review D (the preprint version is available here), I can tell you exactly where the idea came from: a final exam.

Friday, April 13, 2018

What is the Universe Made Of?

Good question.  I gave a public lecture (more specifically, the Lois McGlothlin Donaldson Endowed Lecture in Physics) on this subject at the University of Memphis last week -- if you are interested you can watch it here.

Tuesday, April 10, 2018

How Did We Survive the 1980s?

I just finished writing a short story that had to be set, for various reasons, in the early 1980s.  And I could feel my characters' pain.  How does one character find out about another one when there's no internet??  I couldn't have anyone type into a computer, call each other on cellphones, or look up facts on Wikipedia.

Friday, March 16, 2018

Stephen Hawking 1942-2018

I met Stephen Hawking a few times over the years -- the most memorable was in the early 1980s when I was a grad student at the University of Chicago. Stephen was visiting the university, but he also wanted to take a side trip out to Fermilab -- a one-hour drive outside of Chicago. This being the days before GPS (back when we had to navigate by the stars) I was assigned by my Ph.D. adviser to ride along with Stephen and his driver and direct them to Fermilab.

I showed up at the hotel in Hyde Park at the appointed hour and went to the lobby, but Stephen was nowhere to be seen. What to do? Had this been an ordinary theoretical physicist, I would simply have asked the hotel clerk to phone his room. But this was Stephen Hawking -- one does not simply go and knock on his door. So I just waited in the lobby, assuming that Stephen would make his appearance when he wished. After quite a bit of time had passed, Stephen's assistant/driver popped into the lobby and asked, "Why didn't you call up to our room? We've been waiting up there for you!"

Meanwhile (I learned later) one of the senior scientists in the astrophysics group at Fermilab was pacing back and forth, muttering that if anything happened to Hawking, he would "send Scherrer to Tuscaloosa" -- presumably a form of internal exile. But Stephen Hawking, his driver, and I finally did make it out to Fermilab (late) and all was forgiven.

Many years later, I finally got a chance to visit Tuscaloosa to speak at the University of Alabama. It's really a very nice town.

Friday, March 9, 2018

Was the Early Universe Lumpy?

When the universe was only a few minutes old, was it smooth, like Cream of Wheat (yum!), or was it lumpy, like oatmeal? (Yuk!)  British cosmologist John Barrow and I explored this question in this paper, posted yesterday. Most cosmologists think that the matter in the early universe was smooth, not lumpy, and there's no compelling reason to believe otherwise, but it's always important to look at alternatives.

How can we even say anything intelligent about the universe when it was only a few minutes old? Our best probe is the production of elements in the early universe, which goes under the tongue-twisting name of "primordial nucleosynthesis." Most of the atomic nuclei on Earth were made in stars, but a small number, including helium, deuterium, and lithium, were manufactured in the first few minutes of the universe. And the amount of each element produced is exquisitely sensitive to the density of protons and neutrons when the universe was just a few minutes old. If the universe were lumpy rather than smooth, then the element abundances would fluctuate up and down in a predictable way, and we can average these out to get a prediction for what we would see today.

Thursday, February 22, 2018

A Bayesian Coin Flip

Anyone who's spent any time at all with the scientific literature has encountered the phrase "Bayesian statistics." What's that all about?  How can there be more than one kind of statistics? Isn't statistics just a branch of mathematics, where everything is cut and dried? Alas, no. In his book Numerical Recipes, Bill Press describes statistics as "that gray area which is as surely not a branch of mathematics as it is neither a branch of science." Statistics is all about using data to derive conclusions, but there's no single "right" way to do this. So the world of statistics resembles Europe during the Reformation, divided into various factions and sects, one of these being the Cult of the Bayesians. The key idea of Bayesian statistics is that one needs to incorporate prior assumptions about reality into any modeling of data.

Here's an example.  Suppose that Alfred flips a coin 20 times, and he gets 20 heads in a row (this is very unlikely -- the probability of 20 heads in a row is less than one in two million).




Now Alfred flips the coin one more time. What is the probability that this coin flip will come up heads?  Is it

(A) Less than 1/2?  Alfred has used up all the heads.
(B) Exactly 1/2?  Past performance tells you nothing about future returns.
(C) Greater than 1/2?  Alfred is on a roll!