According to the International Astronomical Union, Pluto is still not a planet, but it is a Plutoid.  If I recall correctly, at the time the original naming decision was made, there was a proposal to call the class of Pluto-like objects “Plutons,” but that was rejected, in part because “Pluton” is already the name of Pluto in various languages, including French.  I guess “Plutoid” solves that problem.

I don’t much care about Pluto no longer being considered a planet, but I do think that the IAU made a poor choice of naming conventions.  According to the new system, Pluto and similar objects are not planets, but they are “dwarf planets.”  That’s right: a dwarf planet is not a planet.  That’s a needlessly confusing naming convention, especially since it’s inconsistent with the terminology in the rest of astronomy: dwarf stars are stars, and dwarf galaxies are galaxies.

That’s old news now, of course: the new wrinkle, namely the introduction of the term “Plutoid,” neither solves nor worsens that problem.

Even though it’s all in the past, here are a couple of observations about the Pluto-classification flap:

1. Obviously, no interesting scientific questions hinge on whether we choose to classify Pluto as a planet.  I recall a news article at the time of the Great Naming Controversy saying that the future of NASA’s New Horizons probe was in doubt because of the reclassification of Pluto as a non-planet.  That’s an obviously ridiculous notion: As Abraham Lincoln could tell you, the nature of a thing doesn’t change because of what we call it.

2.  The justification for the Great Renaming was to have a precise physical definition of the word “planet.”  Mike Brown has argued against the need for such a definition: Why not just consider the word “planet” to mean the nine bodies that it has traditionally meant?  By way of analogy, the word “continent” refers to a conventional set of seven land masses.  We don’t really need to justify why it’s that list of seven (Why aren’t Europe and Asia considered as one? Why not include Greenland?).   Often, science needs precise, objective definitions in order to proceed.  But it’s not clear that in this case anyone was being hampered by the arbitrary nine-body definition of the word “planet.”  What, exactly, was the problem that the IAU solved?

And still more on electability

Here’s the latest data on the electability of the Democratic presidential candidates, as determined by the political futures market Intrade.

Probability of getting nomination Probability of winning election Electability

Clinton 17.1% 13% 76%

Obama 80.1% 46% 57.4%

Remember that electability is defined to be the probability that a candidate wins the election, given that the candidate gets the nomination. Intrade lets people bet on which candidate will get the nomination and on which candidate will win the election. In both cases, the odds can be interpreted as probabilities, which are the numbers listed above. The ratio of the probabilities, by Bayes’s Theorem, is the electability.

At the bottom of this post I’ll put a graph showing electability over time. The market “thinks” that Clinton is significantly more electable than Obama, and it has thought so pretty much all the time for the past couple of months.

From conversations with various people it’s become clear that I should be more explicit about what it even means to say what the market thinks a candidate’s electability is. Here’s one way to put it: If you think that Obama’s electability is significantly different from the above value, then you can place bets on Intrade whose odds favor you, and similarly for Clinton. After the page break below, I’ll give excruciatingly explicit details about how you’d place these bets.

Lots of people are confident that the futures market has gotten this wrong — in particular, partisans of each of the two Democrats seem to think that the other one’s electability is very low. A natural question to ask these people is this: have you put your money where your mouth is?

OK. Here’s the electability graph. Data were taken from Slate’s Political Futures pages on every day when I remembered to do it.

Electability april 28

Continue reading And still more on electability

Will physics destroy the world?

Apparently a couple of guys are suing to stop the Large Hadron Collider, the new particle accelerator being built at CERN.  They’re worried about the possibility that the collisions will produce something like miniature black holes or other exotic objects that would then destroy the Earth.

This sort of worry has come up a bunch of times before.  Sometimes the worry is about the possibility that the state of matter that we know and love is only a metastable state, not the most stable state.  The idea then would be that, if you produce a single nugget of the true stable state, everything else would collapse into that new state.  It’d be like having a supersaturated sugar solution: as soon as you give it a nucleation point, everything crystallizes out.  Think Vonnegut’s ice-nine.

So should we be worried about the LHC destroying the world?  The short answer is no.  This sort of thing is logically possible, so it’s certainly worth considering the possibility, given the enormous downside of destroying the world.  But people have considered it very carefully and have shown quite convincingly that there is no risk.  There’s a short overview here, with links to the technical reports.

There’s one argument that dispenses with a lot of the various doomsday scenarios.  The sorts of collisions that will happen in the LHC happen regularly in the Earth’s upper atmosphere, as ultra-high-energy cosmic rays strike the Earth.  You can work out that, over the Earth’s 5-billion-year history, the number of times these events have occurred naturally is many times larger than the number of times they will occur at the LHC.   So the fact that the Earth is still around is very strong evidence that this sort of catastrophic scenario is impossible.

As the NY Times article points out, there’s a loophole in this argument.  The collisions in the upper atmosphere are fast-moving particles colliding with particles that are essentially at rest.  Because of conservation of momentum, anything produced in such a collision would be moving at close to the speed of light, so it wouldn’t stick around long enough to do any damage.  In contrast, the collisions in the collider will be of particles moving in opposite directions with essentially equal speeds, so the resulting detritus will be produced nearly at rest.  There is a big difference between a micro-black hole whizzing through the Earth at nearly the speed of light, which would have essentially no effect, and one that’s moving slow enough to stick around.   To see why you still shouldn’t worry, you have to read the technical reports.

A yard of snow

This post on the nominal illusion, in which the units of measure we use affect our psychological perception of a quantity, reminded me of something else interesting about the way we perceive units.

When I was in college, my roommate told me about a skiing trip he’d been on, where the snow was “a yard deep.” He’s European, so naturally he was thinking “a meter deep” and converting it for my American ears. To any native speaker of American English, of course, that sounds all wrong: we would say “three feet deep.” The question is why?

As far as I can tell, the answer is that yards are units of horizontal measure, not vertical measure. It’s a bit funny that we have units of length that are only used in certain directions, but once you start looking out for them, there are actually a bunch of them. Miles are horizontal (the exceptions are “the mile-high city” and “the mile-high club,” but I think that in both cases the “incorrect” unit is being deliberately used to make the phrase sound funny or memorable). In aviation, feet are vertical. In the ocean, fathoms are vertical and leagues are horizontal (before saying that “20,000 Leagues Under the Sea” proves this wrong, check out what the title actually means: it’s not how far down they went; it’s how far across.)

Of course, in our everyday lives, we experience horizontal distances quite differently from vertical distances, so maybe we shouldn’t be too surprised that there are different units of measure for them.

There’s a nice analogy here to the theory of relativity. In relativity, we learn to think about spacetime, as opposed to thinking of space and time separately. In doing this, it’s much easier to use a system of units in which distance and time are equivalent (and the speed of light has the value 1). Maybe some day in the future, when we’re all zipping around at close to the speed of light in our personal spacecraft, we’ll all have a strong intuitive grasp of relativity. It’ll seem perfectly natural to us to use the same units for distance and time, and the fact that people used to use different units for the two will seem quaint and archaic, like fathoms and leagues.


Update: I thought of one more exception to the statement that miles are horizontal: the Byrds song “Eight Miles High.”  But I think that’s in the same category as the others.  Anyway, they were a bunch of hippie stoners, so who cares what they think?

Anybody out there?

Since hardly anybody knows I have a blog, I assume that hardly anyone’s reading this. If you’re reading this, and I don’t know it, post a comment to let me know you’re out there. I’ll be much more motivated to post things if I think anyone’s watching.

Also, in case you’re wondering, I really enjoy playing Dr. Science, so if you have any science questions you think I might be able to answer, drop me an e-mail. I’ll answer here on the blog if I can. My main areas of expertise are big-bang cosmology and relativity, but you can try me on other topics in astrophysics and physics too.

More on electability

There seems to be more and more talk about which Presidential candidate is most “electable,” at least on the Democratic side. As I noted in an earlier post, the political futures market gives a way to assess electability. Here are some graphs showing what the market has thought about the electabilities of the two leading candidates in each party over the past few weeks.

First, the probabilities of winning the party nomination:

Nomination Probability

(Click on the images to get better-looking versions.)

Next the probabilities of winning the Presidency:


The ratio of these two is the electability (probability of a candidate’s winning the election given that he or she gets the nomination):


By the way, I think that the big jump in Romney’s electability in the last couple of days is probably just a fluctuation. The other big features you can see here are South Carolina on the Democratic side, Florida on the Republican side, and the current uncertainty in interpreting Super Tuesday on the Democratic side. Perhaps the most important thing, from the point of view of voters concerned about electability, is that Clinton and Obama are consistently very close to each other.

In the end, what you make of these results is up to you. Some people seem to attribute to the futures markets an almost magical “wisdom-of-crowds-style” ability to discern the Truth. It’s certainly possible to exaggerate that sort of thing, but at the very least what we’re looking at here is the average view of a community of people who pay a lot of attention to this subject and who are willing to put money behind their beliefs. If the numbers here strongly disagree with what you think about the candidates’ electabilities, then I have two questions for you:

1. Are you confident that you know more than this group of people?

2. If so, why don’t you bet on it?

After all, if you think that the probabilities shown here are wrong, then you can place bets on the futures markets (specifically, Intrade, which is where the above data comes from) where the odds are weighted in your favor.


I’m not going to write about politics in this blog — there’s plenty of that out there already. And I’m certainly not going to engage in any sort of political advocacy. But I thought of an interesting application of probability theory to the upcoming election, and I thought I’d summarize it here.

Remember back in 2004, when it seemed like every Democratic voter was basing his or her choice on which candidate was most “electable”? People’s gut feelings about this sort of thing are generally pretty unreliable, so it’s kind of interesting to look for a data-based answer to the electability question. It occurred to me recently that the various political futures markets provide a good way to answer that question for the upcoming election.

For those who don’t know about the political futures markets, they’re basically a way that people can bet on various political events, including the upcoming US Presidential race. Slate has a good description with lots of nice graphs. The odds on all of these bets can be interpreted as giving the probabilities of various outcomes in the race, as estimated by the community of bettors. These probabilities give enough raw data to measure each candidate’s “electability.”

By electability I mean the probability that a candidate will win in the general election, given that he (or she) gets the nomination. One of the futures markets (Intrade) lets people bet on both who will get the nomination and who will win the general election. The ratio of these for any given candidate is the electability. It’s just Bayes’s Theorem:

P(Hillary wins the presidency) =

P(Hillary gets the nomination) * P(Hillary wins the presidency | Hillary wins the nomination).

[In case it’s not familiar notation, P(y | x) means the probability that y occurs given that x occurs.]

The last factor on the right is Hillary’s electability. The futures market tells us the other two probabilities for each candidate. So we can find the electabilities of all the candidates by simple division. Before looking below, take a guess about which leading candidates in the two parties are most electable.

Continue reading Electability


This blog is for discussion of my scientific interests, which mostly focus on big-bang cosmology and general relativity. I’ll let you know what my research students and I are up to and comment from time to time on news in astronomy and cosmology. If you have anything you want to say, leave a comment on the blog or send me e-mail.