vote for carlo

Mars One plans to establish a permanent settlement on Mars, funded through a reality show about the training process and the one way trip to get there. Potential settlers are posting videos on the Mars One homepage which can be rated by anyone who visits it. The team to be trained will be picked from amongst the top rated videos.

Perhaps unsurprisingly, response is quite diverse, ranging from obvious nutjobs, through undergrads with no prospects, to young career entrepreneurs with science or tech backgrounds. Included amongst applicants appears to be Carlo Rovelli, one of the most philosophically subtle theoretical physicists alive today. Rovelli works on "Loop Quantum Gravity," the prime competitor against the more widely known String Theory for reconciling the apparent contradictions between General Relativity and Quantum Mechanics. The motivation for his endorsement of Loop Quantum Gravity over the sexier String Theory is the philosophical position that scientific progress is made by building upon previous insights rather than making sharp and radical breaks with past theory.

In his Mars One video, Rovelli is self effacing and does not play up his importance in the field. Nevertheless, the frontier for humanity deserves to have intellects like his paving the way. See him here, and Vote for Carlo!

the physics of posters?

A week ago, I put two posters (both from the same source) on my wall with thumbtacks. I was very careful to smooth the posters along the wall so that they would lie flat.

Then, a couple days later, I was very dismayed to notice that posters hanging somewhat slack. I even considered moving the thumbtacks in order to stretch and smooth them more so that they would lie flat.

Luckily, laziness prevailed. Today, the posters are lying flat again, just as I hung them.

So, what's the cause here? When the posters hung slack, I assumed that either a) my recollections of hanging them were inaccurate, or b) the posters were heavy enough to widen the tack holes, and thus hang more loosely.

Now that the posters are lying flat again, the natural assumption is that some kind of expansion and contraction occurred. And, the posters are hung in a small room with a small window that is frequently open, so environment inside changes with that outside.

Here's the puzzle, though: In general, temperature change causes bodies to expand w/ increased warmth and contract with cooling. But, if anything, the intermediary period between the first hanging of the posters and today was cooler than either their hanging or today. What to make of this? Perhaps not temperature, but moisture is to blame? How exactly would this cause such contraction and expansion in a wall poster?

numbers

Apparently, the current federal debt in dollars is orders of magnitude greater than the age of the universe in years.

I find this deeply disturbing.

probability and public policy II: subjective bayes and the "fair coin"

[part one of this series here]

Subjective Bayes is the view that probabilistic statements reflect our state of uncertainty; as such, they are subjective, a mere reflection of a particular human's internal state of belief.

for example: Consider a hypothetical "fair coin": what does it mean to say this coin is "fair"? As it turns out, if we know the precise conditions under which a coin is flipped, we can confidently predict the outcome of a coin toss. The physics itself is relatively simple (comparatively), and if one knows the initial conditions one can predict the outcome (i.e. if one knows the physical properties of the coin and the details of the force which set it spinning (as well as air resistence, wind velocity, etc.), one can calculate precisely which side will land face up).

Consider briefly a simplified model where we assume that air resistance and wind play no role and that the mass of the coin is uniformly distributed. We can graph the outcome of a coin toss against the upward velocity at the time of release (V, in feet per second) and the angular velocity (ω, in revolutions per second). Assuming the coin starts heads up, we can treat grey as heads and white as tails:

Persi Diaconis, "A Place for Philosophy? The Rise of Modeling in Statistical Science," 1998.

The above analysis is from work by Persi Diaconis. He discusses where in the above graph the "typical" coin toss falls:

For a typical one-foot toss, experiments show that coins go up at about 5 m.p.h. and turn over 35-40 revolutions per second. In the units of the picture the velocity is concentrated at about 0.2 on the velocity scale. This is close to zero in the picture. Fortunately, the spin is concentrated at about 40 units up on the ω axis.

For additional details see "The Probability of Heads" by Joseph Keller.

The point here is just that the "probability," or fairness, in a coin toss is not a property of the physical setup, but rather of the observer's belief state about the outcome. Once we are privy to the initial conditions of the coin toss, there is no more uncertainty and thus no more "probability" or "chance" in the event.

Another way of thinking about subjective Bayes is in terms of one's willingness to bet upon an event the outcome of which is as yet undetermined. de Finetti used this thought experiment as the basis of his theory of probability and demonstrated that any set of odds which cannot be subjected to a Dutch Book is a valid probability distribution (a "Dutch Book" is a set of bets devised by the bookie such that you lose no matter what the outcome of the event is).

If probability is just a measure of one's uncertainty, how can we use it to interact with the world? To put it another way: how can I get my subjective probability assignments to match up with the "actual" proportion of outcomes due to the underlying mechanism at work? Or, if the coin isn't "fair, if I'm being cheated, how do I figure that out? The answers to these questions in our next installment.

next: Bayes' Rule