Current Reading

This blog is primarily for me to blog my responses to books that I'm reading. Sometimes I blog about other stuff too, though.

I'm currently re-reading Systems of Survival by Jane Jacobs.

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Monday, May 16, 2016

Galileo had balls

Prompted by a post by Gene Callahan's recent post, I have thought more about Galileo's thought experiment concerning the two balls (of different mass) tied to each other.  Galileo attempted to prove the inconsistency of Aristotelian physics by arguing that two balls tied to each other are a composite object of greater mass than either, so they must fall faster than either constituent ball, but at the same time the lighter ball has a natural tendency to fall slowly and so it will try to slow down the heavier ball.  Based on this, Galileo concludes that the model is inconsistent and so the balls must accelerate at a mass-independent rate.

This is, of course, wrong in one very important sense:  In a world with air resistance things do fall at mass-dependent rates!  Yet, at the same time, we know that Galileo is right about purely gravitatinal effects.  So, how did he get that right, and was it a coincidence?

Well, first, he got it right about purely gravitational effects because of an embedded assumption of inertia.  He is assuming that the lighter ball is trying to retard the motion of the heavier ball, which means that he's assuming that it's harder to move if you're pulling something.  While that's not a fully Newtonian model, it has a general idea of inertia in there.  If you assume a mass-dependent resistance to changes in motion then adding mass to an object should slow its acceleration, which can (if the dependencies are the same) cancel the mass-dependent pull of the earth.

Galileo got this right, but it was partly a coincidence because these cancellations require that mass enter into inertial and gravitational effects in the same way, so that you can divide it out.  Still, he got the qualitative idea.  I wouldn't recommend acceptance of his theoretical arguments at a journal today, but I give him full marks for groping toward the right physical idea and making a major leap in 17th century physics.

The other reason Galileo got this right is that he made the Aristotelian assumptions explicit while keeping inertia implicit.  When your suppositions lead to a contradiction it means that at least one supposition is wrong.  If you only make one assumption explicit then you'll reject that assumption, rather than concluding that there's a tension between two assumptions and we need more investigation to figure out which is wrong.  Galileo may not have realized to what extent he was invoking an assumption of inertia.  I'd probably need to read Two New Sciences to see how well he understood inertia.

Advantage: Advantaged.

A few years ago the Obama administration announced that it was going to put out a "college scorecard" showing data that would help students evaluate colleges.  Supposedly this was Going To Change Everything.

Well, the Chronicle of Higher Education has looked at early results, and it appears that the students who made the most use of this information are students from advantaged backgrounds.
The earnings data, however, did make a difference. (While there have been other efforts to measure postcollege income, this particular metric was new.) Colleges with higher-than-median earnings saw higher-than-expected growth in scores sent in the months after the September release, while those with lower-than-median earnings had lower-than-expected growth. The researchers estimate that for every 10-percent increase in earnings, the number of scores sent rose 2.4 percent.
But when the researchers dug deeper, they found that only some students had been swayed by the new earnings information. "The impact," they wrote, "is driven almost entirely by well-resourced high schools and students." When the researchers looked at parental education, the Scorecard’s impact was concentrated among students whose parents had at least some college. When they looked at race, it was concentrated among students who were white and Asian. And when they looked at school type, it was concentrated among those attending well-resourced public — and even more so, private — schools. 
"The subgroups of students expected to enter the college-search process with the most information and most cultural capital," the researchers wrote, "are exactly the students who responded most strongly to the Scorecard."
If you think about it, it makes sense:  The people who will make the greatest use of information will be those who are the most sophisticated at using information or have people to help them make sophisticated use of information, and those will mostly be students who are already pretty advantaged.  It's not so different from Kentaro Toyama's idea of a "Law of Amplification" in his book Geek Heresy.

But, rest assured dear reader, that the next set of metrics and information resources will Change Everything and totally level the playing field.  Mark my words.  It's coming.  None of this has happened before...or something.