Sunday, October 21, 2007

David Kennedy of Stanford wrote:

The Conscience of a Liberal - Paul Krugman - Books - Review - New York Times: [M]aybe [Paul] Krugman is not really an economist — at least not according to the definition offered more than a century ago by Francis Amasa Walker, the first president of the American Economic Association, who wrote that laissez-faire “was not... the test of economic orthodoxy, merely.... [But] used to decide whether a man were an economist at all.”

I think this approach can be applied to other fields. I should google to find some physicist who, more than a century ago, claimed that accepting Newton's laws of motion was not just orthodoxy but necessary to be considered a physicist at all. The only challenge is that, about a century ago, this was so true that no one felt the need to say it.

Then I would claim that, say the latest Nobel prize winning physicists aren't physicists at all, because they reject f=ma on the grounds that m is not a constant property of an object but depends on the relative velocity of the object and the observer.

downdate: Brad DeLong looked up the actual Walker quote and notes that Walker didn't say what Kennedy claimed he said. Oh well saved me some googling.


Molnar said...

Actually, Newton didn't get it wrong: he said force equals the time derivative of momentum, which is true whether mass is constant or not. I'm not claiming he thought the mass would change, but his formulation is consistent with special relativity. Here's a reference.

Robert said...

I sure didn't expect to end up arguing about translations from Latin here. In any case, the physicist from about a century ago would have agreed that all physicists think f=ma which is false and one paragraph from a book written by Newton doesn't change that.

Robert said...

In particular, in the wikipedia, the possibility that mass changes is introduced before Newton's discussion of how acceleration is force divided by something.

Thus the paragraph used to justify the idea that Newton's views are consistent with special relativity is not only a brief quote but one which removes the relevant statement (a context whose meaning is distorted by removal of the statement). Newtons thoughts on the subject were in word to word translation

"The changes made by these actions are equal, not in the velocities but in the motions of the bodies; that is to say, if the bodies are not hindered by any other impediments. For, because the motions are equally changed, the changes of the velocities made toward contrary parts are reciprocally proportional to the bodies."

Motion is now called momentum and the bodies are the masses of the bodies. Newton clearly did not consider "the bodies" to depend on the relative velocity of the observer. I mean "the bodies" has to be an intrinsic property of the bodies. Thus Newton did not anticipate special relativity.

The physicist from about a hundred years ago would have been much clearer to us than Newton (I mean by googling I could find a quote in English).