Yesterday I was reminded of what got me interested in economics. I’ll preface this by saying that I make my living as a substitute teacher in Toronto. It’s not glamorous, but it pays the bills. It gives me time to do research from outside academia. When I’m in high school classrooms, I always browse the posters on the wall. It’s funny what you see.

Real GDP is key to macroeconomics. But is it a valid measure of economic scale?

In science, being wrong is call for a celebration. Now that you think I’m crazy, let me explain. Science is about the search for truth . Logically, this means we should celebrate when we are right . But here’s the catch. How do we know when we are right? We can’t. Science is about conditional truth. We have ideas that we think are right because they have not yet been proven wrong. Science is a bit like evolution.

This Thanksgiving, I’d highlight something positive. Recently, Bob Sanders wrote a paper pointing out that gas rich galaxies are strong tests of MOND. The usual fit parameter, the stellar mass-to-light ratio, is effectively negligible when gas dominates. The MOND prediction follows straight from the gas distribution, for which there is no equivalent freedom.

In the last post, I noted some of the sociological overtones underpinning attitudes about dark matter and modified gravity theories. I didn’t get as far as the more scientifically  interesting part, which  illustrates a common form of reasoning in physics. About modified gravity theories, Bertone & Tait state Leaving aside just which observations need to be mimicked so precisely (I expect they mean power spectrum;

Like the Milky Way, our nearest giant neighbor, Andromeda (aka M31), has several dozen dwarf satellite galaxies. A few of these were known and had measured velocity dispersions at the time of my work with Joe Wolf, as discussed previously. Also like the Milky Way, the number of known objects has grown rapidly in recent years – thanks in this case largely to the PAndAS survey.

The Milky Way and its nearest giant neighbor Andromeda (M31) are surrounded by a swarm of dwarf satellite galaxies. Aside from relatively large beasties like the Large Magellanic Cloud or M32, the majority of these are the so-called dwarf spheroidals. There are several dozen examples known around each giant host, like the Fornax dwarf pictured above.

As soon as I wrote it, I realized that the title is much more general than anything that can be fit in a blog post. Bekenstein argued long ago that the missing mass problem should instead be called the acceleration discrepancy, because that’s what it is – a discrepancy that occurs in conventional dynamics at a particular acceleration scale. So in that sense, it is the entire history of dark matter.

Note: this is a guest post by David Merritt, following on from his paper on the philosophy of science as applied to aspects of modern cosmology. Stacy kindly invited me to write a guest post, expanding on some of the arguments in my paper . I’ll start out by saying that I certainly don’t think of my paper as a final word on anything.

David Merritt recently published the article “Cosmology and convention” in Studies in History and Philosophy of Science . This article is remarkable in many respects. For starters, it is rare that a practicing scientist reads a paper on the philosophy of science, much less publishes one in a philosophy journal.