The Most Elegant Solution In All Of Physics

A day or two ago, I posted my nomination for the greatest mystery in all of physics: why is it that the “gravitational charge” (i.e. how strongly you couple to the gravitational field) is identically equal to your inertial mass (i.e. how strongly you resist being pushed around by any kind of force)?

Einstein’s General Relativity is our modern theory of gravity, and it answers this question in an extremely satisfying and elegant manner. Specifically, gravity is not a force at all; it’s the geometry of spacetime. All objects move through spacetime in as straight a line as they can; if they deviate from a straight line, it’s simply because of the curvature of spacetime. Objects of different mass are moving through the same spacetime geometry, so they all will move in the same manner.

This, to me, is an amazingly simple and elegant solution to what seems to be a great conundrum. Yes, it’s often convenient to talk about gravity as a force, but when we recognize it not as a force but just as the background of what’s out there, the conundrum completely goes away. Quantum Mechanics is in many ways a more successful theory than GR, in that it has been much more widely tested, and its tests are more precise. But I find at least the “gravity is the curvature of spacetime” part of GR to be far more elegant and beautiful than quantum mechanics.

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The Myth of the Meritocracy

I don’t know if it was intended for me, but somebody printed out and stacked with my airline reservations a scan of a letter by Smith & Smith (from Arlington, Virginia), from the 2006 issue of Physics Today (letters to the editor). The scan also included a number of penned comments written by a highly cynical and annoyed person commenting on the letter.

The letter is objecting to an earlier article about the “pipeline problem” in physics, where at higher and higher levels, women represent a smaller and smaller fraction of physicists. The conclusion of their letter reads:

Once society has fixed its problems, the optimal solution will percolate throughout the physics community so long as we maintain our unbiased meritocracy.

Whoever annotated the letter underlined “maintain our unbiased meritocracy” and wrote “Ha Ha Ha!” in thick black Sharpee letters right next to it. I tend to agree with the sentiments of the annotator, and here’s why.

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The Greatest Mystery In All Of Physics

Because this is me, I must start with a lot of disclaimers. First, the title is catchy, but many would disagree with the mystery I’ve identified. Even I might. So, please try to avoid flaming me for my choice. Second, very shortly I will post “The Most Elegant Solution In All Of Physics,” a post that might allow one to argue that what I’m about to identify as the greatest mystery isn’t a mystery at all! Groundwork laid, here we go….

In physics, there is this quantity “mass” that we use to describe how much “stuff” there is in a particle. Technically speaking, “mass” is the energy content of an object measured by an observer when that object is at rest with respect to the observer, and when the observer is viewing that object as a closed system from the outside. (In other words, we don’t know anything about “internal energy,” because the object is just a thing.)

The thing is, there really are two different kinds of mass. First, there is inertial mass, which describes how much an object resists being pushed around by any kind of force. Second, there is gravitational mass, which describes how strongly an object couples to the gravitational field. And, yet, to the best precision we’ve been able to measure, these two kinds of mass are exactly the same. So much so that in introductory physics classes we just call it “mass”, and students may not even realize that there’s anything surprising about the fact that the two are the same!

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Science is Not Just a Game

A week ago, a colleague pointed me to this New York Times article about Marcus Ross. Ross is an individual whom I personally have a hard time respecting, given what he’s done. He’s a Young-Earth Creationist who has managed to get a PhD in geosciences studying a species that vanished 65 million years ago… and all along maintaining as if he believed what he was doing.

This has been written about elsewhere in the blogosphere; I’ll just point you to Janet’s blog entry on the matter, and you can jump forward from there.

Here’s my take on the matter: Ross is not intellectually honest, at least not given the ground assumptions that make science worth doing.

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And So It Begins

Hello everybody! This is the new home of Galactic Interactions. To those of you who have not heard of me before, you can find older posts at my blog’s former location.

I’m an assistant professor of Physics & Astronomy at Vanderbilt University who is still learning how to keep his mouth shut. Or, rather, not learning, hence the blog (among other things). My favorite things to blog about are astronomy and astronomy education & outreach. Every so often, I will get a bee in my bonnet and attempt to explain some concept or another from astronomy, Physics or cosmology. However, I will also rant on about science in culture, the conflict between science and religion (and the places where there not need be so much of a conflict as there is), “free culture” issues… and about the despair of being on the tenure track without adequate funding, and the assumption of impending doom that results.

I’m looking forward to my new role as a science blogger at, and feel honored to be among the august company that is the other bloggers on this site.