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Why Teach General Relativity?

In my Modern Physics class, we’re currently covering some of the basic ideas of general relativity (GR), Einstein’s powerful (and mathematically complicated!) theory of gravitation. Since most of my research has touched on GR one way or another, it’s a subject very dear to my heart. It’s also one of those topics that got me into theoretical physics in the first place—it may not be my first scientific love (that would be dinosaurs), but it’s my first grown-up scientific love.

A lot of modern physics courses skip general relativity; some textbooks don’t have anything on it, and others put it into an “optional” section. I certainly didn’t learn anything about GR in my modern courses (I had two!), but my undergraduate advisor had a strong interest, so we started discussing it as soon as I knew enough to carry on one end of a conversation.

Beyond my personal affection for GR, I think it’s a good topic to teach physics students even if they never actually use it. My reasoning: at some point in their lives, a coworker or family member or someone at a bar is going to ask them a question like “Why doesn’t the black hole at the center of the galaxy just suck everything in?” or “Why does gravity bend light?” or even “What’s the big deal about Einstein anyway?” You don’t have to be an expert in general relativity to answer any of those questions, but unless you read well-written popularizations of GR or teach yourself, you might get the answers wrong.

(A short explanation of why we don’t have to worry about black holes: they don’t “suck” things in. The difference between a black hole and a star of the same mass is the size: black holes concentrate all of their mass into a very tiny space, so the gravitational field is much stronger nearby compared to a star. This is why light can’t escape a black hole, but has no trouble leaving a star.)

(A short explanation of why gravity bends light: Einstein showed that gravity can be described by geometry instead of ordinary forces. Think about the surface of Earth: if two people start walking north from the equator but at different longitude values, they’ll both end up at the North Pole (we’ll pretend there are no oceans in their way!). No force was acting on them, yet they ended up in the same place as though a force was attracting them to each other! Light paths are like the paths of the two people walking, and gravity is like the curvature of the Earth. This is a highly simplified version, but it’s correct in its general outline.)

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