When I wrote that post on crackpots, one of my inspirations was a particularly annoying Twitter conversation. The guy I was talking to had convinced himself that general relativity was a mistake. He was especially pissed off by the fact that, in GR, energy is not always conserved. Screw Einstein, energy conservation is just common sense! Right?
Think a little bit about why you believe in energy conservation. Is it because you run into a lot of energy in your day-to-day life, and it’s always been conserved? Did you grow up around something that was obviously energy? Or maybe someone had to explain it to you?
Maybe you learned about it…from a physics teacher?
A lot of the time, things that seem obvious only got that way because you were taught them. “Energy” isn’t an intuitive concept, however much it’s misused that way. It’s something defined by physicists because it solves a particular role, a consequence of symmetries in nature. When you learn about energy conservation in school, that’s because it’s one of the simpler ways to explain a much bigger concept, so you shouldn’t be surprised if there are some inaccuracies. If you know where your “common sense” is coming from, you can anticipate when and how it might go awry.
Similarly, if, like one of the commenters on my crackpot post, you’re uncomfortable with countable and uncountable infinities, remember that infinity isn’t “common sense” either. It’s something you learned about in a math class, from a math teacher. And just like energy conservation, it’s a simplification of a more precise concept, with epsilons and deltas and all that jazz.
It’s not possible to teach all the nuances of every topic, so naturally most people will hear a partial story. What’s important is to recognize that you heard a partial story, and not enshrine it as “common sense” when the real story comes knocking.
Don’t physicists use common sense, though? What about “physical intuition”?
Physical intuition has a lot of mystique behind it, and is often described as what separates us from the mathematicians. As such, different people mean different things by it…but under no circumstances should it be confused with pure “common sense”. Physical intuition uses analogy and experience. It involves seeing a system and anticipating the sorts of things you can do with it, like playing a game and assuming there’ll be a save button. In order for these sorts of analogies to work, they generally aren’t built around everyday objects or experiences. Instead, they use physical systems that are “similar” to the one under scrutiny in important ways, while being better understood in others. Crucially, physical intuition involves working in context. It’s not just uncritical acceptance of what one would naively expect.
So when your common sense is tingling, see if you can provide a source. Is that source relevant, experience with a similar situation? Or is it in fact a half-remembered class from high school?
4 gravitons 
Well, for what it’s worth, “physical intuition” as you describe it sounds just like common sense, but, well, for physicists.
“Physical intuition uses analogy and experience. It involves seeing a system and anticipating the sorts of things you can do with it”. Common sense works in the same way. You get yourself hurt with a knife, you learn to be careful. You see something pointy and sharp, you think back to your knife experience, and through analogy you decide to be careful with this new object.
I’m not trying to argue you are wrong (I understand your point and agree with you), but I think it could have been expressed better.
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Yeah, you’re right, I could have expressed myself better. The distinction I was trying to draw is that, in the case of physical intuition, we’re explicit about what we’re drawing from, at least on some level: “I have X experience of Y, which is appropriate because Z.” Usually when common sense is invoked it isn’t coupled to that kind of explicit reasoning.
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Physicists always try to know what they’re building upon, but exactly when they talk about “physical intuition”, the sources of their reasoning and conclusions are murkier than when they don’t talk about the “physical intuition”, right? So in that case, they’re closer to the laymen (who are willing to randomly spit results according to their hormonal moods and call it “common sense”) than ever. 😉
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Agree completely. I really enjoy reading your posts, maybe not always understand When I went to school there were only 4 elements, earth, fire, air and water.
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Lloyd, you must have been a schoolkid before the era of Aristotle (fourth century before Christ) because it was Aristotle who introduced the fifth element, the aether, although he didn’t use that name (others renamed it later). 😉
More seriously, I hope that you were were jokingly exaggerating about your education because elements in the modern sense were largely starting in 1661 when Robert Boyle proposed atoms similar to those we know; and in 1789 when Antoine Lavoisier published a book that already contained 32 modern elements we agree with. You were born and studying later, weren’t you? 😉
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Hi 4Gravitons,
Here’s something I’d like to know as a physics enthusiast! How do we actually discover particles and interactions? I mean, we can only measure amplitudes, not what the actual contributing Feynman diagrams are, so how can we be so sure that those are the strong and the weak interactions SU(2), SU(3) etc. Our detectors work electromagnetically right?
If we have a model and the relevant coupling constants that results in in an infinite series of paths that when summed over them is an amplitude(it has contributions from all known and unknown interactions right?). But what if a completely different model and completely different symmetry breakings(instead of the Higgs mechanism), coupling constants, etc. could lead to completely different Feynman diagrams and the same observed amplitude for the EM interaction at the detector in the end?
How can we be so sure(or somewhat sure) that SM is responsible for the interactions at the accessible energies? If I were a theoretical physicist I would be paranoid about the idea that different forces may be the “true forces”, but I clearly miss something.
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It’s a good question, but I think you’re underestimating just how hard it is to find two models that fit all of the relevant data. If we were just trying to match to one amplitude, maybe it could be possible, but in practice there are thousands of different processes at a wide range of energies, some measured with ten or more digits of precision. We don’t have a theorem that proves that that’s “enough” (and in some sense it can’t be, since there should be some beyond-the-Standard Model physics), but it makes it quite unlikely that we’re missing something big at accessible energies.
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I’ll also say more about this in this week’s post, so tune in for that.
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