Arguing with Galileo


Many of us have tried to imagine what it was like to be Galileo. Dangerously perceptive, unjustly persecuted and posthumously vindicated -- though these struggles were undoubtedly an aching burden on the banned and jailed astronomer himself, they certainly make for interesting contemplation after the fact. After all, Galileo is remembered not just for his contributions to physics and astronomy, but perhaps most importantly for his struggle against the prevailing (incorrect) theories and beliefs of his day.

We who have the perspective provided by satellites, moon walks and deep space probes can see so easily that of course Galileo was right: The heliocentric model of the universe articulated by Nicolaus Copernicus, though not perfect, is a far more accurate model of the solar system than the geocentric design advocated by the church and by the philosophers of antiquity. The moon and other planets are clumps of matter, just like the Earth. The sun doesn't orbit our planet; we're the ones in orbit around the sun. It can be proved a dozen ways. It's a scientific truth as clear as running water.

But we have to remember that back in the 17th century, in Catholic Europe, there was no Voyager probe. No one had traveled beyond Earth's atmosphere, and true astronomy was still an adolescent science. Though Copernicus and Galileo were ultimately proved correct, and though it now seems so obvious to us, we must remind ourselves that it really wasn't obvious back then. Most people simply didn't know these things. The principles of science that today seem so firm must have sounded strange, uncertain and even counterintuitive when they were new. Thus, while I am curious what it was like to be Galileo, I'm perhaps even more curious what it was like to be one of his rivals and detractors.

There are so many ways one can build a view of the physical universe. If you simply use a bit of observation and common sense, you might come up with a model not that different from what many of the ancients thought -- one that tells you the Earth is stationary and the heavens revolve around it. To put it plainly, the ground doesn't feel like it's going anywhere. If you have ever ridden on the deck of a boat or on the back of a horse, you know that movement can easily be sensed by your body. Your legs, your abdomen, the tiny machines of your inner ear all conspire to let your brain know when you're in motion or at rest. If the whole planet is in motion, shouldn't we be able to sense it as one intuitively grasps the sensation of riding in a vehicle?

Come to think of it, why don't we feel the Earth hurtling through space at hundreds of kilometers per second? The answer has to do with reference frames. Despite our intuitions, the human body actually doesn't sense movement -- it senses acceleration. When you feel the feeling of riding some type of vehicle, what you're actually sensing is many continuous events of acceleration and deceleration in various directions, as well as forces of friction, such as wind resistance. But imagine you were riding a magic space-horse through an empty interstellar expanse, and this space-horse was able, because it is magic, to travel at an exactly constant speed in the same direction, with no forces of friction acting upon it. Galilean relativity tells us that you wouldn't sense this type of motion at all. Not only that, but there would be no way of scientifically detecting that you were in motion, without visual reference to the passing of other objects in the galaxy. Experiments performed within an inertial frame of reference are reliable, as if "still," as long as the velocity and direction of the reference frame doesn't change. For this reason, a person on the surface of a fast-moving planet doesn't feel the rhythm and doesn't have to dance. It might as well be as solid as it seems.

We know all of this now, but before we knew it, who could blame us for using our common sense to disagree with Galileo? My point is that it wasn't just dogmatic religious persecution that caused people to disbelieve heliocentrism -- though that surely was what led to Galileo's forced recantation and house arrest. If you were ignorant of Galilean relativity, the idea that the Earth was in a high-velocity orbit around another celestial body was just absurd. It flew in the face of everything you observed about the world around you. In other words, you could use your common sense and still come out wrong, just because you didn't understand a very simple principle of physics.

This makes me wonder: Where is our common sense leading us astray today? Are there principles which we today take as clear and obvious, but next year could be proved wrong in ways that violate all of our most basic intuitions? If so, what are they? Will we end up laughing at the next Galileo?


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