…The untiring flying of those butterflies — indifferent to the motion of the ship — pointed out a profound truth: Galileo said — “being in the cabin and looking at those butterflies, you can’t tell whether the ship is at rest (un-moving) or is moving with a fixed speed in a given direction!”
What’s so special about those butterflies? We ask: Can we perform a proper ‘experiment’ INSIDE the cabin of the ship and looking at the results, find out whether the ship is moving uniformly or not.
On that, Galileo says: “all the best — but you won’t be able to”…
Now you’re playing the billiard-ball game with Galileo — in that same cabin of the ship — on a nice smooth, frictionless table with many coloured balls. With your stick you hit the ‘striker’ ball on one of those and see the whole event — as the ‘striker’ ball approaches the other one, they both collide smoothly, and after the collision, move apart…in such a manner that the whole momentum of the striker — BEFORE colliding the other one, gets distributed among them AFTER the collision. You exclaim — what a nice demo of the conservation of momentum! — ‘the total momentum of the balls’ BEFORE the collision is the same as ‘the total momentum of those balls’ AFTER the collision!…
Galileo is watching…He agrees with you and says — “Yes, the momentum IS conserved. Law of conservation of momentum works here, in this cabin of our ship. But tell me, the ship is moving or not?”
And you suddenly realize — you can’t answer his question — for whether the ship is uniformly moving or not — the collision of those billiard balls would take place by conserving the total momentum! You look at those butterflies — they are still flying here & there…seemingly unbothered about the state of motion of the ship.
Now Galileo generalizes — he says: there’s nothing that distinguishes the “physics of the state of rest” from the “physics of the state of uniform motion”.
As a consequence of this fact, it turns out that it’s not possible to claim — in ‘absolute’ sense — that a ‘point’ in space — is the same point now as it was a minute ago. This realization resulted in Galileo’s acceptance of the Copernican idea of moving (as opposed to stationary) Earth, even though its motion was (and still is) unnoticeable.
So our ‘fixed cinema-screen’ analogy doesn’t work in the Galilean idea of spacetime: According to Galileo, the ‘time’ is absolute (in agreement with Aristotle), but the ‘space’ isn’t. There doesn’t exist any way, or an experimental procedure to claim that there is an ‘absolutely fixed’, ‘un-changing with time’ background space…
This spacetime structure led to a fundamental principle — known as the “Galilean Principle of Relativity”: The laws of physics are the same in all the uniformly moving ‘reference frames’ — in the above example, the ‘ship’ represents such a reference frame, and for all practical purposes, our Earth too.
Yes, the theory of relativity existed — even before Einstein. It was ‘Galilean Relativity’ — expressed in terms of the above principle. Newtonian formulation supports Galilean Relativity.
Butterflies took a long time — about twenty centuries — to fly from Ancient Greece to Modern Italy. But they didn’t stop there.
Within a much lesser period of only about two and a half centuries, they reached Germany, where Einstein welcomed them.
Exploration continues…
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