Galileo Galilei described an interesting situation, which possibly helped him develop his idea of spacetime.
In brief, abbreviated (and slightly modified) form it goes something like this: “Imagine you are in the main cabin below decks on some large ship, and having with you some butterflies (in a bottle)…Let the ship proceed with any speed, ‘steadily’ in a given direction…now,
you open the bottle into a wide vessel beneath it and let the butterflies come out of it…you will notice that they keep flying along any direction, apparently without being tired of keeping up with the motion of the ship…”
What does this situation have to do with Galilean spacetime?
Before looking for an answer, let’s go a long long time back in history — from the 17th century (AD) in Italy to the 4th century (BC) in ancient Greece, where Aristotle lived…
The Aristotelian picture of reality was based on two tenets:
The first one implied that the ‘reality’ is like the screen in a cinema theatre, where the locations (the points) on the screen are fixed and un-moving, maintaining their identity from one moment to another, throughout the process of ‘projection of the film’.
Such a picture followed from the tenet of the ‘absoluteness of space’ — a belief that a thing that’s appearing as ‘being at a fixed location’ to you, should appear as ‘being at the same fixed location’ to me as well — even though I am ‘moving’ with respect to you!
The second one was about the ‘absoluteness of time’. Its consequence is the notion of ‘simultaneity’: A belief that if two different things appear as happening ‘simultaneously’ to you, at some moment in time, then those things should appear happening ‘simultaneously’ to me as well and that too at the ‘same’ moment in time — even though I am ‘moving’ with respect to you!
These Aristotelian tenets led to a particular picture of reality — in which the arena (the ‘stage of drama’) represented absolute, unique spacetime — there wasn’t anything relative!
Actually, while engaging in daily chores and activities, I think we all (maybe unknowingly) live with this Aristotelian picture of reality only:
For you’ll say — whether I move or not, things at rest ARE at rest; whether I move or not, things happening simultaneously ARE HAPPENING simultaneously!
We all agree: Space is the same for you and me, flow of time is the same for you and me, and so on…and, that’s not wrong at all!
However, there are ‘more correct’ versions of it, provided by Galileo Galilei and Albert Einstein.
Galileo Galilei, by observing those indifferently flying butterflies, rejected the first of the Aristotelian tenets.
Albert Einstein rejected both!
Exploration continues…
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