I am wondering if Einstein gave us the concepts of “relative motion” and/or “frame of reference”? While the concept behind relative motion was probably well known starting with Galileo or Newton, I assume, I wonder if men of early science may have had a different term for this? (It’s often forgetten, but Galileo did quite a lot of experiments with simple motion, too.)
Along the line of the OP, did these early men of science understand that a ball thrown forward off a wagon moving forward would have a cumulative total velocity of the two (Vwagon+Vtoss) relative to the ground? Or, was this whole idea of relative velocities fuzzy, at best?
We take this for granted today, but did Newton understand this?
Sure, they understood that if you were running on top of a train, your speed was the combination or your running velocity and the trains velocity. This is why is was so hard for them to grasp the concept of Einstein’s theory on the speed of light. It had been accepted that the light coming from the train’s headlights was going faster than the light from your lantern on the ground. Einstein suggested that all light traveled at the same speed no matter what. Even if it was on top of a train.
OK, even if they realized this, did they use these exact terms frame of reference to explain differences in relative velocities between a person inside the train and a person on the ground?
Especially “frame of reference”…is that a more recent concept, even if just very late 19th century, perhaps? - Jinx
No, they were well established terms. There had been debates on whether or not there existed an absolute frame of reference for a long time before Einstein.
This is an instance where we have to carefully distinguish between when the concepts were developed and when the language was introduced.
As the modern name suggests, the notion of Galilean invariance goes back to at least Galileo. His most famous example is his thought experiment about throwing something back and forth inside a ship in the Dialogue. The essential idea is there - it doesn’t matter what uniform velocity the ship has - but he didn’t introduce any particular terminology in explaining it.
As far as understanding the role of relative motion and Galilean invariance in classical mechanics, it’s mostly all there in Principia. The issue of absolute and relative motions comes up right at the beginning, with Newton discussing the difference in some detail in the Scholium to the opening Definitions. He even notes that relative motions are what are useful in practice. But he argues that there are absolute motions (and hence an absolute notion of space) on philosophical grounds. Plus the famous example of the swinging bucket. Famously, those arguments proved controversial and so you get the run-in between Leibnitz and Clarke on the issue. But both Newton and Leibnitz understood what Galileo had been getting at.
When people later came to give the notion a more formal expression, the way they tended to express it was by talking about “systems of coordinates” and the like. That’s the sort of terminology Mach used. Similarly, Poincare in 1905 in Science and Hypothesis:
and he dubs this the “Principle of Relative Motion”. Einstein’s language in the first 1905 paper is very similar. He talks about the “Principle of Relativity” and:
The later systems are “moving” ones. Einstein sticks with “systems of coordinates” elsewhere, though “inertial system” does appear in the later writings to mean the same thing. Other physicists also kept with the pre-1905 terminology for a good long time.
So when do “frames” appear on the scene. Before the 1940s at the latest, since Joseph Larmor talks about a “frame of inertia” in his undated notes to a reprint of Maxwell’s Matter and Motion and he died in 1942. He attributes the name to James Thomson, but I’ve no idea who that is (and, no, it’s not J.J.).
I suspect use of the term only became widespread in the 1960s.
To tie up the loose ends in my post above, the Stanford Encyclopedia of Philosophy entry on Space and Time: Inertial Frames includes a discussion of the 19th century attempts to formalise the notion. It quotes Thomson introducing the term “reference frame” in 1884.
Though they don’t say so, this will be the James Thomson (1822-92) who was Lord Kelvin’s elder brother.
And, for completeness, Larmor’s edition of Matter and Motion was originally published in 1920 (by the SPCK, of all people).