Here's a short story:

Philosophers in antiquity used the concept of force in the study of stationary and moving objects and simple machines, but thinkers such as Aristotle and Archimedes retained fundamental errors in understanding force. In part this was due to an incomplete understanding of the sometimes non-obvious force of friction, and a consequently inadequate view of the nature of natural motion. A fundamental error was the belief that a force is required to maintain motion, even at a constant velocity. Most of the previous misunderstandings about motion and force were eventually corrected by Galileo Galilei and Sir Isaac Newton. With his mathematical insight, Sir Isaac Newton formulated laws of motion that were not improved-on for nearly three hundred years. By the early 20th century, Einstein developed a theory of relativity that correctly predicted the action of forces on objects with increasing momenta near the speed of light, and also provided insight into the forces produced by gravitation and inertia. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: Who corrected misconceptions about motion and force?
Guess: Galileo Galilei and Sir Isaac Newton

Question: Who compiled laws of motion?
Guess: Sir Isaac Newton

Question: Were they ever improved?
Guess: yes

Question: After how long?
Guess: three hundred years

Question: Who furthered knowledge of gravity and inertia?
Guess: Einstein

Question: What did his theory of relativity predict?
Guess: the action of forces on objects with increasing momenta near the speed of light

Now answer the following question in the same format.

Question: When was it developed?

early 20th century