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How does the principle of relativity apply to a game of table tennis on a moving train?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

According to the principle of relativity—one of the fundamental postulates of special relativity—the laws of physics are identical in all inertial frames of reference. An inertial frame is a system moving at a constant velocity, such as a train traveling smoothly along a straight track.12

Because the train moves at a constant velocity, a game of table tennis played inside the train is physically indistinguishable from a game played while the train is at rest. Within the train's reference frame, the ball obeys the same mechanical laws, and the players experience the game exactly as they would on solid ground. An observer on the train never perceives changes in space or time within their own reference frame, regardless of the train's speed.13

While a trackside observer would see the ball moving with a different velocity due to the train's motion, the underlying physical principles governing the game remain invariant. These relativistic effects only become significant at speeds approaching the speed of light, which is far beyond the speeds of any train.123