The Observer Effect

Relativity: One Stubborn Fact and Everything That Follows

Light measures the same speed however fast you are moving. Take that seriously and distance and time have to bend — which is why your phone's position fix depends on the theory working.

The Observer Effect: Measuring Something Touches It

Checking a tyre's pressure lets out a little air, so the reading is never quite what was there before. With objects that hardly matters. With people it can be larger than what you were trying to measure.