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Know where your map stops
Two maps of physics
For hundreds of years, Newtonian physics was a wide-ranging map of how the world works, from gravity to celestial motion. In 1905, Albert Einstein's theory of special relativity replaced the understanding handed down by Isaac Newton and gave physics a new map.
Neither map is complete
Newtonian physics did not stop being useful. It still predicts the movement of objects large and small reliably, with some limitations that Einstein pointed out. Einstein's physics is not totally complete either: with every year that goes by, physicists grow more frustrated at being unable to tie it into small-scale quantum physics, so another map may yet come.
Exploring new ground
What physicists do well, and most people do poorly, is carefully delimit what each map can explain. When they hit uncharted territory like quantum mechanics, they explore it carefully instead of assuming the maps they already have can explain it all.
Why does Newton still work for engineers?
Einstein's equations reduce to Newton's when speeds are far below the speed of light and gravity is weak. The gap between the two predictions scales with the square of an object's speed divided by the speed of light, so at everyday speeds it is vanishingly small. Engineers designing bridges, cars, and most spacecraft trajectories therefore keep using Newtonian mechanics. The equations are simpler to calculate and give answers accurate to the precision such work needs.
Why does GPS need Einstein's physics?
GPS satellites orbit about 20,000 kilometers up, where gravity is weaker, and general relativity says clocks tick faster there. Their speed makes the clocks tick slower under special relativity. The net effect is that a satellite clock gains roughly 38 microseconds each day compared with one on the ground. Without correction, position errors would add up to kilometers per day.
Why won't relativity and quantum physics combine?
General relativity describes gravity as the smooth curvature of space and time. Quantum theory describes the other forces as exchanges of discrete packets of energy. When physicists apply the standard quantum methods to gravity, the calculations produce infinities that cannot be removed the way they are for the other forces. Approaches such as string theory and loop quantum gravity are attempts to close that gap.
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