So it was natural to think of the gravitational field as a field just like Maxwell’s electromagnetic field. It is a field that mediates a force, and it is a field that carries energy and momentum. And whereas the charges that are the sources of the electromagnetic field are electrostatic charges, the “charges” that are the sources of the gravitational field are masses.
So far so good, but the gravitational field has another special property that the electromagnetic field does not. Its “charge” is just the inertial mass. The magnitude of the gravitational force acting on a body is proportional to its mass; the magnitude of the inertial force by which the body resists that force is also proportional to its mass. So the body’s mass cancels out of the equations and we’re left with universal gravitation: All bodies accelerate at the same rate in a gravitational field. Which means that when you fall freely in a gravitational field, freefalling bodies near you will either float still or move in a straight line at uniform speed relative to you. In other words, you cannot tell if you’re freely falling or just floating in empty space without an external reference.
The recognition of this was what Einstein supposedly described as his “happiest thought”
just as a charged body is the source of the electromagnetic field.
Yeah but the Josephson Junction is used to measure the ORIGIN of volts due to a "spontaneous symmetry breaking" of time-frequency energy - aka noncommutative phase!!
Oops.
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