In Newtonian mechanics, time is absolute: all observers agree on whether two distant events happen at the same moment (simultaneity). However, in Einstein’s theory of relativity, simultaneity is relative. This means whether two distant events occur at the same time depends on the observer’s frame of reference. Different observers moving relative to each other may disagree on the timing of events.

In Newtonian physics, forces such as gravity act instantaneously across a distance. For example, if the Sun were to suddenly disappear (hypothetically), Earth would “immediately” feel the absence of the Sun’s gravitational pull. This implies that information (in this case, gravitational influence) travels instantaneously.

Einstein’s special relativity asserts that information, including the influence of forces, cannot travel faster than the speed of light. If simultaneity of distant events cannot be universally agreed upon, then the idea of instantaneous action across distances becomes incoherent. Why?

• Imagine an observer in one frame of reference sees an event (e.g., a particle exerting a force) happen at a particular time. In another frame of reference, due to the relativity of simultaneity, the timing of that event is shifted. If forces or effects were instantaneous, it would violate causality because different observers would disagree on the order of cause and effect.
  
• To preserve the consistency of physical laws across all reference frames, interactions between objects must propagate at a finite speed—specifically, no faster than light.
  

The impossibility of defining a universal simultaneity undermines the Newtonian idea of instantaneous “action at a distance.” If there’s no absolute way to say that two events are simultaneous, then there’s no consistent way for one event to instantaneously cause a change at a distant location. This leads to the conclusion that forces or influences must be mediated by fields (e.g., electromagnetic or gravitational fields) that propagate at finite speeds.

Einstein’s rejection of action at a distance is a natural consequence of relativity’s rethinking of space and time. The relative nature of simultaneity ensures that causality and the finite speed of information transfer are preserved, making Newtonian instantaneity incompatible with the relativistic framework.