>>12697873>>12697873I wouldn't call simultaneity a variable. It's merely something we have intuition about that's no longer valid in relativity. The speed scales at which this occur are orders of magnitude faster than the fastest object humans have ever made. The factor by which time dilates is where is your speed as a fraction of the speed of light. Theoretically, humans are about to launch a rocket that will, at its fastest speed while sling shotting around the sun, be about 400,000 mph. The speed of light is about 700,000,000 mph. This yields a time dilation factor of about 1.0000002.
The absolute limit is the speed of light, where the spacetime interval is formally zero everywhere. This means photons will experience the birth and death of the universe simultaneously. Remember, simultaneity is relative in accordance with speed. The sign of the spacetime interval determines whether the distance between two objects is time-like (part of observable universe) or space-like (causally disconnected regions of spacetime).
>https://physics.stackexchange.com/questions/10756/difference-between-timelike-and-spacelike-vectors 
