Why does time slow down the faster you move?

 My Answer on Quora.

It doesn’t slow down at least in the sense that this question is worded. This is a misconception. Lets look a bit more at the bigger picture that comes from Einstein’s Special Theory of Relativity (1905).

Time is a relative concept. There is no such thing as absolute time in the broader scope of non-Galilean relativity. The measurement of time is specific to the frame of reference of the person making the measurement. Within the same frame of reference all observers agree on the same time measurement. However if one frame of reference is moving with respect to the other then there will be a disagreement in how much time has elapsed between events.

You see the only ‘absolute’ here is that the laws of physics hold across all frames of reference. The speed of light in a vacuum as measured by all is the SAME. We call it c. One cannot add or subtract onto c. It is what it is. There is no such thing as c + v or c-v (which Galilean relativity argues for).

Which means that something has to give. Actually several entities do, including absolute time and absolute length. The relative motion of one frame of reference to the other has to be taken into account and it is corrected for by invoking the Lorentz or Gamma factor.

where v = speed of the one frame of reference relative to the other.

For example if Bob boards a space ship and travels at a constant speed v relative to his Earthbound cousin Ann. Bob will experience what is called proper time (a poorly worded term). Ann will be the benefactor of relativistic or measured time. Both measurements of time are correct within their specific frame of reference.

Now the two times are related by the Gamma factor as shown below:

where delta t prime = measured time and delta t is proper time. Gamma is included in this equation and is always greater than or equal to 1. As v approaches c gamma tends to infinity so that the discrepancy between measured time and proper time ramps up considerably. We don’t see this as much in the day-to-day as v is so small compared to c. However the phenomenon is real….we call it TIME DILATION.

Consequently for Ann it will seem that Bob’s clock is running VERY slow. However from Bob’s perspective life is normal and there is nothing wrong with his clock. He will likely argue that Ann’s clock is running too fast. Both are correct in their own frame of reference.

Worth noting is that there is an additional assumption built into this analysis. Both frames of reference are not-accelerating. That is they are Inertial. For accelerating Frames of reference we need to bring in ideas from Einstein’s Theory of General Relativity (1915).