The fact that time isn’t consistent for everyone is a very troubling idea, and even physicists have a hard time learning about it initially. Imagine how difficult it was for Einstein to convince his peers about it originally! He didn’t even have the benefit of science fiction to lean upon for sparking the imagination!
In this week’s index-card summaries [submitted by students in a class called “Physics and Society: The Present”, for which I am a teaching assistant], I received an excellent comment that expresses this difficulty very well:
Time is a span (a standard for all) that is not measured by movement of light — it occurs in the dark too. Time dilation is not logical to me.
Before I go on, please consider this: what would happen if time took a break, and just stopped for a few moments? Would anyone notice? If time wasn’t changing, could anything else change?
Let’s think of time as just a way to measure how things change. Then time is used to measure when something starts to change, how long the change takes, and when the change stops occurring. This measurement of change is always relative to some kind of clock that is held adjacent to the thing that is changing.
Time dilation comes into play only when comparing information from one reference frame to another. Information can only be transferred via some kind of physical signal, and every form of communication currently known is based on electromagnetic waves. Even visually witnessing something is subject to how the light reflects off the situation!
Since the signals carrying information can only travel at a constant speed (the speed of light), then two observers in relative motion won’t necessarily get the signals at the same time, and thus they would disagree over what moment a change starts or stops occurring, as well as how long the change takes place.
The fact that they disagree about time is due to the fact that light takes time to travel, not that time can be “measured by movement of light”. Time itself isn’t changing, but our perception of it may be different than someone who is moving at relative speed to us, if we took the time to compare stories.
It’s definitely difficult to envision the high speeds and/or huge distances where time dilation will have an effect, but experiments have verified that it is a real effect. This may not make it any easier to believe at first, but with more and more evidence of the reality of this effect, it can even be fun!