Time is the fourth dimension. It is not a physical dimension like length, width and height, but it is still measurable. Or is it? Our entire society functions around time. The sun and moon divide day and night and clocks tell us the hours, minutes and seconds. But, how accurate is our measurement of it?

Did you know that, in higher elevations, time goes slightly faster? This phenomenon has to do with Einstein’s theory of general relativity, which predicted that a clock at a higher elevation would run faster than a clock closer to the Earth’s surface. This is because there is a weaker gravitational force the higher in elevation you go.

However, the difference in the passage of time in higher elevations is very minimal, so much so that a difference of one foot in elevation would only make you older by about 100 billionths of a second over 79 years of life.

Altitude is not the only factor that influences the passage of time. Your emotions also play a role. Take, for instance, the phrase, “Time flies when you are having fun,” which also, uncoincidentally, originated from Albert Einstein.

When we are doing enjoyable activities, time seems to go by faster. In contrast, when we are in a situation that causes fear, time perception slows down. Studies have suggested that fear distorts our perception of time to prepare us to act as quickly as possible in dangerous situations.

According to Adrian Bejan, a mechanical engineering professor at Duke University, everyone has their own “mind time” that differs from the relatively-constant physical time. Everyone perceives the passage of time differently.

If you feel like time passes more quickly for you now than it did when you were younger, you are not alone.

Part of what influences your “mind time” is your age. According to Bejan, there’s an inversely proportional relationship between stimuli processing and the sense of time passage. When you are younger and all the stimuli you experience are new, time appears to pass more slowly. As you get older, there are less new stimuli to process, which makes time seem to pass more rapidly.

Although time is measurable, it is not as concrete as the physical dimensions. Time is always moving forward, but at a different pace in every person’s mind.