Time impacts every aspect of our lives. Yet, despite our seemingly intimate relationship with time, many would be hard-pressed to define it. So, exactly what is time?
For millennia, philosophers and scientists have proposed and argued theories about time, but very few have resulted in any kind of consensus. And while there still remains no complete definition, in its strictest sense, time is a unit of measure. We use it to describe an event, or the duration of or between them. This is true both in everyday matters, as well as in mathematics. Colloquially, we might say something like “time marches on” when speaking about life’s forward movement, while physicists would characterize this perceived experience as the “arrow of time.”
Beyond time’s symptomatic effects, however, the true essence of the phenomenon remains controversial and mysterious. While physicists can use units of time within their equations, questions such as "How does time work?" continue to confound the scientific community. Indeed, as the superficial layers of time get pulled back, common sense and intuitive ideas are challenged, making it difficult to talk about the existential without flirting with something more metaphysical. As a result, the riddle of time has actually led some philosophers and scientists to rethink the entire model of our universe.
In that spirit, let’s explore some scientific theories about time that challenge our own views about reality.
Time Is An Illusion
You may have once heard – from a cool science teacher or over-exuberant guest at a "hip" dinner party – that time isn’t real. And while this statement isn't necessarily untrue, the core precept behind it (sometime called the B-theory of time) actually states that time is an illusion.
Put more specifically, our experience of time (past, present, and future) has more to with our own subjectivity than any foundational feature of existence. Most modern physicists now subscribe to this view, explaining that the time we perceive through our human minds at a local level is vastly different than time’s innate behavior at a universal scale.
Time Is A Dimension
We all understand the basic concept of three-dimensional space. Technically, it means that every point in our universe has a physical address, tractable by a coordinate along three perpendicular planes (X, Y, and Z for those who remember high school trigonometry). It's just one of the many interesting things about space. But does a point in space really "exist" without a time in which to do so?
There is no known physical law requiring time to be a dimension, but Albert Einstein’s special theory of relativity led to just that proposition. Referred to as the fourth dimension, time is now widely accepted to be seamlessly woven with three-dimensional space in a continuum scientists refer to as spacetime .
Time Is Relative
Einstein’s assertions didn’t stop with the interconnectedness of spacetime. Special relativity introduced the idea that time and speed are directly correlated. Known as "time dilation," the principle states that time slows for things in motion — the faster something goes, the slower time passes. Countless experiments have proven the theory, showing that clocks in motion tick more slowly than their synchronized counterparts at rest.
While undetectable at human speeds, the effects of time dilation become greater the faster one travels. In fact, taking a two-year roundtrip spaceflight from Earth at the speed of light would be the equivalent to 30 years here on the ground. Perhaps, then, humankind’s elusive fountain of youth is not a place, but a journey.
Time Is Objective
Contradicting Einstein’s special relativity, and the greater scientific community’s current understanding of spacetime, some physicists and philosophers imagine time as an independent and fundamental property of our universe.
The Newtonian view states that time is not merely an interpretation, nor is it affected by external factors. Also known as "absolute time and space," it posits that these features of our universe act as a sort of stage and backdrop for everything contained within it.