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 Travel Could Be Possible
One indisputable law of existence states that when the topic of time comes up, talk of time travel will soon follow. And while there have been innumerable books, movies, and TV shows dedicated to the subject, science has been short on evidence of time travel’s plausibility.
Einstein’s theory of relativity once again comes into play here, revealing that time travel is indeed possible, though it would require moving close to the speed of light — something currently outside of our engineering reach. Still, if that technical hurdle proved solvable, the math only supports time travel into the future, not the past.
Another popular concept that could allow for both forward and backward travel is through the warping of spacetime. Theoretically, this is thought to be possible by way of a wormhole. Again though, the option presents some currently insurmountable obstacles; namely, how does one create a wormhole? As the holy grail of travel, however, the concept is slowly transitioning from the realm of fiction to areas of hard science.
Still, its potential legitimacy leads to many paradoxes and questions like: If time travel will one day be possible, why has no one come back from the future to tell us how to do it yet?
It May Be Impossible To Know What Happened Before The Big Bang, Since Time Didn't Exist
By aiming their telescopes into the distant universe, cosmologists have the ability to look back into the past. This is possible because light takes time to travel, only revealing to us what was happening in the environment when the light left. While modern astronomers can peer back almost 13.8 billion years, they’re shy about 380,000 years of seeing our universe’s Big Bang.
If we’re actually able to see back to the beginning of time one day, it’s doubtful we’ll ever know what came before it. One reason has to do with our limited methods of investigation. They rely (out of necessity) on the known laws of physics, and are thought to breakdown outside of our universe. Another reason is the nature of the universe itself.
It’s widely accepted that the Big Bang was the catalyst for spacetime, which probably means there wasn't space or time before it. Well, at least not in a form we can currently understand.
Time Could Exist Elsewhere
A growing number of scientists believe that our universe could be one among many. The Multiverse hypothesis proposes that there could be one to an infinite number of other universes outside of our own. Some predict these universes would be identical or differ only slightly from ours, while others are imagined to be totally unique.
In the former case, time is likely to exist and behave like it does in our universe. In a radically different universe, however, something similar to time, or perhaps something even more mind-bending and exotic, could exist. In that case, it's close to unfathomable to imagine how their universe would function.
Time Has No Present
The most common sense views on time are related to how we, as a species, directly experience it. We are in a present moment, that moment passes, becomes the past, and the future becomes our new present. This series of perceived moments goes on and continues without delay or fail. However, drawing upon special relativity, Eternalism proposes that we inhabit a "block universe" where dimensional time doesn’t flow at all. In this view, past, present, and future are all part of one, tenseless spacetime mixture.