Time. We can’t get enough of it. We are desperate to make it flow faster or slower, and yet we are reminded again and again to live in the now. When it comes to big philosophical questions, the concept of time and related ideas like past, present, and future are among the big hitters. Is the future already written? What do we mean by the present? Does the past exist?  

From the perspective of physics, time is just as troublesome, but for different reasons. Time is employed pretty much everywhere but in physics, it's not obvious why it has a particular direction. Time is the progression of events from the past into the future via the present. It is the fourth dimension in our universe, together with the three dimensions of space that make up the space-time continuum.

The space-time continuum

The space-time continuum is a theoretical construct that helps explain the very fabric of our existence. The four dimensions are length, width, height – or up/down, left/right, and forward/backward – and the fourth, time.

Through developing his theories of special and general relativity, Albert Einstein looked at the laws of physics as they relate to the speed of light, ultimately positing nothing can move faster than the speed of light in a vacuum. To Einstein, space and time were not separate and unrelated phenomena, but interwoven into a single continuum: the space-time continuum.

Nothing that contains energy is outside of time.

Why does time flow?

We experience time as something that unavoidably passes, going in a very specific direction that we consider forward. Many physical laws do not seem to have a preference on whether time flows forward or backward, however, so scientists have been searching for an explanation for those physical laws that only seem to work one way. The most famous of them is the second law of thermodynamics.

According to this law, in an isolated system (like our universe) left to evolve, entropy – the idea that physical systems increase in randomness, moving from order to disorder – always increases. So we can distinguish the past from the future by looking at the entropy. This is one of the ways scientists explain time marching on as the so-called "arrow of time"; the more disordered a system becomes, the less it can regain order, and the stronger the arrow of time.  

But time can pass differently. Einstein's relativity posits that gravity is not just an invisible force that attracts objects but a warping of space-time; the more massive an object, the more it warps space-time around it. Thus, time is not constant everywhere, as gravity and acceleration can change the way time travels. This can most clearly be seen where, thanks to gravity slowing down the clock for 4.5 billion years, Earth’s core is about 2.5 years younger than the surface.

What is the past?

Employing Einstein’s special relativity also helps answer another question about time: Is the past real? If we only live in the now and the past is inaccessible to us how can we say that the past is real?

The answer comes from the notion of "now" Einstein’s famous theory. As mentioned above, clocks move differently in different environments, so the concept of now is observer-dependent.

The concept of "now" depends on where you are, where are you going, and how fast you are getting there. Two events might happen at the same time for one observer but at different times for another. So what seems to be now for one person, is the past for another. The past is still there, inaccessible to us, but very much real.   

Is the future already written?

Now, that is a question for the ages. Call it destiny, fate, or free will, humanity across the world has tackled this question. Relativity has set the past in stone and challenges the idea that there is a specific now. So how can there be a future? Present, past, and future must co-exist. This is the "block universe" that Einstein envisions in relativity. Past, present, and future are just slices of time, like snapshots of reality all co-existing.

But not everyone is happy with this deterministic view of the universe. Quantum mechanics, for example, is not very deterministic, despite what Einstein said. But the block universe idea doesn’t have to have a predetermined future. Physicist George Ellis actually came up with a neat formulation of Einstein’s idea that preserves the block universe but doesn’t extend it into the future. While "now" is subjective there is a universal present, the boundary of the future that continues to expand in the direction of time (which may be different from the local arrow of time).

So, the past is written and the future is up for grabs. But carpe diem, seize the day and trust very little in tomorrow. We do have some answers about time but not all of them are satisfactory; we still lack a full understanding of this dimension. If we’ll ever get one, well, only time will tell.