Time. We can’t get enough. We despair of making it flow faster or slower, and yet we are reminded again and again to live in the present. When it comes to big philosophical questions, the concept of time and related ideas like past, present, and future are among the big names. Is the future already written? What do we mean by present? Does the past exist?
From a physics perspective, time is just as troublesome, but for different reasons. Time is used almost everywhere, but in physics it is not obvious why it has a particular direction. Time is the progression of events from the past to the future via the present. It is the fourth dimension of our universe, along 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 top/bottom, left/right and front/back – and the fourth, time.
In developing his theories of special and general relativity, Albert Einstein examined the laws of physics as they relate to the speed of light, ultimately claiming that nothing can travel faster than the speed of light in a vacuum. For Einstein, space and time were not separate and unrelated phenomena, but intertwined in a single continuum: the space-time continuum.
Nothing that contains energy is out of time.
Why is time flowing?
We experience time as something that inevitably passes, going in a very specific direction that we consider to be moving forward. However, many physical laws do not seem to have a preference as to whether time flows forwards or backwards, so scientists have been looking for an explanation for these physical laws which only seem to work in one direction. The most famous of these is the second law of thermodynamics.
According to this law, in an isolated system (like our universe) that is allowed to evolve, entropy – the idea that physical systems increase randomly from order to disorder – always increases. So we can distinguish the past from the future by looking at entropy. This is one of the ways scientists explain that time moves forward as the so-called “arrow of time”; the more a system becomes disordered, the less it can reorganize and the stronger the arrow of time.
But time can pass differently. Einstein’s relativity postulates that gravity is not just an invisible force that attracts objects, but a distortion of spacetime; the more massive an object, the more it distorts the space-time around it. Thus, time is not constant everywhere, because gravity and acceleration can change the way time moves. This is most clearly seen where, thanks to gravity slowing down the clock for 4.5 billion years, the Earth’s core is about 2.5 years younger than the surface.
What is the past?
Einstein’s use of special relativity also helps answer another question about time: is the past real? If we only live in the present and the past is inaccessible to us, how can we say that the past is real?
The answer comes from the concept of “now” of the famous theory of Einstein. As mentioned above, clocks move differently in different environments, so the concept of now depends on the observer.
The concept of “now” depends on where you are, where you are going, and how quickly you get there. Two events can occur 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 real.
Is the future already written?
Now, that’s a question for the ages. Call it fate, fate or free will, humanity across the world has grappled with this issue. Relativity has set the past in stone and challenges the idea that there is a specific present. So how can there be a future? Present, past and future must coexist. It is the “block universe” that Einstein envisions in relativity. Past, present and future are just slices of time, like snapshots of reality that coexist.
But not everyone is satisfied with this deterministic view of the universe. Quantum mechanics, for example, is not very deterministic, despite what Einstein said. But the idea of a blocky universe 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 blocky universe but doesn’t expand 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).
Thus, the past is written and the future is up for grabs. But carpe diem, seize the day and trust tomorrow very little. We have some answers regarding time, but not all of them are satisfactory; we still lack a full understanding of this dimension. If we ever get one, well, only time will tell.