What is time and how do we know it exists?

What is time and how do we know it exists?

Time. We can’t get enough of him. We are desperate to make 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 hitters. Is the future already written? What do we mean by the 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 through the present. It is the fourth dimension in 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 up/down, left/right and forward/back – and the fourth, time.

By developing his theories of special and general relativity, Albert Einstein looked at the laws of physics in relation to the speed of light, ultimately postulating that nothing can move 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 outside of time.

Why does time flow?

We live time as something that inevitably passes, going in a very specific direction that we consider as before. However, many physical laws do not seem to have a preference for whether time flows forward or backward, so scientists have sought an explanation for those physical laws that seem to work only one way. The most famous of them is the second law of thermodynamics.

According to this law, in an isolated system (such as our universe) allowed to evolve, entropy—the idea that physical systems grow randomly, going from order to disorder—always increases. So we can tell the past from the future by looking at entropy. This is one way scientists explain the passage of time as the so-called “arrow of time”; the more disordered a system becomes, the less order it can regain and the stronger the arrow of time.

But time can pass differently. Einstein’s relativity assumes that gravity is not just an invisible force that attracts objects, but a warping of space-time; the more massive an object is, the more it warps the spacetime around it. Thus, time is not constant everywhere because gravity and acceleration can change the way time passes. This can be seen most clearly where, due to gravity slowing down the clock for 4.5 billion years, the Earth’s core is about 2.5 times 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 live only in the present 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 you’re going, and how fast you’re getting there. Two events can happen at the same time to one observer but at different times to 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 destiny, fate or free will, humanity around the world has addressed 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? The present, the past and the future must coexist. This is the “block universe” that Einstein sees in relativity. The past, present, and future are just pieces of time, like snapshots of co-existing reality.

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 clear formulation of Einstein’s idea that keeps the universe block but does not 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).

So the past is written and the future is up for grabs. But carpe diem, seize the day and have very little faith in tomorrow. We have some answers about 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.

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