Over the years, scientists and philosophers have come up with different ways to help think about how time, existence, and the now fit together. In a recent paper, Carlo Rovelli of the Centre de Physique Théorique (CPT) of the Aix-Marseille University suggests that perhaps we need a new way to define reality and the now.
Previously, there have been two main ways to understand the progression of time.
In the first, there is an objectively real present. Events in the past and in the future do not exist. The only reality, the only thing that is real, is the present. This idea is called Presentism.
This idea, however, runs into some serious problems when you start taking into account relativity. To see why let’s do a thought experiment.
Let’s imagine that you are standing on the ground in the middle of a lightning storm. You see two bolts of lightning hit the ground, one to your left and one to your right. Both bolts were the same distance from you. Because of this, the light from both bolts had the same distance to travel to you. This leads you to believe that the bolts hit the ground simultaneously.
At the same time, however, there is an airplane traveling through the clouds. Passengers on this airplane also see the lightning bolts as their airplane is exactly midway between them. But what they perceive is different from you.
Because they are traveling towards one of the lightning bolts and away from the other, they, in essence, meet the light from the bolt ahead of them. They see this bolt first. They are traveling away from the bolt behind them, so it looks like this bolt strikes later.
But remember one key fact – light always, independent of reference frame, travels at 299,792,458 meters per second. The passengers on the airplane are midway between the bolts, and, within their reference frame, it appears that the light from the two bolts traveled the same distance. The only conclusion that the passengers on the airplane can make is that the bolt ahead of them happened first.
This is the problem of “simultaneity” and is one of the conclusions from special relativity – different frames of reference may not agree on the order of events. What we call the “present” is actually subjective.
So much for Presentism.
What other options do we have? An alternate idea of how to think of time is called Eternalism. Instead of thinking of reality as three dimensional, we can instead add in the fourth dimension, time. After Einstein formulated relativity, Hermann Minkowski came up with the idea of four-dimensional Minkowski spacetime. In this four-dimensional space, simultaneity is no longer a problem. Rather than solely measuring the time between events, observers on Minkowski spacetime would instead measure the spacetime between events. This spacetime distance should be the same for all observers.
In this four-dimensional world, events in the past and in the future are all equally real. Because of this, there is no special moment called the “present”. There is no moment that is happening now. One could argue that there is nothing “happening” and there is nothing “becoming”. Everything, from objects to events, simply exists. Our definition of “now” doesn’t exist in the Eternalist Universe.
In his paper, Dr. Rovelli challenges us to look at time in a different way. Physics, he says, is all about understanding becoming. Physics explains how things happen – why a ball rolls down a ramp, how a car skids to a stop, or why the Earth revolves around the sun. Without change, physics does not exist. He argues for a third possibility.
“A civilisation that thinks the Earth is flat may have a notion for ‘up’ for which all ‘up’ directions are parallel. On learning that the Earth is round, the notion of ‘up’ must somewhat be revised: ‘up’ in Sydney points to a different star than ‘up’ in London. This does not mean that ‘up’ is illusory: it means that it works a bit differently than we thought.”