In The Order of Time, Carlo Rovelli sticks to the theme that reality is not what it seems, in this case focusing entirely on the concept of time.
In a sense, the history of science can be described as an uncompromising assault on intuition and common sense. The Earth appears flat and stationary, but in reality spins and soars through space at 67,000 miles per hour. Humans appear to be a species of their own special creation, but in reality are simply naturally evolved primates.
These ideas are familiar by now, but they were revolutionary at the time of their discovery. What Rovelli is pointing out is that science may not be done with us yet—the next assault on our common sense might be the revelation of a world without time.
Rovelli points out that what we label as “common sense” is often the intellectual work of past thinkers—who may or may not have gotten things right. How we think about time—our common sense notions of it—have in fact been shaped by past thinkers, the two most prominent being Aristotle and Isaac Newton.
Isaac Newton thought that time was universal and absolute, that the variable “t” applied to everyone everywhere and continued to tick on even if everything in the universe stood still.
Aristotle, in contrast, thought of time simply as the measure of change, and that without change it made no sense to talk about time in any absolute or universal sense.
Because Aristotle got so much wrong in so many fields, you might think you ought to favor Newton in this debate, but according to Rovelli you’d be wrong to do so. Aristotle’s picture of time is closer to reality, as confirmed by the work of Albert Einstein.
Einstein demonstrated that time is relative, not absolute. We now know that time moves slower at lower altitudes and at higher speeds. Someone living in the plains will age less than someone living in the mountains and someone traveling at high speeds will age less than someone stationary. This counterintuitive notion does not sit well with common sense, but that’s the point: reality is not, and has never been, what it seems.
What all of this adds up to is that there is no such thing as universal time; there is only time for you, from your perspective, in relation to other objects. Reality is a world of events and changing quantities, not of shapes and universal space and time. And this is precisely what quantum physics demonstrates, where the equations that work have no time variable at all.
So the flow of time is simply a psychological construct, an illusion, and one we may need to discard if we are to penetrate further into the depths of reality.
But, if time is not embedded in the fabric of reality, it does beg the question: where does our concept of time come from?
In a word: entropy. The second law of thermodynamics tells us that entropy can never decrease, and increasing entropy is the way our minds mark the passing of time. Since entropy cannot be reversed, time flows in one direction from past to future. And so, entropy is part of the basic fabric of reality and time is simply our way of measuring it. That is, unless we’re talking quantum mechanics, which needs no time variable at all.
What should we make of this claim? Unless you are working in the field of quantum physics, it would be difficult to refute what Rovelli is saying. If the quantum equations do not require time, then either time is an illusion or else we don’t really understand the quantum world. You’ll have to read the book to get a handle on the more technical aspects that I won’t hesitate to admit flew right over my head.
One point of contention I can offer, however, is regarding Rovelli’s claim that the concept of a “universal present” is nonsensical. Here’s how he describes it:
Hypothetically, if you had a friend on another planet four light-years away, would both of you be experiencing the same present moment? According to Rovelli, there is no such present moment that applies to both of you.
If you were to look at your friend through a telescope, you would be seeing him four years ago, because that’s how long the light would take to travel back to you. So your “now” and his “now” cannot be said to be occurring at the same present moment.
But is this right? I’m not so sure. Yes, it’s true that, looking through the telescope, you would be seeing your friend four years earlier, due to the limitation of the speed of light. But, if you were to continue to look through the telescope for four more years, you would see your friend’s “now” moment precisely four years later, and that would only be true if there was an initial present moment that was the same for both of you.
So in a sense there is a universal present, it’s just not accessible to us perceptually due to the constraints of the speed of light. Maybe this is a small point, but I think it shows that a universal present has to exist, even if, due to the nature of the universe, time can only be perceived locally.
In any case, it’s interesting to contemplate that time is simply the psychological measure of change, and that the underlying changes can speed up or slow down depending on your position and speed relative to other objects. This is the world of relativity and reality, which banishes Newton’s universal clock and redeems Aristotle’s notion of change.
A final point to consider is that, if time is only a psychological construct, and is not in any objective sense part of the universe, then this could be the final nail in religion’s casket: without time, without a beginning, there is no need for “creation.”
More by Carlo Rovelli:
Seven Brief Lessons on Physics – a concise and clearly written examination of the biggest ideas in physics.
Reality Is Not What It Seems: The Journey to Quantum Gravity – explores how the idea of reality has changed over time and our current conception of quantum physics.