Physics tells us that time doesn’t flow like a river, as Heraclitus claimed. Why then do we feel like we’re swept along?
As you read this article, time will seem to pass. Right now, you are reading these words, but now you are reading these ones. What was present just an instant ago seems to have already slipped into the past. You will carry this feeling with you – as objects change and move, as thoughts run through your head, as feelings ebb and flow – until you fall asleep tonight. Heraclitus thought that time was like a river: ‘Everything flows and nothing abides; everything gives way and nothing stays fixed.’ Our experience of the world seems to back this up. It certainly feels as if time is sweeping us along. Yet, physicists and philosophers will tell you that Heraclitus was wrong. Time, they say, does not actually pass. In his book The Order of Time (2018), the Italian theoretical physicist Carlo Rovelli writes:
What could be more universal and obvious than this flowing?
And yet things are somewhat more complicated than this. Reality is often very different from what it seems. The Earth appears to be flat but is in fact spherical. The Sun seems to revolve in the sky when it is really we who are spinning. Neither is the structure of time what it seems to be: it is different from this uniform, universal flowing.
So, what is the real structure of time? Well, it’s complicated. Some think that time is like space: the past, present and future are all equally real locations. Some, like Rovelli, think time emerges directly from the laws of thermodynamics and quantum mechanics. Physicists and philosophers may have different approaches to the structure of time, but what unites them is a rejection of the notion that that there is a ‘now’, a present moment, that moves from the past toward the future. If that is true, and time does not really move, we are left with a question: why does it seem to pass? We would never mistake a frozen river for a running one, so, if nothing flows and everything abides, why does it feel as if time is rushing by?
Perhaps it’s just an illusion. Our senses tell us that time is passing, but we are perceiving something that isn’t really there. To see an illusion is to see a way the world could be, but isn’t: the Earth looks flat when it is actually round; optical illusions can make identical lines appear to be different lengths. These illusions present real possibilities – it’s easy to find places in the world where you could experience something flat that wasn’t an illusion, or experience one line that really was longer than another. But if Rovelli and others are right, there is nowhere you could go to truly experience the flowing time that Heraclitus talked about. It is not a real possibility. Just as the world is not set up for someone to hallucinate a square circle, the world is not set up for the illusion of time passing. So if the flow of time is not an illusion, what is it?
Thinking a thought is not like watching an ice cream melt, or a chameleon change colour
Some philosophers say that time seems to pass due to the way we perceive change. They argue that moving objects appear ‘dynamic’, and that we mistake this dynamism for time passing. To see what they mean, imagine watching a movie where each frame was shown for two full seconds. You would see a series of static scenes with people and objects in slightly different positions. First Jackie Chan’s fist is here, then there, then it is in contact with someone’s face. Although it would be clear that each image shows the actor in a slightly different position, you would not see him moving. Now, imagine seeing those frames at the speed they would be played in a cinema: 24 frames per second. Suddenly, the sense that you are looking at a series of static scenes disappears, and you can’t see where one frame ends and the next begins. Chan’s fist is no longer simply there, then there, then there; he now appears to be punching someone. You are looking at the very same series of static frames in both the slow case and the fast case. But the flowing ‘dynamism’ you see in a cinema is a quality added by your perceptual system.
What does this have to do with time seeming to pass? Our perceptual systems do not just add a dynamic look to things we see in movies, but also to the things we see in the real world. Change in the world appears to flow smoothly because our perceptual systems transform moving objects in the same way they do static frames of Jackie Chan: they superimpose a dynamism on to them that they do not possess. Because we fail to recognise that this is a product of our minds rather than a feature of reality, we have come to believe that the world is dynamic, and that time really flows. That’s one theory, anyway.
But what about those moments when we are not perceiving change? The room you are currently in might be entirely still, yet time seems to continually flow. In such cases, we might be tempted to say that time still passes – that the world seems dynamic – because we continue to experience our thoughts changing just like we experience objects changing in the world. However, though thoughts rush through our heads from the moment we wake up to the moment we sleep, we don’t experience individual thoughts changing in the same way that we perceive changes in objects. Thinking a thought is not like watching an ice cream melt, or a chameleon change colour. As an experiment, try to think a single thought – make sure you don’t accidentally think two thoughts. Keep it in your mind, and observe whether it changes. I suspect that you found this exercise difficult. Thoughts jump around, and it is hard to know where one ends and the next begins. The experience of thinking is nothing like seeing objects change. Our inner thoughts don’t explain why time still seems to pass even when we are not perceiving differences in the world. Appealing to our perceptions of change to explain the feeling of flowing time might not be such a promising approach after all. Something else is going on here.
The arguments above – that flowing time is an illusion or a result of how we experience changing objects – appeal to our perception of the world to explain why time seems to pass. We see, smell, hear or feel things moving and changing, but perhaps the feeling of time passing is not related to our experience of sensing the world. We also feel pain in our bodies; feel emotions, intuitions and yearnings. The important word here is ‘feel’. In these cases, we are not perceiving the outside world. These non-perceptual experiences include the feeling of doing things, of making changes in the world: we feel ourselves walking and running, opening doors and tapping screens, talking and listening. I think time passing is a result of how we experience the changes that we make in our daily lives.
The possibility of performing no bodily or mental action whatsoever is never an option
When you reach for your coffee cup or stand up from your desk, you have a sense that you are causing your body to move. In a similar way, you experience yourself as the author of most of the bodily movements that you make. You can also experience yourself performing mental actions: you can deliberately shift your attention away from these words to the sound of the traffic outside or consciously try to remember the last place you saw your house keys. The sense that we are causing our bodily or mental actions can be thought of as a unique type of change experience. However, this kind of change is not one we perceive out there in the world (like watching Jackie Chan punch someone or hearing a plane fly through the air). It’s a kind of change we feel ourselves causing.
When you move your body, you feel yourself making changes in the world around you; when you refocus your thoughts, you experience yourself changing the landscape of your mind. We could call this ‘agentive change’ – change that an agent (like you, for example) experiences themselves as causing – and it is pervasive in a way that perceptual experiences of change in our external environment are not. As long as you are awake, you won’t stop thinking, meaning that the feeling of making mental changes persists (even in a sensory deprivation tank).
Tied up with our sense that we are the cause of our actions is the feeling that we can stop doing whatever it is we are doing and start doing something different. If you wanted, you could close this browser tab right now and get up from where you are sitting. But, though we can change our behaviour, the possibility of performing no bodily or mental action whatsoever is never an option. As long as you are awake, you will never feel as if you can stop causing change. Jean-Paul Sartre declared that mankind was ‘condemned to be free’; similarly, we find ourselves at every waking moment condemned to act. Of course, we stop acting when we fall asleep but, as any insomniac will tell you, sleep is something you must wait for, not something you do. You can hasten sleep’s arrival, but you cannot switch yourself off like a laptop.
I believe that this leads us to mistake the feeling of doing – moving, thinking, focusing – for the feeling of time passing. We experience ourselves as perpetually, helplessly active. This is likely a product of our neurophysiology. Brains don’t stop: information is continually being received, recalled, processed and responded to, so it is not surprising that we always find ourselves doing something. But we are not consciously aware of this fact. In fact, consciousness does not provide any explanation as to why we find ourselves in such a state. We are driven to keep making changes. And it is here that we make a mistake. Rather than blaming our neurophysiology for the feeling that we must constantly act, we blame the world outside: we mistakenly think that some outside force (like a flowing river of time) is responsible for the ever-present feeling that we are being ‘pushed along’.
We are condemned to act. It is not, as Heraclitus imagined, that ‘everything flows and nothing abides.’ Instead, the feeling of being swept along is the result of our brains’ constant churning. We mistake our own momentum for that of the world. Time does not flow. We do.
