Take a moment to clap your hands together. Do you hear the sound of the clap right as your hands come into contact? This does appear to occur at the same time. Yet the sound of the clap travels much slower than the light from your hands, and your brain differs in the time taken to process sound and light. So how does the clap appear to be in sync?

Our ability to measure time is the glue that holds our perception of the world together. It ties our senses, our memories and the events of our lives into a coherent narrative. Yet this system is rarely thought about, and, in many ways, peculiarly disconnected from reality. For instance, time tends to flow faster when we feel excited (1), slow down when we move slowly (2) and even seems to flow differently when we look at the colour red (3). Our window of time tends to expand when we’re taking in a high density of important information (4) and contracts when we are in a state of flow (5). Overall, our subjective experience of time is malleable, ebbing in and out of alignment with real, objective time.

This indicates our perception of time is shaped by our environment and internal state rather than a direct readout of physical time, and our best neuroscientific theories of time perception support this. Though scientists have theorised our brain uses a central clock or metronome, more recent evidence suggests our mechanisms for perceiving time are distributed across our brain (6). For example, there seem to be distinct mechanisms involved in tracking time of less than a second, compared to more than a second (7). Each sense also seems to have its own timing systems, meaning vision, hearing and touch modalities are able to track their own time (8). Rather than syncing to a central clock, many researchers believe the measurement of time is implicit in the timing of neural processes and inferred from external signals (9). It’s not a metronome – it’s an orchestra without a conductor, each player keeping the other in check. This means our flow of time is dynamic, stitched together from our environment, alertness and the neuronal activity of the brain itself.

Our subjective experience of time and the inner workings of time in the brain are very different from the steady, constant flow we perceive physical time to be. Yet, time as an objective feature of the universe is dynamic in its own way.

We all share the basic experience of “being” in a present moment. According to our best understanding of physics, however, time is tied to space, with no point in spacetime being uniquely privileged (10). This means there is no singular present moment we all share. Rather, depending on their position and motion through space, different people can experience different chains of events in time. In essence – different people experience different presents (11). Time is also inherently directionless. Fundamental equations in physics are time-symmetric, meaning the laws of physics work in reverse (12). Our experience of time as a directional flow is fundamental to how we see the world, but this flow is a product of entropy (13). This refers to how arrangements of particles in a system are overwhelmingly likely to progress from states of order into states of increasing disorder. An apple decays and doesn’t revitalise. Ice cubes melt and don’t reform. But this is also not a fundamental force, like we perceive the flow of time to be. It is a statistical tendency that emerges only on the large-scale interactions of an uncountable number of particles. In summary, time in physics is far from an independent arrow. It is interweaved with space and has direction only through the relationships between particles. Yet it remains an integral aspect of our reality. 

If objective time is so different from our intuitions, how do we explain our experience of time? Why do we experience a seemingly shared present moment, and a sense of time flowing forward steadily? Ultimately, this is because our experience of time is constructed. We need the experience of a present moment to draw together events in the world (14). The clap of your hands, in the truest sense, is a collection of particles. But by interweaving the myriad streams of brain activity and sensory stimuli, the mind places this clap within a moment. Just as we, as a species, place ourselves within a moment. 

Time in the brain is represented through a shifting, organised chaos of neural activity and interconnected systems. Within physics, it is bound with space and progresses forward through a dance of particles organised through thermodynamics. Collectively, we tell stories and plan futures through a shared sense of time that has been somehow ordered from the chaos. If you’re ever without a clock and wondering how much time has passed, remember, you are not alone.

References

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2. De Kock R, Zhou W, Joiner WM, Wiener M. Slowing the body slows down time perception. eLife. 2021. doi: 10.7554/eLife.63607

3. Shibasaki M, Masataka N. The color red distorts time perception for men, but not for women. Sci Rep. 2014;4(1):5899. doi: 10.1038/srep05899

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9. Buhusi CV, Meck WH. What makes us tick? Functional and neural mechanisms of interval timing. Nat Rev Neurosci. 2005 Oct;6(10):755-65. doi: 10.1038/nrn1764

10. Buonomano D, Rovelli C. Bridging the neuroscience and physics of time. arXiv. 2021. doi: 10.48550/arXiv.2110.01976

11. Baron S, Miller K. An Introduction to the Philosophy of Time. 1st ed. Polity; 2018. 280 p. 

12. Carrol S. Time. In: The Biggest Ideas In The Universe: Space, Time and Motion. Dutton; 2022. p. 304. 

13. Buonomano D. Your Brain Is a Time Machine: The Neuroscience and Physics of Time. 1st ed. W. W. Norton & Company; 2017. 304 p. 

14. Eagleman DM. Human time perception and its illusions. Curr Opin Neurobiol. 2008;18(2):131–136. doi: 10.1016/j.conb.2008.06.002