Your Brain is a Time Machine, with Dean Buonomano

In this podcast episode, Neil deGrasse Tyson and his co-hosts Gary Riley and Chuck Nice engage in a deep and lively discussion with Dean Buonomano, a professor of neurobiology and psychology at UCLA, about the nature of time from both a neuroscience and physics perspective. The conversation touches on how the brain perceives and processes time, the concept of mental time travel, biological clocks ranging from circadian rhythms to neural dynamics, the relationship between time and human culture, and the philosophical and scientific questions surrounding time travel and consciousness.

Mental Time Travel and Human Cognition

One of the distinguishing features of human cognition discussed is the ability known as mental time travel—our capacity to mentally project ourselves backward into the past and forward into the future. This ability underpins many fundamental human behaviors, such as the invention of agriculture, which requires anticipating future outcomes from present actions. The podcast highlights how this leap was significant because it involved connecting cause and effect over extended temporal scales, a cognitive challenge that most animals do not solve. The discussion also touches on the idea that mental time travel may have played a crucial role in human awareness of mortality, leading to the development of religion as a cultural response to death's inevitability. While some animals, like squirrels hoarding nuts, appear to anticipate the future, their behavior is largely driven by instinct rather than conscious future planning.

Clocks in Nature and the Brain's Timekeeping

Dean Buonomano explains that human-made clocks have evolved through history to become extremely precise devices based on counting repetitive oscillations, from sundials and pendulums to atomic clocks. These mechanical and electronic clocks rely on consistent, repeating physical phenomena as time bases. By contrast, the brain does not keep time this way. Instead, neural timekeeping arises from the brain's complex dynamical system and patterns of neural activity that evolve over time, more akin to an hourglass or continuous process rather than a ticking oscillator. Different biological clocks exist on different timescales: circadian rhythms that align with the 24-hour day, regulated by molecular feedback loops primarily in the suprachiasmatic nucleus, and neural clocks that govern second-to-second timing for moments within conscious experience.

Circadian Rhythms and the Role of Light

The podcast delves into the biological foundations of the circadian clock, emphasizing its molecular mechanism involving transcription-translation feedback loops. The master circadian clock resides in the brain's suprachiasmatic nucleus, located near optic nerve crossover points, explaining why light input largely entrains circadian rhythms. The importance of environmental cues is underscored by experiments where sensory isolation or blindness can cause drift in natural circadian cycles. Intriguingly, this timing mechanism exists even in simple life forms like cyanobacteria, which need to anticipate day and night to regulate photosynthesis effectively. Experiments show that cyanobacteria with circadian rhythms misaligned with their environment are outcompeted by those whose internal clocks match the environmental cycle.

Time as a Fundamental Dimension in Science and Society

The conversation touches on the historical and scientific significance of time in human society and scientific development. From Galileo's observations of pendulum motion—originally noticed during a church service—to the synchronization needs of factories in the industrial revolution powered by reliable clocks, timekeeping has been essential for coordinating human activities. Time's incorporation into physics represents a later development compared to geometry and mathematics, which were historically static disciplines. It required the emergence of calculus and concepts of dynamics to fully describe physical phenomena changing over time. Time is seen as fundamentally more complicated than space due to its intrinsic directionality and experiential qualities.

Neuroscience of Time and Temporal Perception

Dean Buonomano discusses how the brain processes time not as discrete ticks but through evolving patterns of neural activity—"neural dynamics"—that encode temporal intervals. These neural sequences or trajectories form the basis for timing events over seconds and subsecond intervals. The concept of associative plasticity, captured in the phrase "neurons that fire together wire together," plays a role in learning the timing of sequences, with more complex rules governing how neurons strengthen connections in temporal order. Temporal processing highlights an added dimension beyond spatial brain mapping, suggesting that time perception is central to cognition and underexamined compared to spatial studies.

The Illusory Flow of Time and Physics Perspectives

The nature of time's flow—the sense that time constantly moves from past to present to future—is an enduring philosophical and scientific question. Neuroscientists acknowledge that conscious perception includes this flow, but physicists often argue that time may be an illusion, pointing to space-time models where past, present, and future coexist equally (eternalism) versus the view that only the present is real (presentism). The podcast explains how relativity theory treats time as relative and symmetric, making the subjective experience of flowing time a challenge to reconcile with physical laws. Buonomano advocates that the brain's experience of time's flow likely reflects a genuine property of the universe at the mesoscopic scale in which humans operate.

Time Travel: Physics and the Limits of Possibility

The discussion moves to whether actual time travel is physically possible. While general relativity permits theoretical constructs like wormholes that could allow travel across time, these remain speculative and constrained by unresolved paradoxes. Dean Buonomano identifies as a presentist skeptical of physical time travel, proposing instead that the brain is the only true "time machine" humans have, accomplished via mental time travel and memory. The episode references Stephen Hawking's "time travel party" thought experiment, where no future visitors appeared, supporting the idea of time travel's improbability or impossibility. This stance frames time travel as a powerful metaphor and cognitive ability, rather than a physical reality.

Human Memory and Its Biological Basis

The conversation addresses how memories are stored in the brain. Unlike digital memory in computers, brain memory arises from changes in synaptic strength—the weights of connections between neurons—as patterns of neural activity encode information. This plasticity supports not only memory but also the sequencing necessary for temporal cognition such as language and musical rhythm. Unlike man-made systems with clear separation between computation and memory, the brain tightly integrates both functions within overlapping networks, making memory storage a dynamic and distributed process.

Synchronization and Temporal Integration in the Brain

The hosts explore how the brain integrates multi-sensory input occurring at slightly different times, such as seeing lips move before hearing speech, by maintaining a "temporal window of integration." This adaptive window allows us to perceive disparate signals as synchronized despite physical delays. Experiments such as the McGurk illusion reveal the brain's active blending of auditory and visual stimuli over this temporal window, illustrating how the brain corrects sensory timing mismatches to create coherent perception. This mechanism reflects the brain's flexibility and precision in managing time-related sensory information.

Neuroscience, Artificial Intelligence, and Limitations in Understanding

Finally, the episode discusses the limits of human cognition and whether the brain can fully understand itself or the universe. Neuroscience is characterized as a recursive science where the subject (the brain) studies itself, creating unique challenges. Buonomano expresses skepticism that humans intuitively understand phenomena like quantum mechanics but acknowledges that mathematical abstraction allows modeling beyond intuitive grasp. He cautions against oversimplified notions that brain-machine interfaces or artificial intelligence can straightforwardly enhance cognition or enable abilities such as "downloading" skills or memories. The conversation concludes on the recognition that time is central to many deep scientific and philosophical issues, including consciousness, free will, and determinism, and that studying temporal dynamics offers a frontier in neuroscience.