Neil deGrasse Tyson hosts this StarTalk Special Edition with co-hosts Gary O'Reilly and Chuck Nice, exploring the neuroscience of time with guest Dean Buonomano, professor of neurobiology and psychology at UCLA.

Buonomano, author of Your Brain Is a Time Machine The Neuroscience and Physics of Time, discusses how the brain tells time differently from mechanical clocks, using neural dynamics rather than oscillations. The conversation covers everything from circadian rhythms in single-celled organisms to the philosophical implications of time's arrow.

The discussion spans multiple timescales - from microseconds needed for sound localization to circadian rhythms that evolved to match Earth's rotation. Key topics include mental time travel as a uniquely human ability, the role of timing in language and cooperation, and whether physical time travel is possible or limited to our minds.

The episode examines how timing enabled human civilization, from agriculture requiring future planning to the Industrial Revolution's need for synchronized factory work. Historical examples include Galileo's pendulum observations during church services and Huygens' development of pendulum clocks documented in Horologium Oscillatorium.

How the Brain Tells Time Without Clocks

Unlike man-made clocks that count oscillations of a time base, the brain uses multiple specialized timing systems for different scales - microseconds for sound localization, seconds for conversation, and circadian rhythms for daily cycles.

"The brain does not work like that. The clocks in our brain that are responsible for seconds, they don't have an hour hand. And the circadian clock doesn't have a second hand" - Dean explains the brain's distributed timing approach.

Neural timing relies on dynamics rather than oscillators - patterns of activity flowing through networks where different neurons being active indicates different time intervals, like a sophisticated hourglass system.

Circadian Clocks and Evolutionary Timing

The master circadian clock sits in the suprachiasmatic nucleus, positioned above the optical nerve crossing to use visual input for entraining the body's 24-hour rhythm to Earth's rotation.

Cyanobacteria experiments demonstrate timing's survival value: those with 22-hour internal clocks dominated in 22-hour light cycles, while 26-hour clock bacteria won in 26-hour environments, proving evolutionary advantage of matching environmental rhythms.

"They need to know when the sun is going to rise because they need to engage the protein machinery to start doing photosynthesis" - Dean explains why even single-celled organisms evolved timing mechanisms.

Mental Time Travel and Human Uniqueness

Mental time travel - the ability to remember the past and imagine the future - distinguishes humans from most animals and enabled crucial developments like agriculture, which requires planting seeds for future harvest.

"Except for man, all animals are immortal, for they are ignorant of death" - Jorge Luis Borges quote illustrating how mental time travel made humans aware of mortality, possibly co-evolving with religious concepts of afterlife.

Language depends heavily on timing - "they gave her cat food" versus "they gave her cat food" demonstrates how pauses alter meaning, showing unconscious temporal processing in communication.

Memory Storage and Neural Plasticity

Memory is stored through changes in connection strength between neurons, fundamentally different from computer memory where storage and processing are separate - in the brain, "it's the activity flowing through these networks that is both the computation and the memory."

Hebbian plasticity follows the principle "neurons that fire together, wire together" - if visual neurons see a face while auditory neurons hear a name, they strengthen connections to enable future recall.

Sequential learning requires additional rules beyond simple association - "neurons that fire first wire to the neurons that fire second" enables prediction and pattern completion in sequences like A, B, C, D.

Time Perception and Brain Integration

The brain has a temporal window of integration (200-400 milliseconds) that synchronizes visual and auditory signals, fixing the mismatch between seeing lips move and hearing speech regardless of distance.

"The sound processing is actually much quicker" than visual processing despite light traveling faster than sound, because "the retina relies on biochemical reactions" while auditory processing is more direct.

The McGurk illusion demonstrates temporal integration - hearing "ba ba ba" while seeing video of "ga ga ga" creates a blended perception, showing how the brain actively constructs unified experiences from separate sensory streams.

Time Travel and Physics Limitations

As a presentist, Dean argues that physical time travel is impossible - only the present is real, making the brain's mental time travel through memory and imagination the closest we'll ever get to a time machine.

Stephen Hawking's time travel prevention conjecture suggests future discovery of physical laws that formally prohibit time travel, supported by his failed party for future time travelers where "nobody showed up."

Brain-machine interfaces like Neuralink face fundamental limitations because "there's no clear separation between the memory and the computation" in the brain, unlike von Neumann computers with distinct CPU and memory modules.

Historical Development of Timekeeping

Galileo discovered pendulum timing principles during a Catholic ceremony by watching a chandelier swing and checking it against his pulse, leading to the development of pendulum clocks by Christiaan Huygens in Horologium Oscillatorium.

"The real engine of the Industrial Revolution was the clock" because factories required synchronized human behavior - getting everyone to work at the same time enabled mass production beyond what steam engines alone could achieve.

Einstein's work on synchronizing clocks across Europe for train schedules contributed to relativity theory, showing how practical timekeeping needs drove fundamental physics breakthroughs.