What is the Smallest Thing You Can Feel? A Deep Dive into Human Sensation and Perception

Overview

This episode investigates the smallest things we can feel across the five senses, arguing that there is no universal absolute threshold. The hosts contrast subjective experience with measurable quantities and share striking numbers that quantify sensation in molecules and photons rather than poetic thresholds.

Key Insights

• Thresholds vary between individuals and contexts, making universal absolutes unlikely.
• Concrete figures illustrate sensory sensitivity, from taste molecules to single photons.
• The discussion spans biology, physics, and phenomenology to examine what it means to feel.

Overview

The Rest Is Science asks a provocative question: what is the smallest thing you can feel? Michael Stevens and Hannah Fry argue that there is no single, universal threshold that marks the boundary of feeling across all senses. They propose a staged investigation, moving from the most molecular scales to the perceptual thresholds our brains can register. The conversation blends practical measurements with philosophical considerations about sensation and perception, emphasizing that the brain and body shape what counts as a detectable event.

Quantifying Sensation

The episode anchors its exploration with concrete, sometimes surprising numbers. For example, lugduname, an extremely potent sweetener, is detectable at about 0.6 parts per billion in water, while a particular odourant is detectable at about 0.3 parts per trillion. On tasting, a threshold involves not just a single molecule but the number of active receptors and the surrounding environment; on hearing, the discussion suggests trillions of air molecules are typically involved before a sound is perceived.

Sensory Modalities Explored

The show walks through hearing, taste, smell, touch, and sight. In hearing, the researchers consider energy release and the practical difficulty of detecting an instantaneous release of energy. In taste, they examine the role of hydronium ions and a powerful artificial sweetener that binds broadly to sweet receptors, illustrating how taste thresholds are far from simple. Smell is highlighted for its direct neural sampling path via the olfactory nerves, enabling remarkable sensitivity, such as detection of certain compounds at extremely low concentrations. Touch is discussed in terms of static indentation thresholds on skin and the limits of tactile perception. Sight is addressed through photon detection by retinal rods, including the debate about whether a single photon can ever be consciously seen.

Home Experiments and Real-World Implications

The hosts describe accessible experiments, such as feeling the texture differences between DVD and Blu-ray discs, and they discuss how tiny changes in physical structure at the nanoscale can be felt as perceptible differences. They also touch on the phenomenon of non-conscious perception versus conscious awareness, using examples from neuroscience to illustrate how the brain registers stimuli without necessarily producing a vivid perceptual experience.

Conclusion

The discussion concludes that there is no single smallest thing we can feel that applies universally. Instead, the minimal detectable event depends on the sense, the organism, the environment, and the state of the observer. The episode ends with an invitation to continue exploring the boundaries of human perception and the science behind it.