When you see something and react, the news has to travel: eye → brain → decision → arm → finger. Nerves are fast, but not instant — the whole relay takes about a quarter of a second, and no amount of trying gets it to zero.
Most people assume their senses are effectively instant. In fact a nerve signal is a wave of chemical gates, topping out near 120 m/s, a million times slower than copper, so seeing and reacting takes about a quarter of a second, no matter how hard you try.
What's actually happening
Your senses feel instantaneous, but they are a postal service, not telepathy. Light striking your retina triggers a chemical cascade that takes tens of milliseconds before a signal even leaves the eye. It then travels to the visual cortex at the back of your head, gets recognised ("green!"), routes through decision circuits, and a command races back down your spinal cord and arm to the finger muscles. Every leg of the relay is fast; none is free. The total, around a quarter of a second, is your reaction time, and the tester above measures yours.
The wiring itself has a speed limit. Nerve signals aren't electricity in a wire (300,000 km/s); they're waves of chemical gates flipping open along the fibre, topping out around 120 metres per second in the best-insulated nerves. That's a million times slower than copper. Evolution's workaround is myelin, fatty insulation that lets the signal leap between gaps, and brute prioritisation: reflex arcs for hot stoves route through the spinal cord and fire your arm muscles before the pain report has even reached your brain.
Because the lag is unfixable, your brain cheats: it predicts. A batter facing a 145 km/h ball starts the swing before the ball's flight could possibly be fully perceived; your brain even edits the lag out of consciousness, backdating events so the world feels live. That's why the tester punishes early taps — a tap before the green isn't fast perception, it's a guess. And it's why athletics disqualifies any sprinter who leaves within 100 ms of the gun: human biology simply cannot hear and act that fast.
There is a hard biological floor of roughly a quarter-second between seeing and acting — which is why your brain predicts ahead and why sprinters under 100 ms are disqualified as guessing.
- 1A friend holds a 30 cm ruler vertically; you hover your open thumb and finger at the zero mark, not touching.
- 2They drop it without warning; you catch. Read the centimetre mark where you caught it.
- 3Distance → time: 10 cm ≈ 143 ms, 15 cm ≈ 175 ms, 20 cm ≈ 202 ms. Compare with your tap result above — rulers don't lie, and neither do nerves.
Common questions
A nerve signal is not electricity in a wire; it is a wave of chemical gates flipping open along the fibre, topping out near 120 metres per second — about a million times slower than copper.
Reflexes take a shortcut. The hot-stove signal loops through the spinal cord and fires your arm muscles before the pain report has even reached your brain.
World Athletics treats any reaction under 100 ms as a guess, not a response, because human nerve biology simply cannot hear the gun and act that fast.