Hold a ruler by one end, let a friend pinch the other end open, and drop it without warning. You will not catch it at the top. Gravity is a stopwatch, and the place where your fingers close is the reading.
Press Start. The ruler hangs between your thumb and finger, and after a wait you cannot predict it drops. Hit Catch! as soon as it moves. Use the space bar if you have a keyboard.
Your screen and browser add a small delay of their own, so read the numbers as a rough guide and compare runs on the same device. Taps before the ruler moves are not counted.
A dropped object starts at rest and speeds up steadily. After a time t it has fallen a distance
d = ½ g t²where g is about 9.81 m/s². Turn it around and you get your reaction time from where you caught the ruler: t = √(2d / g). No stopwatch needed, only a ruler.
| Reaction time | Ruler falls |
|---|---|
| 0.10 s | 4.9 cm |
| 0.15 s | 11.0 cm |
| 0.20 s | 19.6 cm |
| 0.25 s | 30.7 cm |
| 0.30 s | 44.1 cm |
The distance grows with the square of the time, so the gaps widen. Going from 0.20 s to 0.25 s costs you 11 cm of ruler, while going from 0.10 s to 0.15 s costs only 6. A small slip in time becomes a big slip on the ruler, which is why the ruler is such a sensitive test.
Between the ruler moving and your fingers closing, a chain has to run. Light is focused on your retina, the signal travels to the brain, the brain recognises the movement and decides to act, and a command travels down your arm to muscles that then have to start pulling. Each step takes time, and none can be skipped.
Sprinters give a view of the floor of this delay. World Athletics counts any start under 0.100 s after the gun as a false start, on the view that nobody can react that fast. The figure traces to a 1990 Finnish study of eight elite male sprinters, whose average reaction was about 0.121 s. In a sound-to-nerve-to-muscle sum, signals reached the calf as early as 60 ms in lab studies, and muscles need a little longer again to produce any force.
A study of the 2008 Beijing Olympics estimated the fastest possible block-measured reaction at about 0.11 s for men and 0.12 s for women. In ruler terms, a catch at 0.10 s is a drop of under 5 cm. Even people who train for this all their lives cannot get near it with a sound cue. Your result is probably a good deal slower, and that is normal.
Try the steady-beat mode, where the ruler always drops after exactly the same wait. You will probably catch it much sooner, and sometimes you will tap before it moves. That is not faster reflexes. You are no longer reacting, you are guessing the time and pressing in advance.
That is why a proper reaction test must keep you in the dark. A random wait forces you to wait for the event itself. The test measures reaction only when the timing cannot be predicted, and every tap before the drop is a guess, not a reaction.
A car at 100 km/h covers about 27.8 metres every second. A quarter of a second of delay, before the foot has even reached the brake, is nearly 7 metres of road. Expecting the surprise, by watching the car in front, closing the gap in the mirror and keeping a longer distance, is the only way to shorten the gap, because the chain inside your head will not get shorter.