Daily Telegraph report (2001)

When is now, then?

17 January 2001

Roger Highfield

Scientists have discovered that what we see often lags behind what we hear, thus explaining why an astronomer was fired by the Royal Observatory in Greenwich more than two centuries ago.

Dr Jim Stone of the Sheffield University, who has been studying what we mean by "now", has found that, if a firecracker goes off at arm's length, the time it takes a person to be aware of the bang is different from that of the flash.

After 1000 "flash bang" trials on 17 people, Dr Stone discovered that there is a wide difference in the time it takes an individual to process sound as opposed to vision.

Some people perceived light up to 21 milliseconds before sound, suggesting they could process light more quickly. However, most people hear sound first; for one it took up to 150 milliseconds – about one seventh of a second – to register light after hearing the sound, so this person experiences a slight lag between hearing a someone's voice and seeing their lips move.

"We found the 'now' is different between individuals, but it is very stable within each individual", said Dr Stone, who reports his findings in the Proceedings of the Royal Society. "I think that it is intriguing that the world we perceive via sight and sound is time-shifted by our sensory processes by up to 150 milliseconds."

Dr Stone said it may help to explain why, in 1794, David Kinnebrook, a new assistant to the Astronomer Royal, Nevil Maskelyne, was sacked.

Kinnebrook was asked to record observations of stars and their locations by the "eye and ear" method, which relied on listening to the ticks of a clock and viewing the time at which a given star drifts between two lines. Maskelyne checked Kinnebrook's work but came to different conclusions, deducing that Kinnebrook was half a second slower. Kinnebrook's attempts to do better failed, and he was sacked.

Two decades later, the influential German astronomer Friedrich Bessel deduced that the differences in observations were due to differences between these scientists, not inferior work. Now Dr Stone has shown how varied our perception of "now" can be.

"Up until that time, it was assumed that, all things being equal, people saw or heard the world the same way" said Dr Stone.