Ask Mr Brain...all will be explained

PUBLISHED : Monday, 13 December, 1999, 12:00am
UPDATED : Monday, 13 December, 1999, 12:00am

Which apples are better, red ones or green ones? There is not much difference between the two - they are both good for you. But apples are somewhat overrated in their nutritional value. They are low in vitamin A and vitamin C.

Can anything travel faster than the speed of light? According to Albert Einstein's theory of relativity, no particle can be accelerated from below the speed of light to faster than the speed of light - 300,000 kilometres per second.

But since the 1960s physicists have been aware of the hypothetical existence of subatomic particles which always travel faster than light. These particles are called tachyons. The existence of tachyons is thought to be consistent with the theory of relativity.

Unlike particles which travel slower than light speed, tachyons could only exist at speeds faster than that of light.

While a particle travelling at less than light speed slows down as it loses energy, a tachyon would lose energy and accelerate until it had zero energy and was travelling at infinite speed.

Travelling at infinite speed and having zero energy, tachyons would be undetectable, which is also the case if they do not exist.

Most physicists now think tachyons do not exist. Astrophysicist Dr John Gribbin of the University of Sussex said the equations showing their hypothetical existence were similar to describing the number of players taking part in a card game as being 'the square root of 16'. The answer could be either +4 or -4, but it is obvious there could not be -4 people playing cards. And to date, those who have searched for evidence of tachyons have done so in vain.

Aside from the fruitless search for tachyons, the August 1998 edition of Discover magazine reported that Professor Raymond Chiao of the University of California at Berkeley had apparently made photons travel faster than the speed of light.

His experiments revolved around the quantum mechanics phenomenon of 'tunnelling'. In quantum mechanics, tunnelling occurs when a particle hits a barrier it cannot go through or around. Instead it disappears and almost instantly reappears on the other side.

Tunnelling is not hypothetical; electrons do it in some electronic components.

Conducting races between photons, Professor Chiao showed that the tunnelling photon arrived before the non-tunnelling photon travelling at the speed of light. The experiment suggested the tunnelling photon was travelling at 1.7 times the speed of light.

However, Professor Chiao said what happened was that tunnelling caused the 'bulge' in the middle of the photon to move to the front, setting off a detector before the non- tunnelling photon. So despite appearances it did not really violate the theory of relativity.