The big question: How does tampering with a cricket ball affect its trajectory?

Why does the question arise now?

Because Pakistan forfeited the fourth Test match against England on Sunday after being accused by match officials of ball-tampering. The team's captain, Inzamam-ul-Haq, refused to lead his players back on to the field after tea in protest against a ruling by the umpires that they had illegally scuffed the surface of the ball. Despite no clear evidence of tampering, Hair and Doctrove ruled that a player on the fielding side must have altered the ball's polished surface during the afternoon session in order to exaggerate its movement in the air. England were awarded the match. They win the series 3-0.

Why is a ball vulnerable to being tampered with?

A standard cricket ball weighs 5½oz. The inside is made from cork, wound round with string; the outside consists of red leather stitched around its circumference, creating a raised seam. Ball tampering is the act of illegally modifying the condition of the ball to affect its flight or its behaviour after bouncing. Players are allowed to polish the waxed leather surface on their body or clothing, and to dry it on a towel. However, they are forbidden from applying any substance other than saliva or sweat to the ball, and are also banned from deliberately scuffing or roughing it, or picking at or lifting the seam.

The nature of the game means that a cricket ball is frequently passing through the hands of players on the bowling side. Tampering can take a matter of seconds, yet it is virtually impossible for umpires to keep their eye on the ball at all times.

How would this give a team an advantage?

When a ball is brand new, the leather is equally shiny on both sides of the seam, and will usually deviate little on its flight through the air. As the innings progresses, the surface deteriorates through contact with the bat, ground, and (occasionally) stadium and advertising hoardings. However, the fielding side will usually attempt to keep one side of the ball as pristine as possible. If it becomes significantly shinier than the other side, it will start to do interesting things after it leaves the bowler's hand. The half that is slick and shiny meets little wind resistance when it is thrown; the other side (which is battered, with small bits of leather protruding from its surface) travels less easily through the air. As a result of the varying wind resistances, the ball will fly in a curved direction, a phenomenon known as "swing". Depending on how a bowler releases the ball from his hand, he can make it "swing" either towards or away from the batsman. Uncertainty over the direction and distance that the ball might deviate makes it harder for the batsman to hit it. Tampering with a ball by illegally roughing it up, or by polishing it with banned substances, will exaggerate the extent of this "swing".

In addition, bowlers will usually attempt to make the ball "pitch" or land on its seam, so that it will deviate slightly on landing. Illegally picking or lifting the seam will exaggerate the extent of this further deviation.

Will tampering help all bowlers?

Not necessarily: illegal scuffing, polishing, and seam lifting will be most helpful for so-called "fast" bowlers, who release the ball at speeds of about 90mph. Slower bowlers attempt to alter its direction by using their fingers and wrists to apply spin.

The holy grail of fast bowling is to achieve "reverse swing". This generally occurs when a ball becomes very old, and its aerodynamics change to make it swing towards the polished side. This is because the air begins to flow turbulently around both sides of the ball. The angled position of the seam causes the airflow to separate earlier on one side than the other, altering its direction accordingly. Reverse swing is hard to achieve, and generally only occurs in hot, dry weather conditions, after a ball is 40 overs old. A great deal of suspected ball-tampering is intended to accelerate the process of deterioration that will allow reverse swing to occur.

What tricks are used?

Players have used countless products - including hair wax, lip salve and sunscreen - to illegally make the unscuffed side of the ball slicker and shinier. They may also chew sugary sweets or chewing gum in the field, before applying sweetened (and therefore shinier) saliva to its surface.

Helping the ball to deteriorate can involve soaking one side in water, since wet leather is more easily scuffed. Players have also been observed picking holes in it, and also rubbing in grit or dirt. The seam can be picked or lifted with a finger nail.

Is this kind of cheating new?

Players have been trying to gain an unfair advantage since WG Grace replaced the bails after being bowled, but it is possible that today's high-stake professional game has increased the temptation. Recent years have seen a rash of controversies, the most infamous involving the then England captain Mike Atherton at Lord's in 1994. He was fined £3,000 after being found to have dirt in his pocket, which he claimed was used to dry one side of the ball.

Last season, Surrey were docked eight championship points after they were twice found to have lifted the seam against Nottinghamshire. The censure saw them relegated from the top flight. It is possible, however, that the advent of TV technology has increased the frequency with which guilty parties are caught.

Can it be stamped out?

Cricket's authorities are making efforts to clamp down upon blatant instances of ball-tampering, but they are likely to find it almost impossible to eradicate from the game altogether.

Umpires make regular inspections of the ball, but as Sunday's incident shows, concrete evidence of foul play is hard to come by. It is almost impossible to prove that damage occurred through illegal means, rather than (say) the ball being whacked into the crowd, unless a player is caught red-handed attempting to scuff its surface. It is even harder to prove that sweetened saliva is being illegally applied to a ball, and impossible to ban players from wearing sunscreen or hair gel.

So while increasing levels of scrutiny may be having an effect, the chaos at the Oval this weekend is unlikely to be the last.

Is ball tampering a serious problem for cricket?

Yes...

* Despite the difficulty of catching players at it, several have been fined or banned for tampering in recent years.

* Lifting the seam can cause a dangerously inconsistent bounce and cause a batsman to be seriously injured.

* The ICC has been forced to introduce new laws allowing five runs to the batting side if umpires believe the ball has been roughed up.

No...

* It is far from clear that some illegal practices, for instance the use of sweetened saliva, have any great effect on the ball's flight.

* Cricket remains a "cleaner" sport than football, where diving and other forms of cheating are endemic.

* Pakistan's main complaint was with the umpire Darrell Hair, whom they believe to have a vendetta against them.