So England now have the momentum, right? England won the last Test, Australia lost it badly, so inevitably England will take the series. That's how the theory of momentum works: once an advantage has been established it's very hard for the other side to get back into the contest.

The theory of momentum has become a significant way in which we interpret the sport we watch. We say "the momentum is now all with Chelsea", or whatever. The problem is that the current Ashes series seems to disprove the theory at every level.

Australia had all the momentum going into the series, having won the last one 5-0. But England won the first match by some distance, so they had the momentum going into the second - and lost by an even greater margin. So Australia had the momentum going into the third - and chucked the match away with two innings of disastrous batting.

It looks as if the momentum required for victory actually comes from defeat. And that can't be right, can it?

Momentum is a term borrowed from physics. Momentum is mass in motion: mass and velocity working together. It takes more effort to stop a rolling cannonball than a runaway ping pong ball. Your ping pong ball has momentum all right: just not as much as the cannonball.

You're doing so well you can't actually believe it - and that prompts a fit of the vapours and suddenly you're on the wrong side of a momentum shift

The sporting theory of momentum is biased towards the cannonball. The idea is that once you have started to win, it's very hard to stop. You rumble towards the goal of victory with the weight of your hard-won advantages powering you on. You win because you're winning.

But your opponents don't give up. They seek a momentum shift. And we who watch love to spot it - a small change of advantage that might change everything. Classic example: The Oval 2005, when Australia were bowling England out cheaply. Then Shane Warne dropped Kevin Pietersen at slip when he was on 15 - and Pietersen's demented charge at Brett Lee changed everything.

It's wonderfully easy to spot a momentum shift once it's all over. This goes back to something Mike Brearley once said: that when we look back at a sporting event we see the result as (a) inevitable and (b) morally appropriate. The entire industry of sports writing destroyed at a stroke.

The trick is to spot a momentum shift while the ball is live. That's all about tuning into the emotional ebb and flow of a match, and it's one of the most thrilling things in sport. It's that hang-on-a-minute moment when a struggling team suddenly realise they might win, when the winning team feel a sudden cold draught of doubt.

At what point did the Australians feel doubt after England followed on at Headingley in 1981? When Graham Dilley started to play that mad axeman front-foot cut shot? Certainly it was that doubt that Bob Willis so famously exploited to win the match the following day.

I was in Istanbul when Liverpool were 0-3 down in the Champions League Final in 2005 and halfway through a piece about their defeat when a great momentum shift took place. Liverpool went on to win a match that's still called the Miracle of Istanbul.

You can sense a momentum shift in tennis when a player saves half a dozen break points. His opponent double-faults in the next game and suddenly the loser is winning. You can see it in a major golf tournament when a player goes on "a birdy charge". He birdies the next hole because he birdied the one before.

That's momentum, the notion - not an outrageous one - that what happened before affects what happens next. In the last Ashes match, Steven Finn and Mitchell Johnson took the next wicket because they had just taken the previous wicket.

A momentum shift is one of the most inspiring things you can ever see in sport: not least because it taps directly into our own lives. We have all suffered misfortune, we have all been outplayed by life, we have all felt the pain of defeat. To watch these terrible things being overcome is a thing of profound joy, and more so when the team doing the improbable comeback is the one you're cheering for.

Managing momentum is one of the great skills in sport. When you have got the momentum you must exploit it for all it's worth. Some players get carried away and believe they are immortal; some of them - but by no means all - are punished for this belief.

The trick is to spot a momentum shift while the ball is live. That's all about tuning into the emotional ebb and flow of a match, and it's one of the most thrilling things in sport

Pietersen was always getting out when trying to reach three figures with a six. But sometimes he made it. And that knock at The Oval would never have come off if he'd had a sane and sober view of his own mortality. Pietersen's inability to understand the phenomenon of momentum was at the heart of his greatest achievements.

Form is an aspect of momentum. You play well because you're playing well. You score runs at Edgbaston because you scored runs at Lord's. Joe Root is in marvellous form, a rumbling cannonball of achievement. It won't last forever - no one knows that better than his captain Alastair Cook, but don't tell him, Pike. Not before The Oval, anyway.

Whose advantage? England's obviously, but this series has made such a nonsense of sporting momentum that it's impossible to be dogmatic. If the series has a pattern it's in each side's ability to reverse the momentum of events.

There's a touch of vertigo operating here. You're doing so well you can't actually believe it, and that prompts a fit of the vapours and suddenly you're on the wrong side of a momentum shift.

In this series, momentum has been more like a ping pong ball than a cannonball: a thing easily diverted, with no sense of an inevitable destination, subject to every caprice and blowing with every breeze. England's job is to turn it into a cannonball, but without James Anderson to bring about the transformation.

Still, we'll understand it all in September. And I promise you this: the final result will be both inevitable and morally appropriate.

Simon Barnes is a former chief sportswriter of the Times and the author of more than 20 books