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Slides, spiders, cracks and a giant reflected sun: all "art-ertainments" that have amused hordes of visitors to the cavernous Turbine Hall of London's Tate Modern.

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In terms of popularity and public impact they are some of the most successful contemporary art commissions of this century, but they have come at a price. The space was conceived as a sculpture gallery, but has become more of a public playground.

Children seemingly levitate on their heelies above the slope from the hall's western entrance; others play tag. Adults pose for snaps and press their noses against the window of the colourful shop; others chomp on sandwiches as they queue for tickets to the latest exhibition.

None of which matters until an artist makes a work that is not playful but serious, as happened last year when the Polish artist Miroslaw Balka installed a vast steel chamber into the far end of the hall. As you walked up a ramp into the structure it quickly became pitch black, causing you to lose your bearings.

The artist intended the work to be a comment on internment; he wanted it to generate feelings of apprehension and intrigue. The public just wanted to have fun. Camera-phones would light up the space and ruin the effect, prompting couples hiding at the back to break into fits of giggles.

This year's commission, by the (from tomorrow until 2 May 2011), is another work laced with serious symbolism. He has covered the east end of the Turbine Hall with a 10cm-thick bed of sunflower seeds. There are over 100 million.

Except they are not sunflower seeds at all, but individually hand-crafted pieces of porcelain, every one of which has also been hand-painted. He wants the visitor to walk on his work, to grind his or her heels into the efforts of the 1,600 Chinese workers who made the tiny pieces.

His message is one of scale - when was the last time you saw 100 million hand-crafted objects? - and a comment on the population of China. It is also about individuality, the idea that all these objects look the same but are all in fact different. The sunflower represents the role of the people in the time of Chairman Mao: he was the sun; they were the sunflowers turning to face him.

And as the people starved, so they ate and shared out sunflower seeds. The porcelain is not an arbitrary choice of material either; it has historically been a major export of China.

Ai Weiwei is a substantial artist with plenty to say. The profile I made for Newsnight last week is below - he is a fascinating man.

Who knows how successful this installation will be? Historically the artworks that have existed only in the far end of the hall - as is the case here - have failed to capture the public imagination in the same way as those that have infiltrated the whole space.

But for the visitors who do make it past the shop and the queue and walk upon the serene landscape that Ai Weiwei has created it will be an enjoyable interaction, provoking the feeling of walking along a shingle beach. I suspect many will also be moved by what the artist is saying with the piece.

If that happens and this helps reclaim the Turbine Hall in the public mind for serious artworks, it would, in a way, be the most successful installation yet in Tate's hangar-like space.