A great experiment is about to get under way, and it will tell us much about the future of journalism and the willingness of readers to pay for it. In Wapping last night, News International showed off the new websites for The Times and Sunday Times which have opened to the public this morning. Four weeks from now, a paywall will go up in front of the sites and, by News international's own calculation, more than 90% of their audience will melt away.

Both new sites look engaging and attractive. They are very different - after years of operating online under the same banner, each newspaper now wants to reassert its identity. The Times has a clean, simple look, much like the paper itself, with the accent on news, complemented by interactive graphics and video.

The Sunday Times looks far more like a magazine, emphasising the kind of material that has a shelf life beyond the day of publication. Among the special online offerings are a weekly satirical look at the news by the video artist Alison Jackson, and a culture planner, which you can use to book theatre or cinema tickets direct from the site, or even to set your Sky Plus recorder.

'If you want a quick hit of in-depth news you go to the Times," a Sunday Times executive explained,"if on a Monday, Tuesday or Wednesday you want to tour a little, you come and snack on what the Sunday Times has to offer."

If, that is, you are willing to pay £1 for a day or £2 for a week's access. This is more than just an experiment in whether people will pay for news, it's a strike against the prevailing philosophy of online journalism, which says that the most important thing is to make your material shareable to the widest possible audience. Look at the bottom of most online news articles these days and you will see a share button encouraging you to pass on what you've just read to friends, via e-mail or a social network.

Go to a site like Twitter and you will see an orgy of self-promotion from journalists tweeting links to their latest stories or blog posts. Over the weekend, the social network was alive with discussion of a piece about Lady Gaga by the Times' extremely entertaining feature writer Caitlin Moran. During the election, the Times' chief leader writer and political columnist Daniel Finkelstein reached a far wider audience via social networks and Google searches than actually paid to read him in the paper.

Now all that will stop. Google searches will no longer turn up Times stories, and links posted on social networks will only take you to the papers' sign-in page. News International has opted for the most extreme form of paywall - others let search engines crawl their sites, or offer non-paying visitors a few free articles to entice them in. The paper says there will be advantages to readers in thinning out the crowds. Online chats with the football or cricket writers, for instance, will be more intimate, and as comments will now be by named readers, there will be an end to the tedious anonymous ranting that plagues many news sites.

I asked Danny Finkelstein whether it bothered him that from now on none of his journalism would "go viral", with the risk that he'd be left invisible on the sidelines as the online debate raged through news sites without paywalls. "No," he insisted,"I want my employer to be paid for my intellectual property."

Executives from The Times and Sunday Times weighed in with the perfectly sound argument that it was unsustainable for the two papers to go on doing what they are doing now - quality journalism is an expensive business and they made a loss of £87m last year.

It will take a lot of subscriptions to fill that hole, but the company is convinced that advertisers will find the smaller audience of committed readers more attractive than the 21 million promiscuous passers-by who flit through the free Times Online site each month at present.

While there's been plenty of sniping from the sidelines by News International's rivals, I suspect they are all glad that someone is at least testing the waters. If the two papers do attract enough paying customers to start eating into the company's losses, that will give heart to the whole newspaper industry. But for Times journalists who are getting used to the idea that they can build their own brands by sharing their wares far and wide online, these are going to be a difficult, and perhaps lonely, few months.