A paper criticizing research posted on the physics preprint server ArXiv has led experts to warn of the potential for future legal actions against the site.

In an article put on the server on 27 October, Alvaro De Rújula, a theoretical physicist at CERN, the European Centre for Particle Physics in Geneva, made a strongly worded attack on Martin Rees, Britain's astronomer royal, and his co-workers.

De Rújula alleged that Rees, who is based at the University of Cambridge, claimed credit for breakthroughs in γ-ray astronomy while ignoring contributions made by other groups. De Rújula has worked with some of those whom he says were ignored, including Arnon Dar of the Israel Institute of Technology in Haifa.

The incident raises some difficult issues for ArXiv, admit its administrators at Cornell University in Ithaca, New York. Rees was asked by Dar in September to change the citations, and says that he had originally intended to wait until the paper had been peer-reviewed before doing so. But two days after De Rújula's posting, Rees modified his paper to cite Dar's work.

But if the criticism is seen as successful, other authors could rush to pursue similar complaints. “We don't want these food fights conducted on a regular basis,” says Paul Ginsparg, the Cornell physicist who created ArXiv.

The sharp language used by De Rújula to describe Rees could cause further problems for ArXiv — which doesn't usually check papers before they are posted — if De Rújula's paper were deemed libellous in a court of law. And some experts think that it could be. “The accusations against Rees are potentially defamatory,” says David Mascord, a media-law specialist at DMC Development, a training company based in Brockenhurst, UK.

Postings on ArXiv are deemed to be published and so are subject to libel law. In the United States, libel actions are hard to pursue because of the right to freedom of speech enshrined in the First Amendment to the constitution.

But because the paper has been published on the web, it is subject to the libel laws of any country in which it can be accessed. Under English law, for example, De Rújula, Ginsparg and Cornell University could all be liable if the De Rújula paper were held to be defamatory.

Ginsparg says that he has not taken legal advice over De Rújula's paper, but that he would remove it if recommended to do so by his lawyers. “ArXiv is just a mindless redistribution system,” he adds. “It's not implemented to be a global police force to detect or enforce professional ethics.”