'Terrorism" has two faces. There are real threats and real terrorists, and then again there is a realm of nameless fears, vague forebodings and irrational responses. The German federal police seem to have succumbed to the latter: on July 31 they raided the flats and workplaces of Dr Andrej Holm and Dr Matthias B, as well as of two other people, all of them engaged in that most suspicious pursuit - committing sociology.

Dr Holm was arrested and flown to the German federal court in Karlsruhe; he has since been put in (pre-trial) solitary confinement in a Berlin jail. Of course the police may have solid, rational knowledge they are withholding, but their public statements belong to the realm of farce. Dr B is alleged to have used, in his academic publications, "phrases and key words" also used by a militant group, among them "inequality" and "gentrification". The police found it suspicious that meetings occurred with German activists in which the sociologists did not bring their mobile phones; the police deemed this a sign of "conspiratorial behaviour".

Thirty years ago Germany had a terrible time with indisputably violent militant groups, and that leaden memory hangs over the police. And it may well be that "gentrification" is a truly terrifying word. But this police action in a liberal democracy seems to fall more into Guantánamo mode than genuine counter-espionage.

Consider the hapless Dr B a little further. He's not actually accused of writing anything inflammatory, but seen rather to be intellectually capable of "authoring the sophisticated texts" a militant group might require; further, our scholar, "as employee in a research institute has access to libraries which he can use inconspicuously in order to do the research necessary to the drafting of texts" of militant groups, though he hasn't writtten any. The one solid fact the cops have on Dr Holm is that he was at the scene of the "resistance mounted by the extreme leftwing scene against the World Economic Summit of 2007 in Heiligendamm", perhaps mistakenly believing he is studying this scene rather than stage-managing it.

These are not reasons for Brits, any more than Americans, to cluck in righteous disapproval; in the long, sad history of the IRA, reality and fantasy entwined in an ever tighter cord. But, apart from hoping that our colleague Dr Holm will be freed if only he promises to carry his mobile phone at all times, we are struck by the grey zones of fragile civil liberties and confused state power that this case reveals.

The liberal state is changing. In the 60s, Germany had the most enlightened rules for refugees and asylum seekers in Europe; the US passed the most sensible laws on immigration in its history; France granted automatic citizenship to all those born on its territory, including all Muslims. Today all these countries have, in the name of the war on terror, revised their rules - the state of emergency prevails. The laws meant for real threats are invoked to counter shapeless fear; in place of real police work, the authorities want to put a name - any name - to what they should dread. States of emergency are dangerous to the legitimacy of states. In cases conducted like this one, a government stands to lose its authority and so its ability to root out actual terrorists.

If our colleagues are indeed dangerous sociologists, they should be prosecuted rationally. But, as in Guantánamo, persecution seems to have taken the place of prosecution.

· Richard Sennett is a sociologist at the London School of Economics; Saskia Sassen is a sociologist at Columbia University

r.sennett@lse.ac.uk