Who hasn't wondered what their life would be like today if some past event had turned out differently - that inconsequential decision, for example, that led you to meet the love of your life. Sometimes, small choices change everything. And that is just as true of world history as it is of your personal life.

Time was when the past was seen as a long march towards an inevitable present. But historians have come to realise that the present is anything but inevitable. And so New Scientist asked a panel of experts to speculate on the scientific pasts that might have been. Follow the links below to find out what might have happened if Darwin had not sailed on the Beagle, or Einstein not had his miracle year, as well as many other scenarios. Or read on while Gregory Radick makes the case for virtual history.

What if…the Nazis had won; Newton had abandoned science; electric motors had pre-dated steam engines; Darwin had not sailed on the Beagle; Charles II had no interest in science and a young Einstein had been ignored?

ANYONE who has read Robert Harris's book Fatherland is familiar with the concept of counterfactual history. The idea is simple: pose a "what if..." about the past, in Harris's case, "what if the Nazis had won the war?", and then answer it with a plausible and entertaining account of what might have been.

Counterfactual history might sound like a frivolous exercise, fit only for airport potboilers and lowbrow TV drama-documentaries. But a growing number of historians consider it an indispensable tool, especially for understanding political events. What kind of world would we be living in now, for example, if Al Gore had been declared the winner of the 2000 US presidential election?

But science is another matter. There is no shortage of tantalising what-ifs: what if Newton had carried out his threat to quit science? What if Darwin hadn't sailed on the Beagle? What if Einstein hadn't found a job that allowed him so much time to daydream? The trouble is that until recently, the answer to these questions seemed to be disappointing: science would look much as it does today.

As far back as the 1820s, British historian Thomas Babington Macaulay concluded that science had a life of its own. Once knowledge had progressed to a certain point, he argued, discoveries became inevitable. We could be confident, he wrote in his essay on the poet Dryden, that "without Copernicus we should have been Copernicans, that without Columbus America would have been discovered, that without Locke we should have possessed a just theory of the origin of human ideas."

Macaulay's general conclusion lives on. Who hasn't heard it said that while only Shakespeare could have written Hamlet, somebody else would have come up with evolution by natural selection if Charles Darwin hadn't? (Famously, Darwin thought somebody else had.)

However humbling for individual scientists, this mindset pays great tribute to science itself, by granting it an authority which nothing else in our culture enjoys. If scientists are bound to arrive at roughly the same conclusions whatever the accidents of history, then science must reveal how nature truly is.

Not everyone has been prepared to accept this cosy conclusion, however. In the 1970s and 1980s, some sociologists and historians of science developed an aversion to it, with results that were often subversive. In his well-known 1984 book Constructing Quarks, for example, Andrew Pickering, now at the University of Illinois, suggested that physicists only came to believe that quarks were real thanks to an entirely arbitrary preference for particular types of particle accelerators, detectors and other hardware. Had they chosen differently, physics might be flourishing just as happily - but without quarks.

Such extreme claims unsurprisingly horrify many scientists and, in the 1990s, they precipitated what are now known as the "science wars". These were regrettable on a number of counts, but they had the salutary effect of rousing scientists and science-watchers alike from their dogmatic slumbers concerning counterfactual history. There is now an acceptance that the question of inevitability in science is not an idle one. On the contrary, it goes to the very heart of our basic understanding of what science is, how it has developed, and how much respect it deserves. And with this acceptance has come a willingness to ask what might have been.

That is not to say that science historians are now all beavering away on "what if" questions. At least one of the old prejudices against counterfactual history remains stubbornly in place. This is the idea that we can never really know what might have happened, so it's pointless to enquire. Despite recent rebrandings as "virtual history" or "rerunning the tape", counterfactual history still looks to its critics like so much worthless speculation.

It's undeniably true that we can usually speak much more confidently about what actually happened than what might have. Suppose I state, for example, that on 26 June 2000, the rough draft of the human genome was announced in Washington DC. If challenged, I could produce stacks of newspaper reports, TV clips, official documents and so on, all corroborating this statement. No counterfactual argument can ever be backed so conclusively, so why bother?

But there is, I believe, a very good reason to bother. Showing conclusively that something happened isn't the be-all and end-all of history. Historians must also try to explain the past. And, whether they like it or not, doing so involves asking and answering "what if" questions. It is widely accepted that the rough draft of the human genome was completed when it was because Craig Venter's private project put pressure on the public one. But this claim has a flip side: were it not for Venter, the sequence would have taken longer. And whatever plausible evidence you can produce in favour of the factual claim doubles up as evidence for its counterfactual counterpart.

There's no opting out, then, from counterfactual history. The choice is between engaging it furtively or openly. So, let us now ask, what if Newton had abandoned science? What if Darwin had not sailed on the Beagle? And, of course, what if the Nazis had won the war?