If author S Frederick Starr could travel back in time to the 11th century, to the era of Ibn Sina and Al Biruni – the two Central Asian polymaths at the heart of his new book – he says he would pick the year 1007 AD when both were residing in Kunya-Urgench, Khwarazm, in modern day Turkmenistan. And, he says, he would like to invite them both to dinner: “We know zero about their interaction there, all that we know is silence, and if I had the chance I would fill that silence.”

Filling the “silence” is something Starr does seamlessly – even without the luxury of time travel. Starr’s previous book in the genre, Lost Enlightenment: Central Asia’s Golden Age from the Arab Conquest to Tamerlane, is a modern classic for those trying to understand the forgotten history of this region, with translations into about two dozen languages, including Uyghur and Korean.

Starr says the book “filled a vacuum”. He adds: “My purpose was simply to acknowledge the existence of this great golden age, and to clarify that this was very specific – this was Central Asia. Yes, there are Turkic peoples there, there are Persianate peoples there, but it isn’t Turkey, it isn’t Iran, it is something very distinct on its own.”

His new book, The Genius of their Age: Ibn Sina, Biruni and the Lost Enlightenment, zooms in much closer into the lives of two of the “greatest thinkers in the world between ancient Greece and Renaissance”.

There is a chapter in the book dedicated to a remarkable (and increasingly heated) series of correspondence letters between Ibn Sina and Al Biruni in which they debated Aristotle, the existence of other worlds, gravity, vacuums in space, evolutionary geology and even the mathematical evidence for elliptical movement of the heavens.

Starr says Ibn Sina and Al Biruni were competitors who “fundamentally disagreed” with each other. “Seven years they were in the same very new capital of a principality, they were there at the same time at the same court, they had to have interacted daily. This is Khwarazm, there is no way they could have avoided it. Even though they had a number of common friends, both of whom wrote about these friends, never mentioning the other guy, of course.”

Al Biruni was an orphan from Khwarazm, where he enjoyed the support of Ibn Iraq, a notable mathematician and astronomer. Ibn Sina, known in the West as Avicenna, was native to Bukhara and was trained in the legal and medical sciences. After early successes in life, both of them lived “extremely difficult lives'', says Starr.

The Samanid empire, under whose patronage the two had flourished, was entering into terminal decline and facing external enemies. “Each of them was condemned to be beheaded. Your average mathematician or chemist or medical doctor may have problems in his career, but being beheaded is not one of them,” says Starr.

Both men were wanted by Mahmud of Ghazni, who was collecting talent for his court and “top of his list were Biruni and Ibn Sina. They were the guys he wanted to get. And he spent a lifetime trying to get them both.” He succeeded in the case of Al Biruni, dubbed Alberonius in Latin, who was taken captive after the murder of his patron, the Shah of Khwarazm. But Ibn Sina continued to elude Mahmud until the end.

Al Biruni and Ibn Sina researched and wrote on a wide array of subjects including logic, astronomy, medicine, geography, mathematics, pharmacology, mineralogy, geometry, metaphysics and mathematics. But Starr says the two have been “sliced up like sausage” by modern scholars: “There is a whole industry connected with Ibn Sina that only is concerned with his metaphysics. Barely touches his medicine, or many other fields he is involved with,” he says. Ibn Sina also had a political career, and even served as vizier, although “none of the records of his public career survive”.

“My point is that these modern experts who know everything about Biruni’s spherical trigonometry, they are not interested in dramatically different aspects of the man,” he says.

Starr, who is founding chairman of the Central Asia-Caucasus Institute and Silk Road Studies Program, travels frequently to the region and is a frequent flyer millionaire.

Years ago, he visited Ghazni, Afghanistan, where the grave of Al Biruni currently lies in a dilapidated state. He has also climbed the famous hill in Nandana, Pakistan, (“a heck of a hike”) from where Al Biruni measured the circumference of the Earth.

“Here he is on a mountain top with the simplest equipment which he had invented and made, and he ends up measuring the diameter and the circumference of the Earth more accurately then anyone had done until the seventeenth century,” he says.

To research for the book, Starr consulted sources in English, German, French, Russian, Latin, Turkish and Uzbek. Although Al Biruni and Ibn Sina were not Arabs, their works were written in Arabic, the language of learning in their era. However, Starr did not study primary Arabic sources: “You can't just take the medieval Arabic, and modern Arabs can't read it, it has to be transformed and edited. I have used that whole body, a century of beautifully translated, trans-edited editions of the Arabic originals.”

Some of the very best writing, particularly on Al Biruni, is from Russia. “I have good Russian, I made extensive use of those resources, which have generally been neglected.”

The book took around four years to write. Starr had to dip into many fields and consult experts. “When I am dealing with astronomy I had to call astronomers and make sure that I got it right. And when I was dealing with metaphysics, I called philosophers dealing with metaphysics.”

Starr says Ibn Sina was the more fortunate of the two, his medical works reaching as far as Europe and India early. His metaphysical theological writings also spread far, and Starr notes the medieval catholic theologian Thomas Aquinas “drew very significantly on Ibn Sina.”

“Ibn Sina created closed systems: ‘I have sorted it out, boom, here it is.’ Al Biruni, by contrast, all his writings are research reports: ‘This may be corrected or changed in a few years, but this is what I am thinking right now.’ And as a result his writings in many ways are astonishingly modern in their form, it is open-ended with him.”

Starr, early in his career, worked as an archaeologist in Turkey. Having that background has helped him. Rather than think of figures like Al Biruni and Ibn Sina in abstract terms, Starr says he wanted to make them “almost tactile” by understanding how they lived, and what type of places they called home.

Despite the fact that three quarters of their works were lost, they both left impressive legacies. Ibn Sina’s Canon of Medicine was the medical standard for half a millennia, in both the East and West. Al Biruni’s work discussed a heliocentric worldview, hypothesised the existence of North and South America, and is considered the father of geodesy and Indology.

“These are very big figures and they deserve a place. If we have a place for the Galileos, the Newtons, the Aristotles, we should have a place for people like this as well,” Starr says.