With "A Brief History of Time" (1988), Stephen Hawking wrote a book on cosmology for people who didn't know anything about complicated scientific theories. It became a best-seller, with over 10 million copies sold in 20 years.

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Shortly afterwards, director Errol Morris made a documentary about the physicist's life, taking a look at the famous scientist's childhood and explaining how he landed paralyzed in a wheelchair, and nevertheless persevered as one of the greatest scientific minds of our time.

At the release party for the home video version of Morris' film, Leonard Nimoy, aka Spock on "Star Trek," found out that Hawking would like to appear on the show. Through the actor's intervention, the cosmologist ended up playing a holographic version of himself in 1993, alongside actors representing Albert Einstein and Isaac Newton.

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A trademark sound

Although Hawking could have easily chosen a more fluid-sounding voice over the years, the distinctive computer-based sound developed specially for him in the late 1980s became his trademark.

In 2014, Queen Elizabeth asked him why he used an American accent; the British scientist joked that his voice was "copyrighted" and he couldn't change it.

Hawking was not the only one who identified strongly with his sci-fi voice — so did the entire world, which is why his cameo appearances in different films, TV shows, ads or songs are so easy to recognize.

Strong last message

The University of Cambridge published on Wednesday a video of Hawking's last public speech he had delivered for his 75th birthday, on January 8, 2017.

His inspirational message went viral and was viewed on different social media platforms over half a million times within three hours.

In his speech, he reminds everyone that despite difficulties faced in life, there is always hope; what matters the most is to never give up.