Media playback is unsupported on your device Media caption Stephen Hawking on God, artificial intelligence and mankind's future

Stephen Hawking's fame was founded on the research he did on general relativity and black holes. But he often stepped outside his own field of research, using his recognition to highlight what he saw as the great challenges and existential threats for humanity in coming decades. His pronouncements drove headlines in the media, which sometimes proved controversial.

Leaving Earth

Hawking was clearly troubled that we were putting all our eggs in one basket - that basket being Earth. For decades, Hawking had been calling for humans to begin the process of permanently settling other planets. It made news headlines again and again.

Hawking's rationale was that humankind would eventually fall victim to an extinction-level catastrophe - perhaps sooner rather than later. What worried him were so-called low-probability, high impact events - a large asteroid striking our planet is the classic example. But Hawking perceived a host of other potential threats: artificial intelligence, climate change, GM viruses and nuclear war to name a few.

In 2016, he told the BBC: "Although the chance of a disaster to planet Earth in a given year may be quite low, it adds up over time, and becomes a near certainty in the next thousand or 10,000 years.

He was confident that humans would spread out into the cosmos by that time (given the chance), but added: "We will not establish self-sustaining colonies in space for at least the next hundred years, so we have to be very careful in this period."

Here, Hawking's views dovetailed with those of entrepreneur Elon Musk, another science superstar whose cogitations attract widespread attention. In 2013, Musk told a conference: "Either we spread Earth to other planets, or we risk going extinct. An extinction event is inevitable and we're increasingly doing ourselves in."

Image copyright SPL Image caption Artwork: Hawking said that settling other planets would mitigate the risk of human extinction

In line with his thoughts on the matter, Hawking also attached his name to a project researching technologies for interstellar travel - the Breakthrough Starshot initiative.

Rise of the machines?

Hawking recognised the great opportunities that arose from advances in artificial intelligence, but also warned about the dangers.

In 2014, he told the BBC that "the development of full artificial intelligence could spell the end of the human race".

Hawking said the primitive forms of artificial intelligence developed so far had already proved very useful; indeed, the tech he used to communicate incorporated a basic form of AI. But Hawking feared the consequences of advanced forms of machine intelligence that could match or surpass humans.

Media playback is unsupported on your device Media caption WATCH: Prof Murray Shanahan provides a brief introduction to AI

Some academics thought the comments drew on outdated science fiction tropes. Others, such as Prof Bradley Love, from UCL, agreed there were risks: "Clever AI will create tremendous wealth for society, but will leave many people without jobs," he told The Conversation.

But he added: "If we are going to worry about the future of humanity we should focus on the real challenges, such as climate change and weapons of mass destruction rather than fanciful killer AI robots."

Tipping point

The Cambridge physicist regarded global warming as one of the biggest threats to life on the planet. Hawking was particularly fearful of a so-called tipping point, where global warming would become irreversible. He also expressed concern about America's decision to pull out of the Paris Agreement.

"We are close to the tipping point where global warming becomes irreversible. Trump's action could push the Earth over the brink, to become like Venus, with a temperature of 250 degrees, and raining sulphuric acid," he told BBC News.

Image copyright Getty Images Image caption Hawking was in plentiful company when warning about the threat from climate change

The UN's Intergovernmental Panel on Climate Change (IPCC) also highlights the potential risk of hitting climate tipping points as temperatures increase - though it also emphasises the gaps in our knowledge.

However, Hawking was in plentiful company in regarding global warming as one of the great challenges of centuries to come.

Shhhh, keep it down

There's a whole field of science, known as Seti (The Search for Extra-Terrestrial Intelligence) dedicated to listening for signals from intelligent beings elsewhere in the Universe. But Hawking cautioned against trying to actively hail any alien civilisations that might be out there.

In 2010, he told the Discovery Channel that aliens might simply raid Earth for resources and then move on.

"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said.

"We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."

Image copyright SPL Image caption Should we broadcast our presence to aliens - or just listen out for them?

At the time, Seth Shostak, from the Seti Institute in California, told the Guardian: "This is an unwarranted fear. If they're interested in resources, they have ways of finding rocky planets that don't depend on whether we broadcast or not. They could have found us a billion years ago."

But others saw the logic in Hawking's comments. Ian Stewart, a mathematician at Warwick University, commented: "Lots of people think that because they would be so wise and knowledgeable, they would be peaceful. I don't think you can assume that."

Controversial headlines

The media attention gave him an unprecedented platform. But some in the scientific community were occasionally less enthusiastic about the resulting headlines than the journalists who wrote them.

Indeed, I've been asked in the past why the British media seemed to hang on Hawking's every word.

Prof Sir Martin Rees, the Astronomer Royal, said: "He had robust common sense, and was ready to express forceful political opinions.

"However, a downside of his iconic status was that that his comments attracted exaggerated attention even on topics where he had no special expertise - for instance philosophy, or the dangers from aliens or from intelligent machines."

But many would also argue that, beyond individual statements or headlines, Hawking had a unique ability to connect with the public.

They would say that the "hype" this sometimes generated was an inevitable by-product of his household name status. Instead, we should focus on a greater good - his ability to bring science to the attention of people who might otherwise never have given it a second thought.

It's testament to his success as a communicator that the mourning for this champion of rational thinking extends far beyond the scientific community.

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