It's the latest project of the Breakthrough Prize Foundation, a Silicon Valley-funded group that currently gives the biggest prizes of any scientific award. The 2015 initiative (which is still ongoing) focused on the detection of Earth-like planets and alien signals. Breakthrough Starshot will fund the development of a light-propelled nanocraft -- a spaceship just a few grams in size that would fly through space like ships in the sea, with sails pushed by the physical force of light particles instead of by wind. Its destination will be Alpha Centauri, our closest neighboring star system. Once there, it could try to sense habitable conditions -- or even life -- on the planets.
"There's one underlying big theme, which is looking for other life in the universe," Milner told The Post. "But this initiative also has a theme of pure exploration behind it."
For now, the most distant human spacecraft is Voyager 1. The spacecraft is technically in interstellar space -- beyond the edge of our own solar system -- but it's not destined for stardom. It will take 40,000 years for Voyager 1 to make its closest pass of another star, and even that brush will only bring it within 1.6 light-years of the alien sun. And even if humans are still kicking around by then, Voyager 1 will be tens of thousands of years past its expiration date. It isn't equipped to go sniffing for life in a foreign star system in any case, so the spacecraft will only help us find alien life if it pretty much crashes into their back yard and inspires them to come looking for us.
Milner feels that light-propelled spacecraft might be the key. He's not the only one showing a new interest in the whimsical technology, which had a big fan in Carl Sagan back in the day. Bill Nye (the science guy) and the Planetary Society recently funded a prototype called LightSail, and they hope to follow up with full missions reliant on the tiny, light-powered satellites within the next few years.
But to get to the binary stars in Alpha Centauri within one generation of launch, which is the goal of Milner and Hawking, their spacecraft would have to travel 100 million miles per hour. Today's fastest spacecraft max out in the tens of thousands of mph.
Even with $100 million in funding funneled to the best and brightest -- a group that Milner says will be selected on a rolling basis -- scaling up to that kind of speed is quite the long shot. But Milner hopes that Moore's Law -- the idea that computer-processing power will continue to double every two years -- will continue to hold true. In theory, that means scientists with enough funding could create gram-scale computers and sensors for the spacecraft within the next decade.
It's worth noting, however, that many experts say the days of this rapid computing growth are over. It's quite possible that scaling down the size of these components will take decades, if not more.
The new project relies on a couple other Moore's-Law-adjacent moonshots as well. The team hopes that the rapid advancement of nanotechnology will allow engineers to create incredibly light solar sails to carry the craft, for example.
And there's one more thing: Even if Milner's funding can create a spaceship tiny enough to hurtle through space with unprecedented efficiency, there's no way the power of sunlight will push it to the speeds he wants to obtain.
"The sun is definitely not going to work for us here," Milner told The Post. "What you need is a much more focused beam."
His solution? "A very, very light sail and a very, very big laser," Milner said. He hopes the researchers he funds can figure out how to produce laser arrays powerful enough to give the spacecraft enough of a starting shove.
In theory, this is all possible. In practice, it's a long shot in the extreme. Even Milner, who insists that this "is not a crazy idea" admits there are "about a dozen problems that need to be solved." They're big problems, too.
But at least now, scientists and engineers who are interested in developing this technology will have a clear (and potentially lucrative) goal.
"The message that Stephen Hawking and I want to send is that for the first time ever, this is an achievable goal," Milner said. "We can stand up and talk about it. Fifteen years ago, it wouldn't have made sense to make this investment. Now we've looked at the numbers, and it does."
At Tuesday's announcement, Hawking spoke of humanity's need for exploration as a driving force behind the project.
"I believe what makes us unique is transcending our limits," he said. "Gravity pins us to the ground, but I just flew to America."
"Today we commit to this next great leap into the cosmos because we are human and our nature is to fly," he said.
There are certainly worse things than funding smaller, faster computers and thinner, more resilient nano-materials. But at the end of the day, Milner, whose net worth hovers around $3 billion, seems to be motivated most by boyish enthusiasm.
"I just think it would be cool to make it happen," he said.