On June 28, 2009, Stephen Hawking sat alone in a college at the University of Cambridge and waited patiently for party guests to arrive, but not one of them showed up. Then he sent out invites.

Sending out invites after a party is a pretty surefire way to make sure your guests don't show up (a little pro tip for all you introverts out there). However, Hawking wasn't looking for any old guests, he was looking for time travelers. 

"Here is the invitation giving you the exact coordinates of time and space," he said, talking about the party in the 2010 Discovery series Into the Universe with Stephen Hawking.

"You are cordially invited to a reception for Time Travelers," the invitation read.

"I am hoping copies of it, in one form or another, will survive for many thousands of years. Maybe one day someone living in the future will find the information and use a wormhole time machine to come back to my party, proving that time travel will one day be possible."

No one did. 

Sending out invites only once the event had passed was a vital element of the experiment; only people who could travel back in time could attend. 

So, why did no one show up?

There are a number of reasons why time travelers didn't show. For example, the invites may not have lasted until time travel was invented, the time travelers might have shown up but Hawking kept it secret to protect the space-time continuum, or perhaps you can only travel back to the point where time travel was invented.

Perhaps time travelers didn't like the look of the party and decided not to go. It's unlikely they wouldn't have heard of Hawking though, as one of the planet's preeminent scientists and science communicators. A Brief History of Time has sold more than 10 million copies worldwide while astronomers are still trying to test his black hole paradox. 

However, Hawking was publicly unoptimistic about what his one-man party meant.

"I have experimental evidence that time travel is not possible," Hawking told Arstechnica. "I sat there a long time, but no one came."

Of course, Hawking knew that his experiment wasn't incontrovertible proof that time travel can't ever exist. He noted that Albert Einstein seemed to offer up the possibility that humans could warp space-time so much that we could travel back in time. Although he also noted this warping could potentially trigger a bolt of radiation that could destroy a spacecraft and possibly even space-time itself. 

"What a shame," he says in Into the Universe with Stephen Hawking. "I was hoping a future Miss Universe was going to step through the door."

An earlier version of this article was first published on IFLScience in March 2018.