Stephen Hawking recently gave a party - for time travellers.
But while he told plenty of people about the date, sent out invitations and waited patiently for them to arrive, no body came.
Of course that might have been because he waited until the party was over to send out the invites.
Professor Hawking explained his failed experiment in a recent interview with various journalists, written up by Ars Technica.
"I have experimental evidence that time travel is not possible," he said.
"I gave a party for time-travellers, but I didn't send out the invitations until after the party. I sat there a long time, but no one came."
Hawking has previously spoken about the party, which was 'held' on 28th June 2009, and produced a video about the experiment - but due to the laws of causality, no one has retrospectively showed up.
Hawking also said that Einstein's theories offer the possibility of travelling backwards in time - but "it is likely that warping would trigger a bolt of radiation that would destroy the spaceship and maybe the space-time itself".
In the interview Hawking was also asked about alien life - and reassuringly said that it isn't likely aliens are coming to visit.
"I'm discounting claims that UFOs contain aliens. Why would they appear only to cranks and weirdos?" he asked.
"Do I believe that there is some government conspiracy to conceal the evidence and keep for themselves the advanced technology the aliens have? If that were the case, they aren't making much use of it.
"Further evidence that there isn't any intelligent life within a few hundred light years comes from the fact that SETI, the Search for Extra Terrestrial Life, hasn't picked up their television quiz shows. It is true that we advertise our presence by our broadcast.
"But given that we haven't been visited for four billion years, it isn't likely that aliens will come any time soon."