Top boffin Stephen Hawking has decided to host a party for time travellers. The only problem is that he has actually held the party four years ago at Cambridge university in 2009 and no one showed up.
Hawking has created a new invitation which will survive for many thousands of years.
"Maybe one day someone in the future will find the information and use a wormhole time machine to come back to my party, proving time travel will, one day, be possible," Hawking said.
The fact that the event has now happened and no one attended would be a fixed point in time. Therefore, if a future time traveller does see the invite now and travels back they will have to change the past.
This act will require a rewriting of physics, because current thinking is that you cannot change the past. It would be possible that some time travellers did see the invite and attend the party and create a parallel universe where this event happened. It would also mean that it would be very difficult for us to prove this every happened.
People assume that time is a strict progression of cause to effect, but actually, from a non-linear, non-subjective viewpoint, "it's more like a big ball of wibbly-wobbly... timey-wimey... stuff", someone else once said. [That is enough leading edge physics.Ed]