You may be thinking of the party he hosted on June 28, 2009 for time travelers. He didn't tell anyone about the party beforehand and only sent out invitations after the party was over. He hoped that sometime in the future, if time travel becomes possible, someone would see the invitations and come back in time to attend the party. Unfortunately, no one came to the party (according to him), and he concluded from that that time travel will not ever become possible.
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