What if time travel is only possible once? Imagine the multiverse. All possible histories. In a possible universe where time travel is invented it is used, using time travel alters the past. The past conditions are necessary for the invention of time travel. Time travel when used immediately facilitates all the possible uses of itself and changes the past so much that it deletes itself.

You can use it, but in doing so you remove the conditions for creating time travel. You can use it once and then you and your entire timeline cease to exist. In such conditions I might reserve the use of time travel for things like preventing an unlivable hellscape and not waste it on attending a party with balloons.

> experimental evidence that time travel was not possible

This is making a large assumption, of course, that time travelers had any interest in attending his party, or that they even would have heard about it.*

* I do not believe in time travel.

The first person to arrive was hawking himself in his patented timetraveling wheelchair, advising himself to not tell us that time travel is possible.

Then a bunch of other people arrived and they had a wild party.

Bebe Williams published this idea in issue #14 of Strange Magazine (1994) and has published the book "Instructions for Meeting Time Travellers"

https://www.artcomic.com/timetravel/intro.html

A time traveler probably would not want to risk exposure, as most governments would want to capture and torture him for information. An interesting question would be, could an event be set up that allows a time traveler to reveal himself while retaining some form of plausible deniability?

An incomprehensibly silly publicity stunt!

Someone was there to take video and put up the balloons.