Thanks to many Zeno's patrons, including these thoughtful replies!
From Mark Young in Canada:
Is her killing grandpa possible?/
Not in /her/ past, clearly -- her grandpa wasn't killed, and so it's impossible for her to be going to had killed him. (I seem to have misplaced my copy of "Tenses for Time Travellers", so please have some mercy on the grammar flames! What I mean is, it's impossible that she, in her personal future, will be killing her grandpa, and that that killing occurred in the past leading to her personal present.)
On the other hand, it's possible (in some logical sense) that in her personal future includes a killing of her grandpa in a time that's parallel to the past leadng to her personal present ("Maybe she's going to have had killed her grandpa.") At some point prior to the killing (probably at the time she arrivesed from the future) the universe split into two streams -- one leading to her birth, and the other leading to her grandpa's death at her hands. In the alternate stream she can then travel forward to see what the effects would be and, if she doesn't like them, go back and kill herself before she kills her grandpa. Depending on what other changes she's made, she might find that travelling forward in time gets her to a third time-stream, or it gets her back to her original present/future (two time streams merge -- perhaps with some quantum indeterminacy about minor events in her past).
Of course, this is already going to have had happened many times.
From Aaron Sloman in the UK:
Regarding your What could I do in the Past?
This is the argument that convinced me many years ago that backward time travel must be impossible.
However, there is something you can do to change the past.
Suppose Guy Fawkes (or modern terrorist) lit a slow fuse connected to some explosives which later blew up a building. At 10am he started the process that caused the building to be destroyed at 11am, after he had escaped.
One of his disciples later tried to do the same with another building, lighting the hour long fuse at 10am.
However, on that occasion a guard came in at 10.30 saw the glowing fuse, and extinguished it. His action at 10.30 caused the disciple not to have started the process at 10am that blew up the second building at 11am.
So you can change the past, but without time travel.
(I think this is related to referential opacity.)
From Mike O'Neill in USA who also added some lines from a play he wrote:
Twenty years ago I wrote and produced a comedy science-fiction radio play titled “Be There Then” (now available in mp3), in the style of the Firesign Theatre, stressing this impossibility of travel to the past by making fun of it.
It’s about a well funded think-tank called the International Inastute for Inapplicable Ideas (or “Four Eyes” for short) that constructed a time machine airplane called “The Procrastinator.” The purpose is to re-capture the youthfulness of the hippie college days.
Below are some pertinent lines from that play. The scene is an introductory briefing given by the captain for the crew before launching the timeship:
Now, unfortunately the way our economy is structured, travel to the future is severely restricted. The price of these chips is heavily regulated in the future to rise so dramatically, that the equipment overtakes its own priced price-cost overruns differential, and this regulatory commission of some really freaked out future generation confiscates and impounds all of our equipment, every time we try to go there.
They say it has something to do with the way we stuck them with all our Social Security bills. So until we work out some kind of treaty with our posterior....posTERity, we are limited to travel in the past. Now, to experience the .. pressure...um.. ah...the.. the PLEASURE of time travel there are a small number of rules to be mesmerized:
NUMBER ONE: You are not supposed to fiddle with the past. This always leads to violins and destruction, and time after time we've seen the warnings provided by Science Faction about this, but somebody always seems to try to get around it. So...
NUMBER TWO: You're not supposed to purchase stocks, bonds, or properties ...without prior permission ...and full partnership approval of The Inastute for Inapplicable Ideas. Fifty percent of all prophets become the property of The Institution and there are absolutely NO REFUNDS. Ah, now... (audience stirs) ...we're (stir- rrr) ...we're not in it...(stirr) ... we're... We're not in it for the Money. We're in it..... for the money we NEED to operate this Institution in a style (sob) and, and (voicecrack) luxury (sigh) that it's been accustombed to. (sob) And i..I'm damn proud of it......So just don't bug me about this money issue.
NUMBER THREE: Ah, your not opposed to transport Federal Lotto Numbers across the Time Barrier. This could get you into a whole lot o'trouble with the U.S. Treasury And Gambling Department.
NUMBER FOUR: .... ATTITUDE.... We've already been touched on this one. We use two primary attitude controllers. The first is the amusement motivators. Um, when the music plays, uh..........uh, when the words are, uh, touched with an emotion. Usually people like sorrow for the past, and....ah a kind of a nostalgia.........And euphoria for the future, a kind of a hope. Ah, anyway when the music plays, you hear the sound to follow, and it's......And the other method to maintain the proper attitude is "Hype Prevention". That's when you use words or slogans like "power to the people", or "be there then", or any of these typical slogans. These help you break out of the present, and ah, by repeating them with heavy breathing. ...ah, ...They...ah...so remember these two controllers, music and hype-preventitilation are very important, and they're j-just critical.
Now, we're just about finished here, so we'll be meeting out on the airstrip, and boarding the Procrastinator, so these final words in summary:
Turning black the sands of time,