If you could go back in a time machine, what would you change?

The following is an excerpt from a novel-in-progress with the working title An American History (From the Perspective of a Time Traveler).

If you could go back in a time machine, what would you change? This is a question humans have pondered for a long time.  If you asked me that when I was a younger man, I would probably talk about a girl who I liked in college who I never had the guts to ask out.  I would have wanted to go back and ask her out.

I conducted an informal poll of the residents of my hometown of Fullerton, and the most common answer to the question, "If you could go back in a time machine, what would you change?" is "9/11."   By this, they are referring of course to the terrorist attacks on the World Trade Center in NewYork City on September 11th, 2001.  When I ask them how they would change things, their answers vary.  Some would call the White House, or major airlines, and warn them.  Others would call the victims and tell them not to board the planes.  Still others would board the planes and physically beat the shit out of the terrorists.  Would any of these things have worked?  It's an interesting metaphysical question.

I too would like to go back to 9/11, but I would like to go back to September 11th, 2008, because that was the day before my favorite writer, David Foster Wallace, hanged himself.  I would like to go back and tell him how much he changed my life, saved it even.  And even if I couldn't stop him from offing himself, I could at least tell him what he meant to me.

The truth is, even though time travel is possible, you cannot change the past, only the future. 

The reason you cannot change the past is because, the way time travel works, is that you are not actually present in the past.  You are only seeing the distant lights and shadows of the past.  Sort of like how Ebenezer Scrooge could look upon Christmas past, but was powerless to change anything.  He was more like a ghost, a detached observer.  The best Scrooge could do was take his newfound knowledge of the past and let it change how he behaved in the present, which would, in turn, change the future.  

Time travel can be very frustrating in this way.