Let's assume ...

[Tender Branson]

there's a time machine that brings you back to "NYC - Sept. 10, 2001" in the next 5 seconds.

When you arrive, you are as old as you are now and have the knowledge that you have now at this moment (no further research before time transport possible).

What are you doing ?

[Tender Branson]

It's very interesting, because if you imagine you had to go there in the next 5 seconds without further research, what do you remember about that day ?

I can't even remember anymore what the flight numbers where, such as UA 175 or something.

Maybe AA 77 ? I have no clue anymore about the exact numbers.

Would someone believe you if you call the police in NYC to warn them ?

What if the airport is actually searched, but none of the hijackers are found ?

...

In another scenario: What happens if I take a flight to Austria and meet myself as a 14 year-old ? That would be weird too (and probably not even possible).

[Mr. Morden]

I'm a believer in the Novikov self-consistency principle, so I wouldn't try to prevent the hijacking, because any attempt would be doomed to failure.

[Donerail]

Nobody'd take you seriously; you're just another crazy shouting on the street. Best case scenario you creep out a few people who work there and they take a day off.

[Mechaman]

I would do nothing.

Why?  Because it would take a lot more than one day in advance to prevent something like 9/11.  And even if I tried to end it, people would think I'm crazy and I'm much more likely to end up in Bellevue by the end of 9/10 then I would at the Pentagon with full details of the future attack.

[CLARENCE 2015!]

Chilling question- I remember United 93 and American 77 so would warn those...I would try to get in touch with John O'Neill who was the former FBI counterterrorism honcho and worked as WTC Director of Security...I'd phone the Pentagon to warn them and would make as big a public scene as possible. Sure- I'd be thought a loon but if any one listened they wouldn't go to work or board a plane that day and I could hopefully save some lives

[Tender Branson]

Calling the police or making a scene in NYC after arriving from the time-travel-portal would probably be a big problem, yes. I fear that the mental facility people would arrive very soon and take me to the mental facility and stuff me with drugs.

The 2 planes that crashed into the WTC started from the Boston airport, right ?

I have thought about the idea to go to Boston Airport and try to see if there are Arab-looking people at the gate.

But if you have only 5 seconds time before you are transported to NYC on Sept. 10, 2001 - where do I get any money from when I arrive in NYC ? I don't carry a wallet with me all the time. So how do I get to Boston without any money ? Hitchhiking ? How long does this take ?

So, if I'm at Boston Airport, what to do ? Call the Airport from a phone cell that there's a (hypothetical) bomb I planted at the gate of the hijackers, so that this gate is evacuated ?

But then again, calling involves money again which I don't have but maybe I have convinced my hitchhiking drivers to give me some, or they have secretly called the mental facility people too and I'm in the mental facility again, stuffed with drugs ...

[Simfan34]

I'd start jumping in front of the nearest television camera and start shouting names- but I'd probably get two or three "Mohammed Atta, Kalid Sheik Mohammed", say "flight eleven, flight 125, flight 93", mention the airports - Boston, EWR, before being carried. off. I'd hopefully make the news and scare off the terrorists.

I get arrested, and tell the police all I know. Hand over my iPod, at least my driver's license, and my phone because it's proof of advanced technology. I'd hopefully have them call over my dad and prove to him I am his son- which makes no sense as I am supposed to be 7. Hopefully I can convince them something's up, and because I've delayed the attacks, they won't happen and the conspirators can be arrested.

[Tender Branson]

Hand over a newspaper from 2012 or at least my driver's license.

Probably not possible. Remember: You only have 5 seconds left until you are transported to NYC.

Unless you are working in an office with newspapers nearby or you are at home with a newspaper.

...

The good thing is, I mostly have my mobile smartphone with me, which is a model that from 2001-perspective looks quite futuristic. Maybe the 2001 police would believe me if I show them my 2011 or so smartphone ...

[Simfan34]

Hand over a newspaper from 2012 or at least my driver's license.

Probably not possible. Remember: You only have 5 seconds left until you are transported to NYC.

Unless you are working in an office with newspapers nearby or you are at home with a newspaper.

...

The good thing is, I mostly have my mobile smartphone with me, which is a model that from 2001-perspective looks quite futuristic. Maybe the 2001 police would believe me if I show them my 2011 or so smartphone ...

Yeah, I changed that. As of now I have my phone in my pocket, while old it's still not what they have in 2001. I have my iPod, so that would work too. If I had 5 sec, I'd grab the nearest person's phone, which is probably newer.

I have $20 dollars in my pocket so I'd take a subway up to Rockefeller Center and shout in front of the Today Show cameras- the goal is to disseminate in front of the widest possible audience and in lieu of the social networks, get attention.

[Tender Branson]

I'd hopefully have them call over my dad and prove to him I am his son- which makes no sense as I am supposed to be 7.

Question to our scientists here:

Aren't there theories that if you meet your younger self in the past through timetravel, the whole timeframe collapses and the universe stops to exist ?

[afleitch]

It’s a tough one really. If you went to the police they would ignore you anyway and haul you in. Since you couldn’t stop it you’d be the prime suspect. All you could say is that you are time traveller, therefore you awould be considered to be mentally ill. You might get listened to if you can make generic predictions that turn out to be true. Given that you have no way of knowing if a world without 9/11 would be better or worse than the one we now have, I would therefore let it happen. Out of curiousity I may even watch. If I’m stuck in 2001, I’d make the most of it and try and make some money out of knowing what happens in the future.

[Simfan34]

I'd hopefully have them call over my dad and prove to him I am his son- which makes no sense as I am supposed to be 7.

Question to our scientists here:

Aren't there theories that if you meet your younger self in the past through timetravel, the whole timeframe collapses and the universe stops to exist ?

I'd just meet my dad. If not him then my mother who was in the city then, or a few uncles who also work in the city.

And here's a script.

The TODAY SHOW, September 10, 2001

Matt Lauer: And so we all like dogs, but the newest-

Simfan34: WAIT!

Matt Lauer: What?

Simfan34: Yes. Yes. Please listen. I mean you no harm. There are people planning an attack on the US. Al-Qaeda. The people behind the Cole attack and the embassy attacks and the WTC attack. Do you hear me, Mr. Lauer?

Matt Lauer: What?

Simfan34: Do you hear me? Please. Please. You can save thousands of lives. Please.

Matt Lauer: Okay, okay, I hear you!

Simfan34: Okay. Please call the cops. They should arrest me. But they plan to fly planes into buildings. The WTC and the Pentagon. There are 17 conspirators- I can think of two, Mohammed Atta, and Kalid Sheik Mohammed. They will use flights 125, flight 77, flight 93, and flight 11. American Airlines and United Airlines.

Matt Lauer: What?

Simfan34: I forget which is which. I can tell you flight 93 is United Airlines- it didn't make it to its target, but it would be hijacked. The planes will leave from Boston, Newark and Washington. Please listen to me. Please arrest these two men and their co-conspirators. Otherwise thousands of Americans will die tomorrow, and thousands more in the years to come.

Matt Lauer: What do you mean?

Simfan34: There will be a war, of course. But I can tell you one more thing: there are no weapons of mass destruction in Iraq. Remember this, and we can save even more lives. There is no reason to invade Iraq. Please arrest me now. Yes, and Enron is cooking its books. It will soon collapse.

And I go to quietly stand in a corner, with all looking on terrified, but perhaps they would ask questions? Of course my knowledge of what is to come will be butterflied away. I can only hope they bother to keep on filming the entire time, or someone else is.

[Erc]

Even if you do manage to stop the Boston planes (whether by creating a security scare of your own, getting lucky and killing Atta yourself, etc.), it's probably much too late to stop the plan as a whole; there are still the two other planes, and I don't know if they could have been called off.

And without the two Boston planes, you run the risk that the passengers of United 93 won't be tipped off as to what is happening, and the hijackers on that plane may successfully complete their mission and destroy the Capitol or the White House.  Of course, presumably many fewer lives will be lost (even if the geopolitical result is likely the same), so it's still worth it to try.

Practically...I have a leg up on a bunch of you since I lived in the NYC area in 2001.  This means I can probably go home and hopefully convince my mother to give me whatever resources I need to carry out some plan (I believe my father was out of town on September 10/11, and not working in 7 WTC as he was the previous week).  Of course, this will definitely freak her (and my younger self) out, but small price to pay.

[Erc]

I'd hopefully have them call over my dad and prove to him I am his son- which makes no sense as I am supposed to be 7.

Question to our scientists here:

Aren't there theories that if you meet your younger self in the past through timetravel, the whole timeframe collapses and the universe stops to exist ?

Only according to the Doc Brown school of time travel.  It's not like the universe knows what a "person" is.

[Tender Branson]

I'd hopefully have them call over my dad and prove to him I am his son- which makes no sense as I am supposed to be 7.

Question to our scientists here:

Aren't there theories that if you meet your younger self in the past through timetravel, the whole timeframe collapses and the universe stops to exist ?

This actually reminds me of the "Sliders" episode called "Guardian":

http://www.thinkwatchthink.com/sliders/guardian-sliders-episode-review.html

In this episode, the 12-year older Quinn meets his 12-year younger self and the younger Quinn doesn't recognize the older Quinn.

I wonder how this would be with myself. If I go back to 2001 and visit my 14 year old self, would he (or I ?) recognize the 24-year old me ?

I guess he (I ?) would ... Looks havn't changed that much and I probably even talk the same way.

[Torie]

I would seek an interview with the FBI, and hopefully impress them with enough detailed knowledge, and that while the whole thing seems quite impossible to believe, I also seem quite sane (it probably helps to be a rather stolid attorney here), and hopefully they will check out some of the details, and when they begin to match ... well maybe that will get their attention. At the very least, when the first tower is hit on schedule, one would think the second at least would be emptied immediately. And at that point I would become a suspect.

[Tender Branson]

I would NEVER EVER talk with the FBI, because I cannot rule out that it was an inside-job by the Bush regime. So, if I talk with the FBI first they are the first to know and the first to liquidate me.

[Jackson]

Truthers itt.

[Mechaman]

These are cute fantasies of yours gentlemen.

HOwever, I think that Clarence's idea is probably the most workable, and that's only because he has the association to pull it off.  The rest of us, however, are just random young men who hang out on a political forum all day.  ONe of us goes back in time and instantly starts trying to prevent 9/11 a day before it happens we aren't going anywhere.

Assuming your time machine is one hundred percent safe and doesn't molest your person, sure you could bring your smartphone with you.  And let's just assume, for some reason, it is in perfect working order.  That it even goes backward in time to 2001.

Problem: 3G networks were in their infancy and you are more likely to only have the more limited 2G networks available.  While the ability to surf the internet on your phone without a telephone hook or digital cable may WOW! the average person, to higher ups in the industry and government it won't come as a surprise.  Given that a lot of smartphones rely on a large wireless network system or Wifi for most of their functions, it is unlikely that it would function at a high enough level to convince people you're from the future and not just a publicity seeking spokesperson for a tech industry company who is trying to raise sells through unconventional means (ie, claiming to be from the future and having a cell phone that looks like something from Star Trek).
And this is without touching the possible paradoxes of you having the same area code and phone number as your younger self (if you had a phone at the time).
Or worse yet, you could possibly be a Ruskie or a Chinese secret agent who is trying to get into the US Government to subvert it through a genius cover story.  After all, the Russians aren't the most open about their technological prowess and it would be conceivable to a few people that the Russians could secretly be developing long range communications devices that would allow them to do everything from find secret entrances to the Pentagon to hacking it's mainframe computer wirelessly to shut down our missile defense system.  The fact that you're Anglo Dutch French Irish American, your driver's license (which might not even resemble the ones available at the time) says you're from Tacoma, Washington, you speak impeccable northwest Pacific Coast American, and you seem to struggle even thinking of a Russian word to say doesn't discount the theory.  After all, it isn't uncommon for Americans to be recruited into the Russian secret service industry.
YOu are more likely going to end up like Chekov did in Star Trek IV: The Voyage Home than you are a great prophet.
Showing up with a futuristic looking phone, claiming that Al-Qaeda is going to attack the Twin Towers with the planes themselves, and then topping it off with how you know because you're from the future isn't going to win many over.

Sure, these assumptions that you might be a tech spokesman or a Russian spy may seem far fetched, but they would be more believable than "oh, he came from the future!"  Seriously, what you would be saying would be as unbelievable as a Jewish man who speaks Aramaic and can perform miracles as being "the Messiah" in 30 AD in Palestine.
And like that episode, you are more likely to end up being considered a great subverter worthy of punishment rather than a bringer of truth.  However, unlike Jesus of Nazareth, you would at least be vindicated a very short time later, maybe even a few hours (depending on the painfulness of interrogation at the time) as the planes DO fly into the Towers.  The only problem here is that now people know time travel is possible..........and absolute chaos occurs in the scientific community at this revelation.

But....BUT let's assume that you are somehow successful and that people somehow listened or that Simfan somehow made it past the security in the NBC building and made a national address and wasn't entirely ignored as a crazy and the government took those threats seriously.  Turns out, some guys really did try to hijack some planes to initiate a plot on September 11th, 2001.  A few possible end scenarios:

1) For all the government knows you could've been in on the operation but after a falling out with the terrorists decided to spill the beans on the conspirators.  While the terrorists might deny your involvement this might not happen, given that they want to avoid humiliation and will probably claim that "grand treachery" prevented them from carrying out their plot.  In other words, the same people you put behind bars will likely be the only people (at the time) who claim to know who you are.  You are likely not to get punished (given that you helped reveal the plot), but if you can't go back to the future you are likely to be regarded with a good deal of societal discrimination as a "terror opportunist", etc. etc. etc.

2) The government concludes that you are indeed from the future.  And, if you went public, the US public now knows too.  This results in a large amount of psychological disturbances as Americans, and the entire world, now know that not only is time travel possible, but that it is possible to alter the future.  Congratulations, you have, by yourself, unleashed a new era of hysteria as people left and right get paranoid about the possibility of time travel happening or already happening that could result in a negative world.  But hey, you saved 3,000 people!  Or, maybe even 3 million people!  Must feel good, right?

3) You are successful in your plans.  However, the lack of 9/11 leaves a vacuum for an even larger and more horrific event that no one saw coming.  A world wide pandemic that claims billions, a nuclear attack on a major urban area by Al-Qaeda, a third World War between the US and China leaves the world in ruins, ends up happening instead of a war in Afghanistan and a misguided war in Iraq.  By 2010 the population of the United States is 20 million, a fraction of what it was just eight years earlier after you stopped the terrorist attack.  The Earth has entered a dark stage of humanity.

So yeah, I would probably just take a few photos of the Twin Towers and then call it a day.

[TJ in Oregon]

I would NEVER EVER talk with the FBI, because I cannot rule out that it was an inside-job by the Bush regime. So, if I talk with the FBI first they are the first to know and the first to liquidate me.

What?!? You're a 911 Truther?

[Torie]

I would NEVER EVER talk with the FBI, because I cannot rule out that it was an inside-job by the Bush regime. So, if I talk with the FBI first they are the first to know and the first to liquidate me.

Seek help.

[© tweed]

get the Hell out of Manhattan, bet against the stock market.

[Mechaman]

get the Hell out of Manhattan, bet against the stock market.

^^^^^
This also might work.

[Tender Branson]

I would NEVER EVER talk with the FBI, because I cannot rule out that it was an inside-job by the Bush regime. So, if I talk with the FBI first they are the first to know and the first to liquidate me.

What?!? You're a 911 Truther?

I would NEVER EVER talk with the FBI, because I cannot rule out that it was an inside-job by the Bush regime. So, if I talk with the FBI first they are the first to know and the first to liquidate me.

Seek help.

Not really. I just said: "I can't rule it out" Can you ?

But I'm "lean-terrorists-are-responsible".

Just like being an agnostic, you cannot say there's a God, or not.