Texans Build World's Most Powerful Laser

The problem is that the laser is pulsed, but they give a continuous unit to measure it's power. A Watt is Joules/Second, so a petawatt implies 10¹² joules of energy discharged per second, but that isn't true, that's what it would be if this pulsed laser were continuously running, which would be impossible given the power requirements.

Reading the article they tell you themselves that the energy of the pulse is about 1 joule (not 1,000,000,000,000 joules), but due to the short duration the energy is highly concentrated. Basically it would be like having a petawatt laser that you turned on for a tiny amount of time 10¹³ seconds and then turned it off.

As far as your question regarding how they can know the time it takes when the time is that short, it's accomplished using a technique called mode-locking. Here is a wikipedia article on the subject if you are interested.

http://en.wikipedia.org/wiki/Modelocking

Believe me when I tell you that nobody is guessing anything when it comes to femtosecond lasers. It is an extremely hard trick to pull off and these guys (and gals I assume) have done a great job creating this laser. Don't let some writers need to sensationalize things turn you off to the good science their doing.