No ET! Ever?

[Jehanne]

Power doesn't require complete seconds.   There is nothing sacred about one second.  We talk about and work with power of event that occur on duration vastly shorter than one second all the time.   The power of a bomb, for example.  The shortest duration of fast gamma ray bursts probably have more to do with physical dimension of the region in which the gamma ray burst event occurs.  It's hard to get an radiation event to occur over less than 2 seconds if the transmitting mechanism, like an accretion disk around a black hole, spans 2 light seconds.  The transmitting mechanism of communication transmitter can be just a few millimeters or centimeters across.

There is no reason I can see why detecting ultra short pulses over open broadcast is physically impossible.  Perhaps the technology to do so hasn't been refined yet for really short bursts.  But given the degree to which we've improved our observational techniques and  sensitivity over the last 25 years when motivated by the search of exoplanets, it seems dangerous to even assume such technology as detecting 10e-18 second pulses over open broadcast can't be perfected in just 2-5 years years should the funding for the effort becomes available.   Granted visible light pulses will stretch out and lose peak intensity due to interstellar scattering.  But that effect ought to be small within a few light years.

It may be better to express things simply in joules.  Fact is that stars are basically point sources which is why they, unlike planets, twinkle on clear nights (planets, unlike stars, have angular size on the night sky); this fact, called scintillation, is going to impact any source of light, and so, assuming that you could produce a narrow, coherent form of laser light and aim such perfectly, it is virtually certain that such will get scattered somewhere in the interstellar medium.