No ET! Ever?

[Jehanne]

But, they would be pointing it at a moving target that is going at 675 times the speed of sound, and then, they would also have to track that target across intergalactic space.  In addition, the radio signal, even if it was a laser, could end-up being scattered away by the zillions upon zillions of atoms, molecules, dust particles, etc., scattered throughout intergalactic space.  In addition, the energy requirements, given the inverse law, would still be enormous, likely, the entire energy output of the US over the course of a year, assuming, that ET wanted their signal to picked-up by something other than the Arecibo telescope, which means, of course, that Arecibo would need to be looking at the right spot at the exact time.

P.S.  Arecibo is not equipped to pick-up laser light.

Your moving target over interstellar distances is effectively static; the stars are in pretty much the same positions as when they were observed by the Greeks / Ancient Chinese.

That's false:



And, the above is not accounting for the proper motion of stars throughout the Galaxy.  And, so, if you wanted to send a message from 100 light-years away using a directional antenna, here is the formula that you would use:



After that, you (or I), am going to have to convert lumens to watts:



Assuming 15 lm/watt, here is what I get:

1 light-year = 9460730472580800 meters

100 light years = 946,073,047,258,080,000 meters

1,000,000,000 watts = 15,000,000,000 lumens

D ^ 2 = 8.950542107481892730061252864e+35 meters squared

B = L / (4 * pi * D ^ 2)

   = 1.333619862188475648988125129197e-27 lumens / meter squared

   = 8.8907990812565043265875008613135e-29 Watts / meter squared

Sensitivity of Arecibo radio receiver:  10−26 watts per square meter.

And, so, Arecibo may be able to pick-up a very strong signal if it was looking at the right spot and the exact moment in time.