3 ms·

That could be some freaky case of Doppler effect. To rule that out, the aliens could send both 21 cm * pi and 21 cm * e.

• nullc 2y ago
  It's ruled out by the signal being modulated. The question the choice is answering is where do you put a signal where other intelligent minds might look for it, yet which isn't at a frequency where the universe is particularly loud in ways that will make detecting your signal harder.
  • TheOtherHobbes 2y ago
    The signal is always going to be modulated, unless the source is maintaining a position with zero relative velocity to the Earth, or deliberately compensating for same - both of which would be far more impressive as a "hello" than a random-ish number which will always be distorted by orbital and proper motion. Otherwise it's going to have a varying frequency - maybe not by much, and maybe not quickly, but certainly not static.
    • nullc 2y ago
      Fair, I should have said "modulated by something other than obvious physical processes".