3 ms·

In Contact the alien beacon arrives at 4.4623 GHz. Pi times the Hydrogen line...

by belter 2y ago

birdiesanders 2y ago
Oh wow. That’s wild.
diego_sandoval 2y ago
But do they know what is a second?
- pajko 2y ago
  Out of the question, it has no definition which is only related to physics. Well, there's the "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" definition, but this was chosen to match the celestial-based unit related to the Earth's rotation (which does not tell anything to extraterrestrials).
- phkahler 2y ago
  Don't need to. It's the hydrogen wavelength / pi.
- luma 2y ago
  Wouldn’t be required. Take any frequency, Hydrogen in this case, and multiply it by pi which is unitless. The resulting frequency is pi times whatever you started with no matter how you count the passage of time.
teraflop 2y ago
Yup. And interestingly enough, that detail wasn't in the original novel by Carl Sagan. It was added for the movie, based on (AFAIK) a 1993 paper by David Blair and Marjan Zadnik: https://articles.adsabs.harvard.edu/pdf/1993A%26A...278..669B https://articles.adsabs.harvard.edu/pdf/1993A%26A...278..669...
nine_k 2y ago
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".
zem 2y ago
"ringworld" had a nicely poetic passage about how the 21cm band had been swept clean by all the hydrogen in the universe and was therefore a natural frequency for aliens to try establishing communication over
WithinReason 2y ago
Intelligent aliens would use Tau, not Pi :)

In Contact the alien beacon arrives at 4.4623 GHz. Pi times the Hydrogen line...

by belter 2y ago

In Contact the alien beacon arrives at 4.4623 GHz. Pi times the Hydrogen line...

birdiesanders 2y ago

Oh wow. That’s wild.

diego_sandoval 2y ago

But do they know what is a second?

pajko 2y ago
Out of the question, it has no definition which is only related to physics. Well, there's the "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" definition, but this was chosen to match the celestial-based unit related to the Earth's rotation (which does not tell anything to extraterrestrials).
phkahler 2y ago
Don't need to. It's the hydrogen wavelength / pi.
luma 2y ago
Wouldn’t be required. Take any frequency, Hydrogen in this case, and multiply it by pi which is unitless. The resulting frequency is pi times whatever you started with no matter how you count the passage of time.

teraflop 2y ago

Yup. And interestingly enough, that detail wasn't in the original novel by Carl Sagan. It was added for the movie, based on (AFAIK) a 1993 paper by David Blair and Marjan Zadnik: https://articles.adsabs.harvard.edu/pdf/1993A%26A...278..669B https://articles.adsabs.harvard.edu/pdf/1993A%26A...278..669...

nine_k 2y ago

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".

zem 2y ago

"ringworld" had a nicely poetic passage about how the 21cm band had been swept clean by all the hydrogen in the universe and was therefore a natural frequency for aliens to try establishing communication over

WithinReason 2y ago

Intelligent aliens would use Tau, not Pi :)