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One quantum transition makes light at 21 cm

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Re: One quantum transition makes light at 21 cm

#71

Post by pajko »

diego_sandoval wrote: post #67
belter wrote: post #36

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

But do they know what is a second?

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).

Re: One quantum transition makes light at 21 cm

#75

Post by LeifCarrotson »

raldi wrote: post #37
amiga386 wrote: post #18

> precisely 21 cm Imprecise use of "precise" in the strapline. According to https://en.wikipedia.org/wiki/Hydrogen_line the best measurement of it so far is 21.106114054160 +/- 0.000000000030 cm

That not just imprecise usage of that term; it’s completely incorrect. The correct term would be its exact opposite, “approximately”.

I expect the non-technical author/editor was playing the telephone game and originally wanted to emphasize that the frequency is always the same value, not that the hydrogen emissions frequency is related by arbitrary factors of 9192631770.000 Hz, 1/299792458.000 seconds, and then exactly 21.000/100.000 to the caesium-133 frequency.

Re: One quantum transition makes light at 21 cm

#76

Post by luma »

diego_sandoval wrote: post #67
belter wrote: post #36

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

But do they know what is a second?

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.

Re: One quantum transition makes light at 21 cm

#77

Post by IshKebab »

BurningFrog wrote: post #64
kccqzy wrote: post #10

Amazing article! It seems incredibly to weird to hear about transitions causing photons at 21cm wavelength; I guess I'm only used to seeing (no pun intended) much shorter wavelengths at hundreds of nanometers.

It does feel a little odd that something the size of 5.29×10⁻¹¹ meter can create something 10 billion times larger. I mean, I understand how and why, but it feels odd.

Wavelength isn't an object though. Like if you walk around the world you haven't made something the size of the world.

Re: One quantum transition makes light at 21 cm

#78

Post by teraflop »

belter wrote: post #36

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

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..669...

Re: One quantum transition makes light at 21 cm

#79

Post by Suppafly »

cogman10 wrote: post #3
Suppafly wrote: post #2

I had a CS professor that used to hold up a length of string roughly that length and talk about how that is how far a bit of data can travel at the speed of light during a clock cycle or something. Honestly don't remember the point he was trying to make.

Probably trying to recreate this lecture by Grace Hopper [1] [1] https://www.youtube.com/watch?v=9eyFDBPk4Yw

I'm sure that's what it was. I probably should have remembered that, but it was such a small part of one of his lectures it didn't resonate as deeply as it should have.
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