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Can anyone explain the 21cm line?


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Hello!

Ive been Wikipedia surfing over the last few weeks and stumbled across lots of stuff about the plaque Carl Sagan designed for Pioneer (and it was used again on the voyager record). What’s really caught my interest is the little diagram that shows the hyperfine transition of hydrogen (always looking for tattoo ideas...) and I’ve been trying to understand it ever since. I get that it is significant but every explanation I read quickly gets too technical for me. Does anyone know of an idiots guide anywhere? Or is anyone good at explaining this sort of thing? I’m not totally daft, but physics for me stopped at GCSE level so anything beyond that gives me a headache! I have a feeling that learning this stuff will help explain a few things about solar viewing too!

What I understand so far is (feel free to jump in!)

  • The universe is 90 per cent hydrogen.
  • In its normal state we have one proton and one electron - that’s H1
  • if it is ionised (help please!) then it is H2 - and that’s the stuff we can see with the naked eye.
  • The electron spins as it orbits the proton. The proton spins too.
  • When they both spin in the same direction, they use a smidge more energy than if the proton and electron spin in opposite directions.
  • so eventually the electron will gets its act together and start spinning the other way.

At this point it gets a bit fuzzy - what I’m reading is that this transition causes the atom to emit a very specific wavelength 1420 or 21cm, but is it the transition that causes this wavelength to be emitted or is it emitted after the transition and then constantly ongoing? My understanding is that everything has a wavelength but other than that it all gets fuzzy. And the implications for what any of this means or why it matters or why or how we know or are interested in it is totally beyond me! And why is this so important that it deserved a spot on Pioneer?

Any help gratefully received! Feel free to dumb it down!

Niall

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Hi Niall, you seem to have most of it pinned down.

When an electron in an atom goes from a higher energy state to a lower one it emits energy in the form of light (a photon). In the case of the 21cm line it is a low energy radio wavelength photon.  It is the precise energy difference in the two hyperfine levels that determines the wavelength.  

The reason it is important is firstly any advanced civilization would know about it and secondly neutral hydrogen is very common in our galaxy but apart from the 21cm emission is very hard to spot. By mapping the strength of the 21cm emissions astronomers were able to map out where it is in our galaxy.

Regards Andrew 

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Maybe worth adding that the transition emits one photon, so it is not ongoing. However, in a large cloud of atomic hydrogen (so not ionised, but hot enough that it has not formed molecular [H2] hydrogen, which is hard to detect - carbon monoxide is used as a proxy) there is enough of this spin flipping going on to emit a continuous stream of radiation at the 21cm wavelength.

Billy.

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On 10/21/2017 at 13:20, andrew s said:

The reason it is important is firstly any advanced civilization would know about it and secondly neutral hydrogen is very common in our galaxy but apart from the 21cm emission is very hard to spot.

In this context what you say is true !

But in  another context ( Drake and SETI et.al.) there was a naive assumption that "we" all knew it and thus "we" would all be listening on this frequency for "others" to be calling us (or for ET to be calling home on it ! **)  ie a lot of intelligent non-stochastic traffic.  Ha! 21cm is weak but is incredibly more noisy than other parts the spectrum.

They, more intelligent than us? would chose more efficient means to communicate.  ??  Unless they were dangling bait, luckily they have failed so far , , , ,  until they find one of Sagan's missives   :)

** a bit like in the olden days when all who were anybody were "calling CQ gud,bud" on 28MHz before moving to perceived better pastures like 2m and other places to escape quarantine zones like designated CB bands

 

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