[djmacarro]

I don't know much about about science, the humanities have always been my field, so if someone could explain this to me simply that would be great:
How is one supposed to be able to translate the Plaque on Pioneer 10 & 11 by understanding the "Hyperfine Transition of Hydrogen"?

[Martin]

someone else will have to give you a more complete answer, I will just start the ball rolling

you know that the sun is mostly made of hydrogen with small percentage of helium and heavier elements

likewise most of our galaxy is hydrogen and there are still huge clouds of hydrogen distributed thru it

stars condense from hydrogen and by fusion they make light and produce heavier nuclei.

ordinary cool neutral hydrogen gas makes a faint radio wave signal with a wavelength of 21 centimeter

this is different from the radiation from hot or ionized gases. hydrogen will glow too if you get it hot. but a big cloud of hydrogen left alone way out in space is NOT hot. it just has very low level action from rare collisions which can energize the atoms slightly and cause this faint tingle of radiofrequency resonance------strictly speaking i guess 21 centimeter would be called "microwave" but I think of it as radiowave.


this neutral hydrogen radio signal is very UNIVERSAL and there is nothing else quite like it and in the Fifties and Sixties the structure of our galaxy was MAPPED by these huge radiotelescope dish antennas you see around places. And MOST OF THE MAPPING of the arms of our galaxy was done by receiving this 21 centimeter signal from the huge clouds of hydrogen that are like a skeleton of our galaxy and from which stars are still forming.

they couldnt map our galaxy with VISIBLE LIGHT because it is blocked by huge collections of dust and by all the nearby starlight and crud. But the 21 centimeter signal goes THRU all that like an x ray and so in that wavelength you can see much farther thru the crud and see the whole layout of the galaxy. (at least its skeleton of clouds of gas)

So humans of the Fifties and Sixties and even Seventies got the idea that this 21 centimeter radio wavelength was something universal and special and wonderful and (like) god-ordained----that it was more prominent and obvious than all the other zillions of wavelengths of light and radio that are in nature.

so when they had to pick a wavelength to listen for ET messages that was the first frequency they tuned to. because they thought it was so special and wonderful that it would be an obvious choice for ET to use as well!

So the length 21 centimeters became kind of a feature of the Culture, as exemplified by leading figures like Carl Sagan and other Space Gurus

Now somebody else can tell you what role 21 centimeters played in more detail and in special cases. this is just background.

and I will not tell you what I privately think but leave it as something for you to decide. Is the 21 cm carrier frequency a good signal for listening and sending interstellar communications? Is it an obvious choice? or not.

[djmacarro]

well i knew there is mostly hydrogen is the Sun and galaxy, but after the third line you lost me with the wave lengths...
its times like this i almost wish i payed attention when my high school teacher to giving notes on this instead of trying to get the girl i sat next to to sleep with me...

[Martin]

djmacarro said:

... attention when my high school teacher to giving notes on this instead of trying to get the girl i sat next to to sleep with me...

think about the girl
the way you get to see the girl at all is when light
scatters off her (bounces off her clothes and skin)
and some of the scattered light finds its way into your eye

how do you imagine light?

do you talk on the cellphone?

how do you imagine the radio signals that carry the incoming and outgoing talk?


If you never bothered to try to imagine the waves you use all the time: the light (by which you see the girl) or the radio signal (by which you talk on the cellphone)...if you never pictured the waves you use to live, then you probably dont need to understand about the waves that atoms (for example) can send out.

On the other hand, if you have tried to imagine what is going on with light and radio then you probably can picture the waves traveling, and rather like waterwaves coming into the beach they have a certain wavelength----the distance from crest to crest

light and radio is basically the same process

do you know what a centimeter is and do you know what a micrometer is (also called a micron, a millionth of a meter)? I am no fan of the metric system but that is how people talk about the wavelengths of light and radio

[Martin]

Quote

ordinary cool neutral hydrogen gas makes a faint radio wave signal with a wavelength of 21 centimeter
...

the hydrogen sends out this 21 cm wave when it is doing a little low-energy transition in itself (like rolling over in bed without waking up) called hyperfine transition of the ground state

the ground state is the lowest-energy state

a accidental collision between two hydrogens even when they are not going very fast is enough to get the hydrogen ready to make this transition-----to give it the necessary energy

because the energy involved in this transition is very small

that is why ordinary cold hydrogen can send out this signal

(it doesnt have to be hot or electrified or ionized etc, the way it has to be if you want it to radiate visible light)

long wavelength waves need less energy to set them off, so 21 cm waves can come from cold neutral (un-electrified, not-ionized) hydrogen

[Martin]

this is not for DJ Macarro because a bit advanced but here is some background

http://66.102.7.104/...esis/09-CHAPTER

the hydrogen atom groundstate has two possible configs
one with electron and proton spins aligned and one with them opposite
and the difference in energy is tiny

and if the atom is in the higher of the two it will eventually decay
but the halflife for the decay is 10 or 11 million years!

for astronomy that is OK because these vast clouds of neutral hydrogen that you map out, and form a kind of skeleton of the galaxy, these clouds have vast numbers of hydrogen atoms, so some are always decaying and sending out their 21 cm signal, which you detect when you point your dish at the cloud

the frequency of the wave the atom sends out is 1420 MHz.

the energy of the transition, which is the photon energy of the wave of that frequency, is 6 millionths of an eevee.

that is a tiny amount of energy

it is beautiful that with those big dish antennas we can pick up the slight transition of a bunch of atoms and hear it coming from way over the other side of the galaxy, like 60,000 lightyears away