What does cold hydrogen gas emit at 1420 MHz?

[bluemoonKY]

I just read Paul Davies book The Eerie Silence. In The Eerie Silence, Paul Davies asserts that the radio astronomers looking for radio signals from extraterrestrial civilizations search for radio signals at the frequency 1420 MHz because that is the emission frequency for cold hydrogen gas. What does cold hydrogen gas emit at 1420 Mhz?

 

[blue_leaf77]

What does cold hydrogen gas emit at 1420 Mhz?

What hydrogen gas emits at 1420 MHz is electromagnetic radiation at 1420 MHz.
This frequency corresponds to the transition between the splitted ground state due to hyperfine structure.

 

[bluemoonKY]

Why does hydrogen gas emit electromagnetic radiation at all?

 

[blue_leaf77]

Why does hydrogen gas emit electromagnetic radiation at all?

There are three main types of interaction between bound state electrons and photons: absorption, stimulated emission, and spontaneous emission. The first one involves annihilation of a photon to raise the electron state to a higher level. The last two, opposite to the first one, involve creation of a photon by decreasing the state of an electron to a lower level, and eventually to end up in the ground state.

When an atom is already in the ground state it won't emit photons/EM radiation no matter how. If it's not in the ground state, it will emit EM radiation through spontaneous or stimulated emission. Spontaneous emission is actually a quantum electrodynamics effect which describes the interaction between electron and the vacuum field. Stimulated emission is a form of perturbation experienced by an atom when an already existing EM radiation disturbs an atom in an excited state.

 

[bluemoonKY]

So is the electromagnetic radiation that cold hydrogen gas emits a stream of photons, or is the electromagnetic radiation that cold hydrogen gas emits an electromagnetic wave?

 

[blue_leaf77]

So is the electromagnetic radiation that cold hydrogen gas emits a stream of photons, or is the electromagnetic radiation that cold hydrogen gas emits an electromagnetic wave?

First, in the loose sense, a "stream" of photons is kind of equivalent to an electromagnetic radiation. Second, the thing which is emitting is the hydrogen atom, not the electromagnetic radiation. Electromagnetic radiation, which is the synonym of electromagnetic wave, is what the atom emits.

 

[Borek]

So is the electromagnetic radiation that cold hydrogen gas emits a stream of photons, or is the electromagnetic radiation that cold hydrogen gas emits an electromagnetic wave?

Both. Have you heard about https://en.wikipedia.org/wiki/Wave–particle_duality?

 

[bluemoonKY]

Blueleaf and Borek, your posts are very informative.

Does 1420 MHz mean a frequency 1420 cycles per second?

When cold hydrogen gas emits electromagnetic radiation at 1420 MHz, how are the cycles measured? Do they measure the frequency from the crest of one electromagnetic wave to the next crest?

 

[blue_leaf77]

Does 1420 MHz mean a frequency 1420 cycles per second?

Yes.

When cold hydrogen gas emits electromagnetic radiation at 1420 MHz, how are the cycles measured? Do they measure the frequency from the crest of one electromagnetic wave to the next crest?

I guess astronomers use radio telescope to detect 21 cm line. I don't really know how this equipment exactly works though, so you should probably google this term or start another thread in astronomy and cosmology section of this forum.

 

[bluemoonKY]

Blueleaf, what is the significance of 21 cm?

 

[blue_leaf77]

Blueleaf, what is the significance of 21 cm?

That's the wavelength of EM radiation of frequency 1420 MHz. As far as I know it's used to map the distribution of hydrogen-rich area with low to moderate density in space.

 

[scanvas]

Does 1420 MHz mean a frequency 1420 cycles per second?

No, 1 cycle per second is 1 Hz
1420 MHz = 1 420 000 kHz = 1 420 000 000 Hz

An old but still valid introduction to this matter
http://www2.jpl.nasa.gov/radioastronomy/radioastronomy_all.pdf

 

[blue_leaf77]

Ah, I was careless in post #9. Yeah, it should be what scanvas said.

 

[bluemoonKY]

Is 1420 MHz the same as 1.42 billion cycles per second?

 

[Borek]

Is 1420 MHz the same as 1.42 billion cycles per second?

M stands for million, G stands for billion (at least in some languages ). So yes, that's correct (in some languages at least).

 

[russ_watters]

Is 1420 MHz the same as 1.42 billion cycles per second?

Hz is the unit Herz, which is cycles per second. The M stands for "mega", which means million.

 

[bluemoonKY]

I sure learn a lot of new stuff at physicsforums. God bless this website!