# 1420 MHz--- the emission frequency of cold hydrogen gas

I recently finished reading Paul Davies book The Eerie Silence, which is a book about the SETI (Search for ExtraTerrestrial Intelligence) project. In The Eerie Silence, Davies says that scientists using radio telescopes to search for radio messages from space aliens set their radio telescopes to search for messages at 1420 MHz because 1420 MHz is the emission frequency of cold hydrogen gas.

I googled this and researched this a little bit both on this message board and on other places on the internet before I created this thread.

It is my understanding that cold hydrogen gas emits electromagnetic waves ( a stream of photons) at 1420 MHz.

It is my understanding that 1420 MHz is the frequency (a unit of time) at which cold hydrogen gas emits electromagnetic waves. The frequency is probably measured from the crest of an electromagnetic wave to the next crest or the trough of an electromagnetic wave to the next trough.

Here is what I don't understand: Why does cold hydrogen gas emit electromagnetic waves at all?

Before I read Davies book, I would have assumed that only sources of light such as the sun (or a light bulb) and sources of sound such as satellite dishes and/or radio antennas emit electromagnetic waves.

Do all gases emit electromagnetic waves?

It is my understanding that all atoms can be transformed to a gas if they get hot enough. So if I heated, say, iron hot enough to make iron into a gas, would the iron emit electromagnetic waves?

Drakkith
Staff Emeritus
The 1420 MHz emission is caused by hydrogen's electron transitioning between two closely spaced energy states. The atom can absorb energy from ambient radiation or collisions, which can cause the electron to jump from the lower energy state to the slightly higher energy state. After a small amount of time this electron falls back down to the lower energy state, which then generates an EM wave of a frequency of 1420 MHz. You can read a much more details explanation here.

Before I read Davies book, I would have assumed that only sources of light such as the sun (or a light bulb) and sources of sound such as satellite dishes and/or radio antennas emit electromagnetic waves.
EM radiation (EM waves) are emitted from either accelerated electric charges or magnetic sources, or when atoms/molecules transition from higher energy to lower energy states. Most of the EM radiation from hot objects, such as the Sun or an incandescent light bulb, are caused by charges being accelerated as they bounce off of each other. In 'cold' gases, the frequency and energy of the collisions are relatively low, so the dominant source of radiation is from these 'electronic transitions' when electrons fall from higher to lower energy states.

It is my understanding that all atoms can be transformed to a gas if they get hot enough. So if I heated, say, iron hot enough to make iron into a gas, would the iron emit electromagnetic waves?
It is always emitting EM radiation by virtue of being warmer than absolute zero. This is called thermal radiation and is caused by the charges bouncing around due to their thermal motion. The hotter the object, the greater the amplitude of the collisions and the greater the average frequency of the emitted radiation. This is why iron (or other objects) glow red when heated to a high temperature. At high temps the frequency of the thermal radiation finally starts to be high enough in the visible spectrum for us to see.

DaveE and timmeister37
Excellent post, Drakkith. But I still don't fully understand this.

The 1420 MHz emission is caused by hydrogen's electron transitioning between two closely spaced energy states. The atom can absorb energy from ambient radiation or collisions, which can cause the electron to jump from the lower energy state to the slightly higher energy state.
What would a hydrogen atom collide with that would cause the hydrogen atom to absorb energy?

After a small amount of time this electron falls back down to the lower energy state, which then generates an EM wave of a frequency of 1420 MHz. You can read a much more details explanation here.
What would cause the electron to fall back down to the lower energy state after a small amount of time? Why would the electron's falling back down to a lower energy state release a wave of electromagnetic radiation?

Most of the EM radiation from hot objects, such as the Sun or an incandescent light bulb, are caused by charges being accelerated as they bounce off of each other.
What do you mean exactly when you used the word charges?

In 'cold' gases, the frequency and energy of the collisions are relatively low, so the dominant source of radiation is from these 'electronic transitions' when electrons fall from higher to lower energy states.
I still don't understand why an electron's dropping to a lower energy state causes the emission of an electromagnetic wave.

It is always emitting EM radiation by virtue of being warmer than absolute zero.
What is "it", iron?

This is called thermal radiation and is caused by the charges bouncing around due to their thermal motion. The hotter the object, the greater the amplitude of the collisions and the greater the average frequency of the emitted radiation. This is why iron (or other objects) glow red when heated to a high temperature. At high temps the frequency of the thermal radiation finally starts to be high enough in the visible spectrum for us to see.
In The Eerie Silence, Paul Davies says that cold hydrogen gas emits radiation at a frequency of 1420 Mhz. If changing the heat causes a change in radiation frequency, how would the aliens know to broadcast at 1420 Mhz to reach humans? For instance, let's say that hydrogen gas emits radiation at a frequency of 1420 Mhz at -10 degrees fahrenheit. And let's say hydrogen gas emits radiation at 1400 Mhz at -5 degrees fahrenheit. And let's say hydrogen gas emits radiation at 1380 Mhz at 2 degrees fahrenheit. How did the SETI project come to assume that the aliens would communicate with radio waves to humans at 1420 Mhz then? It sounds arbitrary. Before I created this thread, I thought that the emission frequency of 1420 Mhz for cold hydrogen gas was not arbitrary.

P.S. I looked at the link you gave me, but it kind of went over my head. From your link: "Because of the quantum properties of of radiation, hydrogen in its lower state will absorb 1420 MHz and the observation of 1420 MHz in emission implies a prior excitation to the upper state."

What will hydrogen in its lower state absorb at a frequency of 1420 MHz?

Drakkith
Staff Emeritus
What would a hydrogen atom collide with that would cause the hydrogen atom to absorb energy?
Anything. Other atoms or molecules, photons, or even large particles of dust.

What would cause the electron to fall back down to the lower energy state after a small amount of time? Why would the electron's falling back down to a lower energy state release a wave of electromagnetic radiation?
The drop to the lower energy state is a intrinsic, spontaneous phenomenon, meaning that it happens on its own without something else causing it. The release of an EM wave happens because the change in energy states causes a change in the EM field that propagates outward as an EM wave.

What do you mean exactly when you used the word charges?
Electric charges. Things like protons, electrons, or ions (atoms or molecules missing electrons or with extra electrons).

What is "it", iron?
Yes.

In The Eerie Silence, Paul Davies says that cold hydrogen gas emits radiation at a frequency of 1420 Mhz. If changing the heat causes a change in radiation frequency, how would the aliens know to broadcast at 1420 Mhz to reach humans?
The change in temperature does not change the frequency emitted from hydrogen at 1420 MHz. This is because this particular emission is not thermal radiation, as it isn't caused by the random collisions of charged particles. Instead this particular emission is caused by the electron dropping from a higher energy state to a lower one in a single hydrogen atom. A similar phenomenon generates emission spectra in all elements, just using different starting and ending energy states. See the balmer series for another example with hydrogen.

What will hydrogen in its lower state absorb at a frequency of 1420 MHz?
It absorbs ambient radiation at the same frequency.

timmeister37
Post #4 on this thread was very informative, but I don't have any further questions about post #4 at the moment.

I would like to ask about something you wrote in post #2 though.

It is always emitting EM radiation by virtue of being warmer than absolute zero. This is called thermal radiation and is caused by the charges bouncing around due to their thermal motion. The hotter the object, the greater the amplitude of the collisions and the greater the average frequency of the emitted radiation. This is why iron (or other objects) glow red when heated to a high temperature. At high temps the frequency of the thermal radiation finally starts to be high enough in the visible spectrum for us to see.
Ok, so iron is always emitting thermal radiation. But is iron also always emitting EM radiation due to the iron's electrons transitioning between two closely spaced energy states (like hydrogen always does)?

Drakkith
Staff Emeritus
Ok, so iron is always emitting thermal radiation. But is iron also always emitting EM radiation due to the iron's electrons transitioning between two closely spaced energy states (like hydrogen always does)?
Yes, but the thermal radiation is overwhelmingly dominant.

timmeister37
DrClaude
Mentor
Ok, so iron is always emitting thermal radiation. But is iron also always emitting EM radiation due to the iron's electrons transitioning between two closely spaced energy states (like hydrogen always does)?
They actually are the same thing.

To recap what has been said in previous posts, when matter is in an excited state, it will eventually decay to the ground state by emission of a photon. For individual atoms or molecules, only certain energies are possible, so only photons of certain energies (or wavelengths or frequencies, they are related to each other for photons) can be emitted. When considering dense matter, such as a block of iron or a star, all energies are possible and photons of all energies can be emitted.

Now, how does matter get excited? Mostly by absorbing radiation (photons), by collision with other matter, or by getting energy from something hotter. So the iron bar emits because it has a temperature (it emits even at room temperature, but we don't see it since it is mostly infra-red light), as all objects around you do. The higher the temperature, the more energetic the light, which is why a hot iron bar glows red or white, as does the Sun. This is because at a higher temperature, there is more energy, meaning higher excited states, meaning higher-energy photons.

Even in the deep outer space, the temperature is not absolute zero, such that cold hydrogen gas still has some energy to be in excited states. But there is so little energy that there is basically a single excited energy level hydrogen can be found in, at an energy corresponding to 1420 MHz from the ground state. This is how we can observe hydrogen all over the universe.

As for SETI, I must admit that I never understood why intelligent aliens would try to communicate precisely at that frequency.

epenguin, hutchphd and timmeister37
Even in the deep outer space, the temperature is not absolute zero, such that cold hydrogen gas still has some energy to be in excited states. But there is so little energy that there is basically a single excited energy level hydrogen can be found in, at an energy corresponding to 1420 MHz from the ground state. This is how we can observe hydrogen all over the universe.

As for SETI, I must admit that I never understood why intelligent aliens would try to communicate precisely at that frequency.
The theory is that aliens would try to communicate with humans at 1420 MHz because hydrogen gas emitting EM radiation at 1420 MHz is ubiquitous throughout the universe. So 1420 MHz would recognized as a special radio frequency throughout the universe by all intelligent life.

DrClaude
Mentor
The theory is that aliens would try to communicate with humans at 1420 MHz because hydrogen gas emitting EM radiation at 1420 MHz is ubiquitous throughout the universe. So 1420 MHz would recognized as a special radio frequency throughout the universe by all intelligent life.
I know, but I still don't get it. "We're intelligent and we want to find other intelligent life, so let's send a signal at the most common natural frequency in the universe, so other intelligent life know it is artificial."

epenguin
I know, but I still don't get it. "We're intelligent and we want to find other intelligent life, so let's send a signal at the most common natural frequency in the universe, so other intelligent life know it is artificial."
Any other frequency would be a shot in the dark.

hutchphd
Homework Helper
I'm with @DrClaude on this one. If I'm in a chorus and everyone is singing middle C, would I sing middle C in order to get heard? I think I would sing F# !!!

DrClaude
DrClaude
Mentor
I'm with @DrClaude on this one. If I'm in a chorus and everyone is singing middle C, would I sing middle C in order to get heard? I think I would sing F# !!!
That's what I do! With no effort at all...

Staff Emeritus
I'm with @DrClaude on this one. If I'm in a chorus and everyone is singing middle C, would I sing middle C in order to get heard? I think I would sing F# !!!
I've worked with a vocalist who thought that way.

hutchphd
That's what I do! With no effort at all...
Bad analogy. When the space aliens'radio telescopes are set for a random number, there is only an infinitesimal chance that we earthlings would be on the right frequency to detect them.

When a choir performs before a human audience right next to them, the audience can hear any human pitch singing.

I know, but I still don't get it. "We're intelligent and we want to find other intelligent life, so let's send a signal at the most common natural frequency in the universe, so other intelligent life know it is artificial."
I still don't get your position. "We're intelligent and we want to find other intelligent life, so let's send a signal at some random frequency. We will use a roullette wheel to randomly pick a frequency. The aliens spin a roulette wheel which randomly lands on frequency 3169 MHz. Ok, we will broadcast at 3169 MHz and hope that some other intelligent life light years away also random selects to check for signals at 3169 MHz ."

Our number system, Arabic numerals, adds a second digit at 10. This is prolly based on the fact that a human has ten fingers. Aliens are just as likely to have 8 fingers and add a second digit at 8. So a logical round number for humans to use for radio like 1,000 is probably not going to be a round number for the aliens. The aliens might just as well have 512 as the equivalent of our 1,000.

What alternative do you have in mind if not using 1420 MHz?

hutchphd
Homework Helper
I like the concept of ##1420\pi MHz##. If they inhabit our dimensions surely they will know ##\pi##. And that presumably gets it out of the schmutz.

epenguin
I like the concept of ##1420\pi MHz##. If they inhabit our dimensions surely they will know ##\pi##. And that presumably gets it out of the schmutz.

I thought of pie before as being a natural thing to check. Pie sounds like another logical radio frequency to check in addition to 1420. So you might be on to something.

Perhaps it is difficult to send radio waves at frequency 3.14 MHz. That might make 3.14 a poor choice of frequency to search at.

Anyway, why complicate things by multiplying 1420 by 3.14?

P.S. what does schmutz mean?

hutchphd
Homework Helper
It is I believe from the Yiddish for something dirty or unwanted...extraneous. Someone moreconversant may correct me....

It is I believe from the Yiddish for something dirty or unwanted...extraneous. Someone moreconversant may correct me....
Why complicate things by multiplying 1420 by 3.14?

Will a lone hydrogen atom in outer space emit EM radiation at 1420 MHz if there are no other atoms around the lone hydrogen atom?

hutchphd
Homework Helper
It would not be "some random frequency" and I think it pretty easy to produce and "see" with small equipment. Because 1420Mhz is ubiquitous, this popped immediately into my head.

mjc123
Homework Helper
I thought of pie before as being a natural thing to check. Pie sounds like another logical radio frequency to check in addition to 1420. So you might be on to something.

Perhaps it is difficult to send radio waves at frequency 3.14 MHz. That might make 3.14 a poor choice of frequency to search at.

Anyway, why complicate things by multiplying 1420 by 3.14?
You can't use pi MHz because their units of time (and frequency) are probably different from ours (think where our "second" comes from, although it's not the modern definition of the second), so they won't recognise the number pi in the signal (or we, if it's from them, at a frequency different from pi MHz). The frequency of the hydrogen emission is a constant - the numerical value will depend on the units used, but the physical frequency is the same - and multiplying it by a recognisable constant like pi can make an intelligent signal, while distinguishing it from natural H emission.

hutchphd and timmeister37
Will a lone hydrogen atom in outer space emit EM radiation at 1420 MHz if there are no other atoms around the lone hydrogen atom?

Yes - a lone hydrogen atom can radiate:
Its lidetime is around 10 million years. Therefore a hydrogen atom that has collided with an atom can travel a long way before radiating.
And the atom can be excited not only by colliding, but also by absorbing a photon. Which means that the atom will eventually emit a new photon, in a new direction.

timmeister37, DrClaude and hutchphd
Yes - a lone hydrogen atom can radiate:
Its lidetime is around 10 million years. Therefore a hydrogen atom that has collided with an atom can travel a long way before radiating.
You mean that a hydrogen atom's lifetime is around 10 million years; correct? I am not an expert on chemistry. Perhaps there is a term called lidetime in chemistry that I am not aware of. That's why I have to ask.

If a hydrogen atom's lifetime is 10 years, why does that mean that a hydrogen atom that has collided with an atom can travel a long way before radiating? If a hydrogen atom collides with another atom, wouldn't the hydrogen atom start radiating electromagnetic radiation immediately (as opposed to traveling a long way before radiating)?

And the atom can be excited not only by colliding, but also by absorbing a photon. Which means that the atom will eventually emit a new photon, in a new direction.
Well, as soon as the hydrogen atom collides with another atom, isn't the hydrogen atom going to instantly be emitting a stream of photons (I'm guessing like billions and billions of photons per second)?

What do you mean , in a new direction? I thought that a hydrogen atom just radiates electromagnetic directions in all directions simultaneously.