One quantum transition makes light at 21 cm
151 comments
·April 24, 2025wasabi991011
petsfed
I've never liked the definition of forbidden transitions as "transitions not predicted under the broader approximation", because its rare that anybody actually lays out why a given approximation is used, and therefore why that approximation is inappropriate for the "forbidden" situation.
The reality is that with e.g. 21 cm Hydrogen, or 500.7 nm Oxygen (which I knew by heart, back in the day), its hard to keep a given atom in the appropriate state long enough for it to relax by emitting the appropriate photon. Indeed, we can't create a pure enough vacuum in a large enough chamber that such things happen frequently enough to be measurable.
Calwestjobs
laser, maser, like, other excitation / energy saturation does not work here ?
mjd
This 21-centimeter transition was chosen by the designers of the Pioneer plaques (https://en.wikipedia.org/wiki/Pioneer_plaque) to explain to any alien readers how big we are. At top left is a cartoon of two hydrogen nuclei in opposite spin orientations, and a ruler in between them marked "1". Over on the far right you can see another ruler that measures the height of the female figure, marked with binary numeral "8" ("|---") to indicate that she is approximately 8×21 = 168 cm tall.
arghwhat
If only we could somehow share a physical entity of known dimensions to reference together with the drawing so that we did not need to use a physics riddle to indicate scale...
aylmao
Unsure what the tone of this message is, so I don't know if you're aware, but that's included too:
> Behind the figures of the human beings, the silhouette of the Pioneer spacecraft is shown in the same scale so that the size of the human beings can be deduced by measuring the spacecraft.
It's good to have redundancy, not just so someone interpreting the plaque can confirm their hypothesis, but also in case one of the messages fail. In this case, the spacecraft could break, but we can assume quantum transitions will always be observable.
arghwhat
I sarcastically referenced the plaque itself, which is a convenient disc of a known size to anyone observing the drawing, unlike the space craft or physics riddles.
Using quantum transitions is quite ridiculous in my opinion due to requiring not only the observer to have a perfectly compatible understanding of physics (even a more advanced understanding might not be compatible - maybe they don't categorize elements by electrons, or even treat elemental particles as a quantifiable entity), combined with the sheer number of deductions required to understand what was meant with two circles and a few lines.
I doubt we would ever have decoded this had we been the recipient rather than author, and that's with a perfectly compatible understanding of physics.
mjd
We did that too. There is a cartoon of Pioneer itself, drawn to the same scale.
arghwhat
(I was sarcastically pointing to the plaque itself, a physical entity of a well known size to anyone capable of observing the drawing on it, unlike the space craft or physics riddles.)
pdabbadabba
Yeah. It would be pretty funny if an alien reader of the plaque concluded that 1 refers to the actual length of the line between the two circle thingies and concluded, therefore, that we're only a few cm tall.
null
linschn
The plaque also provided a drawing of the probe itself next to the two human figured, at scale.
null
kccqzy
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.
null
BurningFrog
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.
IshKebab
Wavelength isn't an object though. Like if you walk around the world you haven't made something the size of the world.
nomel
Wavelength is a time thing though. To make something that low frequency (1.4GHz or 7ps), things have to happen pretty slowly.
pixl97
At the same time any one individual walking around the world is a highly improbable event.
justlikereddit
My subwoofer is approximate cubic with 30 cm to a side.
But the wavelength of sound it makes at 20Hz is approximately 17 meter.
Wavelength is merely a human conceptualization. If we reconceptialize it as peak-to-peak interval it suddenly stops being length and becomes a time instead
ttoinou
The sound pressure wave does take 17 meters in the air to make a full cycle, no ? It’s real, same for the photon
AnimalMuppet
Yeah, it's weird to me that an atomic transition can create something with a wavelength so much longer than the atomic radius.
(Yeah, I know that it's a really low-energy transition, and I know about the relationship between energy and wavelength. But the net result I still find highly counter-intuitive.)
jpmattia
> Yeah, it's weird to me that an atomic transition can create something with a wavelength so much longer than the atomic radius.
Then it will be even weirder during an MRI: The protons in your body produce a wavelength that can be of order 1-10 meters.
strongpigeon
What helps me is thinking of it in term of period instead given that the wavelength is the spatial propagation of a change in field. It’s big, but that’s because C is high.
arthurcolle
Segmentation fault! Core dumped
hackrmn
I find it disturbing/puzzling that there is this fundamental physical behaviour like emission of light with wavelength of _exactly_ 21cm -- assuming one centimeter wasn't based on any such property but was just a "random" unit measure that stayed with us historically and through sheer volume of use (in U.S. inches filled the same niche; still do). I mean what are the odds that the wavelength is _exactly_ (the word used in the article) 21cm?
allemagne
The article does say "precisely 21cm" in the subtitle, repeats it in the "key takeaways" section, and then close to the end of the article these's this:
>By measuring light of precisely the needed wavelength — peaking at precisely 21.106114053 centimeters
Which I assume is the actual measurement every time "21cm" is brought up in this article.
petsfed
No more probable than any other value, whole or otherwise. In particular, its (per wikipedia) 21.106cm.
Its funny how our brains find nice whole numbers unsettling in the natural world. I was always sort of weirded out by the distance light travels in a nanosecond: just shy of 1 foot. How weird it is that it flops between systems!
nemomarx
isn't a cm now defined based on the distance light travels in a vacuum in a very small period of time?
so it's not arbitrary really, or rather it probably goes the other way around. a cm used to be based on an arbitrary physical distance but was I think redefined to avoid needing to keep a standard meter cube in Paris.
hnuser123456
It started with the grandfather clock. Everyone's clock pendulum needed to be the same length to have the same length of a second. So a meter also happens to (approximately, this was before we could easily be precise to several decimal places) be the length of pendulum that cycles at 0.5 hz (each swing back and fourth is a second) in 9.8 m/s^2 gravity.
geuis
It started with the French.
https://en.m.wikipedia.org/wiki/History_of_the_metric_system
The meter was originally based on the measured dimensions of the Earth.
SiempreViernes
The standard metre was a rod 1 metre long, you might be thinking of the standard kilo which is a compact cylinder?
joemag
Loved this article! I initially was confused by how this transition would work with the conservation of angular momentum (since the electron would be flipping from spin ±½ to the opposite one). But then remembered that photons are spin 1 particles, so the math works out. Neat.
mcswell
The 21cm wavelength is also the wavelength that was proposed for potential SETI radio communication, I guess because of its distinctiveness. Of course modern SETI searches look at a wide range of frequencies.
Calwestjobs
if that frequency can be generated by natural phenomena then why it is so good for seti ? should not signal distinctive from natural fenomena be used instead ?
wiml
SETI uses the gap between the H line and the OH line (the "water hole"). The whole region of the spectrum is kind of quiet for reasons, and the H/OH area is (perhaps parochially) thought to be a natural place for other water-chemistry life forms to look.
imoverclocked
It’s neat to see theory that allows us to practically see further into the past.
lud_lite
Precisely 21cm or a precise amount that is approximately 21cm?
Calwestjobs
it is not 21 in binary number system nor in mayan, so does not matter, it is just number. same importance for physics as horoscopes, numerology ...
but i was pretty annoyed after i read in article - "exactly 21 cm" and then inside of first diagram - "v=1.4GHz" ...
arthurcolle
I posted this thread to o3 and found the results interesting. https://chatgpt.com/share/680aad8d-ce54-800c-8973-df4258bbe1...
CamperBob2
From the article:
Of course, there’s another possibility that takes us far beyond astronomy when it comes to making use of this important length: creating and measuring enough spin-aligned hydrogen atoms in the lab to detect this spin-flip transition directly, in a controlled fashion. The transition takes about ~10 million years to “flip” on average, which means we’d need around a quadrillion (1015) prepared atoms, kept still and cooled to cryogenic temperatures, to measure not only the emission line, but the width of it. If there are phenomena that cause an intrinsic line-broadening, such as a primordial gravitational wave signal, such an experiment would, quite remarkably, be able to uncover its existence and magnitude.
Isn't that basically an H-maser? Not something found every day on eBay, but not really all that exotic either. Every VLBI site has one or more.
Given a suitable state selection mechanism, which is what masers rely on, I don't see why it would be necessary to flip the states "manually" through ionization or any other mechanism. Keeping the state-selected atoms away from the container walls is the real trick.
wwarner
I’m reading that an H-maser emits 1.4GHz. Maybe you mean something besides it’s emission frequency?
CamperBob2
1.4 GHz = 21 cm. Masers use the same transition AFAIK.
In fact, natural H-masers have been found: https://www.cfa.harvard.edu/news/hydrogen-masers-space
belter
In Contact the alien beacon arrives at 4.4623 GHz. Pi times the Hydrogen line...
teraflop
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...
diego_sandoval
But do they know what is a second?
phkahler
Don't need to. It's the hydrogen wavelength / pi.
luma
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.
pajko
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).
zem
"ringworld" had a nicely poetic passage about how the 21cm band had been swept clean by all the hydrogen in the universe and was therefore a natural frequency for aliens to try establishing communication over
nine_k
That could be some freaky case of Doppler effect. To rule that out, the aliens could send both 21 cm * pi and 21 cm * e.
nullc
It's ruled out by the signal being modulated.
The question the choice is answering is where do you put a signal where other intelligent minds might look for it, yet which isn't at a frequency where the universe is particularly loud in ways that will make detecting your signal harder.
TheOtherHobbes
The signal is always going to be modulated, unless the source is maintaining a position with zero relative velocity to the Earth, or deliberately compensating for same - both of which would be far more impressive as a "hello" than a random-ish number which will always be distorted by orbital and proper motion.
Otherwise it's going to have a varying frequency - maybe not by much, and maybe not quickly, but certainly not static.
birdiesanders
Oh wow. That’s wild.
amiga386
> 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
raldi
That not just imprecise usage of that term; it’s completely incorrect. The correct term would be its exact opposite, “approximately”.
boothby
Indiscreet discrete mathematician checking in. If they said "exactly" we'd have a real problem. Instead, "precisely" in this context means "human eye cannot distinguish from exact value at a stone's throw."
Yes, physicists and engineers hate me, why do you ask?
LeifCarrotson
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.
hexhu
so the claim is inaccurate by 1mm and missing precision data. I'd call it inaccurate and imprecise XD
cluckindan
The exact opposite would be ”imprecise” or ”inaccurate”
raldi
Accuracy and precision are orthogonal concepts. “Approximately 0 light years” is accurate but not precise.
barbazoo
Would have been odd if it had magically matched the arbitrary distances we use in the metric system. It's not that 1m is in any way a "natural" distance that was chosen for anything but practical reasons.
dcrazy
I was expecting some spectacular revelation that the definition of the second, the period of a Cesium atom, and the speed of light were somehow related to the definition of a meter by a factor of 0.21.
timewizard
If our system was based on Planck units then it would be interesting. It would also cause tons of other fundamental constants to be greatly simplified to either integers or integer multiples of known transcendental constants.
mota7
Yes that bugged me too. If you replace 'precisely' with 'approximately' everywhere in the article it becomes much improved ;)
rwmj
On the other hand, since it's a property of the universe maybe now's the time to define 21 cm as this value.
lud_lite
But everyone's hand is precisely 21cm long, of course
dang
Ok, we've made the title not be precise now.
null
For those who feel weird about the whole "forbidden transitions being only possible with quantum tunneling" thing and want an alternative interpretation:
It's only true that the transitions are forbidden under a given simplified model of the atom. It is very much possible to calculate the transition probabilities under a more realistic model, and the previously "forbidden" transitions are now just regular transitions that occur with lower probability.
In this case, the simplified model is that of the electric dipole approximation, where the atom is taken to be an electric dipole (reasonable when the wavelength of light emitted during an atomic transition is much larger than the size of the atom).This means it interacts with electromagnetic radiation only through electric dipole interactions, which implies that energy transitions must change orbital angular momentum, hence the 21cm transition is "forbidden". However, in reality, the atom is not truly an electric dipole, and so the 21cm transition is possible by the magnetic dipole interaction, just with low probability. (This low probability is due to the relative strength of the magnetic interaction compared to the electric interaction).