[HN Gopher] One quantum transition makes light at 21 cm
___________________________________________________________________
One quantum transition makes light at 21 cm
Author : Brajeshwar
Score : 179 points
Date : 2025-04-24 16:38 UTC (6 hours ago)
(HTM) web link (bigthink.com)
(TXT) w3m dump (bigthink.com)
| Suppafly wrote:
| I had a CS professor that used to hold up a length of string
| roughly that length and talk about how that is how far a bit of
| data can travel at the speed of light during a clock cycle or
| something. Honestly don't remember the point he was trying to
| make.
| cogman10 wrote:
| Probably trying to recreate this lecture by Grace Hopper [1]
|
| [1] https://www.youtube.com/watch?v=9eyFDBPk4Yw
| MomsAVoxell wrote:
| I still have my nanowire, received directly from Grace
| herself during one of her last lectures I attended in the
| 80's.
|
| Of course, it's in among about a thousand other wires and
| cables and nonsense.
|
| One of these days I should sort it out and try to identify it
| by length.
|
| She had a very firm handshake, and a very definite glint in
| her eye as she handed those out to her star struck fans ..
| Suppafly wrote:
| I'm sure that's what it was. I probably should have
| remembered that, but it was such a small part of one of his
| lectures it didn't resonate as deeply as it should have.
| pvg wrote:
| That's a different thing, the signal travel length in a
| nanosecond, roughly. This is about the 21 cm RF wave that glows
| from the sky - https://en.wikipedia.org/wiki/Hydrogen_line. One
| of the (hyper) finest names of things in nerddrom - "hyperfine
| transition".
| Someone wrote:
| https://youtu.be/9eyFDBPk4Yw: Admiral Grace Hopper Explains the
| Nanosecond
| Night_Thastus wrote:
| I suppose it's interesting to think about. At today's clock
| rates, the distance between the CPU and RAM actually adds a
| small, but still significant delay.
| cogman10 wrote:
| It's ultimately what killed having a memory controller on the
| northbridge of a motherboard. Having the CPU talk to a
| separate chip to ultimately talk to the RAM simply added too
| much latency into the entire process.
| Night_Thastus wrote:
| And it may end up causing CAMM2 to end up being the next
| standard. The physical layout of the chips on the board
| means the traces can be shorter - leading to lower latency
| and higher stability.
| cogman10 wrote:
| I really hope CAMM2 takes off. It'd be a rare standard
| that could be used for both laptops and desktops. Having
| upgradable memory in a laptop again would be great. Using
| the same standard a desktop would make it easy to find
| sticks as time goes on.
| jpollock wrote:
| Admiral Hopper[1] used to use string to demonstrate how long
| pieces of time are:
|
| https://www.youtube.com/watch?v=9eyFDBPk4Yw
|
| [1] https://en.wikipedia.org/wiki/Grace_Hopper
| ColinWright wrote:
| She didn't use string, she used wires, and would sometimes
| hand them out after lectures.
| ivape wrote:
| Woah. Imagine extrapolating that for life. What does it mean
| to throw away a day?
| dleeftink wrote:
| Or to be given one more
| whartung wrote:
| Well everyone knows if you want your network to be twice as
| fast, just cut all of the cables in half.
| James_K wrote:
| The point of how fast computers are, and why you need to make
| them smaller to make them faster. Think about the bus between
| the CPU and GPU, not much shorter than that. Information cannot
| travel faster than the speed of light, so there is a hard
| constraint on how quickly the GPU can respond to commands. The
| same is true for RAM and even within the CPU, signals take time
| to propagate across it. The total length of your circuitry for
| a single instruction can't be longer than 21cm if that's how
| far light travels.
| kccqzy wrote:
| 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.
| BurningFrog wrote:
| It does feel a little odd that something the size of 5.29x10-11
| meter can create something 10 billion times larger.
|
| I mean, I understand how and why, but it _feels_ odd.
| IshKebab wrote:
| Wavelength isn't an object though. Like if you walk around
| the world you haven't made something the size of the world.
| pixl97 wrote:
| At the same time any one individual walking around the
| world is a highly improbable event.
| nomel wrote:
| Wavelength _is_ a time thing though. To make something that
| low frequency (1.4GHz or 7ps), things have to happen pretty
| slowly.
| tlb wrote:
| * 714 ps
| justlikereddit wrote:
| 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 wrote:
| The sound pressure wave does take 17 meters in the air to
| make a full cycle, no ? It's real, same for the photon
| MaxikCZ wrote:
| It's not about measuring peak to peak in distance, it's
| about measuring how long it takes for one spot to
| encounter second peak after first. The fact that the
| first peak traveled some distance is irrelevant, as its
| entirely dependent on propagation speed, which doesn't
| affect the frequency, only vawelenght.
| ttoinou wrote:
| Would you then say that the wavelength is meaningful for
| the sound example as its properties are really of a wave
| propagating, and meaningless for the light as the wave
| analogy isn't a full description of the light phenomenon
| behavior ?
| AnimalMuppet wrote:
| 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.)
| strongpigeon wrote:
| 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.
| jpmattia wrote:
| > _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_.
| arthurcolle wrote:
| Segmentation fault! Core dumped
| amiga386 wrote:
| > 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 wrote:
| That not just imprecise usage of that term; it's completely
| incorrect. The correct term would be its exact opposite,
| "approximately".
| cluckindan wrote:
| The exact opposite would be "imprecise" or "inaccurate"
| raldi wrote:
| Accuracy and precision are orthogonal concepts.
| "Approximately 0 light years" is accurate but not precise.
| boothby wrote:
| 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?
| hexhu wrote:
| so the claim is inaccurate by 1mm and missing precision data.
| I'd call it inaccurate and imprecise XD
| LeifCarrotson wrote:
| 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.
| barbazoo wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Yes that bugged me too. If you replace 'precisely' with
| 'approximately' everywhere in the article it becomes much
| improved ;)
| rwmj wrote:
| On the other hand, since it's a property of the universe maybe
| now's the time to define 21 cm as this value.
| xnickb wrote:
| then cm will become a bit longer and it'll break many things
| ncoco wrote:
| Like the width of an A4 sheet of paper.
| dang wrote:
| Ok, we've made the title not be precise now.
| lud_lite wrote:
| But everyone's hand is precisely 21cm long, of course
| belter wrote:
| In Contact the alien beacon arrives at 4.4623 GHz. Pi times the
| Hydrogen line...
| birdiesanders wrote:
| Oh wow. That's wild.
| diego_sandoval wrote:
| But do they know what is a second?
| pajko wrote:
| 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).
| phkahler wrote:
| Don't need to. It's the hydrogen wavelength / pi.
| luma wrote:
| 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.
| teraflop wrote:
| 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...
| nine_k wrote:
| 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 wrote:
| 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 wrote:
| 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.
| nullc wrote:
| Fair, I should have said "modulated by something other
| than obvious physical processes".
| zem wrote:
| "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
| imoverclocked wrote:
| It's neat to see theory that allows us to practically see further
| into the past.
| dang wrote:
| [stub for offtopicness]
|
| [come on you guys]
| pixelbeat wrote:
| 42/2
| baxtr wrote:
| When I replied, the article had been posted 42 minutes ago
| and had 42 upvotes.
|
| That can't be a coincidence.
| fragmede wrote:
| The people running our simulation are just fucking with us
| at this point.
| mystifyingpoi wrote:
| [flagged]
| nine_k wrote:
| The article gives the answer to the question in the title in
| the first paragraph. This is really commendable. (Suspense is--
| wait for it!--overrated.)
| dang wrote:
| We've replaced the magic title with the more informative
| subtitle now. Standard mod trick.
| mightysashiman wrote:
| [flagged]
| dang wrote:
| Please don't do this here.
| mightysashiman wrote:
| Reddit gave me bad habits. My apologies
| quantumHazer wrote:
| Though 13 is totally the average!
| ddalex wrote:
| How many inches is that
| imoverclocked wrote:
| It is 21.1061... cm worth of them.
|
| Fun fact, your computer is really good at answering this
| question for you. So is (say) Google Search.
| barbazoo wrote:
| Or an LLM, or even just using MacOS spotlight search
| autocompletes "21cm how many inches " to the right answer.
| staticelf wrote:
| Chickens take 21 days to hatch and give us the most protein
| rich food you can eat. Does this mean that chickens are the
| magic creature of the universe?
| wafflemaker wrote:
| Pretty sure newly hatched chicks are not the most protein
| rich food you can eat. But the crunch must be amazing.
|
| Is this why cows and horses eat them?
| hnthrowaway0315 wrote:
| Damn that's 42 / 2!
| zhengyi13 wrote:
| ... so the _question_ is "What is twice the magic quantum
| light length?"
|
| Doesn't seem all that great, but I'm probably missing
| something.
| omegacombinator wrote:
| Ah, so the answer to life, the universe, and everything was
| actually 42/2 ...
| simiones wrote:
| Is this going to be t vs p all over again?
| o11c wrote:
| I recently did a bit of programming exploration using tau
| exclusively instead of pi (and `sintau` instead of the new
| `sinpi` function etc.)
|
| In almost every aspect this was far simpler, but there was
| the curious case of the constant `M_2_SQRTPI = 2 /
| sqrt(p)`. Even after looking up what weird formula that
| constant is used in, it wasn't at all clear to me where
| would be the most _sensible_ place to put the constant.
| m3kw9 wrote:
| [flagged]
| pineaux wrote:
| Its clearly only half of the answer.
|
| The complete answer of the universe would of course be 42cm.
| wasabi991011 wrote:
| 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).
| petsfed wrote:
| 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 wrote:
| laser, maser, like, other excitation / energy saturation does
| not work here ?
| joemag wrote:
| 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 +-1/2 to the
| opposite one). But then remembered that photons are spin 1
| particles, so the math works out. Neat.
| CamperBob2 wrote:
| 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 wrote:
| I'm reading that an H-maser emits 1.4GHz. Maybe you mean
| something besides it's emission frequency?
| CamperBob2 wrote:
| 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
| mjd wrote:
| 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 8x21 = 168 cm tall.
| arghwhat wrote:
| 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...
| mjd wrote:
| We did that too. There is a cartoon of Pioneer itself, drawn
| to the same scale.
| arghwhat wrote:
| (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.)
| aylmao wrote:
| 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.
|
| [1] https://en.wikipedia.org/wiki/Pioneer_plaque
| arghwhat wrote:
| 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.
| spullara wrote:
| Do you think that physics is somehow subjective? We
| absolutely would have decoded the message.
| cuttothechase wrote:
| Our understanding evolves, course corrects, spins off
| etc., we can use some static value as purported from the
| dark ages or by newtonian or later einstenian points of
| view. They all are measurably correct for the problems
| that they are trying to solve for the people who lived
| during those times. A million years from now would these
| values still be relevant or be considered as having the
| same value of importance or will they be replaced by even
| more finer and precise and contextually different values
| that could be more precise and more accurate etc.,
| petsfed wrote:
| I submit that if the concept of quantum transitions is
| alien to whatever recipient of that probe (if ever), then
| any attempts to communicate are hopeless anyway. That is,
| if the recipient's physical reality is so different from
| our own that they can't at least get back to "oh, this
| distance means _that_ transition, now the rest of the
| plaque makes sense ", then no asynchronous communication
| will bridge that gap.
| arghwhat wrote:
| There is no relation between the ability to communicate
| and a shared understanding of our concept of quantum
| transitions - case in point, our invention of the
| technology we use to communicate with deep space far
| predates us learning these concepts ourselves.
|
| I'd also hold that the only thing this plaque could ever
| give is clear sign of artificial creation, and by virtue
| the (possibly past) existence of some entity capable of
| creating it. Maybe they'll get a vague idea of what we
| look like, but if "their" culture does not commonly
| depict themselves in 2D as we do, or "they" have vastly
| different morphologies, even that would be unclear. The
| context needed to understand our attempt at showing our
| location might also be lost if the thing went far enough.
| m4rtink wrote:
| What if you end up with a picture of the record &
| everything else gets lost - that riddle will still work.
| Say the civilisation that found it collapses & leaves
| behind some garbled data, including a picture of the
| record.
|
| Or even future human data archeologists digging through a
| mix of 20 & 21 century data heavily polluted by AI slop.
| ;-)
| arghwhat wrote:
| Making the data fault tolerant to the discovery by
| another civilization, its collapse and later rediscovery
| by _another_ civilization seems a bit of a stretch goal.
| :)
| linschn wrote:
| The plaque also provided a drawing of the probe itself next
| to the two human figured, at scale.
| pdabbadabba wrote:
| 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.
| hackrmn wrote:
| 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?
| nemomarx wrote:
| 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 wrote:
| 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 wrote:
| It started with the French.
|
| https://en.m.wikipedia.org/wiki/History_of_the_metric_syste
| m
|
| The meter was originally based on the measured dimensions
| of the Earth.
| hnuser123456 wrote:
| Ah yes, you're right. Another nice coincidence that a
| seconds pendulum is less than 1% away from 1/10 millionth
| the distance between the equator and poles.
| SiempreViernes wrote:
| The standard metre was a rod 1 metre long, you might be
| thinking of the standard kilo which is a compact cylinder?
| petsfed wrote:
| 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!
| allemagne wrote:
| 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.
| arthurcolle wrote:
| I posted this thread to o3 and found the results interesting.
| https://chatgpt.com/share/680aad8d-ce54-800c-8973-df4258bbe1...
| mcswell wrote:
| 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 wrote:
| 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 ?
| lud_lite wrote:
| Precisely 21cm or a precise amount that is approximately 21cm?
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