13 ms·
One quantum transition makes light at 21 cm
- Suppafly 2y agoI 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 2y agoProbably trying to recreate this lecture by Grace Hopper [1] [1] https://www.youtube.com/watch?v=9eyFDBPk4Yw https://www.youtube.com/watch?v=9eyFDBPk4Yw
- MomsAVoxell 2y agoI 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 2y agoI'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 2y agoThat'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 https://en.wikipedia.org/wiki/Hydrogen_line. One of the (hyper) finest names of things in nerddrom - "hyperfine transition".
- deleted 2y ago[deleted]
- Someone 2y agohttps://youtu.be/9eyFDBPk4Yw https://youtu.be/9eyFDBPk4Yw: Admiral Grace Hopper Explains the Nanosecond
- Night_Thastus 2y agoI 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 2y agoIt'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 2y agoAnd 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 2y agoI 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.
- deleted 2y ago[deleted]
- fennecbutt 2y agoAnd capacitance, etc too.
- jpollock 2y agoAdmiral Hopper[1] used to use string to demonstrate how long pieces of time are: https://www.youtube.com/watch?v=9eyFDBPk4Yw https://www.youtube.com/watch?v=9eyFDBPk4Yw [1] https://en.wikipedia.org/wiki/Grace_Hopper https://en.wikipedia.org/wiki/Grace_Hopper
- ColinWright 2y agoShe didn't use string, she used wires, and would sometimes hand them out after lectures.
- ivape 2y agoWoah. Imagine extrapolating that for life. What does it mean to throw away a day?
- dleeftink 2y agoOr to be given one more
- whartung 2y agoWell everyone knows if you want your network to be twice as fast, just cut all of the cables in half.
- James_K 2y agoThe 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 2y agoAmazing 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 2y agoIt 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 2y agoWavelength isn't an object though. Like if you walk around the world you haven't made something the size of the world.
- justlikereddit 2y agoMy 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
- amiga386 2y ago> precisely 21 cm Imprecise use of "precise" in the strapline. According to https://en.wikipedia.org/wiki/Hydrogen_line https://en.wikipedia.org/wiki/Hydrogen_line the best measurement of it so far is 21.106114054160 +/- 0.000000000030 cm
- raldi 2y agoThat not just imprecise usage of that term; it’s completely incorrect. The correct term would be its exact opposite, “approximately”.
- cluckindan 2y agoThe exact opposite would be ”imprecise” or ”inaccurate”
- raldi 2y agoAccuracy and precision are orthogonal concepts. “Approximately 0 light years” is accurate but not precise.
- boothby 2y agoIndiscreet 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?
- kbelder 2y ago"It's precisely 21 cm." "Is it 21 cm?" "No."
- hexhu 2y agoso the claim is inaccurate by 1mm and missing precision data. I'd call it inaccurate and imprecise XD
- LeifCarrotson 2y ago
- belter 2y agoIn Contact the alien beacon arrives at 4.4623 GHz. Pi times the Hydrogen line...
- birdiesanders 2y agoOh wow. That’s wild.
- diego_sandoval 2y agoBut do they know what is a second?
- pajko 2y agoOut 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 2y agoDon't need to. It's the hydrogen wavelength / pi.
- luma 2y agoWouldn’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 2y agoYup. 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..669B https://articles.adsabs.harvard.edu/pdf/1993A%26A...278..669...
- nine_k 2y ago
- imoverclocked 2y agoIt’s neat to see theory that allows us to practically see further into the past.
- dang 2y ago[stub for offtopicness] [come on you guys]
- pixelbeat 2y ago42/2
- karol 2y ago[flagged]
- bhaskara2 2y agoApt
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- mystifyingpoi 2y ago[flagged]
- wasabi991011 2y agoFor 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 2y agoI'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 2y agolaser, maser, like, other excitation / energy saturation does not work here ?
- petsfed 2y agoNo, because you need ultra-cold or ultra-low density (or both, as with 21 cm hydrogen) gas. If your mean free path divided by your mean molecular velocity is significantly less than the relaxation time, then the the atom/molecule gets knocked out of the necessary high-energy state well before the transition occurs with sufficient frequency. With [O III] in particular, it only gets into the necessary state via collisions (that's the easy part) occurring in extremely low density plasma, but then it relaxes via photon emission (that's the hard part). So if it gets knocked around by another collision, then the photon never gets emitted in the first place.
- joemag 2y agoLoved 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.
- apognwsi 2y agothat's not right. if photons were truly spin 1, there would be 3 spin eigenstates available, but in fact there are only 2 (Sz=0 is unavailable). the pithy argument invokes the absence of a stationary frame of reference. for all practical purposes, photons are behave like spin 1/2 particles (despite being bosons). see, for example, the jones algebra / calculus.
- CamperBob2 2y agoFrom 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 2y agoI’m reading that an H-maser emits 1.4GHz. Maybe you mean something besides it’s emission frequency?
- CamperBob2 2y ago1.4 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 https://www.cfa.harvard.edu/news/hydrogen-masers-space
- GIVEMEGO 2y ago[dead]
- mjd 2y agoThis 21-centimeter transition was chosen by the designers of the Pioneer plaques (https://en.wikipedia.org/wiki/Pioneer_plaque 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 2y agoIf 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 2y agoWe did that too. There is a cartoon of Pioneer itself, drawn to the same scale.
- arghwhat 2y ago(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 2y agoUnsure 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 https://en.wikipedia.org/wiki/Pioneer_plaque
- hackrmn 2y agoI 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 2y agoisn'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 2y agoIt 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 2y agoIt started with the French. https://en.m.wikipedia.org/wiki/History_of_the_metric_system https://en.m.wikipedia.org/wiki/History_of_the_metric_system The meter was originally based on the measured dimensions of the Earth.
- hnuser123456 2y agoAh 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.
- arthurcolle 2y agoI posted this thread to o3 and found the results interesting. https://chatgpt.com/share/680aad8d-ce54-800c-8973-df4258bbe1a6 https://chatgpt.com/share/680aad8d-ce54-800c-8973-df4258bbe1...
- mcswell 2y agoThe 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 2y agoif 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 2y agoSETI 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.
- lud_lite 2y agoPrecisely 21cm or a precise amount that is approximately 21cm?
- Calwestjobs 2y agoit 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" ...
- mkl 2y agoIt's a badly worded subtitle, probably by an editor. From later in the article: "By measuring light of precisely the needed wavelength — peaking at precisely 21.106114053 centimeters".