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).
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.
> 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.
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.
I don't know if aliens would decode it but it's not right that humans wouldn't decode it.
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.
Maxwell published in 1873. The double slit experiment was 1803, subatomic theory developed throughout the 1800s, and Planck proposed quanta in 1900. The first radio transmission across the Atlantic came approximately 2 years after Planck's theory.
I doubt it is plausible to develop anything resembling industrial technology without stumbling across certain fundamental truths in the process because doing so requires a sufficiently accurate model of physics.
It's easy to imagine that another species might have never conceptualized electrons as little balls orbiting around a nucleus. They are neither balls nor are they flying in circles, those are simply abstractions we like because they appeal to the way we perceive reality. The way we conceptualize electrons leads to issues like the wave-particle duality, so it's likely just a local optimum we got stuck in. Another species might not even think of Electrons as being distinct entities, maybe they think of the electron field as one large ocean with some waves in it, or they subscribe to the single electron theory, or something we have never thought of and might never imagine from our perspective.
It is also very highly constrained by how _we_ observe the universe. Beings with different sensory/cognitive capacities could develop very different models.
> equivalences between apparently dissimilar modes of presentation
If there was some mathematical equivalence between their models and ours, which is already a leap to assume, there is still a question about whether the specific measure used would be translated to something equivalent to our object length measure in their model, which gets much stronger than just some equivalence assumption. And it’s even stronger to assume that this equivalence could just be inferred without any other information apart from the disk.
How could we ever be certain than another intelligence (whatever that means) would be capable of understanding the intended message? Unless of course we are already starting off with the major assumption that the only things that can be intelligent are things like us. I'm not even sure that intelligent has any meaning aside from denoting behavior "similar to us".
Or to them, an atom is as large an arbitrary macro structure as proteins are to us, and so they would never consider two empty circles with a single line to represent something so big and chaotic. Or maybe they had the crazy idea of building everything of vibrating strings!
Who knows what the abstractions and approximations would be when the foundation of it all isn't "getting hit on the head by an apple".
Or even future human data archeologists digging through a mix of 20 & 21 century data heavily polluted by AI slop. ;-)
(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.)
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.
I mean, I understand how and why, but it feels odd.
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
>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.
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!
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.
https://en.m.wikipedia.org/wiki/History_of_the_metric_system
The meter was originally based on the measured dimensions of the Earth.
but i was pretty annoyed after i read in article - "exactly 21 cm" and then inside of first diagram - "v=1.4GHz" ...
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.
In fact, natural H-masers have been found: https://www.cfa.harvard.edu/news/hydrogen-masers-space
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.
Otherwise it's going to have a varying frequency - maybe not by much, and maybe not quickly, but certainly not static.
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
Yes, physicists and engineers hate me, why do you ask?
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 ..
[come on you guys]
The complete answer of the universe would of course be 42cm.
In almost every aspect this was far simpler, but there was the curious case of the constant `M_2_SQRTPI = 2 / sqrt(π)`. 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.
Doesn't seem all that great, but I'm probably missing something.
Is this why cows and horses eat them?
Fun fact, your computer is really good at answering this question for you. So is (say) Google Search.