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One quantum transition makes light at 21 cm

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Re: One quantum transition makes light at 21 cm

#111

Post by allemagne »

hackrmn wrote: post #103

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 us…

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.

Re: One quantum transition makes light at 21 cm

#112

Post by linschn »

arghwhat wrote: post #99
mjd wrote: post #97

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 "…

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...

The plaque also provided a drawing of the probe itself next to the two human figured, at scale.

Re: One quantum transition makes light at 21 cm

#113

Post by hnuser123456 »

nemomarx wrote: post #108
hackrmn wrote: post #103

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 us…

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.

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.

Re: One quantum transition makes light at 21 cm

#114

Post by arghwhat »

aylmao wrote: post #105
arghwhat wrote: post #99

Earlier quoted context omitted.

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...

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 th…

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.

Re: One quantum transition makes light at 21 cm

#115

Post by wwarner »

CamperBob2 wrote: post #90

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, k…

I’m reading that an H-maser emits 1.4GHz. Maybe you mean something besides it’s emission frequency?

Re: One quantum transition makes light at 21 cm

#116

Post by arghwhat »

mjd wrote: post #102
arghwhat wrote: post #99

Earlier quoted context omitted.

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...

We did that too. There is a cartoon of Pioneer itself, drawn to the same scale.

(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.)

Re: One quantum transition makes light at 21 cm

#117

Post by petsfed »

wasabi991011 wrote: post #81

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…

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.

Re: One quantum transition makes light at 21 cm

#118

Post by ncoco »

xnickb wrote: post #69
rwmj wrote: post #51

Earlier quoted context omitted.

On the other hand, since it's a property of the universe maybe now's the time to define 21 cm as this value.

then cm will become a bit longer and it'll break many things

Like the width of an A4 sheet of paper.

Re: One quantum transition makes light at 21 cm

#119

Post by geuis »

hnuser123456 wrote: post #113
nemomarx wrote: post #108

Earlier quoted context omitted.

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.

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.

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.

Re: One quantum transition makes light at 21 cm

#120

Post by zem »

belter wrote: post #36

In Contact the alien beacon arrives at 4.4623 GHz. Pi times the Hydrogen line...

"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
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