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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

#181

Earlier quoted context omitted.

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…

laser, maser, like, other excitation / energy saturation does not work here ?

No, 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.

Re: One quantum transition makes light at 21 cm

#182

Earlier quoted context omitted.

The period you're describing is that of old quantum theory, which was hugely inconsistent and predates our theories of modern quantum mechanics which is post 1925 or so. The inventor of the arc converter was 18 at the time radio waves were discovered, 34 at the time he invented the arc converter, but 56 and with only 17 years left till his death when the era of modern quantum mechanics started with the invention of w…

(Not to mention that the hydrogen line was only discovered in 1951, as a result of years of hearing it using radio equipment invented half a century prior. Even things as basic as the proton took until 1932 to discover.)

So a period of 25 years to go all the way from "weird fringe theory that fixes some issues we've been grappling with for a long time now" to "got it all figured out". The start of that period aligns very closely with the initial invention of radio. And a variable effect due to the hydrogen line can potentially be observed by anyone operating a radio in the relevant band.

The only way this doesn't work is if the aliens who retrieve the plaque from deep space somehow stabilize in the long term at a point where they've developed rocketry and general space travel but not radio or an understanding of quantum mechanics.

However the above would seem to imply that they don't do radio astronomy, don't have a very good understanding of light (since it's all photons), and don't have high frequency electronic circuits (since designing those requires accounting for RF interference). I guess their understanding of optics is also lacking and their understanding of chemistry is rudimentary and stagnant over the long term.

In other words, aliens permanently stuck at a late 1800s technology level that have nonetheless developed the ability to travel across interstellar distances. And spotted voyager (a very small cold object in deep space). And retrieved it intact.

That's undoubtedly a very cool premise for a scifi story but as far as real life goes I think your time would be better spent worrying that voyager might be eaten by a species of space fairing wale.

Re: One quantum transition makes light at 21 cm

#183

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.

The standard metre was a rod 1 metre long, you might be thinking of the standard kilo which is a compact cylinder?

Was, they made the smoothest silicon sphere, Avogadro project. And now apparently they define it via physics as mentioned in ops article, "namely a specific transition frequency of the caesium-133 atom, the speed of light, and the Planck constant"

Re: One quantum transition makes light at 21 cm

#184
post #2

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.

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.

And capacitance, etc too.

Re: One quantum transition makes light at 21 cm

#185

Earlier quoted context omitted.

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.

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.

Nope.

The so called Planck units are the worst system of units conceivable and they could never be used in practice. This has nothing to do with the values of the Planck units, but with their uncertainties.

When Planck has suggested that system of units, as a possible improvement over the system of natural units proposed by Maxwell a quarter of century before him, by removing 2 somewhat arbitrary choices required by the Maxwell system of units (of 2 kinds of atoms, one for providing a frequency unit and one for providing a mass unit), that was before the development of quantum mechanics and before of the discovery of several quantum effects that are useful in metrology.

The reason why the Planck system of units is bad is because it defines the Newtonian constant of gravitation as an exact constant.

However, the Newtonian constant of gravitation can be measured only with an extreme uncertainty, many, many orders of magnitude greater than the uncertainty for measuring any other fundamental physical quantity.

By forcing the Newtonian constant of gravitation to be exact, its uncertainty does not disappear. That uncertainty just moves into the values of all other physical quantities that include mass in their dimensional formulae.

This means that in Planck's system of units most absolute values of physical quantities have uncertainties far too great to be usable. In Planck's system of units, for most quantities only the ratio between 2 quantities can be accurate, not also their absolute values.

Nevertheless, not all is bad in Planck's system of units. Only using the Newtonian constant of gravity is bad. Using the Planck constant to provide a unit of mass instead of using the mass of some arbitrary atom is good.

By combining Maxwell's system of units with the good part of Planck's system of units, you can obtain a system of natural units where there is only one arbitrary choice, of an atomic transition that can provide a unit of frequency. All the other "fundamental constants" can be defined as 1, with the exception of 2 constants that must be measured experimentally, and which provide the intensity of the gravitational interaction, i.e. the Newtonian constant of gravitation, and the intensity of the electromagnetic interaction, i.e. the so-called constant of the fine structure, a.k.a. Sommerfeld constant.

After its last revision, the International System of Units has actually become equivalent with such a Maxwell-Planck system of natural units, except that this is masked for historical reasons by the use of a large number of "fundamental constants" that are inserted into the relationships between physical quantities, and which are exact, but instead of being equal to 1 they have various weird values.

For theoretical work, or inside some simulation programs, it can be more convenient to use a system of units where all "fundamental constants" are 1, and where the unit of time is taken to be the period of the electromagnetic wave corresponding to the cesium 133 transition on which the SI is based (i.e. about 0.109 nanoseconds), so that any value in such a system of units can be converted by an exact factor into a value in SI, e.g. for displaying the results. (Actually that is what I always do.)

Re: One quantum transition makes light at 21 cm

#186

Earlier quoted context omitted.

(Not to mention that the hydrogen line was only discovered in 1951, as a result of years of hearing it using radio equipment invented half a century prior. Even things as basic as the proton took until 1932 to discover.)

So a period of 25 years to go all the way from "weird fringe theory that fixes some issues we've been grappling with for a long time now" to "got it all figured out". The start of that period aligns very closely with the initial invention of radio. And a variable effect due to the hydrogen line can potentially be observed by anyone operating a radio in the relevant band. The only way this doesn't work is if the alien…

Our timeline is not an ordered series of preconditions followed by inevitabilities. It does not hold that if some other civilization says "B", it must first have said "A" and follow by "C".

RF equipment is not built or designed by people working with or having understanding of quantum mechanics. Understanding waves could come from observing liquids, or if audio exists to them, that. Designing around noise can be done empirically as we do. Quantum mechanics in this area only matter due to our current chip manufacturing process, but who says they use semiconductors?

On the other hand they could also have a better understanding of light and photons and never build RF into their core technology stack, thereby not having people observe and analyze the hydrogen line.

The only strict order is that we know now that our previous theories were complete garbage, and we are still waiting to discover why our current theory is complete garbage.

Re: One quantum transition makes light at 21 cm

#187

Earlier quoted context omitted.

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. :)

Compared to a cold object being detected and then picked up from somewhere out in deep space?

Maybe you could detect the directional radio beam we used to talk to it first ? Would be likely doable with a stellar scale radio telescope array.

Re: One quantum transition makes light at 21 cm

#188

Earlier quoted context omitted.

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. ;-)

I really wonder how future archaelogists are ever going to decode our timeline. Imagine a meteor strikes, civilization falls apart, and in 20,000 years they dig up a data centre. Even if they get the computers to work and the hard drives are still readable, everything will be encrypted.

Or maybe you just have what was stored in various nuclear shelters at that time ? That could be even more confusing!

In one unused shelter here in Brno the numbered stones of a medieval chapel are stored since it was demolished long ago. In another shelter in Prague you can find the complete archive records of the Prague 4 city administration.

Any aliens discovering this would inevitably reach the conclusion that humans had a lot of respect to both honoring their past & for comprehensive bureaucracy. So much indeed, that when the end came, they decided to forego the temporary safety of their shelters and let this legacy of their culture survive instead!

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