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One quantum transition makes light at 21 cm

bigthink.com

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Re: One quantum transition makes light at 21 cm

#81

Post by wasabi991011 »

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).

Re: One quantum transition makes light at 21 cm

#82

Post by AnimalMuppet »

kccqzy wrote: post #10

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.

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.)

Re: One quantum transition makes light at 21 cm

#83

Post by pixl97 »

IshKebab wrote: post #77
BurningFrog wrote: post #64

Earlier quoted context omitted.

It 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.

Wavelength isn't an object though. Like if you walk around the world you haven't made something the size of the world.

At the same time any one individual walking around the world is a highly improbable event.

Re: One quantum transition makes light at 21 cm

#84

Post by joemag »

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 ±½ to the opposite one). But then remembered that photons are spin 1 particles, so the math works out. Neat.

Re: One quantum transition makes light at 21 cm

#87

Post by [deleted] »

Night_Thastus wrote: post #22
cogman10 wrote: post #9

Earlier quoted context omitted.

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.

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.

[deleted]

Re: One quantum transition makes light at 21 cm

#88

Post by strongpigeon »

AnimalMuppet wrote: post #82
kccqzy wrote: post #10

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.

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.)

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.

Re: One quantum transition makes light at 21 cm

#89

Post by timewizard »

barbazoo wrote: post #45
amiga386 wrote: post #18

> 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

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.

Re: One quantum transition makes light at 21 cm

#90

Post by CamperBob2 »

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.

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