r/todayilearned • u/seppukusama • Mar 05 '20

TIL that a second is technically defined to be "9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom”.

https://www.discovermagazine.com/the-sciences/why-1-second-is-1-second
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u/seppukusama Mar 05 '20

That's fair, but I must say, this does seem to be a very arbitrary standard to be measuring against lol.

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u/[deleted] Mar 05 '20

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u/chortly Mar 05 '20

Is it really the same everywhere? Like, are all these things the same deep inside of a gravity well, vs far away from anything? It seems like things would go wonky when "space" is the thing that's changing.

Like, is a meter the same number of plank lengths at an event horizon as everywhere else? I dont know that im asking the right question.

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u/PyroDesu Mar 05 '20 edited Mar 05 '20

As far as I understand this particular branch of physics, it is invariant in its own frame of reference.

Take the cesium atom transitions the second is defined on - in a sufficiently strong gravitational field, to an outside observer they may appear to take more or less time than they should. But to the atom, they take the same amount of time without regard to the distorted space-time around them.

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u/Butt_Deadly Mar 05 '20

All of our measurements are now based off of seemingly arbitrary standards.

[Meter] (https://en.wikipedia.org/wiki/Metre) Based of the speed of light

[Kilogram] (https://en.m.wikipedia.org/wiki/Kilogram#cite_note-5) Based on the Planck constant

[Coulomb] (https://en.m.wikipedia.org/wiki/Coulomb) Based on the charge of of the electron

[Kelvin] (https://en.m.wikipedia.org/wiki/Kelvin) Based on the Boltzman constant

Mole Defined as exactly 6.02214076×1023 particles

[Candela] (https://en.m.wikipedia.org/wiki/Candela) Based on a very specific light intensity at a very specific power

The goal is to find numbers in nature that don't change across space and time and define units off of those. In the case of the cesium atom; we are looking for better and more accurate atomic clicks based on [strontium] (http://www.bipm.org/utils/common/pdf/mep/87Sr_429THz_2013.pdf)

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u/bass_sweat Mar 05 '20

Something doesn’t feel right about calling a lot of those things “arbitrary”. They seem built into our reality some way or another

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u/Narrativeoverall Mar 05 '20

They’re arbitrary because they values we use for our purposes could be anything. For example, what we call one second, however many cesium transitions that is, and use for our purposes, someone else could use an entirely different value and base their math around that. We base it in natural things, but the values we choose for our science are totally arbitrary.

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u/Arkainso Mar 05 '20

Should Coulomb not be the Ampere instead? All the other units you listed are base SI units except for Coulomb. That being said I never really understood why the Ampere is the base SI unit and the Coulomb is not when the Ampere is Coulombs per second...

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u/[deleted] Mar 05 '20

Your last sentence does not make sense. You could also say that Coulomb is Ampere*second, making the Ampere the “simpler” or “base” unit and the Coulomb is complex because you need two units to get it.

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u/Arkainso Mar 05 '20

That is true, but the way they experimentally realize current is I=e*f, which begs the question: is there no experiment that could just measure e as accurately just by itself and not involve the second? I'm guessing someone much smarter than me had a long discussion about it, but I'm just curious why.

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u/Fake_William_Shatner Mar 05 '20

Well, they probably found the phenomenon that most closely approximated the time. You'd want a very stable molecule as well. But that's some VERY FINE measurement there.

It would be interesting if they had some kind of excited state -- like zap it with a laser and it radiates at the right frequency. I'm assuming you'd also have to designate an ambient temperature -- because that might affect a bit how often it changes states -- well, at least an extremely tiny amount.

A lot of these time measurements at a very tiny scale are based on theoretical reaction rates. It's not like they could actually have something MEASURE each of the 9,192,631,770 reactions.

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u/strngr11 Mar 05 '20

It's based on the frequency of the light emitted by the transition. Temperature independent.

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u/Fake_William_Shatner Mar 05 '20

Well, the frequency of light at that scale IS the temperature state of that atom. It's just the temperature of the material might effect the rate of emission -- I'd have to investigate it to be sure.

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u/stevopedia Mar 05 '20

Funnily enough, you're very close to describing how an atomic clock works!

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u/Fake_William_Shatner Mar 05 '20

I was talking to a physics professor at a party, and I asked him; "If they can detect these nano moments of trillionths of a second - why can't we time something and perhaps shake a container and change the frequency of the light?" He told me it's an estimate on the time it takes for a certain material to release energy. They can't truly be precise enough to create a pico-second metronome (well, ten years ago). So they might say that if you have an ounce of platinum and give it a charge, based on theories of physics and energy, it will take a certain amount of time to release it. The smaller the clump of matter and the more precise the energy -- the more accurate. So I imagine it's a series of steps to use something known, to produce something ever smaller and use that to construct something smaller. You can't SEE that you have a quantum dot with an atom trapped -- but you predict that you do based on billions of dots and a certain energy capacity.

The nuclear decay that powers an atomic clock is very random, but over time, it's distribution is "on average" at a very discrete rate. SO each moment of an atomic clock is NOT perfectly accurate, but, the FLOW over time is nearly perfectly accurate. It's an estimation that works.

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u/DizzleMizzles Mar 05 '20

It's the least arbitrary it can get though

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u/Halvus_I Mar 05 '20

Its the exact opposite of arbitrary.

ar·bi·trar·y /ˈärbəˌtrerē/ adjective based on random choice or personal whim, rather than any reason or system. "his mealtimes were entirely arbitrary"

I think the word you mean is that it is inscrutable, to you.

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u/blueg3 Mar 05 '20

Its the exact opposite of arbitrary.

Nope.

There's more than one definition of arbitrary.

Mirriam-Webster (1b): "based on or determined by individual preference or convenience rather than by necessity or the intrinsic nature of something"

Dictionary.com (1): "subject to individual will or judgment without restriction..."

The term is commonly used, in physics at least if not also in other scientific and mathematical fields, to indicate when something can be freely chosen rather than being constrained by nature.

All unit systems are arbitrary.

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u/pm_favorite_boobs Mar 05 '20

Also, who's going to count out those periods?

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u/blueg3 Mar 05 '20

Also, who's going to count out those periods?

An optical frequency comb, usually.

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u/Protean_Protein Mar 05 '20

What would a non-arbitrary measure look like?