Twilight Sparkle and Star Swirl the Bearded discuss the redefinition of the SI system of units · 10:05pm May 20th, 2019
Happy World Metrology Day everyone! The international system of units is now redefined. Isn't this exciting? Are you excited ‘cause I'm excited? I've never been so excited... The new kilogram starts today. Well maybe the precise way in which we measure the weight of coffee doesn’t send everyone over the moon, but for those of us working in instrumentation and metrology, this is exciting.
I think it’s time to revisit The Abbey of the Time Turners, the scene of my mini-epic of Equestrian Metrology: Time on Their Hooves, with Twilight and Star Swirl and let them talk about it:
Sources: Background, Twilight.
Twilight Sparkle stood next to the old bearded stallion by the stone wall separating the plaza before the abbey from the cliff edge. The wind gently shook Star Swirl’s hat making the bells tinkle.
“I’m amazed they found a way to define the kilogram as anything other than the mass of an object!” he said. “I did toy with the idea of defining a mass as the weight of a precise volume of water, but it didn’t work well—it depended on too many other things - the temperature and so on. The only reliable way was using a lump of platinum, supplied from the refineries of Princess Platinum herself—that helped to placate her after I declined her suggestion that the unit of length should be the width of her hoof—and it worked pretty well, as long as we kept it hidden from Discord. Mass was actually the easy one. Time—now that was more difficult to measure…”
“We know,” replied Twilight. “Accurately measuring time and defining the Second has been a problem for ponies since the beginning of time as was told in the story. Fortunately we now have a pretty reliable way to do it using atomic clocks, so we can define the unit of time in terms of the hyperfine transition frequency of caesium.”
“How accurate is that?”
“Pretty good. They only lose one second in over a billion years.”
Star Swirl’s mouth dropped open. “A billion years!”
“Yes.”
“As in one thousand million solar orbits?”
“Yes. Assuming the princess in charge of the sun does her job properly. Of course Celestia does, but she’s only been doing for a thousand one hundred and eleven years or so…”
“That is truly incredible. Things have come a long way since the old water clocks.” He looked across the plaza at the centuries old mechanism used to fix the schedule of Equestria in the early years of the abbey. “That led to all sorts of problems. I was always envious of other worlds where the movement of the sun and moon wasn’t down to the whims of alicorn princesses and you could set your clock by them.”
Twilight smiled. “And we can now define length as the distance travelled by light in a fixed amount of time.”
“You don’t need a platinum metre rod?”
“No. The meter is the distance travelled by light in one divided by 299,792,458 of a second
“How do you measure that?”
“Using a laser interferometer—by looking at how a beam of light reflected off a mirror is shifted, we can measure the distance to it to a fraction of the wavelength of the light.”
Star Swirl looked puzzled. This explanation was omitting a few details. “One day you must explain to me how a laser system works.”
“I think Pinkie Pie might do a better job at that.”
“You have told how to measure time and distance. But tell me again about weight? How is it possible to weigh a kilogram, without balancing it against a platinum standard?”
“It’s complicated,” admitted Twilight. “That’s why it has taken so long to get this far. But they have now built a balance which can compare the weight of an object to the electrical energy needed to hold it, and with a series of other instruments, you can use it to fix the mass of the kilogram to the value of Planck’s constant.”
“Planck’s constant?”
“It’s a fundamental constant in quantum physics. Quantum physics showed that energy carried by light is not a continuum, but is divided by into individual quanta. Light is emitted as individual photons, and energy carried by a photon is related to the frequency of the light by the Planck constant.”
Star Swirl shook his head. “Once I have finished studying the magic of friendship, I really should learn some more about modern physics.”
Twilight grinned. “You can always do both at the same time.”
See also: The Problem of Weighing the Kilogram
"You know, when I published my trilogy on the subject, How To Measure Things, Measuring More Things, and Measuring Things Even More Carefully: What The Future Holds, they were commercial failures. I always knew I would be vindicated someday."
Just don't give Discord the idea of fiddling with Planck's constant. That might have some side effects.
This is relevant. Trust me!
5061708
I suspect even Discord won't fiddle with things that don't leave things to fiddle with if they're fiddled with.
5061707
I know just how you feel. Of course you appreciate that commercial success is a fundamentally flawed measure of quality.
5061708
Would 'fiddling' include changing by many orders of magnitude? That does give a new perspective on the measurement problem in quantum mechanics.
5061716
Measure for Measure.
I like the idea of using Aluminium Ion clocks placed at distances from the mass inquestion and taking the differential gravitational temporal rates as the measure of the mass. Then the kilo just becomes that point mass which causes so many seconds per second of change in rate of time between the two clocks at the two radii. With all the appropiate corrections, Including hopefully new ones that such measurements would create.
I mean, you couldnt do it with a pair of 1kg atomic clocks because then they would always be equidistant.
Wouldnt a measurement in freefall in a strong gravitational field be different than a measurement in freefall in deep intergalactic void?
What is the gravitational field strength, and tidal rate, at the expected event horizon of that Gigamass black hole they imaged?
This is related but not about SI-Units:
With time, computers have to have finer ticks (seconds, milliseconds, microseconds, nanoseconds, et cetera). Rather than the unitconversion, made more difficult because on nonterminating binarial representation of fractions of powers of 10 every time we go to finer ticks, we should use PlanckTimeUnits. a 256-bit signed representation would have a total range of 6.8*10^24 Julian Years. That is from 3.4*10^24 years before the Big Bang to 3.4*10^24 years into the future. We would not have the problem of making the ticks finer in computerscience again.
5061735
That would work in principle... but the uncertainties!
5061911
Designing your system to last 1024 years... That is really future proofing.
5063123
And every tick can be signed with a SHA 256 hash.
5063123
With the tick-length of Planck-Time, 128 bits would be a fraction of a second. the 10^24 years just falls out of using Planck-Time and needing 256 bits for the counter. Frankly, it surprises me that you, as a Physicist, do not advocate 512 bits for simulating the evaporation of supermassive black holes (almost 10^102 years if we use 512 bits for representing time).
5063335
Ha. A computationally expensive hash which cannot be doe in a tick, which we shall not save. Of course, the ticks are not even direct ticks anyway:
We have nothing ticking at Planck-Time-Units, so we would measure our period phenomenon and multiply it to get the ticks. The Planck-Time-Units would be a universal currency of time. We would have an invariant count from our arbitrary 0-Time; let us say, 00000-01-01T00:00:00 HC (Holocene-Calendar (it is 12019-05-24T09:57:38Z)). It would be a signed integer, so the clock can easily handle negative dates.
5063538
From my rough estimations, due to the general slowing down of time in gravitational fields, we in the galactic supercluster are a million years approximately behind that of a clock in the middle of The Great Void. I would love to see an Isochronous map equivalent of the universe done by experts. Then we can really start to argue just when Now is.
5063541
This is getting into the weeds, but we should use Barycentric Coordinate Time.