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Redefinitions of SI units

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dr.bob
1304344.  Fri Nov 23, 2018 9:36 am Reply with quote

GuyBarry wrote:
If you don't like the word "assumption", substitute "postulate".


And what if I don't like the word "postulate"? OED says:

postulate /ˈpɒstjʊlət/ [noun]: A thing suggested or assumed as true

And we're back to an assumption again.

GuyBarry wrote:
I'm sure it would have been a hell of a lot easier in practical terms for the BIPM to phase in the new definition of the kilogram over a period of several years, rather than choosing a specific date for it to come into force.


Would it? Why?

GuyBarry wrote:
That doesn't mean that people won't start unofficially adopting the new definition of measurement before 20 May 2019, of course, and relative to that system the mass of the IPK falls short of 1 kg. But it still officially remains 1 kg right up until that date.


"Officially" is the correct word here. Pragmatically, everyone knows that the IPK doesn't really weigh exactly a kilogram, and the only people who would insist it still does are the kind of crazy, anally-retentive people who would insist on following a set of rules despite the fact that they're clearly bollocks*

GuyBarry wrote:
I've re-read the Guardian article now and I must apologize to Dr Terry Quinn. He did not say, as you claimed above, that "measurements over the past century have shown that the international prototype has lost around 50 micrograms". Those are the words of the journalist Alok Jha, who wrote the article.


Yes, you're absolutely right, and I apologise fully and completely to Dr Quinn.

However, here's an article in which Dr Richard Davis (Principal Research Physicist Emeritus, BIPM Consultant, winner of this year's William A. Wildhack Award for outstanding contributions to the field of metrology and measurement science, and the owner of a damn fine moustache!), whilst talking about the IPK, states:

Quote:
we estimate the mass has changed by about 50 parts per billion over 100 years


To which Dr Quinn adds:

Quote:
We want to have our reference standard better than that - units need to be defined to at least the precision with which measurements can be made.



*Or "mathematicians" as they're otherwise known ;-)

 
dr.bob
1304345.  Fri Nov 23, 2018 9:40 am Reply with quote

Alexander Howard wrote:
If Hyperloop ever build a tube five miles long and straight, with a hard vacuum, then I'd say go for it.


No point. Back in 1676, Ole RÝmer explained how you can easily measure the speed of light using a minimum of 587 million kilometres of hard vacuum.

 
GuyBarry
1304369.  Fri Nov 23, 2018 10:40 am Reply with quote

dr.bob wrote:
GuyBarry wrote:
If you don't like the word "assumption", substitute "postulate".


And what if I don't like the word "postulate"?


It seems to be good enough for this site: "one of the basic postulates of special relativity".

http://www.einstein-online.info/dictionary/constancy-of-the-speed-of-light.html

But pick your own word if you like - I can't see that it makes any difference. The point is that it's an established principle that underpins a scientific theory, and which is generally accepted as true. What word would you prefer?

Quote:
GuyBarry wrote:
I'm sure it would have been a hell of a lot easier in practical terms for the BIPM to phase in the new definition of the kilogram over a period of several years, rather than choosing a specific date for it to come into force.


Would it? Why?


Well I don't suppose everyone working in every laboratory across the world is going to be able to re-calibrate their equipment on the same day.

Quote:
Pragmatically, everyone knows that the IPK doesn't really weigh exactly a kilogram, and the only people who would insist it still does are the kind of crazy, anally-retentive people who would insist on following a set of rules despite the fact that they're clearly bollocks


So why didn't the BIPM abandon the link between the IPK and the kilogram twenty years ago when it was suspected that the IPK was losing mass? Why has it taken them all this time to bring in a new standard? Despite apparent discrepancies in the mass of the IPK, no one has been able until now to agree on a new standard that was more accurate than the IPK. Here's Wikipedia:

Wikipedia wrote:
After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures (CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, h. The decision was originally deferred until 2014; in 2014 it was deferred again until the next meeting. CIPM has proposed revised definitions of the SI base units, for consideration at the 26th CGPM. The formal vote, which took place on 16 November 2018, approved the change, with the new definitions coming into force on 20 May 2019.


As far as I can gather, it was the development of the Kibble balance that allowed scientists to indirectly measure mass with a sufficiently high precision. Flawed as it may have been, the IPK was actually the best standard for mass available until the Kibble balance was refined sufficiently. It wouldn't have lasted since 1889 otherwise.

dr.bob wrote:
However, here's an article in which Dr Richard Davis (Principal Research Physicist Emeritus, BIPM Consultant, winner of this year's William A. Wildhack Award for outstanding contributions to the field of metrology and measurement science, and the owner of a damn fine moustache!), whilst talking about the IPK, states:

Quote:
we estimate the mass has changed by about 50 parts per billion over 100 years


That's fine. But it has not changed by "50 micrograms". If it's lost 50 ppb relative to some other reference mass, then, by the definition of the kilogram, that other reference mass has increased by around 50 micrograms. There's no way of genuinely knowing whether the mass of the IPK has gone down or the other mass has gone up. All the evidence may point to the theory that the mass of the IPK has gone down; but, until an independent standard has been agreed, it can't be stated for certain.

Quote:
We want to have our reference standard better than that - units need to be defined to at least the precision with which measurements can be made.


Sure. I am not proposing that the IPK should be retained as the standard for mass now that a better one has been found!

 
tetsabb
1304379.  Fri Nov 23, 2018 12:09 pm Reply with quote

William A Wildhack -- what a superb name!

 
crissdee
1304426.  Fri Nov 23, 2018 2:28 pm Reply with quote

dr.bob wrote:
Back in 1676, Ole RÝmer explained how you can easily measure the speed of light using a minimum of 587 million kilometres of hard vacuum.


Hang on, I've got some old gas pipe and a bike pump, leave it with me and I'll get back to you!

 
dr.bob
1305034.  Thu Nov 29, 2018 6:41 am Reply with quote

GuyBarry wrote:
But pick your own word if you like - I can't see that it makes any difference.


I can't be arsed to argue semantics as it'd make a dull thread even duller. All I want to say about this is that the reason why the constancy of the speed of light has been accepted as a basis for relativity is because of the vast wealth of corroborating data. It doesn't matter how elegant a theory relativity is, or how much people like it, without the corroborating data to back it up, it wouldn't be given the time of day.

GuyBarry wrote:
Well I don't suppose everyone working in every laboratory across the world is going to be able to re-calibrate their equipment on the same day.


Is this what you genuinely think will happen? If it is, then it certainly explains why you're getting quite so het up about the definitions.

I'm pretty sure I've explained previously, both on this thread and the last one, that that's simply not how science is done in practise.

GuyBarry wrote:
So why didn't the BIPM abandon the link between the IPK and the kilogram twenty years ago when it was suspected that the IPK was losing mass?


Because they didn't have a better alternative. They knew the IPK was shit, but they hadn't yet come up with something that was less shit.

GuyBarry wrote:
Why has it taken them all this time to bring in a new standard?


Because it's not an easy thing to define in absolute terms and, as I explained above, they were waiting for the corroborating data that suggests that the new definition is a good way to go.

GuyBarry wrote:
There's no way of genuinely knowing whether the mass of the IPK has gone down or the other mass has gone up. All the evidence may point to the theory that the mass of the IPK has gone down; but, until an independent standard has been agreed, it can't be stated for certain.


I don't know how many times I can say "yes, but everyone already realised that the official definition was bollocks" in different ways. It may not have been possible to state for certain that the IPK lost exactly 50 micrograms, but it was possible to state for certain that, as an absolute definition of mass, it was a bit shit.

 
GuyBarry
1305069.  Thu Nov 29, 2018 9:18 am Reply with quote

dr.bob wrote:

I can't be arsed to argue semantics as it'd make a dull thread even duller. All I want to say about this is that the reason why the constancy of the speed of light has been accepted as a basis for relativity is because of the vast wealth of corroborating data. It doesn't matter how elegant a theory relativity is, or how much people like it, without the corroborating data to back it up, it wouldn't be given the time of day.


Well I know that and you know that. What are you arguing about?

I used the word "assumption". You didn't like it, so I substituted "postulate". You didn't like that either. You still haven't come up with a suitable word. I'm quite happy to use whatever word you feel comfortable with, but just saying "it's not an assumption" or "it's not a postulate" without saying what it actually is is just silly. There must be some word to describe an unproven but generally accepted scientific principle such as the constancy of the speed of light.

Incidentally this page uses the word "assumption", and plenty of others use the word "postulate". I can't see that it's anything to get bothered about.

Quote:
GuyBarry wrote:
Well I don't suppose everyone working in every laboratory across the world is going to be able to re-calibrate their equipment on the same day.


Is this what you genuinely think will happen?


No. That's why the sentence begins "I don't suppose..."

I think we're just arguing about words and not about anything substantial here.

Quote:
GuyBarry wrote:
So why didn't the BIPM abandon the link between the IPK and the kilogram twenty years ago when it was suspected that the IPK was losing mass?


Because they didn't have a better alternative. They knew the IPK was shit, but they hadn't yet come up with something that was less shit.


Exactly! That was my point.

Quote:
GuyBarry wrote:
There's no way of genuinely knowing whether the mass of the IPK has gone down or the other mass has gone up. All the evidence may point to the theory that the mass of the IPK has gone down; but, until an independent standard has been agreed, it can't be stated for certain.


I don't know how many times I can say "yes, but everyone already realised that the official definition was bollocks" in different ways.


Why do you have to keep saying it?

I genuinely don't know what you're arguing about. I think you're just arguing for the sake of arguing. You seem to agree with everything I say but are acting as though you disagree. It's weird.

I actually said in my earlier post:

Quote:
Despite apparent discrepancies in the mass of the IPK, no one has been able until now to agree on a new standard that was more accurate than the IPK.


What you're saying is equivalent to my statement, yet you're going on at me as though I said the opposite.

I've come across people before who are so intent on having an argument that they'll carry on banging away at a point even when it's exactly the same point that the other person is making. It makes me feel quite uncomfortable.

I think I'll withdraw from this thread now.

EDIT: All I have ever tried to say during this discussion is that although in practice scientists may be using inconsistent definitions of the units, the official definitions must always remain internally consistent. For most practical purposes, if you decide to switch to the new definition of the kilogram while sticking to the old definition of (say) the watt ( = 1 kg m^2 s^−3 ), it's not going to make any difference, because the old and new definitions have been calculated to be as close to each other as possible. But you cannot have a coherent system of units where the official definition of the kilogram is based on a fixed value for the Planck constant and the official definition of the watt is still based on the IPK.

 
dr.bob
1305196.  Fri Nov 30, 2018 9:59 am Reply with quote

GuyBarry wrote:

Quote:
GuyBarry wrote:
Well I don't suppose everyone working in every laboratory across the world is going to be able to re-calibrate their equipment on the same day.


Is this what you genuinely think will happen?


No. That's why the sentence begins "I don't suppose..."


I clearly asked the wrong question here. Do you think that "everyone working in every laboratory across the world" is going to have to re-calibrate their equipment as a result of the new SI definitions?

If no, why bring it up?

GuyBarry wrote:
I think we're just arguing about words and not about anything substantial here.


Quite the opposite, I'm trying to get an understanding of what difference you think a new SI definition will have to actual scientists working in actual labs.

GuyBarry wrote:
I genuinely don't know what you're arguing about. I think you're just arguing for the sake of arguing. You seem to agree with everything I say but are acting as though you disagree. It's weird.


I'm not arguing for the sake of arguing, I'm genuinely trying to understand what you think the practical effect a new definition of SI units will be to "everyone working in every laboratory across the world".

GuyBarry wrote:
I actually said in my earlier post:

Quote:
Despite apparent discrepancies in the mass of the IPK, no one has been able until now to agree on a new standard that was more accurate than the IPK.


What you're saying is equivalent to my statement, yet you're going on at me as though I said the opposite.


I have absolutely no problem agreeing completely with the statement above.

GuyBarry wrote:
I've come across people before who are so intent on having an argument that they'll carry on banging away at a point even when it's exactly the same point that the other person is making. It makes me feel quite uncomfortable.


That is absolutely not what I am doing.

GuyBarry wrote:
EDIT: All I have ever tried to say during this discussion is that although in practice scientists may be using inconsistent definitions of the units, the official definitions must always remain internally consistent.


If that's all you have ever tried to say during this discussion, then we would have been in total agreement.

Instead, you've said an awful lot of other things. I've gone back over the thread to figure out what they were, and it's helped a lot to understand where we differ.

For instance, when you said:

GuyBarry wrote:
We had a discussion on the forum a while back where it was suggested that it might be possible to "un-fix" the speed of light if experimental evidence pointed in that direction. This change strikes me as further indication that scientific thinking is moving in the opposite direction.


It seemed to me that you were implying that the new definition of SI units would make it impossible to make any measurements that would show that they were wrong. This is clearly nonsense. I also challenged your use of the phrase "further indication", since this new SI definition has been in the pipeline for years and it's recent acceptance was a mere formality.

You later repeated this point saying that:

GuyBarry wrote:
from next May it will become impossible to frame the hypothesis that Planck's constant (or the Boltzmann constant, Avogadro constant or elementary charge) is variable. They will become "axioms" of the system.


This is also untrue, since Physics doesn't work with "axioms". This probably explains my over-sensitivity about precise use of words like "assumption". Theories are always tested with experimental data.

When I pointed out that the SI units have been redefined in the past, and would be able to be redefined in the future, you said:

GuyBarry wrote:
If you're operating within any coherent system of units, then of course it's impossible to make an experimental observation that invalidates the definitions of that system of units. If it were possible to do so, the system of units would be inconsistent with itself.


Whilst this is true, it neglects to accept the idea that Physicists are clearly happy to operate outside of the coherent system of units. You claimed that "within the SI, it was impossible to frame the hypothesis that the IPK was losing mass, because the mass of the IPK was 1 kg by definition," and yet I pointed out members of the BIPM who clearly stated that the mass of the IPK was changing. Thus, they must've been operating outside of the coherent set of SI units and simply taking a pragmatic stance that the SI definition isn't perfect.

Just as they've always done and will continue to do.

 
GuyBarry
1305205.  Fri Nov 30, 2018 11:00 am Reply with quote

I did say I was going to withdraw, so this is just a brief response out of courtesy. The SI is clearly not perfect and has been revised many times as scientists have become able to make ever more accurate measurements. I think the changes that have been agreed for next May are not just a tweaking of the definitions, though, but a fundamental change in philosophy, where the values of certain physical constants are "hard-wired" into the system in a way that they haven't been before. That doesn't mean that the system can never been revised again, of course, but it will become noticeably harder to do so.

And that's all I have to say for now.

 
dr.bob
1305213.  Fri Nov 30, 2018 11:27 am Reply with quote

GuyBarry wrote:
II think the changes that have been agreed for next May are not just a tweaking of the definitions, though, but a fundamental change in philosophy, where the values of certain physical constants are "hard-wired" into the system in a way that they haven't been before.


I completely agree with this statement, though I'll just quickly point out again that these changes were agreed in principle years ago subject to sufficiently convincing experimental evidence being gathered to back up such a change in philosophy. All that's changed in the last year is that the evidence is now sufficiently convincing.

GuyBarry wrote:
That doesn't mean that the system can never been revised again, of course


I completely agree with this statement.

GuyBarry wrote:
but it will become noticeably harder to do so.


I don't agree with this statement, but I have been unable to convince you of my argument either on this thread or the previous one.

 
Alexander Howard
1305244.  Fri Nov 30, 2018 4:11 pm Reply with quote

Conceptually I think we doubt the wisdom of defining units by what we cannot see with our eyes and feel with our hands. That is a human reaction. The measurement of c is intangible to the senses but pretty much as exactly proven quantity as can be.

It's not like carbon-dating, all depending on someone's calibration-by-guesswork-fifty-years-ago.

I have said we should run tests to show the measurements are accurate using tangible distances, but there is no doubt in my mind that every test in a vacuum with exact, calibrated equipment will come up with the figure of 1.8026175 ◊ 10^12 furlongs per fortnight

 
dr.bob
1305541.  Mon Dec 03, 2018 9:46 am Reply with quote

Alexander Howard wrote:
I have said we should run tests to show the measurements are accurate using tangible distances


The beauty of Physics is how so many things are inter-connected, which provides you with dozens of different ways to check the same thing.

If you were asked to measure the speed of light, most people would guess the best way would be to measure the time it takes to cover a certain distance. However, look under the hood of Physics, and you'll see the speed of light popping up almost everywhere.

One or two of you might be familiar with a certain equation: E = mc^2. It's very simple to see that, if the speed of light changes, then the amount of energy associated with a certain mass would also change. This would have measurable effects from, on the large scale, the way our Sun converts matter into energy to, at the other end of the scale, how subatomic particles behave in the LHC.

There's also the less-famous and, coincidentally, much harder Einstein Field Equation, which looks like this:



Once again, there's the speed of light popping up as the constant "c" on the right hand side. This equation governs how spacetime bends in the presence of mass, so changing the speed of light here would have massive implications for the paths of any object orbiting another object. Yet again, something that could be tested and measured.

The reason why Physicists are confident enough to change the SI units is because all the different ways of measuring the speed of light consistently come back with the same result. As soon as any of the varied methods shows some discrepancy, then people would be looking very closely at it indeed.

 
GuyBarry
1322339.  Tue May 21, 2019 12:33 pm Reply with quote

It happened yesterday and we all missed it!

https://www.npl.co.uk/campaigns/world-metrology-day

Oh well. One day late, may I bid farewell to the old definitions of the kilogram, the ampere, the kelvin and the mole. It was nice knowing you.

 
dr.bob
1322385.  Wed May 22, 2019 7:57 am Reply with quote

I wonder what they'll do with the old kg reference weight. Stick it in a museum, or melt it down for scrap? :)

 
suze
1322413.  Wed May 22, 2019 11:19 am Reply with quote

It appears that they haven't decided yet! For now it is still in its vault, with 24 hour security and its own designated custodian.

The Bureau International des Poids et Mesures does maintain a small museum (visitors by appointment only) at its base in the suburbs of Paris, and that museum already holds the platinum bar which used to be the meter. It seems likely that the kilogram will join it there, although French politicians occasionally say that the meter ought to be removed to a more public museum.

 

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