stupid physics questions

[pogo]

Of course they are “more or less the same”. It would not do for fundamental units to suddenly change, so all rulers, clocks, scales needed to be recalibrated. As far as almost everyone is concerned the units are unchanged.

The problem is the previous definitions were too imprecise. E.g. the metre used to be defined as the length of a metal bar in Paris. But metal shrinks and grows with changes in temperature. It loses mass to chemical processes, atomic processes. Even if you take steps to minimise these effects a metal bar in Paris is enormously inconvenient, especially as to protect it few people can be given first hand access. Most people have to work with copies, copies of copies or worst, when making the tools you use to measure.

The worst thing though was measuring a metal bar is inherently imprecise. There is a hard limit on how accurately you can measure anything, to do with the laws of physics. Normally this does not matter but when you are doing physics it becomes important. So it had to change, to the current exact definitions.

The kilogram has taken longest, still being based on a standard kilogram, as there is no simple approach like the second and metre. But arguably the case for redefining it is even stronger. There is a standard kg but kg is a unit of mass which you cannot measure directly. You can only measure its weight, i.e. mass x gravitational acceleration, but gravitational acceleration (g) can vary significantly, so a standard weight can be very inaccurate.

[5th Earth]

Zip, you're not listening.

The meter is not defined by the size of the Earth.

It hasn't been since 1960. The current definition is the distance travelled by light in vacuum in
1/299792458 second. In turn, a second is defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

These are, based on our current understanding of physics, totally invariant universal constants. All of the other seven SI units are defined in similar ways, except the kilogram. The kilogram itself hadn't been defined in terms of the meter since 1889, when the international prototype kilogram (a chunk of 90/10 platinum/iridium alloy) was made. And as I said, efforts are already being made to fix this problem.

Just look up "International System of Units" on Wikipedia.

[gravy]

That's all really interesting, the Wikipedia article about the kilogram is a really good read

https://en.wikipedia.org/wiki/Kilogram

[KlaxonCow]

In fairness, I think it's us who's not listening intently enough, as I feel it's more of a philosophical than scientific question. That the complaint is about the arbitrariness of these units.

For example, the lightyear. Sure, the speed of light in a vacuum is a universal constant, but a year is to do with the orbit of the Earth around the Sun. It's "Earth-based".

And you might say "well, how about a lightsecond, then?". But a second is an arbitrary length of time. It's man-made. It doesn't relate to any universal measure like the speed of light in a vacuum does. We could have - as Napoleon proposed - gone with "decimal seconds" instead. Divide the day not into 24 hours, 60 minutes and 60 seconds, but into 10 "decimal hours", 100 "decimal minutes" and 100 "decimal seconds". Also, of course, the 24 hour period is "Earth-based" too.

If I'm understanding the complaint correctly, then this is not about the strict definition of these units, but the choice of using them.

The speed of light in a vacuum is a universal constant. The Planck length is a universal constant. Couldn't we define a set of units that are defined solely with universal constants, so that they are genuinely universal metrics. Not man-made. Not "Earth-based". Not human-biased.

Kind of like how Fahrenheit was perfectly arbitrary. The temperature at which water freezes is 32F and boils at 212F.

32? 212? What strange arbitrary numbers.

But along comes Celsius / Centigrade to take some of that arbitrariness away. Zero is freezing and 100 is boiling. There, more sensible and memorable numbers.

But, wait, it's still not good enough. Still arbitrary. Because why consider water special? More precisely, why consider water at sea level special? Because the freezing and boiling points change according to pressure - at the top of Everest, you can boil water at a much lower temperature - so the zero and one hundred thing is so very "Earth-based", when you require it to be at sea level pressures.

So, along comes degrees Kelvin. This sets its zero as Absolute Zero. So, the origin of the scale is properly non-arbitrary - not "Earth-based" - as it's based on something universal. But, then again, the actual scale of the units thereafter is the same as Celsius / Centigrade, so that aspect is still arbitrary. Because though the origin might be universal, the scale is still based off dividing the difference between the freezing and boiling of water at sea level pressures by 100.

And I feel that what's being asked here is - as you can see in this little tale of temperature scales, how the "arbitrariness" is gradually removed and things locked to more universal concepts - why have we not gone whole hog and made a new set of properly universal units?

Units that have got bugger all to do with humans, the Earth, arbitrary substances at arbitrary pressures and so forth, so that they are entirely universal.

A set of measurements where, if aliens showed up, we could just say our unit is "10^40 Planck lengths" and they'd get it. They wouldn't have to know anything about how long the Earth takes to orbit the Sun or what "sea level" pressure might be or anything like that. Purely defined in universal terms.


Mind you, after understanding the complaint, my objection to the objection would be that, as everything's relative, you couldn't really ever do it perfectly. For example, with "10^40 Planck lengths", why are you privileging base 10 positional notation? That's based on ten fingers and toes. A perfectly human sort of measurement.

And, well, everything's relative. You can't really universally measure anything. Not really. Because, as relativity tells us, the local frame of reference of the observer matters.

I'm sitting here in a chair. Not moving.

Or, at least, not moving with respect to my own local frame of reference. But, of course, the Earth is spinning on its axis, and so I'm actually moving at quite some tangential speed - faster than the speed of sound - at this particular latitude. Because, yeah, the local latitude matters. The poles of the rotation aren't moving anywhere (within this particular rotational frame of reference) but a point on the surface perpendicular to the poles - "the Equator", loosely speaking - is travelling ~40,000 miles every ~24 hours. That's a good ~1,600 mph or so. And at latitudes in-between, our speed is somewhere in-between, according to the Cosine rule as this is the surface of a (near) sphere.

So, depending on your frame of reference, I'm not moving at all in my chair or I'm spinning around at a sizeable chunk of ~1,600 mph.

But even that's relative. The Earth also moves around the Sun too, of course. So if you consider my position relative to the geometric centre of the Sun, I'm doing a loop of loops. The Earth spinning and rotating around - and this is why I carefully said "geometric centre of the Sun" - the common centre of gravity of the Sun-Earth system, because we (and the Sun) actually orbit that common centre of gravity, in truth.

But, hey, the Sun isn't sitting still either. It's orbiting the galaxy. And, hey, the galaxy is moving around with respect to other galaxies too - we're getting close to the Andromeda galaxy (or it's getting closer to us or we're mutually getting closer to one another, depending on how you want to relatively view this situation).

There is no universal "centre" from which to hang all this.

And, of course, that's only considering space. But we exist in spacetime. Moving clocks run slower. Gravity warps spacetime. Distances actually expand and contract at relativistic speeds.

So, really, it's a relativistic impossibility to fully divorce an observation from the local "arbitrary" conditions.

And the thing is - even if you could do that, or at least get as damned close to "universal" as is logically possible - then, underlying it all, would any of this be remotely practical? The Planck length might be nicely universal but, simply, its scale is all wrong. If you want to measure more everyday things - someone's height, the distance from John O'Groats to Land's End, etc. - in terms of the Planck length, then the scale is silly. The numbers massive and, thus, horrible to deal with.

And, in the end, for all the theory and philosophising about it, it does boil down to the notion that if the units are pragmatically a bit useless, then no-one's going to give a shit.

I mean, when I go to the pub, I order a pint of beer. Not a litre of beer. The road signs still have miles and not kilometres. The newspapers still report that it's "over 100 degrees" on a hot Summer's day because that makes for a better headline.

Any unit is about commonality and consistency. That's what they're ultimately for. So if they're not sufficiently practical then it doesn't matter how theoretically "better" a set of units might be, no-one's going to arsed with them.

Or, as we do in the UK, we "context switch" to all manner of different units for different things. Because "tradition". Because when all you know is feet and inches, and Fahrenheit, then all this new-fangled Napoleonic "metric" bullshit - as your brain, apparently, is still living in the glorious days of Empire - is why you voted for that there Brexit, gorram it!

[pogo]

For example, the lightyear. Sure, the speed of light in a vacuum is a universal constant, but a year is to do with the orbit of the Earth around the Sun. It's "Earth-based".

No. Physicists are not going to base such an important unit upon something as imprecise as the time it takes to go around the sun. The lightyear is the distance light travels in 365.25 days. And as a day is 24 hours, an hour 3600 seconds, this defines it as am exact number of metres, so it is as well defined as the metre.

365.25 is pretty close to the length of the year but not quite. If it were exact we would have leap years every 4 years, but every 100 years a year which is a multiple of four goes without a leap year, except every fourth century the leap year is inserted. This happened in 2000, which exceptionally was a leap year unlike 1900 and 2100.

But probably more of a problem is that the year length changes. Very slowly but noticeably enough that long term the rule I described above will need adjusting. In theory given enough time the Earth would spiral into the sun with faster and faster orbits. But the sun will have died and burned away the inner planets long before that happens.

[zip accident]

In fairness, I think it's us who's not listening intently enough, as I feel it's more of a philosophical than scientific question. That the complaint is about the arbitrariness of these units.

Yeah, that's more or less what I'm asking. I'm not exactly complaining, more wondering. We are the fish moving out of the water (land) onto land (space). In time, our decendants' decendants will use this notation, why aren't we adopting an early variant of it now, or fairly soon?

If I'm understanding the complaint correctly, then this is not about the strict definition of these units, but the choice of using them.

More or less yeah, but it's really not intended as a complaint.

And I feel that what's being asked here is - as you can see in this little tale of temperature scales, how the "arbitrariness" is gradually removed and things locked to more universal concepts - why have we not gone whole hog and made a new set of properly universal units?

Units that have got bugger all to do with humans, the Earth, arbitrary substances at arbitrary pressures and so forth, so that they are entirely universal.

Yeah, that's about right.

Mind you, after understanding the complaint, my objection to the objection would be that, as everything's relative, you couldn't really ever do it perfectly. For example, with "10^40 Planck lengths", why are you privileging base 10 positional notation? That's based on ten fingers and toes. A perfectly human sort of measurement.

Base 12 for the win.

And the thing is - even if you could do that, or at least get as damned close to "universal" as is logically possible - then, underlying it all, would any of this be remotely practical? The Planck length might be nicely universal but, simply, its scale is all wrong. If you want to measure more everyday things - someone's height, the distance from John O'Groats to Land's End, etc. - in terms of the Planck length, then the scale is silly. The numbers massive and, thus, horrible to deal with.

It needs an exponent.

And, in the end, for all the theory and philosophising about it, it does boil down to the notion that if the units are pragmatically a bit useless, then no-one's going to give a shit.

Sure.

I mean, when I go to the pub, I order a pint of beer. Not a litre of beer. The road signs still have miles and not kilometres. The newspapers still report that it's "over 100 degrees" on a hot Summer's day because that makes for a better headline.

Try ordering a pint in the EU. Do EU newspapers report over 100 degrees on hot summers days? (you set 'em up, I knock 'em down).

Any unit is about commonality and consistency. That's what they're ultimately for. So if they're not sufficiently practical then it doesn't matter how theoretically "better" a set of units might be, no-one's going to arsed with them.

Or, as we do in the UK, we "context switch" to all manner of different units for different things. Because "tradition". Because when all you know is feet and inches, and Fahrenheit, then all this new-fangled Napoleonic "metric" bullshit - as your brain, apparently, is still living in the glorious days of Empire - is why you voted for that there Brexit, gorram it!

Preach it.

No.

Yes. It may be defined in terms of lightspeed, but it's still originally based on the year, even if not to the exact second.

The parsec is also based on Earth units, even if a bit more abstractly.

[Absolute_Zero]

This is more silly than 'stupid'.

Complaining that our daily metrics are arbitrary is just... [redacted]

Any useful, human-scale implementation of Planck units in ordinary use would require massive multipliers... the values of which would be almost [or] entirely arbitrary. Guess what? That's exactly what we have.

Usain Bolt holds the world record for 6.25e+36 Planck length in 1.777180849e+44 Planck time.

And really, that should be in binary.

[gumboot]

I look forward to something doing to length what mass does to weight.

[KlaxonCow]

Physicists are not going to base such an important unit upon something as imprecise as the time it takes to go around the sun.

I know.

You're still not listening.

I was clarifying what zip accident was asking, as everyone was erroneously taking it - as you are right now with what I said - as "our units of measurement are not precise enough". But that was not at all what was being said.

What was actually being asked was "why are our units of measurement still derived from such parochial concerns as ourselves, our planet and our daily experience, when they could - arguably should - all be derived from universal constants?".

Because units derived from universal constants would be context-free and timeless. And, well, not so up our own arses either.

I know that the year in "lightyear" is, of course, very precisely defined. This is not a question about scientific accuracy, but one more about the philosophy of how and why we choose the units of measurement that we chose.

I'm not exactly complaining, more wondering.

Yeah, to call it "a complaint" was more a figure of speech, not an accusation that you were moaning.

Like how in debates, you call what someone puts forth "an argument", but that doesn't mean they're like a married couple having an angry barney.

By "complaint", I just mean that it is a rhetorical objection. An argument to the contrary of the orthodox view. That's all.

We are the fish moving out of the water (land) onto land (space). In time, our decendants' decendants will use this notation, why aren't we adopting an early variant of it now, or fairly soon?

Basically, I think the ultimate reason for the parochial view is simply that we're still a very parochial species in the overall.

Some of us are reaching for the stars. But, like, several billion of us regularly talk to our imaginary magical friend in the sky. Arguments about "filthy foreign brown people stealing all our jobs" actually still holds plenty of currency with enough people to win a referendum - and it's not just the UK, as such extreme politics is popping up all over the globe. Tyrants in Turkey, terrorists in Nice, Trump's wall to keep out the Mexicans. And so on and so forth.

It's parochial because, collectively, we are still a very parochial species.

Base 12 for the win.

But, though we jest about that, it's kind of a measure of why what you're asking is too much. At least for now, as things currently stand.

We could never agree to change to base 12 - even though there's good arithmetical reasons to do so - so what chance getting everyone to agree to start measuring things in binary orders of magnitude of the Planck constants?

At least the average person understands what the number 12 actually is. There would be at least some hope of getting the concept of base 12 over to them.

But, well, "Planck constants? What the fuck are those when they're at home? Right, so this unit is an 'octillion' of these very tiny units. What does 'octllion' mean again?" et cetera, et cetera.

It needs an exponent.

But then it is imprecise.

And, as Absolute_Zero demonstrated, can you really imagine the BBC Sports commentators uttering "Usain Bolt holds the world record for 6.25e+36 Planck length in 1.777180849e+44 Planck time"? To remind us of that "it needs to be practical or people will not adopt it" argument I mentioned.

Any useful, human-scale implementation of Planck units in ordinary use would require massive multipliers... the values of which would be almost [or] entirely arbitrary.

I already covered that.

And really, that should be in binary.

And that one too.

[merman]

I look forward to something doing to length what mass does to weight.

That's what she said...

[Absolute_Zero]

I already covered that.

Very likely...

[it might be the proverbial day of rest™ but I've got an impending code deadline]

PS to Zip. Planck isn't the only system of 'natural' units in use, albeit the most fundamental. Different systems can simplify the math, better model physicality etc in certain fields. i.e. the mass and charge of an electron.

https://en.wikipedia.org/wiki/Natural_units

[5th Earth]

One small bit of pedantry: the Fahrenheit scale was not as arbitrary as it seems at first glance. Zero is the freezing point of a saturated saltwater solution and 100 was supposed to be average human body temperature. The main problems are that saturated saltwater isn't very common in normal life and he got normal body temperature wrong.

[KlaxonCow]

I don't know. A "saturated saltwater solution" sounds pretty bloody arbitrary to me.

And, of course, "average human body temperature" - even if he had got it right - would still fall foul of the "Earth-based / human-based" thing that was the original complaint here.

But, sure, I respect well-placed pedantry. So your mild rebuke is humbly accepted.

[gumboot]

Also, ten is a stupid base. If we move to fundamental units we should also move to a fundamental base.

[5th Earth]

Looking at wikipedia, I was wrong about the definition points for Fahrenheit. Zero was 1:1:1 water/ice/ammonium chloride and 32 was 1:1 water and ice, and body temp was supposed to be 96.

This sounds weird, but the chemical mixture has the advantage that it naturally equilibriates to a specific temperature no matter what temperature the components started as, so it's actually very reproducible. Not intuitive, but easy to make and measure.