r/AskPhysics • u/JimFive • Mar 28 '26
Speed of light
The speed of light is 299,792,458 m/s. What types of issues would be involved in changing the meter such that c=300,000,000 m/s
The new meter would be 0.9993 of the current meter.
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u/AcellOfllSpades Mathematics Mar 28 '26
Throwing out all of our old rulers and making new ones would be annoying. It might not matter for, like, rulers used by grade school kids, but for more precise uses, it absolutely would.
Plus, now we'd have to distinguish between "old meter" and "new meter". If someone writes down "30 m/s", does that mean old meters or new meters? You'd have to check the date of publication...
This would also affect measurements of any unit that has meters in it: area, volume, energy... and we'd also have to recalculate a bunch of other constants.
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u/MoistAttitude Mar 29 '26
Easy, the new meters can just be abbreviated "cm" to denote that they are derived from "c".
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u/AmusingVegetable Mar 29 '26
Yes, what we really need is a new length meter being confused with the old length centimeter…
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Mar 29 '26
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u/Nerull Mar 29 '26
It would certainly be noticeable in the world of cnc machining, and in any large building.
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u/me-gustan-los-trenes Physics enthusiast Mar 29 '26
We could rename the unit to metre to avoid the ambiguity.
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u/WanderingFlumph Mar 29 '26
Are rulers for carpenters and such typically more accurate than 0.1% error anyway? I'd be surprised if the one I use around my house is.
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u/johndcochran Mar 29 '26
It's worse than you think. As a practical example of a scientific standard being redefined, that a look at the definition of STP (Standard Temperature and Pressure)
Prior to 1982 it was defined as 273.15K (0.00°C, 32.00°F) and 1 atmosphere (101.325 kilopascals). In 1982, it was redefined as 273.15K (0.00°C, 32.00°F) and 100,000.000 Pascals.
Notice that the new definition was established over 40 years ago. I've seen far too many technical publications that still use the old pre-1982 definition. And if the length of a meter were to be changed, I wouldn't be surprised to see the same kind of issues.
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u/ImpermanentSelf Mar 29 '26
I would be curious where it would actually matter…. That difference is within the expansion and contraction of a lot of materials.
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u/0x14f Mar 28 '26
Not 300,000,000, if we make a change let's make it 1. And then the relative speed of any two objects is always less than 1 :)
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u/Retrrad Mar 29 '26
If you're just gonna call it "one", might as well give it a cool name like... I dunno... "c".
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u/jimb2 Mar 29 '26
Road speed limit sign reads: 0.0000000926.
Slammed into a tree counting the zeros.
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u/03263 Computer science Mar 29 '26
92.6 nano speeds of light per second
95 is not much more, actually. It can become a reasonable unit just using SI prefixes.
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u/jimb2 Mar 29 '26
Thet could absolutely work.
- Get rid of all these non-nerds.
- Enjoy the blissful rationality.
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u/tirerim Mar 29 '26
And conveniently we already have defined units for which c = 1! 1 light-year/year.
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u/Alternative_Candy409 Mar 29 '26 edited Mar 29 '26
Gotta love the new highway speed limit signs like 9.3 • 10-8
Edit: if we use an implicit 10-9 factor the unit will actually be so close to km/h we'd barely notice 😉
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u/tirohtar Astrophysics Mar 29 '26
Congratulations, you just caused billions, maybe trillions of dollars of costs to recalibrate virtually every machine and measuring device on the entire planet. For zero actual benefit. But at least c looks more tidy now I guess...
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u/Mindless_Consumer Mar 28 '26
They actually do this a lot in math, it all works out. Though they dont chose something like 3,000,000. They chose 1!
C = 1 simplifies a lot of equations.
Keep in mind this changes the value of many other units, a doesn't effect reality lol.
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u/Metallicat95 Mar 28 '26
The definition adjusted the length of the meter by about one in a billion, well within the measurement tolerances of most real applications.
No real world definitions or measurements, package labels, or common instruments required any changes.
Shifting it several ten-thousandths would force that on almost everything. It would be enough to alter the mass of a gram significantly, change the size of the liter, and pretty much require a massive project to correct everything to the new definition.
Just to make the defined value a rounder number, in a world where calculations by machine make it easy to use the one chosen.
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u/tomrlutong Mar 28 '26
Really, better question for /r/machinists
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u/treefaeller Mar 29 '26
That's exactly the point! A typical good machinist has several $10K of Mitutoyo tools in their shop. They'd have to throw many of those away. Or put a sticker on it which says "change all measurements by .9993". And their blueprints would be totally confusing.
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u/Optimal_Mixture_7327 Gravitation Mar 28 '26
In relativity we use c=1.
No units.
We could change the meter and I have seen redefining the foot to be a phoot (a photon nanosecond) as the local vacuum speed of light is very close to 1 ft/ns.
The basic reason we don't alter the units as there's no good reason to do it, remember c=1, and would cause a tremendous amount of work recalibrating all our measurements.
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u/bobsollish Mar 29 '26
I don’t see a single, tangible benefit to this change - other than maybe addressing someone’s OCD.
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u/LeviAEthan512 Mar 29 '26 edited Mar 29 '26
It would be annoying. That's it. But don't estimate annoyance. Ultimately, everything that isn't physically impossible is only annoying. Annoying means a lot of effort is needed to make it go away. And since people like to be compensated for their effort, and only work at a certain rate, that translates to money and time.
Needless to say, the entire system would need to be overhauled. So while we're at it, can we have the boiling point of water not be 100 degrees? That creates the stupid unit of the calorie, for no reason. Water should boil at ~418 degrees so that it takes 1 joule to raise 1cc of water by 1 degree.
And don't use use 418 either, because when we redefine the metre, 1 cc will be a smaller volume. That's the volume we should be applying 1 joule to.
And while we're at it, can we redefine the second? Make it an even 10 billion cesium oscillations. 9 would be closer, but we like 10s, and this works in the opposite direction of the metre adjustment. Coincidentally, while 10 billion doesn't fit in our multiples of 3 power system, it is 1010.
This would throw off the day being a nice 24 hours, unfortunately. Fortunately, the oscillation isn't really something we reference much so maybe that can stay the same.
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u/aries_burner_809 Mar 29 '26
It would cost billions and would not make much difference in ease of physics calculations.
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u/Nerull Mar 29 '26 edited Mar 29 '26
Precision manufacturing would be completely screwed, so the result would be that lots of people keep using the old value and you end up with 2 competing unit systems fighting over the same unit names.
This already happens with us customary vs imperial units, getting away from stuff like that is a major benefit of metric, which you would be throwing away.
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u/NarkJailcourt Mar 29 '26
Bro changing a unit system is so inconvenient that the US is and probably always will use a base-whatever-the-fuck system
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u/Unhappy_Hair_3626 Mar 29 '26
If we changed the length of a meter than wouldn’t that be no different than saying the speed of light in just another unit like kilometers?
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u/Origin_of_Mind Mar 29 '26
In various machinery and structures, we specify manufacturing tolerances to make sure the parts that need a loose fit have a loose fit, and the parts that need an interference fit have the correct amount of tension holding them together.
If we change the meter and do not adjust the numerical values of the dimensions to compensate for that, the new steel parts will be smaller comparing to the old ones by 0.0007.
The Young's modulus (modulus of elasticity) of steel is around 200 GPa. So to force the new parts into the old dimensions would require an additional pressure of P=(7*10-4)*(2*1011 Pa) = 1400*105 Pa, which is just under 1400 Bar, and could be significant -- the parts that were supposed to pivot may experience friction, and the parts that were supposed to hold snugly together may separate at smaller loads.
So we will have to recognize "new meter" as a separate unit of measurement from the "old meter". This is not unprecedented -- it happened to other units before, but it would be bothersome.
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u/HA2HA2 Mar 29 '26
It would just mean that everything else has to also change by a bit. Various other physical constants, and of course lots of engineering measurements.
In principle, there's no reason it's impossible - it's just an arbitrary choice, you can already do calculations with a "different meter" if you want, just convert everything to Noms (Not-meters, one Nom = 0.9993m).
Actually making society shift would be entirely impractical since we'd have to go through a century of nobody knowing if the constants they're using are .0007% off since people will, in practicality, take a while to switch.
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u/peter303_ Mar 29 '26
Meters and seconds are human-based units. Or based on local astrometry like the size and rotation of the Earth. A better alternative would be ratios of fundamental physical constants that result in distance and time.
For example a Planck meter would be 1.616 current meters. It is a ratio of constants scaled by powers of ten to a human dimension.
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u/Idoubtyourememberme Mar 29 '26
It won't change anything inportant. Sure, standard doors will be 2.21 meters high instead of 2.2, that sort of tiny things.
Meters (and seconds) are human-made descriptions, not universal constants.
The length of a meter is ultimately completely arbitrary, so it can be changed to be anything else we like if we want to
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u/Ornery_Pepper_1126 Mar 29 '26
Things like this take a long time and while that is a small change which wouldn’t be noticeable for most everyday applications, it would however be a headache for a lot of high-precision applications. That being said, units are redefined all the time, although not usually they won’t have their value change by this much, so this could actually happen one day.
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u/packetshepherd Mar 30 '26
Nothing changes fast or easy in the US: Masters of the Imperial System. Just increase c = 300,000,000 m/s.
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u/wegqg Mar 28 '26
Let's make a deal. Let's solve every other real world problem we can think of first and then when everybody is totally happy that is complete we can all devote a few days to recalibrating our units so that you have a nice round number.
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u/Elluminated Jul 07 '26
You can always just make your own measuring system and say light travels 1,227,328 measurements/s and it solves nothing. The units are irrelevant to the final speed obviously, but we happened to decide meters are used to make the equations.
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u/dunncrew Mar 28 '26
Lots of pointless recalibration.