Microblog Memes

You can't measure the cesium frequency without having other units defined.

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

The classical behaviour of the electromagnetic field is described by Maxwell's equations, which predict that the speed c with which electromagnetic waves (such as light) propagate in vacuum is related to the distributed capacitance and inductance of vacuum, otherwise respectively known as the electric constant ε0 and the magnetic constant μ0, by the equation

c = 1/(ε_0*μ_0)

Based on this observation we chose what count of cesium oscillations to use to define the second and meter.

But those constants are also defined relative to the others;

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

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

https://pmc.ncbi.nlm.nih.gov/articles/PMC5907514/

Vacuum permeability μ0 = 4π × 10−7 Newton/Ampere^2

Expressed in terms of SI base units, it has the unit kg⋅m⋅s−2⋅A−2. It can be also expressed in terms of SI derived units, N⋅A−2, H·m−1, or T·m·A−1, which are all equivalent.

The new definition of the ampere fixes the value of e instead of μ0, and as a result, μ0 must be determined experimentally [BIPM 2016]. Similarly, the permittivity of vacuum ε0 = 1/μ0c2 must be determined experimentally (as it was before c was fixed in 1983). The product ε0μ0 = 1/c2 remains exact.

And here;

https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html

After Maxwell published his theory of electromagnetism, it became possible to calculate the speed of light indirectly by instead measuring the magnetic permeability and electric permittivity of free space. This was first done by Weber and Kohlrausch in 1857. In 1907 Rosa and Dorsey obtained 299,788 km/s in this way. It was the most accurate value at that time.

Many other methods were subsequently employed to further improve the accuracy of the measurement of c, so that it soon became necessary to correct for the refractive index of air since c is light's speed in a vacuum. In 1958 Froome obtained a value of 299,792.5 km/s using a microwave interferometer and a Kerr cell shutter. After 1970 the development of lasers with very high spectral stability and accurate caesium clocks made even better measurements possible. Up until then, the changing definition of the metre had always stayed ahead of the accuracy in measurements of the speed of light. But by 1970 the point had been reached where the speed of light was known to within an error of plus or minus 1 m/s. It became more practical to fix the value of c in the definition of the metre and use atomic clocks and lasers to measure accurate distances instead. Nowadays, the speed of light in vacuum is defined to have an exact fixed value when given in standard units. Since 1983 the metre has been defined by international agreement as the distance travelled by light in vacuum during a time interval of 1/299,792,458 of a second. This makes the speed of light exactly 299,792.458 km/s. (Also, because the inch is now defined as 2.54 centimetres, the speed of light also has an exact value in imperial units.) This definition only makes sense because the speed of light in vacuum is measured to have the same value by all observers;

Observe how every measurement of c measures it relative to something else!

The very fundamental constants in which we define c requires experimental measurements which we only can perform by already having other units defined and measuring things like forces relative to them! Even our choice to fix our units so c has a value more precise than we can measure simply means we choose to transfer the measurement errors into the less precise units, the non-fixed units

This does not make our measurements of c perfectly precise, it still retains measurement errors of c, but whenever we increase the accuracy in measuring c we choose to force that deviation from prior measurements into the other units depending on it. Similar to how you could first make an inaccurate calculation with Pi = 3.14 and then repeat it with hundreds of decimals, you choose to not let other numbers change Pi and let Pi change every else

You're also, separately, wrong about mass because it depends on measuring the gravitational force. The definition of mass has changed;

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

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.

The Joule and Newton and Planck constant are all defined relative to each other!

... But you can also define Joule in terms of Ohm, in turn defined by Volt, defined by... The meter, second, Ampere (defined as a given number of charges per second), and... The kilogram.

And if you look at vacuum permeability again, which defines the speed of light, that too depends on the damn Newton which once again is defined circularly as ratios to other measured constants!

Tldr: Yes some properties of spacetime are absolute.

However, they are absolute RATIOS defined relative to each other. We took something we could measure reliably and then fixed some of these values relative to our most accurate measurements, and then derived all other values from those based on our measurements of what the ratios must be.