Measurement and observation
Are We Measuring It, or Is It Defining the Ruler?
Every ruler we own is made of local matter and every clock counts local transitions. There is no measuring stick suspended outside the universe.
UnknownWhat we actually know
General relativity gives no absolute ruler. Locally your metre stick behaves normally, because you, the room, the clock and the atoms in all three respond to the same local conditions. We define the metre through the speed of light and the second through caesium — excellent standards, but determined with clocks and rulers occupying the same spacetime as the thing being measured.
Where the extrapolation begins
This is why unitless ratios are harder to fool: double every ruler in existence and the relationship between two axes is unchanged. But it also exposes the gap in the experiment. We can say our measurements of the object remain consistent here. That is a narrower statement than saying the object is not changing.
What the novel invents
Rourke asks the question the room cannot answer: do we have an experiment that would tell us if it altered all our instruments together? Mbane's reply is 'not yet.' The novel leaves three live readings — a rigid structure built with technology we do not understand, an object preserving its dimensions through curved spacetime, or an object adjusting its dimensions to preserve some deeper relationship — and declines to choose.
Questions to think about
- — Design an experiment that could detect a change affecting every instrument equally.
- — Why does 'our measurements are consistent' fall short of 'the object is unchanged'?
- — Which parts of the metre are natural and which are decisions people made?
Where to read next
- — SI redefinition and the physical constants underpinning it
- — Local versus global measurement in general relativity
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