Question 1

What are the SI units actually based on now?

Accepted Answer

Since a redefinition that took effect in 2019, **all seven base units are defined by fixing the numerical values of physical constants** — not by physical objects, and not by properties of materials. It is the most significant change in measurement since the metric system was created.

**The problem it solved.** The kilogram was, until 2019, defined by a **physical cylinder** of platinum-iridium held near Paris. Its mass was the kilogram by definition — so if it changed, the unit changed with it. And it did: comparisons with official copies showed **divergence of tens of micrograms** over a century, and nobody could say which had drifted. **The last artefact-based unit was an embarrassment**, because the standard could be dropped, contaminated or lost.

**How the units are defined now:**

**The second** — by the frequency of a specific caesium atomic transition.

**The metre** — by fixing the **speed of light** at an exact value, making the metre the distance light travels in a defined fraction of a second.

**The kilogram** — by fixing the **Planck constant**, realised in practice with a Kibble balance that relates mass to electrical quantities.

**The ampere** — by fixing the **elementary charge**.

**The kelvin** — by fixing the **Boltzmann constant**, replacing a definition based on the triple point of water, which depended on the isotopic composition of the water used.

**The mole** — by fixing the **Avogadro constant** as an exact number.

**The candela** — by fixing a luminous efficacy value.

**Why this is better:**

**The definitions cannot drift**, because constants do not.

**They are reproducible anywhere**, by anyone with the right apparatus, including in principle off Earth — no pilgrimage to a vault required.

**Precision can improve indefinitely** as measurement technology advances, without redefining anything.

**What did not change:** the size of any unit. A kilogram remains a kilogram — the definitions were chosen to be continuous with the old ones, so no measurement anywhere became wrong overnight.

Question 2

What causes earthquakes, and how is magnitude measured?

Accepted Answer

The sudden release of elastic strain accumulated in rock, and magnitude is measured on a logarithmic scale — which is why the numbers behave in ways people consistently misjudge. The mechanism:

Question 3

How does the geological time scale work?

Accepted Answer

A hierarchy of named intervals dividing Earth's history, defined by changes in the rock and fossil record rather than by round numbers — which is why the boundaries fall at irregular dates. The

Question 4

What is the difference between mass and weight?

Accepted Answer

Mass is the amount of matter in an object and does not change with location. Weight is the force gravity exerts on that mass, and it changes with the strength of the gravitational field. An object

Question 5

How do we know how far away stars are?

Accepted Answer

Through a ladder of methods, each calibrated against the one below it — because no single technique works across the enormous range of distances involved, and the whole structure is only as sound as

Question 6

What is an effect size, and why does it matter more than significance?

Accepted Answer

A measure of how large a difference or relationship is, expressed in a standardised or interpretable form — and it answers the question significance testing does not: not whether an effect exists,

Question 7

How do we know how old the Earth is?

Accepted Answer

Principally from radiometric dating of meteorites, which give an age of approximately 4.54 billion years — and the reason meteorites are used rather than Earth rocks is itself the interesting part.

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