Category
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
Two theories from Einstein, built on the observation that the speed of light is the same for everyone regardless of how they are moving — a fact that sounds harmless and forces enormous conclusions.
A planet orbiting a star other than the Sun. Thousands are now confirmed, and the striking thing is that almost none were detected by seeing them — planets are overwhelmingly outshone by their stars,
Because the definition of "planet" was written for the first time in 2006, and Pluto fails one of its three conditions — not because anything about Pluto changed, but because what we knew about its
A form of matter that does not interact with light — it neither emits, absorbs nor reflects it — but which has mass and therefore gravity. We infer it because several completely independent
Stars form when gravity overcomes pressure in a cloud of gas and dust, and they die when they run out of fuel to resist gravity with. The entire life of a star is a standoff between those two forces,
Mostly through tectonic collision thickening the crust, and they stop growing because erosion and gravity eventually balance uplift — so a mountain range's height reflects an equilibrium rather than
A mineral is a naturally occurring inorganic solid with a definite chemical composition and crystal structure; a rock is an aggregate of one or more minerals. Minerals are the ingredients; rocks are
Groundwater is water held beneath the surface in the spaces within rock and sediment; an aquifer is a body of rock permeable enough to yield usable quantities of it. Where it sits. Rainfall that
A combination of orbital variations setting the rhythm and feedbacks within the climate system amplifying them — with the underlying precondition being a continental arrangement that permits ice
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
Rarely, and through several different processes — the important point being that fossilisation requires an unusual sequence of events, which is why the fossil record is a biased sample of past life
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.
Because of the viscosity of the magma and how much dissolved gas it contains — and both are determined largely by silica content, which in turn depends on the tectonic setting. The mechanism. Magma
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:
The theory that the Earth's outer shell is divided into rigid plates that move relative to one another over a weaker layer beneath — explaining earthquakes, volcanoes, mountain ranges and the
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,
Through the accumulation of independent, mutually reinforcing evidence across many researchers and methods, until a position becomes the working assumption of a field — and it is a description of the
A study exploiting a situation where circumstances have effectively assigned people to different conditions without the researcher doing it — allowing causal conclusions where a deliberate experiment
That every member of the population has a known, non-zero chance of being included — which is a far stronger requirement than the sample merely being large, and it is why enormous samples can be
What probability means. To a frequentist, probability is the long-run frequency of an event over many repetitions. To a Bayesian, it is a degree of belief, which can be updated as evidence arrives.
The tendency for an extreme measurement to be followed by a less extreme one, simply because extreme values partly reflect chance — and failing to account for it produces confident conclusions about
Power is the probability that a study will detect an effect if one genuinely exists. Low power means a real effect is likely to be missed — and, far less intuitively, it also makes the findings that
A third factor associated with both the supposed cause and the supposed effect, creating an apparent relationship between them that is not causal — the central reason correlation does not establish