Science words, plainly

Plain meanings for the words used across this site, from science and philosophy to belief, starting with the words of method: how evidence is weighed, how ideas are tested and how claims are labeled. Every definition follows a checked Library answer or a named source and links to answers where the word does its work. Labels describe the evidence for a claim, not the people who hold it. No physics degree required.

How we know

Established

Strong, repeated evidence and broad scientific agreement. Example: “The universe is about 13.8 billion years old” carries this label. Read more: How old is the universe, and how do we know? · What is a black hole, really? · Does science ever prove anything for certain?

Supported, still debated

Good evidence; the details are still argued. Example: “Many Sun-like stars have a rocky planet in the habitable zone” carries this label. Read more: Is life likely to be rare or common in the universe? · Can meditation change the brain? · Is the universe fine-tuned for life?

Open question

Nobody knows yet. Serious ideas compete. Example: “Life exists somewhere beyond Earth” carries this label. Read more: Is life likely to be rare or common in the universe? · Did time begin at the Big Bang? · Is the mind just the brain?

Not supported

Checked, and the evidence does not back it. Example: “Black holes suck in everything around them, like cosmic vacuum cleaners” carries this label. Read more: What is a black hole, really? · Does astrology work? What happens when it is tested? · Have UFOs been shown to be alien spacecraft?

Author’s hypothesis

Ricardo Maldonado’s own proposal. Unconfirmed, and testable. Example: HD-Blast, the idea that our hot beginning may be the aftermath of a burst of energy in a larger space, carries this label. Read more: What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong?

Beyond science

Philosophy, values or belief. Science can inform it but not settle it. Example: “God exists, or God does not exist.” Evidence can’t settle it either way; the label is not a verdict against it. Read more: What are the main arguments for God's existence, and how do people respond to them? · What is the meaning of life, and who gets to decide? · Is there life after death? What can science say?

Anecdote

A personal story about one case, such as “it cured my diabetes”; a striking story is one case, not a number, and no substitute for scientific evidence. Example: Before trusting a striking story, ask for the base rate: how common is this to begin with? Read more: What are the most common thinking traps, and how do we avoid them? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Apophenia

A tendency to see meaningful connections in random events, a leaning toward false-positive errors; Michael Shermer calls it patternicity. Example: Shermer’s example: a rustle taken for a predator costs little if it was wind; the opposite mistake might cost a life. Read more: What are the most common thinking traps, and how do we avoid them?

Assumption

Something taken as true in order to run or read a test; if it is wrong, the result can be misread, but assumptions can be tested too. Example: Uranus strayed from Newton’s predictions; assuming Newton was right, Adams and Le Verrier proposed an unseen planet, and Neptune was found right there. Read more: Does science ever prove anything for certain? · What future measurement could prove the HD-Blast hypothesis wrong?

Barnum (Forer) effect

The tendency to accept general statements that fit almost anyone as a special description of ourselves. Example: In Forer’s 1949 class, 39 students rated one shared sketch, taken largely from an astrology book, 4.26 out of 5 for fitting them. Read more: Does astrology work? What happens when it is tested? · What are the most common thinking traps, and how do we avoid them?

Base rate (prior)

How common something is to begin with, or how likely a claim was before new evidence came in, based on everything already known. Example: People often give base rates too little weight, though a 1996 review found they almost always use them to some degree. Read more: What are the most common thinking traps, and how do we avoid them? · Why do extraordinary claims need extraordinary evidence? · Does science ever prove anything for certain?

Bayesian reasoning

A way of updating belief: start with the odds you had before, then raise or lower them by how strong the new evidence is. Example: A claim that clashes with much well-tested knowledge starts with low odds, so it needs strong evidence to become likely. Read more: Why do extraordinary claims need extraordinary evidence? · Does science ever prove anything for certain?

Bias

In research, a flaw in how a study is designed or how information is collected or interpreted that can lead to wrong conclusions; in thinking, a predictable lean in judgment. Example: A researcher who knows which patients got the real drug may assess them differently without realizing it; double-blind designs guard against this. Read more: What are the most common thinking traps, and how do we avoid them? · Do psychic abilities like telepathy or seeing the future hold up when tested? · Can meditation change the brain?

Calibration

Setting a curve or model using data already seen; matching that same data later is not a prediction. Example: The HD-Blast curve (Author’s hypothesis) was set using data already in hand, so matching that data is calibration, not a successful prediction. Read more: What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong?

Confirmation bias

The habit of seeking or reading evidence in ways that favor what we already believe; its close cousin, motivated reasoning, is reaching the conclusion we want and then finding reasons. Example: In Wason’s 1960 number-rule test, people mostly tried sets that fit their guess, so every “yes” seemed to confirm it. Read more: What are the most common thinking traps, and how do we avoid them? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Control group

In a study, a group treated the same way as the group getting the treatment being tested, except that it does not get that treatment, so the two can be compared. Example: A 2022 study compared a mindfulness course with an active control, a matched health program, and found no evidence of extra change in brain structure. Read more: Can meditation change the brain? · When does grief need more help? · How can you tell good science from bad science?

Correlation vs. causation

A correlation is a link between two things that tend to go together; it does not show which causes which, or whether either causes the other. Example: People who attend religious services at least monthly more often say they are very happy, but Pew says it is unclear whether religion causes this. Read more: What makes a life well lived? · How can you tell good science from bad science? · How do I tell my family I have doubts about my faith?

Cross-sectional study

A study that compares different groups at one moment; it can show a difference but not what caused it. Example: Many studies compare long-time meditators with non-meditators at one moment, but such people may have differed from the start. Read more: Can meditation change the brain?

Deepfake

A video or audio clip made or changed by AI so that it looks or sounds real. Example: In a 2024 study of 1,276 people, the average ability to tell real media from AI-made media was close to chance. Read more: How can I check a viral video before I share it?

DOI

Short for digital object identifier: a long-lasting ID for a paper, data set or other work, usable as a link that finds it wherever it is kept. Example: A DOI is not a stamp of approval: a Zenodo DOI preserves a public record but does not mean peer review. Read more: What future measurement could prove the HD-Blast hypothesis wrong? · What are near-death experiences, and what do they tell us?

Double-blind test

A test in which neither the people taking part nor those judging the results know who got which treatment, or which answer is right, so hopes and hints can’t steer the result. Example: In a 1985 double-blind test, astrologers picked the right personality profile for a birth chart about one time in three, about what chance gives. Read more: Does astrology work? What happens when it is tested?

Effect size

A number for how big an effect or link is, not just whether it beats chance; in the astrology studies, 0 means no link and 1 means perfect prediction. Example: The time-twin study found an effect size of 0.00, give or take 0.03, where astrologers’ claims imply at least 0.5. Read more: Does astrology work? What happens when it is tested? · Can meditation change the brain? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Evidence

Observations and test results that can count for or against an idea; raw data become evidence once they are interpreted in a way that bears on whether the idea is right. Example: Stars at a galaxy’s edge orbiting too fast are evidence of unseen mass; what that mass is remains unknown. Read more: Does science ever prove anything for certain? · How can you tell good science from bad science? · Why do extraordinary claims need extraordinary evidence?

Extraordinary claims need extraordinary evidence

The principle that the more a claim conflicts with well-tested knowledge, the stronger the evidence it needs before we accept it; the person making the claim must supply that evidence. Example: Drifting continents and the ulcer germ once sounded extraordinary and were accepted when the evidence grew strong; cold fusion in 1989 did not hold up. Read more: Why do extraordinary claims need extraordinary evidence? · Do psychic abilities like telepathy or seeing the future hold up when tested? · Does science ever prove anything for certain?

Falsifiable

Able to clash with some possible observation, so that evidence could show the idea is wrong; scientists often mean simply “testable.” Example: Einstein’s theory of gravity risked a clear prediction, that light passing a massive body would bend; Eddington’s 1919 observations bore it out. Read more: Does science ever prove anything for certain? · What future measurement could prove the HD-Blast hypothesis wrong? · What are the most common thinking traps, and how do we avoid them?

Heuristic

A mental shortcut or rule of thumb that gives quick answers; heuristics usually work but can lead to predictable errors. Example: The availability heuristic judges how common something is by how easily examples come to mind; classic tests of it have a mixed record. Read more: What are the most common thinking traps, and how do we avoid them?

Hypothesis

A proposed explanation that can be tested against observations, stated in a way that allows it to be rejected. Example: Hypotheses do not grow up into theories: the two differ in how much they cover, not in how well they are supported. Read more: Does science ever prove anything for certain? · How can you tell good science from bad science? · What is the HD-Blast hypothesis?

Inference

A conclusion drawn from what was observed, as opposed to the observation itself. Example: The step from “I experienced this” to “a spirit caused it” is an inference, a natural one that still needs checking. Read more: Are ghosts real? What does the evidence say? · How can you tell good science from bad science? · Is life likely to be rare or common in the universe?

Lateral reading

Checking a source by leaving it and opening new tabs to see what other sources say about it, as professional fact-checkers do. Example: In one study, 10 professional fact-checkers reached better-supported judgments, in less time, by reading what others said about a site. Read more: How can I check a viral video before I share it?

Lines of evidence

Evidence from one kind of test or observation; when several independent lines agree, confidence grows, because one flaw is less likely to explain them all. Example: Fast-orbiting stars, tightly bound galaxy clusters, bent light and the oldest light all point to unseen mass. Read more: How can you tell good science from bad science? · How do we know dark matter exists if we cannot see it? · Does science ever prove anything for certain?

Longitudinal study

A study that follows the same people over many years, instead of taking one snapshot. Example: Since 1938, the Harvard Study of Adult Development has followed 724 men. Read more: What makes a life well lived?

Meta-analysis

A study that pools the results of many earlier studies to estimate an overall effect and get a clearer answer. Example: A 2024 meta-analysis pooled 78 ganzfeld telepathy studies and reported a small effect; critics say unpublished failures could explain such results. Read more: Do psychic abilities like telepathy or seeing the future hold up when tested? · Can meditation change the brain? · When does grief need more help?

Methodological naturalism

The working rule of science: explain things using only natural causes and natural laws, a rule that says nothing about whether anything supernatural exists. Example: People of faith and of no faith do science side by side under this rule. Read more: Can someone love science and still believe in God?

Observation

Noting or recording a result or event, either directly with our own senses or indirectly with tools. Example: “A bright light moved fast across the sky” reports an observation; “it was an alien craft” is an inference that needs its own evidence. Read more: How can you tell good science from bad science? · Have UFOs been shown to be alien spacecraft? · Are ghosts real? What does the evidence say?

P-value

A measure of how often chance alone would give a result at least as strong as the one observed. Example: Ertel’s p-value of .054 for the astrologers means pure guessing would do at least that well about 5 times in 100. Read more: Does astrology work? What happens when it is tested? · Does science ever prove anything for certain? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Peer review

Checking of a research paper by other specialists, often before a journal publishes it; useful scrutiny, but not a guarantee that a finding will hold up. Example: A former BMJ editor reports that in tests with deliberately inserted errors, most reviewers spotted only about a quarter of them. Read more: How can you tell good science from bad science? · Is life likely to be rare or common in the universe? · Could a machine ever be conscious?

Placebo

A dummy pill or treatment with no active medicine, used as a comparison for a real treatment so that people in a study do not know which one they got. Example: In a 2016 grief trial, 82.5% given grief-focused therapy plus a placebo pill were rated much improved, against 54.8% given the placebo pill alone. Read more: When does grief need more help?

Placebo effect

An improvement, or apparent improvement, that comes from receiving some kind of treatment rather than from the treatment itself, which can make even ineffective treatments look helpful. Example: That is why many trials compare a group given a placebo with a group given the new treatment. Read more: When does grief need more help?

Prediction

What we would expect to observe if an idea were true, worked out before the test; a fit to data already in hand is not a successful prediction. Example: A real pattern should predict something you have not seen yet, so test it on new data. Read more: What are the most common thinking traps, and how do we avoid them? · What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong?

Preprint

A research paper shared before peer review; it may still be useful, but its status should be clear. Example: A 2025 preprint, not yet peer-reviewed, found only marginal signs of the gas DMS in four new Webb observations of K2-18b. Read more: How can you tell good science from bad science? · Is life likely to be rare or common in the universe? · Could a machine ever be conscious?

Preregistration

Posting a study’s plan and analysis before collecting data, so the results cannot shape the method after the fact. Example: A 23-lab preregistered replication with 2,141 people found the claimed “ego depletion” effect near zero. Read more: What are the most common thinking traps, and how do we avoid them? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Proof

In mathematics and logic, a chain of reasoning that shows a statement must follow from agreed starting rules; science does not produce proofs in this sense. Example: Andrew Wiles’s 1995 proof of Fermat’s Last Theorem covers every case at once, which no amount of checking examples could do. Read more: Does science ever prove anything for certain?

Publication bias

When studies with positive results get published more often than failed ones, so the published record looks stronger than the full picture. Example: A 2006 review of 380 mind-over-machine studies found a very small effect that publication bias could in principle explain. Read more: Do psychic abilities like telepathy or seeing the future hold up when tested? · Can meditation change the brain? · When does grief need more help?

Randomized trial

A study in which people are assigned to treatments by chance, so the groups can be compared fairly. Example: A 2014 review of 47 randomized trials found mindfulness programs gave small-to-moderate relief from anxiety, depression and pain. Read more: When does grief need more help? · Can meditation change the brain? · Is it possible to make peace with the idea of dying?

Replication

Repeating a study or observation with new data, ideally by an independent team, to see whether the result holds up; rerunning the same data and code is called reproducibility. Example: When 100 psychology studies were repeated, 97% of the originals but only 36% of the repeats had statistically significant results. Read more: How can you tell good science from bad science? · Does science ever prove anything for certain? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Reverse image search

A search that starts from a picture instead of words and finds where else that picture appears online. Example: Screenshots of key frames from a video can be searched like photos to find older copies. Read more: How can I check a viral video before I share it?

Sample size

The number of people, cases or measurements a study includes; all else being equal, a larger sample gives more confidence that it represents the whole group. Example: Pew’s 2023–24 Religious Landscape Study surveyed 36,908 U.S. adults. Read more: What is the difference between an atheist, an agnostic and a “none”? · Can meditation change the brain? · Do psychic abilities like telepathy or seeing the future hold up when tested?

Scientific law

A compact description of a pattern in data, a statement of what we expect to happen, which can have exceptions. Example: The ideal gas law links a gas’s pressure, volume and temperature, yet real gases do not follow it exactly. Read more: Does science ever prove anything for certain?

Scientific method

A name for the ways science tests ideas against evidence; the National Academies say there is no fixed set of steps that every scientist follows. Example: A lab may change one factor while holding others steady; an astronomer compares light from objects she cannot bring into a lab. Read more: How can you tell good science from bad science? · Does science ever prove anything for certain?

Sigma (σ)

A measure of how unlikely a result would be if only random noise or chance were at work; bigger numbers mean a fluke is less likely. Example: Five sigma, about 1 in 3.5 million by noise alone, is a common discovery bar in particle physics; hidden errors must still be ruled out. Read more: Does science ever prove anything for certain? · Will the universe end, and if so, how? · Is life likely to be rare or common in the universe?

Statistical significance

A result is statistically significant when it would be unlikely if chance alone were at work; it does not mean the effect is strong. Example: A p-value depends heavily on how big a study is, so a tiny edge can be significant in a big enough pile of trials. Read more: Do psychic abilities like telepathy or seeing the future hold up when tested? · Does science ever prove anything for certain? · What makes a life well lived?

Survivorship bias

Drawing conclusions only from the cases that made it through, because the ones that didn’t are not there to be seen. Example: In World War II, Abraham Wald built methods to reason about the planes that never came back, not only those that did. Read more: What are the most common thinking traps, and how do we avoid them?

Theory

In science, a comprehensive explanation of some part of nature that is supported by a vast body of evidence, such as cell theory or plate tectonics. Example: In everyday talk “theory” can mean a hunch, so calling evolution “just a theory” misreads the word. Read more: Does science ever prove anything for certain? · How can you tell good science from bad science?

Uncertainty

The range of possible values or explanations left by limits in a measurement or study. Example: Planck gives the universe’s age as 13.787 billion years, plus or minus 0.020 billion; error bars show such ranges on graphs. Read more: How can you tell good science from bad science? · How old is the universe, and how do we know? · Does science ever prove anything for certain?

Zenodo record

A dated entry in Zenodo, an open research archive run at CERN, that fixes a date and its contents; it preserves work but is not peer review or an independent test. Example: The HD-Blast benchmark (Author’s hypothesis) was frozen in a public Zenodo record dated 17 December 2025. Read more: What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong? · Did time begin at the Big Bang?

↑ All groups

The universe

Antimatter

Particles paired with matter particles, with the same mass and opposite charge when charged; when a matching pair meets, their energy becomes other particles and radiation. Example: Tests find some differences between matter and antimatter, such as CP violation, but they cannot yet explain the early surplus of matter. Read more: Why is there any matter left in the universe at all?

Bell’s theorem

John Bell’s 1964 result that any theory with local hidden variables must obey a limit, called a Bell inequality, that quantum theory predicts can be broken. Example: Tests from 1972 onward, including loophole-free tests in 2015, broke that limit; Aspect, Clauser and Zeilinger shared the 2022 Nobel Prize. Read more: Did Einstein lose his argument that God does not play dice? · Can quantum entanglement send messages faster than light? · Is anything truly random?

Big Bang

The hot, dense early state of the universe about 13.8 billion years ago, and the expansion and cooling that followed; it was not an explosion at one place in space. Example: It happened everywhere at once, as space itself began to stretch. Read more: Did time begin at the Big Bang? · How old is the universe, and how do we know? · If the universe is expanding, what is it expanding into?

Big freeze (heat death)

An ending in which space expands forever and the universe grows colder, darker and emptier until almost nothing happens. Example: It is what we expect if dark energy, the unknown cause of the speeding-up expansion, stays constant. Read more: Will the universe end, and if so, how?

Black hole

A huge amount of matter packed into a very small space, with gravity so strong that nothing crossing its event horizon can get back out, not even light. Example: Black holes do not suck things in like vacuum cleaners; from far enough away, they pull like any object of the same mass. Read more: What is a black hole, really?

Brane and bulk

In some physics theories, a brane, short for membrane, is a sheet-like universe such as ours, and the bulk is the larger space with extra dimensions it sits in; none has been confirmed by experiment. Example: Some ideas picture the Big Bang as a collision of branes; like other ideas about before the Big Bang, it remains an open question. Read more: Could there be other universes beyond ours? · Did time begin at the Big Bang? · What is the HD-Blast hypothesis?

Chaos

Behavior of a system that follows fixed rules but is so sensitive to its starting point that long-range prediction fails. Example: Dice, coins and weather follow fixed physical rules; their randomness comes from details we can’t track. Read more: Is anything truly random?

Constant of nature

A number in the laws of physics that has to be measured, such as the strength of gravity or the fine-structure constant. Example: The fine-structure constant, a pure number that sets the strength of electromagnetism, is about 1/137; no accepted theory explains why. Read more: Is the universe fine-tuned for life? · What are the main arguments for God's existence, and how do people respond to them?

Cosmic inflation

A proposed burst of extremely fast expansion in the universe’s first instant, before the hot Big Bang; the leading picture of that moment, it helps explain why the universe is so smooth and flat. Example: Eternal inflation is a version in which the fast expansion never stops everywhere, leaving pocket universes. Read more: Could there be other universes beyond ours? · Why is there something rather than nothing? · What is the HD-Blast hypothesis?

Cosmic microwave background

The faint glow left from when the universe was about 380,000 years old, the oldest light we can observe, filling the whole sky. Example: Its tiny hot and cold spots record what the early universe contained and help measure the shape of space. Read more: How old is the universe, and how do we know? · How do we know dark matter exists if we cannot see it? · Is the universe infinite?

Cosmological constant

A term in Einstein’s gravity equations that can act like a fixed energy of empty space; it is the simplest model of dark energy, a steady push that never changes. Read more: Why is the expansion of the universe speeding up? · Will the universe end, and if so, how?

Dark energy

The name for whatever is making the expansion of the universe speed up; it makes up about 70% of the universe, and its nature is unknown. Example: In 2025 the DESI survey hinted that dark energy may be weakening, short of the 5-sigma discovery standard; newer checks are mixed. Read more: Why is the expansion of the universe speeding up? · Will the universe end, and if so, how?

Dark matter

Matter that does not give off, absorb or reflect light, known so far only through its gravity. Example: Rotation curves, graphs of orbit speed against distance from a galaxy’s center, show edge stars moving about as fast as inner ones. Read more: How do we know dark matter exists if we cannot see it? · Why is the expansion of the universe speeding up?

Decoherence

The loss of quantum interference when a system interacts with its surroundings, so that its quantum states fade; it is at the center of the debate over quantum effects in the warm brain. Example: Max Tegmark calculated in 2000 that brain quantum states would fade in under a trillionth of a second; Orch OR supporters dispute his model. Read more: What does quantum physics really say about consciousness and healing?

Entanglement

A link between particles that have interacted, so that measurements on them stay related even far apart; it cannot carry messages faster than light. Example: Einstein called it “spooky action at a distance”; the 2022 Nobel Prize in Physics honored experiments with entangled photons. Read more: Can quantum entanglement send messages faster than light? · Did Einstein lose his argument that God does not play dice? · What does quantum physics really say about consciousness and healing?

Event horizon

The boundary around a black hole: once anything crosses it, even light, it cannot get back out, though it is not a solid surface. Read more: What is a black hole, really?

Fine-tuning

When a result, such as a universe able to support life, depends sensitively on some numbers falling in narrow ranges; the word does not by itself imply a tuner. Example: In a 2000 study, changing the strong force by more than about 0.5% cut the carbon or oxygen stars make by 30 to 1,000 times. Read more: Is the universe fine-tuned for life? · What are the main arguments for God's existence, and how do people respond to them?

Flat space

Space where the angles of a triangle add up to 180 degrees and parallel lines never meet, as on a sheet of paper. Example: Space is flat to within about 0.2%, but a flat space can still loop back on itself, like a video game screen. Read more: Is the universe infinite?

General relativity

Einstein’s theory of gravity, in which mass and energy bend space and time. Example: GPS satellite clocks are set to run slower before launch; without such relativity fixes, GPS would not work. Read more: Did time begin at the Big Bang? · Is time travel possible? · What is a theory of everything, and will we ever have one?

Gravitational lensing

The bending of light by mass, which astronomers use to map where mass is, whether it shines or not. Example: In the Bullet Cluster, lensing maps show the mass follows the galaxies, not the hot gas that holds most normal matter. Read more: How do we know dark matter exists if we cannot see it?

Gravitational wave

A ripple in space and time that spreads out when heavy objects, such as black holes, speed up or collide; the slowest ones take years to pass by. Example: Since the first detection on September 14, 2015, detectors have logged 390 signals, many from colliding black holes. Read more: What is a black hole, really? · What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong?

Gravitational-wave background

Many overlapping gravitational waves blended into a steady murmur, the “hum,” instead of one loud event. Example: In 2023, NANOGrav reported evidence, not yet a firm detection, for a slow hum in data on 67 pulsars. Read more: What future measurement could prove the HD-Blast hypothesis wrong? · What is the HD-Blast hypothesis?

Hawking radiation

A very faint glow that Stephen Hawking predicted black holes should give off because of quantum effects near the event horizon; it has not been observed. Example: The claim that black holes slowly give off heat this way, and could in time evaporate, is labeled Supported, still debated. Read more: What is a black hole, really?

HD-Blast

An Author’s hypothesis by Ricardo Maldonado, author of GOD PLAYS DICE: our hot beginning may be the aftermath of a brief burst of energy in a larger space with extra dimensions. Example: No one has observed such an event. No test of the frozen curve against later, independent pulsar-timing data has been published. Read more: What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong? · Did time begin at the Big Bang?

Hubble constant

How fast space is stretching today, usually given in kilometers per second per megaparsec, a distance of about 3.26 million light-years. Example: Astronomers measure the expansion by comparing galaxies’ distances with the stretching of their light. Read more: How old is the universe, and how do we know?

Hubble tension

The disagreement between two ways of measuring how fast space is stretching today: nearby galaxies give one rate, and the early-universe prediction gives another. Example: It is an open puzzle about today’s expansion rate, not a sign that the Big Bang is wrong. Read more: How old is the universe, and how do we know?

Hubble–Lemaître law

The pattern that galaxies move away from each other at speeds proportional to their distance; the International Astronomical Union recommended this name in 2018. Example: In 1927 Georges Lemaître, a Belgian physicist and Catholic priest, proposed that the universe is expanding. Read more: Was the Big Bang first proposed by a priest?

Interstellar object

An object, such as a comet, that comes from outside our solar system. Example: 3I/ATLAS, found on July 1, 2025, is the third known, after ‘Oumuamua in 2017 and 2I/Borisov in 2019. Read more: Was interstellar comet 3I/ATLAS an alien probe?

Knee

The bend in the HD-Blast benchmark curve near one cycle per year, part of the Author’s hypothesis, not a measured feature. Example: If later pulsar data, fully analyzed by the teams, show a straight line with no bend there, this version of the idea fails. Read more: What is the HD-Blast hypothesis? · What future measurement could prove the HD-Blast hypothesis wrong?

Lambda-CDM

The standard model of cosmology: a universe of ordinary matter, cold dark matter and dark energy (Lambda), described by six basic numbers. Example: Planck’s age of 13.787 billion years assumes this standard model is right. Read more: How old is the universe, and how do we know?

Local hidden variables

Pre-set instructions a particle might carry that fix its results in advance, with no influence faster than light; Bell tests rule out this local kind. Example: Einstein hoped hidden details, never faster than light, decided each result; experiments from 1972 to 2015 went against that local version. Read more: Did Einstein lose his argument that God does not play dice? · Is anything truly random? · Can quantum entanglement send messages faster than light?

Loop quantum gravity

A candidate theory that builds a quantum version of general relativity directly and suggests space comes in tiny separate pieces; it has no confirmed test yet. Read more: What is a theory of everything, and will we ever have one?

Many-worlds interpretation

A reading of quantum physics in which every possible outcome of a quantum event happens, each in its own world. Example: A decisive test between it and other readings is beyond today’s technology. Read more: Could there be other universes beyond ours? · Did Einstein lose his argument that God does not play dice? · Is anything truly random?

Measurement (in quantum physics)

An interaction between a particle and its surroundings, such as a detector, that gives a definite result; it does not require a watching mind. Example: Caltech calls the word “observation” misleading here, because it suggests a mind. Read more: What does quantum physics really say about consciousness and healing? · Is anything truly random?

MOND

Modified Newtonian Dynamics, a proposal by Mordehai Milgrom (1983) that gravity changes at very low accelerations, instead of there being unseen mass. Example: Some scientists think gravity itself needs changing, but no such change yet explains all the evidence without dark matter. Read more: How do we know dark matter exists if we cannot see it?

Multiverse

A proposed collection of many universes, possibly with different laws or constants; different theories mean very different things by it, and none has been observed. Example: A 2011 search of the Big Bang’s afterglow for scars from other bubble universes found no evidence for them. Read more: Could there be other universes beyond ours? · Is the universe fine-tuned for life? · What are the main arguments for God's existence, and how do people respond to them?

Observable universe

The part of the universe whose light has had time to reach us since the Big Bang, a sphere centered on the observer; its edge is a horizon, not a wall. Example: Its edge is now about 46 billion light-years away, and the whole universe is much bigger. Read more: Is the universe infinite? · If the universe is expanding, what is it expanding into?

Primeval atom

Lemaître’s name for the single, dense starting state he proposed in 1931, from which he thought the whole universe began; today’s Big Bang theory differs from it in many ways. Read more: Was the Big Bang first proposed by a priest?

Pulsar timing array

Dozens of steadily spinning dead stars, timed together for years with radio telescopes, used as a galaxy-sized detector for very slow gravitational waves. Example: Pairs of giant black holes are the leading explanation for the slow hum these arrays study. Read more: What future measurement could prove the HD-Blast hypothesis wrong? · What is the HD-Blast hypothesis?

Quantum gravity

A hoped-for theory joining gravity with quantum physics, the rules of the very small; several candidates exist, but none is finished or confirmed. Example: Most ideas about quantum gravity predict new effects only at energies far beyond any machine we can build. Read more: What is a theory of everything, and will we ever have one? · Did time begin at the Big Bang?

Quantum physics

The rules of the very small, in which tiny particles can be in blends of states and be entangled, and theory gives only the odds for many results. Example: Standard quantum physics does not say your mind creates reality, and it does not show that “quantum” products heal. Read more: What does quantum physics really say about consciousness and healing? · Is anything truly random? · Did Einstein lose his argument that God does not play dice?

Quantum vacuum

The lowest-energy state of the quantum fields that fill space: it has no particles, but it is not empty. Example: Philosophers say this “nothing” of physics is still something, since it is full of fields and energy. Read more: Why is there something rather than nothing?

Redshift

The stretching of light toward longer, redder wavelengths; in an expanding universe it grows in proportion to a galaxy’s distance, so astronomers use it to trace expansion. Read more: Was the Big Bang first proposed by a priest? · If the universe is expanding, what is it expanding into? · Why is the expansion of the universe speeding up?

Singularity

A point where Einstein’s equations give infinite values, as at a black hole’s center; it may be real, or a sign that the theory has reached its limit. Example: At the Big Bang the math breaks down; that marks the edge of the theory and does not settle whether time began. Read more: What is a black hole, really? · Did time begin at the Big Bang?

Speed of light

Exactly 299,792,458 meters per second in a vacuum, the same for every local observer and the limit for signals and objects moving through space. Example: A particle with rest mass can approach this speed but would need unbounded energy to reach it. Read more: Why is the speed of light the universe's speed limit? · Can quantum entanglement send messages faster than light?

Standard Model

The tested theory of the known basic particles and three of the four forces (electromagnetic, strong and weak); it leaves out gravity. Example: It has explained almost all experimental results, yet Hawking counted 24 of its numbers that have to be measured. Read more: What is a theory of everything, and will we ever have one? · Why is there any matter left in the universe at all?

String theory

A candidate theory of quantum gravity in which the basic objects are tiny vibrating strings instead of points; it has no confirmed test yet. Read more: What is a theory of everything, and will we ever have one?

Superdeterminism

The idea that a shared cause in the past set up both the particles and the experimenters’ choices in a Bell test. Example: The Stanford Encyclopedia says Bell tests leave local hidden instructions only this way out, and critics call it “conspiratorial.” Read more: Did Einstein lose his argument that God does not play dice? · Is anything truly random?

Superposition

Describing a tiny object as a combination of several possible states at once; a measurement gives one result, with chances set by the math. Read more: What does quantum physics really say about consciousness and healing?

Theory of everything

A hoped-for single set of rules for all four forces of nature and all particles; nobody knows yet whether one can be found. Example: In 2002, Stephen Hawking said he had come to doubt that a final theory could be written as a finite set of rules. Read more: What is a theory of everything, and will we ever have one?

Time dilation

The slowing of a clock, compared with another clock, because it moves fast or sits deeper in gravity. Example: Astronaut Scott Kelly spent a year in space and aged 5 milliseconds less than his twin brother on Earth. Read more: Is time travel possible?

Type Ia supernova

A class of exploding star whose calibrated brightness helps astronomers estimate great distances. Example: Distant supernovae first showed that the universe had expanded more than expected in a gravity-only picture. Read more: Why is the expansion of the universe speeding up?

Vacuum decay

A possible change of empty space to a lower-energy state, starting as a bubble that spreads at the speed of light. Example: An ending through vacuum decay is labeled Open question. Read more: Will the universe end, and if so, how?

Weak anthropic principle

Brandon Carter’s point that we can only observe conditions that allow observers like us to exist, a selection effect that can bias what we conclude. Example: Life began early on Earth, but an early start may be needed for anyone to be here asking, which weakens that clue. Read more: Is the universe fine-tuned for life? · Is life likely to be rare or common in the universe?

Wormhole

A possible tunnel through space linking two distant places; wormholes appear in the math of general relativity, but holding one open would need material with properties no known material has. Example: Einstein’s equations also allow closed timelike curves, paths looping into their own past, on paper; nobody knows how to create them. Read more: Is time travel possible?

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Life

Abiogenesis

The natural process by which life might arise from non-living chemistry. Example: In the 1950s, Stanley Miller made amino acids by sending electric sparks through a mix of gases and water. Read more: How did life on Earth begin?

Amino acid

A small molecule that living things link into long chains to make proteins; life on Earth uses 20 main kinds. Example: Samples from asteroid Bennu contain 14 of the 20 amino acids life uses to build proteins. Read more: How did life on Earth begin?

Biosignature

A sign, such as a gas or a pattern in a rock, that might be made by life; a “potential” biosignature needs more data before anyone can conclude life was present. Example: A 2025 claim of possible signs of life on the planet K2-18b fell short of the usual bar for a discovery. Read more: Is life likely to be rare or common in the universe?

Contingency (in evolution)

When an outcome depends on the particular history that came before it, so small early events can change what happens later. Example: By 2008, only one of Lenski’s 12 populations had evolved the ability to use citrate, and only after earlier “potentiating” changes made it possible. Read more: If we could replay the history of life, would evolution turn out the same?

Convergent evolution

When unrelated living things evolve similar traits on their own. Example: Eyes capable of true vision arose separately several times; in parallel evolution, related populations, like Lenski’s 12 flasks, evolve the same changes separately. Read more: If we could replay the history of life, would evolution turn out the same?

Copernican principle

The working assumption of astronomy that we do not live in a special place; applied to life, it suggests that if Earth is ordinary, life may be ordinary too. Example: The rival Rare Earth argument says simple life might be widespread but complex life rare; observation has settled neither. Read more: Is life likely to be rare or common in the universe?

Drake equation

A 1961 formula by Frank Drake that multiplies seven factors to estimate how many detectable civilizations our galaxy holds; it organizes what we would need to know. Read more: Is life likely to be rare or common in the universe? · What is the Fermi paradox: if aliens exist, where is everybody?

Exoplanet

A planet that orbits a star other than our Sun. Example: NASA’s Exoplanet Archive listed 6,372 confirmed planets beyond our solar system on September 25, 2026. Read more: Is life likely to be rare or common in the universe? · What is the Fermi paradox: if aliens exist, where is everybody?

Fermi paradox

The puzzle that our galaxy has many planets and has existed a long time, yet we have no confirmed sign of alien life or technology. Example: Life might be rare, tools might be short-lived or hard to spot, or we may have searched too little to know. Read more: What is the Fermi paradox: if aliens exist, where is everybody?

Habitable zone

The range of distances from a star where temperatures could allow liquid water on a planet’s surface; it is a first cut and does not show a planet can support life. Example: Kepler data suggest about half of Sun-like stars could have a rocky planet where water could stay liquid, with wide error bars. Read more: Is life likely to be rare or common in the universe?

Last universal common ancestor (LUCA)

The most recent population from which all life on Earth today descends; it was already a cell, so it came after life began. Read more: How did life on Earth begin?

Mutation bias

The fact that some kinds of mutations happen more often than others. Example: A 2026 study found it can shape which fixes win, even in large populations; the idea is still debated. Read more: If we could replay the history of life, would evolution turn out the same?

RNA world

The idea that early life used RNA both to store genetic information and to speed up chemical reactions, before DNA and proteins took over those jobs. Example: Samples from asteroid Bennu contain all five nucleobases, the chemical “letters” found in DNA and RNA. Read more: How did life on Earth begin?

SETI

The Search for Extraterrestrial Intelligence, often focused on possible signs of technology. Example: Radio searches cover only some stars, radio bands, times and signal strengths, so silence so far is hard to read. Read more: What is the Fermi paradox: if aliens exist, where is everybody?

Stromatolite

A layered rock mound built up over time by mats of microbes; some still grow today, for example in Shark Bay, Western Australia. Example: By about 3.48 billion years ago, microbes were building stromatolites in what is now Western Australia. Read more: How did life on Earth begin?

Technosignature

A possible observable sign of technology made by someone elsewhere, such as a narrow radio signal with features unlikely to come from nature. Example: Four radio telescopes searched 3I/ATLAS and found no artificial signals; one picked up natural radio signals from gas, as expected from a comet. Read more: What is the Fermi paradox: if aliens exist, where is everybody? · Was interstellar comet 3I/ATLAS an alien probe?

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Mind and body

Attachment theory

The study of the deep bonds people form with those they depend on, and of how they react when such a bond is broken. Read more: Is it normal to feel that someone who died is still with me?

Bereavement

The period of grief and mourning after the death of someone close. Example: For most bereaved people, grief symptoms lessen between 6 months and 2 years after the loss. Read more: Why does grief hurt so much, even physically? · Is it normal to feel that someone who died is still with me? · When does grief need more help?

Brain death

Death declared when all functions of the entire brain, including the brainstem, have stopped for good; it is not the same as a coma. Example: Under the U.S. legal standard, death means an irreversible loss of heart and lung function or of all brain function. Read more: What happens in the brain as a person dies? · Is there life after death? What can science say?

Broken-heart syndrome

A sudden, usually temporary weakening of the heart’s main pumping chamber after severe stress. Example: Sudden emotional stress, including the loss of a loved one, can trigger it; doctors also call it takotsubo cardiomyopathy. Read more: Why does grief hurt so much, even physically?

Chinese Room

John Searle’s 1980 thought experiment about a man who follows rules to answer Chinese messages without understanding Chinese. Read more: Could a machine ever be conscious?

Compatibilism

The view that free will can exist even if determinism is true. Example: In a 2020 survey, 59.2% of the philosophers who answered favored it. Read more: Is free will real, or does the brain decide before we do?

Consciousness

Having experience: a being is conscious if there is something it is like to be it, such as seeing red or feeling pain. Example: Whether the right kind of computation is enough for it, a view called computational functionalism, is still open. Read more: Could a machine ever be conscious? · Is the mind just the brain?

Continuing bonds

The idea that keeping a connection with someone who died can be a normal part of grief, rather than something that must be cut. Read more: Is it normal to feel that someone who died is still with me?

Death anxiety

Worry or dread about dying or being dead; some is common, and when it takes over daily life, talking therapies can help. Example: In two studies of adults aged 18 to 87, fear of death peaked in people’s 20s and then went down. Read more: Is it possible to make peace with the idea of dying?

Determinism

The idea that everything that happens is the result of the laws of nature and the state of the world in the past. Example: Some readings of quantum theory keep the world fully determined. Read more: Is free will real, or does the brain decide before we do? · Is anything truly random?

Dualism

The view that mind and body are, in some basic way, two different kinds of things. Read more: Is the mind just the brain?

EEG

Electroencephalography: recording the brain’s electrical activity with sensors on the scalp or on the brain itself, which shows up as wavy lines. Read more: Is free will real, or does the brain decide before we do? · What happens in the brain as a person dies?

Gamma waves

Fast brain waves that researchers link with alert, conscious processing; some dying brains show a brief burst of them. Example: No study has tied such bursts directly to what near-death survivors describe. Read more: What happens in the brain as a person dies? · What are near-death experiences, and what do they tell us?

Hallucination

Seeing, hearing or feeling something without a matching source in the outside world; in grief, the word does not by itself mean illness. Read more: Is it normal to feel that someone who died is still with me? · What is sleep paralysis, and why do people sometimes sense a presence in the dark?

Hard problem of consciousness

The puzzle, named by the philosopher David Chalmers in 1995, of why brain processing comes with felt experience at all. Read more: Is the mind just the brain? · Could a machine ever be conscious?

Hospice

Care at the end of life, usually for people expected to live six months or less, aimed at peace, comfort and dignity. Read more: Is it possible to make peace with the idea of dying? · What happens in the brain as a person dies?

Integrated information theory

A theory, first proposed by the neuroscientist Giulio Tononi, that links consciousness to how much information a system holds as a whole, beyond what its parts hold. Example: Its backers say digital computers are unlikely to be conscious. Read more: Could a machine ever be conscious?

Libertarian free will

The view that we have free will and that it requires determinism to be false; it has nothing to do with political libertarianism. Example: In the same 2020 survey, 18.8% of the philosophers who answered favored it. Read more: Is free will real, or does the brain decide before we do?

Mindfulness

Keeping attention on the present moment without judging it, a word from Buddhist practice now also used in medicine and psychology. Example: MBSR, an eight-week course introduced in the late 1970s, teaches it through meditation, yoga and body awareness. Read more: Can meditation change the brain?

Near-death experience (NDE)

A vivid inner experience some people report when their life is in danger, often with peace, light, a sense of leaving the body, or meeting people who have died. Example: In a Dutch study of 344 people revived after cardiac arrest, 62 (18%) reported one. Read more: What are near-death experiences, and what do they tell us? · Is there life after death? What can science say? · What happens in the brain as a person dies?

Neural correlate of consciousness

Brain activity that goes along with a particular conscious experience; finding one is a step toward an explanation, not the explanation itself. Read more: Is the mind just the brain?

Orch OR

“Orchestrated objective reduction,” Roger Penrose and Stuart Hameroff’s proposal that consciousness arises from quantum events in microtubules, tiny protein tubes inside brain cells; it is unconfirmed. Example: A 2020 test ruled out the natural, “parameter-free” version of the Diósi–Penrose collapse idea that Orch OR builds on. Read more: What does quantum physics really say about consciousness and healing?

Palliative care

Care that eases the symptoms and stress of serious illness, which can be given at any stage, alongside treatment. Example: Hospice and palliative care teams treat fear and spiritual concerns as part of care, not only bodily symptoms. Read more: Is it possible to make peace with the idea of dying?

Physicalism

The view that everything, including the mind, is physical or rests completely on physical things. Example: In the 2020 PhilPapers Survey, 51.9% of philosophers who answered leaned toward it about the mind, and 32.1% leaned against. Read more: Is the mind just the brain?

Prolonged grief disorder

Grief that stays intense nearly every day and disrupts daily life long after a loss; the U.S. manual requires at least 12 months for adults, the World Health Organization’s at least 6. Example: The American Psychiatric Association estimates it affects 4% to 15% of bereaved adults, and grief-focused therapy helps many of them. Read more: When does grief need more help? · Why does grief hurt so much, even physically?

Readiness potential

A slow electrical rise, seen in averaged EEG recordings, that comes before a voluntary movement; what it reflects is still debated. Example: In 2012, researchers showed that random ups and downs in brain activity, averaged together, can reproduce that slow rise. Read more: Is free will real, or does the brain decide before we do?

REM sleep

Rapid eye movement sleep, the stage when the eyes move quickly and vivid dreams are most common, while the body’s muscles normally stay still. Example: When dream features, such as dream images or feeling unable to move, break into waking life, it is called REM intrusion. Read more: What is sleep paralysis, and why do people sometimes sense a presence in the dark? · What are near-death experiences, and what do they tell us?

Sensed presence

The strong feeling that someone is near, even though no one can be seen or heard. Example: In a 2023 Pew survey, 34% of U.S. adults said they had felt the presence of a dead relative in the past year. Read more: Is it normal to feel that someone who died is still with me? · What is sleep paralysis, and why do people sometimes sense a presence in the dark?

Sleep paralysis

A short spell, while falling asleep or waking up, when the mind is awake but the body cannot move or speak; it can bring a sense of presence or chest pressure. Example: About 7.6% of people have had at least one episode; the NHS calls it harmless, though it can be scary. Read more: What is sleep paralysis, and why do people sometimes sense a presence in the dark? · Are ghosts real? What does the evidence say?

Terror management theory

A psychology theory that says people cope with knowing they will die by holding on to their worldview and their sense of self-worth. Example: The claim that reminders of death push people to defend their beliefs and self-worth is labeled Supported, still debated. Read more: Is it possible to make peace with the idea of dying?

Turing test

Alan Turing’s 1950 imitation game, in which a judge chats with a person and a machine and tries to tell which is which; it tests behavior, not feeling. Example: Machines have now done well at this game, but talking like a person is weak evidence of feeling. Read more: Could a machine ever be conscious?

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Belief and meaning

Afterlife

Any form of existence a person might have after death, such as heaven, a world to come or rebirth. Example: In a 2021 Pew survey, 73% of U.S. adults said they believe in heaven, and 17% in no afterlife at all. Read more: Is there life after death? What can science say?

Agnostic

A person who believes neither that God exists nor that God does not exist, often because they think we do not, or cannot, know. Example: The biologist Thomas Henry Huxley first used the word in public in 1869. Read more: What is the difference between an atheist, an agnostic and a “none”? · What did Einstein actually believe about God?

Apostasy

The act of abandoning one’s faith; Pew counted 22 countries with laws against it in 2019. Read more: How do I tell my family I have doubts about my faith?

Atheist

In the broad sense, a person who lacks belief in God; in the narrow sense used in philosophy, a person who holds that God does not exist. Example: Einstein rejected a personal God yet refused to call himself an atheist. Read more: What is the difference between an atheist, an agnostic and a “none”? · What did Einstein actually believe about God?

Awe

The feeling we get in front of something vast that our usual ideas cannot easily take in. Example: Americans with and without a religion feel deep wonder about the universe at nearly the same rate: 42% and 39% at least weekly. Read more: Why do we feel awe under the night sky, and is it a spiritual experience?

Brute fact

A fact that has no further explanation; some thinkers say the existence of the universe is one. Example: Bertrand Russell answered that the universe is “just there, and that’s all.” Read more: Why is there something rather than nothing? · What are the main arguments for God's existence, and how do people respond to them?

Concordism

The effort to line up scientific findings with the words of scripture, or to use science to back up a religious text. Example: Lemaître tried a mild form as a young man and later left it behind, keeping his science and faith distinct. Read more: Was the Big Bang first proposed by a priest?

Consequentialism

The approach to ethics that judges actions by their results, such as how much well-being they produce. Read more: Where do right and wrong come from?

Contingent

Able to have not existed, like people, planets and stars; a necessary being, by contrast, could not fail to exist. Example: Cosmological arguments from contingency conclude that a necessary being exists; their critics reply that the universe may need no explanation. Read more: What are the main arguments for God's existence, and how do people respond to them? · Why is there something rather than nothing?

Cosmological argument

A family of arguments that reason from the existence of the universe, or its beginning, to a first cause or necessary being, usually identified with God. Example: Muslim, Christian and other thinkers built versions; the Stanford Encyclopedia says every claim in the debate is seriously disputed. Read more: What are the main arguments for God's existence, and how do people respond to them? · Why is there something rather than nothing?

Deism

Belief in a God who made the world and its laws but does not step into events. Read more: What did Einstein actually believe about God?

Deontology

The approach to ethics that stresses duties and moral rules, whatever the results. Read more: Where do right and wrong come from?

Design argument

An argument that order, beauty and purpose in nature could not come from blind accident, so they point to a designer; modern versions often turn to fine-tuning in physics. Example: William Paley compared nature to a watch found on the ground in 1802; David Hume had already raised sharp objections in 1779. Read more: What are the main arguments for God's existence, and how do people respond to them? · Is the universe fine-tuned for life?

Divine command theory

The view that the source and binding force of morality, or at least of moral duty, come from God’s will or command. Read more: Where do right and wrong come from?

Euthyphro dilemma

A question from Plato: is something good because God, or the gods, love it, or do they love it because it is good? Example: Defenders such as Robert Adams reply that God’s commands flow from God’s good nature, so they are neither arbitrary nor dependent on an outside standard. Read more: Where do right and wrong come from? · What are the main arguments for God's existence, and how do people respond to them?

God of the gaps

Using God to explain whatever science cannot explain yet, which the U.S. National Academies say weakens both science and religion. Read more: Can someone love science and still believe in God?

Hedonic and eudaimonic well-being

Two ideas of happiness: hedonic well-being is feeling good, while eudaimonic well-being is living well through meaning, growth, good relationships and using your abilities. Example: Eudaimonic comes from Aristotle’s eudaimonia, often translated as happiness or flourishing; philosophers still disagree about which kind matters most. Read more: What makes a life well lived?

Humanism

A way of life built on human reason, ethics and care for others, without relying on God. Read more: What is the difference between an atheist, an agnostic and a “none”?

Involuntary doubt

In Catholic teaching, hesitation in believing or difficulty with objections to the faith, as opposed to a deliberate refusal to believe. Read more: Is it okay to question what you were raised to believe?

Just-world theory

The psychologist Melvin Lerner’s idea that people want to believe the world is fair, and may blame victims to protect that belief. Read more: Why do bad things happen to good people?

Kalam argument

A cosmological argument, developed by medieval Muslim theologians and defended today by William Lane Craig: whatever begins to exist has a cause, the universe began, so it has a cause. Example: Its second step, that the universe began, is labeled Open question; critics also doubt that the first step applies to the whole universe. Read more: What are the main arguments for God's existence, and how do people respond to them?

Kalama Sutta

A Buddhist teaching in which the Buddha tells people not to accept ideas only because of tradition or authority, but to test them in practice. Read more: Is it okay to question what you were raised to believe?

Karma

The teaching that good and bad actions bring good and bad results, often in a later life. Read more: Is there life after death? What can science say?

Meaning in life

What makes one person’s life matter, which philosophers separate from the meaning of life, the point of humanity or the universe as a whole. Example: In a 2017 Pew survey, 69% of Americans mentioned family when asked what makes life meaningful. Read more: What is the meaning of life, and who gets to decide?

Moral argument

An argument that there are objective moral facts, such as that cruelty for fun is wrong, and that God is their best explanation, so God probably exists. Example: C. S. Lewis opened Mere Christianity (1952) with a version; critics such as Erik Wielenberg argue moral truths need no God. Read more: What are the main arguments for God's existence, and how do people respond to them? · Where do right and wrong come from?

Natural law

A tradition, best known from Thomas Aquinas, that sees basic moral principles as built into human nature and knowable through reason. Read more: Where do right and wrong come from?

Naturalism

The philosophical view that nature is all there is and nothing supernatural exists; in debates about meaning, the view that a meaningful life is possible without God or a soul. Example: Among naturalists about meaning, subjectivists say it depends on what a person cares about; objectivists say some things are worth doing in themselves. Read more: Can someone love science and still believe in God? · What is the meaning of life, and who gets to decide?

Nihilism

The view that no life has meaning, because what would make a life meaningful cannot exist for any of us. Read more: What is the meaning of life, and who gets to decide?

Nones (religiously unaffiliated)

People who describe their religion as atheist, agnostic or “nothing in particular” in a survey; many still believe in God or a higher power. Example: In Pew’s 2023–24 study, 29% of U.S. adults were nones: 5% atheist, 6% agnostic and 19% nothing in particular. Read more: What is the difference between an atheist, an agnostic and a “none”? · Is it okay to question what you were raised to believe? · Why do we feel awe under the night sky, and is it a spiritual experience?

Ontological argument

An argument that tries to show from the idea of God alone that God must exist. Example: In the eleventh century, Anselm defined God as “that than which no greater can be conceived”; Kant objected that existence is not a real property. Read more: What are the main arguments for God's existence, and how do people respond to them?

Overview effect

The deep awe and sense of connection that many astronauts report when they see Earth from space. Read more: Why do we feel awe under the night sky, and is it a spiritual experience?

Pantheism

The view that God and the universe are one and the same. Example: Whether pantheist, deist or agnostic best fits Einstein’s view is labeled Open question. Read more: What did Einstein actually believe about God?

Pascal’s wager

Blaise Pascal’s argument that, since reason alone cannot decide whether God exists but we must still choose, the wise bet is on God. Example: Diderot noted in 1746 that an imam could reason the same way; defenders reply that we cannot avoid choosing, so we should choose with care. Read more: What are the main arguments for God's existence, and how do people respond to them?

Personal God

A God pictured as a person who has plans, hears prayers, and rewards or punishes people. Example: Einstein did not believe in a personal God who answers prayers or judges people. Read more: What did Einstein actually believe about God?

Principle of sufficient reason

The idea, defended by Leibniz, that everything has a reason or cause; it is powerful and still debated. Example: Leibniz asked why there is something rather than nothing in 1714, and his answer was God. Read more: Why is there something rather than nothing?

Problem of evil

The puzzle of how an all-powerful, all-knowing and all-good God could allow evil and suffering, whether from people’s choices or from nature, like a hurricane. Example: Many philosophers agree evil does not logically rule out God, but they still argue over whether it makes God less likely. Read more: Why do bad things happen to good people?

Rebirth (reincarnation)

The belief that beings are born again and again, in new lives, after they die. Example: Hindu, Buddhist, Jain and Sikh traditions teach rebirth. Read more: Is there life after death? What can science say?

Religious doubt

Questions or uncertainty about religious beliefs, which many believers live with. Example: In Pew’s 2023–24 survey, 83% of U.S. adults believed in God or a universal spirit, but only 54% held that belief with absolute certainty. Read more: How do I tell my family I have doubts about my faith? · Is it okay to question what you were raised to believe?

Religious switching

Pew’s term for having a different religious identity now than the one you were raised with, including moving from no religion to a religion or the reverse. Example: In Pew’s 2023–24 study, 35% of U.S. adults had switched from their childhood religious identity; 85% of switchers did so by age 30. Read more: Is it okay to question what you were raised to believe? · How do I tell my family I have doubts about my faith?

Resurrection

Being raised from death to new life; in Christian, Jewish and Islamic teaching it often means a future raising of the dead. Read more: Is there life after death? What can science say?

Skeptical theism

The view that, if God exists, humans should not expect to see all of God’s reasons for allowing suffering. Read more: Why do bad things happen to good people?

Spinoza’s God

The philosopher Baruch Spinoza’s idea of “God or Nature”: God as nature and its laws, with no human-like purposes and no miracles. Example: Einstein said in 1929 that he believed in Spinoza’s God, and in 1930 he described a “cosmic religious feeling” of awe at nature’s order. Read more: What did Einstein actually believe about God?

Spiritual but not religious

A self-description for people who feel a connection to something beyond themselves but do not see themselves as religious. Example: In Pew’s 2023–24 study, 21% of all U.S. adults said they are spiritual but not religious. Read more: What is the difference between an atheist, an agnostic and a “none”?

Supernaturalism

In debates about the meaning of life, the view that God or a soul is needed for meaning in life, or at least for its deepest kind. Example: Many religious philosophers now grant that a life can have some meaning without God, while arguing God would add more. Read more: What is the meaning of life, and who gets to decide?

Theist

A person who believes that God, or gods, exist. Read more: What is the difference between an atheist, an agnostic and a “none”?

Theodicy

An attempt to explain why God allows evil; a defense is more modest, trying only to show that God and evil could both exist. Read more: Why do bad things happen to good people?

Virtue ethics

The approach to ethics that puts good character, such as kindness and honesty, at the center of morality. Read more: Where do right and wrong come from?

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Strange claims

AARO

The All-domain Anomaly Resolution Office, the U.S. Department of Defense office set up in July 2022 to collect and study UAP reports. Example: In 2024, it said it had found no evidence that any sighting was alien technology. Read more: Have UFOs been shown to be alien spacecraft? · Are governments hiding evidence of aliens? What do the records show?

Apparent retrograde motion

When a planet seems to move backward across the sky for a while, an effect of our viewpoint from a moving Earth, not a real change in the planet’s path. Example: Mercury “going retrograde” is this viewing effect; the claim that it affects people’s lives is labeled Not supported. Read more: Does astrology work? What happens when it is tested?

Birth chart

A map of where the Sun, Moon and planets were at the moment and place of a person’s birth, which astrologers read to describe a person. Example: Many people enjoy astrology just for fun, and that is their choice. Read more: Does astrology work? What happens when it is tested?

ESP (extrasensory perception)

Getting information without using the known senses; telepathy, clairvoyance and precognition are forms of it, and the wider term psi also covers mind over matter. Example: In a 2005 Gallup poll, 41% of U.S. adults said they believed in ESP. Read more: Do psychic abilities like telepathy or seeing the future hold up when tested?

Ganzfeld

German for “whole field”: a test in which a relaxed receiver, eyes covered and soft noise in headphones, tries to pick a hidden target, usually from four choices. Example: Chance alone gives 25% hits; pooled studies report hit rates a few percentage points above that, which critics dispute. Read more: Do psychic abilities like telepathy or seeing the future hold up when tested?

Infrasound

Sound below about 20 hertz (vibrations per second), usually too low for people to hear. Example: Whether infrasound adds to eerie feelings in “haunted” places is still being tested. Read more: Are ghosts real? What does the evidence say?

Motion parallax

An optical effect in which a slow or still object seems to move fast because the observer, such as a jet with a camera, is moving. Read more: Have UFOs been shown to be alien spacecraft?

Non-gravitational acceleration

A small push on a comet’s path that gravity does not explain, usually caused by gas escaping from the comet like a weak rocket. Read more: Was interstellar comet 3I/ATLAS an alien probe?

Pareidolia

Seeing a face or figure in something that has none, like a pattern in wood or a shadow. Read more: Are ghosts real? What does the evidence say?

Precession

The slow wobble of Earth’s axis, like a spinning top, which NASA puts at about 25,770 years per cycle and which shifts which stars line up with the Sun on a given date. Example: The Sun no longer lines up with the same constellations on the same dates as 3,000 years ago. Read more: Does astrology work? What happens when it is tested?

Satellite flaring

A bright flash when sunlight reflects off a satellite at just the right angle, which can look like a glowing orb and last from a split second to a few minutes. Example: In its report covering mid-2024 to mid-2025, AARO solved 238 reports as sunlight flashing off satellites. Read more: Have UFOs been shown to be alien spacecraft?

Simulation argument

Nick Bostrom’s 2003 argument that at least one of three claims is true: civilizations like ours rarely become highly advanced, advanced ones rarely run ancestor simulations, or we are almost surely simulated. Example: An ancestor simulation would be a detailed computer model of a civilization’s own ancestors, complete with minds like ours. Read more: Are we living in a computer simulation?

Substrate-independence

The idea that a mind could run on something other than a biological brain, such as a computer, if the right activity were copied in enough detail. Example: The simulation argument depends on it, and it is still debated. Read more: Are we living in a computer simulation?

Suggestion

The way hints, stories or expectations shape what people notice, feel and remember. Read more: Are ghosts real? What does the evidence say?

Time twins

Unrelated people born at nearly the same time and place, whom astrology predicts should be alike, which makes them a strong test. Example: A study of 2,101 people born in London in March 1958, on average 4.8 minutes apart, found no astrological likeness across 110 traits. Read more: Does astrology work? What happens when it is tested?

UAP

Unidentified anomalous phenomena, the official U.S. term for something seen in the air, at sea or in space that could not be identified; it makes no claim about what the thing is. Example: Unexplained means the answer is missing; it does not mean the answer is aliens. Read more: Have UFOs been shown to be alien spacecraft? · Are governments hiding evidence of aliens? What do the records show?

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Reviewed 2026-09-29. Missing a word, or found a definition that could be clearer? Tell us. For the big open questions, see what we know, and what we don’t know yet.