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A black hole is a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. Although it has an enormous effect on the fate and circumstances of an object crossing it, according to general relativity it has no locally detectable features. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe directly.
An extinction event is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. The number of major mass extinctions in the last 540 million years are estimated from as few as five to more than twenty. These differences stem from confusion as to what constitutes an extinction event as "major", and the data chosen to measure past diversity.
Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in hertz (Hz) which is equal to one event per second. The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period, T—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.
The Mesozoic Era meaning "middle life" is the middle of the three geological eras of the Phanerozoic Eon. It lasted from about 252 to 66 million years ago. It is also called the Age of Reptiles and the Age of Conifers.
The Permian–Triassic extinction event, also known as the P–Tr extinction, the P–T extinction, the End-Permian Extinction, and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as well as between the Paleozoic and Mesozoic eras, approximately 251.9 million years ago. It is the Earth's most severe known extinction event, with around 81% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It was the largest known mass extinction of insects. Some 57% of all biological families and 83% of all genera became extinct.
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
- The laws of physics are invariant in all inertial frames of reference.
- The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer.
Independence is a fundamental notion in probability theory, as in statistics and the theory of stochastic processes.
In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. The fabric of space-time is a conceptual model combining the three dimensions of space with the fourth dimension of time. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive differently where and when events occur.
The decathlon is a combined event in athletics consisting of ten track and field events. The word "decathlon" was formed, in analogy to the word "pentathlon", from Greek δέκα and ἄθλος. Events are held over two consecutive days and the winners are determined by the combined performance in all. Performance is judged on a points system in each event, not by the position achieved. The decathlon is contested mainly by male athletes, while female athletes typically compete in the heptathlon.
Eventing is an equestrian event where a single horse and rider combine and compete against other competitors across the three disciplines of dressage, cross-country, and show jumping. This event has its roots in a comprehensive cavalry test that required mastery of several types of riding. The competition may be run as a one-day event (ODE), where all three events are completed in one day or a three-day event (3DE), which is more commonly now run over four days, with dressage on the first two days, followed by cross-country the next day and then show jumping in reverse order on the final day. Eventing was previously known as Combined Training, and the name persists in many smaller organizations. The term "Combined Training" is sometimes confused with the term "Combined Test", which refers to a combination of just two of the phases, most commonly dressage and show jumping.
In probability theory and statistics, Bayes' theorem, named after the Reverend Thomas Bayes, describes the probability of an event, based on prior knowledge of conditions that might be related to the event. For example, if the risk of developing health problems is known to increase with age, Bayes' theorem allows the risk to an individual of a known age to be assessed more accurately than simply assuming that the individual is typical of the population as a whole.
A gravitational singularity, spacetime singularity or simply singularity is a location in spacetime where the density and gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system. The quantities used to measure gravitational field strength are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. Since such quantities become infinite at the singularity point, the laws of normal spacetime break down.
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, though atmospheres mitigate many surface impacts through atmospheric entry. Impact craters and structures are dominant landforms on many of the Solar System's solid objects and present the strongest empirical evidence for their frequency and scale.
Hawking radiation is black-body radiation that is predicted to be released by black holes because of quantum effects near the black hole event horizon. It is named after the physicist Stephen Hawking, who developed a theoretical argument for its existence in 1974.
In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them or to a difference in gravitational potential between their locations. When unspecified, "time dilation" usually refers to the effect due to velocity.
A heptathlon is a track and field combined events contest made up of seven events. The name derives from the Greek hepta (seven) and ἄθλος. A competitor in a heptathlon is referred to as a heptathlete.
Canoeing has been featured as competition sports in the Summer Olympic Games since the 1936 Games in Berlin, although they were demonstration sports at the 1924 Games in Paris. There are two disciplines of canoeing in Olympic competition: slalom and sprint.
The Paleocene, or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek palæo- meaning "old" and the Eocene Epoch, translating to "the old part of the Eocene".
In probability theory, conditional probability is a measure of the probability of an event occurring, given that another event has already occurred. If the event of interest is A and the event B is known or assumed to have occurred, "the conditional probability of A given B", or "the probability of A under the condition B", is usually written as P(A|B), or sometimes PB(A) or P(A/B). For example, the probability that any given person has a cough on any given day may be only 5%. But if we know or assume that the person is sick, then they are much more likely to be coughing. For example, the conditional probability that someone unwell is coughing might be 75%, in which case we would have that P(Cough) = 5% and P(Cough|Sick) = 75%.
The Cretaceous–Paleogene (K–Pg) extinction event was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the exception of some ectothermic species such as sea turtles and crocodilians, no tetrapods weighing more than 25 kilograms survived. It marked the end of the Cretaceous period, and with it the Mesozoic Era, while heralding the beginning of the Cenozoic Era, which continues to this day.
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