Of the four fundamental interactions, gravitation is the dominant at astronomical length scales. Gravity effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on astronomical length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.

This diagram shows Earth location in the universe on increasingly larger scales. The images, labeled along their left edge, increase in size from left to right, then from top to bottom.

The size of the universe is somewhat difficult to define. According to the general theory of relativity, far regions of space may never interact with ours even in the lifetime of the universe due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe were to exist forever: space may expand faster than light can traverse it.

Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.

Spacetimes are the arenas in which all physical events take place. The basic elements of spacetimes are events. In any given spacetime, an event is defined as a unique position at a unique time. A spacetime is the union of all events in the same way that a line is the union of all of its points, formally organized into a manifold.

Spacetime events are not absolutely defined spatially and temporally but rather are known to be relative to the motion of an observer. Minkowski space approximates the universe without gravity; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity.

An important parameter determining the future evolution of the universe theory is the density parameter, Omega, defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.

Observations, including the Cosmic Background Explorer, Wilkinson Microwave Anisotropy Probe, and Planck maps of the CMB, suggest that the universe is infinite in extent with a finite age, as described by the Friedmann–Lemaître–Robertson–Walker (FLRW) models.

Because we cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.

The universe is composed almost completely of dark energy, dark matter, and ordinary matter. Other contents are electromagnetic radiation (estimated to constitute from 0.005% to close to 0.01% of the total mass-energy of the universe) and antimatter.

The observable universe is isotropic on scales significantly larger than superclusters, meaning that the statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvins.

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields having only a slight amount of spatial inhomogeneity would be difficult to distinguish from a cosmological constant.

Ordinary matter commonly exists in four states: solid, liquid, gas, and plasma. However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates.

A photon is the quantum of light and all other forms of electromagnetic radiation. It is the force carrier for the electromagnetic force, even when static via virtual photons. The effects of this force are easily observable at the microscopic and at the macroscopic level because the photon has zero rest mass; this allows long distance interactions. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit wave–particle duality, exhibiting properties of waves and of particles.

With the assumption of the cosmological principle that the universe is homogeneous and isotropic everywhere, a specific solution of the field equations that describes the universe is the metric tensor called the metric,

Some speculative theories have proposed that our universe is but one of a set of disconnected universes, collectively denoted as the multiverse, challenging or enhancing more limited definitions of the universe. Scientific multiverse models are distinct from concepts such as alternate planes of consciousness and simulated reality.

The Indian philosopher Kanada, founder of the Vaisheshika school, developed a notion of atomism and proposed that light and heat were varieties of the same substance.

By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when Earth rotational axis is aligned with its orbital axis. In the Northern Hemisphere, winter solstice currently occurs around 21 December; summer solstice is near 21 June, spring equinox is around 20 March and autumnal equinox is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.

A planet that can sustain life is termed habitable, even if life did not originate there. Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.

Earth has resources that have been exploited by humans.

Large deposits of fossil fuels are obtained from Earth crust, consisting of coal, petroleum, and natural gas. These bodies form concentrated sources for many metals and other useful elements.

Large areas of Earth surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 human deaths per year. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion and erosion.

There is a scientific consensus linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.

Cartography, the study and practice of map-making, and geography, the study of the lands, features, inhabitants and phenomena on Earth, have historically been the disciplines devoted to depicting Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

• Earth human population reached approximately seven billion on 31 October 2011.
• It is estimated that one-eighth of Earth surface is suitable for humans to live on.
• Three-quarters of Earth surface is covered by oceans, leaving one-quarter as land.
• Half of that land area is desert (14), (82N) The southernmost is the Amundsen–Scott.
• South Pole Station, in Antarctica, almost exactly at the South Pole. (90S)

A country may be an independent sovereign state or part of a larger state, as a non-sovereign or formerly sovereign political division, a physical territory with a government, or a geographic region associated with sets of previously independent or differently associated people with distinct political characteristics. It is not inherently sovereign.

Countries can refer both to sovereign states and to other political entities, such as Vatican City, the largest country in the world is Russia, while the most populous is China, followed by India and the United States of America. The newest country with widespread international recognition as a sovereign state is South Sudan.

The word country comes from Old French which derives from Vulgar Latin (terra) from contra (“against, opposite”). It most likely entered the English language after the Franco-Norman invasion during the 11th century.

A version of “country” can be found in the modern French language as contre, based on the word cuntrée in Old French, that is used similarly to the word “pays” to define non-state regions, but can also be used to describe a political state in some particular cases. The modern Italian contrada is a word with its meaning varying locally, but usually meaning a ward or similar small division of a town, or a village or hamlet in the countryside.

The term “country” can refer to a sovereign state. There is no universal agreement on the number of “countries” in the world since a number of states have disputed sovereignty status. By one application of the declarative theory of statehood and constitutive theory of statehood, there are 206 sovereign states; of which 193 are members of the United Nations, two have observer status at the UN (the Holy See and Palestine), and 11 others are neither a member nor observer at the UN. The latest proclaimed state is South Sudan since 2011.

The degree of autonomy of non-sovereign countries varies widely. Some are possessions of sovereign states, as several states have overseas territories (such as French Polynesia or the British Virgin Islands), with citizenry at times identical and at times distinct from their own. Such territories, with the exception of distinct dependent territories, are usually listed together with sovereign states on lists of countries, but may nonetheless be treated as a separate “country of origin” in international trade, as Hong Kong is.

Several organizations seek to identify trends in order to produce country classifications. Countries are often distinguished as developing countries or developed countries.

The UN additionally recognizes multiple trends that impact the developmental status of countries in the World Economic Situation and Prospects. The report highlights fuel-exporting and fuel-importing countries, as well as small island developing states and landlocked developing countries. It also identifies heavily indebted poor countries.

ANALYSIS/OPINION:

Just in case we needed a reminder, the debate over the desirability and duration of stay-at home orders during the current coronavirus pandemic has exposed the uncomfortable relationship that exists between science and democracy. Science points in the direction of the rule of the expert while democracy is rooted in the principle of popular sovereignty.

While the American people and scientists have competing claims to rule (popular consent vs. scientific knowledge), each actually needs the other to survive peaceably and comfortably, so neither can afford to completely discredit or abandon the other.

Modern, rights-oriented democracies like the United States desire much of what science has to offer, including a real shot at comfortable preservation for the many and not just for the few. There are good reasons, then, for democracies to be perplexed when it comes to the relationship they should seek with persons who have mastered the physical and natural sciences.  

The tension between competing claims to rule based on popular consent on the one side, and scientific knowledge on the other side, only came to a head once the democratic model of government was shown to be defensible, principally by the Founding generation in the United States.

Living with the tension between democracy and science in a responsible way requires that we distinguish the respective spheres within which “political” reasoning and “scientific” reasoning should prevail. This is easier asserted than accomplished, but doing this intelligently is critical to having a decent and competent democratic nation.

One major distinction between the realm of politics and the scientific realm is that the political arena is dominated by matters that are fundamentally variable, something that is not true in the realm of science. Additionally, political decision-makers by definition are persons of practical action who are expected to deliberate and issue commands whereas scientists are not obligated to take action or issue commands.

Political officials who are reputed to be “prudent” are persons who excel at adjusting policies to changing circumstances, including changing public opinions, and who recognize that “contingency” is a characteristic of political life. The prudent decision-maker understands that promoting the common good requires the continuous adjustment of a complex array of matters (e.g., political, economic, cultural, etc.) that are highly variable in nature.  

The most esteemed political decision-makers are persons who are adept at adjusting policies in such a way as to promote what is good for human beings, which requires a recognition that some ways of behaving and conducting human affairs are better than other ways. Knowledge of this kind benefits more from good intuition and extensive experience than from the mastery of some body of scientific knowledge. 

Admittedly, national and state officials who seek to act prudently need the knowledge that scientists possess in order to give people a good shot at enjoying the full benefits of living in a modern, right-oriented democracy. Political decision-making, however, ultimately should prevail in a democracy over the knowledge of scientists when it comes to matters bearing on the quality of life of the people, for example, deciding whether churches provide “essential” services.

A political community that abdicates decision-making to experts on fundamental issues related to the way of life of the people is no longer a democratic community. Additionally, the kind of knowledge required to make intelligent decisions about what constitutes a healthy life for human beings (e.g., risk taking as community service and as a source of personal distinction) transcends the realm of purely scientific knowledge in the direction of morality.

While good scientific knowledge may help when it comes to knowing how to survive, the consummate political knowledge of a George Washington or Winston Churchill makes survival with dignity possible. Consider how Founding-era Americans distinguished themselves for all time by following the prudent counsel of Washington and James Madison. 

All of this accentuates the enormous responsibilities that fall to the people in a democracy. Nurturing a citizenry that is up to the task of selecting persons for public office who are capable of deliberating and acting prudently when it comes to the common good is enormously difficult. It was among the greatest challenges facing the Founders.  

Carrying out this task well requires that citizens appreciate the extremely complex decision-making that is required to protect, restrain and/or balance diverse religious, racial, ethnic, economic and political interests, while also recognizing that political decision-making at its best should satisfy human longings that transcend creature comforts and mere survival.

Despite the protestations of many journalists and political pundits, nothing about all of this is easy. Preserving the integrity and vitality of the nation during the COVID-19 pandemic will require that the American people possess the knowledge and the courage to insist on a prudential relationship between science and politics that preserves the democratic bona fides of our constitutional republic and also points us in the direction of rational, dignified existence — an existence that requires the discipline to control our fears and face adversity with the dignity appropriate to a free people — at this point, we have left the realm of scientific knowledge.     

• David Marion is Elliott Emeritus Professor of Government and a faculty fellow at the Wilson Center for Leadership in the Public Interest at Hampden-Sydney College.

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