Politics of outer space

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Politics_of_outer_space 

The first page of the 1963 treaty that banned nuclear tests in the atmosphere, outer space and under water.

The politics of outer space includes space treaties, law in space, international cooperation and conflict in space exploration, international economics and the hypothetical political impact of any contact with extraterrestrial intelligence.

Astropolitics, also known as astropolitic, has its foundations in geopolitics and is a theory that is used for space in its broadest sense.

International cooperation on space projects has resulted in the creation of new national space agencies. By 2005 there were 35 national civilian space agencies.

Treaties and policies related to outer space

Outer Space Treaty

  Parties

  Signatories

  Non-parties

The Outer Space Treaty, formally the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, is a multilateral treaty that forms the basis of international space law. Negotiated and drafted under the auspices of the United Nations, it was opened for signature in the United States, the United Kingdom, and the Soviet Union on 27 January 1967, entering into force on 10 October 1967. As of August 2023, 114 countries are parties to the treaty—including all major spacefaring nations—and another 22 are signatories.

The Outer Space Treaty was spurred by the development of intercontinental ballistic missiles (ICBMs) in the 1950s, which could reach targets through outer space. The Soviet Union's launch of Sputnik, the first artificial satellite, in October 1957, followed by a subsequent arms race with the United States, hastened proposals to prohibit the use of outer space for military purposes. On 17 October 1963, the U.N. General Assembly unanimously adopted a resolution prohibiting the introduction of weapons of mass destruction in outer space. Various proposals for an arms control treaty governing outer space were debated during a General Assembly session in December 1966, culminating in the drafting and adoption of the Outer Space Treaty the following January.

Key provisions of the Outer Space Treaty include prohibiting nuclear weapons in space; limiting the use of the Moon and all other celestial bodies to peaceful purposes; establishing that space shall be freely explored and used by all nations; and precluding any country from claiming sovereignty over outer space or any celestial body. Although it forbids establishing military bases, testing weapons and conducting military maneuvers on celestial bodies, the treaty does not expressly ban all military activities in space, nor the establishment of military space forces or the placement of conventional weapons in space. From 1968 to 1984, the OST birthed four additional agreements: rules for activities on the Moon; liability for damages caused by spacecraft; the safe return of fallen astronauts; and the registration of space vehicles.

OST provided many practical uses and was the most important link in the chain of international legal arrangements for space from the late 1950s to the mid-1980s. OST was at the heart of a 'network' of inter-state treaties and strategic power negotiations to achieve the best available conditions for nuclear weapons world security. The OST also declares that space is an area for free use and exploration by all and "shall be the province of all mankind". Drawing heavily from the Antarctic Treaty of 1961, the Outer Space Treaty likewise focuses on regulating certain activities and preventing unrestricted competition that could lead to conflict. Consequently, it is largely silent or ambiguous on newly developed space activities such as lunar and asteroid mining. Nevertheless, the Outer Space Treaty is the first and most foundational legal instrument of space law, and its broader principles of promoting the civil and peaceful use of space continue to underpin multilateral initiatives in space, such as the International Space Station and the Artemis Program.

Moon Treaty

The Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, better known as the Moon Treaty or Moon Agreement, is a multilateral treaty that turns jurisdiction of all celestial bodies (including the orbits around such bodies) over to the participant countries. Thus, all activities would conform to international law, including the United Nations Charter.

It has not been ratified by any state that engages in self-launched human spaceflight (e.g. the United States, Russia (or its predecessor the Soviet Union), or the People's Republic of China) since its creation on December 18, 1979, and thus it has little to no relevancy in international law. As of January 2022, 18 states are parties to the treaty.

Artemis Accords

The Artemis Accords builds on a number of treaties that affect the conduct of States and their commercial industries in the exploration and use of space, including the 1967 Outer Space Treaty, the 1972 Liability Convention and the 1975 Registration Convention. NASA has stated that in leading the Artemis program, international partnerships will prepare for a historic human mission to Mars while playing a key role in achieving a sustainable and robust presence on the Moon. The core of the Artemis agreement requires that all activities be conducted for peaceful purposes, consistent with the principles of the Outer Space Treaty. International cooperation under the Artemis Agreement aims not only to promote space exploration, but also to strengthen peaceful relations between nations.

Post-detection policy

A post-detection policy (PDP), also known as a post-detection protocol, is a set of structured rules, standards, guidelines, or actions that governmental or other organizational entities plan to follow for the "detection, analysis, verification, announcement, and response to" confirmed signals from extraterrestrial civilizations. Though no PDPs have been formally and openly adopted by any governmental entity, there is significant work being done by scientists and nongovernmental organizations to develop cohesive plans of action to utilize in the event of detection. The most popular and well known of these is the "Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence", which was developed by the International Academy of Astronautics (IAA), with the support of the International Institute of Space Law. The theories of PDPs constitute a distinct area of research but draw heavily from the fields of SETI (the Search for Extra-Terrestrial Intelligence), METI (Messaging to Extra-Terrestrial Intelligence), and CETI (Communication with Extraterrestrial Intelligence).

Scientist Zbigniew Paptrotny has argued that the formulation of post-detection protocols can be guided by three factors: terrestrial society's readiness to accept the news of ET detection, how the news of detection is released, and the comprehensibility of the message in the signal. These three broad areas and their related subsidiaries comprise the bulk of the content and discourse surrounding PDPs.

Politics of the ISS

  Primary contributing nations

  Formerly contracted nations

Politics of the International Space Station have been affected by superpower rivalries, international treaties and funding arrangements. The Cold War was an early factor, overtaken in recent years by the United States' distrust of China. The station has an international crew, with the use of their time, and that of equipment on the station, being governed by treaties between participant nations.

In 1972 a milestone was reached in co-operation between the United States and the Soviet Union in space with the Apollo-Soyuz Test Project. The project occurred during a period of détente between the two superpowers, and led in July 1975 to Soyuz 19 docking with an Apollo spacecraft.

From 1978 to 1987, the USSR's Interkosmos program included allied Warsaw Pact countries, and countries which were not Soviet allies, such as India, Syria and France, in crewed and uncrewed missions to Space stations Salyut 6 and 7. In 1986, the USSR extended its co-operation to a dozen countries in the Mir program. From 1994 to 1998, NASA Space Shuttles and crew visited Mir in the Shuttle–Mir program.

In 1998, assembly of the space station began. On 28 January 1998, the Space Station Intergovernmental Agreement (IGA) was signed. This governs ownership of modules, station usage by participant nations, and responsibilities for station resupply. The signatories were the United States of America, Russia, Japan, Canada and eleven member states of the European Space Agency (Belgium, Denmark, France, Germany, Italy, The Netherlands, Norway, Spain, Sweden, Switzerland, and the United Kingdom). With the exception of the United Kingdom, all of the signatories went on to contribute to the Space Station project. A second layer of agreements was then achieved, four memoranda of understanding between NASA and ESA, CSA, RKA and JAXA. These agreements are then further split, such as for the contractual obligations between nations, and trading of partners' rights and obligations. Use of the Russian Orbital Segment is also negotiated at this level.

In 2010, the ESA announced that European countries which were not already part of the program would be allowed access to the station in a three-year trial period.

In March 2012, a meeting in Quebec City between the leaders of the space agencies of Canada, Japan, Russia, the United States and involved European nations resulted in a renewed pledge to maintain the space station until at least 2020. NASA reports to be still committed to the principles of the mission but also to use the station in new ways, which were not elaborated. CSA President Steve MacLean stated his belief that the station's Canadarm will continue to function properly until 2028, alluding to Canada's likely extension of its involvement beyond 2020.

On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to collaborate on the development of a replacement for the current ISS. Igor Komarov, the head of Russia's Roscosmos, made the announcement with NASA administrator Charles Bolden at his side. In a statement provided to SpaceNews on 28 March, NASA spokesman David Weaver said the agency appreciated the Russian commitment to extending the ISS, but did not confirm any plans for a future space station.

On 30 September 2015, Boeing's contract with NASA as prime contractor for the ISS was extended to 30 September 2020. Part of Boeing's services under the contract related to extending the station's primary structural hardware past 2020 to the end of 2028.

There have also been suggestions in the commercial space industry that the station could be converted to commercial operations after it is retired by government entities.

In July 2018, the Space Frontier Act of 2018 was intended to extend operations of the ISS to 2030. This bill was unanimously approved in the Senate, but failed to pass in the U.S. House. In September 2018, the Leading Human Spaceflight Act was introduced with the intent to extend operations of the ISS to 2030, and was confirmed in December 2018.

On 12 April 2021, at a meeting with Russian President Vladimir Putin, then-Deputy Prime Minister Yury Borisov announced he had decided that Russia might withdraw from the ISS programme in 2025. According to Russian authorities, the timeframe of the station’s operations has expired and its condition leaves much to be desired.

In January 2022, NASA announced a planned date of January 2031 to de-orbit the ISS and direct any remnants into a remote area of the South Pacific Ocean.

On 24 February 2022, NASA said that American and Russian astronauts currently aboard the ISS would continue normal operations despite the 2022 Russian invasion of Ukraine. British Prime Minister Boris Johnson commented on the current status of cooperation, saying "I have been broadly in favour of continuing artistic and scientific collaboration, but in the current circumstances it's hard to see how even those can continue as normal." On the same day, Roscosmos Director General Dmitry Rogozin insinuated that Russian withdrawal could cause the International Space Station to de-orbit due to lack of reboost capabilities, writing in a series of tweets, "If you block cooperation with us, who will save the ISS from an unguided de-orbit to impact on the territory of the US or Europe? There's also the chance of impact of the 500-ton construction in India or China. Do you want to threaten them with such a prospect? The ISS doesn't fly over Russia, so all the risk is yours. Are you ready for it?" Rogozin later tweeted that normal relations between ISS partners could only be restored once sanctions have been lifted, and indicated that Roscosmos would submit proposals to the Russian government on ending cooperation. NASA stated that, if necessary, US corporation Northrop Grumman has offered a reboost capability that would keep the ISS in orbit.

On 26 July 2022, Borisov, who had become head of Roscosmos, submitted to Putin his plans for withdrawal from the programme after 2024. However, Robyn Gatens, the NASA official in charge of space station operations, responded that NASA had not received any formal notices from Roscosmos concerning withdrawal plans. The United States Congress, in its CHIPS and Science Act signed by President Joe Biden on 9 August, approved extending NASA's funding for the ISS through 2030.

On 21 September 2022, Borisov stated that Russia was "highly likely" to continue to participate in the ISS programme until 2028.

Politics of asteroid mining

In recent years the advancement in technology and engineering have made mining asteroid seem like an attainable endeavour. The global space mining market is estimated to be worth USD 14.71 billion by 2025, as indicated by market research. Although the industry could be years away from successfully mining asteroids, this renewed interest in asteroid mining for metal extraction has the potential to influence the global market of rare metal and create a new geopolitical order.

Outer space has been a ground for geopolitical competition since the Cold War, and the expected growth in asteroid mining could bring about a new geopolitical order organized around resources extraction. Access to rare metals from asteroids could position nation-states and their private sectors competitively. Countries with technological capacity to explore space and finance a new and costly asteroid mining endeavour are better positioned to dominate the global supply chain for such metals, while a handful of countries will lag behind. This threatens to drive a resource race in outer space and could create similar patterns of conflict around resources extraction to what has been experienced on earth.

The expansion of resource extraction in outer space will require the development of a legal regulatory framework that adequately governs asteroid mining activities. The two current treaties that govern activities in outer space are the Outer Space Treaty and the Moon Treaty. Ratified by 98 countries in 1976, the Outer Space Treaty prevents sovereign or private ownership of outer space and its resources, asserting it belongs to all mankind but does not prevent exploitation of its resources. Ratified in 1979, the Moon Treaty clears some of the vague language surrounding the heritage for humankind outline in the Outer Space Treaty. Similar to the Outer Space Treaty, its provisions outline the Moon and other celestial bodies are “not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means” It also introduces a number of provisions that limit activities in outer space. While those provisions have implications for the development of space mining industry, Article 11.7 of the treaty has serious implications for countries that wish to assert monopoly over the emerging asteroid mining industry.

Article 11.7: The main purposes of the international regime to be established shall include:

1.  The orderly and safe development of the natural resources of the moon;

2.  The rational management of those resources;

3.  The expansion of opportunities in the use of those resources;

4.  An equitable sharing by all States Parties in the benefits derived from those resources, whereby the interests and needs of the developing countries, as well as the efforts of those countries which have contributed either directly or indirectly to the exploration of the moon, shall be given special consideration.

Given the limitation the Moon treaty place on states, it has not been ratified by key players in the space frontier, such as the US, China, and Russia.

Both the Outer Space Treaty and the Moon Treaty lack a robust enforcement mechanism that holds states and private entities accountable for their violation of the agreements. The lack of clarity over claims for ownership could result in conflicts among countries and private companies. While International efforts to reconsolidate a regulatory framework to govern future mining activities are much needed, they have been slow. Establishing rules on transparency, resource sharing, and mechanisms to guide conflict resolutions are needed to support the growing economy of asteroid mining.

Several countries have conducted research missions to asteroids. While the US, EU, Japan, Russia, and China have all had successful asteroid missions, only the US and Japan were able to bring samples from an asteroid. With geological surveys for metals often preceding the militarization of territories and the expansion of nation-states in terrestrial context, research missions to asteroids allude to the expansion of state ambition to dominate a large-scale extra-terrestrial extractive regime.

While asteroid mining remains in its infancy, countries are competing to dominate it. However, the difficulty of reaching consensus on global treaties has led countries to branch out to legitimatize the economic exploration and exploitation of asteroids, through passing national law, and relying on loopholes in the international law.

In 2015, the US passed the U.S Commercial Space Launch Competitiveness Act. While the act does not position the US as a state to have authority or ownership in outer space, it positions its citizens to have ownership over resources acquired from space. This allows the U.S to adhere to Outer Space Treaty but also allow to its private entities to carry out mining activities on Asteroids, once feasible. AstroForge, a US start-up with a mission focused on developing technology for asteroid mining, announced two commercial missions to asteroids to be launched in 2023.

In 2016, the government of Luxembourg introduced a legal framework that support and guide the private activities of mining asteroids. To grant certainty investors, it passed a law that explicitly permits private entities to own and sell resources extracted from asteroids. The government also pledged to support research and start-ups focused on space exploration and extra-terrestrial resource extraction with a funding of approximately USD $225 million. Relative to the US, the Luxembourg Space Act provides more clarity and position the country more competitively to engage in asteroid mining. Given its strategic location in Europe, Luxembourg intends to establish itself as a Silicon Valley for space activities.

Despite the difference in passed legislations, the goal is the same: to emerge as a leader in the new asteroid mining frontier and obtain the economic benefit associated with it. Current and future development in asteroid mining do not intel on whether such competition in this frontier will lead to positive changes in international law or allow a harmonization of notational laws among states. Yet, the steady growth in asteroid mining will have implications for geopolitics of terrestrial and extra-terrestrial resource extraction.

Colonialism and imperialism

Further information: Transport divide

Gemini 5 mission badge (1965) connecting spaceflight to colonial endeavours

The logo and name of the Lunar Gateway references the St. Louis Gateway Arch, which some see as associating Mars with the American frontier and the manifest destiny mentality of American settler colonialism

Space colonization has been discussed as postcolonial continuation of imperialism and colonialism, calling for decolonization instead of colonization. Critics argue that the present politico-legal regimes and their philosophic grounding advantage imperialist development of space and that key decisionmakers in space colonization are often wealthy elites affiliated with private corporations, and that space colonization would primarily appeal to their peers rather than ordinary citizens. Furthermore, it is argued that there is a need for inclusive and democratic participation and implementation of any space exploration, infrastructure or habitation. According to space law expert Michael Dodge, existing space law, such as the Outer Space Treaty, guarantees access to space, but does not enforce social inclusiveness or regulate non-state actors.

Particularly the narrative of the "New Frontier", has been criticized as unreflected continuation of settler colonialism and manifest destiny, continuing the narrative of exploration as fundamental to the assumed human nature. Joon Yun considers space colonization as a solution to human survival and global problems like pollution to be imperialist, as such others have identified space as a new sacrifice zone of colonialism.

Natalie B. Trevino argues that not colonialism but coloniality will be carried into space if not reflected on.

More specifically the advocacy for territorial colonization of Mars opposed to habitation in the atmospheric space of Venus has been called surfacism, a concept similar to Thomas Golds surface chauvinism.

More generally space infrastructure such as the Mauna Kea Observatories have also been criticized and protested against as being colonialist. Guiana Space Centre has also been the site of anti-colonial protests, connecting colonization as an issue on Earth and in space.

In regard to the scenario of extraterrestrial first contact it has been argued that being used to employ colonial language would endanger such first impressions and encounters.

Furthermore spaceflight as a whole and space law more particularly has been criticized as a postcolonial project by being built on a colonial legacy and by not facilitating the sharing of access to space and its benefits, too often allowing spaceflight to be used to sustain colonialism and imperialism, most of all on Earth instead.

Neoliberal advocacy

The trend towards the economicisation of outer space under neoliberalism is having a profound impact on the political ecology of outer space.

Outer space is becoming a space for capitalism. A new era of space commercialisation aims to profit from satellite launches, space tourism, asteroid mining and related ventures. This era, driven by private companies such as Elon Musk's SpaceX and Jeff Bezos' Blue Origin, has been dubbed "the new space" by industry insiders." Spatial justice in outer space increasingly means the 'justice' of capital, with capitalism replacing humanity."

Since the mid-20th century, space expansionism has become a popular ideology and, thanks to science, the emergence of technological civilization, and the spread of neoliberalism, it has become possible for a wide range of actors, such as national armies and government agencies, scientists and private companies, to carry out a variety of space activities, such as the regulation of outer space through international law, the deployment of missile and anti-satellite weapons, the establishment of exploration, communication and navigation satellites, and space travel for tourism and habitat expansion. Since the mid-twentieth century, spatial expansionism has gone hand in hand with the concept of the world as a 'planetary earth' - going beyond the concept of a 'global earth' associated with the industrial revolution.

There are always costs and benefits in environmental change and these are unevenly distributed along lines of class, race, ethnicity, gender, and geography (among other axes of difference). The environmental geopolitics of outer space is similarly multi-scale, manifesting itself in contemporary debates on pollution issues such as orbital debris and planetary protection agreements. The cultural, legal, budgetary, and infrastructural footprints experienced in the contemporary space race have measurable environmental footprints on Earth and in outer space. The question of where these footprints fall is arbitrated by larger issues of geopolitical power and vulnerability, which means that human participation in outer space is also a matter of environmental justice.

Pond

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Pond

Pond at Cornjum, Netherlands

A man made pond at sunset in Montgomery County, Ohio.

Stereoscopic image of a pond in Central City Park, Macon, GA, circa 1877.

A pond is a small, still, land-based body of water formed by pooling inside a depression, either naturally or artificially. A pond is smaller than a lake and there are no official criteria distinguishing the two, although defining a pond to be less than 5 hectares (12 acres) in area, less than 5 metres (16 ft) in depth and with less than 30% with emergent vegetation helps in distinguishing the ecology of ponds from those of lakes and wetlands. Ponds can be created by a wide variety of natural processes (e.g. on floodplains as cutoff river channels, by glacial processes, by peatland formation, in coastal dune systems, by beavers), or they can simply be isolated depressions (such as a kettle hole, vernal pool, prairie pothole, or simply natural undulations in undrained land) filled by runoff, groundwater, or precipitation, or all three of these. They can be further divided into four zones: vegetation zone, open water, bottom mud and surface film. The size and depth of ponds often varies greatly with the time of year; many ponds are produced by spring flooding from rivers. Ponds may be freshwater or brackish in nature. 'Ponds' consisting of saltwater, with a direct connection to the sea to maintain full salinity, are normally regarded as part of the marine environment. These bodies of water do not support fresh or brackish water-based organisms, and are not considered to be ponds.

Ponds are usually by definition quite shallow water bodies with varying abundances of aquatic plants and animals. Depth, seasonal water level variations, nutrient fluxes, amount of light reaching the ponds, the shape, the presence of visiting large mammals, the composition of any fish communities and salinity can all affect the types of plant and animal communities present. Food webs are based both on free-floating algae and upon aquatic plants. There is usually a diverse array of aquatic life, with a few examples including algae, snails, fish, beetles, water bugs, frogs, turtles, otters and muskrats. Top predators may include large fish, herons, or alligators. Since fish are a major predator upon amphibian larvae, ponds that dry up each year, thereby killing resident fish, provide important refugia for amphibian breeding. Ponds that dry up completely each year are often known as vernal pools. Some ponds are produced by animal activity, including alligator holes and beaver ponds, and these add important diversity to landscapes.

Ponds are frequently manmade or expanded beyond their original depths and bounds by anthropogenic causes. Apart from their role as highly biodiverse, fundamentally natural, freshwater ecosystems ponds have had, and still have, many uses, including providing water for agriculture, livestock and communities, aiding in habitat restoration, serving as breeding grounds for local and migrating species, decorative components of landscape architecture, flood control basins, general urbanization, interception basins for pollutants and sources and sinks of greenhouse gases.

Classification

The technical distinction between a pond and a lake has not been universally standardized. Limnologists and freshwater biologists have proposed formal definitions for pond, in part to include 'bodies of water where light penetrates to the bottom of the waterbody,' 'bodies of water shallow enough for rooted water plants to grow throughout,' and 'bodies of water which lack wave action on the shoreline.' Each of these definitions are difficult to measure or verify in practice and are of limited practical use, and are mostly not now used. Accordingly, some organizations and researchers have settled on technical definitions of pond and lake that rely on size alone.

Vegetated pond within the sand dunes of the Lençóis Maranhenses National Park, Brazil

Some regions of the United States define a pond as a body of water with a surface area of less than 10 acres (4.0 ha). Minnesota, known as the "land of 10,000 lakes", is commonly said to distinguish lakes from ponds, bogs and other water features by this definition, but also says that a lake is distinguished primarily by wave action reaching the shore. Even among organizations and researchers who distinguish lakes from ponds by size alone, there is no universally recognized standard for the maximum size of a pond. The international Ramsar wetland convention sets the upper limit for pond size as 8 hectares (80,000 m²; 20 acres). Researchers for the British charity Pond Conservation (now called Freshwater Habitats Trust) have defined a pond to be 'a man-made or natural waterbody that is between 1 m² (0.00010 hectares; 0.00025 acres) and 20,000 m² (2.0 hectares; 4.9 acres) in area, which holds water for four months of the year or more.' Other European biologists have set the upper size limit at 5 hectares (50,000 m²; 12 acres).

In North America, even larger bodies of water have been called ponds; for example, Crystal Lake at 33 acres (130,000 m²; 13 ha), Walden Pond in Concord, Massachusetts at 61 acres (250,000 m²; 25 ha), and nearby Spot Pond at 340 acres (140 ha). There are numerous examples in other states, where bodies of water less than 10 acres (40,000 m²; 4.0 ha) are being called lakes. As the case of Crystal Lake shows, marketing purposes can sometimes be the driving factor behind the categorization.

The Pond in Central Park in Manhattan, New York City

In practice, a body of water is called a pond or a lake on an individual basis, as conventions change from place to place and over time. In origin, a pond is a variant form of the word pound, meaning a confining enclosure. In earlier times, ponds were artificial and utilitarian, as stew ponds, mill ponds and so on. The significance of this feature seems, in some cases, to have been lost when the word was carried abroad with emigrants. However, some parts of New England contain "ponds" that are actually the size of a small lake when compared to other countries. In the United States, natural pools are often called ponds. Ponds for a specific purpose keep the adjective, such as "stock pond", used for watering livestock. The term is also used for temporary accumulation of water from surface runoff (ponded water).

There are various regional names for naturally occurring ponds. In Scotland, one of the terms is lochan, which may also apply to a large body of water such as a lake. In the South Western parts of North American, lakes or ponds that are temporary and often dried up for most parts of the year are called playas.  These playas are simply shallow depressions in dry areas that may only fill with water on certain occasion like excess local drainage, groundwater seeping, or rain.

Formation

Pond formation through seeping groundwater in South Tufa, California

Any depression in the ground which collects and retains a sufficient amount of water can be considered a pond, and such, can be formed by a variety of geological, ecological, and human terraforming events.

Ornamental pond with waterfall in Niagara Falls Rock Garden

Natural ponds are those caused by environmental occurrences. These can vary from glacial, volcanic, fluvial, or even tectonic events. Since the Pleistocene epoch, glacial processes have created most of the Northern hemispheric ponds; an example is the Prairie Pothole Region of North America. When glaciers retreat, they may leave behind uneven ground due to bedrock elastic rebound and sediment outwash plains. These areas may develop depressions that can fill up with excess precipitation or seeping ground water, forming a small pond. Kettle lakes and ponds are formed when ice breaks off from a larger glacier, is eventually buried by the surrounding glacial till, and over time melts. Orogenies and other tectonic uplifting events have created some of the oldest lakes and ponds on the globe. These indentions have the tendency to quickly fill with groundwater if they occur below the local water table. Other tectonic rifts or depressions can fill with precipitation, local mountain runoff, or be fed by mountain streams. Volcanic activity can also lead to lake and pond formation through collapsed lava tubes or volcanic cones. Natural floodplains along rivers, as well as landscapes that contain many depressions, may experience spring/rainy season flooding and snow melt. Temporary or vernal ponds are created this way and are important for breeding fish, insects, and amphibians, particularly in large river systems like the Amazon. Some ponds are solely created by animals species such as beavers, bison, alligators and other crocodilians through damning and nest excavation respectively. In landscapes with organic soils, local fires can create depressions during periods of drought. These have the tendency to fill up with small amounts of precipitation until normal water levels return, turning these isolated ponds into open water.

Manmade ponds are those created by human intervention for the sake of the local environment, industrial settings, or for recreational/ornamental use.

Uses

Many ecosystems are linked by water and ponds have been found to hold a greater biodiversity of species than larger freshwater lakes or river systems. As such, ponds are habitats for many varieties of organisms including plants, amphibians, fish, reptiles, waterfowl, insects and even some mammals. Ponds are used for breeding grounds for these species but also as shelter and even drinking/feeding locations for other wildlife. Aquaculture practices lean heavily on artificial ponds in order to grow and care for many different type of fish either for human consumption, research, species conservation or recreational sport.

A small agricultural retention pond in Swarzynice, Poland

In agriculture practices, treatment ponds can be created to reduce nutrient runoff from reaching local streams or groundwater storages. Pollutants that enter ponds can often be mitigated by natural sedimentation and other biological and chemical activities within the water. As such, waste stabilization ponds are becoming popular low-cost methods for general wastewater treatment. They may also provide irrigation reservoirs for struggling farms during times of drought.

As urbanization continues to spread, retention ponds are becoming more common in new housing developments. These ponds reduce the risk of flooding and erosion damage from excess storm water runoff in local communities.

Siddha Pokhari, a reservoir pond in Bhaktapur, Nepal

Experimental ponds are used to test hypotheses in the fields of environmental science, chemistry, aquatic biology, and limnology.

Some ponds are the life blood of many small villages in arid countries such as those in sub-Saharan Africa where bathing, sanitation, fishing, socialization, and rituals are held. In the Indian subcontinent, Hindu temple monks care for sacred ponds used for religious practices and bathing pilgrims alike. In Europe during medieval times, it was typical for many monastery and castles (small, partly self-sufficient communities) to have fish ponds. These are still common in Europe and in East Asia (notably Japan), where koi may be kept or raised.

In Nepal artificial ponds were essential elements of the ancient drinking water supply system. These ponds were fed with rainwater, water coming in through canals, their own springs, or a combination of these sources. They were designed to retain the water, while at the same time letting some water seep away to feed the local aquifers.

Pond biodiversity

Azalea flowers around a still pond in London's Richmond Park

A defining feature of a pond is the presence of standing water which provides habitat for a biological community commonly referred to as pond life. Because of this, many ponds and lakes contain large numbers of endemic species that have gone through adaptive radiation to become specialized to their preferred habitat. Familiar examples might include water lilies and other aquatic plants, frogs, turtles, and fish.

Common freshwater fish species include the Large Mouth and Small Mouth Bass, Catfish, Bluegill, and Sunfish such as the Pumpkinseed Sunfish shown above

Often, the entire margin of the pond is fringed by wetland, and these wetlands support the aquatic food web, provide shelter for wildlife, and stabilize the shore of the pond. This margin is also known as the littoral zone and contains much of the photosynthetic algae and plants of this ecosystem called macrophytes. Other photosynthetic organisms such as phytoplankton (suspended algae) and periphytons (organisms including cyanobacteria, detritus, and other microbes) thrive here and stand as the primary producers of pond food webs. Some grazing animals like geese and muskrats consume the wetland plants directly as a source of food. In many other cases, pond plants will decay in the water. Many invertebrates and herbivorous zooplankton then feed on the decaying plants, and these lower trophic level organisms provide food for wetland species including fish, dragonflies, and herons both in the littoral zone and the limnetic zone. The open water limnetic zone may allow algae to grow as sunlight still penetrates here. These algae may support yet another food web that includes aquatic insects and other small fish species. A pond, therefore, may have combinations of three different food webs, one based on larger plants, one based upon decayed plants, and one based upon algae and their specific upper trophic level consumers and predators. Hence, ponds often have many different animal species using the wide array of food sources though biotic interaction. They, therefore, provide an important source of biological diversity in landscapes.

Opposite to long standing ponds are vernal ponds. These ponds dry up for part of the year and are so called because they are typically at their peak depth in the spring (the meaning of "vernal" comes form the Latin word for spring). Naturally occurring vernal ponds do not usually have fish, a major higher tropic level consumer, as these ponds frequently dry up. The absence of fish is a very important characteristic of these ponds since it prevents long chained biotic interactions from establishing. Ponds without these competitive predation pressures provides breeding locations and safe havens for endangered or migrating species. Hence, introducing fish to a pond can have seriously detrimental consequences. In some parts of the world, such as California, the vernal ponds have rare and endangered plant species. On the coastal plain, they provide habitat for endangered frogs such as the Mississippi Gopher Frog.

Often groups of ponds in a given landscape - so called 'pondscapes' - offer especially high biodiversity benefits compared to single ponds. A group of ponds provides a higher degree of habitat complexity and habitat connectivity.

Stratification

Lakes are stratified into three separate sections: I. The Epilimnion II. The Metalimnion III. The Hypolimnion. The scales are used to associate each section of the stratification to their corresponding depths and temperatures. The arrow is used to show the movement of wind over the surface of the water which initiates the turnover in the epilimnion and the hypolimnion.

Many ponds undergo a regular yearly process in the same matter as larger lakes if they are deep enough and/or protected from the wind. Abiotic factors such as UV radiation, general temperature, wind speed, water density, and even size, all have important roles to play when it comes to the seasonal effects on lakes and ponds. Spring overturn, summer stratification, autumn turnover, and an inverse winter stratification, ponds adjust their stratification or their vertical zonation of temperature due to these influences. These environmental factors affect pond circulation and temperature gradients within the water itself producing distant layers; the epilimnion, metalimnion, and hypolimnion.

A pond in winter experiencing inverse stratification

Each zone has varied traits that sustain or harm specific organisms and biotic interactions below the surface depending on the season. Winter surface ice begins to melt in the Spring. This allows the water column to begin mixing thanks to solar convection and wind velocity. As the pond mixes, an overall constant temperature is reached. As temperatures increase through the summer, thermal stratification takes place. Summer stratification allows for the epilimnion to be mixed by winds, keeping a consistent warm temperature throughout this zone. Here, photosynthesis and primary production flourishes. However, those species that need cooler water with higher dissolved oxygen concentrations will favor the lower metalimnion or hypolimnion. Air temperature drops as fall approaches and a deep mixing layer occurs. Autumn turnover results in isothermal lakes with high levels of dissolved oxygen as the water reaches an average colder temperature. Finally, winter stratification occurs inversely to summer stratification as surface ice begins to form yet again. This ice cover remains until solar radiation and convection return in the spring.

Due to this constant change in vertical zonation, seasonal stratification causes habitats to grow and shrink accordingly. Certain species are bound to these distinct layers of the water column where they can thrive and survive with the best efficiency possible.

For more information regarding seasonal thermal stratification of ponds and lakes, please look at "Lake Stratification".

Conservation and management

Artificial pond in front of the Haus der Kulturen der Welt, Berlin, Germany

Ponds provide not only environmental values, but practical benefits to society. One increasingly crucial benefit that ponds provide is their ability to act as greenhouse gas sinks. Most natural lakes and ponds are greenhouse gas sources and aid in the flux of these dissolved compounds. However, manmade farm ponds are becoming significant sinks for gas mitigation and the fight against climate change. These agriculture runoff ponds receive high pH level water from surrounding soils. Highly acidic drainage ponds act as catalysis for excess CO₂ (carbon dioxide) to be converted into forms of carbon that can easily be stored in sediments. When these new drainage ponds are constructed, concentrations of bacteria that normally break down dead organic matter, such as algae, are low. As a result, breakdown and release of nitrogen gases from these organic materials such as N₂O does not occur and thus, not added to our atmosphere. This process is also used with regular denitrification in anoxic layer of ponds. However, not all ponds have the ability to become sinks for greenhouse gasses. Most ponds experience eutrophication where faced with excessive nutrient input from fertilizers and runoff. This over-nitrifies the pond water and results in mass algae blooms and local fish kills.

Some farm ponds are not used for runoff control but rather for livestock like cattle or buffalo as watering and bathing holes. As mentioned in the use section, ponds are important hotspots for biodiversity. Sometimes this becomes an issue with invasive or introduced species that disrupt pond ecosystem dynamics such as food-web structure, niche partitioning, and guild assignments. This varies from introduced fish species such as the Common Carp that eat native water plants or Northern Snakeheads that attack breeding amphibians, aquatic snails that carry infectious parasites that kill other species, and even rapid spreading aquatic plants like Hydrilla and Duckweed that can restrict water flow and cause overbank flooding.

During the last thirty years of his life, the main focus of Claude Monet's artistic production was a series of about 250 oil paintings depicting the lily pond in his flower garden.

Ponds, depending on their orientation and size, can spread their wetland habitats into the local riparian zones or watershed boundaries. Gentle slopes of land into ponds provides an expanse of habitat for wetland plants and wet meadows to expand beyond the limitation of the pond. However, the construction of retaining walls, lawns, and other urbanized developments can severely degrade the range of pond habitats and the longevity of the pond itself. Roads and highways act in the same manor, but they also interfere with amphibians and turtles that migrate to and from ponds as part of their annual breeding cycle and should be kept as far away from established ponds as possible. Because of these factors, gently sloping shorelines with broad expanses of wetland plants not only provide the best conditions for wildlife, but they help protect water quality from sources in the surrounding landscapes. It is also beneficial to allow water levels to fall each year during drier periods in order to re-establish these gentile shorelines.

In landscapes where ponds are artificially constructed, they are done so to provide wildlife viewing and conservation opportunities, to treat wastewater, for sequestration and pollution containment, or for simply aesthetic purposes. For natural pond conservation and development, one way to stimulate this is with general stream and river restoration. Many small rivers and streams feed into or from local ponds within the same watershed. When these rivers and streams flood and begin to meander, large numbers of natural ponds, including vernal pools and wetlands, develop.

Decision theory

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Decision_theory

The mythological judgement of Paris required selecting from three incomparable alternatives (the goddesses shown).

Decision theory (or the theory of choice; not to be confused with choice theory) is a branch of applied probability theory and analytic philosophy concerned with the theory of making decisions based on assigning probabilities to various factors and assigning numerical consequences to the outcome.

There are three branches of decision theory:

1. Normative decision theory: Concerned with the identification of optimal decisions, where optimality is often determined by considering an ideal decision-maker who is able to calculate with perfect accuracy and is in some sense fully rational.
2. Prescriptive decision theory: Concerned with describing observed behaviors through the use of conceptual models, under the assumption that those making the decisions are behaving under some consistent rules.
3. Descriptive decision theory: Analyzes how individuals actually make the decisions that they do.

Decision theory is a broad field from management sciences and is an interdisciplinary topic, studied by management scientists, medical researchers, mathematicians, data scientists, psychologists, biologists, social scientists, philosophers and computer scientists.

Empirical applications of this theory are usually done with the help of statistical and discrete mathematical approaches from computer science.

Normative and descriptive

Normative decision theory is concerned with identification of optimal decisions where optimality is often determined by considering an ideal decision maker who is able to calculate with perfect accuracy and is in some sense fully rational. The practical application of this prescriptive approach (how people ought to make decisions) is called decision analysis and is aimed at finding tools, methodologies, and software (decision support systems) to help people make better decisions.

In contrast, descriptive decision theory is concerned with describing observed behaviors often under the assumption that those making decisions are behaving under some consistent rules. These rules may, for instance, have a procedural framework (e.g. Amos Tversky's elimination by aspects model) or an axiomatic framework (e.g. stochastic transitivity axioms), reconciling the Von Neumann-Morgenstern axioms with behavioral violations of the expected utility hypothesis, or they may explicitly give a functional form for time-inconsistent utility functions (e.g. Laibson's quasi-hyperbolic discounting).

Prescriptive decision theory is concerned with predictions about behavior that positive decision theory produces to allow for further tests of the kind of decision-making that occurs in practice. In recent decades, there has also been increasing interest in "behavioral decision theory", contributing to a re-evaluation of what useful decision-making requires.

Types of decisions

Choice under uncertainty

Further information: Expected utility hypothesis

The area of choice under uncertainty represents the heart of decision theory. Known from the 17th century (Blaise Pascal invoked it in his famous wager, which is contained in his Pensées, published in 1670), the idea of expected value is that, when faced with a number of actions, each of which could give rise to more than one possible outcome with different probabilities, the rational procedure is to identify all possible outcomes, determine their values (positive or negative) and the probabilities that will result from each course of action, and multiply the two to give an "expected value", or the average expectation for an outcome; the action to be chosen should be the one that gives rise to the highest total expected value. In 1738, Daniel Bernoulli published an influential paper entitled Exposition of a New Theory on the Measurement of Risk, in which he uses the St. Petersburg paradox to show that expected value theory must be normatively wrong. He gives an example in which a Dutch merchant is trying to decide whether to insure a cargo being sent from Amsterdam to St Petersburg in winter. In his solution, he defines a utility function and computes expected utility rather than expected financial value.

In the 20th century, interest was reignited by Abraham Wald's 1939 paper pointing out that the two central procedures of sampling-distribution-based statistical-theory, namely hypothesis testing and parameter estimation, are special cases of the general decision problem. Wald's paper renewed and synthesized many concepts of statistical theory, including loss functions, risk functions, admissible decision rules, antecedent distributions, Bayesian procedures, and minimax procedures. The phrase "decision theory" itself was used in 1950 by E. L. Lehmann.

The revival of subjective probability theory, from the work of Frank Ramsey, Bruno de Finetti, Leonard Savage and others, extended the scope of expected utility theory to situations where subjective probabilities can be used. At the time, von Neumann and Morgenstern's theory of expected utility proved that expected utility maximization followed from basic postulates about rational behavior.

The work of Maurice Allais and Daniel Ellsberg showed that human behavior has systematic and sometimes important departures from expected-utility maximization (Allais paradox and Ellsberg paradox). The prospect theory of Daniel Kahneman and Amos Tversky renewed the empirical study of economic behavior with less emphasis on rationality presuppositions. It describes a way by which people make decisions when all of the outcomes carry a risk. Kahneman and Tversky found three regularities – in actual human decision-making, "losses loom larger than gains"; persons focus more on changes in their utility-states than they focus on absolute utilities; and the estimation of subjective probabilities is severely biased by anchoring.

Intertemporal choice

Main article: Intertemporal choice

Intertemporal choice is concerned with the kind of choice where different actions lead to outcomes that are realised at different stages over time. It is also described as cost-benefit decision making since it involves the choices between rewards that vary according to magnitude and time of arrival. If someone received a windfall of several thousand dollars, they could spend it on an expensive holiday, giving them immediate pleasure, or they could invest it in a pension scheme, giving them an income at some time in the future. What is the optimal thing to do? The answer depends partly on factors such as the expected rates of interest and inflation, the person's life expectancy, and their confidence in the pensions industry. However even with all those factors taken into account, human behavior again deviates greatly from the predictions of prescriptive decision theory, leading to alternative models in which, for example, objective interest rates are replaced by subjective discount rates.

Interaction of decision makers

Military planners often conduct extensive simulations to help predict the decision-making of relevant actors.

Some decisions are difficult because of the need to take into account how other people in the situation will respond to the decision that is taken. The analysis of such social decisions is often treated under decision theory, though it involves mathematical methods. In the emerging field of socio-cognitive engineering, the research is especially focused on the different types of distributed decision-making in human organizations, in normal and abnormal/emergency/crisis situations.

Complex decisions

Other areas of decision theory are concerned with decisions that are difficult simply because of their complexity, or the complexity of the organization that has to make them. Individuals making decisions are limited in resources (i.e. time and intelligence) and are therefore boundedly rational; the issue is thus, more than the deviation between real and optimal behaviour, the difficulty of determining the optimal behaviour in the first place. Decisions are also affected by whether options are framed together or separately; this is known as the distinction bias.

Heuristics

Main article: Heuristics in judgment and decision-making

The gambler's fallacy: even when the roulette ball repeatedly lands on red, it is no more likely to land on black the next time.

Heuristics in decision-making is the ability of making decisions based on unjustified or routine thinking. While quicker than step-by-step processing, heuristic thinking is also more likely to involve fallacies or inaccuracies. The main use for heuristics in our daily routines is to decrease the amount of evaluative thinking we perform when making simple decisions, making them instead based on unconscious rules and focusing on some aspects of the decision, while ignoring others. One example of a common and erroneous thought process that arises through heuristic thinking is the Gambler's Fallacy — believing that an isolated random event is affected by previous isolated random events. For example, if a fair coin is flipped to tails for a couple of turns, it still has the same probability (i.e., 0.5) of doing so in future turns, though intuitively it seems more likely for it to roll heads soon. This happens because, due to routine thinking, one disregards the probability and concentrates on the ratio of the outcomes, meaning that one expects that in the long run the ratio of flips should be half for each outcome. Another example is that decision-makers may be biased towards preferring moderate alternatives to extreme ones. The Compromise Effect operates under a mindset that the most moderate option carries the most benefit. In an incomplete information scenario, as in most daily decisions, the moderate option will look more appealing than either extreme, independent of the context, based only on the fact that it has characteristics that can be found at either extreme.

Alternatives

A highly controversial issue is whether one can replace the use of probability in decision theory with something else.

Probability theory

Advocates for the use of probability theory point to:

• the work of Richard Threlkeld Cox for justification of the probability axioms,
• the Dutch book paradoxes of Bruno de Finetti as illustrative of the theoretical difficulties that can arise from departures from the probability axioms, and
• the complete class theorems, which show that all admissible decision rules are equivalent to the Bayesian decision rule for some utility function and some prior distribution (or for the limit of a sequence of prior distributions). Thus, for every decision rule, either the rule may be reformulated as a Bayesian procedure (or a limit of a sequence of such), or there is a rule that is sometimes better and never worse.

Alternatives to probability theory

The proponents of fuzzy logic, possibility theory, quantum cognition, Dempster–Shafer theory, and info-gap decision theory maintain that probability is only one of many alternatives and point to many examples where non-standard alternatives have been implemented with apparent success; notably, probabilistic decision theory is sensitive to assumptions about the probabilities of various events, whereas non-probabilistic rules, such as minimax, are robust in that they do not make such assumptions.

Ludic fallacy

Main article: Ludic fallacy

A general criticism of decision theory based on a fixed universe of possibilities is that it considers the "known unknowns", not the "unknown unknowns": it focuses on expected variations, not on unforeseen events, which some argue have outsized impact and must be considered – significant events may be "outside model". This line of argument, called the ludic fallacy, is that there are inevitable imperfections in modeling the real world by particular models, and that unquestioning reliance on models blinds one to their limits.

Temporal paradox

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Temporal_paradox

A temporal paradox, time paradox, or time travel paradox, is a paradox, an apparent contradiction, or logical contradiction associated with the idea of time travel or other foreknowledge of the future. While the notion of time travel to the future complies with the current understanding of physics via relativistic time dilation, temporal paradoxes arise from circumstances involving hypothetical time travel to the past – and are often used to demonstrate its impossibility. Temporal paradoxes fall into three broad groups: bootstrap paradoxes, consistency paradoxes, and Newcomb's paradox.

Types

"Causal loop" redirects here. For the cause and effect diagram, see causal loop diagram. For the plot device, see time loop.

Temporal paradoxes fall into three broad groups: bootstrap paradoxes, consistency paradoxes, and Newcomb's paradox. Bootstrap paradoxes violate causality by allowing future events to influence the past and cause themselves, or "bootstrapping", which derives from the idiom "pull oneself up by one's bootstraps." Consistency paradoxes, on the other hand, are those where future events influence the past to cause an apparent contradiction, exemplified by the grandfather paradox, where a person travels to the past to kill their grandfather. Newcomb's paradox stems from the apparent contradictions that stem from the assumptions of both free will and foreknowledge of future events. All of these are sometimes referred to individually as "causal loops." The term "time loop" is sometimes referred to as a causal loop, but although they appear similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting.

Bootstrap paradox

A boot-strap paradox, also known as an information loop, an information paradox, an ontological paradox, or a "predestination paradox" is a paradox of time travel that occurs when any event, such as an action, information, an object, or a person, which ultimately causes itself, as a consequence of either retrocausality or time travel.

Backward time travel would allow information, people, or objects whose histories seem to "come from nowhere." Such causally looped events then exist in spacetime, but their origin cannot be determined. The notion of objects or information that are "self-existing" in this way is often viewed as paradoxical. Everett gives the movie Somewhere in Time as an example involving an object with no origin: an old woman gives a watch to a playwright who later travels back in time and meets the same woman when she was young, and gives her the same watch that she will later give to him.

Information loop

A second class of bootstrap paradoxes is related to information being created from nothing. Think of it like this a math teacher goes into the future and steals a groundbreaking theorem. He then gives it to a promising student. That student grows up to be the exact person the math teacher steals the theorem from in the first place. This is a casual consistent loop. This keeps happening throughout time and makes sense.

Predestination paradox

Smeenk uses the term "predestination paradox" to refer specifically to situations in which a time traveler goes back in time to try to prevent some event in the past.

Grandfather paradox

Top: original billiard ball trajectory. Middle: the billiard ball emerges from the future, and delivers its past self a strike that averts the past ball from entering the time machine. Bottom: The billiard ball never enters the time machine, giving rise to the paradox, putting into question how its older self could ever emerge from the time machine and divert its course.

The consistency paradox or grandfather paradox occurs when the past is changed in any way, thus creating a contradiction. A common example given is traveling to the past and intervening with the conception of one's ancestors (such as causing the death of the parent beforehand), thus affecting the conception of oneself. If the time traveler were not born, then it would not be possible for them to undertake such an act in the first place. Therefore, the ancestor lives to offspring the time traveler's next-generation ancestor, and eventually the time traveler. There is thus no predicted outcome to this. Consistency paradoxes occur whenever changing the past is possible. A possible resolution is that a time traveller can do anything that did happen, but cannot do anything that did not happen. Doing something that did not happen results in a contradiction. This is referred to as the Novikov self-consistency principle.

Variants

The grandfather paradox encompasses any change to the past, and it is presented in many variations, including killing one's past self, Both the "retro-suicide paradox" and the "grandfather paradox" appeared in letters written into Amazing Stories in the 1920s. Another variant of the grandfather paradox is the "Hitler paradox" or "Hitler's murder paradox", in which the protagonist travels back in time to murder Adolf Hitler before he can instigate World War II and the Holocaust. Rather than necessarily physically preventing time travel, the action removes any reason for the travel, along with any knowledge that the reason ever existed.

Physicist John Garrison et al. give a variation of the paradox of an electronic circuit that sends a signal through a time machine to shut itself off, and receives the signal before it sends it.

Newcomb's paradox

Main article: Newcomb's paradox

Newcomb's paradox is a thought experiment showing an apparent contradiction between the expected utility principle and the strategic dominance principle.

The thought experiment is often extended to explore causality and free will by allowing for "perfect predictors": if perfect predictors of the future exist, for example if time travel exists as a mechanism for making perfect predictions, then perfect predictions appear to contradict free will because decisions apparently made with free will are already known to the perfect predictor. Predestination does not necessarily involve a supernatural power, and could be the result of other "infallible foreknowledge" mechanisms. Problems arising from infallibility and influencing the future are explored in Newcomb's paradox.

A notable fictional example of a self-fulfilling prophecy occurs in the classical play Oedipus Rex, in which Oedipus becomes the king of Thebes and in the process unwittingly fulfills a prophecy that he would kill his father and marry his mother. The prophecy itself serves as the impetus for his actions, and thus it is self-fulfilling. The movie 12  Monkeys heavily deals with themes of predestination and the Cassandra complex, where the protagonist who travels back in time explains that he can't change the past.

Proposed resolutions

Logical impossibility

Even without knowing whether time travel to the past is physically possible, it is possible to show using modal logic that changing the past results in a logical contradiction. If it is necessarily true that the past happened in a certain way, then it is false and impossible for the past to have occurred in any other way. A time traveler would not be able to change the past from the way it is; they would only act in a way that is already consistent with what necessarily happened.

Consideration of the grandfather paradox has led some to the idea that time travel is by its very nature paradoxical and therefore logically impossible. For example, the philosopher Bradley Dowden made this sort of argument in the textbook Logical Reasoning, arguing that the possibility of creating a contradiction rules out time travel to the past entirely. However, some philosophers and scientists believe that time travel into the past need not be logically impossible provided that there is no possibility of changing the past, as suggested, for example, by the Novikov self-consistency principle. Dowden revised his view after being convinced of this in an exchange with the philosopher Norman Swartz.

Illusory time

Consideration of the possibility of backward time travel in a hypothetical universe described by a Gödel metric led famed logician Kurt Gödel to assert that time might itself be a sort of illusion. He suggests something along the lines of the block time view, in which time is just another dimension like space, with all events at all times being fixed within this four-dimensional "block".

Physical impossibility

Sergey Krasnikov writes that these bootstrap paradoxes – information or an object looping through time – are the same; the primary apparent paradox is a physical system evolving into a state in a way that is not governed by its laws. He does not find these paradoxical and attributes problems regarding the validity of time travel to other factors in the interpretation of general relativity.

Self-sufficient loops

A 1992 paper by physicists Andrei Lossev and Igor Novikov labeled such items without origin as Jinn, with the singular term Jinnee. This terminology was inspired by the Jinn of the Quran, which are described as leaving no trace when they disappear. Lossev and Novikov allowed the term "Jinn" to cover both objects and information with the reflexive origin; they called the former "Jinn of the first kind", and the latter "Jinn of the second kind". They point out that an object making circular passage through time must be identical whenever it is brought back to the past, otherwise it would create an inconsistency; the second law of thermodynamics seems to require that the object tends to a lower energy state throughout its history, and such objects that are identical in repeating points in their history seem to contradict this, but Lossev and Novikov argued that since the second law only requires entropy to increase in closed systems, a Jinnee could interact with its environment in such a way as to regain "lost" entropy. They emphasize that there is no "strict difference" between Jinn of the first and second kind. Krasnikov equivocates between "Jinn", "self-sufficient loops", and "self-existing objects", calling them "lions" or "looping or intruding objects", and asserts that they are no less physical than conventional objects, "which, after all, also could appear only from either infinity or a singularity."

Novikov self-consistency principle

Main article: Novikov self-consistency principle

The self-consistency principle developed by Igor Dmitriyevich Novikov expresses one view as to how backward time travel would be possible without the generation of paradoxes. According to this hypothesis, even though General relativity permits some exact solutions that allow for time travel that contain closed timelike curves that lead back to the same point in spacetime, physics in or near closed timelike curves (time machines) can only be consistent with the universal laws of physics, and thus only self-consistent events can occur. Anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency. The authors concluded that time travel need not lead to unresolvable paradoxes, regardless of what type of object was sent to the past.

Physicist Joseph Polchinski considered a potentially paradoxical situation involving a billiard ball that is fired into a wormhole at just the right angle such that it will be sent back in time and collides with its earlier self, knocking it off course, which would stop it from entering the wormhole in the first place. Kip Thorne referred to this problem as "Polchinski's paradox". Thorne and two of his students at Caltech, Fernando Echeverria and Gunnar Klinkhammer, went on to find a solution that avoided any inconsistencies, and found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each case. Later analysis by Thorne and Robert Forward showed that for certain initial trajectories of the billiard ball, there could be an infinite number of self-consistent solutions. It is plausible that there exist self-consistent extensions for every possible initial trajectory, although this has not been proven. The lack of constraints on initial conditions only applies to spacetime outside of the chronology-violating region of spacetime; the constraints on the chronology-violating region might prove to be paradoxical, but this is not yet known.

Novikov's views are not widely accepted. Visser views causal loops and Novikov's self-consistency principle as an ad hoc solution, and supposes that there are far more damaging implications of time travel. Krasnikov similarly finds no inherent fault in causal loops but finds other problems with time travel in general relativity. Some physicists suggest that causal loops only exist on the quantum scale. Another conjecture, the cosmic censorship hypothesis, suggests that every closed timelike curve passes through an event horizon, which prevents such causal loops from being observed.

Parallel universes

The interacting-multiple-universes approach is a variation of the many-worlds interpretation of quantum mechanics that involves time travelers arriving in a different universe than the one from which they came; it has been argued that, since travelers arrive in a different universe's history and not their history, this is not "genuine" time travel. Stephen Hawking has argued for the chronology protection conjecture, that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history so that time travelers remain within their world rather than traveling to a different one.

David Deutsch has proposed that quantum computation with a negative delay—backward time travel—produces only self-consistent solutions, and the chronology-violating region imposes constraints that are not apparent through classical reasoning. However Deutsch's self-consistency condition has been demonstrated as capable of being fulfilled to arbitrary precision by any system subject to the laws of classical statistical mechanics, even if it is not built up by quantum systems. Allen Everett has also argued that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time, with different particles emerging in different worlds.