Human mission to Mars

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

 

NASA concept for first humans on Mars, with a habitat and rover, 2019

 

Concept for Mars base with ice dome, pressurized rover, and Mars suits, 2016

 

Simulation of an astronaut on Mars

 

Artist's concept of a spacecraft transporting crew to Mars

A human mission to Mars has been the subject of science fiction, aerospace engineering and scientific proposals since the 20th century. Plans include landing on Mars for exploration at a minimum, with the possibility of sending settlers and terraforming the planet or exploring its moons Phobos and Deimos also considered.

The exploration of Mars has been a goal of national space programs for decades. Conceptual work for missions that would involve human explorers has been ongoing since the 1950s, with planned missions typically being stated as taking place anywhere between 10 and 30 years from the time they are drafted. The list of crewed Mars mission plans shows the various mission proposals that have been put forth by multiple organizations and space agencies in this field of space exploration. Plans have varied from scientific expeditions, in which a small group (between two and eight astronauts) would visit Mars for a period of a few weeks or year, to the permanent colonization of Mars.

In the 2010s, numerous US, European, and Asian agencies were developing proposals for human missions to Mars. In fiction, Mars is a frequent target of exploration and settlement in books, graphic novels, and films.

Travel to Mars

The minimum distance between the orbits of Mars and Earth from 2014 to 2061, measured in astronomical units

 

Three views of Mars, Hubble Space Telescope, 1997

The energy needed for transfer between planetary orbits, or "Delta-v", is lowest at intervals fixed by the synodic period. For Earth / Mars trips, this is every 26 months (2 years and 2 months), so missions are typically planned to coincide with one of these launch periods. Due to the eccentricity of Mars' orbit, the energy needed in the low-energy periods varies on roughly a 15-year cycle with the easiest periods needing only half the energy of the peaks. In the 20th century, there was a minimum in the 1969 and 1971 launch periods and another low in 1986 and 1988, then the cycle repeated. The next low-energy launch period occurs in 2033.

Several types of mission plans have been proposed, including opposition class and conjunction class or the Crocco flyby. The lowest energy transfer to Mars is a Hohmann transfer orbit, which would involve an approximately 9 month travel time from Earth to Mars, about five hundred days at Mars to wait for the transfer window to Earth, and a travel time of about 9 months to return to Earth.

Shorter Mars mission plans have round-trip flight times of 400 to 450 days, but would require significantly higher energy. A fast Mars mission of 245 days round trip could be possible with on-orbit staging. In 2014 ballistic capture was proposed, which may reduce fuel cost and provide more flexible launch windows compared to the Hohmann.

In the Crocco grand tour, a crewed spacecraft would get a flyby of Mars and Venus in under a year in space. Some flyby mission architectures can also be extended to include a style of Mars landing with a flyby excursion lander spacecraft. Proposed by R. Titus in 1966, it involved a short stay lander-ascent vehicle that would separate from a "parent" Earth-Mars transfer craft prior to its flyby of Mars. The Ascent-Descent lander would arrive sooner and either go into orbit around Mars or land, and depending on the design offer perhaps 10–30 days before it needed to launch itself back to the main transfer vehicle. (see also Mars flyby).

Aerobraking at Mars was estimated in the 1980s to cut the mass of a Mars mission lifting off from Earth by half. As a result, Mars missions have designed interplanetary spacecraft and landers capable of aero-braking.

Landing on Mars

Insets depict observation and analysis to find a safe landing site

Landed spacecraft on the surface of Mars:

• Mars 3
• Viking 1 and Viking 2
• Mars Pathfinder and its Sojourner rover
• Spirit and Opportunity rovers
• Phoenix lander
• Curiosity rover
• InSight lander
• Beagle2

Orbital capture

When an expedition reaches Mars, braking is required to enter orbit. Two options are available: rockets or aerocapture. Aerocapture at Mars for human missions was studied in the 20th century. In a review of 93 Mars studies, 24 used aerocapture for Mars or Earth return. One of the considerations for using aerocapture on crewed missions is a limit on the maximum force experienced by the astronauts. The current scientific consensus is that 5g, or 5 times Earth gravity, is the maximum allowable acceleration.

Survey work

Conducting a safe landing requires knowledge of the properties of the atmosphere, first observed by Mariner 4, and a survey of the planet to identify suitable landing sites. Major global surveys were conducted by Mariner 9 and Viking 1 and two orbiters, which supported the Viking landers. Later orbiters, such as Mars Global Surveyor, 2001 Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter, have mapped Mars in higher resolution with improved instruments. These later surveys have identified the probable locations of water, a critical resource.

Funding

The largest limiting factor for sending humans to Mars is funding. The estimated cost is roughly 500 billion U.S. dollars, though the actual costs are likely to be more. Starting in the late 1950s, the early phase of space exploration was conducted as much to make a political statement as to make observations of the solar system. However, this proved to be both wasteful and unsustainable, and the current climate is one of international cooperation, with large projects such as the International Space Station and the proposed Lunar Gateway being built and launched by multiple countries.

Critics argue that the immediate benefits of establishing a human presence on Mars are outweighed by the immense cost, and that funds could be better redirected towards other programs, such as robotic exploration. Proponents of human space exploration contend that the symbolism of establishing a presence in space may garner public interest to join the cause and spark global cooperation. There are also claims that a long-term investment in space travel is necessary for humanity's survival.

Medical

Comparison of radiation doses – includes the amount detected on the trip from Earth to Mars by the RAD inside the MSL (2011–2013).Vertical axis is in logarithmic scale, so the dose over a Mars year is about 15 times the DOE limit, not less than twice, as a quick glance might suggest. The actual dose would depend on factors such as spacecraft design and natural events such as solar flares.

There are several key physical challenges for human missions to Mars:

• Health threat from cosmic rays and other ionizing radiation. In May 2013, NASA scientists reported that a possible mission to Mars may involve great radiation risk based on energetic particle radiation measured by the RAD on the Mars Science Laboratory while traveling from the Earth to Mars in 2011–2012. The calculated radiation dose was 0.66 sieverts round-trip. The agency's career radiation limit for astronauts is 1 sievert. In mid-September 2017, NASA reported temporarily doubled radiation levels on the surface of Mars, with an aurora 25 times brighter than any observed earlier, due to a massive unexpected solar storm.

Artistic vision of spacecraft providing artificial gravity by spinning. (see also Centrifugal force)

• Adverse health effects of prolonged weightlessness, including eyesight impairment.  (Depends on mission and spacecraft design.) In November 2019, researchers reported that astronauts experienced serious blood flow and clot problems while on board the International Space Station, based on a six-month study of 11 healthy astronauts. The results may influence long-term spaceflight, including a mission to the planet Mars, according to the researchers.

• Psychological effects of isolation from Earth and, by extension, the lack of community due to lack of a real-time connection with Earth. (Compare Hermit.)
  
• Social effects of several humans living under cramped conditions for more than one Earth year, and possibly two or three years, depending on spacecraft and mission design.
• Lack of medical facilities.
• Potential failure of propulsion or life-support equipment.

Some of these issues were estimated statistically in the HUMEX study. Ehlmann and others have reviewed political and economic concerns, as well as technological and biological feasibility aspects.

While fuel for roundtrip travel could be a challenge, methane and oxygen can be produced using Martian H₂O (preferably as water ice instead of liquid water) and atmospheric CO₂ with mature technology.

Planetary protection

Robotic spacecraft to Mars are currently required to be sterilized. The allowable limit is 300,000 spores on the exterior of general craft, with stricter requirements for spacecraft bound to "special regions" containing water. Otherwise there is a risk of contaminating not only the life-detection experiments but possibly the planet itself.

It is impossible to sterilize human missions to this level, as humans are host to typically a hundred trillion (10¹⁴) microorganisms of thousands of species of the human microbiota, and these cannot be removed. Containment seems the only option, but it is a major challenge in the event of a hard landing (i.e. crash). There have been several planetary workshops on this issue, but with no final guidelines for a way forward yet. Human explorers would also be vulnerable to back contamination to Earth if they become carriers of microorganisms.

Mission proposals

Over the past seven decades, a wide variety of mission architectures have been proposed or studied for human spaceflights to Mars. These have included chemical, nuclear and electric propulsion, as well as a wide variety of landing, living, and return methodologies.

20th century

Fuel is mined from Phobos with the help of a nuclear reactor.

Over the last century, a number of mission concepts for such an expedition have been proposed. David Portree's history volume Humans to Mars: Fifty Years of Mission Planning, 1950–2000 discusses many of these.

Wernher von Braun proposal (1947 through 1950s)

Wernher von Braun was the first person to make a detailed technical study of a Mars mission.  Details were published in his book Das Marsprojekt (1952, published in English as The Mars Project in 1962) and several subsequent works. Willy Ley popularized a similar mission in English in the book The Conquest of Space (1949), featuring illustrations by Chesley Bonestell. Von Braun's Mars project envisioned nearly a thousand three-stage vehicles launching from Earth to ferry parts for the Mars mission to be constructed at a space station in Earth orbit. The mission itself featured a fleet of ten spacecraft with a combined crew of 70 heading to Mars, bringing three winged surface excursion ships that would land horizontally on the surface of Mars. (Winged landing was considered possible because at the time of his proposal, the Martian atmosphere was believed to be much denser than was later found to be the case.)

In the 1956 revised vision of the Mars Project plan, published in the book The Exploration of Mars by Wernher Von Braun and Willy Ley, the size of the mission was trimmed, requiring only 400 launches to put together two ships, still carrying a winged landing vehicle. Later versions of the mission proposal, featured in the Disney "Man In Space" film series, showed nuclear-powered ion-propulsion vehicles for the interplanetary cruise.

U.S. proposals (1950s to 1970s)

Artist's conception of the Mars Excursion Module (MEM) proposed in a NASA study in 1963. Crew wear Mars suits on surface EVA from the module.

From 1957 to 1965, work was done by General Atomics on Project Orion, a proposal for a nuclear pulse propulsion spacecraft. Orion was intended to have the ability to transport extremely large payloads compared to chemical rocketry, making crewed missions to Mars and the outer planets feasible. One of the early vehicle designs was intended to send an 800-ton payload to Mars orbit. The Partial Nuclear Test Ban Treaty of 1963 made further development unviable, and work ended in 1965.

In 1962, Aeronutronic Ford, General Dynamics and the Lockheed Missiles and Space Company made studies of Mars mission designs as part of NASA Marshall Spaceflight Center "Project EMPIRE". These studies indicated that a Mars mission (possibly including a Venus fly-by) could be done with a launch of eight Saturn V boosters and assembly in low Earth orbit, or possibly with a single launch of a hypothetical "post Saturn" heavy-lift vehicle. Although the EMPIRE missions were never proposed for funding, they were the first detailed analyses of what it would take to accomplish a human voyage to Mars using data from actual NASA spaceflight, laying the basis for future studies, including significant mission studies by TRW, North American, Philco, Lockheed, Douglas, and General Dynamics, along with several in-house NASA studies.

Following the success of the Apollo Program, von Braun advocated a crewed mission to Mars as a focus for NASA's crewed space program. Von Braun's proposal used Saturn V boosters to launch NERVA-powered upper stages that would propel two six-crew spacecraft on a dual mission in the early 1980s. The proposal was considered by President Richard Nixon but passed over in favor of the Space Shuttle.

In 1975, von Braun discussed the mission architecture that emerged from these Apollo-era studies in a recorded lecture and while doing so suggested that multiple Shuttle launches could instead be configured to lift the two Nuclear Thermal Rocket engine equipped spacecraft in smaller parts, for assembly in-orbit.

Soviet mission proposals (1956 through 1969)

The Martian Piloted Complex or "'MPK'" was a proposal by Mikhail Tikhonravov of the Soviet Union for a crewed Mars expedition, using the (then proposed) N1 rocket, in studies from 1956 to 1962. The Soviets sent many probes to Mars with some noted success stories including Mars atmospheric entry, but the overall rate of success was low.

Heavy Interplanetary Spacecraft (known by the Russian acronym TMK) was the designation of a Soviet Union space exploration proposal in the 1960s to send a crewed flight to Mars and Venus (TMK-MAVR design) without landing. The TMK spacecraft was due to launch in 1971 and make a three-year-long flight including a Mars fly-by at which time probes would have been dropped. The project was never completed because the required N1 rocket never flew successfully. The Mars Expeditionary Complex, or "'MEK"' (1969) was another Soviet proposal for a Mars expedition that would take a crew from three to six to Mars and back with a total mission duration of 630 days.

Case for Mars (1981–1996)

Following the Viking missions to Mars, between 1981 and 1996 a series of conferences named The Case for Mars were held at the University of Colorado at Boulder. These conferences advocated human exploration of Mars, presented concepts and technologies, and held a series of workshops to develop a baseline concept for the mission. It proposed use of in-situ resource utilization to manufacture rocket propellant for the return trip. The mission study was published in a series of proceedings volumes. Later conferences presented alternative concepts, including the "Mars Direct" concept of Robert Zubrin and David Baker; the "Footsteps to Mars" proposal of Geoffrey A. Landis, which proposed intermediate steps before the landing on Mars, including human missions to Phobos; and the "Great Exploration" proposal from Lawrence Livermore National Laboratory, among others.

NASA Space Exploration Initiative (1989)

Artist's conception of a human mission on the surface of Mars
(1989 painting by Les Bossinas of Lewis Research Center for NASA)

In response to a presidential initiative, NASA made a study of a project for human lunar- and Mars exploration as a proposed follow-on to the International Space Station. This resulted in a report, called the 90-day study, in which the agency proposed a long-term plan consisting of completing the Space Station as "a critical next step in all our space endeavors," returning to the Moon and establishing a permanent base, and then sending astronauts to Mars. This report was widely criticized as too elaborate and expensive, and all funding for human exploration beyond Earth orbit was canceled by Congress.

Mars Direct (early 1990s)

Because of the greater distance, the Mars mission would be much more risky and expensive than past Moon flights. Supplies and fuel would have to be prepared for a 2-3 year round trip and the spacecraft would need at least partial shielding from ionizing radiation. A 1990 paper by Robert Zubrin and David A. Baker, then of Martin Marietta, proposed reducing the mission mass (and hence the cost) by using in situ resource utilization to manufacture propellant from the Martian Atmosphere. This proposal drew on concepts developed by the former "Case for Mars" conference series. Over the next decade, Zubrin developed it into a mission concept, Mars Direct, which he presented in a book, The Case for Mars (1996). The mission is advocated by the Mars Society, which Zubrin founded in 1998, as practical and affordable.

International Space University (1991)

In 1991 in Toulouse, France, the International Space University studied an international human Mars mission. They proposed a crew of 8 traveling to Mars in a nuclear-powered vessel with artificial gravity provided by rotation. On the surface, 40 tonne habitats pressurized to 10 psi (69 kPa) were powered by a 40 kW photovoltaic array.

NASA Design reference missions (1990s)

NASA Mars habitat concept for DRA 1.0, derived from the Mars Direct Architecture, 1995

In the 1990s NASA developed several conceptual level human Mars exploration architectures. One of these was NASA Design reference mission 3.0 (DRM 3.0) to stimulate further thought and concept development.

Selected other US/NASA studies (1988–2009):

1. 1988 "Mars Expedition"
2. 1989 "Mars Evolution"
3. 1990 "90-Day Study"
4. 1991 "Synthesis Group"
5. 1995 "DRM 1"
6. 1997 "DRM 3"
7. 1998 "DRM 4"
8. 1999 "Dual Landers"

21st century

Artist's concept of crew members setting up weather monitoring equipment on the surface of Mars

NASA Design reference missions (2000+)

The NASA Mars Design Reference Missions consisted of a series of conceptual design studies for human Mars missions, continued in the 21st century Selected other US/NASA plans (1988–2009):

11. 2000 SERT (SSP)
12. 2001 DPT/NEXT
13. 2002 NEP Art. Gravity
14. 2009 DRA 5

MARPOST (2000–2005)

The Mars Piloted Orbital Station (or MARPOST) is a Russian proposed crewed orbital mission to Mars, using a nuclear reactor to run an electric rocket engine. Proposed in October 2000 as the next step for Russia in space along with participation in the International Space Station, a 30-volume draft project for MARPOST was confirmed as of 2005. Design for the ship was proposed to be ready in 2012, and the ship itself in 2021.

ESA Aurora programme (2001+)

Artwork featuring astronauts enduring a Mars dust storm near a rover

In 2001, the European Space Agency laid out a long-term vision of sending a human mission to Mars in 2033. The project's proposed timeline would begin with robotic exploration, a proof of concept simulation of sustaining humans on Mars, and eventually a crewed mission. Objections from the participating nations of ESA and other delays have put the timeline into question, and currently ExoMars, delivered an orbiter to Mars in 2016, have come to fruition.

ESA/Russia plan (2002)

Another proposal for a joint ESA mission with Russia is based on two spacecraft being sent to Mars, one carrying a six-person crew and the other the expedition's supplies. The mission would take about 440 days to complete with three astronauts visiting the surface of the planet for a period of two months. The entire project would cost $20 billion and Russia would contribute 30% of these funds.

USA Vision for Space Exploration (2004)

Project Constellation included an Orion Mars Mission.

On 14 January 2004, George W. Bush announced the Vision for Space Exploration, an initiative of crewed space exploration. It included developing preliminary plans for a lunar outpost by 2012 and establishing an outpost by 2020. By 2005, precursor missions that would help develop the needed technology during the 2010s were tentatively outlined. On 24 September 2007, Michael Griffin, then NASA Administrator, hinted that NASA would be able to launch a human mission to Mars by 2037. The needed funds were to be generated by diverting $11 billion from space science missions to the vision for human exploration.

NASA has also discussed plans to launch Mars missions from the Moon to reduce traveling costs.

Mars Society Germany – European Mars Mission (EMM) (2005)

The Mars Society Germany proposed a crewed Mars mission using several launches of an improved heavy-lift version of the Ariane 5. Roughly 5 launches would be required to send a crew of 5 on a 1200 days mission, with a payload of 120,000 kg (260,000 lb). Total project was estimated to cost 10 to 15 billion Euros.

China National Space Administration (CNSA) (2006)

Sun Laiyan, administrator of the China National Space Administration, said on July 20, 2006 that China would start deep space exploration focusing on Mars over the next five years, during the Eleventh Five-Year Plan (2006–2010) Program period. The first uncrewed Mars exploration program could take place between 2014–2033, followed by a crewed phase in 2040–2060 in which crew members would land on Mars and return home. The Mars 500 study of 2011 prepared for this crewed mission.

Mars to Stay (2006)

The idea of a one-way trip to Mars has been proposed several times. In 1988, space activist Bruce Mackenzie proposed a one-way trip to Mars in a presentation at the International Space Development Conference, arguing that the mission could be done with less difficulty and expense without a return to Earth. In 2006, former NASA engineer James C. McLane III proposed a scheme to initially colonize Mars via a one-way trip by only one human. Papers discussing this concept appeared in The Space Review, Harper's Magazine, SEARCH Magazine,and The New York Times.

NASA Design Reference Mission 5.0 (2007)

NASA released initial details of the latest version conceptual level human Mars exploration architecture in this presentation. The study further developed concepts developed in previous NASA DRM and updated it to more current launchers and technology.

Martian Frontier (2007–2011)

Mars 500, the longest high fidelity spaceflight simulation, ran from 2007 to 2011 in Russia and was an experiment to assess the feasibility of crewed missions to Mars.

NASA Design Reference Mission Architecture 5.0 (2009)

Concept for NASA's Design Reference Mission Architecture 5.0 (2009)

NASA released an updated version of NASA DRM 5.0 in early 2009, featuring use of the Ares V launcher, Orion CEV, and updated mission planning. In this document.

NASA Austere Human Missions to Mars (2009)

Extrapolated from the DRMA 5.0, plans for a crewed Mars expedition with chemical propulsion. Austere Human Missions to Mars

Mars orbit by the mid-2030s (2010)

In a major space policy speech at Kennedy Space Center on 15 April 2010, Barack Obama predicted a crewed Mars mission to orbit the planet by the mid-2030s, followed by a landing. This proposal was mostly supported by Congress, which approved cancelling Project Constellation in favor of a 2025 Asteroid Redirect Mission and orbiting Mars in the 2030s. The Asteroid Redirect Mission was cancelled in June 2017 and "closed out" in September of the same year.

Russian mission proposals (2011)

A number of Mars mission concepts and proposals have been put forth by Russian scientists. Stated dates were for a launch sometime between 2016 and 2020. The Mars probe would carry a crew of four to five cosmonauts, who would spend close to two years in space.

In late 2011, Russian and European space agencies successfully completed the ground-based MARS-500. The biomedical experiment simulating crewed flight to Mars was completed in Russia in July 2000.

2-4-2 concept (2011–2012)

In 2012, Jean-Marc Salotti published a new proposal for a crewed Mars mission. The '2-4-2' concept is based on a reduction of the crew size to 2 astronauts and the duplication of the entire mission. There are 2 astronauts in each space vehicle, there are 4 on the surface of Mars and there are 2 once again in each return vehicle. If one set of hardware runs into trouble, there are 2 astronauts ready to help the 2 others (2 for 2). This architecture simplifies the entry, descent and landing procedures by reducing the size of the landing vehicles. It also avoids the assembly of huge vehicles in LEO. The author claims that his proposal is much cheaper than the NASA reference mission without compromising the risks and can be undertaken before 2030.

Boeing Conceptual Space Vehicle Architecture (2012)

In 2012, a conceptual architecture was published by Boeing, United Launch Alliance, and RAL Space in Britain, laying out a possible design for a crewed Mars mission. Components of the architecture include various spacecraft for the Earth-to-Mars journey, landing, and surface stay as well as return. Some features include several uncrewed cargo landers assembled into a base on the surface of Mars. The crew would land at this base in the "Mars Personnel Lander", which could also take them back into Mars orbit. The design for the crewed interplanetary spacecraft included artificial gravity and an artificial magnetic field for radiation protection. Overall, the architecture was modular to allow for incremental R&D.

Mars One (2012-2019)

In 2012, a Dutch entrepreneur group began raising funds for a human Mars base to be established in 2023. The mission was intended to be primarily a one-way trip to Mars. Astronaut applications were invited from the public all over the world, for a fee.

The initial concept included an orbiter and small robotic lander in 2018, followed by a rover in 2020, and the base components in 2024. The first crew of four astronauts were to land on Mars in 2025. Then, every two years, a new crew of four would arrive. Financing was intended to come from selling the broadcasting rights of the entire training and of the flight as a reality television show, and that money would be used to contract for all hardware and launch services. In April 2015, Mars One's CEO Bas Lansdorp admitted that their 12-year plan for landing humans on Mars by 2027 is "mostly fiction". The company went bankrupt in January 2019.

Inspiration Mars Foundation (2013)

In 2013, the Inspiration Mars Foundation founded by Dennis Tito revealed plans of a crewed mission to fly by Mars in 2018 with support from NASA. NASA refused to fund the mission.

Boeing Affordable Mission (2014)

On December 2, 2014, NASA's Advanced Human Exploration Systems and Operations Mission Director Jason Crusan and Deputy Associate Administrator for Programs James Reuthner announced tentative support for the Boeing "Affordable Mars Mission Design" including radiation shielding, centrifugal artificial gravity, in-transit consumable resupply, and a lander which can return. Reuthner suggested that if adequate funding was forthcoming, the proposed mission would be expected in the early 2030s.

NASA Moon to Mars (2015–present)

Artist's rendering of SLS Block 1/Orion

On October 8, 2015, NASA published its strategy for human exploration and colonization of Mars. The concept operates through three distinct phases leading up to fully sustained colonization.

The first stage, already underway, is the "Earth Reliant" phase. This phase continues using the International Space Station until 2024; validating deep space technologies and studying the effects of long duration space missions on the human body.

The second stage, "Proving Ground," moves away from Earth reliance and ventures into cislunar space for most of its tasks. The proposed Lunar Gateway would test deep space habitation facilities, and validate capabilities required for human exploration of Mars.

Finally, phase three is the transition to independence from Earth resources. The "Earth Independent" phase includes long term missions on the lunar surface with surface habitats that only require routine maintenance, and the harvesting of Martian resources for fuel, water, and building materials. NASA is still aiming for human missions to Mars in the 2030s, though Earth independence could take decades longer.

In November 2015, Administrator Bolden of NASA reaffirmed the goal of sending humans to Mars. He laid out 2030 as the date of a crewed surface landing, and noted that planned 2020 Mars rover would support the human mission.

In March 2019, Vice President Mike Pence declared "American Astronauts will walk on the Moon again before the end of 2024, 'by any means necessary'." This reportedly prompted NASA to accelerate their plans to return to the Moon's surface by 2024. NASA says it will use the Artemis lunar program in combination with the Lunar Gateway as stepping stones to make great scientific strides "to take the next giant leap - sending astronauts to Mars".

SpaceX Mars transportation infrastructure (2016-)

Since 2016, SpaceX publicly proposed a plan to begin the colonization of Mars by developing a high-capacity transportation infrastructure.

The ITS launch vehicle design was a large reusable booster topped by a spaceship or a tanker for in-orbit refueling. The aspirational objective is to advance the technology and infrastructure such that the first humans to Mars could potentially depart as early as 2024.

On 29 September 2017, Elon Musk announced an updated vehicle design for the Mars mission at the International Astronautical Congress. The replacement vehicle for this mission was called BFR (Big Falcon Rocket) until 2018, when it was renamed "Starship". Starship will provide the capability for on-orbit activity like satellite delivery, servicing the International Space Station, Moon missions, as well as Mars missions. There are two phases for the first human mission to Mars via Starship:

• In 2022, at least 2 Starship cargo vehicles will land on Mars.
  • They will confirm water resources and identify hazards.
  • They will place power, mining and life support infrastructure for future missions.
• In 2024, 2 Starship crew vehicles will take the first people to Mars.
  • 2 Starship cargo vehicles will bring more equipment and supplies.
  • They will place a propellant production plant.
  • They will build up a base to prepare for expansion.

Mars Base Camp (2016)

Mars Base Camp (MBC) is a US spacecraft concept that proposes to send astronauts to Mars orbit as early as 2028. The vehicle concept, developed by Lockheed Martin, would utilize both future and heritage technology as well as the Orion spacecraft built by NASA.

Deep Space Transport (2017)

Artist impression of the Deep Space Transport, about to dock with the Lunar Gateway

The 'Deep Space Transport (DST), also called Mars Transit Vehicle, is a crewed interplanetary spacecraft concept by NASA to support science exploration missions to Mars of up to 1,000 days. It would be composed of two elements: an Orion capsule and a propelled habitation module. As of April 2018, the DST is still a concept to be studied, and NASA has not officially proposed the project in an annual U.S. federal government budget cycle.

The DST vehicle would depart and return from the Lunar Gateway to be serviced and reused for a new Mars mission.

Current intentions by nations and space agencies

Artist's rendering of the planned Orion/DSH/Cryogenic Propulsion Module assembly.

A number of nations and organizations have long-term intentions to send humans to Mars.

• The United States has several robotic missions currently exploring Mars, with a sample-return planned for the future. The Orion Multi-Purpose Crew Vehicle (MPCV) is intended to serve as the launch/splashdown crew delivery vehicle, with a Deep Space Habitat module providing additional living-space for the 16-month-long journey. The first crewed Mars Mission, which would include sending astronauts to Mars, orbiting Mars, and a return to Earth, is proposed for the 2030s.
   
• Technology development for US government missions to Mars is underway, but there is no well-funded approach to bring the conceptual project to completion with human landings on Mars by the mid-2030s, the stated objective. NASA is under presidential orders to land humans on Mars by 2033, and NASA-funded engineers are studying a way to build potential human habitats there by producing bricks from pressurized Martian soil.
• The European Space Agency has a long-term goal to send humans but has not yet built a crewed spacecraft. It has sent robotic probes like ExoMars in 2016 and plans to send the next probe in 2022.
• Indian Space Research Organisation successfully placed an uncrewed Mars Orbiter Mission (also called Mangalyaan) satellite in Mars orbit in 2014. ISRO plans a larger follow-up mission called Mangalyaan 2 between 2024 and 2026. This mission will likely consist of a lander and a Mars rover. No plans for an Indian human mission to Mars have been made public.
  
• Japan has sent one robotic mission to Mars in 1998, the Nozomi, but it failed to achieve Mars orbit. JAXA has proposed a rover mission called MELOS for an engineering demonstration of precision landing, and to look for possible biosignatures on Mars in 2020 or 2022. No plans for a Japanese human mission to Mars have been made public.
• China's first attempted mission to Mars, the Yinghuo-1 space probe, was lost with Russia's sample return mission to Phobos, Fobos-Grunt in 2011–2012. China plans to develop and launch an orbiter, lander and rover to Mars in July or August 2020 with a Long March 5 heavy lift rocket.
• Russia plans to send humans in the 2040–2045 timeframe.

Technological innovations and hurdles

Depiction of plants growing in a Mars base. NASA plans to grow plants for space food.

 

NASA has stated that robots will prepare an underground base for a human surface mission.

Significant technological hurdles need to be overcome for human spaceflight to Mars.

Entry into the thin and shallow Martian atmosphere will pose significant difficulties with re-entry and for a spacecraft of the weight needed to carry humans, along with life support, supplies and other equipment. Should a heat shield be used, it would need to be very large. Retro rockets could be used, but would add significant further weight.

A return mission to Mars will need to land a rocket to carry crew off the surface. Launch requirements mean that this rocket would be significantly smaller than an Earth-to-orbit rocket. Mars-to-orbit launch can also be achieved in single stage. Despite this, landing an ascent rocket on Mars will be difficult. Reentry for a large rocket will be difficult.

In 2014 NASA proposed the Mars Ecopoiesis Test Bed.

Intravenous fluid

One of the medical supplies that might be needed is a considerable mass of intravenous fluid, which is mainly water but contains other substances so it can be added directly to the human blood stream. If it could be created on the spot from existing water, this would reduce mass requirements. A prototype for this capability was tested on the International Space Station in 2010.

Breathing gases

While it is possible for humans to breathe pure oxygen, usually additional gases like nitrogen are included in the breathing mix. One possibility is to take in-situ nitrogen and argon from the atmosphere of Mars; however, they are hard to separate from each other. As a result, a Mars habitat may use 40% argon, 40% nitrogen, and 20% oxygen.

An idea for keeping carbon dioxide out of the breathing air is to use re-usable amine bead carbon dioxide scrubbers. While one carbon dioxide scrubber filters the astronaut's air, the other is vented to the Mars atmosphere.

Precursor missions

Some missions may be considered a "Mission to Mars" in their own right, or they may only be one step in a more in-depth program. An example of this is missions to Mars' moons, or flyby missions.

Missions to Deimos or Phobos

Many Mars mission concepts propose precursor missions to the moons of Mars, for example a sample return mission to the Mars moon Phobos – not quite Mars, but perhaps a convenient stepping stone to an eventual Martian surface mission. Lockheed Martin, as part of their "Stepping stones to Mars" project, called the "Red Rocks Project", proposed to explore Mars robotically from Deimos.

Use of fuel produced from water resources on Phobos or Deimos has also been proposed.

Mars sample return missions

Artist concept of SCIM gathering a sample of the Martian atmosphere.

 

Sample return mission concept

An uncrewed Mars sample return mission (MSR) has sometimes been considered as a precursor to crewed missions to Mars' surface. In 2008, the ESA called a sample return "essential" and said it could bridge the gap between robotic and human missions to Mars. An example of a Mars sample return mission is Sample Collection for Investigation of Mars (SCIM). Mars sample return was the highest priority Flagship Mission proposed for NASA by the Planetary Decadal Survey 2013–2022: The Future of Planetary Science. However, such missions have been hampered by complexity and expense, with one ESA proposal involving no less than five different uncrewed spacecraft.

Sample return plans raise the concern, however remote, that an infectious agent could be brought to Earth. Regardless, a basic set of guidelines for extraterrestrial sample return have been laid out depending on the source of sample (e.g. asteroid, Moon, Mars surface, etc.)

At the dawn of the 21st century, NASA crafted four potential pathways to Mars human missions. Of those four, three included a Mars sample return as a prerequisite to human landing; however one did not.

Currently, the rover Perseverance is equipped with a device that will allow it to pick up and seal samples of rock from Mars, to be returned at a later date by another mission. Perseverance is part of the Mars 2020 mission and is set to launch on top of an Atlas V rocket on 30 July 2020.

Crewed orbital missions

Starting in 2004, NASA scientists have proposed to explore Mars via telepresence from human astronauts in orbit.

A similar idea was the proposed "Human Exploration using Real-time Robotic Operations" (HERRO) mission.

 

Self-ownership

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Self-ownership 

Self-ownership, also known as sovereignty of the individual or individual sovereignty, is the concept of property in one's own person, expressed as the moral or natural right of a person to have bodily integrity and be the exclusive controller of one's own body and life. Self-ownership is a central idea in several political philosophies that emphasize individualism, such as libertarianism, liberalism, and anarchism.

Definitional issues

The self

Discussion of the boundary of self with respect to ownership and responsibility has been explored by legal scholar Meir Dan-Cohen in his essays on The Value of Ownership and Responsibility and the Boundaries of the Self. The emphasis of this work illuminates the phenomenology of ownership and our common usage of personal pronouns to apply to both body and property—this serves as the folk basis for legal conceptions and debates about responsibility and ownership. Another view holds that labor is alienable because it can be contracted out, thus alienating it from the self. In this view, the choice of a person to voluntarily sell oneself into slavery is also preserved by the principle of self-ownership.

Labour markets and private property

For anarcho-socialist political philosopher L. Susan Brown: "Liberalism and anarchism are two political philosophies that are fundamentally concerned with individual freedom yet differ from one another in very distinct ways. Anarchism shares with liberalism a commitment to individual freedom while rejecting liberalism's competitive property relations". Scholar Ellen Meiksins Wood says that "there are doctrines of individualism that are opposed to Lockean individualism... and non-Lockean individualism may encompass socialism".

Libertarian capitalist conceptions of self-ownership extend the concept to include control of private property as part of the self. According to Gerald Cohen, "the libertarian principle of self–ownership says that each person enjoys, over himself and his powers, full and exclusive rights of control and use, and therefore owes no service or product to anyone else that he has not contracted to supply".

Philosopher Ian Shapiro says that labor markets affirm self-ownership because if self-ownership were not recognized, then people would not be allowed to sell the use of their productive capacities to others. He says that the individual sells the use of his productive capacity for a limited time and conditions but continues to own what he earns from selling the use of that capacity and the capacity itself, thereby retaining sovereignty over himself while contributing to economic efficiency. A common view within classical liberalism is that sovereign-minded individuals usually assert a right of private property external to the body, reasoning that if a person owns themselves, they own their actions, including those that create or improve resources, therefore they own their own labour and the fruits thereof.

In Human Action, Austrian School economist Ludwig von Mises argues that labor markets are the rational conclusion of self-ownership and argues that collective ownership of labor ignores differing values for the labor of individuals:

Of course, people believe that there is an essential difference between the tasks incumbent upon the comrades of the socialist commonwealth and those incumbent upon slaves or serfs. The slaves and serfs, they say, toiled for the benefit of an exploiting lord. But in a socialist system the produce of labor goes to society of which the toiler himself is a part; here the worker works for himself, as it were. What this reasoning overlooks is that the identification of the individual comrades and the totality of all comrades with the collective entity pocketing the produce of all work is merely fictitious. Whether the ends which the community's officeholders are aiming at agree or disagree with the wishes and desires of the various comrades, is of minor importance. The main thing is that the individual's contribution to the collective entity's wealth is not requited in the shape of wages determined by the market.

Nevertheless, there can be defense of self-ownership which can be critical of the idea of private property, specifically within the socialist branch of anarchism. The anarchist Oscar Wilde said:

For the recognition of private property has really harmed Individualism, and obscured it, by confusing a man with what he possesses. It has led Individualism entirely astray. It has made gain not growth its aim. So that man thought that the important thing was to have, and did not know that the important thing is to be. The true perfection of man lies, not in what man has, but in what man is...With the abolition of private property, then, we shall have true, beautiful, healthy Individualism. Nobody will waste his life in accumulating things, and the symbols for things. One will live. To live is the rarest thing in the world. Most people exist, that is all".

Within anarchism, the concept of wage slavery refers to a situation perceived as quasi-voluntary slavery, where a person's livelihood depends on wages, especially when the dependence is total and immediate. It is a negatively connoted term used to draw an analogy between slavery and wage labor by focusing on similarities between owning and renting a person. The term "wage slavery" has been used to criticize economic exploitation and social stratification, with the former seen primarily as unequal bargaining power between labor and capital (particularly when workers are paid comparatively low wages, e.g. in sweatshops) and the latter as a lack of workers' self-management, fulfilling job choices and leisure in an economy. With the advent of the Industrial Revolution, thinkers such as Pierre-Joseph Proudhon and Karl Marx elaborated the comparison between wage labor and slavery in the context of a critique of societal property not intended for active personal use while Luddites emphasized the dehumanization brought about by machines. Emma Goldman famously denounced "wage slavery" by saying: "The only difference is that you are hired slaves instead of block slaves".

Within left-libertarianism, scholars such as Hillel Steiner, Peter Vallentyne, Philippe Van Parijs, Michael Otsuka and David Ellerman root an economic egalitarianism in the classical liberal concepts of self-ownership and land appropriation, combined with geoist or physiocratic views regarding the ownership of land and natural resources (e.g. those of John Locke and Henry George). Left-libertarians "maintain that the world's natural resources were initially unowned, or belonged equally to all, and it is illegitimate for anyone to claim exclusive private ownership of these resources to the detriment of others. Such private appropriation is legitimate only if everyone can appropriate an equal amount, or if those who appropriate more are taxed to compensate those who are thereby excluded from what was once common property". This position is articulated in contrast to the position of other libertarians who argue for a right to appropriate parts of the external world based on sufficient use, even if this homesteading yields unequal results. Some left-libertarians of the Steiner–Vallentyne type support some form of income redistribution on the grounds of a claim by each individual to be entitled to an equal share of natural resources.

History

John Locke wrote in his Two Treatises on Government that "every man has a Property in his own Person". Locke also said that the individual "has a right to decide what would become of himself and what he would do, and as having a right to reap the benefits of what he did". Josiah Warren was the first who wrote about the "sovereignty of the individual".

 

Freedom of choice

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Freedom_of_choice

Freedom of choice describes an individual's opportunity and autonomy to perform an action selected from at least two available options, unconstrained by external parties.

In politics

In the abortion debate, for example, the term "freedom of choice" may emerge in defense of the position that a woman has a right to determine whether she will proceed with or terminate a pregnancy.  Similarly, other topics such as euthanasia, vaccination, contraception and same-sex marriage are sometimes discussed in terms of an assumed individual right of "freedom of choice". Some social issues, for example the New York "Soda Ban" have been both defended and opposed with reference to "freedom of choice".

In economics

The freedom of choice on which brand and flavor of soda to buy is related to market competition

In microeconomics, freedom of choice is the freedom of economic agents to allocate their resources as they see fit, among the options (such as goods, services, or assets) that are available to them. It includes the freedom to engage in employment available to them.

Ratner et al., in 2008, cited the literature on libertarian paternalism which states that consumers do not always act in their own best interests. They attribute this phenomenon to factors such as emotion, cognitive limitations and biases, and incomplete information which they state may be remedied by various proposed interventions. They discuss providing consumers with information and decision tools, organizing and restricting their market options, and tapping emotions and managing expectations. Each of these, they state, could improve consumers' ability to choose.

However, economic freedom to choose ultimately depends upon market competition, since buyers' available options are usually the result of various factors controlled by sellers, such as overall quality of a product or a service and advertisement. In the event that a monopoly exists, the consumer no longer has the freedom to choose to buy from a different producer. As Friedrich Hayek pointed out:

Our freedom of choice in a competitive society rests on the fact that, if one person refuses to satisfy our wishes, we can turn to another. But if we face a monopolist we are at his absolute mercy...

— Friedrich Hayek, The Road to Serfdom, "Can planning free us from care?"

As exemplified in the above quote, libertarian thinkers are often strong advocates for increasing freedom of choice. One example of this is Milton Friedman's Free to Choose book and TV series.

There is no consensus as to whether an increase in economic freedom of choice leads to an increase in happiness. In one study, the Heritage Foundation's 2011 Index of Economic Freedom report showed a strong correlation between its Index of Economic Freedom and happiness in a country.

Measuring freedom of choice

The axiomatic-deductive approach has been used to address the issue of measuring the amount of freedom of choice (FoC) an individual enjoys. In a 1990 paper, Prasanta K. Pattanaik, and Yongsheng Xu presented three conditions that a measurement of FoC should satisfy:

1. Indifference between no-choice situations. Having only one option amounts to the same FoC, no matter what the option is.
2. Strict monotonicity. Having two distinct options x and y amounts to more FoC than having only the option x.
3. Independence. If a situation A has more FoC than B, by adding a new option x to both (not contained in A or B), A will still have more FoC than B.

They proved that the cardinality is the only measurement that satisfies these axioms, what they observed to be counter-intuitive and suggestive that one or more axioms should be reformulated. They illustrated this with the example of the option set "to travel by train" or "to travel by car", that should yield more FoC than the option set "to travel by red car" or "to travel by blue car". Some suggestions have been made to solve this problem, by reformulating the axioms, usually including concepts of preferences, or rejecting the third axiom.

Relationship with happiness

A 2006 study by Simona Botti and Ann L. McGill showed that, when subjects were presented with differentiated options and had the freedom to choose between them, their choice enhanced their satisfaction with positive and dissatisfaction with negative outcomes, relative to nonchoosers.

A 2010 study by Hazel Rose Markus and Barry Schwartz compiled a list of experiments about freedom of choice and argued that "too much choice can produce a paralyzing uncertainty, depression, and selfishness". Schwartz argues that people frequently experience regret due to opportunity costs for not making an optimal decision and that, in some scenarios, people's overall satisfaction are sometimes higher when a difficult decision is made by another person rather than by themselves, even when the other person's choice is worse. Schwarts had written a book and given speeches criticizing the excess of options in modern society, though acknowledging that "some choice is better than none".

Structure formation

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Many-worlds_interpretation 

 

The quantum-mechanical "Schrödinger's cat" paradox according to the Many-Worlds interpretation. In this interpretation, every quantum event is a branch point; the cat is both alive and dead, even before the box is opened, but the "alive" and "dead" cats are in different branches of the universe, both of which are equally real, but which do not interact with each other.

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wavefunction collapse. This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations, such as the Copenhagen interpretation, the evolution of reality as a whole in MWI is rigidly deterministic. Many-worlds is also called the relative state formulation or the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957. Bryce DeWitt popularized the formulation and named it many-worlds in the 1960s and 1970s.

In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence. Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s, and have become quite popular. MWI is now considered a mainstream interpretation along with the other decoherence interpretations, collapse theories (including the Copenhagen interpretation), and hidden variable theories such as Bohmian mechanics.

The many-worlds interpretation implies that there are very many universes, perhaps infinitely many. It is one of many multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realised. This is intended to resolve some paradoxes of quantum theory, such as the EPR paradox and Schrödinger's cat, since every possible outcome of a quantum event exists in its own universe.

History

In 1952 Erwin Schrödinger gave a lecture in Dublin in which at one point he jocularly warned his audience that what he was about to say might "seem lunatic". He went on to assert that while what the equation that won him a Nobel prize seems to be describing is several different histories, they are "not alternatives but all really happen simultaneously". This is the earliest known reference to many-worlds.

MWI originated in Everett's Princeton Ph.D. thesis "The Theory of the Universal Wavefunction", developed under his thesis advisor John Archibald Wheeler, a shorter summary of which was published in 1957 under the title "Relative State Formulation of Quantum Mechanics" (Wheeler contributed the title "relative state"; Everett originally called his approach the "Correlation Interpretation", where "correlation" refers to quantum entanglement). The phrase "many-worlds" is due to Bryce DeWitt, who was responsible for the wider popularisation of Everett's theory, which was largely ignored for a decade after publication.

Overview of the interpretation

The key idea of the many-worlds interpretation is that unitary quantum mechanics describes the whole universe. In particular, it describes a measurement as a unitary transformation, without using a collapse postulate, and describes observers as ordinary quantum-mechanical systems. This stands in sharp contrast to the Copenhagen interpretation, on which a measurement is a "primitive" concept, not describable by quantum mechanics, the universe is divided into a quantum and a classical domain, and the collapse postulate is central. MWI's main conclusion is that the universe (or multiverse in this context) is composed of a quantum superposition of an infinite or undefinable amount or number of increasingly divergent, non-communicating parallel universes or quantum worlds.

The many-worlds interpretation makes essential use of decoherence to explain the measurement process and the emergence of a quasi-classical world. Wojciech H. Zurek, one of decoherence theory's pioneers, stated: "Under scrutiny of the environment, only pointer states remain unchanged. Other states decohere into mixtures of stable pointer states that can persist, and, in this sense, exist: They are einselected." Żurek emphasizes that his work does not depend on a particular interpretation.

The many-worlds interpretation shares many similarities with the decoherent histories interpretation, which also uses decoherence to explain the process of measurement or wavefunction collapse.MWI treats the other histories or worlds as real since it regards the universal wavefunction as the "basic physical entity" or "the fundamental entity, obeying at all times a deterministic wave equation". Decoherent histories, on the other hand, needs only one of the histories (or worlds) to be real.

Several authors, including Wheeler, Everett and Deutsch, call many-worlds a theory, rather than just an interpretation. Everett argued that it was the "only completely coherent approach to explaining both the contents of quantum mechanics and the appearance of the world." Deutsch dismissed the idea that many-worlds is an "interpretation", saying that to call it that "is like talking about dinosaurs as an 'interpretation' of fossil records."

Formulation

In Everett's formulation, a measuring apparatus M and an object system S form a composite system, each of which prior to measurement exists in well-defined (but time-dependent) states. Measurement is regarded as causing M and S to interact. After S interacts with M, it is no longer possible to describe either system by an independent state. According to Everett, the only meaningful descriptions of each system are relative states: for example the relative state of S given the state of M or the relative state of M given the state of S. In DeWitt's formulation, the state of S after a sequence of measurements is given by a quantum superposition of states, each one corresponding to an alternative measurement history of S.

Schematic illustration of splitting as a result of a repeated measurement.

For example, consider the smallest possible truly quantum system S, as shown in the illustration. This describes for instance, the spin-state of an electron. Considering a specific axis (say the z-axis) the north pole represents spin "up" and the south pole, spin "down". The superposition states of the system are described by a sphere called the Bloch sphere. To perform a measurement on S, it is made to interact with another similar system M. After the interaction, the combined system can be regarded as a quantum superposition of two "alternative histories" of the original system S, one in which "up" was observed and the other in which "down" was observed. Each subsequent binary measurement (that is interaction with a system M) causes a similar split in the history tree. Thus after three measurements, the system can be regarded as a quantum superposition of 8 = 2 × 2 × 2 copies of the original system S.

Relative state

In his 1957 doctoral dissertation, Everett proposed that rather than modeling an isolated quantum system subject to external observation, one could mathematically model an object as well as its observers as purely physical systems within the mathematical framework developed by Paul Dirac, John von Neumann and others, discarding altogether the ad hoc mechanism of wave function collapse.

Since Everett's original work, a number of similar formalisms have appeared in the literature. One is the relative state formulation. It makes two assumptions: first, the wavefunction is not simply a description of the object's state, but is entirely equivalent to the object—a claim it has in common with some other interpretations. Second, observation or measurement has no special laws or mechanics, unlike in the Copenhagen interpretation, which considers the wavefunction collapse a special kind of event that occurs as a result of observation. Instead, measurement in the relative state formulation is the consequence of a configuration change in an observer's memory described by the same basic wave physics as the object being modeled.

The many-worlds interpretation is DeWitt's popularisation of Everett, who had referred to the combined observer–object system as split by an observation, each split corresponding to the different or multiple possible outcomes of an observation. These splits generate a tree, as shown in the graphic above. Subsequently, DeWitt introduced the term "world" to describe a complete measurement history of an observer, which corresponds roughly to a single branch of that tree.

Under the many-worlds interpretation, the Schrödinger equation, or relativistic analog, holds all the time everywhere. An observation or measurement is modeled by applying the wave equation to the entire system comprising the observer and the object. One consequence is that every observation can be thought of as causing the combined observer–object's wavefunction to change into a quantum superposition of two or more non-interacting branches, or split into many "worlds". Since many observation-like events have happened and are constantly happening, there are an enormous and growing number of simultaneously existing states.

If a system is composed of two or more subsystems, the system's state will be a superposition of products of the subsystems' states. Each product of subsystem states in the overall superposition evolves over time independently of other products. Once the subsystems interact, their states have become correlated or entangled and can no longer be considered independent. In Everett's terminology each subsystem state was now correlated with its relative state, since each subsystem must now be considered relative to the other subsystems with which it has interacted.

Properties

MWI removes the observer-dependent role in the quantum measurement process by replacing wavefunction collapse with quantum decoherence. Since the observer's role lies at the heart of most if not all "quantum paradoxes," this automatically resolves a number of problems, such as Schrödinger's cat thought experiment, the EPR paradox, von Neumann's "boundary problem", and even wave-particle duality.

Since the Copenhagen interpretation requires the existence of a classical domain beyond the one described by quantum mechanics, it has been criticized as inadequate for the study of cosmology. MWI was developed with the explicit goal of allowing quantum mechanics to be applied to the universe as a whole, making quantum cosmology possible.

MWI is a realist, deterministic, and local theory. It achieves this by removing wavefunction collapse, which is indeterministic and non-local, from the deterministic and local equations of quantum theory.

MWI (like other, broader multiverse theories) provides a context for the anthropic principle, which may provide an explanation for the fine-tuned universe.

MWI depends crucially on the linearity of quantum mechanics. If the final theory of everything is non-linear with respect to wavefunctions, then many-worlds is invalid.. While quantum gravity or string theory may be non-linear in this respect, there is no evidence of this as yet.

Interpreting wavefunction collapse

As with the other interpretations of quantum mechanics, the many-worlds interpretation is motivated by behavior that can be illustrated by the double-slit experiment. When particles of light (or anything else) pass through the double slit, a calculation assuming wavelike behavior of light can be used to identify where the particles are likely to be observed. Yet when the particles are observed in this experiment, they appear as particles (i.e., at definite places) and not as non-localized waves.

Some versions of the Copenhagen interpretation of quantum mechanics proposed a process of "collapse" in which an indeterminate quantum system would probabilistically collapse down onto, or select, just one determinate outcome to "explain" this phenomenon of observation. Wavefunction collapse was widely regarded as artificial and ad hoc, so an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.

Everett's Ph.D. work provided such an interpretation. He argued that for a composite system—such as a subject (the "observer" or measuring apparatus) observing an object (the "observed" system, such as a particle)—the claim that either the observer or the observed has a well-defined state is meaningless; in modern parlance, the observer and the observed have become entangled: we can only specify the state of one relative to the other, i.e., the state of the observer and the observed are correlated after the observation is made. This led Everett to derive from the unitary, deterministic dynamics alone (i.e., without assuming wavefunction collapse) the notion of a relativity of states.

Everett noticed that the unitary, deterministic dynamics alone entailed that after an observation is made each element of the quantum superposition of the combined subject–object wavefunction contains two "relative states": a "collapsed" object state and an associated observer who has observed the same collapsed outcome; what the observer sees and the state of the object have become correlated by the act of measurement or observation. The subsequent evolution of each pair of relative subject–object states proceeds with complete indifference as to the presence or absence of the other elements, as if wavefunction collapse has occurred, which has the consequence that later observations are always consistent with the earlier observations. Thus the appearance of the object's wavefunction's collapse has emerged from the unitary, deterministic theory itself. (This answered Einstein's early criticism of quantum theory, that the theory should define what is observed, not for the observables to define the theory.) Since the wavefunction merely appears to have collapsed then, Everett reasoned, there was no need to actually assume that it had collapsed. And so, invoking Occam's razor, he removed the postulate of wavefunction collapse from the theory.

Testability

In 1985, David Deutsch proposed a variant of the Wigner's friend thought experiment as a test of many-worlds versus the Copenhagen interpretation. It consists of an experimenter (Wigner's friend) making a measurement on a quantum system in an isolated laboratory, and another experimenter (Wigner) who would make a measurement on the first one. According to the many-worlds theory, the first experimenter would end up in a macroscopic superposition of seeing one result of the measurement in one branch, and another result in another branch. The second experimenter could then interfere these two branches in order to test whether it is in fact in a macroscopic superposition or has collapsed into a single branch, as predicted by the Copenhagen interpretation. Since then Lockwood (1989), Vaidman and others have made similar proposals. These proposals require placing macroscopic objects in a coherent superposition and interfering them, a task now beyond experimental capability.

Probability and the Born rule

Since the many-worlds interpretation's inception, physicists have been puzzled about the role of probability in it. As put by Wallace, there are two facets to the question: the incoherence problem, which asks why we should assign probabilities at all to outcomes that are certain to occur in some worlds, and the quantitative problem, which asks why the probabilities should be given by the Born rule.

Everett tried to answer these questions in the paper that introduced many-worlds. To address the incoherence problem, he argued that an observer who makes a sequence of measurements on a quantum system will in general have an apparently random sequence of results in their memory, which justifies the use of probabilities to describe the measurement process. To address the quantitative problem, Everett proposed a derivation of the Born rule based on the properties that a measure on the branches of the wavefunction should have. His derivation has been criticized as relying on unmotivated assumptions. Since then several other derivations of the Born rule in the many-worlds framework have been proposed. There is no consensus on whether this has been successful.

Frequentism

DeWitt and Graham and Farhi et al., among others, have proposed derivations of the Born rule based on a frequentist interpretation of probability. They try to show that in the limit of infinitely many measurements no worlds would have relative frequencies that didn't match the probabilities given by the Born rule, but these derivations have been shown to be mathematically incorrect.

Decision theory

A decision-theoretic derivation of the Born rule was produced by David Deutsch (1999) and refined by Wallace (2002–2009) and Saunders (2004). They consider an agent who takes part in a quantum gamble: the agent makes a measurement on a quantum system, branches as a consequence, and each of the agent's future selves receives a reward that depends on the measurement result. The agent uses decision theory to evaluate the price they would pay to take part in such a gamble, and concludes that the price is given by the utility of the rewards weighted according to the Born rule. Some reviews have been positive, although these arguments remain highly controversial; some theoretical physicists have taken them as supporting the case for parallel universes. For example, a New Scientist story on a 2007 conference about Everettian interpretations quoted physicist Andy Albrecht as saying, "This work will go down as one of the most important developments in the history of science." In contrast, the philosopher Huw Price, also attending the conference, found the Deutsch–Wallace–Saunders approach fundamentally flawed.

Symmetries and invariance

Żurek (2005) has produced a derivation of the Born rule based on the symmetries of entangled states; Schlosshauer and Fine argue that Żurek's derivation is not rigorous, as it does not define what probability is and has several unstated assumptions about how it should behave.

Charles Sebens and Sean M. Carroll, building on work by Lev Vaidman, proposed a similar approach based on self-locating uncertainty. In this approach, decoherence creates multiple identical copies of observers, who can assign credences to being on different branches using the Born rule. The Sebens–Carroll approach has been criticized by Adrian Kent, and Vaidman himself does not find it satisfactory.

The preferred basis problem

As originally formulated by Everett and DeWitt, the many-worlds interpretation had a privileged role for measurements: they determined which basis of a quantum system would give rise to the eponymous worlds. Without this the theory was ambiguous, as a quantum state can equally well be described (e.g.) as having a well-defined position or as being a superposition of two delocalised states. The assumption that the preferred basis to use is the one from a measurement of position results in worlds having objects in well-defined positions, instead of worlds with delocalised objects (which would be grossly incompatible with experiment). This special role for measurements is problematic for the theory, as it contradicts Everett and DeWitt's goal of having a reductionist theory and undermines their criticism of the ill-defined measurement postulate of the Copenhagen interpretation. This is known today as the preferred basis problem.

The preferred basis problem has been solved, according to Saunders and Wallace, among others, by incorporating decoherence in the many-worlds theory. In this approach, the preferred basis does not have to be postulated, but rather is identified as the basis stable under environmental decoherence. In this way measurements no longer play a special role; rather, any interaction that causes decoherence causes the world to split. Since decoherence is never complete, there will always remain some infinitesimal overlap between two worlds, making it arbitrary whether a pair of worlds has split or not. Wallace argues that this is not problematic: it only shows that worlds are not a part of the fundamental ontology, but rather of the emergent ontology, where these approximate, effective descriptions are routine in the physical sciences. Since in this approach the worlds are derived, it follows that they must be present in any other interpretation of quantum mechanics that does not have a collapse mechanism, such as Bohmian mechanics.

This approach to deriving the preferred basis has been criticized as creating a circularity with derivations of probability in the many-worlds interpretation, as decoherence theory depends on probability, and probability depends on the ontology derived from decoherence. Wallace contends that decoherence theory depends not on probability but only on the notion that one is allowed to do approximations in physics.

Reception

MWI's initial reception was overwhelmingly negative, with the notable exception of DeWitt. Wheeler made considerable efforts to formulate the theory in a way that would be palatable to Bohr, visited Copenhagen in 1956 to discuss it with him, and convinced Everett to visit as well, which happened in 1959. Nevertheless, Bohr and his collaborators completely rejected the theory. Everett left academia in 1956, never to return, and Wheeler eventually disavowed the theory.

One of MWI's strongest advocates is David Deutsch. According to Deutsch, the single photon interference pattern observed in the double slit experiment can be explained by interference of photons in multiple universes. Viewed this way, the single photon interference experiment is indistinguishable from the multiple photon interference experiment. In a more practical vein, in one of the earliest papers on quantum computing, he suggested that parallelism that results from MWI could lead to "a method by which certain probabilistic tasks can be performed faster by a universal quantum computer than by any classical restriction of it". Deutsch has also proposed that MWI will be testable (at least against "naive" Copenhagenism) when reversible computers become conscious via the reversible observation of spin.

Asher Peres was an outspoken critic of MWI. A section of his 1993 textbook had the title Everett's interpretation and other bizarre theories. Peres argued that the various many-worlds interpretations merely shift the arbitrariness or vagueness of the collapse postulate to the question of when "worlds" can be regarded as separate, and that no objective criterion for that separation can actually be formulated.

Some consider MWI unfalsifiable and hence unscientific because the multiple parallel universes are non-communicating, in the sense that no information can be passed between them. Others claim MWI is directly testable.

Victor J. Stenger remarked that Murray Gell-Mann's published work explicitly rejects the existence of simultaneous parallel universes. Collaborating with James Hartle, Gell-Mann had been, before his death, working toward the development a more "palatable" post-Everett quantum mechanics. Stenger thought it fair to say that most physicists dismiss the many-worlds interpretation as too extreme, while noting it "has merit in finding a place for the observer inside the system being analyzed and doing away with the troublesome notion of wave function collapse".

Philosophers of science James Ladyman and Don Ross state that the MWI could be true, but that they do not embrace it. They note that no quantum theory is yet empirically adequate for describing all of reality, given its lack of unification with general relativity, and so they do not see a reason to regard any interpretation of quantum mechanics as the final word in metaphysics. They also suggest that the multiple branches may be an artifact of incomplete descriptions and of using quantum mechanics to represent the states of macroscopic objects. They argue that macroscopic objects are significantly different from microscopic objects in not being isolated from the environment, and that using quantum formalism to describe them lacks explanatory and descriptive power and accuracy.

Polls

A poll of 72 "leading quantum cosmologists and other quantum field theorists" conducted before 1991 by L. David Raub showed 58% agreement with "Yes, I think MWI is true".

Max Tegmark reports the result of a "highly unscientific" poll taken at a 1997 quantum mechanics workshop. According to Tegmark, "The many worlds interpretation (MWI) scored second, comfortably ahead of the consistent histories and Bohm interpretations."

In response to Sean M. Carroll's statement "As crazy as it sounds, most working physicists buy into the many-worlds theory", Michael Nielsen counters: "at a quantum computing conference at Cambridge in 1998, a many-worlder surveyed the audience of approximately 200 people... Many-worlds did just fine, garnering support on a level comparable to, but somewhat below, Copenhagen and decoherence." But Nielsen notes that it seemed most attendees found it to be a waste of time: Peres "got a huge and sustained round of applause…when he got up at the end of the polling and asked 'And who here believes the laws of physics are decided by a democratic vote?'"

A 2005 poll of fewer than 40 students and researchers taken after a course on the Interpretation of Quantum Mechanics at the Institute for Quantum Computing University of Waterloo found "Many Worlds (and decoherence)" to be the least favored.

A 2011 poll of 33 participants at an Austrian conference found 6 endorsed MWI, 8 "Information-based/information-theoretical", and 14 Copenhagen; the authors remark that MWI received a similar percentage of votes as in Tegmark's 1997 poll.

Debate whether the other worlds are real

Everett believed in the literal reality of the other quantum worlds. His son reported that he "never wavered in his belief over his many-worlds theory".

According to Martin Gardner, the "other" worlds of MWI have two different interpretations: real or unreal; he claimed that Stephen Hawking and Steven Weinberg both favour the unreal interpretation. Gardner also claimed that most physicists favour the unreal interpretation, whereas the "realist" view is supported only by MWI experts such as Deutsch and DeWitt. Hawking has said that "according to Feynman's idea", all other histories are as "equally real" as our own,  and Gardner reports Hawking saying that MWI is "trivially true". In a 1983 interview, Hawking also said he regarded MWI as "self-evidently correct" but was dismissive of questions about the interpretation of quantum mechanics, saying, "When I hear of Schrödinger's cat, I reach for my gun." In the same interview, he also said, "But, look: All that one does, really, is to calculate conditional probabilities—in other words, the probability of A happening, given B. I think that that's all the many worlds interpretation is. Some people overlay it with a lot of mysticism about the wave function splitting into different parts. But all that you're calculating is conditional probabilities." Elsewhere Hawking contrasted his attitude towards the "reality" of physical theories with that of his colleague Roger Penrose, saying, "He's a Platonist and I'm a positivist. He's worried that Schrödinger's cat is in a quantum state, where it is half alive and half dead. He feels that can't correspond to reality. But that doesn't bother me. I don't demand that a theory correspond to reality because I don't know what it is. Reality is not a quality you can test with litmus paper. All I'm concerned with is that the theory should predict the results of measurements. Quantum theory does this very successfully." For his own part, Penrose agrees with Hawking that QM applied to the universe implies MW, but he believes the lack of a successful theory of quantum gravity negates the claimed universality of conventional QM.

Speculative implications

Quantum suicide thought experiment

Quantum suicide is a thought experiment in quantum mechanics and the philosophy of physics. Purportedly, it can distinguish between the Copenhagen interpretation of quantum mechanics and the many-worlds interpretation by means of a variation of the Schrödinger's cat thought experiment, from the cat's point of view. Quantum immortality refers to the subjective experience of surviving quantum suicide.

Most experts believe that the experiment would not work in the real world, because the world with the surviving experimenter has a lower "measure" than the world prior to the experiment, making it less likely that the experimenter will go on to experience their survival.

Absurdly improbable timelines

DeWitt has stated that "[Everett, Wheeler and Graham] do not in the end exclude any element of the superposition. All the worlds are there, even those in which everything goes wrong and all the statistical laws break down."

Max Tegmark has affirmed that absurd or highly unlikely events are inevitable but rare under the MWI. To quote Tegmark, "Things inconsistent with the laws of physics will never happen—everything else will... it's important to keep track of the statistics, since even if everything conceivable happens somewhere, really freak events happen only exponentially rarely."

Ladyman and Ross state that, in general, many of the unrealized possibilities that are discussed in other scientific fields will not have counterparts in other branches, because they are in fact incompatible with the universal wavefunction.