Category Archives: Space Travel

Space-A travel is a means by which members of United States Uniformed Services (United States Military, reservists and retirees, United States Department of Defense civilian personnel (under certain circumstances), and these groups' family members, are permitted to travel on aircraft under the jurisdiction of the United States Department of Defense when excess capability allows.

Space available travel is a privilege that derives, in part, from United States Code, title 10, section 4744, which states, "officers and members of the Military Departments, and their families, when space is available, may be transported on vessels operated by any military transport agency of the Department of Defense". Space available travel is defined as "travel aboard DoD owned or controlled aircraft and occurs when aircraft are not fully booked with passengers traveling under orders".

It is a privilege offered to United States Uniformed Services members. Retired members are given the privilege in recognition of their career and because they are eligible for recall to active duty. The criteria for extending the privilege to other categories of passengers is their support to the mission being performed by Uniformed Services members and to the enhancement of active duty Service members' quality of life.

There are rules and guidelines which apply to such travel. Uniformed personnel may only travel Space-A while on leave or pass for the full duration of their Space-A trip, and Space-A travel can not be used in conjunction with travel required by the service. Space A travel may not be used for personal financial gain or in connection with business enterprises or employment. Other nations' laws and policies, as well as U.S. foreign policy, may limit the ability to travel using Space-A.

Aside from members of the United States Marine Corps, travelers do not have to be in uniform for their flights.

Eligible passengers wanting to travel using DoD Space-A travel are required to sign up at the departing location and are then placed on a locally managed Space-A register. The registration process varies depending on the location, but most locations allow signups via electronic mail, fax, or postal mail.

Each location's passenger service center maintains their own Space-A register. Each person signing up is placed on this register using category of travel, signup date and signup time.

Based on status (active duty military, retired military, emergency traveler, etc.), Space-A travel applicants are assigned a category of travel from 1 to 6, which categorizes their priority of movement, 1 being the highest priority. Thus, an applicant with priority 1 will gain a place on an available aircraft over an applicant with priority 4, for example.

The number of space-available seats may not be known until the flight's "Roll Call" just prior to the flight departs. After sorting the signup register by category of travel and signup date, the passenger terminal personnel follow a selection procedure. If there is sufficient seating for everyone desiring a seat, then everyone boards; otherwise, a cutoff point is determined.

The branches of service eligible for Space-A travel are:

Space-A travel is not without its pitfalls. Unlike traditional commercial air traffic, military flights are not always assigned predictable takeoff times. Many factors go into planning a military flight, with space-required cargo and passengers forming the basis of planning. There is no consideration given to potential Space-A travelers during the planning process.

The majority of flights that passengers take occur on: C-5, C-17, C-40, C-130, KC-10, and KC-135 aircraft.

Space-A travelers might meet abrupt, sometimes even in-flight, changes in travel. This need for pre-planning has given rise to a small industry surrounding such travel. Non-governmental enterprises (for the most part, publishers) produce products, initially through books and maps, with more recent incarnations as websites which provide travelers with information regarding Space-A travel.

The following information Space-A links are hosted by volunteer retired military:

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Space-A travel - Wikipedia, the free encyclopedia

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Space Travel and Exploration

This article is about paying space travellers. For other commercial spacefarers, see Commercial astronaut.

Space tourism is space travel for recreational, leisure or business purposes. A number of startup companies have sprung up in recent years, such as Virgin Galactic and XCOR Aerospace, hoping to create a sub-orbital space tourism industry. Orbital space tourism opportunities have been limited and expensive, with only the Russian Space Agency providing transport to date.

The publicized price for flights brokered by Space Adventures to the International Space Station aboard a Russian Soyuz spacecraft have been US $2040 million, during the period 20012009 when 7 space tourists made 8 space flights. Some space tourists have signed contracts with third parties to conduct certain research activities while in orbit.

Russia halted orbital space tourism in 2010 due to the increase in the International Space Station crew size, using the seats for expedition crews that would have been sold to paying spaceflight participants.[1][2] Orbital tourist flights are planned to resume in 2015.[3]

As an alternative term to "tourism", some organizations such as the Commercial Spaceflight Federation use the term "personal spaceflight". The Citizens in Space project uses the term "citizen space exploration".[4]

As of September 2012[update], multiple companies are offering sales of orbital and suborbital flights, with varying durations and creature comforts.[5]

The Soviet space program was aggressive in broadening the pool of cosmonauts. The Soviet Intercosmos program included cosmonauts selected from Warsaw Pact members (from Czechoslovakia, Poland, East Germany, Bulgaria, Hungary, Romania) and later from allies of the USSR (Cuba, Mongolia, Vietnam) and non-aligned countries (India, Syria, Afghanistan). Most of these cosmonauts received full training for their missions and were treated as equals, but especially after the Mir program began, were generally given shorter flights than Soviet cosmonauts. The European Space Agency (ESA) took advantage of the program as well.

The U.S. space shuttle program included payload specialist positions which were usually filled by representatives of companies or institutions managing a specific payload on that mission. These payload specialists did not receive the same training as professional NASA astronauts and were not employed by NASA. In 1983, Ulf Merbold from ESA and Byron Lichtenberg from MIT (engineer and Air Force fighter pilot) were the first payload specialists to fly on the Space Shuttle, on mission STS-9.[6][7]

In 1984, Charles D. Walker became the first non-government astronaut to fly, with his employer McDonnell Douglas paying $40,000 for his flight.[8]:7475 NASA was also eager to prove its capability to Congressional sponsors. Senator Jake Garn was flown on the Shuttle in 1985,[9] followed by Representative Bill Nelson in 1986.[10]

During the 1970s, Shuttle prime contractor Rockwell International studied a $200300 million removable cabin that could fit into the Shuttle's cargo bay. The cabin could carry up to 74 passengers into orbit for up to three days. Space Habitation Design Associates proposed, in 1983, a cabin for 72 passengers in the bay. Passengers were located in six sections, each with windows and its own loading ramp, and with seats in different configurations for launch and landing. Another proposal was based on the Spacelab habitation modules, which provided 32 seats in the payload bay in addition to those in the cockpit area. A 1985 presentation to the National Space Society stated that although flying tourists in the cabin would cost $1 to 1.5 million per passenger without government subsidy, within 15 years 30,000 people a year would pay $25,000 each to fly in space on new spacecraft. The presentation also forecast flights to lunar orbit within 30 years and visits to the lunar surface within 50 years.[11]

As the shuttle program expanded in the early 1980s, NASA began a Space Flight Participant program to allow citizens without scientific or governmental roles to fly. Christa McAuliffe was chosen as the first Teacher in Space in July 1985 from 11,400 applicants. 1,700 applied for the Journalist in Space program, including Walter Cronkite, Tom Brokaw, Tom Wolfe, and Sam Donaldson. An Artist in Space program was considered, and NASA expected that after McAuliffe's flight two to three civilians a year would fly on the shuttle.[8] After McAuliffe was killed in the Challenger disaster in January 1986 the programs were canceled. McAuliffe's backup, Barbara Morgan, eventually got hired in 1998 as a professional astronaut and flew on STS-118 as a mission specialist.[8]:8485 A second journalist-in-space program, in which NASA green-lighted Miles O'Brien to fly on the space shuttle, was scheduled to be announced in 2003. That program was canceled in the wake of the Columbia disaster on STS-107 and subsequent emphasis on finishing the International Space Station before retiring the space shuttle.

With the realities of the post-Perestroika economy in Russia, its space industry was especially starved for cash. The Tokyo Broadcasting System (TBS) offered to pay for one of its reporters to fly on a mission. For $28 million, Toyohiro Akiyama was flown in 1990 to Mir with the eighth crew and returned a week later with the seventh crew. Akiyama gave a daily TV broadcast from orbit and also performed scientific experiments for Russian and Japanese companies. However, since the cost of the flight was paid by his employer, Akiyama could be considered a business traveler rather than a tourist.

In 1991, British chemist Helen Sharman was selected from a pool of 13,000 applicants to be the first Briton in space.[12] The program was known as Project Juno and was a cooperative arrangement between the Soviet Union and a group of British companies. The Project Juno consortium failed to raise the funds required, and the program was almost cancelled. Reportedly Mikhail Gorbachev ordered it to proceed under Soviet expense in the interests of international relations, but in the absence of Western underwriting, less expensive experiments were substituted for those in the original plans. Sharman flew aboard Soyuz TM-12 to Mir and returned aboard Soyuz TM-11.

At the end of the 1990s, MirCorp, a private venture that was by then in charge of the space station, began seeking potential space tourists to visit Mir in order to offset some of its maintenance costs. Dennis Tito, an American businessman and former JPL scientist, became their first candidate. When the decision to de-orbit Mir was made, Tito managed to switch his trip to the International Space Station (ISS) through a deal between MirCorp and U.S.-based Space Adventures, Ltd., despite strong opposition from senior figures at NASA; from the beginning of the ISS expeditions, NASA stated it wasn't interested in space guests.[13] Nonetheless, Dennis Tito visited the ISS on April 28, 2001, and stayed for seven days, becoming the first "fee-paying" space tourist. He was followed in 2002 by South African computer millionaire Mark Shuttleworth. The third was Gregory Olsen in 2005, who was trained as a scientist and whose company produced specialist high-sensitivity cameras. Olsen planned to use his time on the ISS to conduct a number of experiments, in part to test his company's products. Olsen had planned an earlier flight, but had to cancel for health reasons. The Subcommittee on Space and Aeronautics Committee On Science of the House of Representatives held on June 26, 2001 reveals the shifting attitude of NASA towards paying space tourists wanting to travel to the ISS. The hearing's purpose was to, "Review the issues and opportunities for flying nonprofessional astronauts in space, the appropriate government role for supporting the nascent space tourism industry, use of the Shuttle and Space Station for Tourism, safety and training criteria for space tourists, and the potential commercial market for space tourism".[14] The subcommittee report was interested in evaluating Dennis Tito's extensive training and his experience in space as a nonprofessional astronaut.

By 2007, space tourism was thought to be one of the earliest markets that would emerge for commercial spaceflight.[15]:11 However, as of 2014[update] this private exchange market has not emerged to any significant extent.

Space Adventures remains the only company to have sent paying passengers to space.[16][17] In conjunction with the Federal Space Agency of the Russian Federation and Rocket and Space Corporation Energia, Space Adventures facilitated the flights for all of the world's first private space explorers. The first three participants paid in excess of $20 million (USD) each for their 10-day visit to the ISS.

After the Columbia disaster, space tourism on the Russian Soyuz program was temporarily put on hold, because Soyuz vehicles became the only available transport to the ISS. On July 26, 2005, Space Shuttle Discovery (mission STS-114) marked the shuttle's return to space. Consequently, in 2006, space tourism was resumed. On September 18, 2006, an Iranian American named Anousheh Ansari became the fourth space tourist (Soyuz TMA-9).[18]) On April 7, 2007, Charles Simonyi, an American businessman of Hungarian descent, joined their ranks (Soyuz TMA-10). Simonyi became the first repeat space tourist, paying again to fly on Soyuz TMA-14 in MarchApril 2009. Canadian Guy Lalibert became the next space tourist in September, 2009 aboard Soyuz TMA-16.

As reported by Reuters on March 3, 2010, Russia announced that the country would double the number of launches of three-man Soyuz ships to four that year, because "permanent crews of professional astronauts aboard the expanded [ISS] station are set to rise to six"; regarding space tourism, the head of the Russian Cosmonauts' Training Center said "for some time there will be a break in these journeys".[1]

On January 12, 2011, Space Adventures and the Russian Federal Space Agency announced that orbital space tourism would resume in 2013 with the increase of manned Soyuz launches to the ISS from four to five per year.[19] However, this has not materialized, and the current preferred option, instead of producing an additional Soyuz, would be to extend the duration of an ISS Expedition to one year, paving the way for the flight of new spaceflight participants. The British singer Sarah Brightman initiated plans (costing a reported $52 million) and participated in preliminary training in early 2015, expecting to then fly (and to perform while in orbit) in September 2015, but in May 2015 she postponed the plans indefinitely.[3][20][21]

Several plans have been proposed for using a space station as a hotel:

No suborbital space tourism has occurred yet, but since it is projected to be more affordable, many companies view it as a money-making proposition. Most are proposing vehicles that make suborbital flights peaking at an altitude of 100160km (6299mi).[38] Passengers would experience three to six minutes of weightlessness, a view of a twinkle-free starfield, and a vista of the curved Earth below. Projected costs are expected to be about $200,000 per passenger.[39]

Under the Outer Space Treaty signed in 1967, the launch operator's nationality and the launch site's location determine which country is responsible for any damages occurred from a launch.[53]

After valuable resources were detected on the Moon, private companies began to formulate methods to extract the resources. Article II of the Outer Space Treaty dictates that "outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means".[54] However, countries have the right to freely explore the Moon and any resources collected are property of that country when they return.

In December 2005, the U.S. Government released a set of proposed rules for space tourism.[55] These included screening procedures and training for emergency situations, but not health requirements.

Under current US law, any company proposing to launch paying passengers from American soil on a suborbital rocket must receive a license from the Federal Aviation Administration's Office of Commercial Space Transportation (FAA/AST). The licensing process focuses on public safety and safety of property, and the details can be found in the Code of Federal Regulations, Title 14, Chapter III.[56] This is in accordance with the Commercial Space Launch Amendments Act passed by Congress in 2004.[57]

In March 2010, the New Mexico legislature passed the Spaceflight Informed Consent Act. The SICA gives legal protection to companies who provide private space flights in the case of accidental harm or death to individuals. Participants sign an Informed Consent waiver, dictating that spaceflight operators can not be held liable in the "death of a participant resulting from the inherent risks of space flight activities". Operators are however not covered in the case of gross negligence or willful misconduct.[58]

A 2010 study published in Geophysical Research Letters raised concerns that the growing commercial spaceflight industry could accelerate global warming. The study, funded by NASA and The Aerospace Corporation, simulated the impact of 1,000 suborbital launches of hybrid rockets from a single location, calculating that this would release a total of 600 tonnes of black carbon into the stratosphere. They found that the resultant layer of soot particles remained relatively localised, with only 20% of the carbon straying into the southern hemisphere, thus creating a strong hemispherical asymmetry.[59] This unbalance would cause the temperature to decrease by about 0.4C (0.72F) in the tropics and subtropics, whereas the temperature at the poles would increase by between 0.2 and 1C (0.36 and 1.80F). The ozone layer would also be affected, with the tropics losing up to 1.7% of ozone cover, and the polar regions gaining 56%.[60] The researchers stressed that these results should not be taken as "a precise forecast of the climate response to a specific launch rate of a specific rocket type", but as a demonstration of the sensitivity of the atmosphere to the large-scale disruption that commercial space tourism could bring.[59]

Several organizations have been formed to promote the space tourism industry, including the Space Tourism Society, Space Future, and HobbySpace. UniGalactic Space Travel Magazine is a bi-monthly educational publication covering space tourism and space exploration developments in companies like SpaceX, Orbital Sciences, Virgin Galactic and organizations like NASA.

Classes in space tourism are currently taught at the Rochester Institute of Technology in New York,[61] and Keio University in Japan.[62]

A web-based survey suggested that over 70% of those surveyed wanted less than or equal to 2 weeks in space; in addition, 88% wanted to spacewalk (only 14% of these would do it for a 50% premium), and 21% wanted a hotel or space station.[63]

The concept has met with some criticism from some, including politicians, notably Gnter Verheugen, vice-president of the European Commission, who said of the EADS Astrium Space Tourism Project: "It's only for the super rich, which is against my social convictions".[64]

As of October 2013, NBC News and Virgin Galactic have come together to create a new reality television show titled Space Race. The show "will follow contestants as they compete to win a flight into space aboard Virgin Galactic's SpaceShipTwo rocket plane. It is not to be confused with the Children's Space TV show called "Space Racers""[65]

Many private space travelers have objected to the term "space tourist", often pointing out that their role went beyond that of an observer, since they also carried out scientific experiments in the course of their journey. Richard Garriott additionally emphasized that his training was identical to the requirements of non-Russian Soyuz crew members, and that teachers and other non-professional astronauts chosen to fly with NASA are called astronauts. He has said that if the distinction has to be made, he would rather be called "private astronaut" than "tourist".[66] Dennis Tito has asked to be known as an "independent researcher",[citation needed] and Mark Shuttleworth described himself as a "pioneer of commercial space travel".[67] Gregory Olsen prefers "private researcher",[68] and Anousheh Ansari prefers the term "private space explorer".[18] Other space enthusiasts object to the term on similar grounds. Rick Tumlinson of the Space Frontier Foundation, for example, has said: "I hate the word tourist, and I always will ... 'Tourist' is somebody in a flowered shirt with three cameras around his neck."[69] Russian cosmonaut Maksim Surayev told the press in 2009 not to describe Guy Lalibert as a tourist: "It's become fashionable to speak of space tourists. He is not a tourist but a participant in the mission."[70]

"Spaceflight participant" is the official term used by NASA and the Russian Federal Space Agency to distinguish between private space travelers and career astronauts. Tito, Shuttleworth, Olsen, Ansari, and Simonyi were designated as such during their respective space flights. NASA also lists Christa McAuliffe as a spaceflight participant (although she did not pay a fee), apparently due to her non-technical duties aboard the STS-51-L flight.

The U.S. Federal Aviation Administration awards the title of "Commercial Astronaut" to trained crew members of privately funded spacecraft. The only people currently holding this title are Mike Melvill and Brian Binnie, the pilots of SpaceShipOne.

A 2010 report from the Federal Aviation Administration, titled "The Economic Impact of Commercial Space Transportation on the U. S Economy in 2009", cites studies done by Futron, an aerospace and technology-consulting firm, which predict that space tourism could become a billion-dollar market within 20 years.[71] In addition, in the decade since Dennis Tito journeyed to the International Space Station, eight private citizens have paid the $20 million fee to travel to space. Space Adventures suggests that this number could increase fifteen-fold by 2020.[72] These figures do not include other private space agencies such as Virgin Galactic, which as of 2014 has sold approximately 700 tickets priced at $200,000 or $250,000 dollars each and has accepted more than $80 million in deposits.[73]

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Space tourism - Wikipedia, the free encyclopedia

Time travel is the concept of movement (such as by a human) between certain points in time, analogous to movement between different points in space, typically using a hypothetical device known as a time machine, in the form of a vehicle or of a portal connecting distant points in time. Time travel is a recognized concept in philosophy and fiction, but traveling to an arbitrary point in time has a very limited support in theoretical physics, and usually only in conjunction with quantum mechanics or EinsteinRosen bridges. In a more narrow sense, one-way time travel into the future via time dilation is a proven phenomenon in relativistic physics, but traveling any significant "distance" requires motion at speeds close to the speed of light, which is not feasible for human travel with current technology.[1] The concept was touched upon in various earlier works of fiction, but was popularized by H. G. Wells' 1895 novel The Time Machine, which moved the concept of time travel into the public imagination, and it remains a popular subject in science fiction.

Some ancient myths depict moving forward in time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is shocked to learn when he returns to Earth that many ages have passed.[2][3]

The Buddhist Pli Canon mentions the relativity of time. In the Payasi Sutta, one of the Buddha's chief disciples, Kumara Kassapa, explains to the skeptic Payasi that, "In the Heaven of the Thirty Three Devas, time passes at a different pace, and people live much longer. "In the period of our century; one hundred years, only a single day; twenty four hours would have passed for them."[4]

In the Japanese tale of "Urashima Tar",[5] first described in the Nihongi (720).,[6] a young fisherman named Urashima Taro visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is ruins, and his family has died.

In the Talmud, Honi ha-M'agel sleeps for 70 years and awakes to find his grandchildren have become grandparents, and his family and friends have died.[7]

In the utopian novel Louis-Sbastien Mercier's L'An 2440, rve s'il en ft jamais ("The Year 2440: A Dream If Ever There Were One"), the protagonist is transported to the year 2440. A popular work, having gone through twenty-five editions since its appearance in 1771, it describes the adventures of an unnamed man who discusses with a philosopher friend the injustices of Paris, then falls asleep and finds himself in a future Paris.

Washington Irving's "Rip Van Winkle" (1819) depicts a man who takes a twenty-year nap on a mountain, waking up in a future where he has been forgotten, his wife has died, and his daughter has grown.[5] Sleep is also used as a means of time travel in H.G. Wells's The Sleeper Awakes, in which a man wakes up after a two-hundred year hibernation.

Like forward time travel, backward time travel has an uncertain origin. Samuel Madden's Memoirs of the Twentieth Century (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future.[8] Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel.".[9] Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present."[8]

In 1836 Alexander Veltman published Predki Kalimerosa: Aleksandr Filippovich Makedonskii (The Forebears of Kalimeros: Alexander, son of Philip of Macedon), which has been called the first original Russian science fiction novel and the first novel to use time travel.[10] The narrator rides to ancient Greece on a hippogriff, meets Aristotle, and goes on a voyage with Alexander the Great before returning to the 19th century.

In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is "Missing One's Coach: An Anachronism", written for the Dublin Literary Magazine[11] by an anonymous author in 1838.[12] While the narrator waits under a tree for a coach to take him out of Newcastle, he is transported back in time over a thousand years. He encounters the Venerable Bede in a monastery and explains to him the developments of the coming centuries. However, the story never makes it clear whether these events are real or a dream.[13]

Some consider Charles Dickens's A Christmas Carol (1843)[14] to be one of the first depictions of time travel in both directions, as the protagonist, Ebenezer Scrooge, is transported to Christmases past and future. However, these might be interpreted as visions rather than as time travel because Scrooge experiences the time periods as an observer rather than as a participant.

A clearer example of backward time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men) by the French botanist and geologist Pierre Boitard, published posthumously. In this story, the protagonist is transported to the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters a Plesiosaur and an apelike ancestor and is able to interact with ancient creatures.[15]

Edward Everett Hale's "Hands Off" (1881) tells the story of an unnamed being, possibly the soul of a person who has recently died, who interferes with ancient Egyptian history by preventing Joseph's enslavement. This may have been the first story to feature an alternate history created as a result of time travel.[16]

One of the first stories to feature time travel by means of a machine is "The Clock that Went Backward" by Edward Page Mitchell,[17] which appeared in the New York Sun in 1881. However, the mechanism borders on fantasy. An unusual clock, when wound, runs backwards and transports people nearby back in time. But the author fails to explain the origin of either the clock or its abilities.[18]

Enrique Gaspar y Rimbau's El Anacronpete (1887)[19] may have been the first story to feature a vessel engineered to travel through time.[20]Andrew Sawyer has commented that the story "does seem to be the first literary description of a time machine noted so far", adding that "Edward Page Mitchell's story 'The Clock That Went Backward' (1881) is usually described as the first time-machine story, but I'm not sure that a clock quite counts."[21]H. G. Wells's The Time Machine (1895) popularized the concept of time travel by mechanical means.[22]

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime or specific types of motion in space might allow time travel into the past and future if these geometries or motions were possible.[23] In technical papers, physicists generally avoid the commonplace language of "moving" or "traveling" through time. "Movement" normally refers only to a change in spatial position as the time coordinate is varied. Instead they discuss the possibility of closed timelike curves, which are world lines that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, such as Gdel spacetime, but the physical plausibility of these solutions is uncertain.

Relativity predicts that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future." According to relativity there is no single objective answer to how much time has really passed between the departure and the return, but there is an objective answer to how much proper time has been experienced by both the Earth and the traveler, i.e., how much each has aged (see twin paradox). On the other hand, many in the scientific community believe that backward time travel is highly unlikely. Any theory that would allow time travel would introduce potential problems of causality. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, by appealing either to the Novikov self-consistency principle or to the notion of branching parallel universes.

Stephen Hawking has suggested that the absence of tourists from the future is an argument against the existence of time travel. This is a variant of the Fermi paradox. Of course, this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never developed or is cautiously never used; and even if it were developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the correct way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so "[t]his picture would explain why" the world hasn't already been overrun by "tourists from the future."[24] This simply means that, until a time machine were actually to be invented, we would not be able to see time travelers. Carl Sagan also once suggested the possibility that time travelers could be here but are disguising their existence, or are not recognized as time travelers.[25]

The theory of general relativity does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed.[26] These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[27] but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.[25][28]:150

Time travel to the past is theoretically allowed using the following methods:[29]

According to the theory of relativity, a signal or matter moving faster than light from one point to another would appear in some inertial frame of reference as moving backwards in time. This is a consequence of the relativity of simultaneity in special relativity, which says that in some cases different reference frames will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events. Technically, these disagreements occur when the spacetime interval between the events is 'space-like', meaning that neither event lies in the future light cone of the other.[30] If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.[30]

However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backward in time. And since one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backward in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves FTL (faster than light) in A's frame but backward in time in B's frame, and then B sends a reply which moves FTL in B's frame but backward in time in A's frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality in every frame. An illustration of such a scenario using spacetime diagrams can be found here.[31] The scenario is sometimes referred to as a tachyonic antitelephone.

According to special relativity, it would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light. Although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory, it seems that it would not actually be possible to use them to transmit information faster than light.[32] There is also no widely agreed-upon evidence for the existence of tachyons; the faster-than-light neutrino anomaly had opened the possibility that neutrinos might be tachyons, but the results of the experiment were found to be invalid upon further analysis.

The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves", and hence time travel into the past.[23] The first of these was proposed by Kurt Gdel, a solution known as the Gdel metric, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have.[23] Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.

Wormholes are a hypothetical warped spacetime which are also permitted by the Einstein field equations of general relativity,[33] although it would not be possible to travel through a wormhole unless it were what is known as a traversable wormhole.

A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less than the stationary end, as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.[34] This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, a trip backward in time as seen by other observers outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine;[35] in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backward in time.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy (often referred to as "exotic matter"). More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions.[36] However, it is known that quantum effects can lead to small measurable violations of the null energy condition,[36] and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics.[37] Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.[38]

In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.[39] Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.[40]

Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum[41] in 1936 and Kornel Lanczos[42] in 1924, but not recognized as allowing closed timelike curves[43] until an analysis by Frank Tipler[44] in 1974. If a cylinder is infinitely long and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string.

Physicist Robert Forward noted that a nave application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time; however, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.[citation needed]

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter). Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,[45] he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."[28]:96 This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon."[46] However, this theorem does not rule out the possibility of time travel (1) by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) and (2) in regions which contain exotic matter (which would be necessary for traversable wormholes or the Alcubierre drive). Because the theorem is based on general relativity, it is also conceivable a future theory of quantum gravity which replaced general relativity would allow time travel even without exotic matter (though it is also possible such a theory would place even more restrictions on time travel, or rule it out completely as postulated by Hawking's chronology protection conjecture).[citation needed]

Certain experiments carried out give the impression of reversed causality but are subject to interpretation. For example, in the delayed choice quantum eraser experiment performed by Marlan Scully, pairs of entangled photons are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double-slit experiment, and depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, and under most interpretations of quantum mechanics the results can be explained in a way that does not violate causality.[citation needed]

The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. But a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light,[47] so this experiment is understood not to violate causality either.

The physicists Gnter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 3ft (0.91m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.[48]

Some physicists have performed experiments that attempted to show causality violations, but so far without success. The "Space-time Twisting by Light" (STL) experiment run by physicist Ronald Mallett attempts to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal. Mallett has some physical arguments that suggest that closed timelike curves would become possible through the center of a laser that has been twisted into a loop. However, other physicists dispute his arguments (see objections).

Shengwang Du claims in a peer-reviewed journal to have observed single photons' precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than c (and, thus, violating causality).[49] Some members of the media took this as an indication of proof that time travel to the past using superluminal speeds was impossible.[50][51]

Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day (2005) or MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. Back in 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.[52][53][54] These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so farno time travelers are known to have attended either event. It is hypothetically possible that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe.[55]

Another factor is that for all the time travel devices considered under current physics (such as those that operate using wormholes), it is impossible to travel back to before the time machine was actually made.[56][57]

There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of their own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. According to relativity, there would be no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on the choice of reference frame.

This form of "travel into the future" is theoretically allowed (and has been demonstrated at very small time scales) using the following methods:[29]

Time dilation is permitted by Albert Einstein's special and general theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well.[59] For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returnsin this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a black hole.

It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing inside a spherical shell with a diameter of 5 meters and the mass of Jupiter.[60] For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into such a structure is not expected to be within our technological capabilities in the near future.

There is a great deal of experimental evidence supporting the validity of equations for velocity-based time dilation in special relativity[61] and gravitational time dilation in general relativity.[62][63][64] A famous and easy-to-replicate example is the observation of atmospheric muon decay.[65][66] With current technologies it is only possible to cause a human traveler to age less than companions on Earth by a very small fraction of a second, the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev. A researcher from the University of Connecticut is attempting to use lasers to warp or loop spacetime.[67]

Time perception can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced.

Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and it is debatable whether they should be called time travel. However time dilation can be viewed as a better fit for our understanding of the term "time travel" than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation (MWI) of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories.[68] These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the MWI is correct, then a time traveler should indeed end up in a different history than the one he started from.[69][70][71] On the other hand, Stephen Hawking has argued 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 own world rather than traveling to a different one.[24] The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues 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 through a wormhole, with different particles emerging in different worlds.[72]

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes.[73][74] The quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.[75] This effect was referred to as "spooky action at a distance" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used.[76] The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. The fact that these quantum phenomena apparently do not allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments.[citation needed] How the rules of quantum mechanics work to preserve causality is an active area of research.[citation needed]

Theories of time travel are riddled with questions about causality and paradoxes. Compared to other fundamental concepts in modern physics, time is still not understood very well. Philosophers have been theorizing about the nature of time since before the era of the ancient Greek philosophers. Some philosophers and physicists who study the nature of time also study the possibility of time travel and its logical implications. The probability of paradoxes and their possible solutions are often considered.

For more information on the philosophical considerations of time travel, consult the work of David Lewis. For more information on physics-related theories of time travel, consider the work of Kurt Gdel (especially his theorized universe) and Lawrence Sklar.

The relativity of simultaneity in modern physics favors the philosophical view known as eternalism or four-dimensionalism (Sider, 2001), in which physical objects are either temporally extended spacetime worms, or spacetime worm stages, and this view would be favored further by the possibility of time travel (Sider, 2001). Eternalism, also sometimes known as "block universe theory", builds on a standard method of modeling time as a dimension in physics, to give time a similar ontology to that of space (Sider, 2001). This would mean that time is just another dimension, that future events are "already there", and that there is no objective flow of time. This view is disputed by Tim Maudlin in his The Metaphysics Within Physics.

Presentism is a school of philosophy that holds that neither the future nor the past exist, and there are no non-present objects. In this view, time travel is impossible because there is no future or past to travel to. However, some 21st-century presentists have argued that although past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present.[77][78]

One subject often brought up in philosophical discussion of time is the idea that, if one were able to go back in time, paradoxes could ensue if the time traveler were to change things. The best examples of this are the grandfather paradox and the idea of autoinfanticide. The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and attempts to kill his paternal grandfather at a time before his grandfather met his grandmother. If he did so, then his father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather. The paradox is sometimes posed with autoinfanticide, where a traveler goes back and attempts to kill himself as an infant. If he were to do so, he never would have grown up to go back in time to kill himself as an infant.

This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually change the past in any way,[79] an idea similar to the proposed Novikov self-consistency principle in physics.

The Novikov self-consistency principle, named after Igor Dmitrievich Novikov, states that any actions, taken by a time traveler or by an object that travels back in time, were part of history all along, and therefore it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox,[80] ontological paradox,[81] or bootstap paradox.[81][82] The term bootstap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps".[83] The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.[84]

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"[85] that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics. Ross uses Somewhere in Time as an example where Jane Seymour's character gives Christopher Reeve's character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour's character 60 years in the past. As Ross states:

The watch is an impossible object. It violates the Second Law of Thermodynamics, the Law of Entropy. If time travel makes that watch possible, then time travel itself is impossible. The watch, indeed, must be absolutely identical to itself in the 19th and 20th centuries, since Reeve carries it with him from the future instantaneously into the past and bestows it on Seymour. The watch, however, cannot be identical to itself, since all the years in which it is in the possession of Seymour and then Reeve it will wear in the normal manner. Its entropy will increase. The watch carried back by Reeve will be more worn than the watch that would have been acquired by Seymour.

On the other hand, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem). In addition, the second law of thermodynamics only states that entropy should increase in systems which are isolated from interactions with the external world, so Igor Novikov (creator of the Novikov self-consistency principle) has argued that in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop.[86]

David Lewis's analysis of compossibility and the implications of changing the past is meant to account for the possibilities of time travel in a one-dimensional conception of time without creating logical paradoxes. Consider Lewis example of Tim. Tim hates his grandfather and would like nothing more than to kill him. The only problem for Tim is that his grandfather died years ago. Tim wants so badly to kill his grandfather himself that he constructs a time machine to travel back to 1955 when his grandfather was young and kill him then. Assuming that Tim can travel to a time when his grandfather is still alive, the question must then be raised: can Tim kill his grandfather?

For Lewis, the answer lies within the context of the usage of the word "can". Lewis explains that the word "can" must be viewed against the context of pertinent facts relating to the situation. Suppose that Tim has a rifle, years of rifle training, a straight shot on a clear day and no outside force to restrain Tim's trigger finger. Can Tim shoot his grandfather? Considering these facts, it would appear that Tim can in fact kill his grandfather. In other words, all of the contextual facts are compossible with Tim killing his grandfather. However, when reflecting on the compossibility of a given situation, we must gather the most inclusive set of facts that we are able to.

Consider now the fact that in Tim's universe his grandfather actually died in 1993 and not in 1955. This new fact about Tim's situation reveals that him killing his grandfather is not compossible with the current set of facts. Tim cannot kill his grandfather because his grandfather died in 1993 and not when he was young. Thus, Lewis concludes, the statements "Tim doesnt but can, because he has what it takes", and, "Tim doesnt, and cant, because it is logically impossible to change the past", are not contradictions; they are both true given the relevant set of facts. The usage of the word "can" is equivocal: he "can" and "can not" under different relevant facts.

So what must happen to Tim as he takes aim? Lewis believes that his gun will jam, a bird will fly in the way, or Tim simply slips on a banana peel. Either way, there will be some logical force of the universe that will prevent Tim every time from killing his grandfather.[87]

Time travel themes in science fiction and the media can generally be grouped into three general categories: immutable timeline; mutable timeline; and alternate histories (as in the many-worlds interpretation).[88][89][90][91] Frequently in fiction, timeline is used to refer to all physical events in history, so that in time travel stories where events can be changed, the time traveler is described as creating a new or altered timeline.[92] This usage is distinct from the use of the term timeline to refer to a type of chart that illustrates a particular series of events, and the concept is also distinct from a world line, a term from Einstein's theory of relativity which refers to the entire history of a single object.

An objection that is sometimes raised[by whom?] against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into exactly the same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date.[citation needed] However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occur at different times[93] (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance".[94]

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Time travel - Wikipedia, the free encyclopedia

Weve all asked this question at some point in our lives: How long would it take to travel to the stars? Could it be within a persons own lifetime, and could this kind of travel become the norm someday? There are many possible answers to this question some very simple, others in the realms of science fiction. But coming up with a comprehensive answer means taking a lot of things into consideration.

Unfortunately, any realistic assessment is likely to produce answers that would totally discourage futurists and enthusiasts of interstellar travel. Like it or not, space is very large, and our technology is still very limited. But should we ever contemplate leaving the nest, we will have a range of options for getting to the nearest Solar Systems in our galaxy.

The nearest star to Earth is our Sun, which is a fairly average star in the Hertzsprung Russell Diagrams Main Sequence. This means that it is highly stable, providing Earth with just the right type of sunlight for life to evolve on our planet. We know there are planets orbiting other stars near to our Solar System, and many of these stars are similar to our own.

In the future, should mankind wish to leave the Solar System, well have a huge choice of stars we could travel to, and many could have the right conditions for life to thrive. But where would we go and how long would it take for us to get there? Just remember, this is all speculative and there is currently no benchmark for interstellar trips. That being said, here we go!

Over 2000 exoplanets have been identified, many of which are believed to be habitable. Credit: phl.upl.edu

As already noted, the closest star to our Solar System is Proxima Centauri, which is why it makes the most sense to plot an interstellar mission to this system first. As part of a triple star system called Alpha Centauri, Proxima is about 4.24 light years (or 1.3 parsecs) from Earth. Alpha Centauri is actually the brightest star of the three in the system part of a closely orbiting binary 4.37 light years from Earth whereas Proxima Centauri (the dimmest of the three) is an isolated red dwarf about 0.13 light years from the binary.

And while interstellar travel conjures up all kinds of visions of Faster-Than-Light (FTL) travel, ranging from warp speed and wormholes to jump drives, such theories are either highly speculative (such as the Alcubierre Drive) or entirely the province of science fiction. In all likelihood, any deep space mission will likely take generations to get there, rather than a few days or in an instantaneous flash.

So, starting with one of the slowest forms of space travel, how long will it take to get to Proxima Centauri?

The question of how long would it take to get somewhere in space is somewhat easier when dealing with existing technology and bodies within our Solar System. For instance, using the technology that powered the New Horizons mission which consisted of 16 thrusters fueled with hydrazine monopropellant reaching the Moon would take a mere 8 hours and 35 minutes.

On the other hand, there is the European Space Agencys (ESA) SMART-1 mission, which took its time traveling to the Moon using the method of ionic propulsion. With this revolutionary technology, a variation of which has since been used by the Dawn spacecraft to reach Vesta, the SMART-1 mission took one year, one month and two weeks to reach the Moon.

So, from the speedy rocket-propelled spacecraft to the economical ion drive, we have a few options for getting around local space plus we could use Jupiter or Saturn for a hefty gravitational slingshot. However, if we were to contemplate missions to somewhere a little more out of the way, we would have to scale up our technology and look at whats really possible.

When we say possible methods, we are talking about those that involve existing technology, or those that do not yet exist, but are technically feasible. Some, as you will see, are time-honored and proven, while others are emerging or still on the board. In just about all cases though, they present a possible, but extremely time-consuming or expensive, scenario for getting to even the closest stars

Ionic Propulsion: Currently, the slowest form of propulsion, and the most fuel-efficient, is the ion engine. A few decades ago, ionic propulsion was considered to be the subject of science fiction. However, in recent years, the technology to support ion engines has moved from theory to practice in a big way. The ESAs SMART-1 mission for example successfully completed its mission to the Moon after taking a 13 month spiral path from the Earth.

SMART-1 used solar powered ion thrusters, where electrical energy was harvested from its solar panels and used to power its Hall-effect thrusters. Only 82 kg of xenon propellant was used to propel SMART-1 to the Moon. 1 kg of xenon propellant provided a delta-v of 45 m/s. This is a highly efficient form of propulsion, but it is by no means fast.

Artists concept of Dawn mission above Ceres. Since its arrival, the spacecraft turned around to point the blue glow of its ion engine in the opposite direction. Image credit: NASA/JPL

One of the first missions to use ion drive technology was the Deep Space 1 mission to Comet Borrelly that took place in 1998. DS1 also used a xenon-powered ion drive, consuming 81.5 kg of propellant. Over 20 months of thrusting, DS1 was managed to reach a velocity of 56,000 km/hr (35,000 miles/hr) during its flyby of the comet.

Ion thrusters are therefore more economical than rocket technology, as the thrust per unit mass of propellant (a.k.a. specific impulse) is far higher. But it takes a long time for ion thrusters to accelerate spacecraft to any great speeds, and the maximum velocity it can achieve is dependent on its fuel supply and how much electrical energy it can generate.

So if ionic propulsion were to be used for a mission to Proxima Centauri, the thrusters would need a huge source of energy production (i.e. nuclear power) and a large quantity of propellant (although still less than conventional rockets). But based on the assumption that a supply of 81.5 kg of xenon propellant translates into a maximum velocity of 56,000 km/hr (and that there are no other forms of propulsion available, such as a gravitational slingshot to accelerate it further), some calculations can be made.

In short, at a maximum velocity of 56,000 km/h, Deep Space 1 would take over 81,000 years to traverse the 4.24 light years between Earth and Proxima Centauri. To put that time-scale into perspective, that would be over 2,700 human generations. So it is safe to say that an interplanetary ion engine mission would be far too slow to be considered for a manned interstellar mission.

Ionic propulsion is currently the slowest, but most fuel-efficient, form of space travel. Credit: NASA/JPL

But, should ion thrusters be made larger and more powerful (i.e. ion exhaust velocity would need to be significantly higher), and enough propellant could be hauled to keep the spacecrafts going for the entire 4.243 light-year trip, that travel time could be greatly reduced. Still not enough to happen in someones lifetime though.

Gravity Assist Method:The fastest existing means of space travel is known the Gravity Assist method, which involves a spacecraft using the relative movement (i.e. orbit) and gravity of a planet to alter is path and speed. Gravitational assists are a very useful spaceflight technique, especially when using the Earth or another massive planet (like a gas giant) for a boost in velocity.

The Mariner 10 spacecraft was the first to use this method, using Venus gravitational pull to slingshot it towards Mercury in February of 1974. In the 1980s, the Voyager 1 probe used Saturn and Jupiter for gravitational slingshots to attain its current velocity of 60,000 km/hr (38,000 miles/hr) and make it into interstellar space.

However, it was the Helios 2 mission which was launched in 1976 to study the interplanetary medium from 0.3 AU to 1 AU to the Sun that holds the record for highest speed achieved with a gravity assist. At the time, Helios 1 (which launched in 1974) and Helios 2 held the record for closest approach to the Sun. Helios 2 was launched by a conventional NASA Titan/Centaur launch vehicle and placed in a highly elliptical orbit.

A Helios probe being encapsulated for launch. Credit: Public Domain

Due to the large eccentricity (0.54) of the 190 day solar orbit, at perihelion Helios 2 was able to reach a maximum velocity of over 240,000 km/hr (150,000 miles/hr). This orbital speed was attained by the gravitational pull of the Sun alone. Technically, the Helios 2 perihelion velocity was not a gravitational slingshot, it was a maximum orbital velocity, but it still holds the record for being the fastest man-made object regardless.

So, if Voyager 1 was traveling in the direction of the red dwarf Proxima Centauri at a constant velocity of 60,000 km/hr, it would take 76,000 years (or over 2,500 generations) to travel that distance. But if it could attain the record-breaking speed of Helios 2s close approach of the Sun a constant speed of 240,000 km/hr it would take 19,000 years (or over 600 generations) to travel 4.243 light years. Significantly better, but still not in the ream of practicality.

Electromagnetic (EM) Drive:Another proposed method of interstellar travel comes in the form of the Radio Frequency (RF) Resonant Cavity Thruster, also known as the EM Drive. Originally proposed in 2001 by Roger K. Shawyer, a UK scientist who started Satellite Propulsion Research Ltd (SPR) to bring it to fruition, this drive is built around the idea that electromagnetic microwave cavities can allow for the direct conversion of electrical energy to thrust.

Whereas conventional electromagnetic thrusters are designed to propel a certain type of mass (such as ionized particles), this particular drive system relies on no reaction mass and emits no directional radiation. Such a proposal has met with a great deal of skepticism, mainly because it violates the law of Conservation of Momentum which states that within a system, the amount of momentum remains constant and is neither created nor destroyed, but only changes through the action of forces.

The EM Drive prototype produced by NASA/Eagleworks. Credit: NASA Spaceflight Forum

However, recent experiments with the technology have apparently yielded positive results. In July of 2014, at the 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference in Cleveland, Ohio, researchers from NASAs advanced propulsion research claimed that they had successfully tested a new design for an electromagnetic propulsion drive.

This was followed up in April of 2015 when researchers at NASA Eagleworks (part of the Johnson Space Center) claimed that they had successfully tested the drive in a vacuum, an indication that it might actually work in space. In July of that same year, a research team from the Dresden University of Technologys Space System department built their own version of the engine and observed a detectable thrust.

And in 2010, Prof. Juan Yang of the Northwestern Polytechnical University in Xian, China, began publishing a series of papers about her research into EM Drive technology. This culminated in her 2012 paper where she reported higher input power (2.5kW) and tested thrust (720mN) levels. In 2014, she further reported extensive tests involving internal temperature measurements with embedded thermocouples, which seemed to confirm that the system worked.

Artists concept of an interstellar craft equipped with an EM Drive. Credit: NASA Spaceflight Center

According to calculations based on the NASA prototype (which yielded a power estimate of 0.4 N/kilowatt), a spacecraft equipped with the EM drive could make the trip to Pluto in less than 18 months. Thats one-sixth the time it took for the New Horizons probe to get there, which was traveling at speeds of close to 58,000 km/h (36,000 mph).

Sounds impressive. But even at that rate, it would take a ship equipped with EM engines over 13,000 years for the vessel to make it to Proxima Centauri. Getting closer, but not quickly enough! and until such time that technology can be definitively proven to work, it doesnt make much sense to put our eggs into this basket.

Nuclear Thermal and Nuclear Electric Propulsion (NTP/NEP):Another possibility for interstellar space flight is to use spacecraft equipped with nuclear engines, a concept which NASA has been exploring for decades. In a Nuclear Thermal Propulsion (NTP) rocket, uranium or deuterium reactions are used to heat liquid hydrogen inside a reactor, turning it into ionized hydrogen gas (plasma), which is then channeled through a rocket nozzle to generate thrust.

A Nuclear Electric Propulsion (NEP) rocket involves the same basic reactor converting its heat and energy into electrical energy, which would then power an electrical engine. In both cases, the rocket would rely on nuclear fission or fusion to generates propulsion rather than chemical propellants, which has been the mainstay of NASA and all other space agencies to date.

Artists impression of a Crew Transfer Vehicle (CTV) using its nuclear-thermal rocket engines to slow down and establish orbit around Mars. Credit: NASA

Compared to chemical propulsion, both NTP and NEC offers a number of advantages. The first and most obvious is the virtually unlimited energy density it offers compared to rocket fuel. In addition, a nuclear-powered engine could also provide superior thrust relative to the amount of propellant used. This would cut the total amount of propellant needed, thus cutting launch weight and the cost of individual missions.

Although no nuclear-thermal engines have ever flown, several design concepts have been built and tested over the past few decades, and numerous concepts have been proposed. These have ranged from the traditional solid-core design such as the Nuclear Engine for Rocket Vehicle Application (NERVA) to more advanced and efficient concepts that rely on either a liquid or a gas core.

However, despite these advantages in fuel-efficiency and specific impulse, the most sophisticated NTP concept has a maximum specific impulse of 5000 seconds (50 kNs/kg). Using nuclear engines driven by fission or fusion, NASA scientists estimate it would could take a spaceship only 90 days to get to Mars when the planet was at opposition i.e. as close as 55,000,000 km from Earth.

But adjusted for a one-way journey to Proxima Centauri, a nuclear rocket would still take centuries to accelerate to the point where it was flying a fraction of the speed of light. It would then require several decades of travel time, followed by many more centuries of deceleration before reaching it destination. All told, were still talking about 1000 years before it reaches its destination. Good for interplanetary missions, not so good for interstellar ones.

Using existing technology, the time it would take to send scientists and astronauts on an interstellar mission would be prohibitively slow. If we want to make that journey within a single lifetime, or even a generation, something a bit more radical (aka. highly theoretical) will be needed. And while wormholes and jump engines may still be pure fiction at this point, there are some rather advanced ideas that have been considered over the years.

Nuclear Pulse Propulsion:Nuclear pulse propulsion is a theoretically possible form of fast space travel. The concept was originally proposed in 1946 by Stanislaw Ulam, a Polish-American mathematician who participated in the Manhattan Project, and preliminary calculations were then made by F. Reines and Ulam in 1947. The actual project known as Project Orion was initiated in 1958 and lasted until 1963.

The Project Orion concept for a nuclear-powered spacecraft. Credit: silodrome.co

Led by Ted Taylor at General Atomics and physicist Freeman Dyson from the Institute for Advanced Study in Princeton, Orion hoped to harness the power of pulsed nuclear explosions to provide a huge thrust with very high specific impulse (i.e. the amount of thrust compared to weight or the amount of seconds the rocket can continually fire).

In a nutshell, the Orion design involves a large spacecraft with a high supply of thermonuclear warheads achieving propulsion by releasing a bomb behind it and then riding the detonation wave with the help of a rear-mounted pad called a pusher. After each blast, the explosive force would be absorbed by this pusher pad, which then translates the thrust into forward momentum.

Though hardly elegant by modern standards, the advantage of the design is that it achieves a high specific impulse meaning it extracts the maximum amount of energy from its fuel source (in this case, nuclear bombs) at minimal cost. In addition, the concept could theoretically achieve very high speeds, with some estimates suggesting a ballpark figure as high as 5% the speed of light (or 5.4107 km/hr).

But of course, there the inevitable downsides to the design. For one, a ship of this size would be incredibly expensive to build. According to estimates produced by Dyson in 1968, an Orion spacecraft that used hydrogen bombs to generate propulsion would weight 400,000 to 4,000,000 metric tons. And at least three quarters of that weight consists of nuclear bombs, where each warhead weights approximately 1 metric ton.

Artists concept of Orion spacecraft leaving Earth. Credit: bisbos.com/Adrian Mann

All told, Dysons most conservative estimates placed the total cost of building an Orion craft at 367 billion dollars. Adjusted for inflation, that works out to roughly $2.5 trillion dollars which accounts for over two thirds of the US governments current annual revenue. Hence, even at its lightest, the craft would be extremely expensive to manufacture.

Theres also the slight problem of all the radiation it generates, not to mention nuclear waste. In fact, it is for this reason that the Project is believed to have been terminated, owing to the passage of the Partial Test Ban Treaty of 1963 which sought to limit nuclear testing and stop the excessive release of nuclear fallout into the planets atmosphere.

Fusion Rockets:Another possibility within the realm of harnessed nuclear power involves rockets that rely on thermonuclear reactions to generate thrust. For this concept, energy is created when pellets of a deuterium/helium-3 mix are ignited in a reaction chamber by inertial confinement using electron beams (similar to what is done at the National Ignition Facility in California). This fusion reactor would detonate 250 pellets per second to create high-energy plasma, which would then be directed by a magnetic nozzle to create thrust.

Like a rocket that relies on a nuclear reactor, this concept offers advantages as far as fuel efficiency and specific impulse are concerned. Exhaust velocities of up to 10,600km/s are estimated, which is far beyond the speed of conventional rockets. Whats more, the technology has been studied extensively over the past few decades, and many proposals have been made.

Artists concept of the Daedalus spacecraft, a two-stage fusion rocket that would achieve up to 12% he speed of light. Credit: Adrian Mann

For example, between 1973 and 1978, the British Interplanetary Society conducted feasibility study known as Project Daedalus. Relying on current knowledge of fusion technology and existing methods, the study called for the creation of a two-stage unmanned scientific probe making a trip to Barnards Star (5.9 light years from Earth) in a single lifetime.

The first stage, the larger of the two, would operate for 2.05 years and accelerate the spacecraft to 7.1% the speed of light (o.071 c). This stage would then be jettisoned, at which point, the second stage would ignite its engine and accelerate the spacecraft up to about 12% of light speed (0.12 c) over the course of 1.8 years. The second-stage engine would then be shut down and the ship would enter into a 46-year cruise period.

According to the Projects estimates, the mission would take 50 years to reach Barnards Star. Adjusted for Proxima Centauri, the same craft could make the trip in 36 years. But of course, the project also identified numerous stumbling blocks that made it unfeasible using then-current technology most of which are still unresolved.

For instance, there is the fact that helium-3 is scare on Earth, which means it would have to be mined elsewhere (most likely on the Moon). Second, the reaction that drives the spacecraft requires that the energy released vastly exceed the energy used to trigger the reaction. And while experiments here on Earth have surpassed the break-even goal, we are still a long way away from the kinds of energy needed to power an interstellar spaceship.

Artists concept of the Project Daedalus spacecraft, with a Saturn V rocket standing next to it for scale. Credit: Adrian Mann

Third, there is the cost factor of constructing such a ship. Even by the modest standard of Project Daedalus unmanned craft, a fully-fueled craft would weight as much as 60,000 Mt. To put that in perspective, the gross weight of NASAs SLS is just over 30 Mt, and a single launch comes with a price tag of $5 billion (based on estimates made in 2013).

In short, a fusion rocket would not only be prohibitively expensive to build, it would require a level of fusion reactor technology that is currently beyond our means. Icarus Interstellar, an international organization of volunteer citizen scientists (some of whom worked for NASA or the ESA) have since attempted to revitalize the concept with Project Icarus. Founded in 2009, the group hopes to make fusion propulsion (among other things) feasible by the near future.

Fusion Ramjet:Also known as the Bussard Ramjet, this theoretical form of propulsion was first proposed by physicist Robert W. Bussard in 1960. Basically, it is an improvement over the standard nuclear fusion rocket, which uses magnetic fields to compress hydrogen fuel to the point that fusion occurs. But in the Ramjets case, an enormous electromagnetic funnel scoops hydrogen from the interstellar medium and dumps it into the reactor as fuel.

Artists concept of the Bussard Ramjet, which would harness hydrogen from the interstellar medium to power its fusion engines. Credit: futurespacetransportation.weebly.com

As the ship picks up speed, the reactive mass is forced into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust through an engine nozzle, thereby accelerating the vessel. Without any fuel tanks to weigh it down, a fusion ramjet could achieve speeds approaching 4% of the speed of light and travel anywhere in the galaxy.

However, the potential drawbacks of this design are numerous. For instance, there is the problem of drag. The ship relies on increased speed to accumulate fuel, but as it collides with more and more interstellar hydrogen, it may also lose speed especially in denser regions of the galaxy. Second, deuterium and tritium (used in fusion reactors here on Earth) are rare in space, whereas fusing regular hydrogen (which is plentiful in space) is beyond our current methods.

This concept has been popularized extensively in science fiction. Perhaps the best known example of this is in the franchise of Star Trek, where Bussard collectors are the glowing nacelles on warp engines. But in reality, our knowledge of fusion reactions need to progress considerably before a ramjet is possible. We would also have to figure out that pesky drag problem before we began to consider building such a ship!

Laser Sail:Solar sails have long been considered to be a cost-effective way of exploring the Solar System. In addition to being relatively easy and cheap to manufacture, theres the added bonus of solar sails requiring no fuel. Rather than using rockets that require propellant, the sail uses the radiation pressure from stars to push large ultra-thin mirrors to high speeds.

IKAROS spaceprobe with solar sail in flight (artists depiction) showing a typical square sail configuration. Credit: Wikimedia Commons/Andrzej Mirecki

However, for the sake of interstellar flight, such a sail would need to be driven by focused energy beams (i.e. lasers or microwaves) to push it to a velocity approaching the speed of light. The concept was originally proposed by Robert Forward in 1984, who was a physicist at the Hughes Aircrafts research laboratories at the time.

The concept retains the benefits of a solar sail, in that it requires no on-board fuel, but also from the fact that laser energy does not dissipate with distance nearly as much as solar radiation. So while a laser-driven sail would take some time to accelerate to near-luminous speeds, it would be limited only to the speed of light itself.

According to a 2000 study produced by Robert Frisbee, a director of advanced propulsion concept studies at NASAs Jet Propulsion Laboratory, a laser sail could be accelerated to half the speed of light in less than a decade. He also calculated that a sail measuring about 320 km (200 miles) in diameter could reach Proxima Centauri in just over 12 years. Meanwhile, a sail measuring about 965 km (600 miles) in diameter would arrive in just under 9 years.

However, such a sail would have to be built from advanced composites to avoid melting. Combined with its size, this would add up to a pretty penny! Even worse is the sheer expense incurred from building a laser large and powerful enough to drive a sail to half the speed of light. According to Frisbees own study, the lasers would require a steady flow of 17,000 terawatts of power close to what the entire world consumes in a single day.

Antimatter Engine:Fans of science fiction are sure to have heard of antimatter. But in case you havent, antimatter is essentially material composed of antiparticles, which have the same mass but opposite charge as regular particles. An antimatter engine, meanwhile, is a form of propulsion that uses interactions between matter and antimatter to generate power, or to create thrust.

Artists concept of an antimatter-powered spacecraft for missions to Mars, as part of the Mars Reference Mission. Credit: NASA

In short, an antimatter engine involves particles of hydrogen and antihydrogen being slammed together. This reaction unleashes as much as energy as a thermonuclear bomb, along with a shower of subatomic particles called pions and muons. These particles, which would travel at one-third the speed of light, are then be channeled by a magnetic nozzle to generate thrust.

The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket. Whats more, controlling this kind of reaction could conceivably push a rocket up to half the speed of light.

Pound for pound, this class of ship would be the fastest and most fuel-efficient ever conceived. Whereas conventional rockets require tons of chemical fuel to propel a spaceship to its destination, an antimatter engine could do the same job with just a few milligrams of fuel. In fact, the mutual annihilation of a half pound of hydrogen and antihydrogen particles would unleash more energy than a 10-megaton hydrogen bomb.

It is for this exact reason that NASAs Institute for Advanced Concepts (NIAC) has investigated the technology as a possible means for future Mars missions. Unfortunately, when contemplating missions to nearby star systems, the amount if fuel needs to make the trip is multiplied exponentially, and the cost involved in producing it would be astronomical (no pun!).

What matter and antimatter might look like annihilating one another. Credit: NASA/CXC/M. Weiss

According to report prepared for the 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit (also by Robert Frisbee), a two-stage antimatter rocket would need over 815,000 metric tons (900,000 US tons) of fuel to make the journey to Proxima Centauri in approximately 40 years. Thats not bad, as far as timelines go. But again, the cost

Whereas a single gram of antimatter would produce an incredible amount of energy, it is estimated that producing just one gram would require approximately 25 million billion kilowatt-hours of energy and cost over a trillion dollars. At present, the total amount of antimatter that has been created by humans is less 20 nanograms.

And even if we could produce antimatter for cheap, you would need a massive ship to hold the amount of fuel needed. According to a report by Dr. Darrel Smith & Jonathan Webby of the Embry-Riddle Aeronautical University in Arizona, an interstellar craft equipped with an antimatter engine could reach 0.5 the speed of light and reach Proxima Centauri in a little over 8 years. However, the ship itself would weigh 400 Mt, and would need 170 MT of antimatter fuel to make the journey.

A possible way around this is to create a vessel that can create antimatter which it could then store as fuel. This concept, known as the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES), was proposed by Richard Obousy of Icarus Interstellar. Based on the idea of in-situ refueling, a VARIES ship would rely on large lasers (powered by enormous solar arrays) which would create particles of antimatter when fired at empty space.

Artists concept of the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES), a concept that would use solar arrays to power lasers that create particles of antimatter to be used as fuel. Credit: Adrian Mann

Much like the Ramjet concept, this proposal solves the problem of carrying fuel by harnessing it from space. But once again, the sheer cost of such a ship would be prohibitively expensive using current technology. In addition, the ability to create dark matter in large volumes is not something we currently have the power to do. Theres also the matter of radiation, as matter-antimatter annihilation can produce blasts of high-energy gamma rays.

This not only presents a danger to the crew, requiring significant radiations shielding, but requires the engines be shielded as well to ensure they dont undergo atomic degradation from all the radiation they are exposed to. So bottom line, the antimatter engine is completely impractical with our current technology and in the current budget environment.

Alcubierre Warp Drive:Fans of science fiction are also no doubt familiar with the concept of an Alcubierre (or Warp) Drive. Proposed by Mexican physicist Miguel Alcubierre in 1994, this proposed method was an attempt to make FTL travel possible without violating Einsteins theory of Special Relativity. In short, the concept involves stretching the fabric of space-time in a wave, which would theoretically cause the space ahead of an object to contract and the space behind it to expand.

An object inside this wave (i.e. a spaceship) would then be able to ride this wave, known as a warp bubble, beyond relativistic speeds. Since the ship is not moving within this bubble, but is being carried along as it moves, the rules of space-time and relativity would cease to apply. The reason being, this method does not rely on moving faster than light in the local sense.

Artist Mark Rademakers concept for the IXS Enterprise, a theoretical interstellar warp spacecraft. Credit: Mark Rademaker/flickr.com

It is only faster than light in the sense that the ship could reach its destination faster than a beam of light that was traveling outside the warp bubble. So assuming that a spacecraft could be outfitted with an Alcubierre Drive system, it would be able to make the trip to Proxima Centauri in less than 4 years. So when it comes to theoretical interstellar space travel, this is by far the most promising technology, at least in terms of speed.

Naturally, the concept has been received its share of counter-arguments over the years. Chief amongst them are the fact that it does not take quantum mechanics into account, and could be invalidated by a Theory of Everything (such as loop quantum gravity). Calculations on the amount of energy required have also indicated that a warp drive would require a prohibitive amount of power to work. Other uncertainties include the safety of such a system, the effects on space-time at the destination, and violations of causality.

However, in 2012, NASA scientist Harold Sonny White announced that he and his colleagues had begun researching the possibility of an Alcubierre Drive. In a paper titled Warp Field Mechanics 101, White claimed that they had constructed an interferometer that will detect the spatial distortions produced by the expanding and contracting spacetime of the Alcubierre metric.

In 2013, the Jet Propulsion Laboratory published results of a warp field test which was conducted under vacuum conditions. Unfortunately, the results were reported as inconclusive. Long term, we may find that Alcubierres metric may violate one or more fundamental laws of nature. And even if the physics should prove to be sound, there is no guarantee it can be harnessed for the sake of FTL flight.

In conclusion, if you were hoping to travel to the nearest star within your lifetime, the outlook isnt very good. However, if mankind felt the incentive to build an interstellar ark filled with a self-sustaining community of space-faring humans, it might be possible to travel there in a little under a century if we were willing to invest in the requisite technology.

But all the available methods are still very limited when it comes to transit time. And while taking hundreds or thousands of years to reach the nearest star may matter less to us if our very survival was at stake, it is simply not practical as far as space exploration and travel goes. By the time a mission reached even the closest stars in our galaxy, the technology employed would be obsolete and humanity might not even exist back home anymore.

So unless we make a major breakthrough in the realms of fusion, antimatter, or laser technology, we will either have to be content with exploring our own Solar System, or be forced to accept a very long-term transit strategy

We have written many interesting articles about space travel here at Universe Today. Heres Will We Ever Reach Another Star?, Warp Drives May Come With a Killer Downside, The Alcubierre Warp Drive, How Far Is A Light Year?, When Light Just Isnt Fast Enough, When Will We Become Interstellar?, and Can We Travel Faster Than the Speed of Light?

For more information, be sure to consult NASAs pages on Propulsion Systems of the Future, and Is Warp Drive Real?

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How Long Would It Take To Travel To The Nearest Star ...