Unidentified flying object

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Photograph of a purported UFO in Passaic, New Jersey, taken on July 31, 1952

An unidentified flying object (UFO) is any perceived aerial phenomenon that cannot be immediately identified or explained. On investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.

Scientists and skeptic organizations such as the Committee for Skeptical Inquiry have provided prosaic explanations for a large number of claimed UFOs being caused by natural phenomena, human technology, delusions, or hoaxes. Small but vocal groups of "ufologists" favour unconventional, pseudoscientific hypotheses, some of which go beyond the typical extraterrestrial visitation claims and sometimes form part of new religions.

While unusual sightings have been reported in the sky throughout history, UFOs did not achieve their current cultural prominence until the period after World War II, escalating during the Space Age. The 20th century saw studies and investigations into UFO reports conducted by governments (such as Projects Grudge and Sign in the United States, and Project Condign in the United Kingdom), as well as by organisations and individuals.

History

Early history before the 20th century

The 1561 celestial phenomenon over Nuremberg as printed in an illustrated news notice. UFO enthusiasts have described the phenomenon as an aerial battle of extraterrestrial origin. Skeptics find the phenomenon likely to have been a sun dog

People have observed the sky throughout history, and sometimes seen unusual sights: such as comets, bright meteors, one or more of the five planets that can be readily seen with the naked eye, planetary conjunctions, and atmospheric optical phenomena such as parhelia and lenticular clouds. One particularly famous example is Halley's Comet: this was recorded first by Chinese astronomers in 240 BC and possibly as early as 467 BC. As it reaches the inner solar system every 76 years, it was often identified as a unique isolated event in ancient historical documents whose authors were unaware that it was a repeating phenomenon. Such accounts in history often were treated as supernatural portents, angels, or other religious omens. While UFO enthusiasts have sometimes commented on the narrative similarities between certain religious symbols in medieval paintings and UFO reports, the canonical and symbolic character of such images is documented by art historians placing more conventional religious interpretations on such images.

Some examples of pre-modern observations of unusual aerial phenomena:

• Julius Obsequens was a Roman writer who is believed to have lived in the middle of the fourth century AD. The only work associated with his name is the Liber de prodigiis (Book of Prodigies), completely extracted from an epitome, or abridgment, written by Livy; De prodigiis was constructed as an account of the wonders and portents that occurred in Rome between 249 and 12 BCE. An aspect of Obsequens' work that has inspired excitement in some UFO enthusiasts is that he makes reference to things moving through the sky. It is possible that it is a description of meteors, and, since Obsequens is writing some 400 years after the events he describes, the text is not an eye-witness account.
• On April 14, 1561, residents of Nuremberg described the appearance of a large black triangular object. According to witnesses, there were also hundreds of spheres, cylinders and other odd-shaped objects that moved erratically overhead. The accounts are consistent with sun dogs and other similar meteorological phenomena. A similar report comes from 1566 over Basel and, indeed, in the 15th and 16th centuries, many leaflets wrote of "miracles" and "sky spectacles".
• On January 25, 1878, the Denison Daily News printed an article in which John Martin, a local farmer, had reported seeing a large, dark, circular object resembling a balloon flying "at wonderful speed". Martin, according to the newspaper account, said it appeared to be about the size of a saucer from his perspective, one of the first uses of the word "saucer" in association with a UFO. In April of that year, reports of such "mystery airships" in various parts of the United States are reminiscent of modern UFO waves. Many signed affidavits. Scores of people even reported talking to the pilots. Reports of strange ships and artificial lights in the sky were published in local newspapers for the next two decades culminating in a mass panic in 1897 where some people feared that Thomas Edison had created an artificial star that could fly around the country. When asked his opinion of such reports, Edison said, "You can take it from me that it is a pure fake."

20th century and after

In the Pacific and European theatres during World War II, round, glowing fireballs known as "foo fighters" were reported by Allied and Axis pilots. Some proposed Allied explanations at the time included St. Elmo's fire, the planet Venus, hallucinations from oxygen deprivation, or German secret weapons. In 1946, more than 2,000 reports were collected, primarily by the Swedish military, of unidentified aerial objects over the Scandinavian nations, along with isolated reports from France, Portugal, Italy and Greece. The objects were referred to as "Russian hail" (and later as "ghost rockets") because it was thought the mysterious objects were possibly Russian tests of captured German V1 or V2 rockets. Most were identified as natural phenomena as meteors.

The popular UFO craze by many accounts began with a media frenzy surrounding the reports on June 24, 1947 that a civilian pilot named Kenneth Arnold reported seeing nine objects flying in formation near Mount Rainier in the United States. At the time, he claimed he described the objects flying in a saucer-like fashion, leading to newspaper accounts of "flying saucers" and "flying discs". Soon, reports of flying saucer sightings became a daily occurrence with one particularly famous example being the Roswell incident where remnants of a downed observation balloon were recovered by a farmer and confiscated by military personnel. The story received scant attention at the time, but interest in it revived in the 1990s with the publicity surrounding the television broadcast of an Alien autopsy video marketed as "real footage" but later admitted to be a staged "re-enactment". Various UFO claimants said that they had interacted with the aliens driving the spacecraft and a few said they had visited the crafts themselves. In 1961, the first alien abduction account was sensationalized when Barney and Betty Hill went under hypnosis after seeing a UFO and reported recovered memories of their experience that became ever more elaborate as the years went by.

As media accounts and speculation were running rampant in the US, by 1953 intelligence officials (Robertson Panel) worried that "genuine incursions" by enemy aircraft "over U.S. territory could be lost in a maelstrom of kooky hallucination" of UFO reports. Media were enlisted to help debunk and discourage UFO reports, culminating in a 1966 TV special, “UFO: Friend, Foe or Fantasy?”, in which Walter Cronkite "patiently" explained to viewers that UFOs were fantasy. Cronkite enlisted Carl Sagan and J. Allen Hynek, who told Cronkite, “To this time, there is no valid scientific proof that we have been visited by spaceships". Fellow NICAP official Donald E. Keyhoe wrote that Vice Admiral Roscoe Hillenkoetter, the first director of the CIA, "wanted public disclosure of UFO evidence".

A 1969 National Academy of Sciences panel reviewed the Condon Report and concurred with its finding, observing that, “While further study of particular aspects of the topic (e.g., atmospheric phenomena) may be useful, a study of UFOs in general is not a promising way to expand scientific understanding of the phenomena.” Referencing the panel's conclusions, the Pentagon announced that it would no longer investigate UFO reports. According to Keith Kloor, the "allure of flying saucers" remained popular with the public into the 1970s, spurring production of such sci-fi films, as Close Encounters of the Third Kind and Alien, which "continued to stoke public fascination". Kloor writes that by the late 1990s, "other big UFO subthemes had been prominently introduced into pop culture, such as the abduction phenomenon and government conspiracy narrative, via best-selling books and, of course, The X-Files".

Notable cases, incidents

Britain

• The Rendlesham Forest incident was a series of reported sightings of unexplained lights near Rendlesham Forest in Suffolk, England in late December of 1980 which became linked with claims of UFO landings.

France

The most notable cases of UFO sightings in France include:

• the Valensole UFO incident in 1965.
• the Trans-en-Provence Case in 1981.

United States

• In the Kecksburg UFO incident, Pennsylvania (1965), residents reported seeing an object crash in the area.
• In 1975, Travis Walton claimed to be abducted by aliens. The movie Fire in the Sky (1993) was based on this event, but greatly embellished the original account.
• The "Phoenix Lights" on March 13, 1997

Astronomer reports

The USAF's Project Blue Book files indicate that approximately 1% of all unknown reports came from amateur and professional astronomers or other telescope users (such as missile trackers or surveyors). In 1952, astronomer J. Allen Hynek, then a consultant to Blue Book, conducted a small survey of 45 fellow professional astronomers. Five reported UFO sightings (about 11%). In the 1970s, astrophysicist Peter A. Sturrock conducted two large surveys of the AIAA and American Astronomical Society (AAS). About 5% of the members polled indicated that they had had UFO sightings.

Astronomer Clyde Tombaugh, who saw six UFOs, including three green fireballs, supported the extraterrestrial hypothesis for UFOs and said scientists who dismissed it without study were "unscientific". Another astronomer, Lincoln LaPaz, headed the United States Air Force's investigation into green fireballs and other UFO phenomena in New Mexico. LaPaz reported two personal sightings, of a green fireball and a disc. (Both Tombaugh and LaPaz were part of Hynek's 1952 survey.) Hynek took two photos through the window of a commercial airliner of a disc that seemed to keep pace with his aircraft.

Astronomer Andrew Fraknoi rejected the hypothesis that UFOs are extraterrestrial spacecraft and responded to the "onslaught of credulous coverage" in books, films and entertainment by teaching his students to apply critical thinking to such claims, advising them that "being a good scientist is not unlike being a good detective". According to Fraknoi, UFO reports "might at first seem mysterious", but "the more you investigate, the more likely you are to find that there is LESS to these stories than meets the eye".

In a 1980 survey of 1800 members of amateur astronomer associations by Gert Helb and Hynek for CUFOS, 24% responded "yes" to the question "Have you ever observed an object which resisted your most exhaustive efforts at identification?"

Famous hoaxes

Main article: List of UFO-related hoaxes

• The Maury Island incident
• George Adamski, over the space of two decades, made various claims about his meetings with telepathic aliens from nearby planets. He claimed photographs of the far side of the Moon taken by the Soviet lunar probe Luna 3 in 1959 were fake, and that there were cities, trees and snow-capped mountains on the far side of the Moon. Among copycats was a shadowy British figure named Cedric Allingham.
• Ed Walters, a building contractor, in 1987 allegedly perpetrated a hoax in Gulf Breeze, Florida. Walters claimed at first having seen a small UFO flying near his home and took some photographs of the craft. Walters reported and documented a series of UFO sightings over a period of three weeks and took several photographs. These sightings became famous, and are collectively referred to as the Gulf Breeze UFO incident. Three years later, in 1990, after the Walters family had moved, the new residents discovered a model of a UFO poorly hidden in the attic that bore an undeniable resemblance to the craft in Walters' photographs. Most investigators, like the forensic photo expert William G. Hyzer, now consider the sightings to be a hoax.

Terminology

The term "UFO" (or "UFOB") was coined in 1953 by the United States Air Force (USAF) to serve as a catch-all for all such reports. In its initial definition, the USAF stated that a "UFOB" was "any airborne object which by performance, aerodynamic characteristics, or unusual features, does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object". Accordingly, the term was initially restricted to that fraction of cases which remained unidentified after investigation, as the USAF was interested in potential national security reasons and "technical aspects" (see Air Force Regulation 200-2).

During the late 1940s and through the 1950s, UFOs were often referred to popularly as "flying saucers" or "flying discs" due to the term being introduced in the context of the Kenneth Arnold incident. The Avro Canada VZ-9AV Avrocar was a concept vehicle produced during the 1950s, which was a functional aircraft with a saucer shape. UFOs were commonly referred to colloquially, as a "Bogey" by Western military personnel and pilots during the cold war. The term "bogey" was originally used to report anomalies in radar blips, to indicate possible hostile forces that might be roaming in the area.

The term UFO became more widespread during the 1950s, at first in technical literature, but later in popular use. UFOs garnered considerable interest during the Cold War, an era associated with a heightened concerns about national security, and, more recently, in the 2010s, for unexplained reasons. Nevertheless, various studies have concluded that the phenomenon does not represent a threat, and nor does it contain anything worthy of scientific pursuit (e.g., 1951 Flying Saucer Working Party, 1953 CIA Robertson Panel, USAF Project Blue Book, Condon Committee).

The Oxford English Dictionary defines a UFO as "An unidentified flying object; a 'flying saucer'". The first published book to use the word was authored by Donald E. Keyhoe.

As an acronym, "UFO" was coined by Captain Edward J. Ruppelt, who headed Project Blue Book, then the USAF's official investigation of UFOs. He wrote, "Obviously the term 'flying saucer' is misleading when applied to objects of every conceivable shape and performance. For this reason the military prefers the more general, if less colorful, name: unidentified flying objects. UFO (pronounced yoo-foe) for short." Other phrases that were used officially and that predate the UFO acronym include "flying flapjack", "flying disc", "unexplained flying discs", and "unidentifiable object".

In popular usage, the term UFO came to be used to refer to claims of alien spacecraft, and because of the public and media ridicule associated with the topic, some ufologists and investigators prefer to use terms such as "unidentified aerial phenomenon" (UAP) or "anomalous phenomena", as in the title of the National Aviation Reporting Center on Anomalous Phenomena (NARCAP). "Anomalous aerial vehicle" (AAV) or "unidentified aerial system" (UAS) are also sometimes used in a military aviation context to describe unidentified targets.

Extraterrestrial hypothesis

Main article: Extraterrestrial hypothesis

While technically a UFO refers to any unidentified flying object, in modern popular culture the term UFO has generally become synonymous with alien spacecraft; however, the term ETV (ExtraTerrestrial Vehicle) is sometimes used to separate this explanation of UFOs from totally earthbound explanations.

Investigations of reports

UFOs have been subject to investigations over the years that varied widely in scope and scientific rigor. Governments or independent academics in the United States, Canada, the United Kingdom, Japan, Peru, France, Belgium, Sweden, Brazil, Chile, Uruguay, Mexico, Spain, and the Soviet Union are known to have investigated UFO reports at various times. No official government investigation has ever publicly concluded that UFOs are indisputably real, physical objects, extraterrestrial in origin, or of concern to national defense.

Among the best known government studies are the ghost rockets investigation by the Swedish military (1946–1947), Project Blue Book, previously Project Sign and Project Grudge, conducted by the USAF from 1947 until 1969, the secret U.S. Army/Air Force Project Twinkle investigation into green fireballs (1948–1951), the secret USAF Project Blue Book Special Report No. 14 by the Battelle Memorial Institute, and the Brazilian Air Force's 1977 Operação Prato (Operation Saucer). France has had an ongoing investigation (GEPAN/SEPRA/GEIPAN) within its space agency Centre national d'études spatiales (CNES) since 1977; the government of Uruguay has had a similar investigation since 1989.

Prosaic explanations

Main article: Identification studies of UFOs

Fata Morgana, a type of mirage in which objects located below the astronomical horizon appear to be hovering in the sky just above the horizon, may be responsible for some UFO sightings.

Studies show that after careful investigation, the majority of UFOs can be identified as ordinary objects or phenomena. The most commonly found identified sources of UFO reports are:

• astronomical objects (bright stars, bolides, bright planets, and the Moon)
• aircraft (including military, civilian, and experimental aircraft as well as such peculiarities as aerial advertising, missile and other rocket launches, artificial satellites, re-entering spacecraft including space debris, kites, and various unmanned aerial vehicles often popularly termed "drones")
• balloons (toy balloons, weather balloons, large research balloons)
• other atmospheric objects and phenomena (birds, unusual clouds, flares)
• light phenomena (mirages, Fata Morgana, ball lightning, moon dogs, searchlights and other ground lights, etc.)
• psychological effects (pareidolia, suggestibility and false memories, mass psychogenic disorders, optical illusions, and hallucinations)
• hoaxes

A 1952–1955 study by the Battelle Memorial Institute for the USAF included these categories. An individual 1979 study by CUFOS researcher Allan Hendry found, as did other investigations, that fewer than one percent of cases he investigated were hoaxes and most sightings were actually honest misidentifications of prosaic phenomena. Hendry attributed most of these to inexperience or misperception.

Americas

Brazil (1952–2016)

Document on sighting of a UFO occurred on December 16, 1977, in the state of Bahia, Brazil

On October 31, 2008, the National Archives of Brazil began receiving from the Aeronautical Documentation and History Center part of the documentation of the Brazilian Air Force regarding the investigation of the appearance of UFOs in Brazil. Currently, this collection gathers cases between 1952 and 2016.

Canada (c. 1950)

In Canada, the Department of National Defence has dealt with reports, sightings and investigations of UFOs across Canada. In addition to conducting investigations into crop circles in Duhamel, Alberta, it still considers "unsolved" the Falcon Lake incident in Manitoba and the Shag Harbour UFO incident in Nova Scotia.

Early Canadian studies included Project Magnet (1950–1954) and Project Second Storey (1952–1954), supported by the Defence Research Board.

United States

Synopsis

U.S. investigations into UFOs include:

• Project Blue Book, previously Project Sign and Project Grudge, conducted by the USAF from 1947 until 1969
• The secret U.S. Army/Air Force Project Twinkle investigation into green fireballs (1948–1951)
• Ghost rockets investigations by the Swedish, UK, U.S., and Greek militaries (1946–1947)
• The secret CIA Office of Scientific Investigation (OS/I) study (1952–53)
• The secret CIA Robertson Panel (1953)
• The secret USAF Project Blue Book Special Report No. 14 by the Battelle Memorial Institute (1951–1954)
• The Brookings Report (1960), commissioned by NASA
• The public Condon Committee (1966–1968)
• The private, internal RAND Corporation study (1968)
• The private Sturrock panel (1998)
• The secret Advanced Aviation Threat Identification Program which was funded from 2007 to 2012.
• The Unidentified Aerial Phenomena Task Force, a continuing program within the United States Office of Naval Intelligence which was acknowledged in 2017.

Thousands of documents released under FOIA also indicate that many U.S. intelligence agencies collected (and still collect) information on UFOs. These agencies include the Defense Intelligence Agency (DIA), FBI, CIA, National Security Agency (NSA), as well as military intelligence agencies of the Army and U.S. Navy, in addition to the Air Force.

The investigation of UFOs has also attracted many civilians, who in the U.S formed research groups such as NICAP (active 1956–1980), Aerial Phenomena Research Organization (APRO) (active 1952–1988), MUFON (active 1969–), and CUFOS (active 1973–).

On November 24, 2021, the Pentagon announced the formation of the Airborne Object Identification and Management Synchronization Group, a new intelligence group to investigate unidentified objects that may compromise the airspace of the United States.

USAAF and FBI response to the 1947 sightings

Following the large U.S. surge in sightings in June and early July 1947, on July 9, 1947, United States Army Air Forces (USAAF) intelligence, in cooperation with the FBI, began a formal investigation into selected sightings with characteristics that could not be immediately rationalized, such as Kenneth Arnold's. The USAAF used "all of its top scientists" to determine whether "such a phenomenon could, in fact, occur". The research was "being conducted with the thought that the flying objects might be a celestial phenomenon," or that "they might be a foreign body mechanically devised and controlled." Three weeks later in a preliminary defense estimate, the air force investigation decided that, "This 'flying saucer' situation is not all imaginary or seeing too much in some natural phenomenon. Something is really flying around."

A further review by the intelligence and technical divisions of the Air Materiel Command at Wright Field reached the same conclusion. It reported that "the phenomenon is something real and not visionary or fictitious," and there were disc-shaped objects, metallic in appearance, as big as man-made aircraft. They were characterized by "extreme rates of climb [and] maneuverability", general lack of noise, absence of a trail, occasional formation flying, and "evasive" behavior "when sighted or contacted by friendly aircraft and radar", suggesting a controlled craft. It was therefore recommended in late September 1947 that an official Air Force investigation be set up. It was also recommended that other government agencies should assist in the investigation.

USAF

Projects Sign (1947–1949), Grudge (1948–1951), and Blue Book (1951–1970)

Main articles: Project Sign, Project Grudge, and Project Blue Book

Project Sign's final report, published in early 1949, stated that while some UFOs appeared to represent actual aircraft, there was not enough data to determine their origin.

The Air Force's Project Sign was created at the end of 1947, and was one of the earliest government studies to come to a secret extraterrestrial conclusion. In August 1948, Sign investigators wrote a top-secret intelligence estimate to that effect, but the Air Force Chief of Staff Hoyt Vandenberg ordered it destroyed. The existence of this suppressed report was revealed by several insiders who had read it, such as astronomer and USAF consultant J. Allen Hynek and Capt. Edward J. Ruppelt, the first head of the USAF's Project Blue Book.

Another highly classified U.S. study was conducted by the CIA's Office of Scientific Investigation (OS/I) in the latter half of 1952 in response to orders from the National Security Council (NSC). This study concluded UFOs were real physical objects of potential threat to national security. One OS/I memo to the CIA Director (DCI) in December read that "the reports of incidents convince us that there is something going on that must have immediate attention ... Sightings of unexplained objects at great altitudes and traveling at high speeds in the vicinity of major U.S. defense installations are of such a nature that they are not attributable to natural phenomena or any known types of aerial vehicles." The matter was considered so urgent that OS/I drafted a memorandum from the DCI to the NSC proposing that the NSC establish an investigation of UFOs as a priority project throughout the intelligence and the defense research and development community. It also urged the DCI to establish an external research project of top-level scientists, now known as the Robertson Panel to analyze the problem of UFOs. The OS/I investigation was called off after the Robertson Panel's negative conclusions in January 1953.

Project Sign was dismantled and became Project Grudge at the end of 1948. Angered by the low quality of investigations by Grudge, the Air Force Director of Intelligence reorganized it as Project Blue Book in late 1951, placing Ruppelt in charge. J. Allen Hynek, a trained astronomer who served as a scientific advisor for Project Blue Book, was initially skeptical of UFO reports, but eventually came to the conclusion that many of them could not be satisfactorily explained and was highly critical of what he described as "the cavalier disregard by Project Blue Book of the principles of scientific investigation". Leaving government work, he founded the privately funded CUFOS, to whose work he devoted the rest of his life. Other private groups studying the phenomenon include the MUFON, a grassroots organization whose investigator's handbooks go into great detail on the documentation of alleged UFO sightings.

USAF Regulation 200-2 (1953–1954)

Air Force Regulation 200-2, issued in 1953 and 1954, defined an Unidentified Flying Object ("UFOB") as "any airborne object which by performance, aerodynamic characteristics, or unusual features, does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object." The regulation also said UFOBs were to be investigated as a "possible threat to the security of the United States" and "to determine technical aspects involved." The regulation went on to say that "it is permissible to inform news media representatives on UFOB's when the object is positively identified as a familiar object" but added: "For those objects which are not explainable, only the fact that ATIC [Air Technical Intelligence Center] will analyze the data is worthy of release, due to many unknowns involved."

Blue Book and the Condon Committee (1968–1970)

Main article: Condon Committee

A public research effort conducted by the Condon Committee for the USAF and published as the Condon Report arrived at a negative conclusion in 1968. Blue Book closed down in 1970, using the Condon Committee's negative conclusion as a rationale, thus ending official Air Force UFO investigations. However, a 1969 USAF document, known as the Bolender memo, along with later government documents, revealed that non-public U.S. government UFO investigations continued after 1970. The Bolender memo first stated that "reports of unidentified flying objects that could affect national security ... are not part of the Blue Book system," indicating that more serious UFO incidents already were handled outside the public Blue Book investigation. The memo then added, "reports of UFOs which could affect national security would continue to be handled through the standard Air Force procedures designed for this purpose." In addition, in the late 1960s a chapter on UFOs in the Space Sciences course at the U.S. Air Force Academy gave serious consideration to possible extraterrestrial origins. When word of the curriculum became public, the Air Force in 1970 issued a statement to the effect that the book was outdated and cadets instead were being informed of the Condon Report's negative conclusion.

Controversy surrounded the report, both before and after its release. It has been observed that the report was "harshly criticized by numerous scientists, particularly at the powerful AIAA ... [which] recommended moderate, but continuous scientific work on UFOs." In an address to the AAAS, James E. McDonald said he believed science had failed to mount adequate studies of the problem and criticized the Condon Report and earlier studies by the USAF as scientifically deficient. He also questioned the basis for Condon's conclusions and argued that the reports of UFOs have been "laughed out of scientific court". J. Allen Hynek, an astronomer who worked as a USAF consultant from 1948, sharply criticized the Condon Committee Report and later wrote two nontechnical books that set forth the case for continuing to investigate UFO reports.

Ruppelt recounted his experiences with Project Blue Book, a USAF investigation that preceded Condon's.

FOIA release of documents in 1978

According to a 1979 New York Times report, "records from the C.I.A., the F.B.I. and other Federal agencies" ("about 900 documents — nearly 900 pages of memos, reports and correspondence") obtained in 1978 through the Freedom of Information Act request, indicate that "despite official pronouncements for decades that U.F.O.'s were nothing more than misidentified aerial objects and as such were no cause for alarm ... the phenomenon has aroused much serious behind‐the‐scenes concern" in the US government. In particular, officials were concerned over the "approximately 10%" of UFO sightings which remained unexplained, and whether they might be Soviet aircraft and a threat to national security. Officials were concerned about the "risk of false alerts", of "falsely identifying the real as phantom”, and of mass hysteria caused by sightings. In 1947, Brigadier General George F. Schulgen of Army Air Corps Intelligence, warned “the first reported sightings might have been by individuals of Communist sympathies with the view to causing hysteria and fear of a secret Russian weapon.”

White House statement of November 2011

In November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited this planet and to disclose any intentional withholding of government interactions with extraterrestrial beings. According to the response:

The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race...no credible information to suggest that any evidence is being hidden from the public's eye....

— Statement by the White House

The response further noted that efforts, like SETI and NASA's Kepler space telescope and Mars Science Laboratory, continue looking for signs of life. The response noted "odds are pretty high" that there may be life on other planets but "the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved."

ODNI report 2021

On June 25, 2021, the Office of the Director of National Intelligence released a report on UAPs. The report found that the UAPTF was unable to identify 143 objects spotted between 2004 and 2021. The report said that 18 of these featured unusual movement patterns or flight characteristics, adding that more analysis was needed to determine if those sightings represented "breakthrough" technology. The report said that "some of these steps are resource-intensive and would require additional investment." The report did not link the sightings to extraterrestrial life.

Uruguay (c. 1989)

The Uruguayan Air Force has conducted UFO investigations since 1989 and reportedly analyzed 2,100 cases of which they regard approximately 2% as lacking explanation.

Europe

France (1977–2008)

In March 2007, the French space agency CNES published an archive of UFO sightings and other phenomena online.

French studies include GEPAN/SEPRA/GEIPAN within CNES (French space agency), the longest ongoing government-sponsored investigation. About 22% of the 6,000 cases studied remain unexplained. The official opinion of GEPAN/SEPRA/GEIPAN has been neutral, stating on their FAQ page that their mission is fact-finding for the scientific community, not rendering an opinion. They add they can neither prove nor disprove the Extraterrestrial Hypothesis (ETH), but their Steering Committee's clear position is that they cannot discard the possibility that some fraction of the very strange 22% of unexplained cases might be due to distant and advanced civilizations. Possibly their bias may be indicated by their use of the terms "PAN" (French) or "UAP" (English equivalent) for "Unidentified Aerospace Phenomenon" (whereas "UAP" is normally used by English organizations stands for "Unidentified Aerial Phenomenon", a more neutral term). In addition, the three heads of the studies have gone on record in stating that UFOs were real physical flying machines beyond our knowledge or that the best explanation for the most inexplicable cases was an extraterrestrial one.

In 2008, Michel Scheller, president of the Association Aéronautique et Astronautique de France (3AF), created the Sigma Commission. Its purpose was to investigate UFO phenomenon worldwide. A progress report published in May 2010 stated that the central hypothesis proposed by the COMETA report is perfectly credible. In December 2012, the final report of the Sigma Commission was submitted to Scheller. Following the submission of the final report, the Sigma2 Commission is to be formed with a mandate to continue the scientific investigation of UFO phenomenon.

Italy (1933–2005)

Alleged UFO sightings gradually increased since the war, peaking in 1978 and 2005. The total number of sightings since 1947 are 18,500, of which 90% are identifiable.

United Kingdom (1951–2009)

The UK's Flying Saucer Working Party published its final report in June 1951, which remained secret for over fifty years. The Working Party concluded that all UFO sightings could be explained as misidentifications of ordinary objects or phenomena, optical illusions, psychological misperceptions/aberrations, or hoaxes. The report stated: "We accordingly recommend very strongly that no further investigation of reported mysterious aerial phenomena be undertaken, unless and until some material evidence becomes available."

Eight file collections on UFO sightings, dating from 1978 to 1987, were first released on May 14, 2008, to The National Archives by the Ministry of Defence (MoD). Although kept secret from the public for many years, most of the files have low levels of classification and none are classified Top Secret. 200 files are set to be made public by 2012. The files are correspondence from the public sent to the British government and officials, such as the MoD and Margaret Thatcher. The MoD released the files under the Freedom of Information Act due to requests from researchers. These files include, but are not limited to, UFOs over Liverpool and Waterloo Bridge in London.

On October 20, 2008, more UFO files were released. One case released detailed that in 1991 an Alitalia passenger aircraft was approaching London Heathrow Airport when the pilots saw what they described as a "cruise missile" fly extremely close to the cockpit. The pilots believed a collision was imminent. UFO expert David Clarke says this is one of the most convincing cases for a UFO he has come across.

A secret study of UFOs was undertaken for the Ministry of Defence between 1996 and 2000 and was code-named Project Condign. The resulting report, titled "Unidentified Aerial Phenomena in the UK Defence Region", was publicly released in 2006, but the identity and credentials of whomever constituted Project Condign remains classified. The report confirmed earlier findings that the main causes of UFO sightings are misidentification of man-made and natural objects. The report noted: "No artefacts of unknown or unexplained origin have been reported or handed to the UK authorities, despite thousands of Unidentified Aerial Phenomena reports. There are no SIGINT, ELINT or radiation measurements and little useful video or still IMINT." It concluded: "There is no evidence that any UAP, seen in the UKADR [UK Air Defence Region], are incursions by air-objects of any intelligent (extraterrestrial or foreign) origin, or that they represent any hostile intent." A little-discussed conclusion of the report was that novel meteorological plasma phenomenon akin to ball lightning are responsible for "the majority, if not all" of otherwise inexplicable sightings, especially reports of black triangle UFOs.

On December 1, 2009, the Ministry of Defence quietly closed down its UFO investigations unit. The unit's hotline and email address were suspended by the MoD on that date. The MoD said there was no value in continuing to receive and investigate sightings in a release, stating that "in over fifty years, no UFO report has revealed any evidence of a potential threat to the United Kingdom. The MoD has no specific capability for identifying the nature of such sightings. There is no Defence benefit in such investigation and it would be an inappropriate use of defence resources. Furthermore, responding to reported UFO sightings diverts MoD resources from tasks that are relevant to Defence." The Guardian reported that the MoD claimed the closure would save the Ministry around £50,000 a year. The MoD said it would continue to release UFO files to the public through The National Archives.

UFO reports, Parlimentary questions, and letters from members of the public were released on August 5, 2010 to the UK National Archives. "In one letter included in the files, a man alleges Churchill ordered a coverup of a WW II-era UFO encounter involving the Royal Air Force".

Studies

Critics argue that all UFO evidence is anecdotal and can be explained as prosaic natural phenomena. Defenders of UFO research counter that knowledge of observational data, other than what is reported in the popular media, is limited in the scientific community and further study is needed. Studies have established that the majority of UFO observations are misidentified conventional objects or natural phenomena—most commonly aircraft, balloons including sky lanterns, satellites, and astronomical objects such as meteors, bright stars and planets. A small percentage are hoaxes. Fewer than 10% of reported sightings remain unexplained after proper investigation and therefore can be classified as unidentified in the strictest sense. According to Steven Novella, proponents of the extraterrestrial hypothesis (ETH) suggest these unexplained reports are of alien spacecraft, however the null hypothesis cannot be excluded; that these reports are simply other more prosaic phenomena that cannot be identified due to lack of complete information or due to the necessary subjectivity of the reports. Novella says that instead of accepting the null hypothesis, UFO enthusiasts tend to engage in special pleading by offering outlandish, untested explanations for the validity of the ETH, which violate Occam's razor.

Scientific

Ufology is not considered credible in mainstream science. The scientific community has generally deemed that UFO sightings are not worthy of serious investigation except as a cultural artifact.

Allen Hynek (left) and Jacques Vallée

Studies of UFOs rarely appear in mainstream scientific literature. When asked, some scientists and scientific organizations have pointed to the end of official governmental studies in the U.S. in December 1969, following the statement by the government scientist Edward Condon that further study of UFOs could not be justified on grounds of scientific advancement.

Jacques Vallée, a scientist and ufologist, claimed there were deficiencies in most UFO research, including government studies. He criticized the mythology and cultism often associated with UFO sightings, but despite the challenges, Vallée contended that several hundred professional scientists — a group both he and Hynek termed "the invisible college" — continued to study UFOs quietly on their own time.

UFOs have become a prevalent theme in modern culture, and the social phenomena have been the subject of academic research in sociology and psychology.

In 2021, astronomer Avi Loeb launched The Galileo Project, intended to collect and report scientific evidence of extraterrestrials or extraterrestrial technology on or near Earth via telescopic observations. While Loeb's initiative does not take a position on the question of whether UFOs were a phenomenon worthy of study, his arguments have been criticized by other scientists for their extravagance.

Sturrock panel categorization

Besides anecdotal visual sightings, reports sometimes include claims of other kinds of evidence, including cases studied by the military and various government agencies of different countries (such as Project Blue Book, the Condon Committee, the French GEPAN/SEPRA, and Uruguay's current Air Force study).

A comprehensive scientific review of cases where physical evidence was available was carried out by the 1998 Sturrock panel, with specific examples of many of the categories listed below.

• Radar contact and tracking, sometimes from multiple sites. These have included military personnel and control tower operators, simultaneous visual sightings, and aircraft intercepts. One such example was the mass sightings of large, silent, low-flying black triangles in 1989 and 1990 over Belgium, tracked by NATO radar and jet interceptors, and investigated by Belgium's military (included photographic evidence). Another famous case from 1986 was the Japan Air Lines flight 1628 incident over Alaska investigated by the Federal Aviation Administration (FAA).
• Photographic evidence, including still photos, movie film, and video.
• Claims of physical trace of landing UFOs, including ground impressions, burned or desiccated soil, burned and broken foliage, magnetic anomalies, increased radiation levels, and metallic traces. (See, e. g. Height 611 UFO incident or the 1964 Lonnie Zamora's Socorro, New Mexico encounter of the USAF Project Blue Book cases.) A well-known example from December 1980 was the USAF Rendlesham Forest incident in England. Another occurred in January 1981 in Trans-en-Provence and was investigated by GEPAN, then France's official government UFO-investigation agency. Project Blue Book head Edward J. Ruppelt described a classic 1952 CE2 case involving a patch of charred grass roots.
• Physiological effects on people and animals including temporary paralysis, skin burns and rashes, corneal burns, and symptoms superficially resembling radiation poisoning, such as the Cash-Landrum incident in 1980.
• Animal/cattle mutilation cases, which some feel are also part of the UFO phenomenon.
• Biological effects on plants such as increased or decreased growth, germination effects on seeds, and blown-out stem nodes (usually associated with physical trace cases or crop circles)
• Electromagnetic interference (EM) effects. A famous 1976 military case over Tehran, recorded in CIA and DIA classified documents, was associated with communication losses in multiple aircraft and weapons system failure in an F-4 Phantom II jet interceptor as it was about to fire a missile on one of the UFOs.
• Apparent remote radiation detection, some noted in FBI and CIA documents occurring over government nuclear installations at Los Alamos National Laboratory and Oak Ridge National Laboratory in 1950, also reported by Project Blue Book director Edward J. Ruppelt in his book.
• Claimed artifacts of UFOs themselves, such as 1957, Ubatuba, Brazil, magnesium fragments analyzed by the Brazilian government and in the Condon Report and by others. The 1964 Lonnie Zamora incident also left metal traces, analyzed by NASA. A more recent example involves a teardrop-shaped object recovered by Bob White and was featured in a television episode of UFO Hunters but was later found to be waste metal residue from a milling machine.
• Angel hair and angel grass, possibly explained in some cases as nests from ballooning spiders or chaff.

Scientific skepticism

A scientifically skeptical group that has for many years offered critical analyses of UFO claims is the Committee for Skeptical Inquiry (CSI).

One example is the response to local beliefs that "extraterrestrial beings" in UFOs were responsible for crop circles appearing in Indonesia, which the government and the National Institute of Aeronautics and Space (LAPAN) described as "man-made". Thomas Djamaluddin, research professor of astronomy and astrophysics at LAPAN stated: "We have come to agree that this 'thing' cannot be scientifically proven. Scientists have put UFOs in the category of pseudoscience."

Governmental

UFO drawing, authenticity unknown, attribution and date unspecified. One of hundreds of files resulting from US President Bill Clinton's 1995 order to the CIA to declassify all documents with “historical value” that were at least 25 years old.

UFOs have been the subject of investigations by various governments who have provided extensive records related to the subject. Many of the most involved government-sponsored investigations ended after agencies concluded that there was no benefit to continued investigation. These same negative conclusions also have been found in studies that were highly classified for many years, such as the UK's Flying Saucer Working Party, Project Condign, the U.S. CIA-sponsored Robertson Panel, the U.S. military investigation into the green fireballs from 1948 to 1951, and the Battelle Memorial Institute study for the USAF from 1952 to 1955 (Project Blue Book Special Report No. 14). Some public government reports have acknowledged the possibility of the physical reality of UFOs, but have stopped short of proposing extraterrestrial origins, though not dismissing the possibility entirely. Examples are the Belgian military investigation into large triangles over their airspace in 1989–1991 and the 2009 Uruguayan Air Force study conclusion (see below).

Claims by military, government, and aviation personnel

In 2007, former Arizona governor Fife Symington claimed he had seen "a massive, delta-shaped craft silently navigate over Squaw Peak, a mountain range in Phoenix, Arizona" in 1997. Apollo 14 astronaut Dr. Edgar Mitchell claimed he knew of senior government employees who had been involved in "close encounters", and because of this, he has no doubt that aliens have visited Earth.

In May 2019, The New York Times reported that American Navy fighter jets had several instances of unidentified instrumentation and tracking data while conducting exercises off the eastern seaboard of the United States from the summer of 2014 to March 2015. The Times published a cockpit instrument video which appeared to show an object moving at high speed near the ocean surface as it appeared to rotate, and objects that appeared capable of high acceleration, deceleration and maneuverability. In two separate incidents, a pilot reported his cockpit instruments locked onto and tracked objects but he was unable to see them through his helmet camera. In another encounter, flight instruments recorded an image described as a sphere encasing a cube between two jets as they flew about 100 feet apart. The Pentagon officially released these videos on April 27, 2020. The United States Navy has said there have been "a number of reports of unauthorized and/or unidentified aircraft entering various military-controlled ranges and designated air space in recent years".

2021 Pentagon UFO Report

In March 2021, news media announced a comprehensive report is to be compiled of UFO events accumulated by the United States over the years.

On April 12, 2021, the Pentagon confirmed the authenticity of pictures and videos gathered by the Unidentified Aerial Phenomena Task Force (UAPTF), purportedly showing "pyramid shaped objects" hovering above the USS Russell in 2019, off the coast of California, with spokeswoman Susan Gough saying "I can confirm that the referenced photos and videos were taken by Navy personnel. The UAPTF has included these incidents in their ongoing examinations." In May 2021, military pilots recalled their related encounters, along with camera and radar support, including one pilot's account noting that such incidents occurred "every day for at least a couple of years", according to an interview broadcast on the news program, 60 Minutes (16 May 2021). Science writer and skeptic Mick West suggested the image was the result of an optical effect called a bokeh which can make out of focus light sources appear triangular or pyramidal due to the shape of the aperture of some lenses.

On June 25, 2021, U.S. Defense and intelligence officials released the Pentagon UFO Report on what they know about a series of unidentified flying objects that have been seen by American military pilots. NASA Administrator Bill Nelson said that the UFO sightings by pilots "may not be extraterrestrial."

Conspiracy theories

See also: UFO conspiracy theory, Steven M. Greer, Men in black, and Brookings Report

UFOs are sometimes an element of conspiracy theories in which governments are allegedly intentionally "covering up" the existence of aliens by removing physical evidence of their presence or even collaborating with extraterrestrial beings. There are many versions of this story; some are exclusive, while others overlap with various other conspiracy theories.

In the U.S., an opinion poll conducted in 1997 suggested that 80% of Americans believed the U.S. government was withholding such information. Various notables have also expressed such views. Some examples are astronauts Gordon Cooper and Edgar Mitchell, Senator Barry Goldwater, Vice Admiral Roscoe H. Hillenkoetter (the first CIA director), Lord Hill-Norton (former British Chief of Defense Staff and NATO head), the 1999 French COMETA study by various French generals and aerospace experts, and Yves Sillard (former director of CNES, new director of French UFO research organization GEIPAN).

It has also been suggested by a few paranormal authors that all or most human technology and culture is based on extraterrestrial contact (see also ancient astronauts).

"Disclosure" advocates

In May 2001, a press conference was held at the National Press Club in Washington, D.C., by an organization called the Disclosure Project, featuring twenty persons including retired Air Force and FAA personnel, intelligence officers and an air traffic controller. They all gave a brief account of their claims that evidence of UFOs was being supressed and said they would be willing to testify under oath to a Congressional committee. According to a 2002 report in the Oregon Daily Emerald, Disclosure Project founder Steven M. Greer is an "alien theorist" who claims "proof of government coverup" consisting of 120 hours of testimony from various government officials on the topic of UFOs, including astronaut Gordon Cooper.

On September 27, 2010, a group of six former USAF officers and one former enlisted Air Force man held a press conference at the National Press Club in Washington, D.C., on the theme "U.S. Nuclear Weapons Have Been Compromised by Unidentified Aerial Objects" in which they claimed they had witnessed UFOs hovering near missile sites and even disarming the missiles.

From April 29 to May 3, 2013, the Paradigm Research Group held the "Citizen Hearing on Disclosure" at the National Press Club. The group paid former U.S. Senator Mike Gravel and former Representatives Carolyn Cheeks Kilpatrick, Roscoe Bartlett, Merrill Cook, Darlene Hooley, and Lynn Woolsey $20,000 each to hear testimony from a panel of researchers which included witnesses from military, agency, and political backgrounds.

Fringe

The void left by the lack of institutional or scientific study has given rise to independent researchers and fringe groups, including the National Investigations Committee on Aerial Phenomena (NICAP) in the mid-20th century and, more recently, the Mutual UFO Network (MUFON) and the Center for UFO Studies (CUFOS). The term "Ufology" is used to describe the collective efforts of those who study reports and associated evidence of unidentified flying objects.

Private

Some private studies have been neutral in their conclusions but argued that the inexplicable core cases call for continued scientific study. Examples are the Sturrock panel study of 1998 and the 1970 AIAA review of the Condon Report.

Ufology

Main article: Ufology

Photograph of an unusual atmospheric occurrence observed over Sri Lanka, forwarded to the UK Ministry of Defence by RAF Fylingdales, 2004

Ufology is a neologism describing the collective efforts of those who study UFO reports and associated evidence.

Researchers

Main article: List of ufologists

Sightings

Main article: List of reported UFO sightings

Organizations

Main article: List of UFO organizations

Categorization

Some ufologists recommend that observations be classified according to the features of the phenomenon or object that are reported or recorded. Typical categories include:

• Saucer, toy-top, or disk-shaped "craft" without visible or audible propulsion.
• Large triangular "craft" or triangular light pattern, usually reported at night.
• Cigar-shaped "craft" with lighted windows (meteor fireballs are sometimes reported this way, but are very different phenomena).
• Other: chevrons, (equilateral) triangles, crescent, boomerangs, spheres (usually reported to be shining, glowing at night), domes, diamonds, shapeless black masses, eggs, pyramids and cylinders, classic "lights".

Popular UFO classification systems include the Hynek system, created by J. Allen Hynek, and the Vallée system, created by Jacques Vallée.

Hynek's system involves dividing the sighted object by appearance, subdivided further into the type of "close encounter" (a term from which the film director Steven Spielberg derived the title of his 1977 UFO movie, Close Encounters of the Third Kind).

Jacques Vallée's system classifies UFOs into five broad types, each with from three to five subtypes that vary according to type.

In popular culture

Main article: UFOs in fiction

A UFO monument at Tenjo, Colombia

UFOs have constituted a widespread international cultural phenomenon since the 1950s. Gallup Polls rank UFOs near the top of lists for subjects of widespread recognition. In 1973, a survey found that 95 percent of the public reported having heard of UFOs, whereas only 92 percent had heard of U.S. President Gerald Ford in a 1977 poll taken just nine months after he left the White House. A 1996 Gallup Poll reported that 71 percent of the United States population believed the U.S. government was covering up information regarding UFOs. A 2002 Roper Poll for the Sci-Fi Channel found similar results, but with more people believing UFOs are extraterrestrial craft. In that latest poll, 56 percent thought UFOs were real craft and 48 percent that aliens had visited the Earth. Again, about 70 percent felt the government was not sharing everything it knew about UFOs or extraterrestrial life.

Another effect of the flying saucer type of UFO sightings has been Earth-made flying saucer craft in space fiction, for example the United Planets Cruiser C57D in Forbidden Planet (1956), the Jupiter 2 in Lost in Space, and the saucer section of the USS Enterprise in Star Trek. UFOs and extraterrestrials have been featured in many movies.

The intense secrecy surrounding the secret Nevada base, known as Area 51, has made it the frequent subject of conspiracy theories and a central component of UFO folklore. In July 2019, more than 2 million people responded to a joke proposal to storm Area 51 which appeared in an anonymous Facebook post. Two music festivals in rural Nevada, "AlienStock" and "Storm Area 51 Basecamp", were subsequently organized to capitalize on the popularity of the original Facebook event.

Space travel in science fiction

From Wikipedia, the free encyclopedia

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

 

Rocket on cover of Other Worlds sci-fi magazine, September 1951

Space travel, or space flight (less often, starfaring or star voyaging) is a classic science-fiction theme that has captivated the public and is almost archetypal for science fiction. Space travel, interplanetary or interstellar, is usually performed in space ships, and spacecraft propulsion in various works ranges from the scientifically plausible to the totally fictitious.

While some writers focus on realistic, scientific, and educational aspects of space travel, for others the concept may be seen as a metaphor for "free[ing] mankind from the prison of the solar system". Though the science-fiction rocket has been described as a 20th-century icon, according to The Encyclopedia of Science Fiction "The means by which space flight has been achieved in sf – its many and various spaceships – have always been of secondary importance to the mythical impact of the theme". Works related to space travel have popularized such concepts as time dilation, space stations, and space colonization.

While generally associated with science fiction, space travel – involving magic or supernatural entities such as angels – has also occasionally featured in fantasy.

History

Science and Mechanics, November 1931, showing a proposed sub-orbital spaceship that would reach a 700-mile altitude on a one-hour flight from Berlin to New York

 

Still from Lost in Space TV series premiere (1965), depicting space travelers in suspended animation

A classic, defining trope of the science-fiction genre is that the action takes place in space, either aboard a spaceship or on another planet. Early works of science fiction, called "proto SF" – such as novels by 17th-century writers Francis Godwin and Cyrano de Bergerac, and astronomer Johannes Kepler – include "lunar romances", much of whose action takes place on the Moon. Science-fiction critic George Slusser also pointed to Christopher Marlowe's Doctor Faustus (1604), in which the main character is able to see the entire earth from high above, and noted the connections of space travel to earlier dreams of flying and air travel, seen as far back as the writings of Plato and Socrates. In such a grand view, space travel and inventions such as various forms of "star drive" can be seen as metaphors for freedom, one of whose aspects is to "free mankind from the prison of the solar system".

In the ensuing centuries, while science fiction addressed many aspects of futuristic science in addition to space travel, the latter proved the most influential with the general public and the genre's writers and readers, evoking their sense of wonder. Most works were mainly intended to amuse readers, but a small number, often by authors with a scholarly background, strove to educate readers about various aspects of space-related science, including concepts from astronomy; the latter aspiration was shown by, among others, the influential American editor Hugo Gernsback, who dubbed this approach "sugar-coated science" and "scientification". Science-fiction magazines, including Gernsback's Science Wonder Stories, alongside works of pure fiction, popularized early studies and discussed the feasibility of space travel; many science-fiction writers published nonfiction on space travel, such as Willy Ley's articles and David Lasser's book, The Conquest of Space (1931).

Roadside replica of Star Trek starship Enterprize

From the late 19th and early 20th centuries onward there was a visible distinction between more "realistic" and scientific fiction (which later would evolve into hard sf), whose writers, often scientists like Konstantin Tsiolkovsky and Max Valier, focused on the more plausible concept of interplanetary travel (to the Moon or Mars), whereas the more grandiose but less grounded in realism were the stories of "escape from Earth into a Universe filled with worlds", which gave rise the genre of space opera, pioneered by E. E. Smith, popularized by the televisions series Star Trek that debuted in 1966. This trend continues to the present day, with some works focusing on "the myth of space flight", others are centered on "realistic examination of space flight"; the difference can be also described as that between the writers' concern on the "imaginative horizons rather than hardware". The success of the 20th century space program, such as the Apollo 11 Moon landing, was often described as "science fiction come true", and it has also served to further "demystify" the concept of space travel within the solar system, as from that point onward, the writers who wanted to focus on the "myth of space travel" were increasingly likely to do so through the concept of interstellar travel. While the theme of space travel is seen as one of the very optimistic in general, some revaluatory stories, often more pessimistic and disillusioned, juxtapose both of those trends, contrasting the romantic myth of space travel with a more down-to-earth reality. George Slusser suggested that "science fiction travel since World War II has mirrored the United States space program: anticipation in the 1950s and early 1960s, euphoria into the 1970s, modulating into skepticism and gradual withdrawal since the 1980s."

On the screen the French movie 1902 A Trip to the Moon, by Georges Méliès, also described as the first science fiction film, linked special effects to the depictions of spaceflight. Together with other early movies such as Woman in the Moon (1929), Thing to Come (1936) they contributed to the early recognition of the concept of the rocket as the iconic and primary means of space travel, decades before the space program begun in earnest. Later milestones in film and television include the Star Trek series and movies, and the 2001: A Space Odyssey film by Stanley Kubrick (1968), which visually advanced the concept of space travel, allowing it to evolve away from the simplistic rocket towards that of a more complex space ship.

Methods of travel

Generic terms for engines enabling spacecraft propulsion in science fiction include those of a space drive and star drive. In 1977 The Visual Encyclopedia of Science Fiction listed the following methods of space travel: anti-gravity, atomic (nuclear), bloater, cannon one-shot, Dean, faster-than-light (FTL), hyperspace, inertialess, ion, photon, plasma, ram-jet, R. force, solar sail, spindizzy and torchship. The 2007 Brave New Words: The Oxford Dictionary of Science Fiction listed the following vocabulary related to the concept of a space drive: gravity drive, hyperdrive, ion drive, jump drive, overdrive, ramscoop (a synonym of a ram-jet), reaction drive, stargate, ultradrive, warp drive and the torchdrive. Several of those terms are entirely fictitious or based on rubber science, while others are based on real scientific theories. Many fictitious ways of travelling through space, in particular, faster than light travel, tend to go against our current understanding of physics, in particular, the theory of relativity. Some works can sport numerous alternative star drives; for example the Star Trek universe, in addition to its iconic warp drive, has introduced concepts such as transwarp, slipstream and the spore drive, among others.

Many writers, particularly of early science fiction, did not address the method of travel in much detail, and many works of the "proto SF" era had to contend with their authors living in the time where knowledge about the "space" was very limited — in fact, many early works did not even consider the concept of vacuum and instead assumed that atmosphere of sorts, composed of air or aether, continues indefinitely. Highly influential in popularizing the "science" part of the science fiction was the 19th-century French writer Jules Verne, whose method of space travel in his 1865 novel, From the Earth to the Moon (and its sequel, Around the Moon), was explained mathematically, and whose vehicle — the gun-launched space capsule — was described as the first such vehicle to be "scientifically conceived" in fiction. Percy Greg's Across the Zodiac (1880) featured a space ship with a small garden, an early prediction of hydroponics. Another writer who attempted to merge concrete scientific ideas with science fiction prose was the turn of the century Russian writer and scientist, Konstantin Tsiolkovsky, known for popularizing the concept of rocketry. George Mann pointed to works such as Robert A. Heinlein's Rocket Ship Galileo (1947) and Arthur C. Clarke's Prelude to Space (1951) as some of the early, influential modern works that focused on the scientific and engineering aspects of space travel. From the 1960s onward, the growing popularity of modern technology with the public also led to increasing depictions of interplanetary spaceships as based on the advanced but plausible extension of real, modern technology.

Interstellar travel

Slower than light

With regard to interstellar travel, in which faster-than-light speeds are mostly considered unrealistic, more realistic depictions of the theme have often focused on the idea of "generation ships" that travel at sub-light speed for many generations before arriving at their destination. Other scientifically plausible concepts of interstellar travel include suspended animation and, less often, ion drive, solar sail, Bussard ramjet, and time dilation.

Faster than light

Some works involve discussion of Einstein's general theory of relativity and challenges it faces from the quantum mechanics, and include concepts of space travel through wormholes or black holes. Many writers, however, gloss over such problems, introducing entirely fictional concepts such as hyperspace (also, subspace, nulspace, overspace, jumpspace or slipstream) travel using inventions such as hyperdrive, jump drive, the warp drive or space folding. Invention of completely made up devices enabling space travel has a long tradition — already in the early 20th century, Verne criticized H. G. Wells' The First Men in the Moon (1901) for abandoning realistic science (his spaceship relied on anti-gravitic material called "cavorite"). Out of the fictitious drives, by mid-70s the concept of travelling through hyperspace has been described as having achieved the most popularity, and would subsequently be further popularized — as hyperdrive — through its use in the Star Wars franchise. While the fictitious drives "solve" many problems related to physics as understood today (namely, the difficulty of faster-than-light travel), some writers introduce new wrinkles — for example, a common trope involves the difficulty of using such drives in close proximity to other objects, in some cases allowing their use only from the outskirts of solar systems.

While usually the method of traveling through space is just a means to an end, for some works, particularly short stories, it is a central plot device. Such works focus on themes such as the mysteries of hyperspace, or the consequences of getting lost after an error or malfunction.

Faster-than-light

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Faster-than-light 

Spacetime diagram showing that moving faster than light implies time travel in the context of special relativity

Faster-than-light (also FTL, superluminal or supercausal) communications and travel are the conjectural propagation of information or matter faster than the speed of light (c). The special theory of relativity implies that only particles with zero rest mass (i.e., photons) may travel at the speed of light, and that nothing may travel faster.

Particles whose speed exceeds that of light (tachyons) have been hypothesized, but their existence would violate causality, and the consensus of physicists is that they do not exist, and their existence would imply time travel. "Apparent" or "effective" FTL, on the other hand, depends on the hypothesis that unusually distorted regions of spacetime might permit matter to reach distant locations in less time than light could in normal ("undistorted") spacetime.

As of the 21st century, according to current scientific theories, matter is required to travel at slower-than-light (also STL or subluminal) speed with respect to the locally distorted spacetime region. Apparent FTL is not excluded by general relativity; however, any apparent FTL physical plausibility is currently speculative. Examples of apparent FTL proposals are the Alcubierre drive, Krasnikov tubes, traversable wormholes, and quantum tunneling.

Superluminal travel of non-information

Main article: Superluminal motion

In the context of this article, FTL is the transmission of information or matter faster than c, a constant equal to the speed of light in vacuum, which is 299,792,458 m/s (by definition of the metre) or about 186,282.397 miles per second. This is not quite the same as traveling faster than light, since:

• Some processes propagate faster than c, but cannot carry information (see examples in the sections immediately following).
• In some materials where light travels at speed c/n (where n is the refractive index) other particles can travel faster than c/n (but still slower than c), leading to Cherenkov radiation (see phase velocity below).

Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifies as FTL as described here.

In the following examples, certain influences may appear to travel faster than light, but they do not convey energy or information faster than light, so they do not violate special relativity.

Daily sky motion

For an earth-bound observer, objects in the sky complete one revolution around the Earth in one day. Proxima Centauri, the nearest star outside the Solar System, is about four light-years away. In this frame of reference, in which Proxima Centauri is perceived to be moving in a circular trajectory with a radius of four light years, it could be described as having a speed many times greater than c as the rim speed of an object moving in a circle is a product of the radius and angular speed. It is also possible on a geostatic view, for objects such as comets to vary their speed from subluminal to superluminal and vice versa simply because the distance from the Earth varies. Comets may have orbits which take them out to more than 1000 AU. The circumference of a circle with a radius of 1000 AU is greater than one light day. In other words, a comet at such a distance is superluminal in a geostatic, and therefore non-inertial, frame.

Light spots and shadows

If a laser beam is swept across a distant object, the spot of laser light can easily be made to move across the object at a speed greater than c. Similarly, a shadow projected onto a distant object can be made to move across the object faster than c. In neither case does the light travel from the source to the object faster than c, nor does any information travel faster than light.

Closing speeds

The rate at which two objects in motion in a single frame of reference get closer together is called the mutual or closing speed. This may approach twice the speed of light, as in the case of two particles travelling at close to the speed of light in opposite directions with respect to the reference frame.

Imagine two fast-moving particles approaching each other from opposite sides of a particle accelerator of the collider type. The closing speed would be the rate at which the distance between the two particles is decreasing. From the point of view of an observer standing at rest relative to the accelerator, this rate will be slightly less than twice the speed of light.

Special relativity does not prohibit this. It tells us that it is wrong to use Galilean relativity to compute the velocity of one of the particles, as would be measured by an observer traveling alongside the other particle. That is, special relativity gives the correct velocity-addition formula for computing such relative velocity.

It is instructive to compute the relative velocity of particles moving at v and −v in accelerator frame, which corresponds to the closing speed of 2v > c. Expressing the speeds in units of c, β = v/c:

${\displaystyle \beta _{\text{rel}}={\frac {\beta +\beta }{1+\beta ^{2}}}={\frac {2\beta }{1+\beta ^{2}}}\leq 1.}$

Proper speeds

If a spaceship travels to a planet one light-year (as measured in the Earth's rest frame) away from Earth at high speed, the time taken to reach that planet could be less than one year as measured by the traveller's clock (although it will always be more than one year as measured by a clock on Earth). The value obtained by dividing the distance traveled, as determined in the Earth's frame, by the time taken, measured by the traveller's clock, is known as a proper speed or a proper velocity. There is no limit on the value of a proper speed as a proper speed does not represent a speed measured in a single inertial frame. A light signal that left the Earth at the same time as the traveller would always get to the destination before the traveller.

Possible distance away from Earth

Main article: Space travel using constant acceleration

Since one might not travel faster than light, one might conclude that a human can never travel further from the Earth than 40 light-years if the traveler is active between the age of 20 and 60. A traveler would then never be able to reach more than the very few star systems which exist within the limit of 20–40 light-years from the Earth. This is a mistaken conclusion: because of time dilation, the traveler can travel thousands of light-years during their 40 active years. If the spaceship accelerates at a constant 1 g (in its own changing frame of reference), it will, after 354 days, reach speeds a little under the speed of light (for an observer on Earth), and time dilation will increase the traveler's lifespan to thousands of Earth years, seen from the reference system of the Solar System ⁠— but the traveler's subjective lifespan will not thereby change. If they were then to return to Earth, the traveler would arrive on Earth thousands of years into the future. Their travel speed would not have been observed from Earth as being supraluminal ⁠— neither for that matter would it appear to be so from the traveler's perspective– but the traveler would instead have experienced a length contraction of the universe in their direction of travel. After the traveler reverses course, the Earth will seem to experience much more time passing than the traveler does. So while the traveler's (ordinary) coordinate speed cannot exceed c, their proper speed, or distance traveled from the Earth's point of reference divided by proper time, can be much greater than c. This is seen in statistical studies of muons traveling much further than c times their half-life (at rest), if traveling close to c.

Phase velocities above c

The phase velocity of an electromagnetic wave, when traveling through a medium, can routinely exceed c, the vacuum velocity of light. For example, this occurs in most glasses at X-ray frequencies. However, the phase velocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely monochromatic) component of the wave at that frequency. Such a wave component must be infinite in extent and of constant amplitude (otherwise it is not truly monochromatic), and so cannot convey any information. Thus a phase velocity above c does not imply the propagation of signals with a velocity above c.

Group velocities above c

The group velocity of a wave may also exceed c in some circumstances. In such cases, which typically at the same time involve rapid attenuation of the intensity, the maximum of the envelope of a pulse may travel with a velocity above c. However, even this situation does not imply the propagation of signals with a velocity above c, even though one may be tempted to associate pulse maxima with signals. The latter association has been shown to be misleading, because the information on the arrival of a pulse can be obtained before the pulse maximum arrives. For example, if some mechanism allows the full transmission of the leading part of a pulse while strongly attenuating the pulse maximum and everything behind (distortion), the pulse maximum is effectively shifted forward in time, while the information on the pulse does not come faster than c without this effect. However, group velocity can exceed c in some parts of a Gaussian beam in vacuum (without attenuation). The diffraction causes the peak of the pulse to propagate faster, while overall power does not.

Universal expansion

History of the Universe - gravitational waves are hypothesized to arise from cosmic inflation, a faster-than-light expansion just after the Big Bang.

The expansion of the universe causes distant galaxies to recede from us faster than the speed of light, if proper distance and cosmological time are used to calculate the speeds of these galaxies. However, in general relativity, velocity is a local notion, so velocity calculated using comoving coordinates does not have any simple relation to velocity calculated locally. (See Comoving and proper distances for a discussion of different notions of 'velocity' in cosmology.) Rules that apply to relative velocities in special relativity, such as the rule that relative velocities cannot increase past the speed of light, do not apply to relative velocities in comoving coordinates, which are often described in terms of the "expansion of space" between galaxies. This expansion rate is thought to have been at its peak during the inflationary epoch thought to have occurred in a tiny fraction of the second after the Big Bang (models suggest the period would have been from around 10⁻³⁶ seconds after the Big Bang to around 10⁻³³ seconds), when the universe may have rapidly expanded by a factor of around 10²⁰ to 10³⁰.

There are many galaxies visible in telescopes with red shift numbers of 1.4 or higher. All of these are currently traveling away from us at speeds greater than the speed of light. Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.

However, because the expansion of the universe is accelerating, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future, because the light never reaches a point where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Comoving and proper distances#Uses of the proper distance). The current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light-years away, but the signal would never reach us if the event was more than 16 billion light-years away.

Astronomical observations

Apparent superluminal motion is observed in many radio galaxies, blazars, quasars, and recently also in microquasars. The effect was predicted before it was observed by Martin Rees and can be explained as an optical illusion caused by the object partly moving in the direction of the observer, when the speed calculations assume it does not. The phenomenon does not contradict the theory of special relativity. Corrected calculations show these objects have velocities close to the speed of light (relative to our reference frame). They are the first examples of large amounts of mass moving at close to the speed of light. Earth-bound laboratories have only been able to accelerate small numbers of elementary particles to such speeds.

Quantum mechanics

Certain phenomena in quantum mechanics, such as quantum entanglement, might give the superficial impression of allowing communication of information faster than light. According to the no-communication theorem these phenomena do not allow true communication; they only let two observers in different locations see the same system simultaneously, without any way of controlling what either sees. Wavefunction collapse can be viewed as an epiphenomenon of quantum decoherence, which in turn is nothing more than an effect of the underlying local time evolution of the wavefunction of a system and all of its environment. Since the underlying behavior does not violate local causality or allow FTL communication, it follows that neither does the additional effect of wavefunction collapse, whether real or apparent.

The uncertainty principle implies that individual photons may travel for short distances at speeds somewhat faster (or slower) than c, even in vacuum; this possibility must be taken into account when enumerating Feynman diagrams for a particle interaction. However, it was shown in 2011 that a single photon may not travel faster than c. In quantum mechanics, virtual particles may travel faster than light, and this phenomenon is related to the fact that static field effects (which are mediated by virtual particles in quantum terms) may travel faster than light (see section on static fields above). However, macroscopically these fluctuations average out, so that photons do travel in straight lines over long (i.e., non-quantum) distances, and they do travel at the speed of light on average. Therefore, this does not imply the possibility of superluminal information transmission.

There have been various reports in the popular press of experiments on faster-than-light transmission in optics — most often in the context of a kind of quantum tunnelling phenomenon. Usually, such reports deal with a phase velocity or group velocity faster than the vacuum velocity of light. However, as stated above, a superluminal phase velocity cannot be used for faster-than-light transmission of information.

Hartman effect

Main article: Hartman effect

The Hartman effect is the tunneling effect through a barrier where the tunneling time tends to a constant for large barriers. This could, for instance, be the gap between two prisms. When the prisms are in contact, the light passes straight through, but when there is a gap, the light is refracted. There is a non-zero probability that the photon will tunnel across the gap rather than follow the refracted path. For large gaps between the prisms the tunnelling time approaches a constant and thus the photons appear to have crossed with a superluminal speed.

However, the Hartman effect cannot actually be used to violate relativity by transmitting signals faster than c, because the tunnelling time "should not be linked to a velocity since evanescent waves do not propagate". The evanescent waves in the Hartman effect are due to virtual particles and a non-propagating static field, as mentioned in the sections above for gravity and electromagnetism.

Casimir effect

Main article: Casimir effect

In physics, the Casimir–Polder force is a physical force exerted between separate objects due to resonance of vacuum energy in the intervening space between the objects. This is sometimes described in terms of virtual particles interacting with the objects, owing to the mathematical form of one possible way of calculating the strength of the effect. Because the strength of the force falls off rapidly with distance, it is only measurable when the distance between the objects is extremely small. Because the effect is due to virtual particles mediating a static field effect, it is subject to the comments about static fields discussed above.

EPR paradox

Main article: EPR paradox

The EPR paradox refers to a famous thought experiment of Albert Einstein, Boris Podolsky and Nathan Rosen that was realized experimentally for the first time by Alain Aspect in 1981 and 1982 in the Aspect experiment. In this experiment, the measurement of the state of one of the quantum systems of an entangled pair apparently instantaneously forces the other system (which may be distant) to be measured in the complementary state. However, no information can be transmitted this way; the answer to whether or not the measurement actually affects the other quantum system comes down to which interpretation of quantum mechanics one subscribes to.

An experiment performed in 1997 by Nicolas Gisin has demonstrated non-local quantum correlations between particles separated by over 10 kilometers. But as noted earlier, the non-local correlations seen in entanglement cannot actually be used to transmit classical information faster than light, so that relativistic causality is preserved. The situation is akin to sharing a synchronized coin flip, where the second person to flip their coin will always see the opposite of what the first person sees, but neither has any way of knowing whether they were the first or second flipper, without communicating classically. See No-communication theorem for further information. A 2008 quantum physics experiment also performed by Nicolas Gisin and his colleagues has determined that in any hypothetical non-local hidden-variable theory, the speed of the quantum non-local connection (what Einstein called "spooky action at a distance") is at least 10,000 times the speed of light.

Delayed choice quantum eraser

Main article: Delayed-choice quantum eraser

The delayed-choice quantum eraser is a version of the EPR paradox in which the observation (or not) of interference after the passage of a photon through a double slit experiment depends on the conditions of observation of a second photon entangled with the first. The characteristic of this experiment is that the observation of the second photon can take place at a later time than the observation of the first photon, which may give the impression that the measurement of the later photons "retroactively" determines whether the earlier photons show interference or not, although the interference pattern can only be seen by correlating the measurements of both members of every pair and so it can't be observed until both photons have been measured, ensuring that an experimenter watching only the photons going through the slit does not obtain information about the other photons in an FTL or backwards-in-time manner.

Superluminal communication

Main article: Faster-than-light communication

Faster-than-light communication is, according to relativity, equivalent to time travel. What we measure as the speed of light in vacuum (or near vacuum) is actually the fundamental physical constant c. This means that all inertial and, for the coordinate speed of light, non-inertial observers, regardless of their relative velocity, will always measure zero-mass particles such as photons traveling at c in vacuum. This result means that measurements of time and velocity in different frames are no longer related simply by constant shifts, but are instead related by Poincaré transformations. These transformations have important implications:

• The relativistic momentum of a massive particle would increase with speed in such a way that at the speed of light an object would have infinite momentum.
• To accelerate an object of non-zero rest mass to c would require infinite time with any finite acceleration, or infinite acceleration for a finite amount of time.
• Either way, such acceleration requires infinite energy.
• Some observers with sub-light relative motion will disagree about which occurs first of any two events that are separated by a space-like interval. In other words, any travel that is faster-than-light will be seen as traveling backwards in time in some other, equally valid, frames of reference, or need to assume the speculative hypothesis of possible Lorentz violations at a presently unobserved scale (for instance the Planck scale). Therefore, any theory which permits "true" FTL also has to cope with time travel and all its associated paradoxes, or else to assume the Lorentz invariance to be a symmetry of thermodynamical statistical nature (hence a symmetry broken at some presently unobserved scale).
• In special relativity the coordinate speed of light is only guaranteed to be c in an inertial frame; in a non-inertial frame the coordinate speed may be different from c. In general relativity no coordinate system on a large region of curved spacetime is "inertial", so it is permissible to use a global coordinate system where objects travel faster than c, but in the local neighborhood of any point in curved spacetime we can define a "local inertial frame" and the local speed of light will be c in this frame, with massive objects moving through this local neighborhood always having a speed less than c in the local inertial frame.

Justifications

Casimir vacuum and quantum tunnelling

Special relativity postulates that the speed of light in vacuum is invariant in inertial frames. That is, it will be the same from any frame of reference moving at a constant speed. The equations do not specify any particular value for the speed of light, which is an experimentally determined quantity for a fixed unit of length. Since 1983, the SI unit of length (the meter) has been defined using the speed of light.

The experimental determination has been made in vacuum. However, the vacuum we know is not the only possible vacuum which can exist. The vacuum has energy associated with it, called simply the vacuum energy, which could perhaps be altered in certain cases. When vacuum energy is lowered, light itself has been predicted to go faster than the standard value c. This is known as the Scharnhorst effect. Such a vacuum can be produced by bringing two perfectly smooth metal plates together at near atomic diameter spacing. It is called a Casimir vacuum. Calculations imply that light will go faster in such a vacuum by a minuscule amount: a photon traveling between two plates that are 1 micrometer apart would increase the photon's speed by only about one part in 10³⁶. Accordingly, there has as yet been no experimental verification of the prediction. A recent analysis argued that the Scharnhorst effect cannot be used to send information backwards in time with a single set of plates since the plates' rest frame would define a "preferred frame" for FTL signalling. However, with multiple pairs of plates in motion relative to one another the authors noted that they had no arguments that could "guarantee the total absence of causality violations", and invoked Hawking's speculative chronology protection conjecture which suggests that feedback loops of virtual particles would create "uncontrollable singularities in the renormalized quantum stress-energy" on the boundary of any potential time machine, and thus would require a theory of quantum gravity to fully analyze. Other authors argue that Scharnhorst's original analysis, which seemed to show the possibility of faster-than-c signals, involved approximations which may be incorrect, so that it is not clear whether this effect could actually increase signal speed at all.

The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Cologne, claim to have violated relativity experimentally by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons — relatively low-energy packets of light — travelled "instantaneously" between a pair of prisms that had been moved up to 3 ft (1 m) apart. Their experiment involved an optical phenomenon known as "evanescent modes", and they claim that since evanescent modes have an imaginary wave number, they represent a "mathematical analogy" to quantum tunnelling. Nimtz has also claimed that "evanescent modes are not fully describable by the Maxwell equations and quantum mechanics have to be taken into consideration." Other scientists such as Herbert G. Winful and Robert Helling have argued that in fact there is nothing quantum-mechanical about Nimtz's experiments, and that the results can be fully predicted by the equations of classical electromagnetism (Maxwell's equations).

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 from the tail at each station along the way, so that the center of the ever-shrinking main 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.

Winful argues that the train analogy is a variant of the "reshaping argument" for superluminal tunneling velocities, but he goes on to say that this argument is not actually supported by experiment or simulations, which actually show that the transmitted pulse has the same length and shape as the incident pulse. Instead, Winful argues that the group delay in tunneling is not actually the transit time for the pulse (whose spatial length must be greater than the barrier length in order for its spectrum to be narrow enough to allow tunneling), but is instead the lifetime of the energy stored in a standing wave which forms inside the barrier. Since the stored energy in the barrier is less than the energy stored in a barrier-free region of the same length due to destructive interference, the group delay for the energy to escape the barrier region is shorter than it would be in free space, which according to Winful is the explanation for apparently superluminal tunneling.

A number of authors have published papers disputing Nimtz's claim that Einstein causality is violated by his experiments, and there are many other papers in the literature discussing why quantum tunneling is not thought to violate causality.

It was later claimed by Eckle et al. that particle tunneling does indeed occur in zero real time. Their tests involved tunneling electrons, where the group argued a relativistic prediction for tunneling time should be 500–600 attoseconds (an attosecond is one quintillionth (10⁻¹⁸) of a second). All that could be measured was 24 attoseconds, which is the limit of the test accuracy. Again, though, other physicists believe that tunneling experiments in which particles appear to spend anomalously short times inside the barrier are in fact fully compatible with relativity, although there is disagreement about whether the explanation involves reshaping of the wave packet or other effects.

Give up (absolute) relativity

Because of the strong empirical support for special relativity, any modifications to it must necessarily be quite subtle and difficult to measure. The best-known attempt is doubly special relativity, which posits that the Planck length is also the same in all reference frames, and is associated with the work of Giovanni Amelino-Camelia and João Magueijo. There are speculative theories that claim inertia is produced by the combined mass of the universe (e.g., Mach's principle), which implies that the rest frame of the universe might be preferred by conventional measurements of natural law. If confirmed, this would imply special relativity is an approximation to a more general theory, but since the relevant comparison would (by definition) be outside the observable universe, it is difficult to imagine (much less construct) experiments to test this hypothesis. Despite this difficulty, such experiments have been proposed.

Spacetime distortion

Although the theory of special relativity forbids objects to have a relative velocity greater than light speed, and general relativity reduces to special relativity in a local sense (in small regions of spacetime where curvature is negligible), general relativity does allow the space between distant objects to expand in such a way that they have a "recession velocity" which exceeds the speed of light, and it is thought that galaxies which are at a distance of more than about 14 billion light-years from us today have a recession velocity which is faster than light. Miguel Alcubierre theorized that it would be possible to create a warp drive, in which a ship would be enclosed in a "warp bubble" where the space at the front of the bubble is rapidly contracting and the space at the back is rapidly expanding, with the result that the bubble can reach a distant destination much faster than a light beam moving outside the bubble, but without objects inside the bubble locally traveling faster than light. However, several objections raised against the Alcubierre drive appear to rule out the possibility of actually using it in any practical fashion. Another possibility predicted by general relativity is the traversable wormhole, which could create a shortcut between arbitrarily distant points in space. As with the Alcubierre drive, travelers moving through the wormhole would not locally move faster than light travelling through the wormhole alongside them, but they would be able to reach their destination (and return to their starting location) faster than light traveling outside the wormhole.

Gerald Cleaver and Richard Obousy, a professor and student of Baylor University, theorized that manipulating the extra spatial dimensions of string theory around a spaceship with an extremely large amount of energy would create a "bubble" that could cause the ship to travel faster than the speed of light. To create this bubble, the physicists believe manipulating the 10th spatial dimension would alter the dark energy in three large spatial dimensions: height, width and length. Cleaver said positive dark energy is currently responsible for speeding up the expansion rate of our universe as time moves on.

Lorentz symmetry violation

Main articles: Modern searches for Lorentz violation and Standard-Model Extension

The possibility that Lorentz symmetry may be violated has been seriously considered in the last two decades, particularly after the development of a realistic effective field theory that describes this possible violation, the so-called Standard-Model Extension. This general framework has allowed experimental searches by ultra-high energy cosmic-ray experiments and a wide variety of experiments in gravity, electrons, protons, neutrons, neutrinos, mesons, and photons. The breaking of rotation and boost invariance causes direction dependence in the theory as well as unconventional energy dependence that introduces novel effects, including Lorentz-violating neutrino oscillations and modifications to the dispersion relations of different particle species, which naturally could make particles move faster than light.

In some models of broken Lorentz symmetry, it is postulated that the symmetry is still built into the most fundamental laws of physics, but that spontaneous symmetry breaking of Lorentz invariance shortly after the Big Bang could have left a "relic field" throughout the universe which causes particles to behave differently depending on their velocity relative to the field; however, there are also some models where Lorentz symmetry is broken in a more fundamental way. If Lorentz symmetry can cease to be a fundamental symmetry at the Planck scale or at some other fundamental scale, it is conceivable that particles with a critical speed different from the speed of light be the ultimate constituents of matter.

In current models of Lorentz symmetry violation, the phenomenological parameters are expected to be energy-dependent. Therefore, as widely recognized, existing low-energy bounds cannot be applied to high-energy phenomena; however, many searches for Lorentz violation at high energies have been carried out using the Standard-Model Extension. Lorentz symmetry violation is expected to become stronger as one gets closer to the fundamental scale.

Superfluid theories of physical vacuum

Main article: Superfluid vacuum theory

In this approach the physical vacuum is viewed as a quantum superfluid which is essentially non-relativistic whereas Lorentz symmetry is not an exact symmetry of nature but rather the approximate description valid only for the small fluctuations of the superfluid background. Within the framework of the approach a theory was proposed in which the physical vacuum is conjectured to be a quantum Bose liquid whose ground-state wavefunction is described by the logarithmic Schrödinger equation. It was shown that the relativistic gravitational interaction arises as the small-amplitude collective excitation mode whereas relativistic elementary particles can be described by the particle-like modes in the limit of low momenta. The important fact is that at very high velocities the behavior of the particle-like modes becomes distinct from the relativistic one - they can reach the speed of light limit at finite energy; also, faster-than-light propagation is possible without requiring moving objects to have imaginary mass.

FTL neutrino flight results

MINOS experiment

Main article: MINOS

In 2007 the MINOS collaboration reported results measuring the flight-time of 3 GeV neutrinos yielding a speed exceeding that of light by 1.8-sigma significance. However, those measurements were considered to be statistically consistent with neutrinos traveling at the speed of light. After the detectors for the project were upgraded in 2012, MINOS corrected their initial result and found agreement with the speed of light. Further measurements are going to be conducted.

OPERA neutrino anomaly

Main article: Faster-than-light neutrino anomaly

On September 22, 2011, a preprint from the OPERA Collaboration indicated detection of 17 and 28 GeV muon neutrinos, sent 730 kilometers (454 miles) from CERN near Geneva, Switzerland to the Gran Sasso National Laboratory in Italy, traveling faster than light by a relative amount of 2.48×10⁻⁵ (approximately 1 in 40,000), a statistic with 6.0-sigma significance. On 17 November 2011, a second follow-up experiment by OPERA scientists confirmed their initial results. However, scientists were skeptical about the results of these experiments, the significance of which was disputed. In March 2012, the ICARUS collaboration failed to reproduce the OPERA results with their equipment, detecting neutrino travel time from CERN to the Gran Sasso National Laboratory indistinguishable from the speed of light. Later the OPERA team reported two flaws in their equipment set-up that had caused errors far outside their original confidence interval: a fiber optic cable attached improperly, which caused the apparently faster-than-light measurements, and a clock oscillator ticking too fast.

Tachyons

Main article: Tachyon

In special relativity, it is impossible to accelerate an object to the speed of light, or for a massive object to move at the speed of light. However, it might be possible for an object to exist which always moves faster than light. The hypothetical elementary particles with this property are called tachyons or tachyonic particles. Attempts to quantize them failed to produce faster-than-light particles, and instead illustrated that their presence leads to an instability.

Various theorists have suggested that the neutrino might have a tachyonic nature, while others have disputed the possibility.

General relativity

General relativity was developed after special relativity to include concepts like gravity. It maintains the principle that no object can accelerate to the speed of light in the reference frame of any coincident observer. However, it permits distortions in spacetime that allow an object to move faster than light from the point of view of a distant observer. One such distortion is the Alcubierre drive, which can be thought of as producing a ripple in spacetime that carries an object along with it. Another possible system is the wormhole, which connects two distant locations as though by a shortcut. Both distortions would need to create a very strong curvature in a highly localized region of space-time and their gravity fields would be immense. To counteract the unstable nature, and prevent the distortions from collapsing under their own 'weight', one would need to introduce hypothetical exotic matter or negative energy.

General relativity also recognizes that any means of faster-than-light travel could also be used for time travel. This raises problems with causality. Many physicists believe that the above phenomena are impossible and that future theories of gravity will prohibit them. One theory states that stable wormholes are possible, but that any attempt to use a network of wormholes to violate causality would result in their decay. In string theory, Eric G. Gimon and Petr Hořava have argued that in a supersymmetric five-dimensional Gödel universe, quantum corrections to general relativity effectively cut off regions of spacetime with causality-violating closed timelike curves. In particular, in the quantum theory a smeared supertube is present that cuts the spacetime in such a way that, although in the full spacetime a closed timelike curve passed through every point, no complete curves exist on the interior region bounded by the tube.