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Information on Travel


A travel is a long journey. The longer it takes the more it costs. So it is convenient to take enough money along, in order to meet expected and unexpected needs.

See Transport.


A traveller (american traveler) is a person or object that travels between two or more locations.

Wikipedia knows several types of traveller:

  1. A role-playing game known as Traveller published by Games Designer's Workshop (GDW) beginning in 1977
  2. A mechanical traveller, part of a ship or machine
  3. The band Blues Traveler
  4. The BBC television series Crime Traveller
  5. A human traveller, for pleasure or profession, for which see the related links to
This is a disambiguation page; that is, one that just points to other pages that might otherwise have the same name. If you followed a link here, you might want to go back and fix that link to point to the appropriate specific page.

Travel survey

A travel survey (or travel diary or travel behavior inventory) is a survey of individual travel behavior. Most surveys collect information about an individual (socio-economic, demographic, etc.), their household (size, structure, relationships), their vehicle (age, make, model), and a diary of their journeys on a given day (their start and end location, start and end time, mode of travel, accompanyment and purpose of travel).

Major travel surveys are conducted in metropolitan areas typically once a decade. Some regions, notably metropolitan Seattle, Washington conduct a panel survey, which interviews the same people year after year, to see how their particular behavior evolves over time.

See also

External links

Travel blending

Travel Blending is a technique, developed in Australia, for encouraging people to make more efficient and environmentally sound transportation choices. This technique originated in Sydney as an effort to improve Sydney's air quality before the 2000 Summer Olympic Games.

External links

Travel behavior

Travel behavior is the study of what people do over space, and how people use transport. The questions studied in travel behavior are broad, and are very much related to activity analysis and time use studies.

  • How many trips do people make?
  • Where do they go? (What is the destination?)
  • What mode do they take?
  • Who accompanies whom?
  • When is the trip made? What is the schedule?
  • What is the sequence or pattern of trips?
  • What route do people take?
  • Why do people travel? (Why can't people stay at home and telecommute or teleshop?)

These questions can be answered descriptively using a travel diary, often part of a travel survey or travel behavior inventory. Large metropolitan areas typically only do such surveys once every decade, though some cities are conducting panel surveys, which track the same people year after year.

That data is generally used to estimate transport planning models, so that transport analysts can make predictions about people who haven't been surveyed. This is important in forecasting traffic, which depends on future changes to road networks, land use patterns, and policies.

Time travel

Time travel is a concept that has long fascinated humanity -- whether it is Merlin experiencing time backwards, or Mohammed's alleged trip to Jerusalem and ascent to heaven returning before a glass knocked over had spilt its contents. Often nowadays it is a plot device used in science fiction to set a character in a particular time not his/her own, and explore the possible ramifications of the character's interaction with the people and technology of that time - a spin on the "country bumpkin comes to the big city" plot (or vice versa). It evolved to explore ideas of change, and reactions to it, and also to explore the ideas of a parallel universes or alternate history where some little event took place or didn't take place, but causes large changes in the future.

Famous fictional time machines include the TARDIS and H. G. Wells' The Time Machine. Wells' novel is meant to predict the likely future of humanity itself, starting with world wars and ending with humans reverting back to a Garden of Eden existence - with a terrifying twist.

In physics, the "thought experiment" of time travel has been often used to examine the consequences of physical theories such as special relativity, general relativity and quantum mechanics. There is no experimental evidence of time travel, and it is not even well understood whether (let alone how) the current physical theories permit any kind of time travel.

Table of contents
1 Physics
2 Time Travel Theme in Science Fiction
3 Time Travel, or Spacetime Travel?


It should be noted that Einstein's Special Theory of Relativity (and, by extension, the General Theory) very explicitly permits a kind of time dilation that would ordinarily be called time travel. The theory holds that time passes more slowly for faster-moving bodies: for example, a moving clock will run slow; as a clock approaches the speed of light its hands will nearly stop moving. However, this effect allows "time travel" only toward the future: only forward, never backward. It is not the most interesting kind, nor the kind typical of science fiction: hereafter "time travel" will refer to travel with some degree of freedom into the past or future.

Many in the scientific community believe that time travel is highly unlikely. This belief is largely due to Occam's Razor. Any theory which would allow time travel would require that issues of causality be resolved. What happens if you try to go back in time and kill your grandfather? -- see grandfather paradox. Also, in the absence of any experimental evidence that time travel exists, it is theoretically simpler to assume that it does not happen. Indeed, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel - a variant of the Fermi paradox, with time travelers instead of alien visitors. However assuming that time travel can happen opens up the question of why and what physical laws exist to prevent time travel from occurring, and how they can be gotten around.

First of all, if one is able to move from one point to another faster than light, then according to Special Relativity time travel may be possible. Einstein predicted that the passage of time slows as one approaches the speed of light, and implies that if one were to pass the speed of light, then time would actually reverse. However, the very notion of "passing the speed of light" is all but a contradiction within Relativity theory.

The General Theory of Relativity extends the Special Theory to cover gravity. It does this by postulating that matter "curves" the space in its vicinity. But under relativity, properties of space are fairly interchangeable with properties of time, depending on one's perspective, so that a curved path through space can wind up being a curved path through time. In moderate degrees, this allows two straight lines of different length to connect the same points in space; in extreme degees, theoretically, it could allow timelines to curve around in a circle and reconnect with their own past. GTR describes the universe under a complex sytem of "field equations," and there exist solutions to these equations that permit what are called "closed time-like curves, " and hence time travel into the past. The first and most famous of these was proposed by Kurt Gödel), but nearly all of them require the universe to have physical characteristics that it does not appear to have.

A proposed time-travel machine using a wormhole would (hypothetically) work something like this: A wormhole is created somehow. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now experienced less subjective passage of time than the stationary end. An object that goes into the stationary end would come out of the other end in the past relative to the time when it enters. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine; in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but hasn't been built yet, so the tourists from the future can't reach this far back in time.

Creating a wormhole of a size useful for macroscopic spacecraft, keeping it stable, and moving one end if it around would require significant energy, many orders of magnitude more than the Sun can produce in its lifetime. Construction of a wormhole would also require the existence of a substance known as 'exotic matter', or 'negative matter', which, while not known to be impossible, is also not known to exist in forms useful for wormhole construction (but see for example the Casimir effect). Therefore it is unlikely such a device will be ever constructed, even with highly advanced technology.

Another approach, developed by Frank Tipler, involves a spinning cylinder. If a cylinder is long, and dense, and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral. However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it doesn't seem to be possible to create a new cosmic string.

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

Quantum mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presumes that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.

Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information, and therefore do not appear to allow for time travel or FTL communication. This misunderstanding seems to be widespread in popular press coverage of quantum teleportation experiments. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem, and how the rules of quantum mechanics work to preserve causality is an active area of research.

Recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes--there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit of many consistent solutions. Things might, however, turn out to be almost unbelievably strange.

Time Travel Theme in Science Fiction

H. G. Wells' "The Time Machine" is considered the literary masterpiece of the genre. Mark Twain's A Connecticut Yankee in King Arthur's Court is another early time travel classic. Probably the most elaborate demonstration of supposed time travel paradoxes is Robert Anson Heinlein's "All You Zombies."

Time travel themes in science fiction can generally be grouped into two types (based on effect--methods are extremely varied and numerous), each of which is further subdivided.

1. The time line is consistent and can never be changed.
1.1 One does not have full control of the time travel. One example of this is The Morphail Effect.
1.2 The Novikov self-consistency principle applies. (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University)
1.3 Any event that appears to have changed a time line has instead created a new one.
2. The time line is flexible and is subject to change.
2.1 The time line is extremely change resistant and requires great effort to change it.
2.2 The time line is easily changed.

Time Travel in a type 1 universe does not allow any paradoxes, although in 1.3, events can appear to be paradoxical.

In 1.1, Time travel is constrained to prevent paradox. If one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. Michael Moorcock uses a form of this principle and calls it The Morphail Effect.

In 1.2, The Novikov Self-consistency Principle asserts that the existence of a method of time travel constrains events to remain self-consistent (i.e. no paradoxes). This will cause any attempt to violate such consistency to fail, even if extremely improbable events are required.

Example: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 PM, then no bits for thirty seconds after that. If you send a bit back to 10:00:00 PM, everything works fine. However, if you try to send a bit to 10:00:15 PM (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 PM. Etc, etc. An excellent example of this kind of universe is found in Timemaster, a novel by Dr. Robert Forward.

In a universe that allows retrograde time travel but no paradoxes, any present moment is the past for a future observer, thus all history/events are fixed. History can be thought of as a filmstrip where everything is already fixed. See block time for a detailed examination of this way of considering the nature of time.

In 1.3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated, unless the mechanics of time travel required that mass-energy be exchanged in precise balance between past and future at the moment of travel. An example of this kind of time travel can be found in David Gerrold's The Man Who Folded Himself.

Time Travel in a type 2 universe is much more difficult to explain. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes so do all memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past.) This would make it hard to tell whether you are in a type 1 universe or a type 2 universe. However, you could infer that you were by knowing that a) communication with the past was possible and b) it appeared that the time line had *never* been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often. An example of this kind of universe is presented in Thrice Upon a Time, a novel by James P. Hogan.

Larry Niven suggests that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number time travelers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered. However, many other "stable" situations may also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.2 scenario.

In The Restaurant at the End of the Universe Douglas Adams does not see a big problem in becoming his own father, since this is nothing a well adjusted family can't deal with. The big problem is grammar - the tense formation for time travellers. Robert Heinlein's story All You Zombies shows the possible results of taking this concept to its logical conclusion ad absurdam: the time travelling protagonist is/was/becomes his/her own father, son, mother and daughter.

In many science fiction books about time travel, there is a physical machine for transporting people through time but there is a minority which involve time travel through mental disciplines. Jack Finney's Time And Again is one such book. Poul Anderson's There Will Be Time portrays time travel as an ability some are born with. Some people affiliated with the UFO movement say that the ability to time-travel lies latent in everybody's brain, and that that ability is "turned on" in the minds of the Greys, who supposedly have the ability to unlock it in human brains too. Other people believe that both time travel and teleportation can be learned through practice in a similar manner.

In 1992 Harry Turtledove published the novel The Guns of the South which became popular with its story about South African white supremacists using a time travel machine to go back to the days of the American Civil War and equip the dispirited rebel army with 20th century weapons such as the AK-47. They soon win every battle and gleefully march into Washington, D.C. to capture Lincoln. The limits of his time travel machine are ludicrous, however, because it can only take people back a set number of years. This allows him to prevent the white supremacists from making another trip to cure the ills of the first, which (ahem) goes wrong at the end.

It can be argued that the Book of Revelation, describes a form of "spiritual time travel". In contrast to most science fiction conceptualizations of time travel, the Revelation states that John (while on the Greek island of Patmos) had a vision that took him, in spirit, to the future end times in world history and that future events were revealed to him by an angel sent by Jesus Christ.

Time Travel, or Spacetime Travel?

The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats the earth like it is stationary in absolute space. The idea that you can go into a machine that sends you to "1865 A.D.", and you exit from a door that leaves you in the same spot in Poughkeepsie that the time machine was when you entered it ignores the issue that the earth is moving through space around the sun, which is moving in the galaxy, etc. So, if you think of spacetime as 4 dimensions, and "time travel" is just "moving" along one of them, you couldn't stay in the same place with respect to the surface of the earth, because the earth is a moving platform with a highly complicated trajectory! If you only moved "ahead" 5 seconds, you might materialize in the air, or inside solid rock, depending on where the earth was "before" and "after". If you moved "behind" one year, you'd end up in cold outer space, where the earth was a year earlier - in the same part of the sun's orbit, yes, but where has the sun gone over that year? So, to really do what they make look so easy in films like Back to the Future and The Time Machine, your time machine would have to be a very powerful spaceship that could move you large distances and that kept track of the earth's motion through space as part of the solar system, galaxy, etc. But how can you decouple the ship from momentum? If you try to move forward in time, is your ship automatically going to be propelled by the momentum gained by riding the earth? Or does it decouple? But doesn't that bring back the idea of absolute space? Again, even to move one millisecond forward or backward in time, the ship would have to be far beyond anything humans can build, not to mention the acceleration-deceleration problems and what that might do to your blood pressure. You might even use this to argue, Zeno-style, for the impossibility of time machines. In 1980 Robert Heinlein published an amusing novel The Number of the Beast about a ship that lets you dial-in the 6 (not 4!) coordinates of space and time and it instantly moves you there - without explaining how such a device might work.

Scientific references include:
Paul Davies, About Time
Paul J. Nahin, Time Machines
Clifford A. Pickover, Time: A Traveler's Guide

Literary references:
The Time Machine (full text), by H. G. Wells
A Connecticut Yankee in King Arthur's Court (full text), by Mark Twain

See also: Anachronism and time travel, anachronism, grandfather paradox.

Interstellar travel

Interstellar space travel is unmanned or manned travel between stars, though the term usually denotes the latter. The concept of interstellar travel has appeared in many science fiction literature and films. There is a major difference between interstellar travel and interplanetary travel, mainly in the distances involved.

As a practical goal interstellar travel has been debated fiercely by various scientists, science fiction authors, hobbyists and enthusiasts.

Many ideas for managing this goal have been suggested, especially in science fiction themes, ranging from Star Trek's warp drive and hyperspace engines of various sorts to trips that involve very long waits and either multi-generation crews (generation ships) or cryogenic freezing (sleeper ships). Other science fiction authors such as Isaac Asimov, Arthur C. Clarke and others, cover such concepts with greater attention given to plausible ideas as to how this could be achieved. As current knowledge of physics stands there is no effective solution to achieving faster than light travel, and the current technological limits to slower-than-light travel make interstellar journeys shorter than centuries unrealistic.

A number of science fiction novels deal with interstellar travel quasi-realistically by using generation ships where generations of people are born, live and die onboard a massive star ship as it travels to its destination. Alternately, the crew of a starship could spend the bulk of the journey frozen in suspended animation on board a sleeper ship, leaving the tedium of interstellar travel to automated systems and awakening unaged at their destination.

While manned interstellar travel may prove difficult or impossible to accomplish, manned interplanetary travel (travel between the planets of the solar system) is technically feasible, though economic factors, health and other risks regarding a person's continueous stay in space, and other motivational factors may suspend the achivement.

Many scientific papers have been published about related concepts. Given sufficient travel time and engineering work, interstellar travel is certainly possible. NASA has been engaging in research into these topics for several years, and has accumulated a number of theoretical approaches. Among the technologies suggested are nuclear engines (nuclear thermal or nuclear electric, primarily). With any of these technologies interstellar travel times would still be very long compared to a single human lifespan, but it would be within the realm of the possible given generation ships or some sort of organic stasis approach as mentioned previously.

Table of contents
1 The problem of interstellar distances

The problem of interstellar distances

Astronomical distances are sometimes measured in the amount of time it would take a beam of light to travel. Light in a vacuum travels in approximately 3×108 metres per second, which is denoted with the letter c, so a light second is app. 3×108 metres.

The distance between Earth and its Moon is about one and a quarter light seconds. With current propulsion technologies, such a trip will typically take about three days for a spacecraft.

The distance from Earth to other planets in the solar system ranges from three light minutes to about five and a half light hours. Depending on the planet and its alignment to earth, for an typical unmanned spacecraft these trips will take from a few months to a little over a decade.

The nearest star to the Sun is the triple system Alpha Centauri. Light radiating from that star takes a bit more than four years to reach Earth. Currently, the fastest spacecraft built can achieve a velocity of about 30 km per second (relative to Earth). At that rate, the journey would take about 40,000 years. Additionally, at current stage of space technology, the longest space missions that have been initiated are expected to have an operational lifetime of about forty years before failure of key components is likely to happen. Significant engineering advances such as automated self-repair may be required to ensure survival.

Special relativity and the speed of light as a limit

In many science fiction stories starships to travel from star to star faster than light. Currently there is no known way to achieve this, and according to the current understanding of physics there may never be one.

There are two objections to faster than light travel, both originate from the theory of relativity which states that the speed of light is constant in all reference frames.

The first is that because the speed of light is constant to all observers, an attempt to overtake a beam of light will fail. No matter how much energy you put into an spaceship, a beam of light will appear from that spaceship to be moving away at the same speed. While this appears to rule out accelerating past a beam of light, it does not immediately appear to prevent one from exploiting particles that already are moving faster than light or perhaps by somehow sending information faster than light without having any physical object move faster than light.

However even this causes problems. According to a section of the theory of relativity called special relativity, travelling faster than light is equivalent to travelling backwards in time, or time travel, according to some observers. In particular, if faster than light travel is possible without too many arbitrary restrictions, it is possible to have events in the future cause events in the past. This is called a causality loop.

This concept has not been verified experimentally, because no-one has yet exceeded the speed of light in the laboratory. However, it seems that this reasoning must apply to any theory in which the speed of light in a vacuum is equal to all observers, something that has been carefully verified in many experiments.

It is not immediately obvious that a causality loop is impossible, but the idea is sufficiently unsettling that many physicists believe it to be so.

(In this section it should be noted that "experimentally verified" only means that several different repeatable experiments have been performed that support the theory. Of course these cannot prove the theory correct, they can only give confidence that the theory appears to work for the cases that were tested. That is about as good as it gets in science.)

"Practical" interstellar travel

Even without faster-than-light travel, multi-generation starships, or dramatic extensions to human lifespan, it may still be feasible in the medium to long-term future to travel to the nearer stars.

A practical short-term approach that has been proposed is nuclear pulse propulsion. In 1957 it was deemed possible to build 8 million ton starships with this type of engine, even though they would be limited to about 1/10 the speed of light. One problem with it is that such a propulsion method uses nuclear explosives as fuel, and may be highly controversial due to the risk of radiation or other hazards in using such a method.

Another early proposal for an interstellar propulsion system was the Bussard ramjet, in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it using a proton-proton fusion reaction, and expel it out the back. As the fuel would be collected en route, the craft could have theoretically accelerated to near the speed of light. Proposed in 1960, later calculations with more accurate estimates suggest that the thrust generated would have been less than the drag caused by any conceivable scoop design.

Fusion-powered starships should be able to reach speeds of approximately 10 percent of that of light. Light sails powered by massive lasers could potentially reach similar or greater speeds. Finally, if energy resources and efficient production methods are found to make antimatter in the quantities required, theoretically it would be possible to reach speeds near that of light, where time dilation would shorten perceived trip times for the travellers considerably (though shielding the spacecraft from stray atoms in interstellar space would become a very serious issue as faster speeds were achieved). Even given the assumption of 10 percent of light speed, this would be enough to reach Alpha Centauri in forty years, only half a present human lifetime.

See also: interstellar communication, relativistic rocket

This page created and maintained by Jamie Sanderson.
© Jamie Sanderson 1999-2005.