First Time Machine
Moving Between Different Points in Time

First Time Machine
Moving Between Different Points in Time

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, might allow time travel into the past and future if these geometries or motions are possible.

Relativity states that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future".

On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that problems of causality be resolved.

The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived?

But some scientists believe that paradoxes can be avoided, either by appealing to the Novikov self-consistency principle or to the notion of branching parallel universes.

Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to some moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate).

The first time travel story to feature time travel by means of a time machine was Enrique Gaspar y Rimbau's 1887 book El Anacronópete.

This idea gained popularity with the H. G. Wells story The Time Machine, published in 1895 (preceded by a less influential story of time travel Wells wrote in 1888, titled The Chronic Argonauts), which also featured a time machine and which is often seen as an inspiration for all later science fiction stories featuring time travel, using a vehicle that allows an operator to travel purposefully and selectively.

The term "time machine", coined by Wells, is now universally used to refer to such a vehicle.

Since that time, both science and fiction have expanded on the concept of time travel.

A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin.

Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance.

For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less than the stationary end, as seen by an external observer.

However, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.

This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry.

For example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, a trip backwards in time as seen by other observers outside. 

One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine; 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 has not yet been built, so the tourists from the future cannot reach this far back in time.

For quite some time, Ronald Mallett has been working on plans for a time machine.

This technology would be based upon a ring laser's properties within the context of Einstein's Theory of Relativity.

Mallett first argued that the ring laser would produce a limited amount of frame-dragging which might be measured experimentally, saying:

"In Einstein's General Theory of Relativity, both matter and energy can create a gravitational field. This means that the energy of a light beam can produce a gravitational field."

"My current research considers both the weak and strong gravitational fields produced by a single continuously circulating unidirectional beam of light. In the weak gravitational field of an unidirectional ring laser, it is predicted that a spinning neutral particle, when placed in the ring, is dragged around by the resulting gravitational field."

In a later paper, he argued that at sufficient energies, the circulating laser might produce not just frame-dragging but also closed timelike curves (CTC), allowing time travel into the past:

For the strong gravitational field of a circulating cylinder of light, I have found new exact solutions of the Einstein field equations for the exterior and interior gravitational fields of the light cylinder.

The exterior gravitational field is shown to contain closed timelike lines. The presence of closed timelike lines indicates the possibility of time travel into the past. This creates the foundation for a time machine based on a circulating cylinder of light.

Funding for his program, now known as The Space-time Twisting by Light (STL) project, is progressing.

Full details on the project, Mallett's theories, a list of upcoming public lectures and links to popular articles on his work can be found at the Professor's UConn web page, and an illustration showing the concept on which Mallett has designed the time machine can be seen here.

He also wrote a book entitled, Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality, co-written with New York Times best-selling author Bruce B. Henderson, that was first published in 2006.

In June 2008, motion picture director Spike Lee's production company announced it had acquired the film rights to Mallett's book. Lee is co-writing the movie script and directing the picture. In 2006 Mallett declared that time travel into the past would be possible within the 21st century and possibly within less than a decade.

Mallett uses Albert Einstein's General Theory of Relativity to attempt to substantiate his claims.

Back to the Future is a 1985 American science-fiction comedy film that tells the story of Marty McFly, a teenager who is accidentally sent back in time from 1985 to 1955. Are such Time Machines possible such as the plutonium-powered DeLorean "time machine" invented by the slightly mad scientist in the fictional movie 'Back to the Future'?

Back to the Future

Marty McFly goes back in time to get hit on by his mother.

He does this by driving a DeLorean that has been turned into a Time Machine by Dr. Emmett Brown.

Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine.

A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine.

Gamma rays projected at it might allow information (not matter) to be sent back in time; however, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter).

How To Time Travel

Late for school? A meeting? Just take a wormhole -- you'll be there before you know it.

Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough, he did not prove this.

But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."

This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "there will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete.

One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon."

However, this theorem does not rule out the possibility of time travel

1) by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine)

2) in regions which contain exotic matter (which would be necessary for traversable wormholes or the Alcubierre drive).

Because the theorem is based on general relativity, it is also conceivable a future theory of quantum gravity which replaced general relativity would allow time travel even without exotic matter (though it is also possible such a theory would place even more restrictions on time travel, or rule it out completely as postulated by Hawking's chronology protection conjecture).

Time Travel: Einstein's Big Idea
Theory of Relativity

The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity.

The theory of relativity enriched physics and astronomy during the 20th century. When first published, relativity superseded a 200-year-old theory of mechanics elucidated by Isaac Newton.

It changed perceptions.

For example, it overturned the concept of motion from Newton's day, into all motion is relative. Time was no longer uniform and absolute, as related to everyday experience.

Furthermore, no longer could physics be understood as space by itself, and time by itself. Instead, an added dimension had to be taken into account with curved space-time.

Time now depended on velocity, and contraction became a fundamental consequence at appropriate speeds. In the field of microscopic physics, relativity catalyzed and added an essential depth of knowledge to the science of elementary particles and their fundamental interactions, along with introducing the nuclear age.

With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.