Back To The Future: 3 Real Life Time Travelers
Time travel is the concept of movement (often by a human) between different points in time in a manner analogous to moving between different points in space, typically using a hypothetical device known as atime machine. Time travel is a recognized concept in philosophy and fiction, but travel to an arbitrary point in time has a very limited support in theoretical physics, usually only in conjunction with quantum mechanics or Einstein–Rosen bridges. Sometimes the above narrow meaning of time travel is used, sometimes a broader meaning. For example, travel into the future (not the past) via time dilation is a well-proven phenomenon in physics (relativity) and is routinely experienced by astronauts, but only by several milliseconds, as they can verify by checking a precise watch against a clock that remained on Earth. Time dilation by years into the future could be done by taking a round trip during which you move at speeds approaching that of light, but this is not currently technologically feasible for manned vehicles.
The concept is popular in science fiction novels; a science fiction novel written in 1895 called The Time Machine, by H. G. Wells, was instrumental in moving the concept of time travel to the forefront of the public imagination, but the earlier short story “The Clock That Went Backward“, by Edward Page Mitchell, involves a clock that, by means unspecified, allows three men to travel backward in time. Non-technological forms of time travel had appeared in a number of earlier stories such as Charles Dickens‘ A Christmas Carol. Historically, the concept dates back to the early mythologies of Hinduism (such as theMahabharata). More recently, with advancing technology and a greater scientific understanding of the universe, the plausibility of time travel has been explored in greater detail by science fiction writers, philosophers, and physicists.
Stephen Hawking has suggested that the absence of tourists from the future is an argument against the existence of time travel: this is a variant of the Fermi paradox. Of course, this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never developed (or is cautiously never used); and even if it were developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the correct way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so “[t]his picture would explain why” the world hasn’t already been overrun by “tourists from the future.” This simply means that, until a time machine were actually to be invented, we would not be able to see time travelers. Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers, because bringing unintentional changes to the time-space continuum might bring about undesired outcomes to those travelers. It might also alter established past events. There is also the possibility that if events were changed, we would never notice it because all events following and our memories would have been instantly altered to remain congruent with the newly established timeline.
Certain experiments carried out give the impression of reversed causality but are subject to interpretation. For example, in the delayed choice quantum eraser experiment performed by Marlan Scully, pairs ofentangled photons are divided into “signal photons” and “idler photons”, with the signal photons emerging from one of two locations and their position later measured as in the double-slit experiment, and depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or “erase” that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, and under most interpretations of quantum mechanics the results can be explained in a way that does not violate causality.
The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. But a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light, so this experiment is understood not to violate causality either.
The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein’s theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled “instantaneously” between a pair of prisms that had been moved up to 3 ft (0.91 m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: “For the time being, this is the only violation of special relativity that I know of.” However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.
Some physicists have performed experiments that attempted to show causality violations, but so far without success. The “Space-time Twisting by Light” (STL) experiment run by physicist Ronald Mallettattempts to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal. Mallett has some physical arguments that suggest that closed timelike curves would become possible through the center of a laser that has been twisted into a loop. However, other physicists dispute his arguments (see objections).
Shengwang Du claims in a peer-reviewed journal to have observed single photons’ precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons’ main bodies, and the precursor traveled at c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than c (and, thus, violating causality). Some members of the media took this as an indication of proof that time travel to the past using superluminal speeds was impossible