Chronology protection conjecture
Encyclopedia
The chronology protection conjecture is a conjecture
by the physicist
Professor Stephen Hawking
that the laws of physics are such as to prevent time travel
on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curve
s. The chronology protection conjecture should be distinguished from chronological censorship under which every closed timelike curve passes through an event horizon, which might prevent an observer from detecting the causal violation.http://dx.doi.org/10.1007/s10701-008-9254-9
The idea of the Chronology Protection Agency appears to be drawn playfully from the Time Patrol or Time Police concept present in works of science fiction
such as Isaac Asimov
's novel The End of Eternity
.
does allow them in certain circumstances (for example, it would allow for a time machine constructed from a wormhole
– see Wormholes and time travel). But attempts to incorporate quantum
effects into general relativity using semiclassical gravity
seem to make it plausible that vacuum fluctuations would drive the energy density
on the boundary of the time machine (the Cauchy horizon
of the region where closed timelike curves become possible) to infinity, destroying the time machine at the instant it was created or at least preventing anyone outside it from entering it. The question then arises: is this apparent censorship of closed timelike curves a global constraint of physics, in the same way as a conservation law
, or is it a series of accidental coincidences?
A definite theoretical decision on the status of the chronology protection conjecture would require a full theory of quantum gravity
as opposed to the semiclassical
arguments that have been mainly used to support it (there are also some arguments from string theory
which seem to support chronology protection, but string theory is not yet a complete theory of quantum gravity). Experimental observation of closed timelike curves would of course demonstrate this conjecture to be false
, but short of that, if physicists had a theory of quantum gravity whose predictions had been well-confirmed in other areas, this would give them a significant degree of confidence in the theory's predictions about the possibility or impossibility of time travel.
Other proposals which allow for backwards time travel but prevent time paradoxes, such as the Novikov self-consistency principle
which would ensure the timeline stays consistent, or the idea that a time traveler is taken to a parallel universe
while his original timeline remains intact, do not qualify as "chronology protection".
Conjecture
A conjecture is a proposition that is unproven but is thought to be true and has not been disproven. Karl Popper pioneered the use of the term "conjecture" in scientific philosophy. Conjecture is contrasted by hypothesis , which is a testable statement based on accepted grounds...
by the physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Professor Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
that the laws of physics are such as to prevent time travel
Time travel
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the...
on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curve
Closed timelike curve
In mathematical physics, a closed timelike curve is a worldline in a Lorentzian manifold, of a material particle in spacetime that is "closed," returning to its starting point...
s. The chronology protection conjecture should be distinguished from chronological censorship under which every closed timelike curve passes through an event horizon, which might prevent an observer from detecting the causal violation.http://dx.doi.org/10.1007/s10701-008-9254-9
Origin
In a 1992 paper, Hawking uses the metaphorical device of a "Chronology Protection Agency" as a personification of the aspects of physics which make time travel impossible at macroscopic scales, thus apparently preventing time paradoxes. He says:- It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.
The idea of the Chronology Protection Agency appears to be drawn playfully from the Time Patrol or Time Police concept present in works of science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
such as Isaac Asimov
Isaac Asimov
Isaac Asimov was an American author and professor of biochemistry at Boston University, best known for his works of science fiction and for his popular science books. Asimov was one of the most prolific writers of all time, having written or edited more than 500 books and an estimated 90,000...
's novel The End of Eternity
The End of Eternity
The End of Eternity by Isaac Asimov is a science fiction novel, with mystery and thriller elements, on the subjects of time travel and social engineering....
.
Quantum corrections
Many attempts to generate scenarios for closed timelike curves have been suggested, and the theory of general relativityGeneral relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
does allow them in certain circumstances (for example, it would allow for a time machine constructed from a wormhole
Wormhole
In physics, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional surface. If this surface is folded along a third dimension, it...
– see Wormholes and time travel). But attempts to incorporate quantum
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
effects into general relativity using semiclassical gravity
Semiclassical gravity
Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical....
seem to make it plausible that vacuum fluctuations would drive the energy density
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
on the boundary of the time machine (the Cauchy horizon
Cauchy horizon
In physics, a Cauchy horizon is a light-like boundary of the domain of validity of a Cauchy problem...
of the region where closed timelike curves become possible) to infinity, destroying the time machine at the instant it was created or at least preventing anyone outside it from entering it. The question then arises: is this apparent censorship of closed timelike curves a global constraint of physics, in the same way as a conservation law
Conservation law
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves....
, or is it a series of accidental coincidences?
A definite theoretical decision on the status of the chronology protection conjecture would require a full theory of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
as opposed to the semiclassical
Semiclassical gravity
Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical....
arguments that have been mainly used to support it (there are also some arguments from string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...
which seem to support chronology protection, but string theory is not yet a complete theory of quantum gravity). Experimental observation of closed timelike curves would of course demonstrate this conjecture to be false
Falsifiability
Falsifiability or refutability of an assertion, hypothesis or theory is the logical possibility that it can be contradicted by an observation or the outcome of a physical experiment...
, but short of that, if physicists had a theory of quantum gravity whose predictions had been well-confirmed in other areas, this would give them a significant degree of confidence in the theory's predictions about the possibility or impossibility of time travel.
Other proposals which allow for backwards time travel but prevent time paradoxes, such as the Novikov self-consistency principle
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general...
which would ensure the timeline stays consistent, or the idea that a time traveler is taken to a parallel universe
Many-worlds interpretation
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...
while his original timeline remains intact, do not qualify as "chronology protection".
External links
- http://www.hawking.org.uk/index.php/lectures/63
- http://plus.maths.org/content/time-travel-allowed — Kip ThorneKip ThorneKip Stephen Thorne is an American theoretical physicist, known for his prolific contributions in gravitation physics and astrophysics and for having trained a generation of scientists...
discusses time travel in general relativity, and the basis in quantum physics for the chronology protection conjecture