# Time travel, quantum-style.

Time travel, quantum-style

The boundary between science fiction and theoretical physics sometimes gets fuzzy, especially when quantum theory is invoked. The latest apparent border-crossing offers the prospect of a "quantum time-translation machine" -- a device capable of moving an event to a different position in time.

The secret of the machine's operation lies in a novel interpretation of the equations governing quantum mechanics. Physicist Yakir Aharonov of the University of South Carolina in Columbia and his collaborators start with a fundamental quantum-mechanical feature called superposition, which expresses the idea of combining a suitable set of "elementary" physical situations to describe a quantum system's physical state -- somewhat analogous to overlapping photographic images to create a composite picture. Such superpositions, when applied to the way a quantum system evolves over time, "may lead to unusual consequences," the researchers state in the June 18 PHYSICAL REVIEW LETTERS.

Aharonov and his colleagues initially apply the superposition principle to physical forces, described by mathematical constructs known as hamiltonians. In a system in which an external control sets the force, they arrange for a superposition of all the situations arising from different force settings. That's equivalent to taking a weighted average of all the possible forces. Because of the way this quantum-mechanical averaging works, the resulting average may occasionally be far larger than the individual forces involved. In that situation, the action of a number of weak forces would correspond to the action of a single, strong force, furnishing a new, quantum-mechanical route for amplifying forces.

Time enters the picture because the strength of a force, as expressed in a hamiltonian, is directly related to the time it takes to produce a given effect. In other words, doubling the hamiltonian has the same net effect as doubling the period of time for which it operates. Thus, the superposition concept used for forces also applies to time. By superposing a number of situations in which different periods of time have elapsed, one could obtain the effect of a passage of time longer than in any of the original situations -- in effect, taking the system into the future. Or, if the superposition produces a negative time, one could move into the past.

To achieve such an effect, one needs to produce a number of coexisting situations in which a given force is allowed to operate for different times. Aharonov's group solves the problem by appealing to the general theory of relatively, in which the time elapsed for an observer in a particular place depends on the strength of the local gravitational field. One can control the passage of time by controlling the gravitational potential, perhaps by enclosing the system in a massive spherical shell and then altering the shellhs radius.

Could one construct such a time-translation machine? Aharonov and his colleagues concede that their machine would work only rarely because it relies on the random processes of quantum mechanics to produce the required superposition state. Furthermore, such a superposition state has never been produced on the scale required for a time-translation machine.

In the Aug. 23 NATURE, mathematician Tony Sudbery of the University of York In England notes that quantum systems develop over time in two distinct ways. "There is the natural development which happens if the system is left undisturbed, and there are the quantum jumps which occur at a measurement, representing the inevitable disturbance caused by observation," he writes. "The proposed machine takes the system into what would be the future (or the past) if only the first of these [evolutionary paths] was operating." That would put a time traveler in a time frame quite different from the actual future or past we experience.

The boundary between science fiction and theoretical physics sometimes gets fuzzy, especially when quantum theory is invoked. The latest apparent border-crossing offers the prospect of a "quantum time-translation machine" -- a device capable of moving an event to a different position in time.

The secret of the machine's operation lies in a novel interpretation of the equations governing quantum mechanics. Physicist Yakir Aharonov of the University of South Carolina in Columbia and his collaborators start with a fundamental quantum-mechanical feature called superposition, which expresses the idea of combining a suitable set of "elementary" physical situations to describe a quantum system's physical state -- somewhat analogous to overlapping photographic images to create a composite picture. Such superpositions, when applied to the way a quantum system evolves over time, "may lead to unusual consequences," the researchers state in the June 18 PHYSICAL REVIEW LETTERS.

Aharonov and his colleagues initially apply the superposition principle to physical forces, described by mathematical constructs known as hamiltonians. In a system in which an external control sets the force, they arrange for a superposition of all the situations arising from different force settings. That's equivalent to taking a weighted average of all the possible forces. Because of the way this quantum-mechanical averaging works, the resulting average may occasionally be far larger than the individual forces involved. In that situation, the action of a number of weak forces would correspond to the action of a single, strong force, furnishing a new, quantum-mechanical route for amplifying forces.

Time enters the picture because the strength of a force, as expressed in a hamiltonian, is directly related to the time it takes to produce a given effect. In other words, doubling the hamiltonian has the same net effect as doubling the period of time for which it operates. Thus, the superposition concept used for forces also applies to time. By superposing a number of situations in which different periods of time have elapsed, one could obtain the effect of a passage of time longer than in any of the original situations -- in effect, taking the system into the future. Or, if the superposition produces a negative time, one could move into the past.

To achieve such an effect, one needs to produce a number of coexisting situations in which a given force is allowed to operate for different times. Aharonov's group solves the problem by appealing to the general theory of relatively, in which the time elapsed for an observer in a particular place depends on the strength of the local gravitational field. One can control the passage of time by controlling the gravitational potential, perhaps by enclosing the system in a massive spherical shell and then altering the shellhs radius.

Could one construct such a time-translation machine? Aharonov and his colleagues concede that their machine would work only rarely because it relies on the random processes of quantum mechanics to produce the required superposition state. Furthermore, such a superposition state has never been produced on the scale required for a time-translation machine.

In the Aug. 23 NATURE, mathematician Tony Sudbery of the University of York In England notes that quantum systems develop over time in two distinct ways. "There is the natural development which happens if the system is left undisturbed, and there are the quantum jumps which occur at a measurement, representing the inevitable disturbance caused by observation," he writes. "The proposed machine takes the system into what would be the future (or the past) if only the first of these [evolutionary paths] was operating." That would put a time traveler in a time frame quite different from the actual future or past we experience.

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Publication: | Science News |
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Date: | Sep 8, 1990 |

Words: | 607 |

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