The Clock Mirage by Joseph Mazur

The Clock Mirage by Joseph Mazur

Author:Joseph Mazur
Language: eng
Format: epub
Publisher: Yale University Press
Published: 2020-06-14T16:00:00+00:00


Looped world line

The illustration here represents the lucky time traveler’s convenient loop of a world line, a CTC that loops in such a way that a now meets with a past. It seems to be a traveling jump in time, yet it is just a meeting. Now is still now and past remains in the past. It’s a fine picture, but could a timelike curve ever meet itself? Stephen Hawking, by his chronology protection conjecture, speculates that a finite CTC would refute some laws of physics. “The laws of physics do not allow the appearance of closed timelike curves,” Hawking wrote in 1992. Evidently, such a curve would have to be infinite, thereby forbidding any kind of normal, finitely scaled time travel through looped timelike curves. He quipped, “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.”7

Those timelike curves that touch themselves in finite loops support time travel opportunities in the genre of science fiction, where jumps into the past or jumps into the future are almost always soaring pole vaults. It’s confusing. The nonfictional astronaut who travels at fantastic speeds for a year comes back to earth younger than if she had stayed on earth. She is not younger than she was when she started her journey. She has not time traveled. She has traveled into the future, but in reality all that had happened was that two clocks—hers and earth’s—were scaled differently. If she wandered the galaxy at a practically impossible speed very close to the speed of light for one second before returning in the next second, she would have returned to earth perhaps a decade later in earth time. But she had not traveled through time into the future. Her time had contracted relative to earth’s time. No time was ever skipped over.

In the real world every experience is connected to places and times. We tend to separate our perceptions of space and time, partly because we learned them in separate experiences. However, we should not be thinking of space and time as separate entities, just as we should not think of letters on a page as separate from words or musical notes separate from timing. We all travel along with time at different rates. One person’s appearance of aging moves faster or slower than another’s.

The confusion comes when we consider the difference between time tA for the astronaut and time te for someone on earth. Time te is the common time calibrated to the motions of our planet and synchronized to all local stationary places on earth. As the scale moves forward for earth time te, so does the scale for the astronaut, just more slowly, always maintaining the inequality tA < te.

The astronaut who returns to earth to find all her friends so much older than before didn’t jump ahead in time, though she might feel as if she had. Her time foreshortened to make things feel as if there was a jump.



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