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Time Cloak Hides Very Brief Events [Animation]

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For years physicists have been refining invisibility cloaks—physical setups that cleverly reroute light around a region in space, effectively concealing any object that might be inside. But now researchers at Cornell University have built the first temporal cloak, a device that obscures an object or event not at a particular point in space but at a specific moment in time.

In a preliminary demonstration, Cornell postdoctoral researcher Moti Fridman and his colleagues shone a laser beam through an experimental apparatus and into a detector. A physical object or even another beam of light in the laser beam’s path could create a change in the laser light that the detector would register. But with some clever optics, Fridman and his colleagues were able to open up a brief time gap in the beam and then close it back up as if the beam had gone undisturbed, and such that the detector did not register the interruption. The gap allows anything that would have otherwise affected the beam to instead slip right through [see animation below], leaving no trace for the detector to pick up.


The researchers used the cloak to obscure an optical pulse that ordinarily interacts with the laser beam to produce a telltale spike at a certain wavelength. When the event was cloaked, however, the telltale spike was basically undetectable.

The cloak, described in the January 5 issue of Nature, relies on the fact that light of different colors moves at different speeds through certain media. (Scientific American is part of Nature Publishing Group.) Using a device that they call a “time lens,” the researchers split a single-color laser beam into a spread of wavelengths, then slowed half of those wavelengths while speeding up the others. That created a very brief time gap that could be closed again before the beam reached the detector by reversing the lensing process, restoring the beam to a single, seemingly undisturbed wavelength.

The gap achieved by Fridman and his colleagues was extremely small—just 50 picoseconds, or 50 trillionths of a second, in duration. The researchers note that it is possible to extend the gap somewhat, but that scattering and dispersion effects limit the scope of the temporal cloak to a few nanoseconds.

Animation by Rose Eveleth

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. jtdwyer 4:07 pm 01/5/2012

    The article states:
    “But now researchers at Cornell University have built the first temporal cloak, a device that obscures an object or event not at a particular point in space but at a specific moment in time.”

    I think this is inaccurate, since the cloaking effect occurs at a particular location (if not a point) in space at a particular moment in time.

    Such a short duration cloak is actually effective for some range of locations at specific moments in time, as the beam gap propagates between the two fibers.

    Actually, some number of balls might be passed through the gap between the fibers as the beam gap progresses, depending on the size of the gap between fibers…

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  2. 2. gmartfin 5:46 pm 01/5/2012

    How is this any different than this:

    I don’t see it as slowing time but slowing light and taking advantage of the gap. Time has not been affected in any way shape or form.

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  3. 3. Whammer2 9:50 pm 01/5/2012

    That is an amazing video. Some questions:

    1. Over what distance could the “cloaking” (non-recognition) effect be sustained?
    2. If the cloaked light pulse were modulated (i.e. contained information) would that information also be undetectable by an outside observer?

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  4. 4. gesimsek 7:50 am 01/6/2012

    Acording to E=m.c2 time, space and energy is the function of each other. Therefore, it is not possible to make things invisible unless you go faster than light. However, it is possible to hide things by sending back unvisible waves of light or slowing down the light, in which case observer will see you in a different location than where you are.

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  5. 5. ianlib 10:53 am 01/6/2012

    It is hoped that the first actual demonstration of a technique originally described theoretically a year ago by Martin McCall and his colleagues at Imperial College London in the Journal of Optics get the deserved respect that is due to them now that this has become a big breakthrough.

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  6. 6. JustLooking 11:05 am 01/6/2012

    I think the Time Cloak is in reference to the viewer. For example, say the viewer was seeing images of an eventt at one frame per second and there were 4 frames in total. This time cloak would cloak frame 2 and 3. The space and time for the viewer for the seconds frames 2 and 3 should have appeared would be lost to the viewer for the event. The viewer would not be able to account for seconds 2 and 3 for the event but, can for seconds 1 and 4. For the viewer a space in time for the event is lost, unaccounted for and that is why it is a time cloak. Time itself is not effected, just the viewers sighting of the event within a time an space of the event.

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  7. 7. leaf6 8:25 pm 01/6/2012

    This method is rather limited in terms of practicality. Unlike a laser pulse, the Sun continuously emits light allowing absolutely no time interval for an object to pass through.

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  8. 8. verdai 5:26 pm 01/10/2012

    the break is in the beam, not in time.
    anyway, pi’coseconds? how we escape there?

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  9. 9. Johnay 7:24 pm 01/11/2012

    I wonder if this will lead to techniques that could undermine the security, or at least tamper-evident nature, of optical communications.

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