A Time Cloak: Did Physicists Really Make Events Undetectable?

Sometimes curious papers appear in dry physics journals, whether Physical Review D, or in this case, The Journal of Optics. For the paper in question, the author – one Martin McCall of Imperial College, London, proposed splitting a beam of light into 2 components moving at differing speeds. Let one component have speed v2 and the other v1 and if v2 > v1 then the first component would build a lead on the slower one, thereby opening up a complete gap of darkness between them. In this way, any event occurring (or deliberately inserted) within the gap would be undetectable.


One would basically have a “time cloak”.

McCall went one step further, proposing the re-integration of the two components before the information-bearing total light beam reached an observer. (Keep in mind it is precisely light which makes events visible in time). In such a scenario, there’d be no way to determine or detect the darkness gap ever existed in the first place.

While McCall didn’t construct this device – he estimated 5-10 years to complete- he was correct in that ultimately a team of physicists led by Alexander Gaeta, did develop a device (which he called a “time lens”) that could alter the speed of light for a specific beam. Recall here, again, from my earlier blog that light alters its speed when passing through a material.

When transiting to a higher density medium, the velocity of light slows, and conversely increases in moving from higher density medium to lower. The time lens combined the two processes by intersecting a pre-existing beam of light with a laser just as the former passed through a glass fiber. Gaeta's original objective, however, had nothing to do with time cloaks. It was to slow down a light beam enough to measure rapidly changing phenomena such as in controlled explosions.

It was only after he’d read McCall’s paper he realized he could also use the time lens to speed up one segment of a light beam and slow down the other. Thus motivated, Gaeta and his crack team assembled a network of optical fibers with lasers and time lenses scattered along the route. On one auspicious say in April, 2011, Gaeta happened to send a light beam into one end of an optical fiber, then through his time lens splitting the beam in 2.


As McCall had earlier theorized, when the leading segment surged ahead a “time gap” was created and widened. By the time the split beam had traversed one kilometer (about five eighths of a mile) the time gap reached 15 trillionths of a second. At this maximal gap point the experimenters fired a laser across the conducting fiber, introducing a fiducial mark or marker.


Now, in the scheme of observation in a normal time sequence, when a laser beam impacts a beam of light, its color change indicating a change of frequency. In Gaeta’s team’s case, however, the created darkness gap worked to perfection. By virtue of the laser transiting the darkness gap or time gap the color of the beam remain unchanged, in other words the putative color change event was cloaked.

Now, the most intriguing part of the experiment: after transiting another time lens which had the reverse effect on the light components (i.e. speeding up the slower segment, slowing the faster one) the re-integrated beam reached its end point and with the exact same properties as when it began. Hence, no outside or independent observer would ever have been aware an intercepting laser was fired. THAT event was “cloaked”.

Nonetheless not all physics research venues were impressed. The editors of the journal Nature (to which Gaeta originally submitted his claim) demanded a time gap be created at least three times larger than the one in the experiment, or 45 trillionths of a second. They deemed the experimental value too low (and likely too much within the standard errors) to be worthy of publication.

Not to worry, on borrowing equipment from six other labs, and scouring Cornell’s Physics labs for amplifiers, Gaeta and his team got the time gap up to 40 millionths of a second, and a light sensor in the experiment confirmed the cloaking of the event. Of course, with all that effort and a fine result, Nature finally accepted the paper for publication.


Gaeta’s next goal? Increasing the time cloak to billionths of a second extent, which will require twenty times more power. Is there a possibility to have macro-scale time cloaks (on the order of minutes) for events, say to enable a team of black ops to carry out an assassination and then cloak the event? Doubtful! The power required would likely be larger than all the physics labs and amplifiers on the planet could provide.

Not to worry, the U.S. Defense Advanced Research Projects Agency (DARPA) is partially funding time cloak research to see what more can be done in terms of expanding the time gaps.


Stay tuned!