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So clear Steve!

This sounds impractical. How big could you make it and what would you use it for? Is it possible to change network communications by using delays and switches, or would you notice slow clocks that are set on other paths?


I think I figured out the reset!

It is starting with mirrors A and D reflecting and E and H transmitting.

To reset turn A transparent. For the length of the obscurity gap light going from A to D will combine with the light that is taking the ABCD path. I don't see any way around the double brightness.

When the light from the ABCD path leaves D, turn D to transparent.

For the time the double intense light is striking E, E must be half transmission and half reflection. I don't see any other way to make this reset work without this halfway state. Does this make sense? I thought about it for a long time:-) Half of the split light travels EFGH and the other half goes from E to H.

As the double light finishes striking E, turn E to full reflection and in the time it takes light to go from E to H, H is turned to full reflection.

My head is spinning:) Does this work? Is there any way to do it without the mirror being half and half transmission? Let me know and I will celebrate with a treat:-)


Outstanding Jheri - you nailed it!

I didn't mention the fact that you need one of the mirrors to be a beamsplitter (half transmission and half reflection), but figured it out. I'm impressed! The next time we get together I'll buy you the pint of gelato at L'Arte del Gelato .. what you normally have:-) In the meantime you have cause for celebration. This is a bit tricky.

As to your earlier comment. Yes - you can do this with any form of communication where the speed of light - or an electric signal - is constant. It would be easier in the case you mention, but perhaps not as visually dramatic.

Jean Russell

Unfortunately the mirror of G and C are observable unless they are also cloaked. If they are cloaked, than their version of G and C are visible to the observer looking in their direction instead of toward the light source. I forget the math for calculating the angle opposite GH, but that would define the limitation to the view of the observer. We can add appropriate blinders in the thought experiment and restrict the observer from turner her head. Also, to cloak (make invisible) the diamond or clock, wouldn't C and F have to become transparent, making the light path ABGH?


There are a two types of cloaks - one cloaks objects and the other events. This is a temporal cloak (thanks to Jean for pointing out the confusion!).

You may or may not be able to see the mirrors, but the point is the observer is looking at the object that is being illuminated by the light source. The apparatus can insert a cloaking time that can't be detected without observing and understanding the apparatus. To the observer looking on the source-A-D-object-E-H path the cloak is a true cloak. It is just a gedanken experiment:-)

Consider the periscopes AB and CD -- together they are just a periscope pair. In a gedanken experiment the mirrors can be perfect. Assume that A and D can switch to transparent. There is no way an observer can determine if the light path is ABCD or AD unless some sort of travel time experiment can be conducted. The only light available is traveling along the path. The mirrors, by themselves, are invisible as no external light is provided. But even if they were visible the observer couldn't determine the path without some extra information.

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