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In Death's End, the final installment of the Remembrance of Earth's Past series, there is a scene in which Cheng Xin and Luo Ji are invited to tea with Sophon, an embodiment of the Trisolaran's sophon.

Luo Ji is permitted a yes or no question, which is "Is it possible to generate a safety broadcast?" In other words, to send a message to the alien civilizations of the Dark Forest that you are absolutely harmless and incapable of producing harm (through technological breakthrough). Sophon merely answers, "Yes."

This concept comes up after Blue Space betrays the Earth and Trisolaris' coordinates, initiating a dark forest strike. Humans consider the 4 contingency options: exodus to space, creating a black domain to hide in, bunkering in Jupiter's shadow, and regressing to pre-industrial era technology. They settle on bunker, as it's decided regression is not an effective safety broadcast.

What is the safety broadcast that Sophon alluded to?

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    Broadcasting that you're turning your area of space into a sublight-speed zone
    – Valorum
    Commented Aug 1, 2022 at 16:53

1 Answer 1

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'Broadcast' is a bit of a misleading term. Converting the solar system in to black domain (reduced lightspeed region) would be recognisable by other civilisations, so it simultaneously rendered the inhabitants incapable of harming anyone outside the domain, made it virtually impossible for anyone outside to harm those inside, and made their harmless nature visible so there was no reason to try to harm them.

How would a distant observer see the Solar System black hole created by lowering the speed of light? There were two possibilities: for technologically primitive observers, the Solar System would simply disappear; and, for technologically advanced observers, they should be able to detect the black hole, but instantly understand that the system was safe. Take a distant star, a barely visible dot. Anyone casually glancing at it would say: Oh, that star is safe; that star will not threaten us. This was the cosmic safety notice. The impossible was possible, after all.

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  • Hmm. Is there anything that suggests that the black domain is untouchable by outside forces?
    – Valorum
    Commented Aug 1, 2022 at 17:10
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    "Any external missile, such as a photoid, would be traveling at a very high velocity to produce the necessary destructive power, and would thus enter the black domain with a speed far in excess of the modified speed of light inside. Under the theory of relativity, such an object would have to proceed at the (new, low) speed of light as soon as it crossed the barrier, and its excess kinetic energy would be converted to mass. The first part of the object entering the black domain would suddenly slow down and acquire a much larger mass,
    – Michael
    Commented Aug 1, 2022 at 17:16
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    while the rest of the object, still moving at the unaltered speed of light, would run into the first part, thereby destroying the entire missile with the impact."
    – Michael
    Commented Aug 1, 2022 at 17:16
  • So resistant rather than completely untouchable.
    – Michael
    Commented Aug 1, 2022 at 17:17

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