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How to contact "Them"? Tachyon Signal (2)
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"Causality" is a fundamental Principle of Physics. If Tachyons can transmit information faster than Light then, according to General Relativity, they should violate the Law of Causality, leading to logical Paradoxes of the "kill your own grandfather" type. This is often illustrated with thought experiments such as the "Tachyon Telephone Paradox" or "Logically Pernicious self-Inhibitor."
The problem can be understood (note: really?) in terms of the Relativity of simultaneity in the Special Relativity Theory, which says that different "inertial reference frames" will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events (technically, these disagreements occur when the spacetime interval between the events is "space-like", meaning that neither event lies in the future Light Cone of the other). If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the Speed of lLght or slower, the Mathematics of Simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.
However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received BEFORE it was sent, so that the signal could be said to have moved backward in time.
Because one of the two fundamental postulates of Special Relativity says that the Laws of Physics should work the same way in every Inertial Frame, if it is possible for signals to move backward in time in any one frame, it must be possible (that they move backward) in all frames. This means that if observer "A" sends a signal to observer "B" which moves faster than light in "A'"s frame but backwards in time in "B'"s frame, and then "B" sends a reply which moves faster than light in "B"'s frame but backwards in time in "A"'s frame, it could work out that "A" receives the reply before sending the original signal, challenging Causality in every frame and opening the door to severe Logical Paradoxes.
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Ma se un "nemico", anche solo potenziale, od una pessima persona (che conoscete come tale), vi chiamasse, così, sine causa, Voi...rispondereste?!? Meditateci su.
Dunque - cellulare scarico a parte - se qualcuno o qualcosa esiste "lungo la strada" e/o vicino a quelle stelle e/o su qualche pianeta che vi orbita attorno ed ascolta, il messaggio è già arrivato.
Se vogliono o volevano farlo penso pure che ci abbiano già risposto, ma qui torniamo al punto di partenza: come (e cioé con quale tipo di Segnale) lo hanno/avrebbero fatto?
Siamo certi che la "risposta" non sia già arrivata mentre noi non stavamo ascoltando oppure non potevamo né possiamo e/o sappiamo ascoltare, per semplice inadeguatezza dei nostri recettori?
La domanda è lecita e, se volete, legittimamente banale, ma la risposta, però, è impossibile darla. Almeno: io non ne sono capace.