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A tachyonic antitelephone is a hypothetical device in theoretical physics that could be used to send signals into one's own past. Albert Einstein in 1907[1][2] presented a thought experiment of how faster-than-light signals can lead to a paradox of causality, which was described by Einstein and Arnold Sommerfeld in 1910 as a means "to telegraph into the past".[3] The same thought experiment was described by Richard Chace Tolman in 1917;[4] thus, it is also known as Tolman's paradox.

https://en.wikipedia.org/wiki/Tachyonic_antitelephone

Suppose that it could be made and we could send message from the future and affect the past. Ignoring the grandfather's paradox, is there a limitation to how far in the past we can change an event, and what determines how far in the past we can send information to?

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    The reason something is called a "paradox" is that its premise is incoherent, and we cannot "suppose" otherwise because it is still incoherent. Even if you "ignore" the grandfather's paradox the law of explosion will manifest the incoherence in multiple other ways. The paradox of signals that are exchanged if and only if they are not exchanged is discussed by your own link, there is Fayngold's paradox, etc.
    – Conifold
    Commented May 25 at 5:15

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The tachyonic telephone is a thought experiment showing that two observers moving slower than light relative to one another and equipped with faster than light communication can arrange to send signals into the past. The exact details of how far into the past depends on the relative motion of the observers and how fast the FTL mechanism is. An upper bound for how far into the past for observers moving apart would be the time at which the observers passed one another (or would have passed one another if their paths are projected backwards).

In general the kinds of time machines envisioned by physics are "closed timelike curves" which can reach back in time only to the point of their creation. Of course due to the grandfather paradox it is generally believed that such devices are impossible. The tachyonic antitelephone is thus an argument against the existence of FTL communication rather than a practical suggestion for a time machine.

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  • If time truly is a fourth spatial dimension, then this would imply that everything is really static (like a 4D film strip) and it’s only our perception traveling along the time axis (somehow) that gives the appearance of change over time, so to me at least physics based time travel in a spatially 4D universe makes no sense. It would have to happen with the actual observer, the observer being the person’s perception, and that discussion is definitely outside the domain of physics. Am I missing something in this discussion that would allow physics based time travel in a 4D world?
    – bob
    Commented May 25 at 14:11
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    Time is not a spatial dimension, but it is a dimension that can "mix" with space in limited ways. There are solutions to Einstein's field equations that allow for closed time like curves ("time travel") but these solutions are generally regarded as unphysical, because they require things like negative energy or a rotating universe which do not appear to exist. As for 4D spacetime in general your concern about the block universe is not unique but the comment section is not the place to discuss it, it probably deserves its own question.
    – Eric Smith
    Commented May 26 at 0:48

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