Would sending a message to the past need faster-than-light travel, and who says so?

"Albert Einstein in 1907 [ 1 ] [ 2 ] presented a thought experiment of how faster-than-light signals can lead to a paradox of causality"

"The same thought experiment was described by Richard Chace Tolman in 1917; [ 4 ] thus, it is also known as Tolman's paradox ."

"A device capable of "telegraphing into the past" was later also called a "tachyonic antitelephone" by Gregory Benford , David Book, and William Newcomb ."

"According to the current understanding of physics, no such faster-than-light transfer of information is possible."

"Physicists posit that faster-than-light particles cannot exist because they are inconsistent with the known laws of physics ."

"If such particles did exist, they perhaps could be used to send signals faster than light and into the past. According to the theory of relativity , this would violate causality , leading to logical paradoxes such as the grandfather paradox ."

"In 1962 [ 5 ] and again in 1969 [ 7 ] Oleksa-Myron Bilanuik, Vijay Deshpande and E. C. George Sudarshan discussed the possibility of a class of faster-than-light particles consistent with special relativity."

"The term tachyon was coined by Gerald Feinberg in a 1967 paper titled "Possibility of faster-than-light particles". He proposed that tachyonic particles could be made from excitations of a quantum field with imaginary mass ."

"It was soon realized that Feinberg's model did not in fact allow for superluminal (faster than light) particles or signals and that tachyonic fields merely give rise to instabilities, not causality violations."

"They call first of them "the primitive causality", and the second one – "the Einstein or relativistic causality". The claim is that SS "violates only the Einstein causality, but not primitive causality.""

"Obviously, the authors ignore the relativity of simultaneity and its unavoidable implications (the Tolman paradox) that would arise from SS."

"The term " tachyon " was coined by Gerald Feinberg in a 1967 paper [ 7 ] that studied quantum fields with imaginary mass. Feinberg believed such fields permitted faster than light propagation , but it was soon realized that this was not the case."

"For closely related reasons, the maximum velocity of signals sent with a tachyonic field is strictly bounded from above by the speed of light. [ 2 ] Therefore, information never moves faster than light regardless of the presence or absence of tachyonic fields."

"Under no circumstances do any excitations of tachyonic fields ever propagate faster than light—the presence or absence of a tachyonic (imaginary) mass has no effect on the maximum velocity of signals, and so unlike faster-than-light particles there is no violation of causality ."

"As noted by Gregory Benford , special relativity implies that tachyons, if they existed, could be used to communicate backwards in time [ 2 ] (see tachyonic antitelephone )."

"Because time travel is considered to be non-physical, tachyons are believed by physicists either not to exist, or else to be incapable of interacting with normal matter."

"G. A. Benford; D. L. Book; W. A. Newcomb (1970). "The Tachyonic Antitelephone". Physical Review D . 2 (2): 263."

"The idea goes back at least to the 1960s, when physicist Gerald Feinberg tried to give faster-than-light particles a formal place in theory."

"If a tachyon could outrun light, then under some circumstances different observers could disagree about the order of events. A particle emitted in one frame of reference could look, in another, like it was absorbed before it was ever sent."

"That kind of reversal fed the old fear that tachyons would demolish causality itself."