Timescape by Gregory Benford
In the future [or at least what was 18 years after the year he wrote the book], Earth is facing environmental crises and related consequences.† A physicist in 1998 has designed equipment that will send tachyon advanced waves in such a way it can affect the results of an obscure physics experiment in the 1960's.† It is possible to cause the "interference" in the 1960's experiment to contain a message in Morse Code.† So the scientists in 1998 try to convey a message that will cause people in the 1960's to reduce their harm to the environment.
Timescape explores issues in the scientific understanding of time and the complexities and paradoxes of attempting to communicate with another period of time.† For instance, there is a discussion between two scientists.† One raises the point that if they send a message to the past telling them not to harm the environment - and the people in the past change what they do so that there isn't an environmental crisis in 1998 - wouldn't that mean that scientists in 1998 would have no reason to send the message...† The other scientist agrees that if the result was no crisis, no message would be sent and that would be a logical inconsistency.† However, he suggests, what if the message causes the people in the 1960's to act in such a way that there is an environmental crisis in 1998 that is serious enough to motivate sending the message, but not as extreme as the crisis would have been if there were no changes made in the 1960's.† In that case, perhaps there is no paradox.† ...Or does communications to the past result in a branching of universes somewhat like in the Many Worlds interpretation of quantum physics?
Later, the book shows how the message from 1998 can cause those receiving the message in the 1960's to publish their results.† The distribution and reading of the discovery is different from how the world would have been without the message, but not necessarily in any large way.† However, a seemingly minor shift could influence other events of historical magnitude, as in one case in Timescape.
The book jumps back and forth between England in 1998 and California in the early 1960's.† We get a flavor of both cultures, plus a look at academic / scientific culture.
Timescape includes a good deal about the personal lives of a few main characters.† Readers looking for more character orientation in their hard SF are likely to appreciate this.† Other readers may feel that the amount of personal-life scenes that don't influence later parts of the book is more than needed.† Even if these are tangential, they don't tend to be particularly long scenes and the writing is good.
As far as my personal tastes go, I would have liked more exploration of the physics of time in the book.† That is higher in my reading priorities than another book on how the personal lives of scientists are influenced by the ecology in immediate crisis or the personal lives of a scientist who has what may be a great discovery but has difficulty building a clear scientific case for it.† Timescape has all of these; itís just a question of the proportions you would prefer them in.
Communications back in time may not be forbidden by the laws of physics as we understand them today, but that doesn't tell us whether or not it is feasible in practice.† In Timescape, communications back in time depend on the existence of tachyons and whether tachyons [and other particles] actually have detectable "advanced waves" (a wave moving backwards in time as well as one moving forward).† This is not forbidden by physics, but neither has it been show to be true.
Even granting the necessary physical phenomena can exist, maintaining the necessary logical consistency for it to function could be tricky.† For instance, in Timescape, scientists send back a message about ecological threats.† The message includes references to specific brand name products.† When the message is received in the past, those name brand products do not yet exist.† If the message were to somehow result in a product of that name never being released, that may constitute an unacceptable logical inconsistency.† Perhaps it's possible for the message to cause the product never to exist, but a "clerical error" in the future results in this "incorrect" name being included in the message.† Maybe not.† Logical consistency presents so many potential barriers to successful communication with the past...
A "Many Worlds" kind of branching of parallel universe may save us from having to worry about logical consistency.† However, if any changes resulting from communications with the past create a separate parallel universe, then you haven't accomplished anything in the version of the future you live in.† You can only have an impact on somebody else's parallel universe.