Starfarers by Poul Anderson
The premise of the story is that in a future in which humans have spread out into the solar system, scientists detect trails of x-ray sources thousands of light years away. It is deducted that these result from large objects moving very close to the speed of light. It is concluded that the most likely explanation is interstellar spaceships using technology humans have not yet developed. A crucial issue in reaching such speeds is the enormous energy needed to accelerate a large object to that speed. Scientists can only think of one likely source for so much energy - zero point energy from the vacuum. Using this inspiration, scientists work towards and achieve such an interstellar drive.
As humans start to explore nearby stars, they also send out a special starship Envoy on a scientific mission to the closest group of x-ray sources - 5000 light years from Earth. Although the crew will only experience a few years passing, when they return to Earth, the world will have changed 10,000 years. Therefore, those who are willing to be on the crew tend to have issues or reasons to leave behind everything they know.
The first 8 chapters of the book are tiny stories of crew members before leaving on this mission. This provides the reader with background on the characters; but (depending on your tastes) it may seem it takes too long in doing so. Even after their flight begins, there are more chapters about the characters or their interrelationships. Eventually, we get to the point where tiny stories are interjected giving the reader an idea what is happening back on Earth and its neighboring star systems. There is an emphasis on changes that are taking place that affect those humans involved in interstellar travel. And the mission begins to have more than just personal and interpersonal material.
The mission detects a decreasing amount of x-ray sources suggesting a decline in interstellar travel by the aliens they have gone to meet. Meanwhile, thousands of years have passed on Earth and the human space travelers are also finding interstellar commerce is not what it once was.
They do find a couple of planets to visit. Readers do get to see more than the lives of the crew. We are even introduced to the idea of an intelligence that is not made of matter, but is somehow based solely on quantum states [see below]. Most of the contact with aliens deals with physical beings that once had an interstellar civilization. However, at this point most members of the alien race now just want a stable, peaceful society on their planet. To them, the presence of the starfaring humans poses potential problems.
The various interpersonal dynamics that have been developing throughout the book lead to a series of events resulting in a crisis on the ship. A serious conflict provides a period of action and excitement in the story.
The book continues with glimpses of the changes occurring in human society on Earth and in neighboring star systems. Envoy returns to an Earth that is dramatically changed.
Anderson presents a scenario that might explain the Fermi Paradox ("If there is intelligent life out there, why havenít we seen any of them?"). There is also a view of an alien species and their society. The book covers a considerable length in distance and time. Youíre likely to find something of interest in the book. However, you may be left with mixed feelings about it.
I can't say the quantum state life sounds so likely to me, but I couldn't say why it conflicts with physics. What I would ask is how can a life-form reach the point of having advanced science without tool use for experimentation, and is it feasible for something that is only quantum states (but not quanta) to have the necessary level of tool use?