Twistor by John Cramer
There's a stereotype that hard SF has flat characters and other limited literary qualities. (And that is certainly true of some.) Twistor is more towards the other end of the scale. There's a lot of character and setting material, and that is on the level of a professional writer. However, readers who want the fullest literary quality may find that this material doesn't have the same spark as in finer literature.
Those of us who can appreciate literary quality, but put a greater importance on the story and the ideas, may feel the book begins slowly. Much of the first 20% of the book is the build-up of the characters and setting, with only foundations for the plot. From there, it develops more of the scientific framework on which the major plot depends. By the time one is 40% or so through the book, we're ready to set off on the adventure that one might see in a synopsis of the novel.
The story begins in and around the physics department at a university campus. David is a post-doc working with Vanessa, a Ph.D. student. Her thesis advisor, Saxon, also has a tech company on the side. The company is having trouble, which is leading to pressure from Megalith, a big corporation that has loaned Saxon money. Megalith is a greedy company with few scruples. David and Vanessa are working on a device intended to do research that could lead to new data storage technology (of a kind Saxon's company is contracted to Megalith for). However, an unexpected phenomenon takes place when the device is run.
At first, it's not clear what is happening. After some efforts, David and Vanessa conclude their device has sent some material from our universe to a parallel universe. As a result of some confusion and caginess by Saxon, Megalith gets the impression advances are being made in storage technology that Saxon wants to keep for himself. This eventually leads to Megalith sending men to steal the device. David responds by programming the device to send itself to the other universe. However, when a friend's children come out of a hiding place in the laboratory, David and the children end up being in the wrong place and are sent to the other universe along with the device.
The device ran on the university's electrical power system, which is not available in the other universe. So it can't be used as-is to return back to our universe. In the following days, David tries to figure out how he can put together equipment to return, Vanessa and others try to figure out what happened and how they can get to David and the children, and Megalith continues its dirty tricks to force information out of Saxon and Vanessa. David and Vanessa had become romantically involved, so each is especially concerned with what has happened to the other.
David and the children are on an Earth-like planet (similar gravity, air, water, etc.) However, they are in a forest of huge trees. The device has actually carved out a room-sized sphere in the trunk of one of these trees. David has to chop a hole to give them an exit from the tree. And it turns out they are a ways up in the tree and have to make a ladder out of cables in order to climb down. They find an interesting, distinctive ecosystem.
David puts together a scaled-down version of the device that permits limited communication between the universes, but doesn’t allow transportation of people. When he learns Vanessa is in danger, he has to figure out what he can do.
Twistor was written in 1989 when superstring theory was newer and the excitement over it was greater. Today's readers may find the talk of some topics that were cutting edge in the 1980's to be less awe-inspiring today.
Cramer has an interesting idea. He put an afterward section at the end of the book. He explains that in good hard SF the speculative science should seamlessly integrate with the well-established science from the readers' point of view. That makes the story feel convincing. However, it has the disadvantage that readers can come away from a book thinking the speculative material is actually currently accepted science. So, for the benefit of readers who want to know which is which, Cramer went over what in his book was established science, which were theories scientists were considering and which were just ideas Cramer invented for the book.
I like that concept. Nevertheless, I didn't find everything in the story convincing. Please don't misunderstand. Cramer is a notable physicist, I am not. If I tell you one thing about physics and Cramer tells you something different in a non-fiction work, believe him not me. But in a work of fiction, Cramer may sometimes choose something less plausible. In Twistor, the story revolves around the invention of a device that can swap a certain volume of space between two parallel universes. At one point, a scaled-down version of the device is built which can only move smaller quantities between the universes. This smaller device operates using a small number of flashlight batteries. That was not convincing for me.