Ah, 1970. The world was full of optimism. Nixon was the president, the oil crisis had not yet begun, and all the science fiction authors thought you could fly to another star in a human life span just by scooping the hydrogen existing in abundance in interstellar space, using it for fusion, and generating infinite 1G of acceleration.
The idea is that the mass ratio problem of interstellar travel disappears if you don't have to carry your own propellant, thus allowing a ship to accelerate to such a large fraction of light speed that the time dilates to the point where hundreds of light years of travel can be experienced in a mere few years of shipboard time. Thus also making the problem of finite crew life span to go away, and as a triple whammy, the magnetic scooping field also protects the ship from impacts and deadly radiation. On top of all this it's mere icing on the cake that with a constant thrust of 1G, you don't have to worry about the effects of microgravity on organisms.
In reality, there are numerous problems with a Bussard ramjet.
Bussard himself noted that just building such a thing would be enormously difficult, after all, one of the components is a fusion reactor (using hydrogen as a fuel supposes proton-proton fusion, which is extra difficult, and might not be possible), and that interstellar hydrogen is not evenly distributed. And then the density of the hydrogen in actual space is lower than he assumed. The density in local space near our Sun is particularly low. You would also have to reach a significant fraction of c just to reach a speed where the ramjet can [i]start[/i] operating, meaning you'd need to carry some fairly exotic secondary form of propulsion in addition to the ram scoop. It might not be possible to build a laser that can ionize a large enough volume of hydrogen to make it responsive to the magnetic scoop, while not using more energy than the fusion reactor provides.
In 1978, T.A. Heppenheimer wrote that the ramjet would actually experience drag, and it most likely wouldn't be possible to build a scoop that wouldn't fail to stress long before the extreme velocities in this novel could be reached. There is a maximum speed for such a ship which is 12% of c.
There are also some cosmological assumptions that might be considered outdated today. But I won't go deeper into those. It's not like we've fully figured out the universe even today.
But, I can't help but feel this is all mere pedantic nitpicking. After all, compared to the question of can a future spaceship ever travel faster than light? (Physics gives us a hard "no") An interstellar space drive concept that seems merely extremely improbable, is still vastly more realistic. In the end, these are engineering problems, not the breaking down of a fundamental physics principle. It's conceivable that a sufficiently advanced civilization could solve these problems and... wait. What's this?
A world ruled by Sweden? This is not science fiction. This is fantasy.
The novel is like those examples in physics textbooks that ask you to imagine a spaceship with a clock inside it moving at near light speed to illustrate the effects of relativity, except instead of being contracted into one paragraph and a diagram, it's stretched out into the length of a full novel. Sound exciting? But fear not, it's at least well written. Poul Anderson is one of the rare breed of science fiction authors who can both write prose and understand physics. I could imagine some of his descriptions of stars and galaxies sounding amazing spoken in Carl Sagan's voice over a backdrop of 1970s synths.
With all joking done and all the issues with the drive concept enumerated, it does not seem like the property of a just universe that this novel should be seemingly less well known and accoladed than Niven's Ringworld that was published in the same year, with jankier writing and far more handwavey science. Just a nomination for Hugo? If I had the power to heap all the awards on Tau Zero, I would. If you're a fusion ram scoop enjoyer, this is the book to read.
>"The conditions are too unnatural. Some of us can adapt, but I've learned that others can't. So we absolutely have to push tau down as low as may be, no matter what the dangers. Not simply to make the trip itself short enough for us to endure. But for the psychological need to do our utmost."
>"How is that?"
>"Don't you see? It's our way of fighting back at the universe. Vogue la galère. Go for broke. Full steam ahead and damn the torpedoes. I think, if I can put the matter to our people in those terms, they'll rally. For a while, anyhow."
>"The wee birdies sing and the wild flowers spring,
>And in sunshine the waters are sleeping —"