Sometimes we have to look in the mirror and say, you know what? Maybe I'm the problem. Watching this video (https://youtu.be/fJ7lYU7l58k?si=xkc87qjjQKSN1JWa) made me realise that what I was experiencing was a skill and experience issue. My brain was just not ready to appreciate this book and I'm gonna DNF @57ish% and come back when I've read more hard sci-fi and Egan in general to see if I can git gud.

I adored Reasons to be Cheerful and Quarantine, but I think I really have a lot to through before building back up to this.

Once again, grabby hands x Audible Included library has failed me.

Also, my brain is mush right now and this is the worst possible time I could have blundered into this book. Avoiding blurbs and diving in sometimes is a blessing and other times it's rally unhelpful, but I am my own worst enemy.

It was weird, but I found the geometry too hard to visualize. ( )

Greg Egan might be my favorite sci-fi author (top-5 for sure), but this is both his worst book to date and a bad book overall. It is built around some completely novel physical geometry and universe dimensionality, poorly explained in the book (to have any hope of understanding, you'll need a guide on Egan's website), but that's fine. The problem is that is really the only positive part of the book; the rest is a stupid/weird morality story about different groups and coexistence (4000/800 had a simplified form of this as well; I hope Egan hasn't gone full SJW) -- the characters themselves are really shallow and unremarkable.

There is a moderate amount of focus required to understand the geometry of the world, but there is no payoff. Skip. ( )

For fans of multiple world-time-lines and especially for fans of Christopher Priest's [b:The Inverted World|142181|The Inverted World|Christopher Priest|https://images.gr-assets.com/books/1245646253s/142181.jpg|2226603], welcome to Dichronoauts.

Walking one way brings you to the future, the other, to the past. But space is still space and time is still time. Thanks to the little creature attached to the people here, we've got a cultural/exploratory thing going on that is the spiritual godchild of Priest's classic novel. Just look at the cover to get an idea. That's a picture of the Earth. As in, the Earth, to these people, is shaped like an hourglass. That makes EVERYTHING pretty messed up. :)

So let's explore! And the characters barely know anything more than us, so we're introduced to many theories that may sound absurd, but the Earth is ALREADY absurd... because time and alternate worldlines are as easily traversed as walking across a field.

As a LITERAL worldbuilding novel, almost all the fun is in exploring and visualizing the world they inhabit.

The rest... well... as okay. Not overly special. The chasm was pretty cool. Bits and pieces elsewhere. But overall, I was not overly invested in any character.

Win some, lose some. The cool aspects are VERY cool, however. :) ( )

Our universe is described as having three physical dimensions, plus one time dimension, where the eigenvalues for the physical dimensions are all the same, but the eigenvalue for time is opposite (x, y, z, t: +, +, +, -) or (x, y, z, t: -, -, -, +) A universe where the eigenvalue of time is equal to that of the spatial dimensions (x, y, z, t: +, +, +, +) has been explored in depth by Greg Egan in his Orthogonal series of books. However, what would happen if the eigenvalues of the physical dimensions were not identical, e.g.: (x, y, z, t: +, +, -, -)? Or even, in a 4-physical-dimensional universe, (w, x, y, z, t: -, +, +, +, -)? In the universe I'm interested in, there is one time dimension with a negative eigenvalue, and 3 or 4 physical dimensions, but one of the physical dimensions has an eigenvalue equivalent to that of the time dimension, while the others have the opposite eigenvalue as is the case in our universe. How would the physics and chemistry of such a universe be different to our own? How would movement in the negative-eigenvalue spatial dimension work? Would life - or even matter - be able to exist? Egan's "Dichronauts" has two spatial dimensions and two of time in its 4-space. This system seems to involve three spatial dimensions and one time, but one of the spatial dimensions and the time dimension has negative value. The two 4-spaces seem similar to the extent that it is difficult to discern if they are the same or different. Me? I'm puzzled. In the special relativity, the 4-distance is calculated on this way: ds2=dx2+dy2+dz2-dt2ds2=dx2+dy2+dz2-dt2.

In your world, having a space-like dimension with -1-1 signature, it would be calculated like ds2=dx2+dy2-dz2-dt2ds2=dx2+dy2-dz2-dt2.

It has far consequences:

The space won't be isotropic any more. Thus, things would behave differently if you rotate them around the zz-axis and any linear combination of xx and yy. The laws of the physics would behave differently in the different (space) directions.

On the Noether theorem, the symmetries of the Universe have a deep connection to its conservation laws. The isotropy of the space results the conservation of angular momentum. In a non-isotropic Universe, the angular momentum isn't a conserved quantity any more. Any effect will be instant around any direction, for which dx2+dy2-dz2=0dx2+dy2-dz2=0. It would effectively mean, that moving any point-like particle on such an axis, you get the same system. This would be a new symmetry, which would result a new conservation law, what doesn't exist in our Universe. To calculate, which conservation law is it, is complex but it doesn't require much more math/physics skills as in the high school. Any physics student from around the second year of his studies can do this for you, although it probably wouldn't be easy to find a cooperative one. Having an additional, conventional space dimension (thus, a 5D space-time) doesn't change this significantly. I am not sure, but this "new conservation law" would probably essentially mean, that any point of the space, for which dx2+dy2-dz2=0dx2+dy2-dz2=0 are the same. If it is correct, then the result is that this negative-signature space dimension "eats" all of the others. Thus, you have essentially a 1D space.

Note: this all depends on if the (non-curved) space-time of your Universe is still governed by the Special Relativity, as ours.

NB: Things would significantly change if you calculate what happens with the gravitation, too. Gravitation changes the geometry of the spacetime, thus the distances wouldn't be calculated like ds2=dx2+dy2-dz2-dt2ds2=dx2+dy2-dz2-dt2, instead you have a tensor (essentially a table) for which
ds2=dr--??????gxxgyxgzxgtxgxygyygzygtygxzgyzgzzgtzgxtgytgztgtt??????dr--
ds2=dr_[gxxgxygxzgxtgyxgyygyzgytgzxgzygzzgztgtxgtygtzgtt]dr_
gn1n2gn1n2 is determined by the mass and impulse distributions, it is essentially the General Relativity analogy of the gravitational field. For small (much lighter as black holes) and slow (much slower as speed of light) you get the Newtonian gravitation from it.

It is possible, that near strongly gravitational objects the space-time would be multidimensional again.
I think this universe is so remarkable, because most universes would not give rise to stars, planets, and intelligent life. From my perspective, this may be one of the crappier universes I’ve ever read. I personally think humans are a scourge. But the fact remains it required a great deal of fine-tuning of physical constants to produce a universe with stars, planets, and at least one intelligent life form. I am a true solipsist. I believe everything we experience could be a virtual reality that the universe we experience does not exist anywhere. Whereas, this is a distinct possibility, I would not say it is probable. If our universe was just a dream, mathematics would not be the helpful too it is. The universe would be more irrational. I am certain the universe we see only exists in our minds. Real objects in our universe do not possess the property of color. Our brains colorize the universe. Moreover, we see large solid objects, when reality is quite different. Every particle is in some manner of speaking is spread-out through all of space-time. We only see a limited number of dimensions. We don't perceive time as spatial dimension. The universe we see with our eyes is nothing the universe that really exists. More than likely the universe is a multidimensional complex field filled with colorless, odorless, tasteless, silent multidimensional waves, branes à la String Theory or what have you. Your body like most matter is mostly empty space spread-out over space-time…

There is no evidence anything infinite or infinitesimal actually exists. More than likely space-time is finite, but unbounded.

I believe micro-states increases with space-time inflation, and decreases with space-time deflation. It is, therefore, space-time inflation that gives rise to the 2LoT and the arrow of time. I believe the number of micro-states decrease, entropy increases, matter and energy congeals and unifies with space-time implosion that we could travel backwards in "time" without revisiting Earth history. Time started at the Big Bang, but there are many ways back to the Big Bang. Our past is only one of those ways. All black holes lead back to the Big Bang, and the beginning of time.

I imagined the nascent in the exact same predicament as Buridan's ass. Buridan's ass was a perfectly logical creature the starved to death between two equidistant bushels of hay.

From one assumption, I have been able to deduce a large part of modern physics. I assume the universe is perfectly logical, that the universe is not capricious, random, or arbitrary. I admit I can be wrong. This is merely a working hypothesis on my part. If it is true, then the universe had a near infinite number of equally good pathways it could have taken. If the universe was not conscious, if it did not have free choice, how could it choose one random path over a near infinite number of equally good paths? I concluded that like electricity, the universe took every possible pathway. Moreover, this is the only way Feynman's path-integral, sum-over-histories solution to QED makes any sense. It is the only way it could be true. Max Tegmark expands on this very concept, though he dismisses my analogy to Buridan's ass.

My conjecture explains why there is so much balance and order in our universe. According to it, the universe has to balance out. The multiverse must have zero energy. It explains why for every action there must be an equal and opposite reaction. It explains the laws of matter-energy conservation, and all the other conservation laws. I believe the universe is completely balanced, that there is a hidden symmetry, that the universe is a perfect unity, that the universe is not capricious, random, or arbitrary. Every scientific discovery with the exception of Borh's and Heisenberg's interpretation of Quantum Mechanics supports my conjecture, that the universe is completely deterministic, unitary, and holographic. "Entanglement" is more evidence for my overly speculative, wild conjecture. The existence of vacuum fluctuations would falsify my theory. Vacuum fluctuations and virtual particles appear to exist. I predict they will disappear as our knowledge of physics increases.

Bottom-Line: How about another 5-dimensional universe Mr. Egan like you did on “Diaspora”, but this time using 5-fold-eigenvalues? Greg Egan keeps on producing SF like no other. 5 stars for the physics, 0 stars for the story which is crap; rounded to 4 stars overall.

NB: Ah! It feels good to be back at reviewing books…

SF = Speculative Fiction. ( )