Ask a human to describe its Universe and you will at least get a reply.
快猫短视频s, in particular, will give well-rehearsed explanations of black holes,
particle physics, plate tectonics and natural selection. The assumption here is
that we are detached enough to grasp what these things are really like鈥攂ut
are we? What would such concepts look like to beings living in a parallel
universe where matter obeyed very different principles?

We reckon it鈥檚 a good thought experiment, but it鈥檚 not easy to set up: just
where do you go to get such a view? Happily, one of us鈥擳erry
Pratchett鈥攈as a ready-made destination, the Discworld universe where his
fantasy tales take place.

This is a world where you鈥檒l see a giant turtle sculling its way across
space. On its back are four elephants supporting a flat disc of a world,
inhabited by humans and not-so-humans. Death rides out on a horse called Binky,
weeding out losers and winners as they struggle to survive in a richly hostile
world.

And there is a city, Ankh-Morpork, a fetid squalor of an urban sprawl whose
river is so far from the liquid-running-splish-splash mental model we have of
water that it achieves another state of matter: somewhere between a gel and a
solid. A body hits its surface and rebounds. And light behaves like a proper
wave should鈥攊t floods across the land and you can surf on it.

The connective tissue of this world, however, is not science but magic.
Discworld鈥檚 place of learning is the Unseen University, where bright young
wizards in the High Energy Magic Building probe the fundamentals of thaumaturgy,
and the older wizards wish that they wouldn鈥檛. Here theories of time hang around
in a book of witches and ghouls, and a librarian adjusts to life as an
orang-utan after too much magic escaped from the padlocked grimoires of the
library shelves.

In short, no bad place to go for an outsider鈥檚 view of our world. Because
Discworld doesn鈥檛 run on science, we needed its wizards to invent a world like
ours if they were going to help us. So they came up with Roundworld鈥攁
thinly disguised replica of our own universe. Obviously this was to be a
science-fiction world鈥攈ow could it be anything else? And yes, many diehard
science purists look askance at science fiction. But such sniffiness is
unfounded. After all, the central tenet of science fiction is nothing as crude
as predicting the future, but dealing with the 鈥渨orlds of if鈥�. At its best, a
science-fiction story is a thought experiment. What would happen if this or that
amazing ingredient turned out to be real?

The good thing about 鈥渋f鈥� is that it does not imply belief. Mathematicians
make perfectly logical deductions from false hypotheses: what, for example, if
there were to be a counterexample to Fermat鈥檚 Last Theorem? It鈥檚 useful for them
to think this way even if the result of such thinking turns out to be that there
isn鈥檛 one. Similarly, science-fiction fans don鈥檛 believe the events in stories
they read about aliens on distant planets any more than they believe that UFOs
have landed on this one. What they do relish is putting their brains in gear to
check the story is true to known science except for an added ingredient or two,
that humanoid robot or this antigravity.

When Larry Niven wrote Ringworld, a story about a gigantic
ring-shaped structure spinning around a star, it won all the big science-fiction
awards. Not long after, however, fans from the Massachusetts Institute of
Technology paraded at Worldcon, a big annual science-fiction convention, waving
placards protesting about a serious scientific error. The error was not that the
amazingly strong material that the Ringworld was composed from doesn鈥檛
exist鈥攖hat鈥檚 a 鈥渨hat if鈥� implicit in postulating Ringworld. No, the
problem was that Niven鈥檚 Ringworld is dynamically unstable. The MIT students did
the mathematics to show that it would slowly drift off centre.

Take another example. What should a really good historian tell you about the
assassination of Abraham Lincoln? A very average historian would name the
murderer and the date. A good historian would tell you what make of gun the
assassin used, how he concealed it under his coat, why he hatched the plot to
begin with. But an excellent historian would tell you what could have happened
had Lincoln only been injured.

The 鈥渨hat if?鈥� questions posed by science fiction explore what mathematicians
might call the 鈥減hase space鈥� of the unrealised possibilities surrounding the
things that actually happen. Those MIT students were worried about Ringworld鈥檚
stability because, as engineers and physicists, they were accustomed to ask
whether disturbing this system in some way, would the disturbance grow or die
out? Stability as a concept involves exploring the region of phase space that
lies near a genuine real-world event.

And science fiction can also help us ask an even bigger kind of what-if. The
world we know emerges from a Universe twisting and turning and growing ever more
complex as it gives birth rise to quarks, quasars, galaxies, planets,
bacteria鈥攁nd, of course, human beings. But what lies in the phase space of
both nearby universes and nearby theories of how universes work? Answering that
via fiction could really open our eyes to features of our own Universe which we
might otherwise miss.

For example, are fundamental particles really
fundamental, or are they just things that you bring into existence when you
smash lumps of matter together very hard? Is DNA-based life the only possible
kind? And鈥攎ore abstractly鈥攁re the explanatory powers of science
really that special? After all, there are alternatives鈥攃ommon sense (in
which the Sun goes round the world because that鈥檚 what it looks as if it鈥檚
doing), religion (in which things happen because that鈥檚 the will of God) and
magic (in which things happen because of spells and rituals).

So, what happened in our Discworld thought experiment? To our surprise, the
wizards began to take over, driving the story in the direction fitted to their
own concerns and prejudices rather than those that the authors might have had in
mind. For example, when Rincewind the wizard sees fish trying to crawl up on to
land inside the Roundworld project, he takes pity on them and throws them back
into the sea. He doesn鈥檛 realise that they are on their way to evolving into
land animals.

The point here is that if you think evolution selects for
fitness in an absolute sense, then the gasping fish on land is unfit and ought
not to survive. But actually evolution selects for relative fitness: since the
gasping fish has no serious competition, it will work fine. Rincewind doesn鈥檛
realise this, but we can use his mistake to dramatise a valid scientific
point.

To Discworlders steeped in magic, the science of Roundworld鈥攐ur
science鈥攄id not always look as alien as you鈥檇 imagine. The wizards, for
example, felt quite at home with genetic engineering, thinking it was a bit like
casting a spell in the sense that you don鈥檛 always get the results you predict
(or want).

What did surprise the wizards, however, was the sight of Roundworld golfers
feeling pride when they got a hole in one. This was unjustified, they felt,
because of all the times the golfers had tried to get a hole in one and failed:
aim hundreds of golf balls at hundreds of holes and one is likely to go in by
chance. On Discworld, billion-to-one chances happen nine times out of ten. This
follows from narrative imperatives: unless the hero succeeds in battling against
impossible odds, there is no story. In Roundworld, what went awry was not the
laws of probability so much as people鈥檚 perceptions of those laws.

The wizards were also horrified to find that chalk hills are made from dead
animals. That trillionfold death doesn鈥檛 move us鈥攖oo many, too small, too
long ago. We also have a problem getting our minds round huge amounts of time.
But those wizards were watching the Earth in fast-forward mode: they got a feel
for deep geological time. Unlike us, they were truly unhappy about how many
times a promising species got wiped out by a comet, asteroid, methane eruption,
ice age, or whatever.

Roundworld, as they said, is 鈥渁 good place to evolve on, but you wouldn鈥檛
want to live there鈥�.

We now think that in a sense they may be right.

I READ them all. All the books with 鈥榯ime鈥�, or quantum鈥� or 鈥榗osmos鈥�
somewhere in the title.

This time, I鈥檓 going to understand. This time I promise not to daydream.
This time I promise not to get lost around chapter three. This time I promise
not to say, 鈥楾hey鈥檙e making it up as they go along, I could get at least
five different answers to a simple quadratic equation when I was at school
and no one called me a physicist, where do people get these jobs?鈥�

But I am haunted by the possibility that, somewhere out there, quiet
people in comfy armchairs are leafing through the very same pages and then
suddenly standing up and shouting: 鈥極f course! I see it all now! At last
I understand!鈥�

It never works like that for me. For a few minutes after the last page
everything seems to hang together, the whole glittering mass of it. Then
it collapses. Like a black hole, probably. And what is left is not knowledge,
but richly textured ignorance.

And now here come Kate Charlesworth and John Gribbin, to give a drink
of water to a drowning man.

Well, you know all about them already, because here between big floppy
covers are Junior Chicken as the cosmic Virgil and Alexis the Quantum Cat
in the role of inquiring thicko, that is, us, or anyway me. Alexis spends
a lot of time in a state of quantum panic. I know how she feels.

Let me say here and now that when it comes to drawing a cat going through
an anxiety attack, no one can beat Charlesworth. And Gribbin is a scientist
who not only reads science fiction but writes it too, so that鈥檚 all right.

They lull you into a false sense of security with practically ordinary
science on page one, and then, well before page 64, here they are running
a Blue Peter programme for cosmic engineers (鈥楬ow to Make a Time Machine
out of a Pair of Old Wormholes and an Everyday Asteroid鈥�). If you don鈥檛
go away and have a serious lie down every half a dozen pages, then the general
effect is of looking into a kaleidoscope while riding the Wall of Death.

The frenetic layout is sometimes too distracting. Several times I found
myself trying to follow the narrative as conflicting signals sent the focus
of attention skidding across the page.

And I was trying to follow the narrative. I admit that phrases such
as 鈥楢n atom is 100 billion times bigger than the seed from which our entire
Universe has sprung鈥� create within me an irrational desire to set fire to
the nearest astrophysicist, but there were moments when I was actually aware
that I was understanding what was going on (the James Clerk Maxwell Wave
Particle Sheep will not be forgotten in a hurry). That is rare.

The Cartoon History of Time is witty and good-looking and informative
and for all I know may be accurate too, although lovely phrases such as
鈥榯he froth on the never ending River of Time may extend indefinitely鈥� may
create an irrational desire on the part of some astrophysicists to set fire
to Gribbin.

With rather more authority I can tell you that the jokes are pretty
good. Oh, all right, very good. I laughed out loud several times. This is
a funny book. It almost can鈥檛 help it.

Hey, it says here 鈥榠n far less than the blink of an eye, a region of
space 10-**17 the size of an atom was inflated into a region the size of
a grapefruit鈥�. Oh, god.

Terry Prachett is the author of comic fantasy books based on Disc-world,
which is flat and rides through space on the back of a huge turtle