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simple universe and cosmology simulations

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Feb 7, 2024, 11:29:56 PMFeb 7
We all need mental models to think about complex things. Computer
people have it easy in this respect -- there is allays "someone" to
bounce your crazy ideas off of to see whether or how many internal
contradictions your latest theory has.

When thinking about "life in the universe" we can either stick to just
the instances we know of and end up believing all swans are white, or
we can go a bit more general and see what e.g. set of simple ideas
leads to behaviour we expect and try and learn what must be so
special about those simple ideas to give those answers.

A little problem I've often set students on in the past is to write a
computer model to simulate some aspect of the universe as we think we
know it. Students generally think this will involve YUGE programs run
on supercomputer clusters to even get a smidgen of any credit. But I'm
generally thinking about 100-line programs. :)

We'll look here at a simple "universe simulation". We'll let the
universe start off with a "seed" and see whether it can grow and in
what way it will grow. We can think of the thing being simulated as
the matter in our little universe or the matter-energy or maybe even
the "information" or "life" in the universe.

As I have told students in the past, there is a simple 1-line change
to a universe simulator that makes it a cosmological simulation.

That's right. The essential thing between the 2 is that a cosmological
simulation must end up with the observer aka "you" in it. It must
entail a set of rules and procedures that can "start from almost
nothing" and lead to you. One of the few things any of us (assuming
you all exist) can know for sure -- I exist. And that selects which
models actually are worthwhile looking at. If your mental model cant be
shown to evolve to a universe with me in it then it must be a busted
model. :)

I've put the simple C++ code and the output files on my web page at

The basic program "t" (I name all programs "t" unless I think I may
need to refer to them in a book sometime) simulates the universe as a
string of letters. A "1 d" universe. "Laws of physics" are simple
rewrite rules. They take one sequence of letters (if it presently
exists in the simulated universe) and change it to another sequence of
letters (or possibly the empty string).

In this way we can simulate typical quantum mechanical operations of
"creation" and "destruction". Or you can think of them as chemical
reactions taking a set of ingredients and producing a set of products.

One basic temple for a "law of destruction" might look like this:

A B C -> A C

Which we can read as "when B is in the company of A and C then it can
disappear leaving only A and C".

A "law of creation" is just the reverse of a law of destruction:

A C -> A B C

Reading "if A and C exist together somewhere, a B can pop up out of nowhere".

Or we can make creation "cost something" by using something like:

A C -> B

"A and C can be totally converted to a B".

We can also have "laws of magic" that can change one thing to another:

A B C -> A D C

Or the transformation can cost something:

A B C -> A D

The simple program at the URL does something a little different that
"thinking about it" -- it just generates a set of random rewrite rules
and sees what happens. Does the "amount of stuff" tend to increase or
decrease. Does (Shannon) entropy increase or decrease? Does the
universe "get stuck" or keep on expanding? Or just collapse?

We can imagine this all depends on the exact "laws" that are used. If
there are more "creation" laws then we might expect the universe to
just keep getting bigger. Small things keep getting converted to "more
things". Contrariwise, if there are more rules of destruction the
universe could be expected to run down quickly or dead-end itself
(i.e. the letters left don't match any laws and the universe freezes to death).

In the case of random laws we expect something interesting will happen.

It turns out it usually means the universe keeps on expanding with
"more stuff" or "more information" or "more life".

For the 2nd program -- as I said a simple mod of the first one :) --
we want to find whether a universe can "create a copy of me". For
"me" we might as well in this case just chose a special sequence of
letters and see whether a random set of rules can create it from a
small random collection of junk.

In this case the string selected to represent something we simply must
have in the final universe at some point DOES get created. But not
the first 20 universes or so.

Adjusting the various parameters can lead to all manner of interesting

It turns out to be extremely fun to play Creator and you begin to see
why She likes all the mucking around so much.

The US govt portrays itself as the world's preeminent
superpower, so to acknowledge that there are things in their
airspace, whatever they are, that are faster and more manoeuvrable
and run rings around fast jets doesn't play very well.
So there's the embarrassment factor, and maybe a little bit of
fear that either an adversary has made a quantum leap in
development, which has left the US in a poor second place, or, as
some believe, this really is extra terrestrial, in which case we're
not at the top of the food chain anymore.
-- Nick Pope, 02 May 2023

We are not afraid to entrust the American people with unpleasant facts,
foreign ideas, alien philosophies, and competitive values. For a nation that
is afraid to let its people judge the truth and falsehood in an open market
is a nation that is afraid of its people.
-- JFK

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