from white on black backgroundMONDAY, AUGUST 29, 2011
My topic - the Universal Dovetailer Argument
This month I'm breaking with tradition and actually *posting early!*This is
for two reasons, 1) because I have some time on my hands being sick at home
today and 2) because the topic may be more challenging than the average so
I'm giving you all some time to get acquainted with the material before we
discuss it. I'm going to try to be as concise as possible but a certain
level of verbosity is unavoidable.

First of all, let me make my own position on this theory clear. I don't
'believe' it as such. In fact I actively disbelieve it, for reasons I won't
go into here - I'll leave that for the day. Nevertheless it does open up
some pretty fascinating philosophical vistas, and ties into a whole bunch
of stuff including the dreaded Cryogenic Paradox, quantum theory, the
objective existence of numbers, the modern multiverse cosmology, and more.
As a backgrounder, I stumbled on this through a Google group called the
Everything List, a discussion group bound together by the concept that
'everything exists', which I'd been referred to by someone who left a
comment on my Cryogenic Paradox post.

But I promised concision, so onwards! The idea I want to present is called
the Universal Dovetailer Argument (UDA), and was proposed by a French
academic by the name of Bruno Marchal <>,
who is a research fellow in AI at the Université Libre de
Here is Marchal's original paper on the
which I attempt to distill this summary). Unfortunately Marchal's
argumentation style is not exactly crystalline in its clarity for a
non-mathematician, a problem exacerbated by his inconsistent conjugation of
English verbs. Nevertheless, I think I've succeeded in deciphering 90% of
the gist. I'll tell you the bit I don't follow (and have my doubts about)
when I get there.

So, the UDA proposes itself as a refutation of materialism, and purports to
"reduce physics to fundamental machine psychology", by which I think he
means the mathematically defined laws by which computing devices (namely,
us) compute their self-consistent states (I'll explain). It relies on the
following assumptions:

   - That a brain (and a consciousness) can be substituted by a
   'Turing-machine' type computational device, i.e., we could theoretically
   have a brain transplant and not know the difference.
   - The 'Church Thesis', which basically says that one type of computer
   can always perfectly emulate another (contradicting my suspicion that my
   old Commodore 64 would struggle with Photoshop CS5, but if you fed the data
   through in small enough blocks, and could maintain state with enough
   memory...). We're talking mathematical inputs and outputs here, not the
   time taken nor the form of presentation.
   - Arithmetical Realism - namely the idea that mathematical relations and
   propositions "are true independently of me, you, humanity, the physical
   universe" etc.

These assumptions sum to what Marchal calls Classical Computationalism (or
just "comp" for brevity's sake). I can personally accept the second of
these assumptions, C64 notwithstanding. The first and last are a lot more
debatable, but as I said, we're "running with it".

So here goes the argument in 8 steps:

   1. "Comp" allows for teleportation, whereby a person is cut at one place
   and pasted at another. (I can't resist relating a joke at this point: Man
   standing in teleporter hears an announcement: "Your duplicate has been
   created at the destination, but due to a technical fault, there has been a
   delay in vaporization at this end. Please stand by...") Now the person at
   the end of the teleportation is a "consistent extension" of the one prior
   to teleportation, so Marchal assumes it is the same
   individual/consciousness. All we have done is add to the individual's
   belief set a new one relating to location. This is in fact irresistible
   unless we try to "stick" consciousness to the physical substrate of the
   "computation" that is the mind, but that violates assumptions 1 (and 2)
   2. Now let us imagine this person keeps a diary which gets teleported
   along with him or her and records what happens. A third-person account of
   the teleportation will at this stage be completely consistent with the
   diary (first person) account. The pronouns will be different, but the
   proposition will be logically identical ("Pierz was teleported from
   Melbourne to Sydney" vs "I was teleported from Melb to Syd"). But imagine
   there is now inserted a time delay between "departure" and "arrival" in the
   teleportation process. If the teleportee is not allowed reference to any
   external cues to the passage of time (eg, a non-teleported calendar), the
   first person account will now differ from the third person account, as the
   diary will not register any delay - the gap is invisible/non-existent from
   the first person perspective.
   3. Now imagine the person's data is transmitted to two destinations, say
   Adelaide as well as Sydney. From the first person perspective, what is the
   chance my diary will say I arrived in Sydney after the experiment is
   complete? It has to be 50%. Even though there is no uncertainty in the
   third-person account, there is an irreducible uncertainty in the first
   person account (I'm going to use the abbreviations 1-p and 3-p from now on
   for these perspectives because I'm lazy). From 3-p, there's a fully
   determined, objective situation; from 1-p, there's a 50% uncertainty.
   Objectively (3-p) the person is going to end up in both places. But there
   is no diary where someone is in both places at once, so you could say from
   the point of view of the traveller, the chance of being in one place or
   another post-teleport is 50% each way.
   4. Now let us introduce a delay in one branch of the teleportation. The
   internet connection to Adelaide is down and they can't email all the
   attachments required for reconstitution for a couple of days, say. In this
   scenario, the probability of ending up in Sydney or Adelaide is *still* 50%
   - the delay is irrelevant because it does not affect the 1-p account.
   5. *Now* imagine we don't vaporize the person in Melbourne, but simply
   email a copy to Sydney and duplicate them. What now is the probability of
   ending up in Melbourne or Sydney from 1-p? It is still 50%, since the
   "original" that stays at home can be considered as having been "teleported"
   to the same place with a zero delay.
   6. Next we can postulate the possibility of "teleporting" a subject into
   a virtual reality rather than an actual one. All the preceding steps can be
   performed with the reconstitution being virtual rather than real, without
   any change in the 1-p account for as long as the simulation is maintained
   and given an accurate enough virtual Sydney.
   7. Now we come to the so-called Universal Dovetailer (UD), and now we
   get contentious (if the above wasn't contentious enough).  The UD is
   basically an algorithm that executes all possible programs. It is called a
   dovetailer because, in order not to "crash" on infinitely executing
   programs (there is in theory no way to exclude such algorithms), it
   executes one instruction from each program in sequence then goes on to the
   next instruction in the next program, and so on, thus doing what a
   multitasking OS with a single core effectively does - simulating
   parallelism. Now suppose that the universe is large enough and robust
   enough to allow such a device to run forever. It follows that will generate
   all possible Turing machine states infinitely often, including all possible
   virtual reconstructions of yourself in all possible locally emulable
   environments. And since we have established (or assumed) that your
   mind/brain is emulable and any such emulation will in fact result in a 1-p
   indeterminacy as to where 'you' end up - this indeterminacy being the
   infinite union of all finite portions of the UD machine where you are
   emulated - we can see that 'you' exist within the UD as one trace through
   the all the existing computational states of this "machine". So physics, as
   the science which gives correct predictions about the probabilities of
    entities being in particular states, can be reduced to "fundamental
   machine psychology", or "some measure of consistent states" from a
   first-person point of view. It becomes an "average of consistent
   histories". GEDDIT??
   8. OK, but here's the obvious objection. What if the universe is not big
   enough to support the UD? Then our normal physical predictions would be
   safe from interference from all the virtual states of this machine.
   According to Marchal, "such a move can be considered ad hoc and
   disgraceful" (!), but he doesn't really explain the disgrace of this
   obvious objection. Instead he goes on to argue that we cannot associate
   inner experiences with physical processing (this was the point I raised
   earlier when introducing teleportation), but instead that inner states must
   correspond to a type or branch of computation in the platonic mathematical
   space of all computation (Arithmetical Realism). But I cannot frankly
   follow his reasoning why (it's on page 11 of the paper if you want to have
   a crack). He draws the conclusion that computation is not performed by a
   physical machine (which the universe may not be able to support), but that
   these computations exist in mathematical space, requiring no physical

OK, so the logic of the last part gets kinda weird and flaky, but consider
this: modern cosmology now suggests the existence of an infinite universe,
or in fact an infinity of universes separated by space expanding faster
than the speed of light and therefore unable to communicate in a classical
sense, but presumably still quantum-entangled. If this picture is true,
then the universe itself could be the UD, creating all possible Turing
states, and if it is the case (solving the Cryogenic Paradox) that a
sufficiently accurately emulated state in effect "blurs into" all other
equal states (i.e., another way of saying there is only one observer), then
something like the above becomes almost plausible. We have a version of
Everett's many universes interpretation of QT, in which the wave function
is the "presence" of all the other universes adjoining ours in information
space or, to use Marchal's lingo, it is the 1-p indeterminacy of all
consistent states in the infinite trace of the UD. You can at this point
dispense with the awkward terminology of the "dovetailer" and simply
consider  the universe as a generator of all possible configurations of
information. If information rather than matter is primary, then something
along the lines outlined above is not as preposterous as it at first sounds.


(I realize this was a mega-crash-course in a some kinda out-there ideas, so
apologies for that. I do recommend having a go at the first 12 pages of
Marchal's paper if anything is unclear.)
AT 11:19 

Guy Montag <> said...

Thanks for this and the link to the paper. I've been lurking on the
everything list for several months now. needless to say Marchal's ideas are
fascinating, regardless of how you feel about them. I for one am slowly
coming to terms with his argument and am becoming increasingly sympathetic
to them, although i do have several reservations.

Does the UDA really require empirical confirmation from modern cosmology?
In the last step Marchal disposes of any concrete UD and only requires a
platonic one. Elsewhere he says

"Once you explain why arithmetical machines are statistically right
to believe in physical laws without any real universe, such a real
universe is redundant.
By Arithmetical Realism and OCCAM razor, there is no need
to run the concrete UD, nor is there any need for a real concrete
(Or you can use the movie graph argument to show that a first
person is not able to distinguish real/virtual/and *Arithmetical*
nature of his own implementations, and this eliminates OCCAM.)"


In any case, it is causing me to challenge many of my assumptions about
OCTOBER 3, 2011 7:30
C. Seligo <> said...

Forestalling an objection by calling an argument a disgrace is so French.
There are much clearer exponents of these ideas. David Deutsch, in The
Fabric of Reality, does a pretty good job of explaining what it would mean
to call the universe "computational" , without ever insisting that it is so
(which is my sense of graceful thinking.) Sci Fi author Greg Egan's short
stories and novels explore these questions, while retaining a sense of how
human beings and error in the system can be very consequential. Read "The
Kidnapping" or "Transition Dreams" or " The extra" to get an idea of how
virtualized consciousness is problematic. There are so many unknowns,
tenuous propositions, and we are generally very bad about thinking about
large numbers, to say nothing of infinity. Any history of science will find
great minds crashing at the cusp.
FEBRUARY 10, 2012 11:59

Post a 
Home <>

You received this message because you are subscribed to the Google Groups 
"Everything List" group.
To post to this group, send email to
To unsubscribe from this group, send email to
For more options, visit this group at

Reply via email to