The original questions were posted here.

From Dr. Seth Lloyd’s answers it is clear that:
1) he is assuming the Deutsch quantum computing model, which is Turing reducible and
2) he is assuming that quantum particles encode a finite amount of information, so that they are completely discrete in every possible sense, including: space/time, mass, energy, momentum, and any other possible physical value.

1 and 2 are, by the way,  standard views in both fields, quantum computing (defined by David Deutsch) and quantum mechanics (as defined by several authors). From 1 and 2 it can be deduced that Seth Lloyd indirectly implies that the universe is Turing computable (since the only difference between a quantum computer and a Turing machine -disregarding the usual fact about the infinite tape- is the run time, a fact borne out by his answers). Regarding 2, most quantum quantities and solutions to equations suggest that there are minimum values, namely the Planck time and the Planck length, a fact which assorts well with a discrete scenario. However the issue is commonly bypassed by physicists and by the theory itself. In other words, quantum mechanics seems to be consistent with both a continuum and a  discrete universe and does not offer final evidence or an ultimate theoretical conclusion one way or the other. In fact it is usual among physicists to think of  superposition as an entanglement in a space continuum, which would allow  a particle to be in an infinite number of  states simultaneously.

From 1 and 2 we can conclude that Church’s thesis–in both its weak and strong non-physical and physical versions, which we will discuss in a separate post– remains intact  even when Dr. Lloyd’s approach is closer to a physical basis (an accepted modern model of the universe) and of course the empirical data (which supports quantum mechanics itself). His chain of reasoning is basicaly as follows:

a&b->c:

a) the universe is completely describable by quantum mechanics
b)  standard quantum computing completely captures quantum mechanics
c) therefore the universe is a quantum computer.

He proved a relation between a and b, which probably puts the standard (or some standard) theory of quantum mechanics and the standard quantum computing model in an isomorphic relation with each other. Here “standard” means that some assumptions were made.

Here is a literal transcription of the answers to my questions given by Dr. Seth Lloyd:
—–
A: A quantum computer differs from a Turing machine in that its bits are quantum bits, and so can exist in a superposition. In addition, it can be instructed to put those bits in superpositions. These two features make a quantum computer apparently much more powerful than an ordinary Turing machine. What a quantum computer does is still Turing computable, but a typical quantum computation of T steps on N qubits requires O(2^N) bits on a classical Turing machine, and O(T 2^2N )logical operations. That is, it takes a Turing machine exponential amounts of time and space to simulate a quantum computer or quantum Turing machine. (Compare the definition of universality: a universal Turing machine can simulate any other Turing machine in polynomial time.)
So a quantum computer is apparently more powerful than a classical computer.
There is a perfectly good definition of a quantum Turing machine (basically, a Turing machine with qubits and extra instructions to put those qubits in superposition, as above). A universal quantum computer is a physical system that can be programmed (i.e., whose state can be prepared) to simulate any quantum Turing machine. The laws of physics support universal quantum computation in a straightforward way, which is why my colleagues and I can build quantum computers. So the universe is at least as powerful as a universal quantum computer. Conversely, a number of years ago I proved that quantum computers could simulate any quantum system precisely, including one such as the universe that abides by the standard model. Accordingly, the universe is no more computationally powerful than a quantum computer.

A: As long as a quantum particle encodes 3, 4, or M states, where M is a finite number, then the computational picture remains the same (this is also true classically). Now, it is a fact that a physical
system with finite energy confined to a finite volume of space has only a finite number of discrete states. So we are OK.
——-
“Now, it is a fact that a physical system with finite energy confined to a finite volume of space has only a finite number of discrete states.”—I wish the obviousness of this final remark were readily apparent to me.

Dr. Seth Lloyd’s work is very compelling, and I am engaged in a project inspired by ideas related to those he expounds here—mining the computational universe to uncover Lloyd’s programmer monkeys. But I find that his theory of the universe– which by the way I agree with, though it may sometimes seem otherwise– assumes no less than any other conception of the universe, which leaves space for continued thinking on evocative hypotheses, including Church’s, even as we attempt to hack the universe.

Lloyd’s thesis adds to the conception of the Universe as a Turing computer an important and remarkable claim (albeit one that depends on the conception of the quantum computer), viz.  that the Universe is not only Turing computable, but because it is constituted by quantum particles which behave according to quantum mechanics, it is a quantum computer computing its future state from its current one. The better we understand and master such theories, the better prepared we would be to hack the universe in order to perform the kind of computations -quantum computations- we would like to perform.

I would agree with Rudy Rucker too as to why Seth Lloyd assigns such an important role to quantum mechanics in this story. Rudy Rucker basically says that being a subscriber to quantum mechanics, Lloyd doesn’t give enough consideration to the possibility of deterministic computations. Lloyd writes, “Without the laws of quantum mechanics, the universe would still be featureless and bare.” However, though I am one among many (including Stephen Wolfram) who agree  that it is unlikely that the universe is a cellular automaton, simply because cellular automata are unable to reproduce quantum behavior from empirical data (but note that Petri and Wolfram himself attempt explanations of quantum processes based on nets), there’s  absolutely no need to rush headlong into quantum mechanics. If you look at computer simulations of physical systems, they don’t use quantum mechanics as a randomizer, and they seem to be able to produce enough variations to feed a computational universe. Non-deterministic randomness is not neccesary; pseudorandomness or unpredictable computation seem to be enough.

Ist das Universum ein Computer?
http://www.dtmb.de/Aktuelles/Aktionen/Informatikjahr-Zuse/
Germany, November 2006, Informatik Jahr
Deutschen Technikmuseum Berlin
From Konrad Zuse’s Invention of the Computer to his “Calculating Space” to Quantum Computing.

Lesson One: For someone with a hammer in his hand the world seems to be a  nail. Joseph Weizenbaun.

Lesson Two: Knowing the input and the transition function of a Turing machine we know everything about it. Marvin Minsky.

Dr. Zuse’s futuristic drawing

- The first talk entitled  ”What Can We Calculate With?” by Prof. Dr. Bernd Mahr from the Technische Universitat of Berlin was a very good introduction to the standard theory of computation based on Turing’s model and classical mathematical logic. His remarks on the time when computing arose from math because the Greeks discovered they were unable to compute the square root of 2 were interesting. He pointed out some evident but not always explicit facts: Calculation has a subject (the individual who calculates), an object (what is calculated),  a medium (how it is calculated), and a symbolic representation (the language -binary, for instance). His use of the Leibniz medallion for explaining   starting points, ending points and transitions in a calculation was elementary but interesting (transitions: intermediate calculations). Further explanations of reversibility and non-linearity using transition nets were also illuminating. The context of a calculation (or computation) and the strong relation between the computation itself and its context is  such that it is sometimes difficult to distinguish them. Since any process can be seen as an object in itself, the context can become the calculation and the context of the context too. In some way, as we know, the concept of calculation is a constraint of a part of a calculation, and then it is defined in terms of an input, an ouput and a transition. He pointed out too that behind many of these complicated systems there is a Turing machine. It  is no longer visible from the top, but it is there.

Dr. Konrad Zuse

- Dr. Horst Zuse’s  talk titled “Konrad Zuse’s Calculating Space (Der Rechnende Raum)”:
Dr. Konrad Zuse’s son, Dr. Horst Zuse made some interesting remarks about his father’s book “Calcualting Space”,  in which  Dr. Zuse proposes studying  the universe as a digital system, specifically a cellular automaton. Dr. Horst Zuse is a professor at the Technische Universitat of Berlin and his personal webpage can be found at: www.cs-tu-berlin.de/~zuse and www.zuse.info

Dr. Konrad Zuse’s son, Dr. Horst Zuse

Dr. Zuse’s father’s main  question was: “Is Nature Digital, Analog or Hybrid?” It seems that he tended to answer “Digital,” proposing a No-Yes value language (binary). His thoughts were published in the Nova Acta Leopoldina. He evidently did acknowledge that there could be problems attempting to reconcile an explanation of the Universe in terms of Cellular Automata with Quantum Mechanics and General Relativity.

According to Konrad Zuse, laws of physics could be explained in terms of laws of switches (in the computational sense). Physical laws are computing approximations captured by the formulas in our models. He saw that differential equations could be solved by digital (hence discrete) systems.

Dr. Carl Adam Petri at the Berlin conference

- Dr. Carl Adam Petri (yes, the creator of the Petri nets!) on “Rechnender Netzraum” or “Computing Net Universe”:
According to Dr. Petri, at the Planck length quantum computing can be described by digital systems using combinatorial models (net models, kennings, combinat), and therefore the universe can be studied using discrete nets which are even capable of explaining quantum and relativistic fundamentals like Bell’s theorem and Heisenberg’s uncertainty principle. That would mean that discrete systems (for instance those proposed by Stephen Wolfram) would suffice to explain even quantum and relativistic phenomena.

According to Petri, measurement is equivalent to counting. For instance in S.I. one second is 9192631770 Cs periods. In fact Norbert Wiener proposed some axiomatics of measurement.

The correspondence of Petri nets with the Heisenberg uncertainty principle arises from the limitations of our observational capacities when carrying out measurements. When two different types of observations are performed, -for example momentum p and position q- we can only see p or q in a chain of succesive events related by a causality net. His nets as well as his explanations of such phenomena are very neat and elegant. The relevant slides  on causality and linear logic may be found  at:
http://www.informatik.uni-hamburg.de/TGI/mitarbeiter/profs/petri/slides/
He also distributed a CD with his slide presentation at the conference.

For Petri, the Universe is a Petri Net.

Dr. Seth Lloyd’s presentation at the conference in Berlin, Germany

- Dr. Seth Lloyd’s talk entitled “The Universe is a Quantum Computer”:
According to Dr. Seth Lloyd, professor at MIT, because quantum mechanics is the most fundamental and foundational theory of the universe,  assuming it would lead us to conclude that  the whole universe is a quantum computer computing itself. The input consists of basically random processes (which he characterizes using the metaphor of  programmer monkeys) and the outcome is all that we see around us. Because an elementary particle interacts with others and changes its state, he argues that each particle can be seen as information, as a qubit (for quantum binary digit), which unlike a bit  can be in 0,1 or both states at the same time according to the quantum property known as  entaglement or superposition. When a particle interacts with other particles they change their states according to a quantum logical gate.

Therefore, his conclusion is that the universe is not only a computer but a quantum computer. However some questions arise:

1. What does he mean by quantum computing? According to the standard model (by Deutsch) quantum computing is Turing computable (disregarding run time). If Lloyd is assuming the standard model then the universe is indeed a quantum computer, but even more remarkably (since we have scientific and philosophical hypotheses like the Turing thesis) it is Turing computable. However, if he is assuming the more general quantum mechanics model, let’s say the standard model in physics (which basically assumes the possibility of harmless continuity rather than inquiring into it) he is saying that the universe is not a computer (since the term derives  from what we mean by Turing computable and hence covers  digital computers too). So the assumptions made are significant and cannot be glossed over if one wishes  to argue convincingly that  the universe is  a computer in some standard sense. If what we  assume to be computing is something that seems to be deterministic or rule-based, the concept becomes fuzzy and  additional remarks need to be made.

2. What if a quantum particle encodes more information than just a 1 or 0 for the spin or any other quantum property? Let’s say a third value, or even worse, a non-computable number of values. In quantum mechanics for example, the superposition of a particle assumes an infinite and non-countable number of places since it is in all the spectra at the same time. If space/time is a continuum then it is evidently in a non -countable number of positions, which leaves us with  a non-computable model, or at least with something that’s neither a Turing-computable model nor a standard quantum-computable (namely Deutsch) model. And this is not a simple assumption since it requires anotherTuring-type thesis which in the final analysis does not answer the most fundamental  question, i.e. whether the universe is a computer or not and if it is, what kind of computer (in the computational power sense) it is.

I raised these questions with Seth Lloyd and I will be posting his answers soon.

Seth Lloyd at the conference in Berlin

A remarkable idea proposed by Seth Lloyd concerned “hacking the universe”. As Charles Bennett used to say, a computer is a handicapped quantum computer. So if Lloyd is right, a computer is not only a handicapped quantum computer but it is not taking advantage of the full computational power of the universe and it is just patching the universe instead of hacking it, as it would be in its power to do. By contrast, a quantum computer uses some particles that are already computing “something” (nothing less and nothing more than the processes in the universe ) to perform the computation that we want it to perform. It can be said to be  ”hacking the universe” in Lloyd’s terms.

On the other hand, if the notion of programmer monkeys is valid it should be possible to test it experimentally. Under the supervision of M. Jean-Paul Delahaye, computer science professor at the University of Lille I (http://www2.lifl.fr/~delahaye/) we are undertaking this task. We are exploring  Lloyd’s quantum computational universe (or at least a handicapped but representative part, the recursive computational universe), applying some complexity measures (universal distribution, average-case complexity or Levin’s measure) in order to uncover the monkeys behind the Universe, or in other terms, to analyse the average distribution of randomly discrete systems with random inputs.

Is Seth Lloyd falling into the carpenter’s problem of thinking that the universe is a nail and the moon made of wood?  Is it because he is a quantum computer scientist that he thinks the universe is a quantum computer? He argues of course that the charge is unfair, but then we have been told by Dr Petri  that the Universe is in fact a Petri Net which probably  needs neither strong randomness nor quantum mechanics!

Here  is a video online in which he explains much of this:
http://www.edge.org/video/dsl/EF02_Lloyd.html

Dr. Zuse’s futuristic drawing 2

- Jurgen Schmidhuber reprised his algorithmic approach to the theory of everything in his talk entitled   “The program that computes all computable universes”.
Jurgen Schmidhuber’s major contribution probably is his Speed Prior concept, a complexity measure similar to Algorithmic Information Complexity, except that it is based on computation speed and not program length. i.e. the fastest way of describing objects rather than the shortest.
There is more information on his website: http://www.idsia.ch/~juergen/ (where he includes an unfavorable review of  NKS) and in his slide presentation on the Speed Prior at: http://www.idsia.ch/~juergen/speedprior/sld001.htm
Of course Schmidhuber himself has identified a problem with the Prior measure: If every possible future exists, how can we predict anything?

Other interesting talks on philosophical issues: If the Universe is a computer, therefore the human mind should be a computer too.
Is “the Universe is a  computer” a metaphor?
My answer: The metaphor is “The Universe is not a Computer”

Lesson Three: Metaphors can be reversed.

Kovas Boguta’s  talk was titled ”Is the Computer a Universe?” In it he pointed out the richness of mining the computational universe of simple programs.

Because we were together during the gala dinner I had an interesting exchange with Dr. Konrad Zuse’s son, Dr. Horst Zuse (Also at our table were the Technikmuseum director Dr. Dirk Bondel and  my colleague Kovas Boguta from Wolfram Research, among others). He shed some light on his father’s interactions with Alan Turing ( none apparently),  with von Neumann (some interaction regarding the controversy over who first built a digital computer and concerning von Neumann’s architecture, which  our current digital computers do not conform to, the ALU being separated from the memory as it is in Zuse’s conception but not in  von Neumann’s original design).

Zuse’s Z1 first computer “the input device, something equivalent to the keyboard, at the Technikmuseum in Berlin”

- Symbolic Visualizations, University of Texas:http://cvcweb.ices.utexas.edu/ccv/projects/VisualEyes/SymbVis/index.php- Proof nets and zero-knowledge proofs.

Kurt Godel workshop for studying his legacy and writings. Lille, France, May 19-21, 2006

My thoughts, ideas, references, comments and informal notes:

- The wheel machine, a machine for real computation which I am proposing -as a thought experiment- in a forthcoming paper  on the Church-Turing thesis -Yes, one more paper on the CT thesis!- with comments on Wilfried Sieg’s paper entitled “Church Without Dogma: Axioms for Computability”

- “In case Cantor’s continuum problem should turn out to be undecidable from the accepted axioms of set theory, the question of its truth would loose its meaning, exactly as the question of the truth of Euclid’s fifth postulate in Euclidian geometry did”. Godel replies: “It has meaning anyway, as Euclid’s fifth postulate gave rise to other now accepted mathematical fields.”

- Godel Gibbs Lecture and his dicotomy on absolutely undecidable propositions and the computational power of the human mind (Turing did great work… but he was wrong when he proposed his formal theory as a model of human thought…)

- New contacts and references: Olivier Souan, Rudy Rucker, Karl Svozil

Mark van Atten’s “On Godel’s awareness of Skolem’s lecture”.
Rick Tieszen

- Herbrand on general recursive functions, letter to Godel.

- Leibniz’ influence on Godel’s arithmetization?

- Sources: Godel Editorial Project. Firestone Library, Princeton University. I.A.S. Marcia Tucker, librarian for Godel papers.

- Godel’s concept of finite procedure as the most satisfactory definition of computation. “A machine with a finite number of parts as Turing did” or “finite combinatorial procedure” as a definition of an algorithm, mechanical or computational procedure.

- Computation’s main constraints: boundness and locality (paper from Hernandez-Quiroz and Raymundo Morado).

- Aphorisms and autoreference (Gabriel Sandu and Hinttika)

- Feferman on Turing

- Is Sieg’s paper and the question of “finite machine=effective procedure” a tautology? In fact such an approach seems to be one of the most strict versions of the Turing Thesis, and even though both Church and Turing probably did propose it in such a strict sense, extensive versions of the thesis have traditionaly covered more content, but even when it is strictly stated that there is still space for a thesis, it is neither proved nor provable from my point of view, and most authors would concur, though some clearly would not. I will comment on this more extensively later, since this was one of my Master’s topics and merits a post by itself.

- Putnam’s thought experiment on cutting all sensorial inputs. Solution: It is impossible in practice. However, machines are an example in a sense, and that is why we do not recognize intelligence in them – they are deprived of  sensorial capabilities.

Yes, Godel found an inconsistency in the U.S. constitution. My answer: One? Certainly a bunch. That’s why we need lawyers, who make them even worse.