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In [[computer science]], the [[Actor model]], first published in [[1973]] {{ref_harvard|Hewitt|Hewitt et al. 1973|-}}, is a mathematical model of [[concurrent computation]]. Many fundamental issues were discussed and debated in the early history of the Actor model. See [[Actor model and process calculi history]] and [[Actor model and process calculi]] for coevolution with [[process calculi]].

In [[computer science]], the [[Actor model]], first published in 1973, is a mathematical model of [[concurrent computation]].

==Event orderings versus global state==

A fundamental challenge in defining the Actor model is that it did not provide for global states so that a computational step could not be defined as going from one global state to the next global state as had been done in all previous models of computation.

In 1963 in the field of [[Artificial Intelligence]], [[John McCarthy (computer scientist)|John McCarthy]] introduced situation variables in logic in the Situational Calculus. In McCarthy and HaysHayes [1969], a situation is defined as "the complete state of the universe at an instant of time." In this respect, the situations of McCarthy are not suitable for use in the Actor model since it has no global states.

From the definition of an Actor, it can be seen that numerous events take place: local decisions, creating Actors, sending messages, receiving messages, and designating how to respond to the next message received. Partial orderings on such events have been axiomatized in the Actor model and their relationship to physics explored (see [[Actor model theory]]}).

==Relationship to physics==

PublicationsAccording onto Hewitt (2006), the Actor model claimed that it wasis based on physics in contrast with other models of computation that were based on mathematical logic, set theory, algebra, ''etc.'' In fact physicsPhysics influenced the Actor model in many ways, especially [[quantum indeterminacy|quantum physics]] and [[Theory of relativity|relativistic physics]]. One issue is what can be observed about Actor systems. The question does not have an obvious answer because it poses both theoretical and observational challenges similar to those that had arisen in constructing the foundations of quantum physics. In concrete terms for Actor systems, typically we cannot observe the details by which the arrival order of messages for an Actor is determined (see [[Indeterminacy in concurrent computation]]). Attempting to do so affects the results and can even push the indeterminacy elsewhere. ''e.g.'', see [[metastability in electronics]]. Instead of observing the insides of arbitration processes of Actor computations, we await the outcomes.

==Models prior to the Actor model==

An early issue was to determine what could be observed about Actor systems. The question did not have an obvious answer because it posed both theoretical and observational challenges similar to those that had arisen in constructing the foundations of quantum physics. [[Werner Heisenberg]] in a quote that he attributed to [[Albert Einstein]] many years after the fact stated [Heisenberg 1971]:

The Actor model builds on previous models of computation.

===Lambda calculus===

:It is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe. You must appreciate that observation is a very complicated process. The phenomenon under observation produces certain events in our measuring apparatus. As a result, further processes take place in the apparatus, which eventually and by complicated paths produce sense impressions and help us to fix the effects in our consciousness. Along this whole path—from the phenomenon to its fixation in our consciousness—we must be able to tell how nature functions, must know the natural laws at least in practical terms, before we can claim to have observed anything at all. Only theory, that is, knowledge of natural laws, enables us to deduce the underlying phenomena from our sense impressions. When we claim that we can observe something new, we ought really to be saying that, although we are about to formulate new natural laws that do not agree with the old ones, we nevertheless assume that the existing laws—covering the whole path from the phenomenon to our consciousness—function in such a way that we can rely upon them and hence speak of “observation.”

The [[lambda calculus]] of [[Alonzo Church]] can be viewed as the earliest [[message passing]] [[programming language]] (see Hewitt, Bishop, and Steiger 1973; [[Structure and Interpretation of Computer Programs|Abelson and Sussman 1985]]). For example, the lambda expression below implements a tree data structure when supplied with parameters for a {{mono|leftSubTree}} and {{mono|rightSubTree}}. When such a tree is given a parameter message {{mono|"getLeft"}}, it returns {{mono|leftSubTree}} and likewise when given the message {{mono|"getRight"}} it returns {{mono|rightSubTree}}.

λ(leftSubTree,rightSubTree)

[[Imre Lakatos]] and [[Thomas Kuhn]] had done extensive work on the '"theory laden" character of observation.

λ(message)

''if'' (message == "getLeft") ''then'' leftSubTree

''else if'' (message == "getRight") ''then'' rightSubTree

However, the semantics of the lambda calculus were expressed using [[mathematical logic|variable substitution]] in which the values of parameters were substituted into the body of an invoked lambda expression. The substitution model is unsuitable for concurrency because it does not allow the capability of [[sharing]] of changing resources. Inspired by the lambda calculus, the [[Interpreter (computing)|interpreter]] for the programming language [[Lisp programming language|Lisp]] made use of a data structure called an environment so that the values of parameters did not have to be substituted into the body of an invoked lambda expression. This allowed for sharing of the [[Side-effect (computer science)|effects]] of updating shared data structures but did not provide for concurrency.

The kind of information about the physical world that is available to us according to Chris Fuchs [2004] is “the potential consequences of our experimental interventions into nature.” So this is where we stand with the information available about Actor systems. Characterizing Actor computations is part of [[Actor model theory]].

===Simula===

The importance of all this is that according to [Fuchs 2004] quantum physics "is a theory whose terms refer predominantely to our interface with the world. ... It is a theory not about observables, not about beables, but about 'dingables.' We tap a bell with our gentle touch and listen for its beautiful ring." In concrete terms for Actor systems, typically we cannot observe the details by which the arrival order of messages for an Actor is determined (see [[Indeterminacy in computation]]). Attempting to do so affects the results and can even push the [[Quantum indeterminacy in computation|indeterminacy]] elsewhere. ''e.g.'', see [[metastability in electronics]]. Instead of observing the insides of arbitration processes of Actor computations, we await the outcomes.

[[SIMULA|Simula 67]] pioneered using message passing for computation, motivated by discrete event simulation applications. These applications had become large and unmodular in previous simulation languages. At each time step, a large central program would have to go through and update the state of each simulation object that changed depending on the state of whichever simulation objects it interacted with on that step. [[Kristen Nygaard]] and [[Ole-Johan Dahl]] developed the idea (first described in an IFIP workshop in 1967) of having [[Method (computer science)|methods]] on each [[Object (computer science)|object]] that would update its own local state based on messages from other objects. In addition they introduced a [[Class (computer science)|class structure]] for objects with [[Inheritance (object-oriented programming)|inheritance]]. Their innovations considerably improved the modularity of programs.

However, Simula used [[coroutine]] control structure instead of true concurrency.

===Smalltalk=== <!-- This section is linked from [[Smalltalk]] -->

[[Alan Kay]] was influenced by message passing in the pattern-directed invocation of [[Planner programming language|Planner]] in developing [[Smalltalk]]-71. Hewitt was intrigued by Smalltalk-71 but was put off by the complexity of communication that included invocations with many fields including ''global'', ''sender'', ''receiver'', ''reply-style'', ''status'', ''reply'', ''operator selector'', ''etc.''

In 1972 Kay visited MIT and discussed some of his ideas for Smalltalk-72 building on the [[Logo programming language|Logo]] work of [[Seymour Papert]] and the [[Little man computer|"little person"]] model of computation used for teaching children to program. However, the message passing of Smalltalk-72 was quite complex. Code in the language was viewed by the interpreter as simply a stream of tokens. As [[Dan Ingalls]] later described it:

:''The first (token) encountered (in a program) was looked up in the dynamic context, to determine the receiver of the subsequent message. The name lookup began with the class dictionary of the current activation. Failing there, it moved to the sender of that activation and so on up the sender chain. When a binding was finally found for the token, its value became the receiver of a new message, and the interpreter activated the code for that object's class.''

Thus the message-passing model in Smalltalk-72 was closely tied to a particular machine model and programming-language syntax that did not lend itself to concurrency. Also, although the system was bootstrapped on itself, the language constructs were not formally defined as objects that respond to '''Eval''' messages (see discussion below). This led some to believe that a new mathematical model of concurrent computation based on message passing should be simpler than Smalltalk-72.

Subsequent versions of the Smalltalk language largely followed the path of using the virtual [[Method (computer science)|methods]] of Simula in the message-passing structure of programs. However Smalltalk-72 made primitives such as integers, floating point numbers, ''etc.'' into [[Object (computer science)|objects]]. The authors of Simula had considered making such primitives into objects but refrained largely for efficiency reasons. [[Java (programming language)|Java]] at first used the expedient of having both primitive and object versions of integers, floating point numbers, ''etc.'' The [[C Sharp (programming language)|C#]] programming language (and later versions of Java, starting with Java 1.5) adopted the less elegant solution of using ''[[Boxing (computer science)|boxing]]'' and ''unboxing'', a variant of which had been used earlier in some [[Lisp programming language|Lisp]] implementations.

The Smalltalk system went on to become very influential, innovating in bitmap displays, personal computing, the class browser interface, and many other ways. For details see Kay's ''The Early History of Smalltalk''.<ref name="kay1996">{{cite journal|last=Kay|first=Alan|authorlink=Alan Kay|title=The Early History of Smalltalk|url=http://www.smalltalk.org/downloads/papers/SmalltalkHistoryHOPL.pdf|journal= ACM SIGPLAN Notices|volume=28|issue=3|date=March 1993|pages=69–75|doi=10.1145/155360.155364|url-status=dead|archiveurl=https://web.archive.org/web/20120205105015/http://www.smalltalk.org/downloads/papers/SmalltalkHistoryHOPL.pdf|archivedate=2012-02-05}}</ref> Meanwhile, the Actor efforts at MIT remained focused on developing the science and engineering of higher level concurrency. (See the paper by Jean-Pierre Briot for ideas that were developed later on how to incorporate some kinds of Actor concurrency into later versions of Smalltalk.)

===Petri nets===

Prior to the development of the Actor model, [[Petri net]]s were widely used to model nondeterministic computation. However, they were widely acknowledged to have an important limitation: they modeled control flow but not data flow. Consequently, they were not readily composable, thereby limiting their modularity. Hewitt pointed out another difficulty with Petri nets: simultaneous action. ''I.e.'', the atomic step of computation in Petri nets is a transition in which tokens ''simultaneously'' disappear from the input places of a transition and appear in the output places. The physical basis of using a primitive with this kind of simultaneity seemed questionable to him. Despite these apparent difficulties, Petri nets continue to be a popular approach to modelling concurrency, and are still the subject of active research.

===Threads, locks, and buffers (channels)===

Prior to the Actor model, concurrency was defined in low-level machine terms of [[Thread (computer science)|threads]], [[Lock (computer science)|locks]] and [[Buffer (computer science)|buffers]]([[Channel (programming)|channels]]). It certainly is the case that implementations of the Actor model typically make use of these hardware capabilities. However, there is no reason that the model could not be implemented directly in hardware without exposing any hardware threads and locks. Also, there is no necessary relationship between the number of Actors, threads, and locks that might be involved in a computation. Implementations of the Actor model are free to make use of threads and locks in any way that is compatible with the laws for Actors.

==Abstracting away implementation details==

An important challenge in defining the Actor model was to abstract away implementation details.

For example, consider the following question: "Does each Actor have a [[Queue (data structure)|queue]] in which its communications are stored until received by the Actor to be processed?" [[Carl Hewitt]] argued against including such queues as an integral part of the Actor model. One considerationsconsideration was that such queues could themselves be modeled as Actors that received messages to <tt>{{mono|enqueue</tt>}} and <tt>{{mono|dequeue</tt>}} the communications. Would such an Actor queue itself have a queue (which would in turn have ...)?! Another consideration was that some Actors would not use such queues in their actual implementation. ''E.g.,'' an Actor might have a network of [[Arbiter (electronics)|arbiters]] instead. Of course, there is a mathematical abstraction which is the ''sequence'' of communications that have been received by an Actor. But this sequence emerged only as the Actor operated. In fact the ordering of this sequence can be [[Quantum indeterminacy|indeterminate]] (see [[Indeterminacy in concurrent computation]]).

Another example of abstracting away implementation detail was the question of [[Interpreter (computing)|interpretation]]: "Should interpretation be an integral part of the Actor model?" The idea of interpretation is that an Actor would be defined by how its program [script] processed <tt>{{mono|eval</tt>}} messages. (In this way Actors would be defined in a manner analogous to [[Lisp programming language|Lisp]] which was "defined" by a meta-circular interpreter procedure named <tt>{{mono|eval</tt>}} written in Lisp.) Hewitt argued against making interpretation integral to the Actor model. One consideration was that to process the <tt>{{mono|eval</tt>}} messages, the program [script] of an Actor would itself have a program [script] (which in turn would have ...)! Another consideration was that some Actors would not use interpretation in their actual interpretation. ''E.g.,'' an Actor might be implemented in hardware instead. Of course there is nothing wrong with interpretation ''per se''. Also implementing interpreters using <tt>{{mono|eval</tt>}} messages is more modular and extensible than the monolithic interpreter approach of Lisp.

==Operational model==

Nevertheless, progress developing the model was steady. In 1975, Irene Greif published the first [[Operational semantics|operational]] model in her dissertation.

==Scheme==

[[Gerald Jay Sussman|Gerald Sussman]] and [[Guy L. Steele, Jr.|Guy Steele]] then took an interest in Actors and published a paper on their [[Scheme (programming language)|Scheme]] interpreter in which they (misleadingly) concluded "we discovered that the 'actors' and the [[lambda calculus|lambda expessionsexpressions]] were identical in implementation." According Theto actual situation is thatHewitt, the lambda calculus is capable of expressing some kinds of parallelism but, in general, ''not'' the concurrency expressed in the Actor model. On the other hand, the Actor model is capable of expressing all of the parallelism in the lambda calculus.

==Laws for Actors==

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==Specifications and proofs==

Aki Yonezawa published his specification and verification techniques for Actors. Russ Atkinson and [[Carl Hewitt]] published a paper on specification and proof techniques for serializers providing an efficient solution to [[Encapsulationencapsulation (computer science)|encapsulating]] shared resources for [[concurrency control]].

==Mathematical characterization using ___domain theory==

Finally eight years after the first Actor publication, Will Clinger (building on the work of [[Irene Greif]] 1975], [[Gordon Plotkin]] 1976], [Michael Smyth 1978], [ [[Henry Baker (computer scientist)|Henry Baker]] 1978], [Francez, [[C. A. R. Hoare|Hoare]], Lehmann, and de Roever 1979], and [[Edward Arthur Milne|Milne]] and Milner[[John 1979Milnor|Milnor]] 1979) published the first satisfactory mathematical [[denotational semantics|denotational]] model incorporating [[unbounded nondeterminism]] using [[___domain theory]] in his dissertation in 1981 (see [[Denotational_semanticsDenotational semantics of the Actor model#Denotational_semantics_of_concurrencyClinger's Model|Clinger's model]]). Subsequently, Hewitt [2006] augmented the diagrams with arrival times to construct a [[Denotational semantics of the Actor model#The Timed Diagrams Model|technically simpler denotational model]] that is easier to understand. See [[History of denotational semantics]]).

==See also==

==Was the Actor model premature?==

* [[Actor model and process calculi history]]

The history of the Actor model raises the question of whether it was [[prematurity|premature]].

* [[History of denotational semantics]]

* [[Actor model middle history]]

* [[Actor model later history]]

==References==

===Original definition of prematurity===

{{reflist}}

As originally defined by Gunther Stent [1972] "A discovery is premature if its implications cannot be connected by a series of simple logical steps to contemporary canonical or generally accepted knowledge." Ilana Lövy [2002] glossed the phrase "series of simple logical steps" in Stent's definition as referring to the "target community's ways of asking relevant questions, of producing experimental results, and of examining new evidence." [[Michael Ghiselin]] [2002] argued that if a "minority of scientists accept a discovery, or even pay serious attention to it, then the discovery is not altogether premature in the Stentian sense." In accord with Ghiselin's argument, the Actor model was not premature. Indeed it enjoyed initial popularity and for a couple of decades underwent steady development.

However, Stent in his original article also referred to a development as premature such that when it occurred contemporaries did not seem to able to do much with or build on. This is what happened after a while with the Actor model. The reasons were twofold:

#For over 30 years after the first publication of the Actor model, widely deployed computer architectures developed in the direction of making a single sequential thread of execution run faster.

#For over 25 years after the first publication, there was no agreed standard by which software could communicate high level data structures across organizational boundaries.

===Before its time?===

According to Elihu M. Gerson [2002], phenomenon that lead people to talk about discoveries being before their time can be analyzed as follows: "We can see the phenomenon of 'before its time' as composed of two separate steps. The first takes place when a new discovery does not get tied to the conventional knowledge of its day and remains unconnected in the literature. The second step occurs when new events lead to the 'rediscovery' of the unconnected results in a changed context that enables or even facilitates its connection to the conventional knowledge of the rediscovering context."

Now both of the above circumstances that held back the Actor model have changed with the development of (1) [[multicore|many-core]] ([http://www.intel.com/technology/techresearch/idf/platform-2015-keynote.htm Platform 2015 Unveiled at IDF Spring 2005]) computer architectures and (2) [[Web Services]]. By the criteria of Gerson, the Actor model might be described by some as ''before its time''.

==Bibliography==

According to Hadasa Zuckerman and [[Joshua Lederberg]] [1986], premature discoveries are those that were made but neglected. By their criteria it remains to be seen whether or not the Actor model was premature!

*{{Cite book|ref={{harvid|Hewitt et al.|1973}}|first1=Carl | last1=Hewitt |first2=Peter | last2=Bishop |first3=Richard | last3= Steiger|chapter=A Universal Modular Actor Formalism for Artificial Intelligence | title=IJCAI'73: Proceedings of the 3rd [[International Joint Conference on Artificial Intelligence]] | year=1973 | pages=235–245|chapter-url=https://dl.acm.org/doi/10.5555/1624775.1624804|series=<!---->}}

* {{cite journal|last=McCarthy|first=John|title=Situations, actions and causal laws|journal=Technical Report Memo|year=1963|issue=2|publisher=Stanford University Artificial Intelligence Laboratory}}

Gerson [2002] argued, "But histories and sociological studies repeatedly show that we do not have a discovery until the scientific community accepts it as such and stops debating about it. Until then the proposed solution is in an intermediate state." By his argument, the Actor model is a discovery but since its practical importance is not yet accepted by the community, its practical importance is not a discovery.

* {{cite journal|last1=McCarthy|first1=John|title=Some Philosophical Problems from the Standpoint of Artificial Intelligence|journal=Machine Intelligence|year=1969|issue=4|pages=463–502|last2=Hayes|first2=Patrick|publisher=[[Edinburgh University Press]]|citeseerx=10.1.1.85.5082}}

* {{cite book|last=Heisenberg|first=Werner|title=Physics and Beyond: Encounters and Conversations|year=1971|publisher=Harper & Row|___location=New York|pages=63–64|isbn=978-0061316227|others=Translated by A. J. Pomerans|url-access=registration|url=https://archive.org/details/physicsbeyondenc00heis}}

==References==

* {{cite book|last1=Hewitt|first1=Carl|date=January 1974|doi=10.1145/512927.512942|last2=Bishop|first2=Peter|last3=Greif|first3=Irene|last4=Smith|first4=Brian|last5=Matson|first5=Todd|last6=Steiger|first6=Richard|title=Proceedings of the 1st annual ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages – POPL '73 |chapter=Actor induction and meta-evaluation |pages=153–168|citeseerx=10.1.1.104.295|s2cid=33611569 }}

*John McCarthy. '''Situations, actions and causal laws''' Technical Report Memo 2, Stanford University Artifcial Intelligence Laboratory. 1963.

* {{cite journal|last=Hewitt|first=Carl|title=Behavioral Semantics of Nonrecursive Control Structure|journal=Proceedings of Colloque Sur la Programmation|date=April 1974|url=http://dl.acm.org/citation.cfm?id=721498|pages=385–407|isbn=9783540068594}}

*John McCarhty and Patrick Hayes. '''Some Philosophical Problems from the Standpoint of Artificial Intelligence''' in Machine Intelligence 4. Edunburgh University Press. 1969.

* {{cite book|last1=Greif|first1=Irene|last2=Hewitt|first2=Carl|title=Proceedings of the 2nd ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages – POPL '75 |chapter=Actor semantics of PLANNER-73 |date=January 1975|doi=10.1145/512976.512984|pages=67–77|s2cid=18178340 }}

*Werner Heisenberg. '''Physics and Beyond: Encounters and Conversations''' translated by A. J. Pomerans (Harper & Row, New York, 1971), pp. 63–64.

* {{cite journal|last=Hewitt|first=Carl|title=How to Use What You Know|journal=Proceedings of the 4th International Joint Conference on Artificial Intelligence|volume=1|date=September 1975|pages=189–198}}

*Gunther Stent. '''Prematurity and Uniqueness in Scientific Discovery''' Scientific American. December, 1972.

* {{Cite thesis |type=Ph.D. |title=Semantics of Communicating Parallel Professes |last=Greif |first=Irene |year= 1975 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

* {{note_label|Hewitt|Hewitt et al. 1973|-}} Carl Hewitt, Peter Bishop and Richard Steiger. '''A Universal Modular Actor Formalism for Artificial Intelligence''' IJCAI 1973.

* {{cite journal|last1=Baker|first1=Henry|last2=Hewitt|first2=Carl|title=The Incremental Garbage Collection of Processes|journal=Proceedings of the Symposium on Artificial Intelligence Programming Languages|date=August 1977|pages=55–59 |doi=10.1145/800228.806932 |hdl=1721.1/41969 |s2cid=1557419 |url=http://www.ncstrl.org:8900/ncstrl/servlet/search?formname=detail\&id=oai%3Ancstrlh%3Amitai%3AMIT-AILab%2F%2FAIM-454|hdl-access=free}}{{Dead link|date=January 2020 |bot=InternetArchiveBot |fix-attempted=yes }}

*Carl Hewitt, ''et. al.'' '''Actor Induction and Meta-evaluation''' Conference Record of ACM Symposium on Principles of Programming Languages, January 1974.

* {{cite journal|last2=Baker|first2=Henry|last1=Hewitt|first1=Carl|title=Laws for Communicating Parallel Processes|journal=International Federation for Information Processing|date=August 1977|hdl=1721.1/41962}}

*Carl Hewitt, ''et. al.'' '''Behavioral Semantics of Nonrecursive Control Structure''' Proceedings of Colloque sur la Programmation, April 1974.

* {{Cite thesis |type=Ph.D. |title=Specification and Verification Techniques for Parallel Programs Based on Message Passing Semantics |last=Yonezawa |first=Aki |year= 1977 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

*Irene Greif and Carl Hewitt. '''Actor Semantics of PLANNER-73''' Conference Record of ACM Symposium on Principles of Programming Languages. January 1975.

* {{Cite thesis |type=Ph.D. |title=Very Large Address Space Modularly Extensible Computer Systems |last=Bishop |first=Peter |year= 1977 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

*Carl Hewitt. '''How to Use What You Know''' IJCAI. September, 1975..

* {{cite journal|last=Hewitt|first=Carl|title=Viewing Control Structures as Patterns of Passing Messages|journal=Journal of Artificial Intelligence|date=June 1977|volume=8 |issue=3 |pages=323–364 |doi=10.1016/0004-3702(77)90033-9 |hdl=1721.1/6272|hdl-access=free}}

*Irene Greif. '''Semantics of Communicating Parallel Professes''' MIT EECS Doctoral Dissertation. August 1975.

* {{Cite thesis |type=Ph.D. |title=Actor Systems for Real-Time Computation |last=Baker |first=Henry |year= 1978 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

*Henry Baker and Carl Hewitt '''The Incremental Garbage Collection of Processes''' Proceeding of the Symposium on Artificial Intelligence Programming Languages. SIGPLAN Notices 12, August 1977.

* {{cite journal|last1=Hewitt|first1=Carl|last2=Atkinson|first2=Russ|title=Specification and Proof Techniques for Serializers|journal= IEEE Transactions on Software Engineering|date=January 1979|doi=10.1109/TSE.1979.234149|pages=10–23|hdl=1721.1/5756|s2cid=15272353 |hdl-access=free}}

*Carl Hewitt and Henry Baker '''Laws for Communicating Parallel Processes''' IFIP-77, August 1977.

* {{Cite thesis |type=Ph.D. |title=A Computational Theory of Animation |last=Kahn |first=Ken |year= 1979 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

*Aki Yonezawa '''Specification and Verification Techniques for Parallel Programs Based on Message Passing Semantics''' MIT EECS Doctoral Dissertation. December 1977.

* {{cite journal|last1=Hewitt|first1=Carl|last2=Attardi|first2=Beppe|last3=Lieberman|first3=Henry|title=Delegation in Message Passing|journal=Proceedings of First International Conference on Distributed Systems|date=October 1979|___location=Huntsville, AL}}

*Peter Bishop '''Very Large Address Space Modularly Extensible Computer Systems''' MIT EECS Doctoral Dissertation. June 1977.

* {{Cite thesis |type=Ph.D. |title=Automatic Verification of Serializers |last=Atkinson |first=Russ |year= 1980 |publisher=[[MIT]]}}

*Carl Hewitt. '''Viewing Control Structures as Patterns of Passing Messages''' Journal of Artificial Intelligence. June 1977.

* {{cite journal|last1=Kornfeld|first1=Bill|last2=Hewitt|first2=Carl|title=The Scientific Community Metaphor|journal=IEEE Transactions on Systems, Man, and Cybernetics|date=January 1981|doi=10.1109/TSMC.1981.4308575|volume=11|pages=24–33|url=https://dspace.mit.edu/bitstream/1721.1/5693/2/AIM-641.pdf|hdl=1721.1/5693|s2cid=1322857 |hdl-access=free}}

*Henry Baker. '''Actor Systems for Real-Time Computation''' MIT EECS Doctoral Dissertation. January 1978.

* {{cite journal|last=Lieberman|first=Henry|title=Thinking About Lots of Things at Once without Getting Confused: Parallelism in Act 1|journal=MIT AI Memo|issue=626|date=May 1981|hdl=1721.1/6351}}

*Carl Hewitt and Russ Atkinson. '''Specification and Proof Techniques for Serializers''' IEEE Journal on Software Engineering. January 1979.

* {{cite journal|last=Lieberman|first=Henry|title=A Preview of Act 1|journal=MIT AI Memo|issue=625|date=June 1981|hdl=1721.1/6350}}

*Ken Kahn. '''A Computational Theory of Animation''' MIT EECS Doctoral Dissertation. August 1979.

* {{Cite thesis |type=Ph.D. |title=Reasoning about Change in Knowledgeable Office Systems |last=Barber |first=Gerry |year= 1981 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

*Carl Hewitt, Beppe Attardi, and Henry Lieberman. '''Delegation in Message Passing''' Proceedings of First International Conference on Distributed Systems Huntsville, AL. October 1979.

* {{Cite thesis |type=Ph.D. |title=Parallelism in Problem Solving |last=Kornfeld |first=Bill |year= 1981 |publisher=[[MIT]] [[Computer engineering|EECS]]}}

*Russ Atkinson. '''Automatic Verification of Serializers''' MIT Doctoral Dissertation. June, 1980.

* {{Cite thesis |type=Ph.D. |title=Foundations of Actor Semantics |last=Clinger |first=Will |year= 1981 |publisher=[[MIT]] [[Mathematics]]}}

*Bill Kornfeld and Carl Hewitt. '''The Scientific Community Metaphor''' IEEE Transactions on Systems, Man, and Cybernetics. January 1981.

*{{cite journal|last=Theriault|first=Daniel|title=A Primer for the Act-1 Language|journal=MIT AI Memo|issue=672|date=April 1982|hdl=1721.1/5675}}

*Henry Lieberman. '''Thinking About Lots of Things at Once without Getting Confused: Parallelism in Act 1''' MIT AI memo 626. May 1981.

* {{cite journal|last1=Lieberman|first1=Henry|last2=Hewitt|first2=Carl|title=A real Time Garbage Collector Based on the Lifetimes of Objects|journal=Communications of the ACM|date=June 1983|doi=10.1145/358141.358147|volume=26|issue=6|pages=419|citeseerx=10.1.1.123.5055|s2cid=14161480 }}

*Henry Lieberman. '''A Preview of Act 1''' MIT AI memo 625. June 1981.

* {{cite journal|last=Theriault|first=Daniel|title=Issues in the Design and Implementation of Act 2|journal=MIT AI Technical Report|issue=728|date=June 1983|hdl=1721.1/6940}}

*Gerry Barber. '''Reasoning about Change in Knowledgeable Office Systems''' MIT EECS Doctoral Dissertation. August 1981.

* {{cite journal|last=Lieberman|first=Henry|title=An Object-Oriented Simulator for the Apiary|journal=Conference of the American Association for Artificial Intelligence|date=August 1983|___location=Washington, D. C.|url=http://www.aaai.org/Papers/AAAI/1983/AAAI83-090.pdf}}

*Bill Kornfeld. '''Parallelism in Problem Solving''' MIT EECS Doctoral Dissertation. August 1981.

* {{cite journal|last1=Hewitt|first1=Carl|last2=de Jong|first2=Peter|title=Analyzing the Roles of Descriptions and Actions in Open Systems|journal=Proceedings of the National Conference on Artificial Intelligence|date=August 1983|hdl=1721.1/5649}}

*Will Clinger. '''Foundations of Actor Semantics''' MIT Mathematics Doctoral Dissertation. June 1981.

* {{cite book|last=Jammer|first=M.|title=Symposium on the Foundations of Modern Physics: 50 years of the Einstein-Podolsky-Rosen Gedanken experiment|year=1985|publisher=World Scientific|___location=Singapore|pages=129–149|editor=P. Lahti, P. Mittelstaedt|chapter=The EPR Problem in Its Historical Development}}

*Daniel Theriault. '''A Primer for the Act-1 Language''' MIT AI memo 672. April 1982.

* {{cite book|last=Fine|first=A.|title=The Shaky Game: Einstein Realism and the Quantum Theory|year=1986|publisher=University of Chicago Press|___location=Chicago|isbn=978-0226249476}}

*Henry Lieberman and Carl Hewitt. '''A real Time Garbage Collector Based on the Lifetimes of Objects''' CACM June 1983.

* {{cite journal|last1=Hewitt|first1=Carl|last2=Lieberman|first2=Henry|title=Design Issues in Parallel Architecture for Artificial Intelligence|journal=MIT AI Memo|issue=750|date=November 1983|hdl=1721.1/5653}}

*Daniel Theriault. '''Issues in the Design and Implementation of Act 2''' MIT AI technical report 728. June 1983.

* {{cite book|last=Fuchs|first=Christopher|title=Quantum Theory: Reconstruction of Foundations|year=2002|publisher= Växjo University Press|___location=Växjo|editor=A. Khrenikov|chapter=Quantum mechanics as quantum information (and only a little more)}}

*Henry Lieberman. '''An Object-Oriented Simulator for the Apiary''' Conference of the American Association for Artificial Intelligence, Washington, D. C., August 1983

* {{cite web|last=Hewitt|first=Carl|title=What is Commitment? Physical, Organizational, and Social|date=April 27, 2006|url=http://www.pcs.usp.br/~coin-aamas06/10_commitment-43_16pages.pdf|work=COIN@AAMAS}}

*Carl Hewitt and Peter de Jong. '''Analyzing the Roles of Descriptions and Actions in Open Systems''' Proceedings of the National Conference on Artificial Intelligence. August 1983.

*M. Jammer '''The EPR Problem in Its Historical Development''' in Symposium on the Foundations of Modern Physics: 50 years of the Einstein-Podolsky-Rosen Gedankenexperiment, edited by P. Lahti and P. Mittelstaedt (World Scientific, Singapore, 1985), pp. 129–149.

*A. Fine '''The Shaky Game: Einstein Realism and the Quantum Theory''' (University of Chicago Press, Chicago, 1986)

*Carl Hewitt and Henry Lieberman. '''Design Issues in Parallel Architecture for Artificial Intelligence''' MIT AI memo 750. Nov. 1983.

*Hadasa Zuckerman and Joshua Lederberg. '''Postmature Scienfic Discovery?''' Nature. December, 1986.

* Christopher Fuchs '''Quantum mechanics as quantum information (and only a little more)''' in A. Khrenikov (ed.) Quantum Theory: Reconstruction of Foundations (Växjo: Växjo University Press, 2002).

*Elihu M. Gerson. '''Prematurity and Social Worlds''' in Prematurity in Scientific Discovery. University of California Press. 2002.

[[Category:Actor model (computer science)]]

[[Category:History of computing|Actor model]]

[[Category:History of software|Actor model]]