By absolute synonyms, I mean words (in the same language) that are interchangeable in all situations.
There can't be differences in register, meaning, or emotional value.
Is there material that treats about this subject, especially showing examples in English? In case they don't exist, I'd like to read material with an explanation or theory why they can't exist in any language.
I would like to make a more general contribution, but still regarding your specific question.
There are many definitions of synonymy. Let's agree in a simple one: it is a semantic relation between the meanings of words or sentences. We now have the problem of defining meaning. In one way or another, the concept of synonymy is understood without requiring absolute identity between words, if there is such thing. Without going in the details, a logical account for absolute synonymy would say that two linguistic forms are synonyms if they are interchangeable salva veritate, that is, keeping the truth value of the expression they are part of. In his attack on the concept of analyticity, Quine (1951) discussed the idea of whether this kind of interchangeability was a condition strong enough for synonymy. Saying that bachelor is a synonym of unmarried man was saying that the proposition
All and only bachelors were unmarried men.
was analytical. But that is circular, and Quine wanted to discuss whether the sufficient condition for cognitive synonymy was interchangeability and not analyticity. Hundreds of articles have been written about this and I cannot review all the arguments, but the point was to illustrate one of the basic ideas about logical synonymy. If you think about the example of the correspondence between bachelor and unmarried man, you will soon realise that there are not really many words like those in natural language.
So, if we talk about words, we think that synonyms are those that can be substituted for each other in sentential contexts, and if we talk about sentences, then they are said to be synonyms if the substitution preserves truth values. But still we have to be careful with the concept of synonymy. For words, we can have different semantic values that may not have a one-to-one correspondence to each other. Think about the words old, ancient, aged, obsolete. They are not interchangeable in all contexts: old/ancient ritual vs. old/?ancient laptop. So it's not only about the meaning of the synonyms, which is unclear, but also about their relations with other words on their contexts of appearance (the basic idea being compositionality in semantics). We can also use euphemisms as synonyms for the words we don't want to use. Here we are opening the door to a whole new set of factors that can influence synonymy, as some previous comments/answers point out. We must then consider that synonymy (either absolute or partial) depends on what are the meanings we are trying to compare and the compositional nature of the relations between words in the context of sentences. This is a matter of debate for any kind of approach to meaning, either in semantics, pragmatics and other subfield.
Let us admit that there can be at least two kinds of synonymy: there might be a full synonymy for words that are logically (salva veritate) interchangeable, but that is not very common. And there is also this everyday use of synonymy that treats sameness of meaning in a more or less unrestricted way. One could see meaning as some sort of continuum and words as mapping certain parts of that continuum, with overlaps between them that allows us to call them synonyms. This can be analysed theoretically and empirically in linguistics. As was mentioned before, definitions depend on the framework used to explain meaning. If the meanings of words are their referents, then it is easy to find synonyms. A second option is to understand that meanings are senses (more or less in he Fregean conception), but then you will find many debates as well. A third option is a psychological perspective were meanings are representations or concepts in the mind, and there you will find even more disagreement. The definition of synonymy is then closely related to the semantic perspective adopted and the definition of meaning endorsed. You can compare words in terms of denotations, semantic features, semantic maps, representations, etc. You can also study synonymy in terms of sociolinguistics, comparing dialects and registers, or even from a cross-linguistic point of view. The use of computational methods and corpus research is also frequent. But at the end, you can only say that there is absolute synonymy if you analyse all the possible meanings/contexts, and that is quite difficult, if not implausible.
In sum, definitions of synonymy are relative to the theoretical framework adopted, and particularly dependent on the definition of meaning. What you call "absolute synonymy" can also be what has been defined as logical or full synonymy, but not everyone will agree. Words like bachelor and unmarried man can be said to be full synonyms, but then again, these examples are extremely rare and there might be differences we don't know yet. That is also the case of gorse and furze, mentioned in a previous answer. Perhaps the reason is that these words have a very narrow range of contexts of use. It might be the case that words with more meanings and more frequently used tend to be less close to the end of the continuum where full synonymy (if it exists) is found. Besides, two or more words that mean exactly the same seem to be not economical for language, and thus it would tend to be avoided. Are there absolute synonyms? There can be, but then again, the answer itself is not absolute. I am sorry I can't provide references, but it seems that's the situation, at least in linguistic semantics. Partial synonymy is assumed for methodological purposes without much discussion, while absolute synonymy is considered rare because it is inefficient and uneconomical.
Quoting an excerpt from Haiman's 1980 paper "The Iconicity of Grammar"
The first type [of iconicity], whose existence is universally (though often only implicitly) recognized in practice, is that of a one-to-one correspondence between the signans and the signatum, whether this be a single word or a grammatical construction. The iconic assumption that such a regular bi-unique correspondence must exist motivates the inclusion under a single heading of the various meanings of a single form in both traditional dictionaries and grammars. It also serves as the unspoken basis for the commonly accepted axiom that no true synonyms exist, i.e. that different forms must have different meanings (cf. Bloomfield 1933:145, Nida 1958:282, Bolinger 1968:127 for representative statements of this axiom). Following Hjelmslev, Kurylowicz, and Martinet, I will refer to this relationship as the iconicity of ISOMORPHISM. (Haiman 1980:515--6)
The citation "Bolinger 1968" is: Entailment and the meaning of structures. Glossa 2.119-28, and "Nida 1958" is: An analysis of meaning and dictionary making. IJAL 24.279-92.
This question triggered a memory of a thread in my favourite language blog.
I couldn't recall exactly the name of the post of what words were discussed, but in my question for it I turned up another thread containing comments about these terms:
Then finally I found the thread I was looking for, which was about the terms:
The only circumstance I'm aware of where absolute synonyms occur is in the situation of name-avoidance taboo.
In many Australian languages it is common practice to avoid saying the names of the deceased. This name avoidance (or taboo) practice includes words which are phonologically similar to a deceased person's name, which can result in words in common use also becoming taboo. These taboo words are typically replaced by some other word, or perhaps by one borrowed from another language. These replacement words are used as total synonyms of the word they replace.
An example of this is given in Dixon (1980:29), involving the death of a man named Ngayunya. This led to proscription of the first singular pronoun ngayu and its replacement with nganku, drawn from the mother-in-law avoidance speech style. As the death name taboo applies most particularly to speech with, and in the presence of, relatives of the deceased, both terms were at all times in use by at least some members of the community.
Based on my understanding of neocortex, there would be a continuous variable representing "how far up" your perceptive hierarchy the two words can get on separate pathways before they result the the same set of activations, if they ever result in the same set of activations.
Here are a few that make no difference to me at a very low perceptive level. Usually I am unconscious of which one was said without being told to listen for it.
"Drunk" and "Drank" as the past tense of drink. "box" and "machine" when referring to a computer "coke" and "soda" both mean soda without further clarification in the Southeast. "bucks" and "dollars" same thing, I'm not even going to notice which one is used.
Two words can be regarded as synonymous just due to a relation in meaning. Though usually, words referred to as "synonyms" are those who mean the same thing. Thing is, semantics isn't just definitions. There's another side to it. Connotations. This is were objectivity starts to fade and you see many different opinions based on race, geography, gender, age demographic, etc. Heck, even individually the connotations had can differ, though you'll still see patterns within the common denominators mentioned.
There is something close to absolute synonyms. Cognitive synonyms, which is one of the "extreme" ends of the spectrum, where the definition is exactly the same. Thing is, these cognitive synonyms still aren't absolute synonyms, due to human nature making associations and connotations inevitable. I guess the closest you can find to absolute synonyms are obscure cognitive synonyms. Due to their obscurity, or better yet, if they're obsolete, there is no preconceived societal connotations with the words. BUT STILL, different connotations will be had, due to the different look, roots, construction and potential etymological origins of the respective words.
Simply put, the human mind doesn't allow absolute synonyms.
There are lots of absolute synonyms among technical terms: Creutzfeld-Jakob disease is absolutely synonymous with Jakob-Creutzfeld disease, and likewise for variants with Kreutzfeld. It's a prion-caused brain disorder similar to mad cow disease. Similarly, it is all one whether you say Simia pygmaeus as Linnaeus did, or Pongo pygmaeus in the modern style: the orangutan of Borneo is meant in either case.
German has a couple of absolute synonyms. Typically, they are regional variants but within their respective regions there is no difference in register, meaning or emotional value. The only difference that is conveyed is ‘I learnt my language in ’.
This is true for example for a number of food items (Krapfen/Berliner/Pfannkuchen for jam doughnut being one of the most well-known). Sometimes, one word is Austrian or Swiss while another is primarily German such as Kissen/Polster (cushion). My favourite one is the word for Saturday which is Sonnabend in the North but Samstag in the South, Austria and Switzerland.
