Talk:Thought experiment

Some thought experiments cannot be performed at all. Einstein's famous "What would I see if I looked at a mirror while riding on a light beam?" provided crucial insights for relativity yet are expressly impossible under the theory it was responsible for generating. User:Two16

Intuition Pump

Intuition pump auto-redirects here which, while not 100% accurate, is fine. But, the mention of intuition pump within the article is inaccurate. Although Dennet's first mention of the term is derogatory (contra Searle's "Chinese Room"), he also applauds their use:

" I coined the term "intuition pump," and its first use was derogatory. I applied it to John Searle's "Chinese room," which I said was not a proper argument but just an intuition pump. I went on to say that intuition pumps are fine if they're used correctly, but they can also be misused. They're not arguments, they're stories. Instead of having a conclusion, they pump an intuition. They get you to say "Aha! Oh, I get it!"

The idea of consciousness as a virtual machine is a nice intuition pump. It takes a while to set up, because a lot of the jargon of artificial intelligence and computer science is unfamiliar to philosophers or other people. But if you have the patience to set some of these ideas up, then you can say, "Hey! Try thinking about the idea that what we have in our heads is software. It's a virtual machine, in the same way that a word processor is a virtual machine." Suddenly, bells start ringing, and people start seeing things from a slightly different perspective."

Plato's cave?

Isn't Plato's cave a metaphor rather than a thought experiment?

I'd agree with you. Plato's cave is simply stating that our explanations of the world are like that of the man who ventures out of the cave. In other words, his questions are not framed in the same way as a thought experiment at all; therefore, I'm removing it. If anyone diagrees feel free to state it here and/or revert. Laura Scudder 00:37, 17 Feb 2005 (UTC)

I guess I'll add something more in defense of removing Plato's allegory of the cave. The phrase thought experiment to me means that the proposed situation needs to have a testable prediction - just like an experiment. For example, Maxwell's demon is a thought experiment because were someone to create a Maxwell's demon, they would see that the temperature in one side of the box gets hotter.

In contrast, Plato's allegory is a metaphor because although it posits a question about people raised chained in a darkened cave, we are meant to interpret the story as saying that we are in a cave and wouldn't be able to understand what someone who travelled outside told us about the "real world". Since we can't get to this "real world" there is no testable prediction to the hypothetical situation, thus it's an allegory rather than a thought experiment.

This is a bit nitpicky, but in this interpretation of the meaning of thought experiment, were the story meant to only say something about people raised in caves with only shadows (similar to Mary's room where a girl is raised without colors) it would qualify, but because it is an allegory about our world it does not. Feel free to disagree. Laura Scudder 01:00, 17 Feb 2005 (UTC)

einstein

what about einstein????

Indeed, what about Einstein?

In fact, three of Einstein's most famous thought experiments, the EPR paradox, galileo's ship and the twin paradox are in the list. There are many many more relativity-related thought experiments (riding in a relativistic elevator with a flashlight and a mirror for instance), and you should feel free to write those up and include them if you think they're noteworthy enough. --Laura Scudder | Talk 20:24, 29 May 2005 (UTC)

Einstein was probably the greatest thought experimenter of all time. Aside from the three noted above, he pioneered a lot more, such as a train being hit by lightning, flying ladders and so on. I'm actually kind of surprised that the Twin paradox isn't listed as a thought experiment.

67.201.192.140 (talk) 18:44, 4 March 2008 (UTC)Ai52487963

Wikipedia and Vandalopedia

See the entry on the latter in BJAODN (Wikipedia:"Bad Jokes? We ain't got no stinking Bad Jokes and Other Deleted Nonsense!"), and Wikigovernment (Wikipedia:Whose Bad Jokes and Other Deleted Nonsense is it Anyway?).

How would Vandalopedia/Wikigovernment actually look like/work?

Needs a link

This is just crying out for a link "but others pre-date Socrates." If you know this to be a fact then you should know enough to create another page informing readers what these other old thought experiments were and link to it.

One reference is: Rescher, N., "Thought Experiment in Pre-Socratic Philosophy", pp.31-41 in Horowitz, T. & Massey, G.J. (eds.), Thought Experiments in Science and Philosophy, Rowman & Littlefield, (Savage), 1991.cogtrue 22:05, 16 May 2006 (UTC)

definition of thought experiment

Warning. At present, the definition of thought experiment is sorely lacking. It currently says the following:

"In philosophy, physics, and other fields, a thought experiment (from the German term Gedankenexperiment, coined by Ernst Mach) is an attempt to solve a problem by using an analogy. These experiments are used to understand something practical through comparison with a hypothetical example. Thought experiments design a hypothetical situation in which our intuitive response is contrary to our actual responses in a similar real situation. Thought experiments create dissonance with the known and accepted, which with time have led to the reformulation or precision of theories."

This is vague and inaccurate. A thought experiment is not in any way an analogy or "comparison". They are not used to "solve problems." They are not used to understand something "practical", whatever that means. In a typical philosophical thought experiment, one reflects on a hypothetical example and has an intuitive assessment of what is morally right, what the reference of a term is, etc, in that situation. This reflection is supposed to show us something that is true in general of that concept (i.e., morality, reference), not what is true in the imaginary example as opposed to reality. It is not generally thought that the response is "contrary to our actual response in a similar real situation." Thought experiments may "create dissonance with the known and accepted", whatever that means, but if they do, that is inessential to what they are as a conceptual tool.

Hopefully, a more useful definition can be crafted. Until then, keep the grain of salt nearby.

--Rldoan 08:53, 7 February 2006 (UTC)

I proposed the following first piece:

"Thought experiments are well-structured and well-defined (rather than ill-defined) hypothetical questions that employ subjunctive reasoning (irrealis moods). Thought experiments often introduce interesting, important and valuable new perspectives on old mysteries and old questions; yet, although they may make old questions irrelevant, they may also create new questions that are not be easy to answer."

And I placed them within the Wikipedia, along with a very long piece elborating on thought experiments -- all of which have been derived from some two year's deep academic study of thought experiments -- and, as well a considerable number of additional, relevant references.

Now, I am not being "precious"; but I simply can not understand how -- or even why -- the original paragraph re-appeared.

The original paragraph is:

"A thought experiment (from the German term Gedankenexperiment, coined by Ernst Mach) in the broadest sense is the use of an imagined scenario to help us understand the way things really are. The understanding comes through reflection on the scenario. Thought experiment methodology is a priori, rather than empirical, in that it does not proceed by observation or experiment."

This contains a number of serious and extremely nmisleading errors that are perpetuated in the literature:

(1) Mach did not "coin" the term Gedankenexperiment. It was, possibly, "coined" by Ørsted; but, according to modern scholarship (see Wittt-Hanson), it was certainly -- as far as the remaining extant literature goes -- was first used by by Ørsted.

(2) Again, I am not being "precious"; but, in the realm of "thought experimentation" the imaginary situations are not scenarios because (a) the term scenario has a very precise meaning when used by Kahn, (b) any usage of the term "scenario", within an article dealing with any process involving the imagination, is very dangerously equivocal, (c) given that some (in the literature) make the mistake of using the term "scenario" as if it were a synonym for "thought experiment", whilst others use the term as if it means "the set of physical (i.e., physical only) circumstances which I would like you to imagine, whilst others use the term to denote all of the circumstances (including any of the protagonists' actions), whillst others, yet again, only use the term "scenario" to describe circumstances in which the least acceptable circumstances would arise -- e.g., speaking about a "worst-case scenarion. For all these reasons and more, the term "scenario" must be avoided, except where it is intentionally referring to the specific usage of Kahn.

I moved the section (slightly adapted) "The understanding comes through reflection on the scenario. Thought experiment methodology is a priori, rather than empirical, in that it does not proceed by observation or experiment" into the part "Thought Experiments in General".

As I am new to all of this, I really don't knw what to do about this sabotage and active mmisinformation.

Can anyone guide me as to what I should do -- or is it something that "just goes with the Wikiterritory" that I must simply accept as an implied condition of being a contributor?

cogtrue 04:32, 6 May 2006 (UTC)

I've made further changes in response to some of your comments. The motivation for restoring the original text is that it allows the article to lead with a clear definition and attribution, rather than a very indirect characterization.

Let me remind you to assume good faith instead of claiming "sabotage and active mmisinformation [sic]" . Al 04:59, 6 May 2006 (UTC)

(1) Thank you for removing the error; and (2) I can see how your re-arragment has made it much more readable. (3) I now recognize that "assuming good faith" means precisely that! Thanks for your promptness cogtrue 05:15, 6 May 2006 (UTC)

I'm glad we'll be able to work together to improve this article. Al 05:55, 6 May 2006 (UTC)

Very interesting content added to the entry lately! I contributed much (but not all) of the previous content, but it was from a merely general familiarity with thought experiments in philosophy. Currently, the header effectively has two definitions of "thought experiment." I have some thoughts about one of them:

"Thought experiments are well-structured and well-defined (rather than ill-defined) hypothetical questions that employ subjunctive reasoning (irrealis moods). Thought experiments often introduce interesting, important and valuable new perspectives on old mysteries and old questions; yet, although they may make old questions irrelevant, they may also create new questions that are not be easy to answer."

(1) I wonder, why the paranthetical "rather than ill-defined"? This seems to anticipate some reader objection. I would think that anticipating and responding to potential objections would be belong in the more detailed discussions. (2) The first sentence seems a bit inaccessible for an encyclopedia ideally targeted (I presume) at the average reader. There is some unexplicated technical language ("subjunctive reasoning"), and it is fairly abrubt. (3) The second sentence seems vague to me and not very informative. Can it be removed without loss?

I second the above comment about leading with a clear definition (and, I would also add, a relatively accessible one). I think the techical point about subjunctive reasoning is valuable, but might that be better placed in a more detailed following discussion? I'm happy to concede the point about use of the term "scenario" (again my familairity is not that of an expert). But if alternate language must be used, I think it ought to be non-technical vocab accessible to the average reader (which is what I was going for).

(Rldoan 07:45, 11 May 2006 (UTC))

Rldoan, Thanks for your useful comments.

(1) I have moved:

Thought experiments often introduce interesting, important and valuable new perspectives on old mysteries and old questions; yet, although they may make old questions irrelevant, they may also create new questions that are not be easy to answer.

To the start of the Practical Application of Thought Experimentation, because it seems to fit better there.

(2) In relation to the definition,

"Thought experiments are well-structured and well-defined (rather than ill-defined) hypothetical questions that employ subjunctive reasoning (irrealis moods).

I agree. I have altered it to read:

"Thought experiments are well-structured hypothetical questions that employ “What if? reasoning“ (see (irrealis moods).

It seems far simpler, much less complex, and far more informative. cogtrue 05:45, 13 May 2006 (UTC)

function and goal of thought experiments

There are two interesting lists of the functions and goals of thought experiments. Here are some thoughts. (1) I don't know what the difference is between functions and goals. I think they are essentially the same teleological concepts, and the items on the lists seem to have the same "function." (2) The list seems a bit long for a general encyclopedia entry. Can some be sacrificed or consolidated?

(Rldoan 07:57, 11 May 2006 (UTC))

Ridoan, Thanks for your comment.

I now recognize that these should be two separate sections beneath the (altered) title Thought experimentation in general, namely (a) The Theoretical Consequences of Thought Experimentation and (b) The Practical Application of Thought Experimentation (perhaps somebody can suggest a better title?). Here (and I may add something later to elaborate on this point) I am trying to distinguish between what I see are two very separate notions: (a) thought experiments as a tool in general, and (b) the specific use for which a specific thought experiment has been created (with the stress on the created) by a specific individual, in a specific and particular case, to serve a specific and particular need.

In relation to your comment on the length of the list: based on a wide study of the literature, I feel it is essential that Wikipedia supply this sort of description to newcomers to the topic, so that they will immediately have a reasonable understanding of the entire field. Otherwise, in my view, in the absence of such direction, one is left with only the philosophy-oriented descriptions of thought experiments that appear in philosophical encyclopedias, or the descriptions within specialist books on, say, quantum physics.

Also, a major driving force of Wikipedia seems to be to foster the interactive aggregation of knowledge, especially through cross-referencing. So, it seems that such lists, wherever they are appropriate -- and in this case, given the overall looseness of the term "thought experiment" itself, and the wide range of applications to which the term thought experiment" is used, I think that it is very appropriate to describe the wide range of applications of "thought experimentation" -- seem to be an essential part of disseminating these sorts of far-less-than-well-known inter-relationships and far-less-than-well-understood facts (that are very well known and very well understood by the experts) to those who are newcomers.

May I suggest the following way to resolve this matter of the length of the list: if it was to be held that the length of the list was detrimental to the article itself (and I would accept such a view 100%) the solution would be for the heading to remain, and beneath that heading a single sentence – i.e., List of Practical Applications of Thought Experimentation – with a link to a separate Wikipedia page (I have noticed that this move is often used in Wikipedia articles). cogtrue 05:04, 13 May 2006 (UTC)

afterthought: It may, also, be much better for the article in its emerging form to have the other lists that appear under Famous thought experiments to be also placed eslewhere in Wikipedia, but linked to this page? cogtrue 06:10, 13 May 2006 (UTC)

types of hypothetical question

A gratuitious comment. This is great stuff! Very useful information!

(Rldoan 08:06, 11 May 2006 (UTC))

Here's something nongratuitous. It is not clear to me how the typical thought experiments in philosophy match up to the list of hypothetical questions here. In philosophy, thought experiments are not typically used for causal reasoning, as the hypothetical questions here seem to be used. In philosophy, we (typically) imagine a scenario which (we will assume) is nomologically possible and decide how to *describe* or *characterize* the scenario. That is, philosophers are not interested in the causal consequences (or antecedents), but what that situation represents, so to speak. Is this a different type of hypothetical question?

(Rldoan 08:17, 11 May 2006 (UTC))

Again, Rldoan, Thanks. The "stuff" that you remark upon was placed in the article specifically because, according to my experience, this sort of thing is entirely well-understood (but is never expressed) by the all of the "experts"; and it is really quite clear from the way they speak and write about things, that these sorts of views are very deeply embedded within their all of their thoughts about "thought experimentation".

In relation to the offered list of hypothetical questions and its relationship to the work of philosophers: I believe that some of this is due to a confusion in relation to the application of the term thought experiment by philosophers to their activities.

Firstly, it is certain that philosophers have been asking hypothetical questions for a couple of thousand years.

Second, Ørsted and Mach, with their gendankenexperimenten and gedankenversuchen were, very specifically, referring to an (otherwise real, physical) experiment that was conducted mentally, and it was conducted as a "proxy" for that real experiment. The term "thought experiment" came into English specifically to precisely denote such a "proxy" mental activity. However, the utility of having such a term was not lost on philosophers, who soon took on the term as an exclusive label for their own pursuits. This is borne out by the fact that, say, Einstein’s physics-oriented imaginary speculations were generally referred to as gendankenexperiments in the English language literature (i.e., rather than as "thought experiments"), and it was not at all unusual to see these sorts of physics-oriented imaginary speculations, when conducted by other than Einstein, referred to as as gedanken experiments (i.e., one word of German, one word of English). Thus, the immediate problem may well be due to the fact that the metaphorical extension of the term "thought experiment" to these sorts of sets of circumstance no longer really demonstrate any coherent sort of source-to-target mapping from Ørsted and Mach’s gendankenexperiments.

In philosophy, many of the questions seem to be of the form “If this was to be the case, what would happen”, or “If this were to exist, what it look like” (both prefactual), with the answer being that there is no way that it could ever be the case. Others, such as the violinist problem are asking a question of the form “if this were to happen, in these circumstances, how would you respond?”.

Perhaps, too, the whole situation is complicated by the use of thought experiments in quantum physics – where it seems that the thought experiment itself is all that we know about the concept, theory, or position (but physics is not my specialty).

Not certain that I have completely answered your question; however, I am certain that we both agree that the philosopher is speculating on a hypothetical situation and that their speculation is entirely within their imagination.cogtrue 06:30, 13 May 2006 (UTC)

What are these?

I came looking for some ideas (which I thought were thought-experiments?) whose names I didn't know. What is the multiple mansions mind-trick (where you try to imagine as much detail in each room until they are as real as a memory)? What's the Door to December or the Cannibalistic Cat (Door to December where you look out the window and it's summer and go out the door within the same room and it's snowing? or the cat who eats his body starting with his tail... and as he gets closer and closer to his own head the mind has more trouble imagining it?). If these aren't thought experiments, what are they? 82.93.133.130 11:15, 11 February 2007 (UTC)

I can't really tell what you are describing. The mansions mind-trick as you describe it sounds like a kind of mental exercise, in which you try to improve your memory. The Door to December sounds like it's just supposed to some kind of mind f*ck (probably intense if you are on drugs, I guess). The Cannibalistic Cat sounds like its supposed to be a sort of paradox. These are not thought experiments. In a thought experiment, you imagine a scenario for the purpose of drawing a conclusion about the world. It's part of an argument. It's not just a difficult or perplexing thing to do with your mind. There are different kinds of thought experiments. To get an idea of what thought experiments are, I would suggest just clicking on the links to particular thought experiments to see how these hypothetical situations are used. These are the kinds of things that have come to be called "thought experiment." Rldoan 09:49, 16 February 2007 (UTC)

In my understanding, none of the sorts of things you have described are amongst those sorts of thing that might be labelled with the term "thought experiment" by a philosopher (and they would most certainly not be called "thought experiments" by any scientist).

Yet, if these sorts of mental adventure are not "thought experiments", the question then rises "What should we call them?".

My view is that it would be best to follow the lead of the Australian philosopher J. L. Mackie and employ the term he used to label the sort of intellectual challenge provoked by Locke’s account of the prince and the cobbler (see here for Locke's text).

Mackie, very deliberately, and in my view very sensibly, called them "puzzle cases".

I believe that the things you speak of are "puzzle cases". For more on "puzzle cases" see Mackie, J.L., Problems from Locke, Oxford University Press, (Oxford), 1976, p.175. Lindsay658 20:37, 16 February 2007 (UTC)

The prince and the cobbler is actually a thought experiment, in the sense in which philosophers talk about thought experiments. Locke adhered to a memory, or consciousness, theory of personal identity. The prince and the cobbler case is used to support that theory. It's a perfectly good philosophical thought experiment. It's argumentative. (I haven't read the Mackie. But thought experiments that philosophers propose and think about really are "puzzle cases". They are cases to reflect on and draw a conclusion about the state of things. They can be hard to figure out and are typically matters of dispute among philosophers. Calling something a puzzle case does not mean it's not a thought experiment. It is, however, probably a cavalier and slightly dismissive way of referring to thought experiments, but they are still thought experiments.)

The funny things described by the user above are not philosophical thought experiments and they are not scientific thought experiments (which are different things). I've suggested above what they really are, at least based on the minimal description given -- memory exercises, mind f***'s, and paradoxes. They are not argumentative. They are fun things to do that are not intended to show anything. Rldoan 09:02, 19 February 2007 (UTC)

Thought experiments involving causal reasoning

The section that lists seven types of thought experiments had said that these are the seven types of thought experiments in general. This is simply not true. The seven types of thought experiments listed concern causal reasoning. Thought experiments in philosophy (typically) are not concerned with causal reasoning. I changed the category title and discussion to reflect this limitation. If someone with a physics orientation wants to change some of the language to better reflect conventional vocabulary, that's fine. But the seven types are not inclusive of all thought experiments (as the term "thought experiment" is currently used). —The preceding unsigned comment was added by Rldoan (talk • contribs) 09:10, 15 February 2007 (UTC).

---

It is worth adding to the bibliography a note that Horowitz and Massey's book can be downloaded as a PDF:

http://philsci-archive.pitt.edu/archive/00003190/

Rosa Lichtenstein (talk) 22:36, 21 June 2008 (UTC)

Merge with Hypotheticals

It has been over a year now since the Hypotheticals article was created, and it still an uncited stub. As it currently exists, it does not present sufficient information to stand on its own, and its topic is barely differentiable from the idea of a thought experiment, if at all. Hypotheticals should be merged into this article. Neelix (talk) 16:22, 6 October 2008 (UTC)

How though? a thought experiment could be considered a type of hypothetical, but not all hypotheticals would necessarily be considered as thought experiments; hypothetically speaking of course (couldn't resist). thought experiments could perhaps be considered as a sub-category of hypotheticals, or as a derived concept, but it doesn't really work the other way around. so how do we merge them? add thought experiment to hypotheticals? when i've given it some more though, i'll see about adding material on the page there; could do a sub-section on thought experiments maybe? Lx 121 (talk) 12:20, 18 May 2009 (UTC)

Opposed. But there is a difference between the two, as stated above. Further, I can't see why Hypotheticals is not self-sufficient, and any similarity is properly addressed by a simple 'see also' link. Martinor (talk) 21:59, 19 May 2009 (UTC)

Opposed. Thought experiments are significant enough to warrant their own article, rather than merely a section in hypotheticals. And making hypotheticals a section of thought experiments wouldn't make sense either, since hypotheticals is a broader category. Thought experiments are hypothetical experiments, which is more specific. "What would the world be like if Germany won World War II?" is a hypothetical, but it would be quite odd to call it a thought experiment. -- Tim314 (talk) 07:19, 14 August 2009 (UTC)

I agree that it would be odd to characterize the hypothetical question, "What would the world be like if Germany won World War II?" as a thought experiment. But I would feel perfectly comfortable calling Philip Dick's novel "The Man in the High Tower" a thought experiment, perhaps because the novel is not a mere restatement of the hypothetical, but is a result of the novelist's work within the hypothetical; which causes me to ask, aren't all thought experiments tools that measure the boundaries of the hypothetical? (a question I don't feel qualified to answer). Perhaps this artistic application of the term has a somewhat different, less scientific (more casual) definition, which perhaps falls outside the scope of the term as here defined, but I do feel its a valid application.

The "thought experiment" in speculative fiction (perhaps the most classic example is Flatland) therefore may deserve some acknowledgement in an encyclopedic article. Bustter (talk) 01:41, 15 August 2009 (UTC)

Philosophical possibility

There is still much debate whether Chalmers experiment should be seen as metaphysically possible. However, I think an addition should be made to address the difference between metaphysically possible and logically possible. The paragraph suggests that Chalmers experiment is also debated as logically possible. Please add some references for this claim or make a clear distinction between metaphysically possible and logically possible.

62.177.201.147 (talk) 12:50, 2 February 2009 (UTC) Remko, student philosophy at the Erasmus University of Rotterdam

hoax?

Casimir cones listed here as a thought experiment is probably a hoax. Any thoughts? Dauto (talk) 20:19, 27 January 2009 (UTC)

Since the wikilink links to an article that is AfD I would concur. I will remove it. --Saddhiyama (talk) 20:59, 27 January 2009 (UTC)

Gedankenexperiment

… is written as one word, even in English. See http://en.wiktionary.org/wiki/gedankenexperiment --77.186.138.131 (talk) 09:54, 25 September 2009 (UTC)

Zeno

Whya are zeno's paradoxes in the philisophical cat they seem more physical to me. Stupidstudent (talk) 06:12, 22 October 2010 (UTC)

EPR

There is a longish section on EPR with stuff like Bohr won the argument by strength of personality.... This reads very oddly, like someone's anti-Bohr tirade. I don't think it really belongs. Bohr was indeed right, and Einstein wrong, and the thought experiment itself was valuable. But all the authority-people stuff should go William M. Connolley (talk) 21:10, 11 January 2011 (UTC)

Objectionable material removed.Chjoaygame (talk) 23:38, 11 January 2011 (UTC)

Yes that is how it was sourced. "Bohr won the argument by strength of personality". Sounds familiar to me here. Opinions removing sourced material. Zulu Papa 5 * (talk) 05:20, 12 January 2011 (UTC)

Undo

Undid this one [1], the remover opinions don't match the relevant and verified text. Zulu Papa 5 * (talk) 05:16, 12 January 2011 (UTC)

Thought experiments are valuable, but we do not help anybody by privileging the incorrect views of one philosopher. It is not true that TE underlie all modern physics, or at least it isn't true that this is unquestionably true. You could equally well assert that careful observation underlies all modern physics. The quote I removed is simply unhelpful William M. Connolley (talk) 08:29, 12 January 2011 (UTC)

Agree. Even if we would include this view, we certainly cannot say: "As the contemporary philosopher Martin Cohen puts it, ...". We should rather say: "The contemporary philosopher Martin Cohen puts that ...". The as-phrase suggests that Wikipedia endorses this view. DVdm (talk) 08:54, 12 January 2011 (UTC)

Is there a source for this statement "It is not true" it's a compelling issue for inclusion that is fairly attributed and could be balanced with additional sources. Zulu Papa 5 * (talk) 14:18, 12 January 2011 (UTC)

See WP:UNDUE William M. Connolley (talk) 15:20, 12 January 2011 (UTC)

Ok again, where (what source) is the counter weight originating from? Zulu Papa 5 * (talk) 15:38, 12 January 2011 (UTC)

Any physics text. jps (talk) 00:28, 13 January 2011 (UTC)