The pros and cons of Gedankenexperimente

  1. Since irony is not so well suited to further compassion and understanding I question in this thread that by categorizing some ideas as a thought experiment there's justification of treating these ideas as well-posed in the scientific sense. I have got three examples:

    1) "Doesn't one, by comitting suicide, not actually die but take ultimate control over reality ?"
    2) "What happens if we travel at the speed of light ?"
    3) "And again I say unto you, It is easier for a camel to go through the eye of a needle, than for a rich man to enter into the kingdom of God ?"

    Since the third thought experiment could be misunderstood either as bigotry or as another violent act of sarcasm I shall explain to you that I am serious about this and it is only meant as a question about which questions make sense and which don't.

    More clearly, what constitutes a valid thought experiment ? Do we have to care about validity at all ?
  2. jcsd
  3. questions (1) and (3) if taken literally are not scientific (in that the scientific method could not apply), nor even possible to phrase in scientific terms.

    question (2) is scientific, however as long as SR is valid (which we currently have every reason to believe that it is) the answer is a firm "irrelevant" since nothing that possesses mass may achieve a velocity equal to the speed of light. more formally, the lorentz transform of the rest mass diverges in the limit:

    [tex]\lim_{v \rightarrow c} \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}}[/tex]
  4. OOO, they closed my thread again! It seems no one wnats to address my issue :-(
    You said "Unless you are able to measure consciousness by some instrument you can't prove any connection between consciousness and wave function collapse. "

    But as far as I know (and may be I am wrong), even theoritical physicists may not always have an instrument to easure things- or do they?
  5. Although it's off-topic, just a short and final answer. Physics deals with things we can measure and how we can relate the these measurements with one another. No more, no less. Although theories about these relations become quite complicated sometimes, in the end it always amounts to the same thing: measurement. If you can't measure it somehow you can't call it physics. Well you can but then you're using your own language.

    This clearly doesn't mean that something which we can't measure doesn't exist. We probably all agree about that consciousness exists, but we can only measure certain primitive aspects of brain activity, or psychologists can measure behaviour. In my opinion consciousness will always be outside of physics and that's the reason why you won't find answers here. Seeking for religion in physics is a blind alley.

    Note by the author: this is an off-topic response that has nothing to do with the initial question.
    Last edited: Oct 3, 2007
  6. Q_Goest

    Q_Goest 2,989
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    Hi OOO,
    With all due respect, the examples you provided are not thought experiments. Per the Stanford Encyclopedia of Philosophy:

    In general, thought experiments do not by themselves constitute proof. They are intended to aid in understanding or to provoke some thoughts surrounding an issue. To actually prove anything, one must use logic and/or analysis of some sort to support any claims.

    For example: Chalmers provides a thought experiment wherein a neuron is replaced by a silicone chip. Each neuron is thus replaced, one at a time with a chip such that every connection to each neuron is maintained and each chip functions as the neuron had. From this, he makes various observations about the replacement and attempts to create an argument in support of functionalism. See "Absent Qualia, Fading Qualia, Dancing Qualia".

    In this example, the argument surrounding the thought experiment seems strong, but comes up short of actually proving anything. Physical thought experiments such as Einstein’s elevator may be a bit more convincing, but they still are not proof of anything. So I suppose the pros are that thought experiments provide a way of looking at something and they give clues to how nature works, but on the con side, a thought experiment alone does not constitute proof of anything.
  7. I agree. Interestingly enough, some people call (1) a thought experiment, see this thread.

  8. What are you talking about? Seriously, is it so hard for you to grasp the concept of something that's not measureable not being existant? I mean jesus if we're going to delve into the imaginary then we're wasting our time.
  9. Can you tell me what part of my question you didn't understand ?

    From what did you draw the conclusion that I didn't grasp something ? The posting of Viva Diva was OT, and I answered it, so what ?

    At least if we want to talk about physics, yes, I'd say we are wasting our time if we are drifting towards confused pseudoscience and crackpottery.
  10. Q_Goest

    Q_Goest 2,989
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    Ok, now I understand what you're refering to. The quantum suicide thought experiment is a valid one, though I'm not too impressed by it. That's a thought experiment intended to consider what might happen given the MWI of QM is true and implies that conscious beings are immortal. I didn't recognize it when you stated: "not actually die but take ultimate control over reality" but I understand what you mean now.

    Perhaps you had other thought experiments in mind in your initial list.
  11. The intention of this thread was to initiate discussion about whether thought experiments are of any use if they can't be done in reality. In my opinion the difference between the examples given at the thread start and Einstein's elevator thought experiments is clearly the fact that the elevator experiments could well be performed in reality.

    The only reason why Einstein labelled them as thought experiments was that they were in some sense beyond his economical resources and also beyond the experimental means of the time. Actually almost all modern tests of General Relativity, e.g. the Pound-Rebka experiment, can be considered as practically feasible variants of Einstein's thought experiments.

    By contrast I'd consider things like the "quantum suicide thought experiment" or "Schrödinger's cat thought experiment" as metaphysical brainf*ck. As a matter of fact Schrödinger only tried to find a vivid metaphor for how practically useless subjective interpretations of quantum mechanics are. Trying to speculate in all seriousness about the physical consequences of the cat experiment (and its nerdy derivative "quantum suicide") is like proving a joke with formal logic.

    But I guess there will always be these two fractions among physicists: the ones that like to do productive work (either by experiment or by theory) and the wise guys. Trying to change this is futile.
  12. Q_Goest

    Q_Goest 2,989
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    Let’s say there’s a philosophy class in which the prof asked folks to create thought experiments. How would we rate them?

    Regarding the quantum suicide TE, I’d rate that one poorly since it offers no additional insight. It doesn’t shed any light whatsoever on why a person only experiences one single ‘world’ instead of many. It also says nothing about why, with all these different worlds we allegedly inhabit, we should go through time being conscious of this particular one that we’re in, so it can’t possibly suggest someone would be concious following one extremely unlikely event in which a person survives an almost sure death. So I’d agree, it’s a useless TE.

    I think they are, but they must as you say, be “beyond [our] economical resources and also beyond the experimental means of the time.” They must also shed some light on nature, and be subject to some kind of analytical proof or logic. Thought experiments should not be used as proof (such as Chalmers TE above), nor try to appeal to our intuitions (such as the China brain by Ned Block). To rate one highly, I think we'd need to be able to carry out some proof on it either using logic or mathematical analysis.
  13. I only noticed on second reading how appropriate your example of the Chalmers thought experiment is in this context. It's surprising how questions like "quantum suicide" and "Schrödinger's cat" cleverly disguise the fact that they curry favor with religion by dealing with things like consciousness and free will.

    Starting by saying "Replace a neuron by a silicon chip that behaves exactly like the neuron itself" guides one directly onto the wrong track (i.e. metaphysics). Nobody knows how to build a silicon chip that behaves exactly like a neuron. Should it simulate the nuclear magnetic moments as well ? Should it simulate the nuclear matter ? Should it even simulate quarks ? What about "physics beyond the standard model" ? Or is the silicon neuron rather a poor-man's model of a neuron with 10000 degrees of freedom instead of infinitely many as in reality.

    The question whether consciousness is mechanistic or not can't be answered because it is impossible to know whether our abstract descriptions of nature are complete. Even if the future laws themselves could somehow be proven to be complete, the boundary conditions can clearly never be. And thus if it's a question that can't be answered at all, it's also a question that is totally unrelated to science.
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