When Einstein started to think about gravitation, he completely created a new theory that no experiment supported. He based his reasoning, as he explained it later, on small thought experiments (Gedankenexperiment) that led him to formulate the equivalence principle. From this equivalence principle, he used the principles of physics and understood that a new space-time geometry was necessary and after years of effort he obtained the Einstein's equation. It is only when he used this equation to compute the anomalous perihelion advance of the planet Mercury that he knew his theory had something to do with the real world (but one could not call this a prediction). Since then, many predictions of General Relativity have been successfully checked experimentally.

Edited question

A thought experiment is an experiment that has not been realized, but that was imagined. The results of such experiment are based on well established physical principles but have never been observed physically. A second example of a thought experiment considers a frictionless movement of a vehicle on a road (see Einstein and Infeld, The Evolution of Physics: The Growth of Ideas From Early Concepts to Relativity and Quanta, chapter 1 on the Galilean relativity). Another example in particle physcis is described in one of the above comments. Thought experiments only draw conclusions from physical considerations, not from mathematical derivations, and can therefore be used as illustrations destined to non-physicists.

Does anyone think of other thought experiments which results have been proved relevant later by physical experiments, either because the thought experiment has been realized or because its results had physically testable consequences ?

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    $\begingroup$ Galileo made a couple of important ones. Probably the most famous one is that the time for an object to fall from a certain height is independent of their mass. More info can be found here: en.wikipedia.org/wiki/… $\endgroup$
    – Hunter
    Jan 15, 2014 at 22:03
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    $\begingroup$ If a thought experiment is completely without seeds from physical reality it is science fiction. Thought experiments may diverge from known real experiments, but they have to do that without contradicting limits from physical experiments. $\endgroup$
    – anna v
    Jan 16, 2014 at 8:57
  • $\begingroup$ @Hunter. (+1) In the book The Evolution of Physics: The Growth of Ideas From Early Concepts to Relativity and Quanta written by Einstein and Infeld, the authors describe very well a thought experiment with the example of Galilean relativity. The experiment you mention is actually based on this relativity principle and on the weak equivalence principle. $\endgroup$
    – Tom-Tom
    Jan 16, 2014 at 9:11
  • $\begingroup$ @annav. A thought experiment is based on physics and on reality. Einstein's lift was never built, yet the experiment makes perfect sense. In my reply to Hunter, I mention another example in which the experiment, although it is impossible to realize (a vehicle moving on a road without friction) the result of this experiment is what one calls principle of inertia. $\endgroup$
    – Tom-Tom
    Jan 16, 2014 at 9:19
  • $\begingroup$ @Dimensio1n0. I do not think this question is too broad or refers to "myths". A thought experiment is something extremely precise. It is an experiment that can not be realized but that one imagines being realized. The results of the experiment are guessed and used to build a physical principle. Example: Einstein's lift experiment is used to motivate the idea that space is curved by the gravitational field. $\endgroup$
    – Tom-Tom
    Jan 16, 2014 at 14:04

1 Answer 1


Dirac prediction of antimatter? Schrodinger formulation of quantum mechanics?

Einstein thought experiments were not entirely devoid of empirical evidence: you have to base your physics on something.

The technique is commonly used in research all the time, and (depending on your definition) predates Einstein - he just coined the phrase. It would be tricky, after the fact, to work out to what extent the theory preceded the discovery or was based on previous discoveries.

  • $\begingroup$ Thank you for your answer. I am actually not aware of any thought experiment made neither by Dirac or Schrödinger. $\endgroup$
    – Tom-Tom
    Jan 16, 2014 at 8:37
  • $\begingroup$ mathematics are higher level thought experiments, and Einstein's thought experiments were based on mathematics anyway. $\endgroup$
    – anna v
    Jan 16, 2014 at 8:49
  • $\begingroup$ @anna-v. I disagree with that. In my opinion, thought experiments are based on physics, as I explained in my reply to your previous comment. Mathematics perform analysis. Mathematics without a physical basis can be completely disconnect from anything tangible (and that also part of their beauty). $\endgroup$
    – Tom-Tom
    Jan 16, 2014 at 9:24
  • $\begingroup$ Dirac: the math showed some terms that look non-physical. The thought experiment was to figure what the consequences would be of not throwing the terms away ... i.e. what would be the properties of these new particles? We have to do something like this every time we design an experiment. But some people will disagree with that as a thought experiment. Please state the definition you want to use and we can talk. $\endgroup$ Jan 17, 2014 at 5:00
  • $\begingroup$ Taking your example, here is a nice thought experiment: Take a neutron in a referential frame $\def\R{\mathfrak R}\R$, and observe the beta decay $\def\n{\bar\nu_e}n\to p+W^-$ followed by $W^-\to e^−+\n$. Consider now a frame $\R′$ in uniform translation w.r.t. $\R$ such that the second event in $\R$ is observed first in $\R′$. In $\R′$ one observes a spontaneous creation $\emptyset\to W^++e^−+\n$ and then $W^++n\to p$. This shows evidence for the existence of a particle $W^+$ that has the same mass as $W^-$ but opposite charge and for the existence of spontaneous creations of particles. $\endgroup$
    – Tom-Tom
    Jan 17, 2014 at 12:23

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