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Thought Experiments Philosophy department, Shandong University WANG Huaping.

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Presentation on theme: "Thought Experiments Philosophy department, Shandong University WANG Huaping."— Presentation transcript:

1 Thought Experiments Philosophy department, Shandong University WANG Huaping

2 Contents Cases 2 What Is Thought Experiments 31 Problems 33 Task 4

3  Thought experiments, have an honored place in the history of natural sciences. These ‘experiments of the mind’ have been used by innovative physicists. A scientist presents us with some problem. We are perplexed. In a few words of simple prose, the scientist then conjures up an experiment, purely in thought. We follow, replicating its elements in our minds, and our uncertainty evaporates. We know the resolution and somehow we sense that we knew it all along. What Is Thought Experiments

4  An example of thought experiments:  When a chain is draped over a double frictionless plane, as in Fig. 1a, how will it move? What Is Thought Experiments Fig. 1a

5  Simon Stevin suggested us to imagine a case as Fig. 1b shows: What Is Thought Experiments Fig. 1b In the imaginary case, the chain does not move. Otherwise, it seems, the system would be in a state of perpetual motion. Then imagine cutting the string at the two lower corners. Since the balls were in equilibrium prior to the cutting, they remain so afterwards: the shorter and the longer string of balls are in balance.

6  Thought experiments are devices of the imagination used to investigate the nature of things. Thought experimenting often takes place when the method of variation is employed in entertaining imaginative suppositions.  Thought experiments employ a methodology that is a priori, rather than empirical, in that they do not proceed by observation or physical experiment.  Most often thought experiments are communicated in narrative form, sometimes through media like a diagram. What Is Thought Experiments

7  The understanding comes through reflection on the situation.  Well-structured hypothetical questions employ "What if?" reasoning. hypothetical  Some TE are counterfactual, which means that they speculate on the possible outcomes of a different past. Sometimes, it is impossible to run the experimental scenario in the real world. impossible  They are prefactual (before the fact).  They are causal reasoning: in TE one reasons from causes to effects, or effects to causes.

8   challenge (or, even, refute) a prevailing theory   confirm a prevailing theory.   establish a new theory. Consequences OF TE

9  This TE attempts to show that space is infinite. Case 1: Lucretius’ Spear  If there is a purported boundary to the universe, we can toss a spear at it. If the spear flies through, it isn't a boundary after all; if the spear bounces back, then there must be something beyond the supposed edge of space, a cosmic wall that stopped the spear, a wall that is itself in space. Either way, there is no edge of the universe; space is infinite.

10   Did Galileo dropped balls from the top of the Leaning Tower of Pisa ? Leaning Tower of Pisa   According to Aristotle 's physics, objects fall at a speed relative to their mass. Aristotle   Galileo thought Aristotle’s theory is wrong.   How did Galileo do to prove it? Case 2: Galileo’s Ball

11   Galileo imagine two objects joined by a light thread.   How fast do they fall? Falling at an intermediate speed ?   This answer contains a contradiction, since the joined single object, with a mass the sum of the two initial objects, should fall, according to Aristotle, faster than either of the single objects. Case 2: Galileo’s Ball A B

12  Van Helmont challenged earlier ideas that food was digested by the body's internal heat.  If such was the case, van Helmont argued, how could cold-blooded animals live?  His own opinion was that digestion was aided by a chemical reagent, within the body, such as inside the stomach.  Van Helmont's idea was "very near to our modern concept of an enzyme. Case 3: Helmont and Digest

13   Newton visualizes a cannon on top of a very high mountain.   If there was no force of gravity (as was thought at the time) the cannonball should follow a straight line away from Earth. Earth Case 4: Newton’s Cannon

14   So long as there is a gravitational force acting on the cannon ball, it will follow different paths depending on its initial velocity.   If the speed is low, it will simply fall back on Earth. If the speed is very high, it will indeed leave Earth.   If the speed equals some threshold orbital velocity it will go on circling around the Earth along a fixed circular orbit just like the moon.   If the speed is higher than the orbital velocity, but not high enough to leave Earth altogether it will continue in an elliptical orbit. Case 4: Newton’s Cannon

15  Einstein: "...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. Case 5: Chasing a Beam of Light

16  From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. For how should the first observer know or be able to determine, that he is in a state of fast uniform motion? One sees in this paradox the germ of the special relativity theory is already contained.  Somehow, this little glitch led Einstein right to the theory of special relativity. Case 5: Chasing a Beam of Light

17 Case 6: Schrodinger’s Cat  A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): In a Geiger counter, there is a tiny bit of radioactive substance, so small that perhaps in the course of the hour, one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges, and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour,

18  one would say that the cat still lives if meanwhile no atom has decayed. Case 6: Schrodinger’s Cat

19  The psi-function of the entire system would express this by having in it the living and dead cat mixed in equal parts. It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. Case 6: Schrodinger’s Cat

20  Maxwell imagines one container divided into two parts, A and B. Both parts are filled with the same gas at equal temperatures and placed next to each other. Observing the molecules on both sides, an imaginary demon guards a trapdoor between the two parts. When a faster-than-average molecule from A flies towards the trapdoor, the demon opens it, and the molecule will fly from A to B. Likewise, when a slower-than- average molecule from B flies towards the trapdoor, the demon will let it pass from B to A. Case 7: Maxwell's Demon

21  The average speed of the molecules in B will have increased while in A they will have slowed down on average. Since average molecular speed corresponds to temperature, the temperature decreases in A and increases in B, contrary to the second law of thermodynamics. Case 7: Maxwell's Demon

22  The infinite monkey theorem states that a monkey hitting keys at random on a typewriter keyboard for an infinite amount of time will almost surely type a given text, such as the complete works of Shakespeare. Case 8: Monkeys and Typewriters

23  Since "thinking" is difficult to define, Turing chooses to "replace the question by another: "Are there imaginable digital computers which would do well in the imitation game? Case 9: Turing Test I propose to consider the question: 'Can machines think?

24  A human judge engages in a natural language conversation with a human and a machine designed to generate performance indistinguishable from that of a human being. All participants are separated from one another. If the judge cannot reliably tell the machine from the human, the machine is said to have passed the test. Case 9: Turing Test

25  Suppose that artificial intelligence research has succeeded in constructing a computer that behaves as if it understands Chinese. It takes Chinese characters as input and produces other Chinese characters, which it presents as output. Suppose that this computer performs its task so convincingly that it comfortably passes the Turing test: To all of the questions that the person asks, it makes appropriate responses, such that any Chinese speaker would be convinced that he or she is talking to another Chinese-speaking human being. Case 10: Chinese Room

26  Supposes that Searle is in a closed room and has a book with an English version of the computer program, along with sufficient paper, pencils, erasers, and filing cabinets. Searle could receive Chinese characters through a slot in the door, process them according to the program's instructions, and produce Chinese characters as output. Case 10: Chinese Room

27  As the computer had passed the Turing test this way, it is fair to deduce that Searle would be able to do so as well, simply by running the program manually.  There is no essential difference between the role the computer plays in the first case and the role he plays in the latter. Each is simply following a program, which simulates intelligent behavior. And yet, Searle points out, "I don't speak a word of Chinese.“ Since he does not understand Chinese, we must infer that the computer does not understand Chinese either. . Case 10: Chinese Room

28  In the Scholium to Book 1 of Principia, Isaac Newton describes an experiment in which a bucket of water hung by a long cord is twisted and released. Initially, only the bucket rotates while the water remains stationary, as indicated by its flat surface. frame for all motion. Problems

29  Gradually, the rotational motion is communicated to the water and centrifugal force distorts its surface into a paraboloid. After the bucket stops turning, the water continues to rotate until frictional forces again bring it to rest, flattening the paraboloid. (Viscosity, which causes this friction, is described by another law proposed by Newton!) Newton cited the rotating-bucket experiment to support his notion of absolute space as the reference frame for all motion. Problems

30 Task  How do you think about the TE of Lucretius’ Spear?


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