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Physics, Metaphysics, & Other Nonsense H.S. Hestevold Handout 14 Spring 2014 Quantum Mechanics: Philosophical Concerns, Part I I. Intrinsic and extrinsic properties Stephen Yablo writes: “You know what an intrinsic property is: it's a property that a thing has (or lacks) regardless of what may be going on outside of itself.”1 David Lewis writes: A sentence or statement or proposition that ascribes intrinsic properties to something is entirely about that thing; whereas an ascription of extrinsic properties to something is not entirely about that thing, though it may well be about some larger whole which includes that thing as part. A thing has its intrinsic properties in virtue of the way that thing itself, and nothing else, is. Not so for extrinsic properties, though a thing may well have these in virtue of the way some larger whole is. The intrinsic properties of something depend only on that thing; whereas the extrinsic properties of something may depend, wholly or partly, on something else. If something has an intrinsic property, then so does any perfect duplicate of that thing; whereas duplicates situated in different surroundings will differ in their extrinsic properties.2 How the distinction between intrinsic and extrinsic properties should be drawn is controversial,3 but we will not involve ourselves with this controversy. Rather, the distinction will be developed informally, leaving us with a rough-and-ready understanding of the distinction suitable enough for the purposes of our discussion. Roughly, an entity’s intrinsic properties are those properties that it has independently of other things that exist. For example, you have the intrinsic properties being human, being conscious, being biologically female (or being biologically male, 1 “Intrinsicness”, Philosophical Topics 26 (1999), 479. 2 “Extrinsic Properties”, Philosophical Studies 44, 111-112. 3 For example, are an entity’s extrinsic properties its relational properties (e.g. being the father of Marilyn Monroe, being larger than an apricot) such that its intrinsic properties are its monadic properties (e.g. having mass, being red hot, having a positive charge)? Are an entity’s intrinsic properties its essential properties -- those properties it has at any time that it exists? (This would suggest that an entity’s extrinsic properties are its non— essential properties -- properties that the entity happens to have, but need not have.) Are an entity’s intrinsic properties those properties that it would have in common with any duplicates of that entity? (This would suggest than an entity’s extrinsic properties are those properties that any of its duplicates could lack.) 1 as the case may be), and having a body with such-and-such mass. That you have these characteristics is a function of you alone: you would have these properties even if every other entity in the universe disappeared. (If a mad scientist created a doppelganger of you, the doppelganger would also have those properties.) You have other properties, however, that don’t involve yourself alone; these other properties in some sense depend on other entities that exist. For example, you now have the properties being a daughter (or being a son, as the case may be), being a sibling (if you have a brother or sister), being an aunt or uncle (if you have a nephew or niece), being a student at UA, and weighing more than Hestevold’s dog. Your being a sibling (if you are) is incidental: you would not have this property had your parents not had at least one child other than yourself. You may now lack the property of being an aunt (or uncle), but you will acquire that property if you have a sibling who becomes a parent. Had individuals never formed The University of Alabama, you would not now have the property of being a student at UA. A proton’s mass or charge would be among its intrinsic properties; whether it is part of a hydrogen atom or boron atom would be an extrinsic feature of the particle. Having a spin of a certain direction is an intrinsic property of the electron (even though its spin may be causally affected by entities other than the electron). If spatial points do exist after all, then presumably every spatial point has the same intrinsic properties as any other spatial point; but spatial points would differ with respect to their extrinsic properties (e.g. with respect to the spatial relations that they bear to certain other spatial points). II. A simple-minded explication of the double-slit experiment Read Lawrence Sklar's description of the double-slit experiment in “The Quantum Picture of the World” in SUP, 165.4-167; study Figure 4.2. A. Experiment #1: What happens when electrons are fired at a double-slit wall with the right {R) slit closed? The electrons form a strike pattern in front of the left (L) hole that is evidence that each electron exhibits the property being a BB like particle. B. Experiment #2: What happens when electrons are fired at a double-slit wall with the L slit closed? The electrons form a strike pattern in front of the R hole that is evidence that each electron exhibits the property being a BB like particle. 2 C. Experiment #3: What happens when electrons are fired at a double-slit wall with the both slits open? The electrons do not form BB-like strike patterns in front of the two slits. Rather, they form an interference pattern -- a pattern that suggests that electrons lack the property being a BB-like particle. D. Regarding Experiment #3, what should we claim about a given electron E between the time it leaves the electron gun and the time it strikes the detection screen? Does E reach the detection via the left slit? The right slit? Both slits? Neither slit? 1. Reminder: If X = Y, then X and Y have all properties in common. 2. "E traveled through the left slit to reach the detection screen." In Experiment #3, electrons that strike the detection wall apparently lack the property being a BB-like particle. But Experiment #1 suggests that those electrons that pass through the left slit have the property being a BB-like particle. So, E cannot be identical with any particle that passes through the left slit. 3. "E traveled through the right slit to reach the detection screen." In Experiment #3, electrons that strike the detection wall apparently lack the property being a BB-like particle. But Experiment #2 suggests that those electrons that pass through the right slit have the property being a BB-like particle. So, E cannot be identical with any particle that passes through the right slit. 4. "E traveled through both the left and right slits to reach the detection screen." First, some would argue that it is metaphysically impossible that a single entity could wholly occupy two distinct places at the same time. Second, if one places a detector by each slit, one detector or the other (but not both) will register every time an electron is fired toward the detection screen and makes it past the double-slit wall. So, there is experimental evidence that no electron passes through both slits at once. And there is no evidence that a half-electron passes through each of the two slits. 5. "E traveled through neither slit to reach the detection screen." This is not a viable option: if 3 one blocks both slits, E would never have reached the detection screen. So, unless one believes in mystical teletransportation, E cannot jump from the electron gun to the detection screen without taking one path or the other. 6. "Between leaving the electron gun and striking the detection screen, E was in superposition:" Between leaving the electron gun at time t and striking the detection screen at time t*, electron E was something that exhibited a mysterious mode of being that did not involve E's having a particular set of intrinsic properties at a particular time and location. (It is false that E travels through the left slit with the property being a BB-like particle; it is false that E travels through the right slit with the property being a NON-BB-like particle.) III. A simple-minded explication of two experiments4 Read Lawrence Sklar's description of the Stern-Gerlach experiment, 168.1-170.1; study Figure 4.4. To simplify matters, abandon talk of spin, momentum, and the like. Instead, let's talk about color and hardness. Assume that every electron is either black or white [B or W]; and assume that every electron is hard or soft [H or S]. A. Color box. There is a color box that detects the color of electrons: if a stream of electrons enters from the west, all B electrons exit to the north, and all W electrons exit to the east. Color determination is repeatable: if color box #1 detects Ellie to be a B electron, then color box #2 will also reveal that Ellie is B. Note: The color box reveals nothing else about an electron but its color; if an electron emerges from the east, we know it is W, but we don't know whether it is spinning up or spinning down, whether it is hard or soft: one would need a spin box or hardness box to detect those characteristics… B. Hardness box. There is also a hardness box that detects the hardness of electrons: if a stream of electrons enters from the west, all H electrons exit to the north, and all S electrons exit to the east. Hardness determination is also repeatable: if hardness box #1 detects Elton to be an S electron, then hardness box #2 4 See David Z. Albert, Quantum Mechanics and Experience, Ch. 1. 4 will also reveal that Elton is S. Note: The hardness box reveals nothing else about an electron but its hardness; if an electron emerges from the north, we know it is H, but we don't know whether it is spinning up or spinning down, whether it is black or white: one would need a spin box or color box to detect those characteristics. C. D. Experiment #4: Are color and hardness correlated? Is it true that most W electrons are also H or that most S electrons are B? Or, is there no correlation between an electron's color and its hardness? To answer these questions, put a color box in sequence with a hardness box (and then vice versa). The results: 1. If you take the B electrons that emerge from the north of a color box and feed them through a hardness box, half the B electrons will be H and half the B electrons will be S. 2. If you take the W electrons that emerge from the east of a color box and feed them through a hardness box, half the W electrons will be H and half the W electrons will be S. 3. If you take the H electrons that emerge from the north of a hardness box and feed them through a color box, half the H electrons will be B and half the H electrons will be W. 4. If you take the S electrons that emerge from the east of a hardness box and feed them through a color box, half the S electrons will be B and half the S electrons will be W. 5. The bottom line: by linking two detection boxes, there appears to be no correlation between color and hardness. Experiment #5: what if one places a hardness box between two color boxes? 1. Step 1: Color box #1 produces a stream of 100% B electrons and a stream of 100% W electrons. 2. Step 2: Take all the B electrons from color box #1 and stream them through the hardness box. What will happen? Just as before, the stream of B electrons will generate a stream of 50% H electrons and 50% S electrons. (Are the H and S 5 electrons all B when they exit the hardness box? One would likely assume that they are all B, but we don't know: the hardness box sorts by hardness, not by color. So, the only thing that we can detect about an electron emerging from a hardness box is that it is H or S; we can't also detect at that time its color.) 3. Step 3: You now experiment with the H electrons that all streamed out of the north (i.e. B) exit of the hardness box. You stream the H electrons through the west opening of color box #2. What results should you expect? Answering this question seems obvious. We know that color detectors are repeatably accurate: if passed through a color box again and again, Ellie the B-electron will be detected to be B again and again. So, if color box #1 generates a stream of all-B electrons, and if that stream of all-B electrons is subsequently divided (by the hardness box) into streams of H and S electrons, then one would assume that that stream of H electrons, when run through color box #2, would generate a stream of B electrons exiting north and no W electrons exiting east. This anticipated outcome is not what happens… The actual results. From the stream of H electrons, color box #2 actually generates two streams of electrons: 50% are B and 50% are W! How can this be? Using only one color box and one hardness box, we produced evidence that color and hardness are independent properties of electrons. Yet, given the three-box experiment, it appears that detecting an electron's hardness can causally affect the color of the electron: the second color box generates two streams of electrons -- one stream of B electrons and another stream of W electrons! (Pause a moment to scratch your head.) [Note: Are the B and W electrons all H when they exit the color box? One would likely assume that they are all H given that they all originated from the north exit of the hardness box, but we don't really know: the color box sorts by color, not by hardness. So, the only thing that we can detect about an electron emerging from color box #2 is that it is then B or W; we can't also detect at that time its hardness.] 6 IV. A simple-minded explication of a more complicated experiment5 A. The two-mirror apparatus. Imagine a square work bench. A hardness box is mounted in the lower left (i.e. southwest [SW]) corner of the bench; if a stream of electrons enters through the west opening of the box, hard electrons stream north following the western edge of the bench, and soft electrons stream east following the southern edge of the bench. A mirror at a 45°-angle at the NW corner changes the direction of the H electrons, sending them traveling along the north edge of the bench toward the northeast [NE] corner. Similarly, a mirror at a 45°-angle at the SE corner changes the direction of the S electrons, sending them traveling along the eastern edge of the bench toward the NE corner. Thus far, the apparatus is made with a hardness box plus two mirrors; it also includes a collection box… Located at the NE corner of the bench is a collection box that collects and recombines the two streams of electrons -- it recombines the electrons that traveled north and then east with those electrons that traveled east and then north. The collection box then shoots the stream of recombined electron away from the bench to the NE. Let 'path h' refer to the path that H electrons take from the SW hardness box north to the NW mirror and then east to the collection box. Let 'path s' refer to the path that S electrons take from the SW hardness box east to the SE mirror and then north to the collection box. B. Preliminary testing. Plenty of H electrons are fed into the hardness box; and they all take path h and emerge from the collection box and are verified to still be H after their journey. A quantity of S electrons are fed into the hardness box; they all take path s and emerge from the collection box and are verified to remain S electrons after their journey. C. Experiment #6: feed all W electrons into the SW hardness box and detect for H/S in the collection box. As expected, half the W electrons follow path h, and half the W electrons follow path s. In the end, half 5 See David Z. Albert, Quantum Mechanics and Experience, Ch. 1. 7 the electrons in the collection box are H and half are S. No surprise here. D. Experiment #7: feed all H electrons into the SW hardness box and detect for B/W in the collection box. As expected, the H electrons all follow path h; no electrons will have followed path s. In the collection box at the end of path h, these electrons are detected to have remained H; and half of these H electrons are B and half of these H electrons are W. Again, no surprises here. Note: the same outcome occurs if one feeds all S electrons into the SW hardness box: after they all follow path s (and none follows path h), they remain soft in the collection box, and 50% are B and 50% are W. 8 E. Experiment #8: feed all W electrons into the SW hardness box and detect for color at the end. 1. 2. What results should you expect? The reasonable expectation is developed in three steps: a. Having used a heretofore unmentioned color box to generate a stream of W electrons, one passes that stream of W electrons through the SW hardness box. Half of those W electrons are H and half are S; so, half the W electrons will follow path h to the collection box and the other half will follow path s to the collection box. In the collection box, then, half the collected electrons will be detected to be H and half will be detected to be S. This is not problematic. b. Reminder: In the earlier three-box Experiment #5, we learned that a stream of single-color electrons passing through a hardness box will produce H and S electrons that are subsequently detected to be half W and half B, suggesting that the mere detection of hardness can alter the color of electrons. c. The bottom line regarding what one should expect from the two-mirror apparatus: if a stream of W electrons is sorted into half H and half S electrons, then given the threebox experiment, one expects that those H and S electrons will be subsequently measured to be half W and half B -- that passing through the hardness box will have causally altered the color of half the incoming W electrons. (There is no reason to believe that the outcome will be any different from the threebox experiment: only W electrons are streamed into the hardness box; half of all W electrons are H; the other half are S; the H and S electrons bounce off mirrors and land in an inert holding container. So, given the three-box experiment, you should expect that, in the collection box, half the combined H/S electrons will be W and half will be B. The actual results. After passing a stream of W electrons through the hardness box and collecting them in the collection box, half are H and half 9 are S. When those combined H and S electrons are then tested for color (using, presumably, a color box #2), one discovers that all collected electrons are W! This outcome conflicts with the outcome of the three-box experiment described earlier! Now it appears that detecting hardness does not affect the color of the incoming W electrons! F. Experiment #9: repeat Experiment #8 with a barrier that can be moved in to block path s and moved out to open path s. When the barrier is moved out, the apparatus operates exactly like Experiment #5. 1. 2. What results should you expect if you stream all W electrons into the hardness box and move the barrier inward to block path s? a. If you stream all W electrons into the hardness box, half are hard and half are soft; so half will follow path h and half will follow path s. So, if you block path s, the total number of electrons that land in the collection box will be decreased by 50%. b. Reminder: With no barrier (as in Experiment #8), all electrons in the collection box are detected to be white. Thus, without the barrier, all the incoming W electrons that take path h are white, and all the incoming electrons that take path s are white. c. The bottom line of what one should expect with the barrier moved inward. Whether the barrier is or isn't blocking path s should have no affect on the color of electrons that follow path h. So, if all the electrons on path h are white without the barrier, there is thereby every reason to conclude that all the electrons along path h will be white with the barrier moved inward. And these path-h electrons are the only electrons that will reach the collection box. So, there is every reason to believe that only half the incoming W electrons will reach the collection box and that they will all be white. The actual results. As predicted, with the barrier in place, only half the incoming W electrons ever 10 reach the collection box (and they reach it, of course, via path h). The surprising result, however, is that the electrons collected are not all W! Half are W and half are B! How can this be? (Take another moment to scratch your head and to exhibit an appropriate expression of disbelief.) This outcome conflicts with the outcome of Experiment #8: Whether a barrier does or doesn't block the path of S electrons should have nothing to do with the color of a separate flow of H electrons! But, apparently, there is a mysterious connection after all. 3. G. Note: The same results occur if one blocks path h instead of path s. What should we claim about white electron E that is inserted into the hardness box and lands in the collection box in Experiment #8? Did E reach the collection box via path h? Path s? Both? Neither? 1. Reminder: If X = Y, then X and Y have all properties in common. 2. "E took path h to the collection box." Every electron in the collection box, including E, has the property of being definitively W. But Experiment #9 suggests that any electron that takes path h lacks the property of being definitively W. So, E cannot be identical with any electron that takes path h. 3. "E took path s to the collection box." This fails for the same reason: every electron in the collection box, including E, has the property of being definitively W. But Experiment #8 suggests that any electron that takes path s (when path h is blocked) lacks the property of being definitively W. So, E cannot be identical with any electron that takes path s. 4. "E took both path h and path s to the collection box." First, some would argue that it is metaphysically impossible that a single entity could wholly occupy two distinct places at the same time. Second, after an electron is inserted into the apparatus, it is always found on path h or path s, but never both; and, no half-electrons are ever found. 11 V. 5. "E took neither path h nor path s to the collection box." This is not a viable option: if one blocks both h and s, E would never have reached the collection box. So, unless one believes in mystical teletransportation, E cannot jump from the hardness box to the collection box without taking one path or the other. 6. "Between entering the hardness box and entering the collection box, E was in superposition:" Between entering the hardness box at time t and landing in the collection box at time t*, electron E was something that exhibited a mysterious mode of being that did not involve E's having a particular set of intrinsic properties at a particular time and location. (It is false that E travels path h with the property being W; it is false that E travels path h with the property being non-W.) Causation and spatiotemporal locality & quantum weirdness If event E1 that occurs at time t1 is the (indirect) sufficient causal condition of event E2 that occurs at later time t5, then we assume that E1 has causal effects at every time between t1 and t5. In short, we assume that an event at one time cannot be causally connected with an event at a later time if there is a time between these two events at which the first event has no causal effect at all. If Socrates drinks hemlock at time t and if Socrates dies at t5 and if the arsenic exhibits no causal effect whatsoever at times t2, t3, and t4, then the hemlock cannot have been the cause of Socrates’ death at t5. The temporal gap would assure us that the two events are causally independent. Similarly, if event E1 that occurs in spatial region S1 is the (indirect) sufficient causal condition of event E2 that occurs in spatial region S5, then we assume that E1 and E2 are spatially local: E1 exhibits some causal effect within every spatial region between S1 and S5. If the cue ball and eight ball are a foot apart on the pool table and if the eight ball begins rolling into a side pocket after you simply touch the top of the cue ball with your finger, your touching the cue ball cannot be the cause of the eight ball’s rolling: your touching the cue ball had no causal effect on any of the spatial regions between the two balls. The spatial gap assures us that the two events are causally independent. A potential problem. “But what about gravity? The Moon’s gravity has an affect on the Earth’s oceans and a powerful magnet 12 can causally effect a nail place several centimeters away. These examples demonstrate the causation need not require spatial locality -- there is such a thing as causation at a distance.” This objection is right on target… unless we agree that (a) there exists something (e.g. a gravitational field) that lies between the Moon and Earth's oceans and (b) there exists something (e.g. a magnetic field) that lies between the magnet and nail. If there does exist a gravitational field between the Moon and oceans, then the Moon is the indirect cause of the oceans' tidal changes: the Moon, via its gravitational field, does exhibit causal effects on every spatial region between the region occupied by the Moon and the region occupied by the oceans. Intrinsic properties. Presumably if an event that involves a certain object at one place and time brings about an event involving some other object at another place and time, then the causal connection is a function of the intrinsic properties had by each of the objects. If the lighting of a cigar causes a forest fire, the causal connection is a function of certain intrinsic (chemical, physical) properties had by the cigar and other intrinsic properties had by the forest’s trees. The weirdness. We want to say that the electron’s approaching the double-slit wall is an (indirect) cause of its striking a particular location on the detector. That is, various facts about the electron as it approaches the wall -- facts that may involve its mass, spin, charge, speed, trajectory, etc. -indirectly determine that it strikes the detector at a particular place. If this is true, then the approaching electron must exert (directly or indirectly) some causal effect involving every region of space that it occupies between its approaching the wall and its striking the wall. If, however, the electron lacks any definitive location and lacks any definitive set of intrinsic properties at any time between its approaching the double-slit wall and its striking the detector, then it would seem that the causal connection between the approaching electron and its striking the wall violates the principle of spatial locality -that this causal connection involves a causal connection that weirdly “travels across” a spatial gap and temporal gap. How can this be? 13