4 ms·

A few thoughts, speaking as a physics professor who asks his relativity students to play both "Velocity Raptor" and "A Slower Speed of Light": * Like many othe

by Steuard 10y ago

A few thoughts, speaking as a physics professor who asks his relativity students to play both "Velocity Raptor" and "A Slower Speed of Light": * Like many others here, I was a bit dismayed to see this implemented as a Java applet. Increasing security concerns have meant that I've been able to use fewer and fewer of those in my classes: it's just not reasonable to ask students to jump through those hoops anymore. * When I look at the menu, it only goes up through level 7. * I have made very little progress on level 8 (the one with the diffraction grating in the middle that's trying to get you to reach 2% transmission at about an 80 degree angle on each side). I have a general idea of what you want me to do, but I get so little visible transmission most of the time that it feels all but impossible to fine-tune my momentum or other aspects of my strategy. (Is there any way to increase the slit widths or number of slits or something to make that easier?) * What exactly do the arrow keys do? Are you imposing a linear potential gradient across the whole screen for a short time? It's hard to judge "accuracy" when I'm not quite sure what I'm looking at. (That also means I'm not entirely sure how to teach it.) * Along similar lines, do you have a sense of what exactly is leading to the visible interference effects during motion? (The ones that you point out can be guides as to which direction the wave is moving?) Maybe I ought to have a direct intuition for this! Is it purely an effect of reflections off of the walls, or would I see the same thing if I started with a Gaussian wave packet in a linear potential without any boundaries at all? (And if it's reflection related, why is it mostly showing the direction of primary motion rather than the direction of the reflections?) Some explanation for this might be nice in a teaching tool. * Do you think you could allow the user to toggle the phase-rainbow mode on and off? (That's the mode that I would expect to always give an indication of the dominant momentum of the wave function.) * The "collapse" mechanic is very helpful for game play, but might deserve some sort of explanation. In particular, I worry that it could train students to believe a critical misconception: that if the probability of a particular state becomes high enough, that state immediately becomes "true". Maybe that wouldn't turn out to be a serious concern, but I'm not certain that beginners are well-equipped to recognize which weird aspects of game play are supposed to reflect real physics and which are mere game enhancements. (The way collapse works here feels a lot like Copenhagen, but not quite, since it doesn't obey the Born probability rule.) * It would allow for significantly richer strategy and game play if the player had a wider menu of potential functions to choose from. (For instance, what if the arrow keys continued to be linear potential functions and a mouse click provided a harmonic oscillator?) * In your explanatory text, I'm not comfortable with your discussion of decoherence. I'm far from an expert on that subject, so I'm hesitant to try to offer specific corrections. But it feels off to me. (In particular, a single quantum particle bouncing off of strict potential energy walls should not be a manifestation of decoherence, because the particle's state isn't becoming entangled with the state of those walls in any way. I don't see any manifestation of decoherence in the game.) In case you're interested, my current favorite intro level textbook on quantum mechanics is Tom Moore's "Six Ideas that Shaped Physics: Unit Q". It doesn't go as far as I'd like for a really thorough course (which is especially true in the recent 3rd edition, though the material that's still there is presented much more clearly), but as part of an introductory sequence it does a great job. Much like Townsend's fantastic Jr/Sr level text, Moore first introduces the "rules" of quantum mechanics in the context of two-state spin systems (in a slightly simplified version of Dirac's bra-ket notation), and only moves to wave functions once the fundamental concepts are established. (It doesn't manage to do a whole lot with time dependent position/momentum systems like the ones you're simulating, though.)

sideshowb 10y ago
Another mega comment: don't know why this isn't more upvoted. Thanks for the bug report. Level 8 is odd. You thought more diffraction would occur with slower speed right? In the simulation it doesn't; bounce off the left wall and hit the grating fast to complete. I have to admit this bothers me. I can only assume this is because sidebands which would otherwise hit the grating itself move inwards to hit the 2% targets or something. Not good at all from a learning point of view. Yes, the arrow keys impose a linear potential gradient while they are being held. It fades in gradually as well so no sharp transients (unless you differentiate). Not sure if that detail was required but it's there now. The interference effects during motion are not wall reflections as far as I can tell. Some levels such as the traps don't have walls but (imperfect) absorbers and they exhibit it just as much. You're right, it's easy to see why this occurs in the rainbow mode but not so much in the amplitude mode. Genuine physics or artifact of simulation - presumably somebody can do some calculus and work that one out! Decoherence, I acknowledge should involve multiple quantum particles. What we do see here though is an evolution from a single particle with coherent phase, which interferes with itself, into one with incoherent phase, that doesn't. Conceptually that seems similar to decoherence. I think all your pedagogical points are good ones btw. Like I said to evanb, if you would like to be acknowledged for your review I'm more than happy to. Best wishes.
- Steuard 10y ago
  I'm very puzzled about level 8, based on what you've said. Certainly to be a good pedagogical tool, I'd want to see a system that very visibly showed a wider and wider diffraction pattern for longer wavelengths (as you've said). [The one advantage of a bounce that I was able to think of while playing was that it might wind up giving a cleaner "plane wave" input, but I'm not even sure that that's an advantage. Now that you mention it, though, perhaps there is some ideal speed for which bouncing off the back and side walls would result in propagation at an angle, so that a meaningful fraction of the probability density would hit the targets in as a central interference maximum.] If I were you, I'd leave out the discussion of decoherence entirely. It's a subtle topic (evanb did it justice, I think), and I'm reasonably convinced that it doesn't really apply to the game. (Even if I might be wrong about that, is that tidbit of background info important enough to your discussion to be worth the effort and/or the risk?) Thanks for a fun take on quantum stuff, regardless! (I don't know what's in the higher levels, but might be fun to see some more examples of things like tunneling through different barriers. And I wonder if there's any way of setting up resonance in a cavity in an interesting way...) Regarding credit: I don't feel like I've contributed that much at this point, but if you wind up with a moderate list of thank you's at some point, you're welcome to include "Steuard Jensen" on the list. [As for my comment not being more upvoted: I think I got some sort of black mark on my record here a few months ago after I tried to insist that recent reports about reactionless propulsion systems are very unlikely to pan out. Ever since then, I've gotten the sense that my comments are showing up with an initial scoring penalty.]

by Steuard 10y ago

sideshowb 10y ago

Another mega comment: don't know why this isn't more upvoted. Thanks for the bug report. Level 8 is odd. You thought more diffraction would occur with slower speed right? In the simulation it doesn't; bounce off the left wall and hit the grating fast to complete. I have to admit this bothers me. I can only assume this is because sidebands which would otherwise hit the grating itself move inwards to hit the 2% targets or something. Not good at all from a learning point of view. Yes, the arrow keys impose a linear potential gradient while they are being held. It fades in gradually as well so no sharp transients (unless you differentiate). Not sure if that detail was required but it's there now. The interference effects during motion are not wall reflections as far as I can tell. Some levels such as the traps don't have walls but (imperfect) absorbers and they exhibit it just as much. You're right, it's easy to see why this occurs in the rainbow mode but not so much in the amplitude mode. Genuine physics or artifact of simulation - presumably somebody can do some calculus and work that one out! Decoherence, I acknowledge should involve multiple quantum particles. What we do see here though is an evolution from a single particle with coherent phase, which interferes with itself, into one with incoherent phase, that doesn't. Conceptually that seems similar to decoherence. I think all your pedagogical points are good ones btw. Like I said to evanb, if you would like to be acknowledged for your review I'm more than happy to. Best wishes.

Steuard 10y ago
I'm very puzzled about level 8, based on what you've said. Certainly to be a good pedagogical tool, I'd want to see a system that very visibly showed a wider and wider diffraction pattern for longer wavelengths (as you've said). [The one advantage of a bounce that I was able to think of while playing was that it might wind up giving a cleaner "plane wave" input, but I'm not even sure that that's an advantage. Now that you mention it, though, perhaps there is some ideal speed for which bouncing off the back and side walls would result in propagation at an angle, so that a meaningful fraction of the probability density would hit the targets in as a central interference maximum.] If I were you, I'd leave out the discussion of decoherence entirely. It's a subtle topic (evanb did it justice, I think), and I'm reasonably convinced that it doesn't really apply to the game. (Even if I might be wrong about that, is that tidbit of background info important enough to your discussion to be worth the effort and/or the risk?) Thanks for a fun take on quantum stuff, regardless! (I don't know what's in the higher levels, but might be fun to see some more examples of things like tunneling through different barriers. And I wonder if there's any way of setting up resonance in a cavity in an interesting way...) Regarding credit: I don't feel like I've contributed that much at this point, but if you wind up with a moderate list of thank you's at some point, you're welcome to include "Steuard Jensen" on the list. [As for my comment not being more upvoted: I think I got some sort of black mark on my record here a few months ago after I tried to insist that recent reports about reactionless propulsion systems are very unlikely to pan out. Ever since then, I've gotten the sense that my comments are showing up with an initial scoring penalty.]