5 ms·

IAmA theoretical physicist. Ask me anything (via reddit)

50 points by d4ft 16y ago

Enlightenment 16y ago
How tall of a ladder do I need to climb over the Walled-Garden in an iPhone?
Jach 16y ago
I stopped reading after it appeared he was advocating collapse theory. About the only non-confusing quantum mechanics I've read so far that's been light on math has been Feynman's stuff like QED and this series: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ On a meta-note, I don't really think this is anything but noise. If you want to really, really understand the physics, you have to do the math, and if you just want a high-level concept I don't see how in the space of a single reddit post you could take away loads of confusion or adequately explain a concept without cheating the person. Here's Feynman's take on magnets (really, on answering 'why' questions): http://www.youtube.com/watch?v=wMFPe-DwULM http://www.youtube.com/watch?v=wMFPe-DwULM
- Maro 16y ago
  Check out Dirac's classic 'The Principles of Quantum Mechanics': http://www.amazon.com/Principles-Quantum-Mechanics-International-Monographs/dp/0198520115/ref=sr_1_3?ie=UTF8&s=books&qid=1275651696&sr=8-3 http://www.amazon.com/Principles-Quantum-Mechanics-Internati... I was shocked how much clearer (and more useful) the exposition is compared to textbooks.
  - pfedor 16y ago
    I strongly second this recommendation, but it's not exactly "light on math". It's light on calculations, but you need to have had some contact at the very least with linear algebra and calculus. Now that I think about it, that probably makes it a perfect book for software engineers, who know the mathematical concepts from studying computer science, but have not much experience actually doing calculations, so typical QM textbooks may be off-putting for them.
  - Jach 16y ago
    Thanks! Added to my wish list, I'll have to see if it's at my library. I'm all for finding textbooks that are actually well-written.
- pfedor 16y ago
  By "collapse theory" I presume you mean the Copenhagen interpretation of Quantum Mechanics? That actually gives the guy more credibility: it suggests he learned physics at a physics department of some university, as opposed to, say, from popular literature or random blogs. The page you linked says: "the many-worlds interpretations wins outright given the current state of evidence". This sentence does not make sense. The MWI, just like the Copenhagen interpretation and all other interpretations of Quantum Mechanics, are just this: interpretations. They don't disagree about the expected outcome of any imaginable measurement. They can't be tested experimentally. Choosing one interpretation or the other is a philosophical choice, not a scientific conclusion. "Evidence" in favor of one interpretation of Quantum Mechanics is of the same kind as "evidence" of the existence (or non-existence) of God, i.e., some people feel very strongly one way or another and will argue with you to death about it, not to mention create web pages which authoritatively state their opinions on the subject, but the disagreement cannot really be resolved by arguments or by empiric observations. Physicists are aware of this and that's why most of them choose the simplest thing and talk about the collapse of the wave function.
  - Jach 16y ago
    I'm not going to go into the arguments about it, I've given you the link so you can read them all yourself. Though I will quote two big things (edited for note: assertions, not arguments for them; arguments are at the link above) against collapse postulates in general, since you probably didn't read them and perhaps you have a response why it doesn't matter: "If collapse actually worked the way its adherents say it does, it would be the only non-linear evolution in all of quantum mechanics, the only non-unitary evolution in all of quantum mechanics, the only non-differentiable (in fact, discontinuous) phenomenon in all of quantum mechanics, the only phenomenon in all of quantum mechanics that is non-local in the configuration space, the only phenomenon in all of physics that violates CPT symmetry, the only phenomenon in all of physics that violates Liouville's Theorem (has a many-to-one mapping from initial conditions to outcomes), the only phenomenon in all of physics that is acausal / non-deterministic / inherently random, and the only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light." Second, now paraphrasing, is that many-worlds, aka macroscopic decoherence (QM applied universally), is a strictly simpler theory and Occam's Razor alone rules out collapse as it's an added detail not needed. Lastly, this is just my own opinion, but just because you learned from a set of standard textbooks or from a physics department at some place, doesn't necessarily mean you learned correctly. Truth isn't what the university teaches, even though there's a lot of overlapping. (I have learned there are apparently still some people teaching that we are evolved from chimps.)
- rsaarelm 16y ago
  Is there really reason to believe that practicing theoretical physicists who advocate wavefunction collapse are so mistaken as to be not worth learning from? I find the lesswrong cultural insistence on going with MWI pretty troubling when it gets to the point of not listening to actual physicists who don't use it, as as far as I can tell, very few people there are actual active quantum physics researchers. MWI could well be a better way to develop understanding of quantum mechanics, but dissing people who actually actively work with the stuff because their established notation has historical baggage doesn't seem like a good idea when you're not actively working with the stuff yourself.
  - Jach 16y ago
    I believe they are very mistaken, just like I believe practicing scientists who also believe in God are very mistaken. There are arguments for and against my beliefs, and I don't want to go into them here. I'm just explaining that to me a spiritual scientist (or a collapse-postulate scientist) is placed in a category where I can't believe that they take their science all that seriously. This doesn't make them impossible to learn from, or not worth it, or wrong about everything, but honestly I'd rather learn from someone more serious and excited about learning like Feynman. Someone who revels in the odd and beautiful nature of reality and doesn't need to invent comfortable intuition-friendly (and complicated) "reasons" why or how something does something. That said, if such scientists that I put into my would-rather-not-learn-from category want to teach me some math or programming or something solid and backed by experiment, if they want to show me an experiment to learn from, I'm all for it! I grew up in Utah, I can't think of a single influential teacher not a Mormon, obviously I didn't think because they were religious they weren't worth learning from.
tca 16y ago
There's no relevant physics without mathematics, one has to work out the math before trying to answer the 'why' questions. There are no short cuts. (Note: I'm also a theoretical physicist :) )
- Confusion 16y ago
  The math follows from the basic assumptions made about nature. Assume translational, rotational and time invariance, add the finite speed of light and some less obvious assumptions and all of classical physics (for those non-phycisists: this includes special and general relativity) follows. Assume charged fundamental particles and EM follows. Assume quantized fundamental units and quantum mechanics follows. Lots of relevant physics can be described and shown with hardly any mathematics at all. The idea that nature is mathematical in essence is a notion that primarily physicists entertain. They mistake intelligence for knowledge and philosophers, that have actual knowledge of the philosophical problems involved, know better. The idea that nature is mathematical in essence, and that we have discovered this fundamental essence, is both hubris and usually utterly devoid of any philosophical sophistication.
  - tca 16y ago
    You could invert the phrase and still make a lot of sense: 'Nature follows from the basic mathematical assumptions.' But you are quite right: Philosophers know better, could know...
    - Confusion 16y ago
      You could invert the phrase and still make a lot of sense: 'Nature follows from the basic mathematical assumptions. It's not just that. It's also that a lot of physics can readily be explained and understood without mathematics. It's just that with mathematics, it's easier to demonstrate the consequences. It's important to remember that Einstein got at relativity by understanding the right assumption to be made. Just to be clear on this: I fully agree that in advanced physics, you sometimes you have to blindly 'do the math' and can determine the physical interpretation of the result only afterwards. However, that doesn't mean that there is no physics without mathematics, it doesn't mean that there isn't a lot of physics that can be understood without mathematics and it certainly doesn't mean nature is mathematical in essence. Hell, it isn't even physical in essence, unless you can reduce our minds to physics. The notion that 'nature is mathematical in essence' is not logically seperated from assumptions about the object/subject and mind/matter dualities (and about whether they are dualities in the first place) that I doubt most of the phycisists espousing these notions are willing to support, because they are rather incompatible with other parts of their worldview.
vrode 16y ago
When did you start actively pursuing you goal? Do you think someone less exposed to science and math in particular can achieve the same level as you?
tjmaxal 16y ago
Whoa this is long!
Jun8 16y ago
There should be more people like him/her who are willing to put time in answering questions about their specialty, although some may be unexplainable to the general public. For another excellent example of such a guy, see Terence Tao.
- dhimes 16y ago
  +1 for Tao
mbh2000 16y ago
Help me with a thought experiment. What do you think the result would be if a target, heated by the National Ignition Facility were to encounter a partical stream from 2 sides such as provided by the LHC.
mbh2000 16y ago
Help me with a thought experiment. What do you think the result would be if a target, heated by the National Ignition Facility were to encounter a partical stream from 2 sides such as provided by the LHC.

50 points by d4ft 16y ago

How tall of a ladder do I need to climb over the Walled-Garden in an iPhone?

Jach 16y ago

I stopped reading after it appeared he was advocating collapse theory. About the only non-confusing quantum mechanics I've read so far that's been light on math has been Feynman's stuff like QED and this series: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ On a meta-note, I don't really think this is anything but noise. If you want to really, really understand the physics, you have to do the math, and if you just want a high-level concept I don't see how in the space of a single reddit post you could take away loads of confusion or adequately explain a concept without cheating the person. Here's Feynman's take on magnets (really, on answering 'why' questions): http://www.youtube.com/watch?v=wMFPe-DwULM http://www.youtube.com/watch?v=wMFPe-DwULM

Maro 16y ago
Check out Dirac's classic 'The Principles of Quantum Mechanics': http://www.amazon.com/Principles-Quantum-Mechanics-International-Monographs/dp/0198520115/ref=sr_1_3?ie=UTF8&s=books&qid=1275651696&sr=8-3 http://www.amazon.com/Principles-Quantum-Mechanics-Internati... I was shocked how much clearer (and more useful) the exposition is compared to textbooks.
- pfedor 16y ago
  I strongly second this recommendation, but it's not exactly "light on math". It's light on calculations, but you need to have had some contact at the very least with linear algebra and calculus. Now that I think about it, that probably makes it a perfect book for software engineers, who know the mathematical concepts from studying computer science, but have not much experience actually doing calculations, so typical QM textbooks may be off-putting for them.
- Jach 16y ago
  Thanks! Added to my wish list, I'll have to see if it's at my library. I'm all for finding textbooks that are actually well-written.
pfedor 16y ago
By "collapse theory" I presume you mean the Copenhagen interpretation of Quantum Mechanics? That actually gives the guy more credibility: it suggests he learned physics at a physics department of some university, as opposed to, say, from popular literature or random blogs. The page you linked says: "the many-worlds interpretations wins outright given the current state of evidence". This sentence does not make sense. The MWI, just like the Copenhagen interpretation and all other interpretations of Quantum Mechanics, are just this: interpretations. They don't disagree about the expected outcome of any imaginable measurement. They can't be tested experimentally. Choosing one interpretation or the other is a philosophical choice, not a scientific conclusion. "Evidence" in favor of one interpretation of Quantum Mechanics is of the same kind as "evidence" of the existence (or non-existence) of God, i.e., some people feel very strongly one way or another and will argue with you to death about it, not to mention create web pages which authoritatively state their opinions on the subject, but the disagreement cannot really be resolved by arguments or by empiric observations. Physicists are aware of this and that's why most of them choose the simplest thing and talk about the collapse of the wave function.
- Jach 16y ago
  I'm not going to go into the arguments about it, I've given you the link so you can read them all yourself. Though I will quote two big things (edited for note: assertions, not arguments for them; arguments are at the link above) against collapse postulates in general, since you probably didn't read them and perhaps you have a response why it doesn't matter: "If collapse actually worked the way its adherents say it does, it would be the only non-linear evolution in all of quantum mechanics, the only non-unitary evolution in all of quantum mechanics, the only non-differentiable (in fact, discontinuous) phenomenon in all of quantum mechanics, the only phenomenon in all of quantum mechanics that is non-local in the configuration space, the only phenomenon in all of physics that violates CPT symmetry, the only phenomenon in all of physics that violates Liouville's Theorem (has a many-to-one mapping from initial conditions to outcomes), the only phenomenon in all of physics that is acausal / non-deterministic / inherently random, and the only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light." Second, now paraphrasing, is that many-worlds, aka macroscopic decoherence (QM applied universally), is a strictly simpler theory and Occam's Razor alone rules out collapse as it's an added detail not needed. Lastly, this is just my own opinion, but just because you learned from a set of standard textbooks or from a physics department at some place, doesn't necessarily mean you learned correctly. Truth isn't what the university teaches, even though there's a lot of overlapping. (I have learned there are apparently still some people teaching that we are evolved from chimps.)
rsaarelm 16y ago
Is there really reason to believe that practicing theoretical physicists who advocate wavefunction collapse are so mistaken as to be not worth learning from? I find the lesswrong cultural insistence on going with MWI pretty troubling when it gets to the point of not listening to actual physicists who don't use it, as as far as I can tell, very few people there are actual active quantum physics researchers. MWI could well be a better way to develop understanding of quantum mechanics, but dissing people who actually actively work with the stuff because their established notation has historical baggage doesn't seem like a good idea when you're not actively working with the stuff yourself.
- Jach 16y ago
  I believe they are very mistaken, just like I believe practicing scientists who also believe in God are very mistaken. There are arguments for and against my beliefs, and I don't want to go into them here. I'm just explaining that to me a spiritual scientist (or a collapse-postulate scientist) is placed in a category where I can't believe that they take their science all that seriously. This doesn't make them impossible to learn from, or not worth it, or wrong about everything, but honestly I'd rather learn from someone more serious and excited about learning like Feynman. Someone who revels in the odd and beautiful nature of reality and doesn't need to invent comfortable intuition-friendly (and complicated) "reasons" why or how something does something. That said, if such scientists that I put into my would-rather-not-learn-from category want to teach me some math or programming or something solid and backed by experiment, if they want to show me an experiment to learn from, I'm all for it! I grew up in Utah, I can't think of a single influential teacher not a Mormon, obviously I didn't think because they were religious they weren't worth learning from.

tca 16y ago

There's no relevant physics without mathematics, one has to work out the math before trying to answer the 'why' questions. There are no short cuts. (Note: I'm also a theoretical physicist :) )

Confusion 16y ago
The math follows from the basic assumptions made about nature. Assume translational, rotational and time invariance, add the finite speed of light and some less obvious assumptions and all of classical physics (for those non-phycisists: this includes special and general relativity) follows. Assume charged fundamental particles and EM follows. Assume quantized fundamental units and quantum mechanics follows. Lots of relevant physics can be described and shown with hardly any mathematics at all. The idea that nature is mathematical in essence is a notion that primarily physicists entertain. They mistake intelligence for knowledge and philosophers, that have actual knowledge of the philosophical problems involved, know better. The idea that nature is mathematical in essence, and that we have discovered this fundamental essence, is both hubris and usually utterly devoid of any philosophical sophistication.
- tca 16y ago
  You could invert the phrase and still make a lot of sense: 'Nature follows from the basic mathematical assumptions.' But you are quite right: Philosophers know better, could know...
  - Confusion 16y ago
    You could invert the phrase and still make a lot of sense: 'Nature follows from the basic mathematical assumptions. It's not just that. It's also that a lot of physics can readily be explained and understood without mathematics. It's just that with mathematics, it's easier to demonstrate the consequences. It's important to remember that Einstein got at relativity by understanding the right assumption to be made. Just to be clear on this: I fully agree that in advanced physics, you sometimes you have to blindly 'do the math' and can determine the physical interpretation of the result only afterwards. However, that doesn't mean that there is no physics without mathematics, it doesn't mean that there isn't a lot of physics that can be understood without mathematics and it certainly doesn't mean nature is mathematical in essence. Hell, it isn't even physical in essence, unless you can reduce our minds to physics. The notion that 'nature is mathematical in essence' is not logically seperated from assumptions about the object/subject and mind/matter dualities (and about whether they are dualities in the first place) that I doubt most of the phycisists espousing these notions are willing to support, because they are rather incompatible with other parts of their worldview.

vrode 16y ago

When did you start actively pursuing you goal? Do you think someone less exposed to science and math in particular can achieve the same level as you?

tjmaxal 16y ago

Whoa this is long!

Jun8 16y ago

There should be more people like him/her who are willing to put time in answering questions about their specialty, although some may be unexplainable to the general public. For another excellent example of such a guy, see Terence Tao.

dhimes 16y ago
+1 for Tao

mbh2000 16y ago

Help me with a thought experiment. What do you think the result would be if a target, heated by the National Ignition Facility were to encounter a partical stream from 2 sides such as provided by the LHC.

mbh2000 16y ago