Back in the days, I took advanced physics at University but if the lectures would have been like this. I probably would have been a researcher!
I have been somewhat hooked to the lectures of Richard Feynman for the past 2 weeks. The whole binge on lectures started when a friend had many theories and he saw a 2-slit experiment and started to explain many things based on what he understood, backing nothing with scientific publication and I had to point out even that one of his source was actually a creationist. Long story, short, I end up watching more than I need, reading through papers and finding my university books under a pile of dust (10 years). Don't get me wrong, I have no intention (yet) to really study this and no claim that I really will understand but somewhat I started to see a lot of real life parallels.
I found it fascinating that based on this lecture back in time when no-one had heard about a laser disc, I could not only understand the reflection mechanics on the disc, the angles but also understand how the reflection will look on the DVD-disc. I didn't even noticed before that difference.
One nice parallel to real life and quantum mechanics I though about was the lottery. Everyone has a ticket, each ticket is a probability to win. You would be now thinking about the money as a wave. Yet, the second the numbers are drawn and you check, the money can be though as particles. As long as you haven't checked, you can't throw out the ticket, even though the numbers are their and the ticket has now a clear value.
There is more, if you start looking at the different way an electron may reach it's destination, everything is so cool and I'm not . Enjoy the show, share references to me, I'll be grateful!
Reviewing quantum mechanics for MCATs right now, absolutely hate it; as cool of a guy Richard Feynman is; no matter who teaches quantum mechanics, I despise the topic.
Quantum mechanics is part of the physics I don't despise too much because with what was asked of me, being good at linear algebra meant one of the easiest A of my life :D
Did you read the recent article about the speed of quantum? They said 10,000 x faster than light. That's astonishingly fast, but if this is true, its no longer classified as instant. Kind of disappointing in a way..
On March 31 2013 03:57 NerdUpgrades wrote: Did you read the recent article about the speed of quantum? They said 10,000 x faster than light. That's astonishingly fast, but if this is true, its no longer classified as instant. Kind of disappointing in a way..
In lieu of the article could you at least explain what you mean by this?
I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
On March 31 2013 04:02 sob3k wrote: I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
On March 31 2013 04:02 sob3k wrote: I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
Because Biology majors like me need to understand Quantum Mechanics to a certain extent for General Chem, and Organic Chem, but don't have the time to take upperdivision math, cuz course load.
On March 31 2013 04:02 sob3k wrote: I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
Because Biology majors like me need to understand Quantum Mechanics to a certain extent for General Chem, and Organic Chem, but don't have the time to take upperdivision math, cuz course load.
This also holds true for people like me who, though they can take and succeed at physics to a certain point, need those analogies to attempt to grasp the material. Though I'm going to be a college freshman, when my teachers began explaining rudimentary quantum to me in math terms I was like... dafuq.
On March 31 2013 04:02 sob3k wrote: I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
Because Biology majors like me need to understand Quantum Mechanics to a certain extent for General Chem, and Organic Chem, but don't have the time to take upperdivision math, cuz course load.
The mathematics involved in quantum mechanics, at least at introductory and undergraduate level, is not in the realm of "upper division" mathematics, but something you'd learn in your first or second year, primarily calculus, linear algebra, and partial differential equations. "cuz course load" of a biology major is hardly a convincing argument either, especially if you say that to physics/mathematics professors.
Real life examples and analogies are fine and extremely helpful to gain a foothold of a concept, but it can never substitute having to go through the frustrations of understanding the mathematics behind it.
On March 31 2013 04:02 sob3k wrote: I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
Because Biology majors like me need to understand Quantum Mechanics to a certain extent for General Chem, and Organic Chem, but don't have the time to take upperdivision math, cuz course load.
The mathematics involved in quantum mechanics is not what in the realm of "upper division" mathematics, but something you'd learn in your first or second year, primarily calculus, linear algebra, and partial differential equations. "cuz course load" of a biology major is hardly a convincing argument either, especially if you say that to physics/mathematics professors.
Real life examples and analogies are fine and extremely helpful to gain a foothold of a concept, but it can never substitute having to go through the frustrations of understanding the mathematics behind it.
But the difference is Organic Chemistry only needs the basic understanding of quantum mechanics, so far as it relates to orbitals, and energy states of electrons. The math part never really a necessity. Also, 2nd year is already the year for Organic Chem encounter, so if 2nd year math is required as a pre-req of understanding quantum mechanics, Ochem would get shoved back to 3rd year of college, and no Bio major would be able to graduate in 4 years; since Ochem is a pre-req for upperdiv biology.
On March 31 2013 03:57 NerdUpgrades wrote: Did you read the recent article about the speed of quantum? They said 10,000 x faster than light. That's astonishingly fast, but if this is true, its no longer classified as instant. Kind of disappointing in a way..
You're slightly misinterpreting this experiment. Theoretically, this process should occur instantaneously. The experiment is putting a *lower* bound on the "speed" of this "interaction." (It's not really an interaction, but that's not important). The main thing to take away from it is that entanglement is a non-local phenomena: there isn't a signal being sent between the two particles.
It's a little subtle, but entanglement can't be used to communicate faster than light. That means that there is no violation of relativity here. This experimental result is pretty impressive technically, but doesn't really give evidence of anything new.
Yeah, I know the feeling, things are much less fun and interesting when you have to work to really understand them. I wanted to point out that even in those situation, having a great teacher can make the whole difference. I passed the first course of physics, then the teacher changed, I struggled to stay interested.
Anyway, I don't find the discussion around mathematics interesting. Sure, Feynman is leaving out all the math, but he does a damn good job at explaining in a way that if you could handle the math, you recognize the tools he would be using. It is not important if the only goal is to be amazed by that stuff. Sure, it's unfair to get all the candy and not having to pay for it, but if you tell me I can't truly enjoy the candy without knowing how it was made, then gtfo
On March 31 2013 04:42 0x64 wrote: Yeah, I know the feeling, things are much less fun and interesting when you have to work to really understand them. I wanted to point out that even in those situation, having a great teacher can make the whole difference. I passed the first course of physics, then the teacher changed, I struggled to stay interested.
Anyway, I don't find the discussion around mathematics interesting. Sure, Feynman is leaving out all the math, but he does a damn good job at explaining in a way that if you could handle the math, you recognize the tools he would be using. It is not important if the only goal is to be amazed by that stuff. Sure, it's unfair to get all the candy and not having to pay for it, but if you tell me I can't truly enjoy the candy without knowing how it was made, then gtfo
As a maths major, the maths inside are the candy, and the physics is the hocus pocus to put me to sleep... :p
Personally I dont prefer the the written version of Feynman's lectures over, say, Landua's series on Theor. Phys. or Messiah's. However, in audio/video format they are some of the best lectures you will ever have, although mathwise they are very "hand wavy", Landua and Messiah have far more rigorous coverage. For math majors I would recommend books like Linear Algebra in QM, The theory of groups and QM, etc
I love Feynman. I honestly don't think it matters what he's explaining, he'd have me enthralled. If it weren't for:
I wouldn't have returned to education to study physics and even gained an appreciation of math too. I found within the two a motivation which I really hadn't known prior to thinking about that video.
On March 31 2013 03:48 micronesia wrote: Math aside, quantum mechanics is absolutely fascinating.
Math not aside, make sure you are good at eigenvectors, second order differential equations, and complex numbers.
Math aside, there is no quantum mechanics. Anything you can express purely using a language like English with regards to Quantum Mechanics is likely to be erroneous or incomplete. Technically, I guess that's true of any hard physical science - but then with Quantum Mechanics it takes it to whole 'nother level.
I think the best sentiment about quantum mechanics you can put into English is "MAGIC!!" (Close enough for the vast majority of people, and there are still some things that might as well be to researchers. But they're working on it.)
That said, yes, it's absolutely fascinating and I have no where near the kind of math skills to begin to appreciate how really freaking weird and wonderful QM is as a field.
On March 31 2013 03:48 micronesia wrote: Math aside, quantum mechanics is absolutely fascinating.
Math not aside, make sure you are good at eigenvectors, second order differential equations, and complex numbers.
Math aside, there is no quantum mechanics. Anything you can express purely using a language like English with regards to Quantum Mechanics is likely to be erroneous or incomplete. Technically, I guess that's true of any hard physical science - but then with Quantum Mechanics it takes it to whole 'nother level.
I think the best sentiment about quantum mechanics you can put into English is "MAGIC!!" (Close enough for the vast majority of people, and there are still some things that might as well be to researchers. But they're working on it.)
That said, yes, it's absolutely fascinating and I have no where near the kind of math skills to begin to appreciate how really freaking weird and wonderful QM is as a field.
You can definitely discuss the basic ideas of quantum mechanics without advanced mathematics (advanced meaning above what you learn in high school). What happens when you shoot a beam of electrons through a double-slit and observe the pattern on a screen? You don't need mathematics to discuss this type of quantum nature.
What happens when a particle is trapped in a square well? You don't need mathematics to discuss how the energy levels are quantized up until the energy exceeds the top of the well.
That's not to say a non-mathematical discussion is sufficient to truly appreciate it, though.
On March 31 2013 04:02 sob3k wrote: I am really mixed about the whole "explain theoretical physics with no math for the masses" movement. On one hand its sort of interesting and makes some people enthusiastic about the actual science. But on the other its honestly a completely load of bullshit analogies masquerading as understanding, with actually no real basis in the science, which is PURE FUCKING MATH.
Especially in quantum, the whole thing is that while the insanely complicated math works out, it doesn't actually make any logical sense to us, because its operating at a level where anything past out most basic logic systems were not designed for and do not work. If it made sense to you then you wouldn't be able to function in real life. If you THINK you understand it, you don't.
Its just math, and no matter how many bouncing balls and rubber sheets and cars and stop watches and shit they analogize, you can't even begin to really learn it without being a mathematician.
A criticism I'd have here is that, just because we can describe something mathematically, doesn't necessarily mean we understand it either. That's ok, we can still get the right answer and use it to build technology, but I think when we falsely believe we understand something that we don't, that's a bad thing. (known as reification)
The nice thing about these analogies is that our own ignorance is obvious to us, so it keeps us honest about what we really understand and what we don't. Because once we think we understand something, we stop asking questions and stop learning.
I used to think quantum physics was awesome until I took a class on it.. Then I realized it was pretty much the standard "here's some equations someone smarter than you derived, manipulate them and do math" type of thing. Also I still have no idea what the conceptual explanation for matrices and eigenvectors are, despite acing the class. To the math purists out there, I'd say that as a non-physics major, the non-math explanations and the pop science I read were both far more engaging and useful than the equations.
On April 01 2013 00:18 iamho wrote: I used to think quantum physics was awesome until I took a class on it.. Then I realized it was pretty much the standard "here's some equations someone smarter than you derived, manipulate them and do math" type of thing. Also I still have no idea what the conceptual explanation for matrices and eigenvectors are, despite acing the class. To the math purists out there, I'd say that as a non-physics major, the non-math explanations and the pop science I read were both far more engaging and useful than the equations.
That's how I feel, I know how university physics is taught and I've seen it been taught almost precisely the same way all around the globe almost from the same books. There is something different when a Nobelist put his mind to work on how he could explain the things he understand to the people that don't need actually to understand the specific.
Need I remind you guys, that this is the definition of a good scientific paper; one should be able to read a paper with a general understanding of the field or even better with a scientific general education.
Well anyway, I tend, with time, to think there is more value on the way the message is delivered than university put effort into. And if a lecturer can't deliver the message better than a great lecturer, then record once the damn lecture and distribute it to the whole world. Have the lecturer use his hours on explaining and answering questions about the lecture instead of trying to produce something similar.
And physics, I believe is one of the most dependent area, where the quality of the lecturer can help you "get it".
Electromagnetism, the math behind, all seems so disconnected. It is possible to learn how to calculate and yet not even understanding what you are doing and why. Scary?
On April 01 2013 04:29 0x64 wrote: And physics, I believe is one of the most dependent area, where the quality of the lecturer can help you "get it".
Electromagnetism, the math behind, all seems so disconnected. It is possible to learn how to calculate and yet not even understanding what you are doing and why. Scary?
It is scarier when this happens in practical settings. For example, an engineer who is strong with computers may use matlab simulations to find solutions to problems without actually understanding the underlying issue causing the problem, and how this solution addresses (or shorts) them.
On April 01 2013 04:29 0x64 wrote: And physics, I believe is one of the most dependent area, where the quality of the lecturer can help you "get it".
Electromagnetism, the math behind, all seems so disconnected. It is possible to learn how to calculate and yet not even understanding what you are doing and why. Scary?
It is scarier when this happens in practical settings. For example, an engineer who is strong with computers may use matlab simulations to find solutions to problems without actually understanding the underlying issue causing the problem, and how this solution addresses (or shorts) them.
ok on the first video, he mention the probability of photons reflecting off the surface of the water is a 4% reflection and 96% of the photons goes straight through, and he said that it's measured result and no one has figured out why yet. did i get what he said right? if i did, and i remember when photons make contact with an atom, if it's the right wave length, it will get absorb, and after absorbing the photon, the atom returns to it's ground stats and release a photon of the same wave length in a random direction. Does that mean out of that 4% reflection, every one of those re-released photons is traveling in the same direction of the reflection?
Nobody here can answer this?
could it be that it is not every one of those 4% reflects into the detector, instead all of the photons who hits an atom gets absorbed if it's the right wave length, and only 4% of them gets absorbed and reflect and that percentage depends on the material since different material compose of different atoms and hence difference electron orbitals that requires different activation energy to make the electron go to excited state and then back to ground state and re-emit a photon in a random direction.
and that 4% of photons gets re-emitted by the reflecting material's atoms will be traveling in random direction, yet because photon have wave property the only place these random directed photons didn't cancel each other's wave out is at the exact angle in which the sum of all vectors of those photons have the shortest path. If you put the detector at that exact place you would count 4% of the total photons and if you place that detector elsewhere you would count less than 4%?
What I think is really scary about Quantum Mechanics (and quantum physics in general) is that when you get out to the bleeding edge and around some effects, even WITH the math, you can see how something happens, describe it... but still be baffled by why it's happening. There are things in quantum theory that even leading researchers will look at, smile, and go "I have no freaking idea" when you ask "Well, why does it do that?"
Which is why I think the math is generally necessary. Some things mathematically let you see the logic behind and with the more imprecise English descriptions.
And the English descriptions can get engaging and go right on into "WTF" land - but are still pretty damned cool.
All else aside, I think QM is a pretty cool area of physics. Much cooler than all that string stuff.
On April 01 2013 04:29 0x64 wrote: And physics, I believe is one of the most dependent area, where the quality of the lecturer can help you "get it".
Electromagnetism, the math behind, all seems so disconnected. It is possible to learn how to calculate and yet not even understanding what you are doing and why. Scary?
It is scarier when this happens in practical settings. For example, an engineer who is strong with computers may use matlab simulations to find solutions to problems without actually understanding the underlying issue causing the problem, and how this solution addresses (or shorts) them.
Hah, now I'm scarred, thanks!
Actually, that sounds like a lot of engineering I know. They don't really care WHY something works the way it does - they are more concerned with what they can do with the properties available to them. If it works in whatever way it works, that's great, they can use that. If it doesn't impact what they are trying to do with something, they really aren't that interested in the deeper questions. A civil engineer only cares about how strong concrete is under specific conditions and if it is suitable for the application they're using it for - not why cement bonds together in the first place.
On April 01 2013 04:29 0x64 wrote: And physics, I believe is one of the most dependent area, where the quality of the lecturer can help you "get it".
Electromagnetism, the math behind, all seems so disconnected. It is possible to learn how to calculate and yet not even understanding what you are doing and why. Scary?
It is scarier when this happens in practical settings. For example, an engineer who is strong with computers may use matlab simulations to find solutions to problems without actually understanding the underlying issue causing the problem, and how this solution addresses (or shorts) them.
Hah, now I'm scarred, thanks!
Actually, that sounds like a lot of engineering I know. They don't really care WHY something works the way it does - they are more concerned with what they can do with the properties available to them. If it works in whatever way it works, that's great, they can use that. If it doesn't impact what they are trying to do with something, they really aren't that interested in the deeper questions. A civil engineer only cares about how strong concrete is under specific conditions and if it is suitable for the application they're using it for - not why cement bonds together in the first place.
Sometimes a lack of actual understanding of what's going on can have serious unforeseen consequences. Maybe this adjustment matlab told you to do makes the bridge stronger, but because you relied solely on the computer simulation and didn't actually analyze it yourself, you didn't realize that the bridge is now much more susceptible to resonance, weathering, or some thing your simulation didn't take into account. While common, this is quite a dangerous way to do science.
On April 01 2013 00:18 iamho wrote: I used to think quantum physics was awesome until I took a class on it.. Then I realized it was pretty much the standard "here's some equations someone smarter than you derived, manipulate them and do math" type of thing. Also I still have no idea what the conceptual explanation for matrices and eigenvectors are, despite acing the class. To the math purists out there, I'd say that as a non-physics major, the non-math explanations and the pop science I read were both far more engaging and useful than the equations.
Don't they at least show how it was derived? I went to a class on classical mechanics and the whole class was about deriving the formulas.
On April 01 2013 00:18 iamho wrote: I used to think quantum physics was awesome until I took a class on it.. Then I realized it was pretty much the standard "here's some equations someone smarter than you derived, manipulate them and do math" type of thing. Also I still have no idea what the conceptual explanation for matrices and eigenvectors are, despite acing the class. To the math purists out there, I'd say that as a non-physics major, the non-math explanations and the pop science I read were both far more engaging and useful than the equations.
Don't they at least show how it was derived? I went to a class on classical mechanics and the whole class was about deriving the formulas.
Well quantum mechanics can's always be derived the way classical mechanics can. For example, the Schrodinger Equation itself was proposed rather than derived.
On April 02 2013 01:13 Recognizable wrote: How do you propose an equation on unimaginable things like these?
I'm not sure how to answer this. They had a pretty good idea of what solutions the Schrodinger equation should give, and how they should behave under some simple situations, I guess. It was just a matter of finding the right differential equation.
I'm not exactly a quantum physics expert (4th year physics student), but basically what happened was:
Several phenomena such as black body radiation, the Stern-Gerlach experiment, the photoelectric effect etc were explained by hypothesizing the existence of quantum behaviour. This is known as "old" quantum physics, and had no theoretical basis. Rather, it was a collection of ad-hoc assumptions for each experiment.
What Dirac, Heisenberg and Shroedinger did was provide a physical theory that explained all this behaviours. What this means is, they provided some very specific (and abstract) postulates, and then demonstrated that by using these mathematical hypothesis it was possible to model quantum behaviour, that is, to have a mathematical model that explained all these phenomena and was able to predict even more things (which essentially means, contructing a theory in the scientific sense of the word).
On April 02 2013 01:13 Recognizable wrote: How do you propose an equation on unimaginable things like these?
I'm not sure how to answer this. They had a pretty good idea of what solutions the Schrodinger equation should give, and how they should behave under some simple situations, I guess. It was just a matter of finding the right differential equation.
Maybe a quantum expert can weigh in more.
From what I heard (I'm only a maths student), Feynman uses in his lectures the analogous interpretation of quantum mechanics to the Lagrangian interpretation of classical mechanics. In Lagrangian mechanics, a particle's trajectory is such that the difference between kinetic and potential energy (summarized through time) is minimal. From this you can derive Newton's second law. The analogous interpretation in quantum would be that you look at all possible trajectories and each will give a certain contribution to the final wave function. From this you should be able to get the Schroedinger equation.
Like Newton's Second law, the Schrödinger equation can be mathematically transformed into other formulations such as Werner Heisenberg's matrix mechanics, and Richard Feynman's path integral formulation
While that is correct, it's also not very precise.
Feynman's formulation of quantum mechanics (which is what you described) is from the 50's ish, it has nothing to do with how it was first written in the 30's.
If you want to be technical, Shroedinger's equation is simply written H |E> = E |E>, and it's a "simple" eigenvalue/eigenvector problem. H is the hamiltonian, |E> is an eigenvector and E the corresponding eigenvalue. The messy thing of course is figuring out how to write it explicitly when you are finding the eigenvalues and eigenvectors in a spite that has infinite dimensions, and sometimes even continous dimensions (the equations in quantum mechanics are written in l2 spaces for the math students out there). That's where all the derivatives and such that you see in how it's usually written come in; those are one way to represent the operators present in the Hamiltonian. This was the approach first used by Dirac, Heisenberg and Shroedinger in the 30's.
Feynman simply introduced yet another way of looking at the math framework of quantum physics, which proved extremely valuable in formulating the more "modern" quantum theories, known as quantum field theory (of which i know next to nothing because it's FUCKING COMPLEX SHIT).
Well it's a postulate, you can't derive it. You just say "well fuck it it's kinda like classical mechanics. Makes sense" afaik.
edit: unless you talk about crazy shit like string theory or other unified field theories. There -might- be some more general principle from which the quantization postulate comes from, but yeah, not many people would be aware of that and even less would understand wtf is written.
I can't seem to load the video, but isn't reflection of light off a boundary dependent on the conductivity? For example, a mirror is very conductive so most of the light reflects off of it, giving you a nice image of yourself. Water is much less conductive than silver or other metals. On the other hand, optical density for dielectrics plays a role, too. The reflectivity will depend on how much the speed decreases as light travels from air to water.
I think (not sure) the reflection thing that Feynman talks about is an effect from quantum electrodynamics, which is the quantum theory of electromagnetic interaction, rather than quantum mechanics, which is the theory first developed in the 30s by Shroedinger etc. Quantum electrodynamics was the first quantum field theory developed (and one of the key areas in Feynman's research in the 50's), and it's some REALLY complex shit. Much more so than quantum mechanics, which is Shroedinger's equation, canonical commutation rules etc.
As far as structure of matter/applied quantum mechanics goes, wether a material is transparent or not depends on it's conductivity, yeah. Here's the general idea: solving Shroedinger's equation for a system means finding which energies that system can have. For some systems, this energy is discrete, so only a select, precise levels are possible. This is the example in atom orbitals: the electrons can't orbit at whatever energy level they want, they must follow precise energy levels (and orbits). On the other hand, other systems can have any energy possible, which is known as a continous spectrum. This is the case, for example, in free photons. When it comes to solids, the energy levels are organized in bands. Some intervals in energy are allowed, others aren't. Insulators are materials in which the bands are all filled completely. This means the electrons need a lot of energy to shift between one band and the other, more than visible light can give; this makes them transparent to radiation which has as much energy as the gap between one band and the other. Additionally it makes it harder to incite any kind of electric current, since you need so much energy to move the electrons. Conductors on the other hand have half filled bands, which means it's possible that the electrons in a half filled band will absorb a smaller amount of energy (for example from visible light) and be excited without jumping from one band to the other. In that sense, conductivity and reflection are dependent on each other. I'm sure there's more complex things going on as well, but that's how it was presented to me in the structure of matter class i took.
If you are interested in learning more about quantum electrodynamics i recommend reading Feynman's "QED". It's a collection of the lessons he gave (i believe they might even be the same as those videos) and it's a very good read, without any math. I read it in 12th grade and was able to understand most of it.
It doesn't really reflects in a sense that we are used to in everyday objects, the photons actually gets absorbed and released back in a random direction. and this process depends on a few things,
first it depend on what kind of light. if the light source is the sun then the photons produced by the sun has a continuous spectrum, meaning it has all different kind of wavelengths. If it's a light source that produce only a specific wavelength of photons then none of the photon will get absorbed unless we have the right kind of atom as the reflecting surface.
second it depend on the type of material the reflective surface is compose of, different kind of materials have different atoms, and different atoms only absorb few specific kind of wavelength photons and letting the other photons pass right through them without absorbing them. And that's because only the photons that have the right amount of energy will excite an atom and make its valence electron jump to the next orbital. Once the atom finish being excited and turn back into ground state, that energy it absorbed from the photon is released and turn back into a photon, but it is released in a random direction compare to the direction of the original photon that was absorbed a fraction of a seconds ago. and then percentage of the "reflected" photons depend on the range of absorb-able wavelength the atoms that made of the surface.
thirdly, out of all those randomly re-emitted photons from the surface, because they are randomly directed, some of them could be re-emitted further into the more atoms and gets absorbed again and again until they make their way out, those that made it out through the bottom will form a refraction, and those who make it out of the surface will from a reflection.
my question is about whether or not i understood what Fraynmen said about the photons that are being reflected and refracted. those 3 points i mention above all consider the photon as a particle, but to understand the reflection and refraction, we need to think of the photon as a wave. and here is what I am not sure about and want someone who know better to clarify or re-affirm, the randomly re-emitted photons also behave like waves and do go all over the place, however the waves of different photons have the same wavelength because they are all re-emitted from the same type of atom, hence if they are spaced apart with the right distance, they would ending up canceling each other out. and therefore the only place we don't see cancellation are where we will find the reflection and refraction.
As far as scattering processes and collisions go, to my knowledge photons are usually treated as a particle rather than a wave. Imagine a ball bouncing off a wall; in this case the reflective solid is the wall and the ball is the photon. The math used actually follows similar rules, ie, conservation of momentum and energy.
I don't think it has to do with interference because in reflection becuase you usually see an individual beam rather than a a interference pattern. Additionally, the light waves in a monochromatic weave woudln't necessarily cancel each other out simply because while they have the same wave length, they'd need to have opposite phases as well.
An example of a monodimensional wave is: f(x) = A*sin(k*x+p);
A is the amplitude, k is the weave number (which related to the frequency, which in turn is related to wavelength), x is the point in space, p is the phase.
If two waves (of identical wavelegth) have the same phase at a fixed point in space they are said to be in phase (i think that's the english expression), so they produce constructive interference: the amplitudes sum each other. If they have opposite phases (for example one is 0 and the other is 180 degrees), they are said to be out of phase and the two amplitudes subtract each other, producing destructive interference. For example, these two functions (which can represent a wave) are out of phase: http://www.wolframalpha.com/input/?i=draw sin(x), sin(x+pi)
If photons are emitted with random phases you will not be able to see any kind of interference pattern. The reason you don't see interference patterns for every beam of light existing is because the phases don't interact with each other at a macroscopical level, since they will be all distributed randomly.
Note that this is classical physics. I don't know how things change in QED, i haven't studied any of it at a college level.
he talk about reflection off of a mirror @28:30 of this video "http://www.youtube.com/watch?v=kMSgE62S6oo"
he talks about how light in nature will find the shortest path to the detector/observer, at around 40 he said something about the path that's not the shortest have their amplitude canceled out. in your equation, A is the amplitude, and only way for the A to be cancel out is to have the sin(k*x+p) to turn out to be opposite signs.
1) Feynman is fucking awesome. 2) Yeah. Basically what is happening is that all the contributions that are further away from the centered, "obvious" path have meaningless contributions, so it's as if the light was following the path everyone "experiences" at a macroscopic scale. Feynman explains it better than i do. The arrows he's talking about aren't related to wavelength though, they are phases (more or less). If you have time to either watch all of those videos or read QED (which is based on those lessons) you'll get it better than i can explain it.
Feynman is a master at explaining his ideas. even ppl like me who have limited understanding and only self taught for most part have a chance to understand what quantum mechanic is about.
we don't need to be scientists to be interested and learn about these, the internet can help us become scientifically literate. I started my investigation about what is dark matter and dark energy, and in order to know that, I go search the history of the discovery of them, and that lead me to learn about the blue and red shift of the spectrum, and then that lead me to learn about what exactly produces spectrum, and that lead me to learn about how photon electron interaction in an atom, and that lead me to learn about how scientists can figure out the temperature of the star based on it's spectrum, and that leads me to understand how we figure out the distance of the stars based on the temperature and spectrum, and the list goes on and on, I have no one to tell me what to learn, I am just following my interest. Feynman's lectures touches on what I learned and it makes a connection when he himself said over and over that even he the one who came up with these and won a Noble prize for it does not understand these at all.
On April 02 2013 09:39 rei wrote: Feynman's lectures touches on what I learned and it makes a connection when he himself said over and over that even he the one who came up with these and won a Noble prize for it does not understand these at all.
That's one of the great things about Feynman. He's an honest scientist, and excels at conveying the wonder without sounding in the least bit condescending towards anyone.
EPT
. Enjoy the show, share references to me, I'll be grateful!
