Feynman's QED: Is it Still Relevant Today?

[KingBigness]

I picked up a copy of Feynman's QED - "A strange theory of light and matter" from the university library today because it looked interesting and I was wondering if this book is still relevant to todays Quantum physics?

It's a fascinating read but I want to make sure I am not learning stuff that has been since disproven.

Any opinions would be greatly appreciated

 

[torquil]

I picked up a copy of Feynman's QED - "A strange theory of light and matter" from the university library today because it looked interesting and I was wondering if this book is still relevant to todays Quantum physics?

It's a fascinating read but I want to make sure I am not learning stuff that has been since disproven.

Any opinions would be greatly appreciated

QED has been found to be a part of the larger Electroweak theory. None of QED has been disproven. If anything, it has been proven more and more to be an accurate theory. Study of QED is an essential step on the way to study the more comprehensive Standard Model oof particle physics.

 

[KingBigness]

I was hoping that would be the reply, that the theory of QED has only been built on top of and further unified as opposed to being disproven.

So you recommend I read it? It's a fascinating theory. Have you got any further reading you recommend after this book?

 

[torquil]

I was hoping that would be the reply, that the theory of QED has only been built on top of and further unified as opposed to being disproven.

So you recommend I read it? It's a fascinating theory. Have you got any further reading you recommend after this book?

I'm a physicist, so I have studied the detailed theory in all its mathematical glory, but still his explanations offer a nice intuitive understanding of the subject.

It also describes quantum physics phenomena in a nice way. So the book doesn't only describe quantum electrodynamics.

The last chapter "Loose Ends" might not be completely up to date, but still worth a read. E.g. his table of quarks doesn't include the top quark, which has been detected.

 

[KingBigness]

I have just finished my first year of a Bach of Physics at university and am hoping to transfer to a double major of maths and physics where I will hopefully get into honors/masters for physics and keep going. So I am not afraid of any maths that will be coming my way.

The quantum subjects this year fascinated me and they were only first year courses, can't wait to see what is planned for the future!

With this in mind, can you recommend any good up to date books that I might read to get an insight into quantum physics and/or astro, eg black holes, before I start to study it in depth at university?

 

[torquil]

So you recommend I read it? It's a fascinating theory. Have you got any further reading you recommend after this book?

Yes I recommend it warmly. Not really a big reader of "popular science" books, but I would recommend reading something that goes into the whole standard model of particle physics. Not sure I would recommend any of the multitudes of "evangelistic" books about string theory, though :-)

 

[torquil]

I have just finished my first year of a Bach of Physics at university and am hoping to transfer to a double major of maths and physics where I will hopefully get into honors/masters for physics and keep going. So I am not afraid of any maths that will be coming my way.

The quantum subjects this year fascinated me and they were only first year courses, can't wait to see what is planned for the future!

With this in mind, can you recommend any good up to date books that I might read to get an insight into quantum physics and/or astro, eg black holes, before I start to study it in depth at university?

As mentioned, I don't know that many non-technical books. The most technical popular science book that I have seen is Penrose's "The Road to Reality". It has nice descriptions of quantum physics, relativity and cosmological matters, as far as I can remember, but also a lot of math. Don't remember if it contains anything on statistical physics. It does contain quite a few speculative chapters towards the end on cosmology and regarding the measurement problem in QM, though (and I disagree with much of that) But the exposition of the established theory is very good if I remember correctly.

 

[KingBigness]

I'll definitely keep my eye out for books on that topic.

Not a believer in string theory?

If you don't mind me asking, what have you done since you left uni in the role of a physicist, eg what jobs have you done?

I only ask because my dad thinks that I would be much better off being an electrical engineer (what I originally wanted to do) and doesn't really see where being a physicist will take me.

I try to explain the multitudes of jobs my physics teachers have had but wouldn't mind some more 'ammunition.

I understand if you can't be bothered to reply =P

 

[KingBigness]

As mentioned, I don't know that many non-technical books. The most technical popular science book that I have seen is Penrose's "The Road to Reality". It has nice descriptions of quantum physics, relativity and cosmological matters, as far as I can remember, but also a lot of math. Don't remember if it contains anything on statistical physics. It does contain quite a few speculative chapters towards the end on cosmology and regarding the measurement problem in QM, though (and I disagree with much of that) But the exposition of the established theory is very good if I remember correctly.

Didn't see this, I'll definitely check the book out and welcome the maths as a challenge.
Would love to know your views and beliefs on the measurement problem. Although I do not think you would be bothered to explain it on a forum like this, and I do not blame you, especially when I probably wouldn't understand what you are saying haha

 

[cbetanco]

I only ask because my dad thinks that I would be much better off being an electrical engineer (what I originally wanted to do) and doesn't really see where being a physicist will take me.

First of all, I would just say to do what makes you happy. I personally find electrical engineering quite boring (though I think its important, and plenty people enjoy the subject), and physics is the only thing that seemed appealing to me. It doesn't matter what jobs you get in the end, as long as what you do gives you personal joy. Also, in terms of jobs, I think a physics degree makes you one of the most employable people out there. You can go into finance, work as a computer programer (believe it or not, you will do a fair amount of programming as a physics major, which I didnt really understand until I went through the program), go into medical physics, work as an engineer, become a teacher, the list goes on and on. Also, if you wish to go to graduate school, and after your degree you decide maybe you do want to do engineering instead, its good to know that physics majors have a higher acceptance rate into enginnering graduate school than engineering majors. You are much more prepared to do cutting edge research, as you have a much more solid foundation in the underlying theory of Electromagnetics, Quantum theory (which engineering majors in general do not take), Statistical and Thermal Physics, and basic mechanics. I think a solid foundation in these is essential in any modern engineering practices, and makes you that much more valueable. I do not mean to put down engineering majors, but I am more trying to highlight the advantages of a physics major.

As far as Feynmans QED book is concerned, I think its a wonderful introduction to QED that does not involve any intesive mathematics. It also will proabablz make you more excited about teaching physics (I know it did for me). Books on string theory can be intersting, but the problem is that string theory is still in its infancy, and has yet to make any concrete predictions that can be tested with todays technology. It is debatable if one would even call it proper science, but this shouldn't stop you from picking up a book on it if it tickles your fancy.

 

[juanrga]

I picked up a copy of Feynman's QED - "A strange theory of light and matter" from the university library today because it looked interesting and I was wondering if this book is still relevant to todays Quantum physics?

It's a fascinating read but I want to make sure I am not learning stuff that has been since disproven.

Any opinions would be greatly appreciated

Very much of Feynman QED is incorrect.

A modern view of QED is given in the textbooks of Mandl and Shaw and in the two volume book by Weinberg.

 

[KingBigness]

Thank you for your reply cbetanco. He by no means is pushing me towards engineering he just thinks being an engineer is more practical, but that is because he has not done to much study in 'pure' science so he does not know where it can take you.

I want to do physics and maths because I want to research and I enjoy it, and after all if you enjoy something then you never have to work a day in your life.

Your stat on physicists having a higher acceptance rate, if true, will greatly annoy my girlfriend who is studying a bach of mechatronic engineering with a masters of biomedical. Not that she will have any trouble getting accepted into post grad.

I understand you are just trying to highlight the advantages of studying physics, which I know, but I it hard to convey to somewhere where it can take you when ultimately they don't understand what it is in the first place.

 

[cbetanco]

Very much of Feynman QED is incorrect.

Which part? I have to admit I read the book a long time ago, and have not reread it after taking field theory

Your stat on physicists having a higher acceptance rate, if true, will greatly annoy my girlfriend who is studying a bach of mechatronic engineering with a masters of biomedical. Not that she will have any trouble getting accepted into post grad.

I will look for the exact numbers and post them when I find them. I also think physics majors have higher pcat and mcat scores, based off the statistics posted on the wall of the physics department at my last university...

 

[KingBigness]

I would also like to know where it goes wrong, I don't want to get the wrong information stuck in my head.

 

[homeomorphic]

The Road to Reality is pretty hard to read, I think, unless you already know the stuff. It's better as a supplement than trying to learn things from it directly. But it's a great supplement.

 

[KingBigness]

The Road to Reality is pretty hard to read, I think, unless you already know the stuff. It's better as a supplement than trying to learn things from it directly. But it's a great supplement.

Thanks for the advice, also at 1100 pages it may take a while to read =P
I was just thinking of brief books I can read before I learn it more in depth in the coming years.

 

[nonequilibrium]

https://www.amazon.com/dp/0691121397/?tag=pfamazon01-20

But definitely also buy the Road to Reality. It might take you a few years to get through (indeed best as a supplement), but it is worth every penny and every second.

All books by Penrose are brilliant. More accessible is his The Emperor's New Mind and also Shadows of the Mind. These two contain a significant amount of quantum mechanics (and Penrose is known for not shying away from the interpretation and meaning in his books) but let me also state that these two books are focused on the connection between quantum physics and consciousness (broadly speaking) and as a result also discussing things like computability (despite its name, also very interesting!), which might or might not be of your interest.

Also, read Einstein's Relativity (cheap Dover edition). It is very accessible (written for the main public, at least the part that doesn't mind an algebraic expression or two) but also very interesting, written by the master himself.

 

[KingBigness]

https://www.amazon.com/dp/0691121397/?tag=pfamazon01-20

But definitely also buy the Road to Reality. It might take you a few years to get through (indeed best as a supplement), but it is worth every penny and every second.

All books by Penrose are brilliant. More accessible is his The Emperor's New Mind and also Shadows of the Mind. These two contain a significant amount of quantum mechanics (and Penrose is known for not shying away from the interpretation and meaning in his books) but let me also state that these two books are focused on the connection between quantum physics and consciousness (broadly speaking) and as a result also discussing things like computability (despite its name, also very interesting!), which might or might not be of your interest.

Also, read Einstein's Relativity (cheap Dover edition). It is very accessible (written for the main public, at least the part that doesn't mind an algebraic expression or two) but also very interesting, written by the master himself.

I have Einstein's Relativity in mind for the next book I read.
I will also check out the others, thank you.

 

[Fredrik]

Don't let anyone tell you that "very much of" Feynman's book is wrong. It's one of the best non-mathematical books on physics, and it's short and very cheap. You'd be crazy not to buy it. If something in it is wrong, it's not going to mess you up more than any other of the non-mathematical books. They are all misleading to some degree because of how they dumb things down to avoid mathematics.

Penrose's books "The emperor's new mind" and "Shadows of the mind" were mentioned above. I enjoyed both of them a long time ago, but they are much more likely to mislead you than for example Feynman's book. It's generally accepted that Penrose's main point in those books (an argument intended to prove that human thought can't be carried out by something like a computer) is wrong. His argument doesn't hold. I also think he's making a few mistakes in his critique of QM. In particular, I don't think it's correct to say that QM is fundamentally time-asymmetric, and when he points out how weird QM is if you think of the wavefunction as a description of what the physical system is actually doing (good points IMO), he concludes that QM needs to be replaced...when he could have just concluded that maybe he shouldn't think of the wavefunction that way. If you are aware of these flaws, these books are still a good read.

You should definitely get "Black holes and time warps: Einstein's outrageous legacy" by Kip Thorne. I'd be surprised if it isn't still the best non-mathematical book about GR. So put it on the "must buy" part of your list, where Feynman's QED should already be.

I haven't read the Feynman lectures on physics, but everyone seems to really like them, so maybe you should consider those.

You might also want to consider one of the easy books on SR. A lot of people have asked for recommendations, so I'll just quote one of George Jones's answers.

I third Taylor and Wheeler, but I like the (red) paperback version of the first edition. I forget why I prefer the first edition over later later edition(s) (I have compared editions). I prefer the paperback version over the hardcover version of the first edition because the paperback edition has solutions (not just answers) to the problems. My battered and beaten copy (I got it while in high school) is in the bottom left of

https://www.physicsforums.com/showthread.php?p=1897989#post1897989.

Another introduction to special relativity that I really like is A Traveler's Guide To Spacetime: An introduction to the Special Theory of Relativity by Thomas Moore. Moore's book is maybe a little easier to read than Taylor and Wheeler.

 

[atyy]

Probably the chief conceptual advance since Feynman's original work (not the book, which is late), is why renormalization works. This is provided by "renormalization group" and "effective field theory ideas". The basic idea is that we don't need theories that are consistent at all energies. They just need to work at low energies. Renormalization flow is the process of seeing what a theory given only its symmetries "looks like" at lower and lower energies. A "renormalizable theory" like QED is one in which the flow converges to a fixed point, about which perturbation theory can be done.

QED is probably inconsistent at high energies as the flow does not converge perturbatively. There is a possibility that it converges non-perturbatively, in which case the theory would be "asymptotically safe".

For some theories, like QCD, the renormalization flow converges perturbatively at higher and higher energies, a feature known as "asymptotic freedom", and the theory is consistent at all energies. This doesn't mean it's true at all energies, it just means that experiment, and not just theory, is needed to show that it fails.

These ideas were roughly known to some in the high energy community such as Gell-Mann and Low, and also sketched to solve apparently completely different problems in condensed matter physics by Kadanoff. Full clarity came with Wilson's work in condensed matter, whose theorists are so incompetent they had to be saved by an outsider from high energy! (Don't kill me, I actually heard this from a condensed matter theorist :tongue2:)

An introduction to the renormalization group is given in http://arxiv.org/abs/0909.0859. It is very important in the postulated relationship, discovered by string theory, between quantum field theories in D dimensions, and gravity theories in D+1 dimensions http://arxiv.org/abs/0909.0518.

 

[juanrga]

Which part? I have to admit I read the book a long time ago, and have not reread it after taking field theory.

Feynman formulation of QED uses Dirac wavefunctions, position operators, negative energy electrons, antiparticles as particles traveling backward in time, and other silly (and invalid) stuff abandoned in modern formulations of QED (see Mandl and Shaw and Weinberg textbooks on quantum field theory for details).

Feynman diagrams (which he believed describing real phenomena at particle level) have now only a pictorial status. Probably the more fantastic piece of nonsense that I read from Feynman is when (in his QED textbook) he tries to convince us that quantum (Dirac) electrons move always at the speed of light.

Moreover, Feynman approach to electrodynamics is very tricky (and without mathematical rigor) and cannot be not used as a basis for QCD and other interactions.

 

[juanrga]

I would also like to know where it goes wrong, I don't want to get the wrong information stuck in my head.

See #21

 

[Fredrik]

Feynman formulation of QED uses Dirac wavefunctions, position operators, negative energy electrons, antiparticles as particles traveling backward in time, and other silly (and invalid) stuff abandoned in modern formulations of QED (see Mandl and Shaw and Weinberg textbooks on quantum field theory for details).

Hm, I don't think the book we're talking about mentions any of that (except maybe the idea that antiparticles can be described as particles going back in time*). Mandl & Shaw and Weinberg are both good books on quantum field theory, but they're much more advanced than what the OP needs. He hasn't even studied QM yet, if I recall correctly.

*) I don't know QFTs well enough to know if this makes sense.

Feynman diagrams (which he believed describing real phenomena at particle level) have now only a pictorial status.

Did he? It's been a long time since I read the book, so I don't remember how he described them. I think that when he talked about paths and probability amplitudes, he described it as a way to calculate probabilities, not as a description of what actually happens. Anyway, now that we have told the OP to think of Feynman diagrams as pictures that represent terms of a series instead of as descriptions of what actually happens, that particular detail is less likely to poison him.

Probably the more fantastic piece of nonsense that I read from Feynman is when (in his QED textbook) he tries to convince us that quantum (Dirac) electrons move always at the speed of light.

Did he say that they move at c "through spacetime"? While not technically wrong, this relies on a definition of "speed through spacetime" that I consider completely useless. I'm still curious because it might explain where Brian Greene picked up that idea. Anyway, now we're talking about a different book (right?).

 

[homeomorphic]

*) I don't know QFTs well enough to know if this makes sense.

Neither do I, but I don't think Feynman would have insisted that they literally move backwards in time, anyway. I think he was just saying you could think of it that way.

I get the feeling that some people just have something against intuition. They find it confusing because they don't think deeply enough about it. They say particles don't go back in time. They don't want to talk about Dirac's sea of electrons. But that was Dirac's inspiration. It's interesting. The solution for me, is not to abandon it, but to just point out it's only for inspiration.

Feynman diagrams may just be pictures of series expansions, but I think it may help to THINK of it as particles zipping around, even if that's not the literal truth. A serious thinker can allow himself these sorts of liberties without being lead astray.

Feynman's book is definitely worth reading, regardless. You won't learn it properly until you do real QED, but the point is just to get an idea, not to learn it properly.

 

[cbetanco]

Feynman diagrams (which he believed describing real phenomena at particle level) have now only a pictorial status.

This is still highly debatable. I know a lot of particle physicist (theorists and experimentalists) who consider them part of reality. I myself think think deep inelastic scattering experiments at the LHC prove their existence, but others aren't convinced.

 

[juanrga]

Hm, I don't think the book we're talking about mentions any of that (except maybe the idea that antiparticles can be described as particles going back in time*). Mandl & Shaw and Weinberg are both good books on quantum field theory, but they're much more advanced than what the OP needs. He hasn't even studied QM yet, if I recall correctly.

*) I don't know QFTs well enough to know if this makes sense.

That book is derived from his lectures. Feynman uses his misunderstanding of Dirac wavefunctions and of position operators for his discussion of amplitudes of paths in that book. In that book he also mentions negative energy states and traveling backward in time.

I forget to say that Feynman also uses the hole theory. A nonsensical theory now best regarded as «historical curiosity» as Weinberg and others emphasize.

Did he? It's been a long time since I read the book, so I don't remember how he described them. I think that when he talked about paths and probability amplitudes, he described it as a way to calculate probabilities, not as a description of what actually happens. Anyway, now that we have told the OP to think of Feynman diagrams as pictures that represent terms of a series instead of as descriptions of what actually happens, that particular detail is less likely to poison him.

The problem is not in the amplitudes per se, but in Feynman usage of solutions of the Dirac equation as if were wavefunctions.

Did he say that they move at c "through spacetime"? While not technically wrong, this relies on a definition of "speed through spacetime" that I consider completely useless. I'm still curious because it might explain where Brian Greene picked up that idea. Anyway, now we're talking about a different book (right?).

What relation exists between what I wrote and your «move at c "through spacetime"» and your «a definition of "speed through spacetime"»

 

[juanrga]

This is still highly debatable. I know a lot of particle physicist (theorists and experimentalists) who consider them part of reality. I myself think think deep inelastic scattering experiments at the LHC prove their existence, but others aren't convinced.

I also know a lot of people who knows little about the subject.

Feynman initially proposed them as real spacetime diagrams, but it is now clear that the spacetime behind QFT is unphysical. For this reason Feynman diagrams are now best regarded as pictorial representations. Or said in more explicit form the x and the t in Feynman diagrams (e.g., in the book read by the OP) have nothing to see with physical space and time. For that reason x was downgraded from operator status to unobservable parameter in QFT, although Feynman was unaware.

 

[cbetanco]

I also know a lot of people who knows little about the subject.

I don't really understand why you said this. What about people who know little about the subject?

Feynman initially proposed them as real spacetime diagrams, but it is now clear that the spacetime behind QFT is unphysical. For this reason Feynman diagrams are now best regarded as pictorial representations. Or said in more explicit form the x and the t in Feynman diagrams (e.g., in the book read by the OP) have nothing to see with physical space and time. For that reason x was downgraded from operator status to unobservable parameter in QFT, although Feynman was unaware.

How does demoting space and time to parameters make Feynman diagrams unphysical? Feynamn diagrams have ALWAYS been pictorial representations, but I don't see how just because they are pictorial representations, that invalidates the physical reality of what the pictures represent. Where did you read that Feynman was not aware that x and t were demoted from operator status to observable status?

 

[juanrga]

How does demoting space and time to parameters make Feynman diagrams unphysical? Feynamn diagrams have ALWAYS been pictorial representations, but I don't see how just because they are pictorial representations, that invalidates the physical reality of what the pictures represent. Where did you read that Feynman was not aware that x and t were demoted from operator status to observable status?

Feynman initially proposed them as real spacetime diagrams, but that now are best regarded as pictorial representations of scattering events.

I have not said the rest of what you say. What I said is that the x and the t in Feynman diagrams have nothing to see with physical space and time, as good references emphasize. Contrary to what you say x and t in Feynman diagrams are not observable, but unobservable parameters. Feynman was plain wrong about this as his articles show.