Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A few questions about QM and QFT.

  1. May 29, 2007 #1
    I'm only going to be a junior next year in high school so bear with me but I've been "studying" quantum mechanics and quantum field theory for a while now. I understand most of what I've read very well but there are a few things that have been bothering me.

    *How does QFT explain how two opposites attract?
    *Does the idea of photon exchange between electrons on the microscopic scale also apply to macroscopic magnets?
    *How does a low-energy microwave photon pass through walls and such but higher-energy visible light photons can't?
    *Is there any experimental evidence for the photon exchange described by quantum field theory?
    *Why does the observer matter so much when trying to apply QM to the entire universe? I don't understand this because the laws of physics are constant everywhere. If I perform an experiment where I live and travel 100 million miles out into space and do the same thing, I'm going to get the same result.
    *I'm reading a book right now that basically said everyone's idea of quantum mechanics is different (other than the math). Does this mean all I know is just someone's opinion and is probably wrong?

    I obviously don't have the luxury of math that you guys have so I would appreciate it if you tried to answer my questions as best you could without it. Thanks.
  2. jcsd
  3. May 29, 2007 #2
    What book is that - and what other books are you using?
    You may need some more reading,
    but at least let the folks here help you avoid any real crackpot metaphysics books.
  4. May 29, 2007 #3
    I'm reading Three Roads to Quantum Gravity right now. Thats the one that said everyone's idea of QM was different. I'm also reading Deep Down Things: The Breathtaking Beauty of Particle Physics. That one is where I get most of my concepts of quantum field theory.
  5. May 29, 2007 #4
    I’ll give three additions over time you will find helpful, don’t expect to be get it all just this Summer;

    Understanding the universe : from quarks to the cosmos
    By Don Lincoln

    Entanglement : the greatest mystery in physics
    By Amir D. Aczel

    And here is an odd one you have to promise to be careful with:
    The dancing Wu Li masters
    by Gary Zukav

    On this one I recommend skimming through some of the material starting at;
    page 212 and page 282
    Then later when you think your ready for those topics come back and read it again.
    [edit; opps - I see this book has many issues and page numbers do not track - - chapters "the Dance" & " the End of Science" a page or two in is where to look]

    BUT here is the deal with Wu Li – it is just full of Metaphysically Philosophical Trash

    I not telling you to not read it, sooner or later you will need to figure out when your reading junk or real science. And that’s just my opinion of the metaphysics anyway.
    Heck even real scientists can publish scientific junk sometimes.
    Although Zukav has a nice way of explaining complex science in a fairly complete way that it can be understood – this also means it get a bit long but hat can be good.
    BUT remember he is also very good at slipping in his own Weird Metaphysically opinions that have nothing to do with science, so just done get hooked into those parts scattered in along with the science.

    Much of what is in these do not relate directly to QM and QFT, but you need to know the physics problems they are working to solve to understand where they are going.
    Last edited: May 30, 2007
  6. May 29, 2007 #5


    User Avatar
    Science Advisor

    Last edited by a moderator: Apr 22, 2017
  7. May 30, 2007 #6
    I have to say that I'm very surprised by the amount of help I'm getting. Most of the members of other forums are jerks and really don't help at all. I really appreciate all you guys' help. Thanks. I'll definitely check out those books especially the one about entanglement and the one written by Feynman.
  8. Jun 11, 2007 #7
    I appreciate the help with books but can someone help me with my questions? Thanks.
  9. Jun 11, 2007 #8
    try the universe its an amazing book
  10. Jun 29, 2007 #9
    Last edited by a moderator: May 2, 2017
  11. Jun 30, 2007 #10
    The funny thing is most QFT textbook does not cover that. They are all about computing scattering amplitudes and decay rates. The best answer to you question can still be foundhttp://www.math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html" [Broken].

    No, then you use classical electromagnetic fields. There is an uncertainty relation, the more precise knowledge you got of your classic electric field, the less do you know about the number of photons exchanged.


    Virtual particles are never seen, most say they are only a metaphor for calcuating procedure in QFT.

    Note when interpreting QM we need to split the world in quantum stuff that we measure and an classical observer. But where and when make that split? Quantum laws hold for the entire universe.

    Note, they all differ on the interpretation of QM, not the theory itself. Very few believe that the theory need to be revised.

    EDIT: Note this real and virtual photon business is very confusing. But picture this, you got an electric wire sending out a static electric field with definite electric field strength. Now the number of (real) photons is completly uncertain. Or from QFT view you say virtual photons are exchanged, but which anyway just a metaphor. But when you start moving the wire and you moving it more and more rapidly, the electric field keeps fluctuating more and more, becomes more uncertain. But know the information of real photons becomes more available, so photons become real. .... well that's how I see
    Last edited by a moderator: May 2, 2017
  12. Jul 1, 2007 #11
    The book said something like how the uncertainty principle can be applied to electric and magnetic fields. If one knows the precise value of the electric field, you can't know anything about the magnetic field. Does that have anything to do with what you are saying?

    Thanks for answering my questions, by the way.
  13. Jul 2, 2007 #12
    I also noticed that QFT textbooks are usually limited to calculations of the S-matrix (=scattering cross-sections, decay rates, and energies of bound states). I haven't seen any rigorous attempts to investigate the finite time evolution of interacting states in QED. I would appreciate any references about that. My guess is that the time evolution cannot be studied within the (renormalized) QFT in its current form, because the Hamiltonian is ill-defined (it contains infinite counterterms).

    The URL provided by you didn't make things clear for me. It looks like pure hand-waving.
    Last edited by a moderator: May 2, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook