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

What attraction/repulsion?

  1. Oct 1, 2007 #1
  2. jcsd
  3. Oct 1, 2007 #2

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    what do you mean "how" ?
     
  4. Oct 2, 2007 #3

    jtbell

    User Avatar

    Staff: Mentor

    The "exchange particles" transfer momentum and energy from one particle to another. Simplistically speaking, you can think of one particle as "emitting" an exchange particle, which carries off some of the first particle's energy and momentum. Then the exchange particle is "absorbed" by the second particle, whose energy and momentum changes as a result. The energy and momentum gained by the second particle equals the energy and momentum lost by the first particle.
     
  5. Oct 2, 2007 #4
    But all of these particles are massless, so how can they possibly CARRY energy?
     
  6. Oct 2, 2007 #5

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    They are not massless?

    And there is the Heisenbergs uncertainty priciple:

    [tex] \Delta E \Delta t \geq \hbar /2 [/tex]
     
  7. Oct 2, 2007 #6

    jtbell

    User Avatar

    Staff: Mentor

    In relativity, the general relationship among mass, energy and momentum of a particle is

    [tex]E^2 = (pc)^2 + (m_0 c^2)^2[/tex]

    When [itex]m_0 = 0[/itex], [itex]E = pc[/itex]. Note that classical electromagnetic waves also have this relationship between the energy and momentum that they carry, so it fits that photons have the same relationship.
     
  8. Oct 3, 2007 #7
    In english?(I am sorry but for once, I want to understand the whole thing, I look at math as a tool, not as a conceptual aid.) After I understand it completely, then I'll be interested in the math.
     
  9. Oct 3, 2007 #8

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    Okay, but the language of modern(quantum) physics is math, that's just the way it is. Langragians, hamiltonians, propagators etc..

    But these are quite easy ones:

    [tex] \Delta [/tex] means uncertaninty,
    E is energy, t is time.
    So the Energy conservation can be violated, for as long as this equation is fulfilled.

    And the equation jtbell wrote just shows the equivalence of mass and energy and momentum. Massless particles DO carry energy, if they are in motion.
     
  10. Oct 3, 2007 #9

    arivero

    User Avatar
    Gold Member

     
  11. Oct 3, 2007 #10
    Although, this time, I understood what you told me..it just made me more confused. How can massless particles carry energy?
     
  12. Oct 3, 2007 #11

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    Thay can because there is "different types" of energy.

    Look at the photon:

    [tex] E_{\gamma} = hf = \dfrac{hc}{\lambda} [/tex]

    No mass in the expression for energy

    Have you somewere motivated WHY you think that massless particles cant carry energy?
     
  13. Oct 4, 2007 #12

    arivero

    User Avatar
    Gold Member

    [tex] F . l [/tex]

    If you can push an object with a force F along a distance l, you are carrying energy. Force is the quantity of momentum you can transfer by unit of time. The photon can transfer momentum, thus it can carry energy.

    Of course this leaves the real fundamental question: how is it that a massless particle can transfer momentum? Here you can start to discuss the foundations of special relativity and quantum mechanics. But do not worry about Force and Energy, they are mathematical concepts with big names.
     
    Last edited: Oct 4, 2007
  14. Oct 4, 2007 #13

    mjsd

    User Avatar
    Homework Helper

    How can massive particles, in your opinion, carry energy then? What makes you think that mass is a necessary criterion to define/carry energy? are you thinking about [tex]E=\frac{1}{2} mv^2[/tex] ?
    look at the sun's radiation, it is full of energy right? but do you actually question whether it is the radiation that carries the energy or something else? Is the radiation massive or massless?
     
  15. Oct 4, 2007 #14
    I guess you are right, I have a mental block here. But then how do we define matter? I used to think anything that has mass or energy is matter. Anything else is vacuum. But now, I have been introduced to new concepts like this.

    How are massless particles created?

    Since anything that has a weight needs infinite energy to travel to the speed of light....and since energy is mass...how can photons carry energy and still travel at the speed of light? Won't energy slow them down?
     
  16. Oct 4, 2007 #15

    arivero

    User Avatar
    Gold Member

    If you want to think in dual terms (matter/vacuum) probably the best framework is to call "matter" to the fermions of the standard model and "fields" to the gauge bosons.

    Another alternative is to call "matter" to any distribution of mass or energy and "space" to the space-time of general relativity.
     
  17. Oct 6, 2007 #16
    There are some, rare, few, who don't believe that particles play catch... I believe it was Newton who first suggested that forces at a distance, through empty space, was ridiculous-and the first to admit that he had no idea. What makes any of us think that we are smarter than Newton? In other words, Newton vocated a medium. How particles passing through space, therein causing two bodies to come closer together, could have ever been proposed- is clear evidence that our intellect is decaying.
     
  18. Oct 6, 2007 #17

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    It is a difference having great knowledge and beeing smart/talented. We have much more knowledge today and we have made very large progresses in both theoretical and experimental physics. Its not beeing smart or not, the only thing that matter is what is true. You can't compare Newton with todays science.
     
  19. Oct 11, 2007 #18
    Question still sparks...how can massless particles carry energy? Since energy is mass in a converted form...won't it slow it down? Since it takes infinite energy to accelerate mass to the speed of light.....I think you are getting the point.

    And why do they transfer energy? What makes them do so? How are these massless particles created? How do they die?
     
  20. Oct 11, 2007 #19

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    you are confusing rest mass and energy. Photon has indeen zero rest mass.
     
  21. Oct 11, 2007 #20
    A photon is never at rest. Once upon a time there was this guy named Aristotle. Aristotle gathered up all of the metaphysical philosophies that preceded and produced his own model for substance, which defined matter. His concept of matter has always remained the underlying foundation of our science.

    Along came a spider... who decided to introduce the concept of energy into that foundation- without qualification. You, Skhan- are the fly, trapped in the inevitable confusion of the illusion that energy is something that can suddenly appear and dissappear, be absorbed and emitted, and existentially transform itself from one thing to another. But that is the science you have chosen to swallow, I mean follow..


    Should I regurgitate a lie, or just tell you the truth?
     
  22. Oct 11, 2007 #21
    Have you ever heard of "pair production" and "annihilation". When an electron and a positron collide they form a photon, also, in cloud chambers these two can suddenly appear from a photon. Now according to W. Heisenberg these two phenomenon are not to be construed in any way as evidence that a photon is composed of said particles... You see a photon is suppose to be truely fundamental "in the stricktest sense". Instead, you are told to believe without qualification that a photon is a mysterious energy source that has the unique ability to transform itself into and from these particles.

    It reminds me of the old theory that rotting meat tranforms itself into flies- prior to the understanding that flies laid eggs first...

    Now, what do you know about the photon. First it doesn't have a very large mass if any at all? Why? Well for something to have mass, we need to be able to move it. It is known that light bends or deflects around the sun- an apparent contradiction- because if it can be deflected it has a mass, and it also ( by vectors) reaches a velocity greater than the speed of light. To circumvent this contradiction, you are told by Einstein that space is curved, without qualification of how or in what way- for if space is curved then it must have structure- as I see it.

    The photoelectric effect is perhaps the most interesting evidence, that photons do possess mass. You see, the phenomenon shows that individual photons are responsible for ejecting electrons from a photoelectric plate. It is this phenomenon which defines the energy of a photon as E=hv. The mass of a photon is suppose to have a mass of < 3x10-33 MeV according to the stable partice list , CRC Handbook for Chemistry and PHysics. The Compton effect also showed that a photon can lose some energy when reflecting. This shows that it can transfer not only quantum amounts, but portions.

    Consider that mass, in the case of particles at least are electromagnetically determined from their trajectories according to their curvature in a cloud chamber and therefore according to the electromagnetic, E, B, fields used. A photon is neutral. It doesn't necessarily interact with a cloud chamber, so we cannot see its path. We can only know that it or another neutral particle was present when two oppositely charged particles suddenly appear. There is one exception, however, for it is know that extremely high energy gamma rays do leave tracks-and we are told that they excite the molecules in the cloud chamber.

    It is interesting that Einstein's photon in a box thought experiment supported the idea that the photon could move the box...

    Now why don't we do a thought experiment of our own. What if a photon is composed of a positron and an electron. Why so many wavelengths? Perhaps the two are spinning. Why does it appear massless- because its neutral. If it spins fast it has lots of 'energy' and a short wavelength, if it spins slow it has low energy and a long wavelength.

    Did you know that light is composed of left and right circularly polarized components? The two components can be spinning either left or right, like left and right helices.

    My point is this, a particle has two types of energy if we use the foundation that rests upon Aristotle: kinetic and potential. If we are only able to see the photon as energy, then we must be concentrating upon the kinetic energy of its internal components. The reason we are forced to do this, and forced to ignore everything else about the photon, is simply because we have not been able to answer one question:"why is there a limit to the speed of light?" Now that is an interesting question...
     
  23. Oct 15, 2007 #22
    If photons have mass then how come it can travel to the speed of light? This might be off topic but since all velocities are relative, how come the speed of light is absolute? Why do things change their shape as they change their speed? Why is time affected by it too?

    Can you recommend may be a few good books on it?(seems I ask a lot of question...I like books that take things from fundamental point of view, reason every claim, etc.)

    Sean...please don't be offended but I am interest in aknowledging your qualifications in physics b/c you bring more questions in my mind than answers.(which I love b/c it opens my mind!...but based on your inability to answer those questions, doubts have been raised.)

    One thing more, are the particles like gravitons a weak theory like string or are they strickly proven.(I know that there is not a fine line btw them but mathematically, do they make sense? What do you think?)
     
    Last edited: Oct 15, 2007
  24. Oct 15, 2007 #23
    Do photons have mass? I believe that the answer is 'undecided'. And this is why I related my interpretation of the different schools-ending with a hypothetical idea, since the matter remains hypothetical, unproven, and often controversial.
     
  25. Oct 15, 2007 #24

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Please read the FAQ in the General Physics forum.

    You may also want to read the PF Guidelines that you have agreed to. Pay particular attention to speculative, unverified theory/post.

    Zz.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook