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Thread: Expansion problem question

  1. #1 Expansion problem question 
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    I have a question about the virtual particle antiparticle pairs created within the lowest energy quantum state, now I understand how and why they destroy each other, but what I don't understand is why if these particles can also have a dielectric effect on the background electromagnetic feild reducing it's affect why this wouldn't then also have major drag on/or slowing effect on expansion?


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    Quote Originally Posted by Chrisgorlitz View Post
    I have a question about the virtual particle antiparticle pairs created within the lowest energy quantum state, now I understand how and why they destroy each other, but what I don't understand is why if these particles can also have a dielectric effect on the background electromagnetic feild reducing it's affect why this wouldn't then also have major drag on/or slowing effect on expansion?
    Vacuum polarization depends on the presence of an electromagnetic field; there is no ( known ) electromagnetic field present which spans the universe globally.


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    Cheers Markus, sorry to be asking daft questions, some of this stuff seems really complicated to get a handle on though. Right next daft question lol, if the Vacuum state isn't producing a electromagnetic field what type of energy is it producing? , I checked this on Wiki and it references "According to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence", which I took to mean would result in an EM field.
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    Chris: take a look at the Aharonov-Bohm effect. That's where when you turn on a solenoid, and it causes a phase shift for passing electrons, even though there's no electric field and no magnetic field outside the solenoid. See this bit of the Global action vs local forces section:

    In fact Richard Feynman complained[citation needed] that he had been taught electromagnetism from the perspective of E and B, and he wished later in life he had been taught to think in terms of the A field instead, as this would be more fundamental.

    There's another bit higher up under Significance that says:

    ...all physical effects were describable in terms of the fields which were the derivatives of the potentials...

    Space isn't one big EM field. If it was, we'd see electrons all moving one way and protons the other. But that's not to say it isn't one big A field. If it's all at the same potential you don't notice it, there's no "slope" anywhere, so the derivatives are zero and there's no discernible electromagnetic field.
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    Quote Originally Posted by Farsight View Post
    Chris: take a look at the Aharonov-Bohm effect. That's where when you turn on a solenoid, and it causes a phase shift for passing electrons, even though there's no electric field and no magnetic field outside the solenoid. See this bit of the Global action vs local forces section:

    In fact Richard Feynman complained[citation needed] that he had been taught electromagnetism from the perspective of E and B, and he wished later in life he had been taught to think in terms of the A field instead, as this would be more fundamental.

    There's another bit higher up under Significance that says:

    ...all physical effects were describable in terms of the fields which were the derivatives of the potentials...

    Space isn't one big EM field. If it was, we'd see electrons all moving one way and protons the other. But that's not to say it isn't one big A field. If it's all at the same potential you don't notice it, there's no "slope" anywhere, so the derivatives are zero and there's no discernible electromagnetic field.
    For once I would tend to agree with you - it does make more sense to formulate the Maxwell equations in terms of the 4-potential, and thus in tensor form :



    which has the obvious advantage of being independent of the choice of coordinates.
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    Thanks Markus. I'm not sure even that's ideal. I look on wikipedia and essentially "defines the four-potential in terms of physically observable quantities". It's kind of like describing what it does rather than what it is. It feels like it isn't getting to the heart of it. Maybe there's guys out there who got it totally sussed, but judging from my conversations with people, it doesn't seem to come through in modern teaching.

    Edit: I should add for Chris's benefit that the thing we call the Aharonov-Bohm effect, which is sometimes described as an example of "quantum weirdness", was actually predicted in Ehrenberg & Siday's 1949 classical paper The Refractive Index in Electron Optics and the Principles of Dynamics.
    Last edited by Farsight; October 3rd, 2012 at 04:59 AM. Reason: nu not u and Ehrenberg & Siday
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    Quote Originally Posted by Farsight View Post
    It's kind of like describing what it does rather than what it is.
    I think what is meant by this is simply that the relation connects the abstract 4-potential to the electromagnetic field tensor, the elements of which are the E and B fields which can be directly measured. One cannot directly measure ( in physical terms ) the potential A, only the resultant fields E and B.
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    The 4-potential might be something of an abstraction, Markus, but there's definitely something real that underlies it. The Aharonov-Bohm effect proved that:

    "The Aharonov–Bohm effect illustrates the physicality of electromagnetic potentials, Φ and A, whereas previously it was possible to argue that only the electromagnetic fields, E and B, were physical and that the electromagnetic potentials, Φ and A, were purely mathematical constructs".

    Which reminds me of that Minskowski quote from Raum und Zeit:

    "Bei der Beschreibung des vom Elektron hervorgerufenen Feldes selbst tritt sodann hervor, daß die Scheidung des Feldes in elektrische und magnetische Kraft eine relative ist mit Rücksicht auf die zugrunde gelegte Zeitachse; am übersichtlichsten sind beide Kräfte zusammen zu beschreiben in einer gewissen, wenn auch nicht völligen Analogie zu einer Kraftschraube der Mechanik."

    It isn't really two fields. E and B merely describe the linear and rotational forces resulting from field interaction, and are another example of what it does rather than what it is. The electron's electromagnetic field is one field, with a "power screw" disposition. And potential is more fundamental than field.

    You know Markus, sometimes I think modern physics is like a man in a boat on the ocean. He can see the whirlpools, and he can see the waves. But he can't see the sea.

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    Cheers for your help guys that was much appreciated.
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    Quote Originally Posted by Farsight View Post

    You know Markus, sometimes I think modern physics is like a man in a boat on the ocean. He can see the whirlpools, and he can see the waves. But he can't see the sea.
    I agree with everything you have said about the reality of the 4-potential, and E and B being only two aspects of the same field. What I don't really understand though is your last sentence, because the textbooks I have used always made it clear that E and B are only aspects of the same underlying thing, which is also why they can easily be unified into just one field tensor.
    Even in Maxwell's original formulations, the equations for E and B are not separate, but belong together in one system of equations.

    Do you think modern science treats E and B as totally separate entities ? I would disagree with such a statement, except maybe in certain circumstances for mathematical convenience.
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    Quote Originally Posted by Markus Hanke View Post
    Do you think modern science treats E and B as totally separate entities?
    No and yes. See wikipedia where in the dynamics section you can read "Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole — the electromagnetic field." There's some misleading statements on that page, see the first line, but it is only wikipedia. What's more important is that we see qualified professionals talking about the electric field and the magnetic field as if they're two separate entities. See arXiv for examples. Then if I dig out bona-fide course notes I can see the radial "electric field" lines here, on this I read the myth that moving charges "produce" magnetic fields, and elsewhere I can see concentric "magnetic field" lines. But it's the electromagnetic field. Field interaction results in linear and/or rotational motion. Nowhere do I see any representation of an electromagnetic field combining those radial and concentric lines into the "power screw" spiral lines. I see that on depictions of a gravitomagnetic field, but not electromagnetic field lines. Overall I think there are some issues with the way electromagnetism is taught and understood.
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    Quote Originally Posted by Farsight View Post
    Quote Originally Posted by Markus Hanke View Post
    Do you think modern science treats E and B as totally separate entities?
    No and yes. See wikipedia where in the dynamics section you can read "Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole — the electromagnetic field." There's some misleading statements on that page, see the first line, but it is only wikipedia. What's more important is that we see qualified professionals talking about the electric field and the magnetic field as if they're two separate entities. See arXiv for examples. Then if I dig out bona-fide course notes I can see the radial "electric field" lines here, on this I read the myth that moving charges "produce" magnetic fields, and elsewhere I can see concentric "magnetic field" lines. But it's the electromagnetic field. Field interaction results in linear and/or rotational motion. Nowhere do I see any representation of an electromagnetic field combining those radial and concentric lines into the "power screw" spiral lines. I see that on depictions of a gravitomagnetic field, but not electromagnetic field lines. Overall I think there are some issues with the way electromagnetism is taught and understood.
    Well, sometimes it is convenient to separate the two, for purely mathematical reasons. My understanding ( and I am not a scientist ) was always that E and B are just two aspects of the same electromagnetic field.
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